Astronomy & Space curriculum

Astronomy studies matter beyond Earth: where objects are, how big they are, how they move, what they are made of and how they change. You cannot bring a star into a lab, so most of that comes from its light. A spectrograph spreads the light into its wavelengths, and the lines in that spectrum show what an object is made of and, from how far they shift, how it moves.

A claim gains weight when independent evidence agrees. Europa's smooth, lightly cratered surface first hinted that something was erasing its craters. Later, the Galileo spacecraft found Jupiter's magnetic field disturbed around Europa in a way best explained by a salty ocean under the ice. Two unrelated lines of evidence pointing the same way beat either one alone.

The astronomical unit, Earth's average distance from the Sun, keeps planetary distances readable: Saturn sits at about 9.5 AU. A light-year is a distance, not a time, about 9.5 trillion km, and suits nearby stars. A parsec, about 3.26 light-years, comes from parallax, the small shift a star shows as Earth circles the Sun, and takes over once distances run to thousands of light-years and beyond.

Writing a number as a value times a power of ten lets you compare scales at a glance. The Sun is about 1.5 times 10 to the 8th km away, while a light-year is about 9.5 times 10 to the 12th km. Put both in AU and a star system 4.3 light-years off turns out to be roughly 270,000 times farther away than the Sun.

Even at the speed of light, sunlight still needs about 43 minutes to cross from the Sun to Jupiter and about 2 hours 40 minutes to reach Uranus. So every view is a view of the past. The stars you see tonight are not even from one moment: light from some left a few years ago, from others hundreds of years ago.

Gravity binds stars into galaxies, and galaxies into groups of about 100 or fewer members or into clusters of thousands. Superclusters, made of groups and clusters, are not held together by gravity; with empty voids and walls of galaxies they form the cosmic web. The Milky Way itself sits in a small group of galaxies just off the edge of a larger cluster.

Most ordinary matter in the universe is plasma, gas whose atoms have lost electrons, so it behaves differently from the air around you. Gravity pulls every mass toward every other and acts across any distance, so it holds moons, planets and stars in their orbits. Inside stars, nuclear fusion turns a little mass into energy, as E = mc squared describes.

Picture the sky as an imaginary sphere around you. Its poles sit where Earth's axis points, and its equator is Earth's equator projected outward. Because Earth spins, every star appears to wheel across the sky from east to west once a day. That daily motion is your own rotation showing, not the stars racing around you.

The sky is divided into 88 official constellations, regions whose boundaries were formalized in 1930, so a constellation is an area, not just a figure. An asterism is an informal star pattern, often only part of a larger official constellation. Stars in one pattern can sit at very different distances, so their light left them at very different times.

Altitude is an object's angle above your horizon, from 0 degrees at the horizon to 90 at the zenith straight overhead. Azimuth is its direction along the horizon, measured from north through east. The frame is tied to your spot on a rotating Earth, so a star's altitude and azimuth change all night and differ from one observer to the next.

Declination works like latitude, running from +90 to -90 degrees north or south of the celestial equator. Right ascension works like longitude but is counted eastward from the vernal equinox point, usually in hours, with 24 hours making a full circle. The grid is fixed to the stars rather than to the ground, so a star keeps its coordinates while its altitude changes.

On the magnitude scale larger numbers mean fainter. The faintest stars you can see without a telescope are about magnitude 6, and the brightest star is about -1.4. Each step of one magnitude is a brightness factor of about 2.512, so five steps make a factor of 100. Apparent magnitude says how bright a star looks from here, not how much light it gives off.

As Earth orbits, the Sun appears to travel a full circle against the background stars each year. That path is the ecliptic, named because eclipses can happen only when the Moon is near it. A given star also crosses your meridian about four minutes earlier each day, and over months that drift brings different constellations into the night sky.

Around 250 BC Eratosthenes compared noon shadows on the summer solstice: the Sun lit a deep well at Syene, while a column at Alexandria showed it 1/50 of a circle from overhead, so Earth's circumference was about 50 times the distance between the towns. Ptolemy kept Earth fixed and fitted a planet's backward loops by carrying it on a small circle, an epicycle, whose center rode a larger circle.

Copernicus put the Sun at the center and showed that a planet's backward, retrograde drift is only apparent, an effect of watching from a moving Earth. Yet he kept circular orbits and even epicycles, so his system was no simpler than Ptolemy's and Mars still did not fit. Tycho Brahe's naked-eye positions, the most accurate of his day, let Kepler finally replace the circles.

Each planet moves on an ellipse with the Sun at one focus, not at the center, so its distance from the Sun keeps changing between perihelion, the nearest point, and aphelion, the farthest. Eccentricity measures how flattened the ellipse is, from 0 for a circle toward 1. Mars gave Kepler the clue because its orbit was the most elliptical in Tycho's data.

A line from the Sun to a planet sweeps out equal areas in equal times. To cover the same area while close, the planet must move faster, so it is quickest at perihelion and slowest at aphelion. The deeper reason is conservation of angular momentum, and on a stretched orbit the effect is large: a comet can move many times faster near the Sun than far out.

The square of a planet's orbital period is proportional to the cube of its orbit's semi-major axis, so periods grow steeply with distance: about 88 days for Mercury, 365 for Earth and 10,759 for Saturn. Newton's general form brings in mass, which is how astronomers weigh moons, double stars, black holes and exoplanets from nothing more than their orbits.

Gravity pulls every mass toward every other, more strongly for bigger masses, and weakens with the square of the distance: double the separation and the pull drops to a quarter. Kepler found his rules from data without knowing what drove them. Newton's gravity supplied the cause, and today spacecraft planners add Einstein's relativity only when they need the extra precision.

An orbiting spacecraft is falling the whole time, but moving sideways so fast that it keeps missing the ground; near the space station's height that takes about 28,000 km per hour. Escape from Earth's surface needs about 11.2 km per second. With enough energy to escape, as the Voyagers had for the Sun, a craft is no longer gravitationally bound.

Planets tug on each other, so no orbit is a perfect ellipse. Uranus straying from its predicted path led to Neptune being found by calculation in 1846. At Lagrange points a small body can keep pace with two large ones; trojan asteroids gather at Jupiter's L4 and L5. In a resonance the periods form a simple ratio, which can stop two bodies from ever passing close even when their orbits cross.

Earth's axis is tilted about 23.4 degrees and keeps pointing the same way in space, so for half the year the north leans toward the Sun and gets higher sunlight and longer days. Distance is not the cause: Earth is nearest the Sun at about the time northern winter begins, and the two hemispheres have opposite seasons at the same moment.

At an equinox Earth's axis tilts neither toward nor away from the Sun, the noon Sun stands overhead at the equator, and day and night are nearly equal at every latitude. At the June solstice the Sun is overhead at the Tropic of Cancer, 23.5 degrees north, giving the longest day north of it; the December solstice puts it over the Tropic of Capricorn.

A solar day, noon to noon, averages 24 hours. During one spin Earth also moves almost a degree along its orbit, so it must turn about 361 degrees to bring the Sun back; one turn relative to the stars, the sidereal day, is shorter by those last few minutes. Everyday clocks follow the Sun, and US time zones are offset from UTC by whole hours.

The cycle of seasons, the tropical year, lasts about 365.2422 days. Adding a leap day every fourth year, the Julian rule, overshoots by about 0.0078 days a year. The Gregorian rule drops leap days in century years not divisible by 400, so 1700 was not a leap year but 2400 will be, and the average year matches the seasons to within half a minute.

The Sun always lights half of the Moon; a phase is how much of that lit half faces you. At new Moon the lit side faces away and the Moon sits near the Sun in your sky; at full Moon it sits opposite the Sun. Earth's shadow plays no part except during a lunar eclipse. A full cycle takes about 29.5 days.

