Stars, Galaxies & the Universe

Star life cycles, black holes, galaxies and the Big Bang — the biggest questions about the universe, fact by fact.

A star is a giant ball of gas that produces light and heat.
The Milky Way is the galaxy that contains our solar system.
A galaxy is a vast system of stars, gas, dust, and dark matter.
A light-year is the distance light travels in one year.
The Big Bang theory describes the origin and expansion of the universe.
Black holes have gravity so strong that not even light escapes.
A nebula is a cloud of gas and dust where stars are born.
The Sun is an average-sized star compared to others in the galaxy.
Supernovae are powerful explosions marking the death of massive stars.
Constellations are patterns of stars as seen from Earth.
The universe has been expanding since the Big Bang.
Our nearest star beyond the Sun is Proxima Centauri.
Telescopes gather light to let us observe distant objects in space.
Stars vary in color depending on their surface temperature.
Gravity holds galaxies together and shapes their structure.
The observable universe contains billions of galaxies.
Stars produce energy through nuclear fusion in their cores.
The Sun fuses hydrogen into helium at its core.
The Sun's core temperature reaches about 15 million degrees Celsius.
The Sun lies roughly 150 million kilometers from Earth.
Light from the Sun takes about eight minutes to reach Earth.
The Sun contains over 99 percent of the solar system's mass.
The Sun is composed mainly of hydrogen and helium.
The Sun is roughly 4.6 billion years old.
The Sun's outer atmosphere is called the corona.
Sunspots are cooler, darker regions on the Sun's surface.
Solar flares are sudden bursts of energy from the Sun.
The solar wind is a stream of charged particles from the Sun.
The Sun's visible surface is called the photosphere.
The Sun will eventually expand into a red giant.
The Sun will end its life as a white dwarf.
Proxima Centauri lies about 4.24 light-years from Earth.
Sirius is the brightest star in Earth's night sky.
Betelgeuse is a red supergiant in the constellation Orion.
Polaris is commonly known as the North Star.
Polaris lies nearly above Earth's northern rotational axis.
Stars form when clouds of gas and dust collapse under gravity.
A protostar is an early stage of star formation.
Main-sequence stars fuse hydrogen into helium in their cores.
The Sun is classified as a main-sequence star.
A star's lifespan depends largely on its mass.
Massive stars burn their fuel faster than smaller stars.
Red dwarfs are the most common type of star in the galaxy.
A red giant is a large, cool star nearing the end of its life.
A supergiant is among the largest types of stars known.
White dwarfs are dense remnants of low-mass stars.
A neutron star is an extremely dense stellar remnant.
A teaspoon of neutron star material would weigh billions of tons.
Pulsars are rapidly rotating neutron stars emitting beams of radiation.
Black holes can form from the collapse of massive stars.
The event horizon marks the boundary of a black hole.
The point of infinite density in a black hole is called a singularity.
Supermassive black holes lie at the centers of many galaxies.
The Milky Way's central black hole is called Sagittarius A-star.
Astronomers classify stars by spectral types O, B, A, F, G, K, and M.
O-type stars are the hottest and bluest stars.
M-type stars are the coolest and reddest stars.
The Sun is classified as a G-type star.
The Hertzsprung-Russell diagram plots stars by luminosity and temperature.
Luminosity measures the total energy a star emits per second.
Apparent magnitude measures how bright a star appears from Earth.
Absolute magnitude measures a star's true brightness at a standard distance.
Binary stars are two stars orbiting a common center of mass.
A star cluster is a group of stars bound by gravity.
Open clusters contain young stars loosely bound together.
Globular clusters are dense, spherical collections of old stars.
The Pleiades is a well-known open star cluster.
Stellar parallax helps measure distances to nearby stars.
The Milky Way is a barred spiral galaxy.
The Milky Way contains hundreds of billions of stars.
The Sun orbits the center of the Milky Way.
The Sun takes about 230 million years to orbit the galaxy once.
The Milky Way is about 100,000 light-years across.
The Andromeda Galaxy is the nearest large spiral galaxy.
The Andromeda Galaxy lies about 2.5 million light-years away.
Andromeda and the Milky Way are moving toward each other.
Spiral galaxies have rotating disks with curved arms.
Irregular galaxies lack a defined shape or structure.
Edwin Hubble created a system for classifying galaxy shapes.
The Large Magellanic Cloud is a satellite galaxy of the Milky Way.