The Moon spins once for each 27.3-day orbit, so the same face always points at Earth. The far side is not dark; it simply gets its daylight while turned away from you, and it is fully lit at new Moon. Early on, tides flexed the hot young Moon and drained its spin until rotation matched orbit, and all the solar system's large moons are locked the same way.

The Moon pulls hardest on the side of Earth facing it and least on the far side, so the oceans bulge on both sides and most coasts see two high tides a day. When Sun and Moon line up, at new and full Moon, you get large spring tides; at the quarter Moons they partly cancel into neap tides. Tidal friction also slowly brakes Earth's spin.

A solar eclipse needs a new Moon and a lunar eclipse a full Moon, yet most months have neither, because the Moon's orbit is tilted against Earth's and the Moon usually passes above or below the Sun. Totality is seen only inside the Moon's narrow umbra; the wider penumbra gives a partial eclipse. Near its farthest point the Moon looks too small and leaves a ring of Sun.

Light travels as an electromagnetic wave, described by its wavelength or frequency, and needs no medium, so it crosses empty space. It also comes in photons, discrete packets of energy, and a shorter wavelength means more energy. A digital camera shows the particle side directly: each photon that lands frees an electron that the detector records.

Radio waves, infrared, visible light, ultraviolet, X-rays and gamma rays are one kind of radiation in order of wavelength, with energy rising as wavelength shortens. Visible light largely passes through the air, but atmospheric gases absorb or reflect much of the rest, so X-ray telescopes must work above the atmosphere. Infrared slips through dust that hides visible light.

The hotter an object, the shorter the wavelength at which its light peaks; the peak wavelength is inversely proportional to temperature. That makes a star's color a thermometer you can read from very far away. A blowtorch flame shifts from reddish to bluish as it is turned up to burn hotter, and a star's color shifts with its surface temperature in the same way.

A blackbody's total output grows with its surface area and with the fourth power of its temperature, so a small rise in temperature means a large rise in output. Size matters as much as heat: Rigel's surface is more than twice as hot as the Sun's, but the star is at least 70 times larger, and it emits about 120,000 times the Sun's luminosity.

A hot, dense object glows with a continuous spectrum, every color with no gaps. A hot, thin gas against a cooler background emits only a few separate wavelengths, an emission-line spectrum. Shine a continuous source through cool, thin gas and it absorbs light at a few separate wavelengths, leaving dark absorption lines. Kirchhoff summarized the three cases in 1859.

An atom absorbs light only when a photon has exactly the energy to lift one of its electrons to a higher level, and it emits that energy when the electron drops back. Each element has its own set of levels, so its pattern of lines works like a fingerprint you can read in starlight. Hydrogen's visible Balmer lines are a classic example of one such set.

Motion toward you shifts every spectral line to shorter, bluer wavelengths; motion away shifts it redward. The shift measures only radial velocity, the part of the motion along your line of sight, so motion straight across the sky does not show up in it. Even so, it reveals planets tugging on their stars. The redshift of very distant galaxies is a separate effect, caused by expanding space.

A refractor bends light through a lens; a reflector gathers it with a concave mirror. In Hubble's Cassegrain design a smaller convex secondary mirror sends the light back through a hole in the primary to the instruments. Glass bends blue light more than red, so a lens brings each color to focus at a slightly different point, which leaves a fringe of color around bright objects.

Mirror size controls two different things. Collecting area decides how faint an object you can detect: Webb's 6.5-meter mirror gathers about six times the light of Hubble's 2.4-meter one. Resolution, the finest detail you can separate, grows with mirror size but shrinks as wavelength gets longer, so Webb, working in the infrared, sees about as sharply as Hubble does in visible light.

Magnification rises with focal length relative to the main mirror's diameter, but it only enlarges the image. What limits how faint you can see is collecting area: a larger mirror gathers more light, and Hubble's gathers about 40,000 times as much as your eye. Compare telescopes by aperture first, because extra magnification cannot add light.

Turbulent air blurs starlight and makes stars twinkle, and that blur limits the sharpness of every telescope on the ground; above the air, Hubble sees about ten times more sharply than even larger ground telescopes usually manage. Adaptive optics uses a deformable mirror to undo some of the blur, but it reduces rather than removes it and so far works only over small fields near bright reference stars.

Modern detectors are electronic: in a digital camera's sensor each arriving photon frees an electron, and the electrons are counted and stored to build the image. A spectrograph spreads an object's light into its wavelengths so its composition, temperature and motion can be analyzed. Spectroscopy, reading the lines in that spread-out light, is a key way to work out what distant objects are made of.

Radio telescopes can observe from the ground, because sunlight, clouds and rain do not affect radio work. The catch is resolution: radio waves are so much longer than visible light that a radio dish must be far larger than an optical telescope to see as sharply. Linking many dishes into an array, as the Very Large Array in New Mexico does, gives the resolution of a telescope as wide as the whole array.

Earth's atmosphere blocks X-rays and much of the ultraviolet, and at most infrared wavelengths it is nearly opaque and glows brightly, so those bands need telescopes in space. Infrared also demands a very cold telescope, because warm instruments glow in it themselves; at the Sun-Earth L2 point, about 1.5 million km away, Webb's sunshield can block the Sun, Earth and Moon all the time.

The Sun holds about 99.8 percent of the solar system's mass, and Jupiter has more than twice the material of everything else combined. Around them orbit the planets, at least five dwarf planets, hundreds of moons, vast numbers of asteroids and perhaps trillions of comets and icy bodies. A dwarf planet is round but has not cleared its orbital path.

The four planets nearest the Sun are rocky worlds of rock and metal with solid surfaces. The four giants are much larger, mostly fluid and have no true surface: Jupiter and Saturn are mainly hydrogen and helium, while Uranus and Neptune are 80 percent or more hot, dense water, methane and ammonia. All four giants carry rings and many moons.

Seen from above the Sun's north pole, every planet orbits counterclockwise, and all the orbits lie close to one plane, the ecliptic. A spinning cloud of gas and dust that collapsed and flattened into a disk before the planets formed explains both features at once, which is why any account of the solar system's origin must reproduce them.

When a young body gets hot enough to melt, dense iron sinks toward the center and lighter rock rises, sorting it into core, mantle and crust. Some asteroids partly melted this way, and on the young Moon lighter rock floated up from a magma ocean to form the crust. Heat can also arrive long after formation: tidal flexing by Jupiter keeps Io hot enough for hundreds of volcanoes.

Impacts pile up over time, so a surface with more craters has generally been exposed longer. Counting craters ranks surfaces by relative age, and rock chemistry and spectra help tie those ages to dates. A world with few craters has been resurfaced recently: Venus by volcanism and shifting crust, Europa by its icy flows.

The solar system began as a giant cloud of gas and dust that collapsed under its own gravity, spun faster and flattened into a disk. Most of the material fell to the center and became the Sun. In the disk, dust and ice gathered into planetesimals, solid bodies larger than about a kilometer that built the planets, and the young Sun's wind blew away much of the leftover gas.

Close to the young Sun only rock and metal could stay solid, which is why the inner planets are rocky. Beyond the snow line it was cold enough for water, carbon dioxide or methane to freeze into ice grains. Each compound has its own line, with water's closer to the star than methane's, and the bodies of the outer solar system, from comets to Kuiper Belt objects, are icy.

Earth has a solid iron-nickel inner core, a liquid outer core about 2,300 km thick, a hot, slowly flowing mantle and a thin crust, about 30 km deep under land and 5 km under the ocean. No one has drilled that deep: seismic waves map the layers, and because S waves cannot travel through a liquid, the way they fail to cross the outer core shows it is molten.

Earth's fast spin and molten nickel-iron core generate a magnetic field, which the solar wind squeezes into a teardrop shape. Charged particles trapped in it collide with air above the magnetic poles and make it glow as aurorae. The field's polarity flips at very irregular intervals, as the rock record shows, with no known harm to life.

Earth's lithosphere, the crust plus upper mantle, is broken into plates that move about as fast as fingernails grow. Earthquakes and mountains form where plates grind past, ride over or pull apart. Along with erosion, this constant reworking erases old impact craters, which is why about 190 are known on Earth while the Moon's stay largely intact.

Earth's air is rich in free oxygen, but that oxygen was made by life. Microbes that make oxygen by photosynthesis released it, and a large rise called the Great Oxidation Event produced an oxygen-rich atmosphere long after Earth formed. Venus and Mars, by contrast, have air that is mostly carbon dioxide.

Greenhouse gases such as carbon dioxide trap heat near the surface and keep Earth's average temperature near 15 C. Take the carbon dioxide away and the greenhouse effect would collapse, cooling the surface by about 33 C. Venus shows the same physics run away: its thick carbon dioxide atmosphere traps so much heat that the surface sits near 467 C.

Impacts are a real geological process on Earth, not just ancient history. Chicxulub, a crater buried under sediment off Mexico's Yucatan Peninsula, is most scientists' smoking gun for the asteroid strike 65 million years ago tied to a mass extinction that included the dinosaurs. Smaller hits are routine: a 10-meter asteroid arrives about once a decade.

Venus, Earth and Mars began as similar rocky planets, and young Venus and Earth may both have held oceans. Today Venus is an inferno under thick carbon dioxide, Mars is a cold desert with thin air, and Earth alone keeps liquid water on its surface. Reading the three side by side is a test of which factors decide whether a rocky planet stays habitable.

The Moon's light highlands and dark maria are rocks of different compositions and ages; the maria are impact basins flooded with lava between about 4.2 and 1.2 billion years ago. Impacts have ground the surface into regolith, a layer of powdery dust and rubble. Rilles are channels or cracks left by long-past volcanic activity or by the crust fracturing.

A crater forms in two stages. In excavation the impactor compresses, crushes and vaporizes rock, and a shock wave sprays ejecta up and out. In modification the ground deforms and collapses, so large craters end up shallow with central peaks, terraces and flat floors, while small ones stay simple bowls. On the Moon the change comes near 15 km across.

Crater counts only rank surfaces; dated rock turns the ranking into years. Apollo crews brought back 382 kg of samples, which show the first crust forming around 4.4 billion years ago and the maria filling with lava between about 4.2 and 1.2 billion years ago. Because the Moon keeps its craters, its record serves as an impact history for the inner solar system.

The favored idea is a giant impact: about 4.5 billion years ago something, perhaps a Mars-sized body, hit the young Earth and threw out the debris that became the Moon. Apollo rocks support it, because their oxygen isotopes match Earth's too closely for coincidence, and the Moon's small iron core fits heavy iron staying behind. Capture and side-by-side formation fell out of favor.

With no air to hold heat, the Moon swings from about 127 C in sunlight to -173 C in darkness, and Mercury from about 430 C to -180 C. With no wind or rain, the surface changes far more slowly than Earth's, so craters survive for billions of years. Crater floors near the poles that never see sunlight stay cold enough to keep water ice.

Mercury circles the Sun on a stretched, egg-shaped orbit and spins only once every 59 days, about three turns for every two trips around the Sun, so one sunrise to the next takes 176 Earth days. Its metal core spans about 85 percent of its radius, and early asteroid impacts carved huge basins, the largest about 1,550 km across.

Gas leaks away from the top of an atmosphere, and lighter atoms escape more readily than heavy ones, so a world's escape speed matters: about 11.2 km per second for Earth, but only 5.0 for Mars and 2.4 for the Moon. Speed is not the only route out. Mars, with no global magnetic field, lost much of its air to the solar wind, including about 65 percent of its argon.

Venus's air is mostly carbon dioxide under clouds of sulfuric acid, and it presses on the surface as hard as the ocean does about 930 meters underwater on Earth. The carbon dioxide traps heat in a runaway greenhouse effect, so the surface sits near 467 C.

Radar from NASA's Magellan orbiter saw through the clouds to volcanoes, mountains and a young surface, perhaps 150 million years old on average, with fewer impact craters than any rocky planet except Earth. Venus also spins backward, so the Sun rises in the west, and one rotation, 243 Earth days, takes longer than its 225-day year.

Mars holds the solar system's largest volcano, Olympus Mons, more than 40 km tall from base to summit, and its largest canyon, Valles Marineris, about 3,870 km long. Its red color is rust: iron in the rocks, soil and dust has oxidized. Water ice sits just under the surface in the polar regions.

Mars's thin air is mostly carbon dioxide, so heat escapes fast: standing at the equator at noon, your feet could feel near 24 C while your head felt near 0 C. Winds sometimes raise dust storms that cover much of the planet for months. River valleys and lakebeds show Mars was once warmer and wetter, under a thicker atmosphere.

Ancient river valleys, deltas, lakebeds and minerals that form only in liquid water show Mars once had water at its surface, and some features point to huge floods about 3.5 billion years ago. Today the air is too thin for liquid water to last. Water survives as ice just under the polar surface and as salty brines that seasonally flow down some slopes.

Each kind of mission answers different questions. An orbiter such as Mars Reconnaissance Orbiter studies terrain and atmosphere from above and relays data for surface craft. A rover like Curiosity drives to rocks to test whether Mars was ever habitable. Signs of past life call for samples studied in labs on Earth, so Perseverance's collecting is meant as the first step of a return.

Similar starts can end far apart. On Venus a thick carbon dioxide atmosphere locked in a runaway greenhouse. Mars has no global magnetic field today, only traces of one in its crust from 4 billion years ago, and the solar wind stripped much of its air. Earth kept liquid water and life. The forks may trace back to the disk in which the planets formed.

Jupiter and Saturn are mostly hydrogen and helium. Deep inside, pressure squeezes hydrogen into a liquid, and deeper still into an electrically conducting metallic form whose motion is thought to drive Jupiter's strong magnetic field. Juno found Jupiter's core is not a small solid ball but large, partly dissolved and fuzzy. Uranus and Neptune hold far more water, methane and ammonia.

Jupiter took more than twice the mass of everything else that formed besides the Sun. Saturn, about nine times wider than Earth, is far less massive than Jupiter, and its average density is lower than water's, so it would float in a large enough bathtub. Both spin in less than half an Earth day; on Jupiter that fast spin drives the jet streams that band its clouds.

Jupiter's fast spin drives strong east-west jets that sort its clouds into dark belts and light zones flowing in opposite directions, and Juno found those jets reach about 3,200 km deep. With no solid surface to slow them, storms last: the Great Red Spot, wider than Earth, has been watched for more than 300 years and reaches hundreds of kilometers down.

Compare the sunlight a planet absorbs with the energy it sends back to space, and any excess must come from inside. Jupiter, Saturn and Neptune emit more heat than they receive, much of it left over from their formation 4.5 billion years ago. Uranus long seemed to have none, but a 2025 analysis found it gives off about 15 percent more than it gets.

Uranus and Neptune are ice giants: 80 percent or more of their mass is a hot, dense fluid of water, methane and ammonia over a small rocky core, and methane gas makes both look blue. Uranus is tipped nearly 98 degrees, perhaps by an ancient collision, so each pole gets a 21-year winter night. Neptune gets little energy from the Sun, yet its winds top 2,000 km per hour.