The Small Magellanic Cloud orbits the Milky Way.
Galaxies often cluster together in groups bound by gravity.
The Milky Way belongs to a collection called the Local Group.
The Local Group contains more than fifty galaxies.
The Milky Way lies within the Virgo Supercluster.
Dark matter does not emit or absorb light.
Dark matter's presence is inferred from its gravitational effects.
Dark matter is thought to make up most of the universe's matter.
Dark energy is associated with the accelerating expansion of the universe.
Ordinary matter makes up only a small fraction of the universe.
The Big Bang occurred about 13.8 billion years ago.
The universe was extremely hot and dense at its beginning.
Cosmic microwave background radiation is leftover heat from the Big Bang.
The cosmic microwave background was discovered in 1965.
Arno Penzias and Robert Wilson detected the cosmic microwave background.
Redshift occurs when light from receding objects shifts toward red.
Blueshift occurs when an object moves toward the observer.
Edwin Hubble discovered that galaxies are moving away from us.
Hubble's law relates a galaxy's distance to its recession speed.
More distant galaxies recede faster than nearer ones.
The expansion of the universe stretches the wavelength of light.
The observable universe is about 93 billion light-years in diameter.
Cosmology is the study of the universe's origin and evolution.
The first stars formed a few hundred million years after the Big Bang.
Heavy elements are forged inside stars and supernovae.
A Type Ia supernova results from a white dwarf in a binary system.
A core-collapse supernova marks the death of a massive star.
Supernova remnants are expanding clouds of debris from explosions.
The Crab Nebula is the remnant of a supernova observed in 1054.
A nova is a sudden brightening of a star in a binary system.
Nebulae come in emission, reflection, and dark varieties.
The Orion Nebula is a bright stellar nursery visible from Earth.
A planetary nebula forms when a dying star expels its outer layers.
Planetary nebulae are unrelated to planets despite their name.
Interstellar medium refers to the gas and dust between stars.
Quasars are extremely luminous centers of distant active galaxies.
Quasars are powered by supermassive black holes consuming matter.
An accretion disk is matter spiraling into a black hole.
Active galactic nuclei emit enormous amounts of energy.
Gravitational waves were first directly detected in 2015.
Albert Einstein predicted gravitational waves in his theory of relativity.
General relativity describes gravity as the curvature of spacetime.
Isaac Newton formulated the law of universal gravitation.
Johannes Kepler described the laws of planetary motion.
Kepler's first law states that planets orbit in ellipses.
Nicolaus Copernicus proposed that planets orbit the Sun.
Galileo Galilei used a telescope to study the night sky.
Jupiter's four largest moons are called the Galilean moons.
The solar system formed from a collapsing cloud of gas and dust.
Mercury is the closest planet to the Sun.
Venus is the hottest planet in the solar system.
Earth is the third planet from the Sun.
Jupiter is the largest planet in the solar system.
Saturn is famous for its prominent ring system.
Uranus rotates on its side relative to its orbit.
Neptune is the farthest recognized planet from the Sun.
Pluto was reclassified as a dwarf planet in 2006.
Dwarf planets orbit the Sun but have not cleared their orbits.
The asteroid belt lies between Mars and Jupiter.
Comets are icy bodies that develop tails near the Sun.
A comet's tail always points away from the Sun.
The Kuiper Belt is a region of icy bodies beyond Neptune.
The Oort Cloud is a distant shell of icy objects around the Sun.
Meteoroids are small rocky or metallic objects in space.
A meteor is the streak of light from a meteoroid burning up.
A meteorite is a space rock that reaches Earth's surface.
The Moon's gravity causes ocean tides on Earth.
A solar eclipse occurs when the Moon blocks the Sun.
A lunar eclipse occurs when Earth's shadow falls on the Moon.
Telescopes can detect visible light, radio waves, and other radiation.
The Hubble Space Telescope was launched in 1990.
The Hubble Space Telescope orbits above Earth's atmosphere.
The James Webb Space Telescope observes mainly infrared light.
The James Webb Space Telescope launched in December 2021.
Space telescopes avoid distortion caused by Earth's atmosphere.
Spectroscopy analyzes light to determine an object's composition.
Astronomers study spectra to learn what stars are made of.
Helium was first identified in the Sun's spectrum.
The element helium is named after the Greek word for the Sun.
Exoplanets are planets orbiting stars other than the Sun.