Jupiter's magnetic field is 16 to 54 times as strong as Earth's, and its magnetosphere trails a tail past Saturn's orbit. Near the planet it traps charged particles and speeds them up into radiation that can damage spacecraft. Some of Saturn's aurorae are driven by particles from its moons rather than the solar wind, and Uranus's magnetic axis is tilted about 60 degrees from its spin.

Saturn's rings are not solid sheets but a swarm of separate icy and rocky pieces, ranging from dust-sized grains to house-sized lumps, and each ring orbits at its own speed. They are thought to be remains of comets, asteroids or moons torn apart by Saturn's gravity inside the Roche limit, the closest a satellite can come before being pulled apart. Small shepherd moons such as Prometheus help confine the narrow F ring.

Io, Europa and Ganymede share a 4:2:1 resonance, and the timed pulls of the outer two keep Io's orbit elliptical. As Io's distance from Jupiter changes, tides flex its surface up and down by as much as 100 meters. Friction from that flexing heats the interior so strongly that Io has hundreds of volcanoes, more activity than any other world in the solar system.

Under an ice shell perhaps 15 to 25 km thick, Europa probably has a salty ocean 60 to 150 km deep, which may hold twice the water of Earth's oceans. Tidal heating could drive volcanic or hydrothermal activity on its seafloor. Any life there would live without sunlight, running on chemical energy instead, and NASA's Europa Clipper mission is headed there to study it.

Ganymede has separated into an iron core, a rock mantle and an icy shell, and its metallic core makes it the only moon known to have its own magnetic field. Callisto went the other way: its surface, about 4 billion years old, is the most heavily cratered in the solar system because so little geologic activity has erased anything.

Titan is the only moon with a dense atmosphere, mostly nitrogen like Earth's with a little methane. It is also the only world besides Earth with standing liquid on its surface: methane and ethane fall as rain, flow in rivers and fill lakes and seas. Sunlight keeps breaking down the methane, so something, perhaps cryovolcanism, must be resupplying it.

Enceladus sprays water vapor and ice from its hidden ocean out through geyser-like jets, letting spacecraft sample the ocean, and the spray feeds Saturn's E ring. Triton is the only large moon that orbits opposite to its planet's spin, a sign it was probably a Kuiper Belt object captured by Neptune; Voyager 2 saw geysers on it despite its deep cold.

Pluto is a dwarf planet: round, but sharing its region beyond Neptune with other objects that cross its path, so it has not cleared its orbit. Its moon Charon is about half Pluto's size, and the two keep the same faces toward each other. New Horizons found plains of frozen nitrogen with no craters, a sign the surface is still being renewed.

Most asteroids orbit in the main belt between Mars and Jupiter, where newly formed Jupiter's gravity stirred up collisions and ended planet building. All of them together have less mass than the Moon. Trojans share a planet's orbit at the L4 and L5 points, and near-Earth objects come within 1.3 times Earth's distance of the Sun.

Dark C-type asteroids, probably clay and silicate rock, are the most common and among the most ancient objects in the solar system. S-types are stony, silicates with nickel-iron, and M-types are metallic nickel-iron. The differences track how far from the Sun each formed, and whether it later partly melted so that iron sank and basaltic lava rose.

Planetary defense starts with finding and tracking objects. An asteroid counts as potentially hazardous when it is at least 140 meters wide and its orbit comes within 7.5 million km of Earth's, which signals possible risk, not a predicted hit. In 2022 NASA's DART spacecraft deliberately struck the moonlet Dimorphos and changed its orbit, a test of deflection by impact.

A comet's nucleus is a frozen body of ice, gas and dust, often a few miles across. Near the Sun its ices turn to gas and form a coma that can stretch hundreds of thousands of km. Sunlight's pressure pushes dust into a curved, bright dust tail, while the solar wind sweeps ions into a straight, faint bluish ion tail, and both stream away from the Sun.

Short-period comets, which take less than 200 years to orbit, come from the Kuiper Belt beyond Neptune, and those Jupiter corrals into loops of 20 years or less are Jupiter-family comets. Long-period comets fall in from the Oort Cloud, a spherical shell of icy bodies far beyond the Kuiper Belt that is too distant for direct images.

A meteoroid is a rock in space, a meteor is the streak of light it makes burning up in the atmosphere, and a meteorite is any piece that reaches the ground. A meteor shower happens when Earth passes through the dusty trail a comet left along its orbit. Comet debris is fragile, so showers have not been shown to leave meteorites behind.

Meteorites are irons, stones or stony-irons. Most that fall are stony, but most found long after falling are irons, because they are heavier and easier to tell from Earth rocks. About 99.8 percent come from asteroids and the rest from Mars and the Moon, and the oldest date back to the materials that built the planets.

More than 6,000 exoplanets are confirmed, and many have no match in the solar system. Hot Jupiters, gas giants orbiting so close that their temperatures reach thousands of degrees, were among the first found. Super-Earths, 2 to 10 times Earth's mass, have no counterpart here. Patterns such as a shortage of planets 1.5 to 2 times Earth's size still await explanation.

Energy made in the core moves outward first as radiation, bouncing around the radiative zone, then in rising bubbles of hot plasma through the convection zone. Above the visible photosphere lie the chromosphere, a transition zone and the corona, which is far hotter than the surface beneath it, a puzzle not yet solved.

The photosphere, at about 5,500 C, is the layer from which most of the Sun's radiation escapes into space, so it looks like a surface even though the Sun has no solid one. It is mottled with granules, the tops of convection cells: the bright centers are hot plasma that has risen from below, and the darker edges are cooler gas sinking back down.

Sunspots form where concentrated magnetic flux pushes up from inside, leaving the gas there cooler, about 7,000 F against about 10,000 F around it, and so darker. Their number rises and falls over roughly 11 years from solar minimum to solar maximum, and about every 11 years the Sun's north and south magnetic poles swap.

A flare is a burst of radiation; it travels at light speed, so its effects on Earth's sunlit upper atmosphere, such as a shortwave radio blackout, start as soon as you see it. A coronal mass ejection is a cloud of plasma and magnetic field thrown off the corona, and even the fastest take 15 to 18 hours to arrive. Magnetic reconnection can release both.

The solar wind is a steady outflow of protons and electrons carrying the Sun's magnetic field, about 400 km per second in the plane of the planets and 500 to 800 from coronal holes. Fast wind and coronal mass ejections stir geomagnetic storms that brighten aurorae, disrupt satellites, GPS and radio, and can damage power grids.

In the Sun's core, hydrogen fuses into helium, but four protons do not merge in a single step. In the pp chain's main branch two protons first make deuterium, releasing a positron and a neutrino; adding a proton makes helium-3; then two helium-3 nuclei make helium-4 and free two protons. The helium has slightly less mass than what went in, and that missing mass is the energy, by E = mc squared.

At every depth in the Sun, outward pressure from the heat that fusion keeps supplying balances the inward pull of gravity and keeps the star from collapsing. Because that balance lasts as long as the core keeps fusing, the Sun is stable for a very long time: it is a little less than halfway through its life, with about 5 billion years left.

Energy leaves the core as radiation that bounces around the dense radiative zone, so its trip outward takes on the order of 170,000 years. In the convection zone, rising bubbles of hot plasma carry it the rest of the way to the photosphere. Neutrinos made in the same reactions barely interact, so they cross the whole Sun in about 2 seconds.

Two probes report on the interior directly. Neutrinos from fusion stream out of the core, but detectors caught fewer than solar models predicted, a gap now explained by neutrinos changing type on the way. Helioseismology reads the Sun's surface oscillations from Doppler shifts to see into the interior, and its sound speeds matched the models, which pointed the blame at the neutrinos rather than the Sun.

Apparent magnitude is how bright a star looks from Earth, on a scale that runs backward: the brightest star in the night sky sits near -1.4, the faintest naked-eye stars near 6, and each step is a factor of about 2.512. Absolute magnitude removes distance by asking how bright the star would look from 10 parsecs, which is why the Sun outshines Rigel in your sky even though Rigel would look far brighter side by side.

Luminosity is the total energy a star sends into space every second, a property of the star itself. Flux is the energy crossing a given area in a given time, which is what your telescope actually collects. The inverse-square law links them: double the distance and the star looks one-fourth as bright, so brightness alone cannot tell you luminosity until you know the distance.

A star’s color tracks its surface temperature because a hotter body’s light peaks at shorter wavelengths, so the hottest stars give off more blue light and the coolest more red. Two cautions apply. Interstellar dust reddens starlight, which can make a star look cooler than it is, and the color names are loose: red giants and red dwarfs actually look more orange than red.

Stars are sorted by the lines in their spectra, which track surface temperature: O is hottest, then B, A, F, G, K and M, and a number subdivides each class, which is how the Sun becomes a G2. The order looks scrambled because the letters were assigned before anyone knew they followed temperature. L, T and Y were later added for brown dwarfs, cooler still than M stars.

Each kind of atom absorbs and emits at its own set of wavelengths, so the pattern of lines tells you what a star is made of. The lines carry more than composition. In a dense white dwarf, high pressure disturbs the atoms’ energy levels and smears hydrogen lines far wider than in a puffy blue giant, and a shift of the whole spectrum toward red or blue reveals motion away from or toward you.

A brown dwarf, at roughly 13 to 80 Jupiter masses, never gathers enough mass to sustain the hydrogen fusion that powers a true star, so the light left from its birth fades until it shines only in infrared. Many form the way stars do, from a collapsing clump of gas, though some may form like planets in a disk. Depending on temperature, brown dwarfs fall in spectral types M, L, T and Y.

A Doppler shift measures only radial velocity, the part of a star’s motion straight toward or away from you: approach shifts its lines toward blue, recession toward red, and a larger shift means a faster speed. Motion across the sky leaves the lines unshifted. Tracking these shifts over time is what reveals a star wobbling around an unseen companion, and the same tool measures how fast a galaxy rotates.

Red dwarfs make up about three-quarters of the Milky Way’s stars, and the closest star to the Sun is one, yet none is bright enough to see with the unaided eye. The brightest-looking stars are a different list, because apparent brightness mixes true output with distance: rare O and B stars are very bright, while the numerous M stars are dim.

More than half of the stars in the sky have one or more partners, held in orbit around a shared center of mass. When the orbit lies edge-on to you, each star passes in front of the other once per orbit, and the changing light during those eclipses can reveal the stars’ masses, diameters and precise orbits. When one partner is a collapsed star pulling gas from the other, the system can shine in X-rays.

A star’s mass shows up in how things orbit it. An orbit’s size follows from its period and the mass at its center, so timing a binary pair and measuring its orbit runs that relation backward to give the stars’ masses. Eclipsing binaries are especially useful, since the changing light as one star crosses the other reveals both masses, and Doppler tracking of a wobbling star weighs an unseen companion.

Star sizes span an enormous range. A neutron star packs more than the Sun’s mass into a ball about 12 miles across, a white dwarf is roughly Earth-sized, and a red giant swells to hundreds of times the Sun’s size. Eclipsing binaries offer a direct way to measure them, because the way the light changes as one star passes in front of the other reveals both stars’ diameters.

The Hertzsprung-Russell diagram plots stars’ luminosity against their surface temperature, often labeled by spectral class instead. Plotted this way, stars are not scattered at random: about 90 percent fall along one band, the main sequence, because they are all in the same stable stage of fusing hydrogen in their cores. Giants sit above that band, brighter than main-sequence stars of the same temperature because they are larger.

A star’s place on the diagram tells you its stage. Main-sequence stars, which is what “dwarf” means on its own, fuse hydrogen in their cores. Giants are brighter and larger than main-sequence stars of the same temperature because their core hydrogen is spent and their outer layers have puffed up. White dwarfs, despite the name, are not main-sequence stars at all but hot, Earth-sized remnants that no longer make energy.

On the main sequence, a star’s mass sets how hard it must burn. A heavier star needs more energy pouring out of its core to hold up its own weight, so it shines far more brightly and spends its fuel far faster. The result is lopsided lifetimes: the most massive stars last only a few million years, while some small red dwarfs will shine for trillions of years.

The astronomical unit, the mean Earth-Sun distance, is the yardstick for the solar system, so its exact length sets the scale of every planetary distance. Radar measures it by timing radio waves bounced off a planet. Radar echoes from Venus timed at Goldstone in 1961 gave the unit with an uncertainty of about 300 miles and put it about 50,000 miles longer than the value from older optical observations.

As Earth moves from one side of its orbit to the other, a nearby star seems to shift against more distant stars; half of that angle is the star’s parallax. A star with a parallax of one arcsecond is one parsec away, about 3.26 light-years. The angle shrinks as distance grows, so the method works best for nearby stars, and it anchors the ladder by calibrating Cepheid variables.

A Cepheid swells and shrinks in a steady rhythm, and the length of its cycle is tied directly to its luminosity, a link Henrietta Leavitt found among stars in the Small Magellanic Cloud. Time the pulse and you know the true output; compare that with how bright the star looks and you have its distance. Because Cepheids are bright and reliable, they carried distance measurement into other galaxies.

No single method spans the universe, so distances are built in steps. Parallax calibrates nearby Cepheid variables, Cepheids in galaxies that also host Type Ia supernovae calibrate those far brighter explosions, and the supernovae reach far enough to measure the expansion rate. Each rung inherits the calibration below it, which is why an independent check matters: the ladder’s expansion rate disagrees with the one from the microwave background.

The space between stars is not empty. It holds gas in ionized, atomic and molecular forms, smoke-like dust grains, light and cosmic rays. The densities differ enormously: cold molecular clouds are dense enough to block the light behind them, while even the most remote space still holds about one atom or ion per cubic centimeter.

An emission nebula glows because ultraviolet light from nearby hot, young stars strips electrons from its gas, mostly hydrogen. When the electrons recombine with protons they give off light at specific wavelengths, above all the red hydrogen-alpha line, which is why these clouds look reddish in natural color. That separates them from reflection nebulae, which shine only by scattering starlight off dust.

Dust can either brighten or hide a cloud. A reflection nebula glows by scattering the light of nearby stars, and because its grains are about the size of blue-light wavelengths, blue is scattered most and the nebula usually looks blue. Pack enough dust and gas together and the cloud blocks visible light from the stars behind it, appearing as a dark blotch against a starry background.

Dust between you and a star both dims and reddens its light, because the grains scatter blue light most efficiently and absorb ultraviolet. The star then looks fainter and cooler than it really is, and since apparent brightness also depends on distance, dimming you have not corrected for makes a star seem farther away. Infrared light slips through much of the dust, so infrared views can reveal stars hidden in visible light.

Molecular clouds are cold, dense regions of the interstellar medium where hydrogen exists mainly as molecules and temperatures can fall below -280 F. They range from about 1,000 to 10 million times the Sun’s mass and can span hundreds of light-years. Their cold lets gas clump into dense pockets, which is why most stars form inside them, and their dust blocks the visible light behind them.