Thousands of exoplanets have been confirmed by astronomers.
The transit method detects exoplanets by dimming starlight.
A habitable zone is the region where liquid water could exist.
An astronomical unit is about 150 million kilometers.
The celestial sphere is an imaginary sphere surrounding Earth.
The zodiac includes constellations along the Sun's apparent path.
Orion is a prominent constellation visible in winter skies.
An asterism is a recognizable pattern within or across constellations.
The ecliptic is the Sun's apparent yearly path across the sky.
Earth rotates once on its axis approximately every 24 hours.
The Milky Way appears as a band of light across the night sky.
Light pollution makes faint stars harder to see from cities.
Stars appear to twinkle because of Earth's atmosphere.
Cepheid variable stars help measure distances to galaxies.
Henrietta Leavitt discovered the period-luminosity relation of Cepheids.
A variable star changes in brightness over time.
Carbon and oxygen in living things originated in stars.
The most abundant element in the universe is hydrogen.
Helium is the second most abundant element in the universe.
Galaxies can collide and merge over billions of years.
Galactic mergers can trigger bursts of new star formation.
A starburst galaxy forms stars at a very high rate.
The galactic halo surrounds a galaxy's main disk.
The galactic bulge is the dense central region of a spiral galaxy.
Spiral arms contain regions of active star formation.
The Sun lies in a spiral arm of the Milky Way.
Astronomers measure angles in the sky using degrees and arcseconds.
A degree of sky is divided into sixty arcminutes.
Right ascension and declination form a celestial coordinate system.
The vernal equinox marks a reference point in the sky.
Precession is the slow wobble of Earth's rotational axis.
Earth's precession changes which star marks the north pole over time.
The speed of light is about 300,000 kilometers per second.
Nothing with mass can travel faster than light.
Looking at distant objects means seeing light from the past.
The cosmic distance ladder estimates distances across the universe.
Tidal forces arise from differences in gravitational pull.
Escape velocity is the speed needed to break free from gravity.
Brown dwarfs are objects too small to sustain hydrogen fusion.
A light source's spectrum reveals its motion through redshift or blueshift.

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Stars, Galaxies & the Universe

Star life cycles, black holes, galaxies and the Big Bang — the biggest questions about the universe, fact by fact.

A star is a giant ball of gas that produces light and heat.
The Milky Way is the galaxy that contains our solar system.
A galaxy is a vast system of stars, gas, dust, and dark matter.
A light-year is the distance light travels in one year.
The Big Bang theory describes the origin and expansion of the universe.
Black holes have gravity so strong that not even light escapes.
A nebula is a cloud of gas and dust where stars are born.
The Sun is an average-sized star compared to others in the galaxy.
Supernovae are powerful explosions marking the death of massive stars.
Constellations are patterns of stars as seen from Earth.
The universe has been expanding since the Big Bang.
Our nearest star beyond the Sun is Proxima Centauri.
Telescopes gather light to let us observe distant objects in space.
Stars vary in color depending on their surface temperature.
Gravity holds galaxies together and shapes their structure.
The observable universe contains billions of galaxies.
Stars produce energy through nuclear fusion in their cores.
The Sun fuses hydrogen into helium at its core.
The Sun's core temperature reaches about 15 million degrees Celsius.
The Sun lies roughly 150 million kilometers from Earth.
Light from the Sun takes about eight minutes to reach Earth.
The Sun contains over 99 percent of the solar system's mass.
The Sun is composed mainly of hydrogen and helium.
The Sun is roughly 4.6 billion years old.
The Sun's outer atmosphere is called the corona.
Sunspots are cooler, darker regions on the Sun's surface.
Solar flares are sudden bursts of energy from the Sun.
The solar wind is a stream of charged particles from the Sun.
The Sun's visible surface is called the photosphere.
The Sun will eventually expand into a red giant.
The Sun will end its life as a white dwarf.
Proxima Centauri lies about 4.24 light-years from Earth.
Sirius is the brightest star in Earth's night sky.
Betelgeuse is a red supergiant in the constellation Orion.
Polaris is commonly known as the North Star.
Polaris lies nearly above Earth's northern rotational axis.
Stars form when clouds of gas and dust collapse under gravity.
A protostar is an early stage of star formation.
Main-sequence stars fuse hydrogen into helium in their cores.
The Sun is classified as a main-sequence star.
A star's lifespan depends largely on its mass.