Cosmic rays are not rays but particles, mostly protons, moving at nearly the speed of light. Those from beyond the solar system were probably accelerated by supernova remnants, whose shock waves boost a particle again and again until it escapes. Being charged, they are bent by the galaxy’s magnetic field, so they do not point back to their source, and one striking the air can set off a shower of particles that reach the ground.

Stars form from interstellar gas, forge heavier elements, and return material to space through planetary nebulae and supernovae, enriching the clouds that build the next generation. That recycling is why heavy elements accumulate over time, though they remain a small share: hydrogen and helium make up about 74 and 24 percent of ordinary matter, and heavier “metals” only about 2 percent of the Milky Way’s disk.

A cloud collapses when gravity overwhelms the gas pressure and magnetic fields holding it up. Cold clouds clump into dense pockets that grow by colliding or gathering matter until gravity wins, and one cloud usually fragments into many stars at once. A push can start the process: the gravity of passing stars, two clouds bumping together, or winds driven into the cloud by a nearby supernova.

A collapsing cloud core keeps the slight spin it inherited, and as it shrinks, that spin flattens the infalling material into a disk around the young star, much as spinning dough flattens into a pizza crust. Planets may later form in this disk. Meanwhile the star’s magnetic field channels gas toward its poles, firing glowing jets, called Herbig-Haro objects, in two opposite directions.

A young star is a protostar until it can power itself by fusing hydrogen. As matter falls in and the core shrinks, it heats up, but even hundreds of thousands of degrees is not enough; the core must reach about 10 million degrees. Once fusion starts, its energy stops the contraction and the star settles onto the main sequence, a process that can take about 40 million years.

An orbiting planet tugs its star back and forth, so the star’s light is alternately squeezed toward blue and stretched toward red as it moves slightly toward and away from you. Tracking that wobble over time reveals an unseen planet and an estimate of its mass. The method found the first planet around a Sun-like star in 1995, and early wobble surveys turned up many large, close-in hot Jupiters.

When a planet crosses the face of its star, it dims the star by a measurable amount once every orbit. The size of the dip gives the planet’s size, the timing gives its period, and the period plus the star’s mass gives the orbit’s size. The catch is geometry: you see transits only when the orbit lies nearly edge-on to you, so a planet whose orbit is tilted away from your line of sight never transits.

The two methods find very different worlds. Direct imaging blocks the star’s glare and has so far mostly caught giant planets that are young and still hot enough to glow on their own. Microlensing needs no light from the planet at all: when a foreground star passes in front of a background star it magnifies that light, and a planet around the foreground star adds a second, smaller spike.

The catalog holds worlds unlike anything in the solar system: hot Jupiters that circle their stars in under 10 days, super-Earths more massive than Earth but well below Neptune, and small gaseous mini-Neptunes. Its makeup also reflects how planets were found. The early wobble method turned up hot Jupiters one after another because big, close planets tug hardest, while later space surveys showed that small planets are very common.

The habitable zone is the range of orbits where a planet’s surface could hold liquid water, given enough atmosphere; for the Sun it runs roughly 0.9 to 1.5 AU. It moves with the star, sitting much closer to a dim red dwarf, where flares can strip a planet’s air. It is also only a surface rule: Europa’s ocean lies well outside the Sun’s zone, kept liquid by tidal heating.

How long a star stays on the main sequence depends on how fast it spends its fuel, and mass sets that rate. More massive stars must burn faster to hold themselves up against their own weight, so their larger fuel supply runs out sooner. The most massive stars live only a few million years, while small red dwarfs will outlast the present age of the universe.

When a star like the Sun runs out of hydrogen in its core, fusion there slows and the core contracts and heats. That heat lights hydrogen fusion in a shell around the core, and eventually helium fusion in the core itself, while the extra energy makes the outer layers swell and cool. The star becomes a red giant, hundreds of times the Sun’s size and more luminous, though its surface looks more orange than red.

Stars in a cluster formed at roughly the same time from the same cloud, so they share an age and a distance, which lets astronomers study how a group of stars evolves together. Open clusters hold up to a few thousand mostly young stars in the disk and drift apart; globular clusters pack up to millions of old stars and last billions of years. Some clusters turn out to hold several generations of stars.

A massive star does not stop at helium. When each fuel runs out its core shrinks, heats and ignites the next: carbon to neon, neon to oxygen, oxygen to silicon and finally silicon to iron, leaving layers of different fusion around the core. Each new fuel buys less time, and silicon burning lasts only days. Iron is a dead end, because fusing it consumes energy instead of releasing it, so the core collapses.

Stars shed mass in several ways. Hot, luminous O and B stars drive far denser and faster winds than the Sun, and aging giants become unstable, pulsating and throwing off parts of their atmospheres in episodic bursts. In a close binary, mass can also change hands: once one partner has collapsed, its strong gravity can drain gas from a companion that orbits close enough.

A planetary nebula is the expanding shell of gas a Sun-like star throws off at the end of its life, made to glow by the hot exposed core left behind, a white dwarf. The name has nothing to do with planets: it comes from the round, planet-like look these nebulae had in early telescopes. The Sun is expected to make one.

A white dwarf is the exposed core of a Sun-like star: roughly the Sun’s mass in a ball about Earth’s size, so dense that a teaspoon would outweigh a pickup truck. It makes no new energy and simply cools over billions of years. There is a theoretical ceiling of about 1.4 solar masses, the Chandrasekhar limit, above which no white dwarf can hold itself up. A fully cooled black dwarf would take longer than the universe has existed.

Once a massive star’s core is iron, fusion can no longer hold it up, because fusing iron would absorb energy. The core collapses until the nuclei themselves push back, then rebounds, sending a shock wave out through the star’s outer layers. The explosion can briefly outshine its whole galaxy, and it leaves behind either a neutron star or a black hole.

A Type Ia supernova is not a collapsing massive star but an exploding white dwarf, set off when it merges with a companion or gathers enough gas from one to pass its stability limit. Because these blasts reach roughly the same peak brightness, they work as standard candles, and they are bright enough to see across the universe. Distant ones looking dimmer than expected revealed that cosmic expansion is speeding up.

Stars and their explosive deaths made most of the nuclei heavier than the light elements from the Big Bang. For elements beyond iron the picture is shared: rapid neutron capture, the r-process, makes about half of them, and the 2017 neutron-star merger was the first direct sighting of it. So at least some of the universe’s gold and platinum was forged in colliding neutron stars.

A neutron star is the collapsed core left by a supernova: more than the Sun’s mass in a sphere about as wide as Manhattan is long, so dense that a teaspoon would weigh as much as Mount Everest. Pressure has squeezed most of its protons and electrons into neutrons. A pulsar is a spinning neutron star whose hot spots sweep in and out of view like a lighthouse beam, so the pulses are rotation, not flashing.

Close binaries produce many of the sky’s sudden events. A white dwarf pulling gas from a companion can ignite that gas in a nova, brightening up to 100,000 times. A neutron star or black hole feeding the same way makes an X-ray binary. Long gamma-ray bursts generally come from collapsing massive stars, while short ones come from colliding neutron stars or black holes, which also give off gravitational waves.

In a vacuum, every object falls with the same acceleration, because the mass that resists a push is the same as the mass gravity pulls on. Einstein took that equality as his starting point: in free fall you cannot tell gravity from simply moving inertially, so gravity can be treated as the shape of spacetime rather than a force. The equivalence holds only locally, since over larger regions tidal effects pull falling bodies toward each other.

General relativity replaces the force of gravity with the curvature of spacetime produced by mass. A freely falling body, such as a planet in orbit, is not being pulled off a straight line; it follows a geodesic, the curve of extremal length through curved spacetime. Light follows the curvature too, which is why a massive galaxy cluster can bend and magnify the light of galaxies behind it.