Massive stars burn their fuel faster than smaller stars.
Red dwarfs are the most common type of star in the galaxy.
A red giant is a large, cool star nearing the end of its life.
A supergiant is among the largest types of stars known.
White dwarfs are dense remnants of low-mass stars.
A neutron star is an extremely dense stellar remnant.
A teaspoon of neutron star material would weigh billions of tons.
Pulsars are rapidly rotating neutron stars emitting beams of radiation.
Black holes can form from the collapse of massive stars.
The event horizon marks the boundary of a black hole.
The point of infinite density in a black hole is called a singularity.
Supermassive black holes lie at the centers of many galaxies.
The Milky Way's central black hole is called Sagittarius A-star.
Astronomers classify stars by spectral types O, B, A, F, G, K, and M.
O-type stars are the hottest and bluest stars.
M-type stars are the coolest and reddest stars.
The Sun is classified as a G-type star.
The Hertzsprung-Russell diagram plots stars by luminosity and temperature.
Luminosity measures the total energy a star emits per second.
Apparent magnitude measures how bright a star appears from Earth.
Absolute magnitude measures a star's true brightness at a standard distance.
Binary stars are two stars orbiting a common center of mass.
A star cluster is a group of stars bound by gravity.
Open clusters contain young stars loosely bound together.
Globular clusters are dense, spherical collections of old stars.
The Pleiades is a well-known open star cluster.
Stellar parallax helps measure distances to nearby stars.
The Milky Way is a barred spiral galaxy.
The Milky Way contains hundreds of billions of stars.
The Sun orbits the center of the Milky Way.
The Sun takes about 230 million years to orbit the galaxy once.
The Milky Way is about 100,000 light-years across.
The Andromeda Galaxy is the nearest large spiral galaxy.
The Andromeda Galaxy lies about 2.5 million light-years away.
Andromeda and the Milky Way are moving toward each other.
Spiral galaxies have rotating disks with curved arms.
Irregular galaxies lack a defined shape or structure.
Edwin Hubble created a system for classifying galaxy shapes.
The Large Magellanic Cloud is a satellite galaxy of the Milky Way.
The Small Magellanic Cloud orbits the Milky Way.
Galaxies often cluster together in groups bound by gravity.
The Milky Way belongs to a collection called the Local Group.
The Local Group contains more than fifty galaxies.
The Milky Way lies within the Virgo Supercluster.
Dark matter does not emit or absorb light.
Dark matter's presence is inferred from its gravitational effects.
Dark matter is thought to make up most of the universe's matter.
Dark energy is associated with the accelerating expansion of the universe.
Ordinary matter makes up only a small fraction of the universe.
The Big Bang occurred about 13.8 billion years ago.
The universe was extremely hot and dense at its beginning.
Cosmic microwave background radiation is leftover heat from the Big Bang.
The cosmic microwave background was discovered in 1965.
Arno Penzias and Robert Wilson detected the cosmic microwave background.
Redshift occurs when light from receding objects shifts toward red.
Blueshift occurs when an object moves toward the observer.
Edwin Hubble discovered that galaxies are moving away from us.
Hubble's law relates a galaxy's distance to its recession speed.
More distant galaxies recede faster than nearer ones.
The expansion of the universe stretches the wavelength of light.
The observable universe is about 93 billion light-years in diameter.
Cosmology is the study of the universe's origin and evolution.
The first stars formed a few hundred million years after the Big Bang.
Heavy elements are forged inside stars and supernovae.
A Type Ia supernova results from a white dwarf in a binary system.
A core-collapse supernova marks the death of a massive star.
Supernova remnants are expanding clouds of debris from explosions.
The Crab Nebula is the remnant of a supernova observed in 1054.
A nova is a sudden brightening of a star in a binary system.
Nebulae come in emission, reflection, and dark varieties.
The Orion Nebula is a bright stellar nursery visible from Earth.
A planetary nebula forms when a dying star expels its outer layers.
Planetary nebulae are unrelated to planets despite their name.
Interstellar medium refers to the gas and dust between stars.
Quasars are extremely luminous centers of distant active galaxies.
Quasars are powered by supermassive black holes consuming matter.
An accretion disk is matter spiraling into a black hole.
Active galactic nuclei emit enormous amounts of energy.
Gravitational waves were first directly detected in 2015.
Albert Einstein predicted gravitational waves in his theory of relativity.