The first experimental test came during the solar eclipse of 1919, when starlight passing the Sun was found to bend by the amount relativity predicted. Gravitational lensing by galaxies and clusters shows the same bending on cosmic scales. Later tests went further: Gravity Probe B used spinning gyroscopes in orbit to check two subtle effects of Earth’s spacetime, and GPS clocks confirm daily that gravity changes the rate of time.

In general relativity, clocks deeper in a gravitational field tick more slowly, and light climbing out of strong gravity is stretched to redder wavelengths. GPS satellites feel both relativity effects at once: their orbital speed makes their clocks fall behind, but the weaker gravity in orbit speeds them up by more, so on balance they run fast by microseconds a day and the system must correct for it.

The event horizon is the boundary where the speed needed to escape a black hole reaches the speed of light, so nothing from inside, not even light, can get out. For a nonrotating, uncharged black hole its radius, the Schwarzschild radius, is set by the mass alone. Because the horizon captures light and spacetime around it bends light, the dark shadow it casts appears about twice its actual size.

Black holes give off no light, so the evidence comes from their surroundings. In X-ray binaries, gas falling onto a collapsed companion shines in X-rays. Speed is telling: gas whirling around the core of galaxy M87 at about 550 kilometers per second could be held only by a black hole billions of times the Sun’s mass, and the Event Horizon Telescope later imaged that black hole’s dark shadow.

Gravitational waves are ripples in spacetime, moving at the speed of light, made when massive objects accelerate, such as two black holes or neutron stars spiraling together. By the time they reach Earth they are so weak that detectors like LIGO must sense changes as small as a fraction of a proton’s width using laser interferometry. When a merger also gives off light, telescopes can pinpoint its home galaxy.

The Milky Way is a barred spiral about 100,000 light-years across, with a main disk only about 1,000 light-years thick. A central bulge of mostly old stars and a bar sit at the middle, around a black hole of about 4 million solar masses, and a roughly spherical halo of old stars, globular clusters and dark matter surrounds it all. The Sun lies about 28,000 light-years from the center.

If spiral arms were fixed groups of stars, the galaxy’s rotation would wind them tighter and tighter until they vanished. Instead, the arms are a pattern of higher density that stars, gas and dust pass through as they orbit, slowing and bunching up on the way in, like cars in a traffic jam. Squeezed gas there collapses into new stars, which is why young, hot stars outline the arms.

If a galaxy’s mass followed its light, stars far from the center should orbit more slowly. In the 1970s Vera Rubin found that stars on the outskirts of spiral galaxies move about as fast as those nearer the center. That requires large amounts of unseen mass in the outer regions, and her result turned dark matter from a fringe idea, first raised for galaxy clusters in 1933, into a widely accepted one.

You weigh a galaxy by its gravity’s hold on the things orbiting in it. Stars far out in spiral galaxies move too fast for the visible matter to hold them, and Hubble measurements used to weigh the Milky Way likewise put most of its mass in dark matter rather than stars and gas. That gap is why the visible galaxy is now pictured as sitting inside a much larger dark matter halo.

The Milky Way’s center lies about 28,000 light-years away toward Sagittarius, hidden in visible light by dust but open to infrared. Its heart is a supermassive black hole, Sagittarius A star, of about 4 million solar masses, surrounded by the densest star cluster in the galaxy, where stars sit only 0.4 to 0.04 light-years apart. For a galaxy its size, that black hole is on the low side of normal.

Galaxies grow partly by absorbing smaller ones. Through gravity, a large galaxy can strip material from its neighbors, and the Milky Way is thought to be pulling a stream of hydrogen from its satellite dwarfs, the Large and Small Magellanic Clouds. When galaxies collide, their stars rarely hit, but the gas is compressed into new bouts of star formation, and galaxies without enough momentum to separate merge.

In 1920 Harlow Shapley and Heber Curtis debated whether spiral nebulae lay inside the Milky Way or were separate island universes. Settling it required a reliable distance. In 1923 Edwin Hubble found a variable star in the largest spiral nebula; its pulsation period marked it as a Cepheid and put it about 1 million light-years away, far beyond the Milky Way. Shapley was nearer right about our galaxy’s large size, Curtis about other galaxies.

Galaxies are sorted mainly by shape. Spirals have a disk, arms and a central bulge, and about two-thirds of them are barred, with a band of stars across the center. Ellipticals range from round to oval, with little gas and mostly old stars on random orbits. Lenticulars have a bulge and disk but no arms, and irregulars fit neither pattern. Shape partly reflects history, since mergers can turn spirals into ellipticals or irregulars.

Galaxies range from a few hundred thousand stars to thousands of billions, and dwarf galaxies, typically with less than a billion Suns’ worth of mass, are the most common type. Stellar content tracks shape: spirals keep gas-rich disks where young, hot stars form, while ellipticals hold little gas and dust and are dominated by older, redder stars, so they form few new ones.

Several independent measurements point to the same unseen mass. Stars far out in spiral galaxies orbit too fast for the visible matter to hold them, galaxies in clusters move too fast to stay bound by their visible mass, and the bending of background light by clusters reveals more mass than the galaxies and gas supply. Dark matter neither emits nor absorbs light, so it is known only by its gravity.

Distances to galaxies use standard candles, objects whose true brightness is known, compared with how bright they look. Cepheid variables work for relatively nearby galaxies; Type Ia supernovae, much brighter, reach about 1,000 times farther once Cepheids in the same galaxies have calibrated them. Each method is only as good as the rung beneath it, so an error in a lower rung carries into every distance built on it.

Hubble’s law says a galaxy’s recession speed is proportional to its distance, speed equals H0 times distance, so a galaxy twice as far recedes twice as fast. The constant H0 is the universe’s current expansion rate, in kilometers per second per megaparsec, and its exact value is unsettled: the distance ladder gives about 70 to 76, while the early-universe microwave background gives about 67 to 68.

Cosmological redshift is not galaxies racing through space but space itself expanding, which stretches light to longer wavelengths during its journey; a Doppler redshift, by contrast, comes from the source’s own motion. The farther apart two galaxies are, the faster they separate, so the view is much the same from any galaxy, like the view from any boat on an open ocean. No galaxy sits at a center.

Quasars were first found in the 1950s and 1960s as “quasi-stellar radio sources” that looked like stars but gave off radio waves. They turned out to be billions of light-years away, which made them a puzzle: a region about the size of the solar system shines 100 to 1,000 times brighter than an entire galaxy. The power source is a supermassive black hole swallowing matter at the galaxy’s core.

Seyfert galaxies, radio galaxies, quasars and blazars may look different, but they appear to share one engine: a supermassive black hole feeding from an accretion disk, often wrapped in a ring of dust. Many differences come from the viewing angle. Seen from the side, the dust hides some features of the core; looking almost straight down a jet, you see a blazar, especially bright and variable.

The central engine is matter spiraling into a supermassive black hole through an accretion disk, where gravity and friction heat the gas to millions of degrees and make it glow from infrared to X-rays. Near the black hole, a small share of the infalling gas is redirected into twin jets moving close to the speed of light. A jet aimed toward you looks far brighter than its partner, an effect called Doppler beaming.

A distant quasar works as a backlight. Each gas cloud between it and you absorbs the light at its own wavelengths, and since the clouds sit at different distances, their lines land at different redshifts, so one spectrum maps the gas along the whole sightline. Hubble used a quasar 7 to 8 billion light-years away this way to trace thin hydrogen filaments of the cosmic web.

When galaxies collide their stars rarely hit, since galaxies are mostly empty space, but the gas clouds are compressed and burst into star formation; starburst galaxies can make 100 or more Sun-like stars a year. If the galaxies lack the momentum to pass each other, they merge, and the result can look nothing like either parent. Astronomers suspect most elliptical galaxies were built this way from merging spirals.

A census of galaxies with the Hubble telescope found that a central black hole’s mass depends on the mass of its galaxy’s central bulge of stars: the larger the galaxy, the larger the black hole. The link suggests the two grew together, though the order is still debated; black holes may have come first, galaxies first, or both may have developed in step.

Gravity has arranged galaxies into a hierarchy. Groups generally hold 100 or fewer galaxies, clusters hold hundreds to thousands, and superclusters, typically not bound together, collect clusters and groups. Walls and filaments of galaxies form the cosmic web, with vast voids between them, as gravity pulls matter into ever larger concentrations.

In a galaxy cluster, the galaxies make up only about 5 percent of the mass and hot X-ray-emitting gas about 15 percent; the rest is dark matter. The Bullet Cluster, formed when two clusters collided, shows it directly: lensing places most of the mass apart from the hot gas that holds most of the ordinary matter. Gravitational lensing and the motions of cluster galaxies give separate lines of evidence.

The expansion rate sets the universe’s age, so measuring it precisely pins the age down. Before the Hubble telescope, estimates ranged from 9.7 to 19.5 billion years; precise distances to Cepheids and supernovae narrowed it to 13.8 billion. Any age must also be older than the oldest stars, and globular clusters hold some of the oldest known stars, so their ages give an independent check.

The cosmological principle states that, on the largest scales, matter is spread the same way everywhere and the universe looks the same in every direction. It means your view of the cosmos is like the view from one boat on an open ocean, much the same as from any other boat, so no place is special. It is an assumption checked against observation, and measurements on the largest scales support it.

The Big Bang model says the universe expanded and cooled from an extremely hot, dense state, and it explains the microwave background, the expansion of space and the observed mix of light elements. It describes an expansion of space everywhere, not an explosion into space. What came before, or whether there was a true beginning, is unknown, and how far back general relativity can be trusted is itself uncertain.

The cosmic microwave background is light released about 380,000 years after the Big Bang, when the universe cooled enough for atoms to form and light could travel freely for the first time. Expansion has stretched it about 1,100 times, so it now arrives as microwaves with a near-perfect blackbody spectrum at about 2.725 K. Its tiny temperature differences mark the density seeds that grew into galaxies.

In the first minutes after the Big Bang, protons and neutrons fused into deuterium and then helium, with traces of lithium. Within about five minutes most of today’s helium had formed, and expansion had cooled the universe enough to stop the process. That brief window explains why helium makes up about 24 percent of the mass of elements, a quantitative test the Big Bang model passes.

In the late 1990s, astronomers expected to measure how much the expansion was slowing. Instead, distant Type Ia supernovae looked dimmer than they should, meaning they were farther away than expected, which only made sense if the expansion is speeding up. “Dark energy” names whatever drives that acceleration; it makes up about 68 percent of the universe’s energy, but its nature is unknown.

Measured by mass and energy, ordinary matter, everything made of atoms, is only about 5 percent of the universe. Dark matter, which pulls by gravity but gives off no light, is about 27 percent, and dark energy, which drives the accelerating expansion, about 68 percent. So the stars, gas and planets you can see are a small minority even of the matter, which is mostly dark.

Inflation proposes that, for a fraction of a second, the early universe expanded faster than light. It would explain why distant regions of the microwave sky share one temperature though they seem never to have been in contact, why space is so close to flat, and where the seeds of galaxies came from, by blowing up tiny quantum fluctuations. What drove it is unknown, and rival ideas about the initial state remain under study.

Life as we know it needs three things: a source of energy, the right chemical ingredients, and a liquid to work in, which for Earth life is water. Those criteria guide where astrobiologists look, but they are a working definition, not a guarantee. One moon has all three, yet it is far colder than anywhere life is found on Earth, and its lakes hold organic compounds rather than water.

Extremophiles live where humans could not: hot springs and hydrothermal vents, inside glaciers, deep underground and in fluids more acidic than battery acid. Ecosystems at deep-sea vents, found in 1977, run on chemical reactions between seawater and rock rather than sunlight. That widens the list of places alien life might exist, including Mars and the ice-covered oceans of some outer-planet moons.

Early Mars had rivers, lakes, a thicker atmosphere and the ingredients and energy life needs, so rovers search its rocks for signs of past life. Europa and Enceladus hide liquid-water oceans under ice, kept warm by tidal flexing from their planets, possibly with seafloor vents. Titan offers a stranger chemistry. A positive result would need a biosignature that nonliving processes cannot produce.

When a planet passes in front of its star, some starlight filters through its atmosphere, and the gases there absorb at their own wavelengths. Telescopes can read those absorption bands to look for gases life might produce, such as oxygen, ozone or methane. The hard part is ruling out other sources: a gas only counts as a biosignature if nonliving processes cannot make it.

In 1961 Frank Drake wrote an equation estimating how many intelligent civilizations might exist in the galaxy, but most of its terms are still rough guesses, so it frames the question more than answers it. The Fermi paradox sharpens the puzzle: if such civilizations exist, why have we not met one? Searches now look for technosignatures such as radio or laser signals, and none has yet been found.

A rocket works by Newton’s third law: it pushes exhaust out the back, and the exhaust pushes the rocket forward. The ideal rocket equation shows the cost. The speed a rocket can gain depends on its exhaust speed and the logarithm of its full-to-empty mass ratio, so reaching orbit takes about 90 percent propellant by weight. Staging helps by discarding empty tanks and engines along the way.

An orbit is a continuous fall around a planet, and its lowest and highest points around Earth are called perigee and apogee. A burn changes the far side of the orbit, not the point where you fire: firing at apogee raises perigee, and firing at perigee raises apogee. Lower orbits are faster, so a spacecraft that speeds up does not just go around faster; it climbs to a higher, slower orbit.

Orbits are chosen for the job. Low Earth orbit, about 160 to 2,000 km up, gives close views and fast laps; many polar orbiters there are Sun-synchronous, passing each spot at the same solar time so lighting stays consistent. At 35,786 km an orbit takes one day, and directly over the equator a geostationary satellite stays fixed over one spot, ideal for watching weather. A medium orbit like those GPS uses takes about 12 hours.

A gravity assist lets a spacecraft gain or lose speed by passing close to a planet and using its gravity; a single pass by Earth has added tens of thousands of kilometers per hour to a probe bound for Saturn. The standard transfer between two circular orbits uses two burns: one to stretch the orbit’s far point out to the new height, and a second at that far point to lift the opposite side and round the orbit off.

Astronauts float not because gravity is gone but because they are falling: at the space station’s height, Earth’s gravity is nearly as strong as at the surface. That constant free fall affects the body. Weight-bearing bones lose about 1 to 1.5 percent of their mineral density a month, muscles waste without exercise, and fluids shift toward the head, which may press on the eyes and affect vision.

A crewed spacecraft must recycle what a closed cabin uses up. Space station life support scrubs carbon dioxide, splits water to make oxygen, holds the cabin at a livable temperature, and has shown it can recover about 98 percent of its water, including from urine. Radiation is harder: galactic cosmic rays are difficult to shield against, and the risk grows on the longer trips to the Moon and Mars.

A few missions reshaped what is known. The twin Voyager probes, launched in 1977, flew past the outer planets and now chart the edge of interstellar space. The Hubble Space Telescope, launched in 1990 and built to be serviced in orbit, refined the universe’s age and helped reveal dark energy. Webb, launched in 2021, orbits the Sun rather than Earth, shielded so it can observe in infrared.