General relativity describes gravity as the curvature of spacetime.
Isaac Newton formulated the law of universal gravitation.
Johannes Kepler described the laws of planetary motion.
Kepler's first law states that planets orbit in ellipses.
Nicolaus Copernicus proposed that planets orbit the Sun.
Galileo Galilei used a telescope to study the night sky.
Jupiter's four largest moons are called the Galilean moons.
The solar system formed from a collapsing cloud of gas and dust.
Mercury is the closest planet to the Sun.
Venus is the hottest planet in the solar system.
Earth is the third planet from the Sun.
Jupiter is the largest planet in the solar system.
Saturn is famous for its prominent ring system.
Uranus rotates on its side relative to its orbit.
Neptune is the farthest recognized planet from the Sun.
Pluto was reclassified as a dwarf planet in 2006.
Dwarf planets orbit the Sun but have not cleared their orbits.
The asteroid belt lies between Mars and Jupiter.
Comets are icy bodies that develop tails near the Sun.
A comet's tail always points away from the Sun.
The Kuiper Belt is a region of icy bodies beyond Neptune.
The Oort Cloud is a distant shell of icy objects around the Sun.
Meteoroids are small rocky or metallic objects in space.
A meteor is the streak of light from a meteoroid burning up.
A meteorite is a space rock that reaches Earth's surface.
The Moon's gravity causes ocean tides on Earth.
A solar eclipse occurs when the Moon blocks the Sun.
A lunar eclipse occurs when Earth's shadow falls on the Moon.
Telescopes can detect visible light, radio waves, and other radiation.
The Hubble Space Telescope was launched in 1990.
The Hubble Space Telescope orbits above Earth's atmosphere.
The James Webb Space Telescope observes mainly infrared light.
The James Webb Space Telescope launched in December 2021.
Space telescopes avoid distortion caused by Earth's atmosphere.
Spectroscopy analyzes light to determine an object's composition.
Astronomers study spectra to learn what stars are made of.
Helium was first identified in the Sun's spectrum.
The element helium is named after the Greek word for the Sun.
Exoplanets are planets orbiting stars other than the Sun.
Thousands of exoplanets have been confirmed by astronomers.
The transit method detects exoplanets by dimming starlight.
A habitable zone is the region where liquid water could exist.
An astronomical unit is about 150 million kilometers.
The celestial sphere is an imaginary sphere surrounding Earth.
The zodiac includes constellations along the Sun's apparent path.
Orion is a prominent constellation visible in winter skies.
An asterism is a recognizable pattern within or across constellations.
The ecliptic is the Sun's apparent yearly path across the sky.
Earth rotates once on its axis approximately every 24 hours.
The Milky Way appears as a band of light across the night sky.
Light pollution makes faint stars harder to see from cities.
Stars appear to twinkle because of Earth's atmosphere.
Cepheid variable stars help measure distances to galaxies.
Henrietta Leavitt discovered the period-luminosity relation of Cepheids.
A variable star changes in brightness over time.
Carbon and oxygen in living things originated in stars.
The most abundant element in the universe is hydrogen.
Helium is the second most abundant element in the universe.
Galaxies can collide and merge over billions of years.
Galactic mergers can trigger bursts of new star formation.
A starburst galaxy forms stars at a very high rate.
The galactic halo surrounds a galaxy's main disk.
The galactic bulge is the dense central region of a spiral galaxy.
Spiral arms contain regions of active star formation.
The Sun lies in a spiral arm of the Milky Way.
Astronomers measure angles in the sky using degrees and arcseconds.
A degree of sky is divided into sixty arcminutes.
Right ascension and declination form a celestial coordinate system.
The vernal equinox marks a reference point in the sky.
Precession is the slow wobble of Earth's rotational axis.
Earth's precession changes which star marks the north pole over time.
The speed of light is about 300,000 kilometers per second.
Nothing with mass can travel faster than light.
Looking at distant objects means seeing light from the past.
The cosmic distance ladder estimates distances across the universe.
Tidal forces arise from differences in gravitational pull.
Escape velocity is the speed needed to break free from gravity.
Brown dwarfs are objects too small to sustain hydrogen fusion.
A light source's spectrum reveals its motion through redshift or blueshift.

Learn this lesson, not just read it

Load these facts straight into the free Class Coupon Speed Learning app — they become fast, playful study activities with audio and translation in 100+ languages.

Get the Speed Learning app →