Astronomy basics: 50 cards on planets, stars and galaxies

All 50 cards

1. 1
   The planets in order from the Sun
   Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune

   Pluto was reclassified as a dwarf planet in 2006.
2. 2
   The IAU definition of a planet
   A body that orbits the Sun, is massive enough for its gravity to make it nearly round, and has cleared the neighborhood around its orbit

   Pluto meets the first two conditions but not the third.
3. 3
   The five dwarf planets recognized by the IAU
   Ceres, Pluto, Haumea, Makemake and Eris

   Ceres is the largest object in the asteroid belt between Mars and Jupiter; the others lie beyond Neptune.
4. 4
   Rocky planets, gas giants and ice giants
   Rocky: Mercury, Venus, Earth and Mars. Gas giants: Jupiter and Saturn, mostly hydrogen and helium. Ice giants: Uranus and Neptune, rich in water, ammonia and methane
5. 5
   Smallest planet, and the closest to the Sun
   Mercury

   It has no moons and almost no atmosphere, so its surface swings from about 430 °C by day to about −180 °C at night.
6. 6
   Hottest planet, and why
   Venus: its thick carbon dioxide atmosphere traps heat in a runaway greenhouse effect, keeping the surface above 450 °C

   It is hotter than Mercury despite being almost twice as far from the Sun.
7. 7
   Planet that takes longer to spin once than to orbit the Sun
   Venus: one rotation takes about 243 Earth days, one orbit about 225

   It also spins backward compared with most planets, so the Sun rises in the west.
8. 8
   Why is Mars red?
   Iron oxide (rust) in its dust and rocks

   Mars has the largest volcano in the solar system, Olympus Mons, and two small moons, Phobos and Deimos.
9. 9
   Largest planet
   Jupiter, more than twice as massive as all the other planets combined

   Its Great Red Spot is a giant storm that has raged for well over a century.
10. 10
    Jupiter’s four largest moons
    Io, Europa, Ganymede and Callisto, the Galilean moons

    Galileo discovered them in 1610. Ganymede is the largest moon in the solar system, bigger than Mercury, and Io is the most volcanically active world.
11. 11
    Planet less dense than water
    Saturn

    Its rings are made mostly of water ice, in pieces from dust grains to house-sized chunks.
12. 12
    Saturn’s largest moon, and what makes it unique
    Titan: the only moon with a thick atmosphere, and the only world besides Earth known to have lakes and seas on its surface

    Its lakes and seas are liquid methane and ethane.
13. 13
    Planet that spins on its side
    Uranus, with an axial tilt of about 98°

    A giant collision early in its history is the leading explanation.
14. 14
    Farthest planet from the Sun
    Neptune, about 30 AU away

    It was found in 1846 close to where mathematicians had predicted from irregularities in Uranus’s orbit. Its winds are the fastest in the solar system, above 2,000 km/h.
15. 15
    Kuiper belt
    A broad ring of icy bodies beyond Neptune, from about 30 to 50 AU from the Sun, home to Pluto and other dwarf planets
16. 16
    What kind of star is the Sun?
    A yellow dwarf (a G-type main-sequence star), about 4.6 billion years old

    It holds about 99.8% of the solar system’s mass.
17. 17
    How does the Sun produce energy?
    Nuclear fusion in its core turns hydrogen into helium, mostly through the proton–proton chain

    The core is about 15 million °C.
18. 18
    Life cycle of a Sun-like star
    Nebula, protostar, main-sequence star, red giant, planetary nebula, white dwarf

    The Sun is about halfway through its roughly 10-billion-year life on the main sequence.
19. 19
    Life cycle of a massive star
    Nebula, protostar, main-sequence star, red supergiant, supernova, then a neutron star or a black hole
20. 20
    Star colors from hottest to coolest
    Blue, white, yellow, orange, red

    The spectral classes in the same order are O, B, A, F, G, K and M.
21. 21
    Hertzsprung–Russell diagram
    A plot of stars’ luminosity against surface temperature; most stars, including the Sun, lie on a diagonal band called the main sequence

    Temperature increases to the left. Giants sit above the main sequence and white dwarfs below it.
22. 22
    White dwarf
    The hot, dense core left behind by a Sun-like star: about the size of Earth, with a mass comparable to the Sun’s

    A white dwarf can’t be heavier than about 1.4 solar masses, the Chandrasekhar limit.
23. 23
    Neutron star
    The collapsed core of a massive star after a supernova, typically about 1.4 solar masses packed into a sphere about 20 km across

    A pulsar is a spinning neutron star whose beams of radiation sweep past Earth.
24. 24
    Black hole
    A region where gravity is so strong that nothing, not even light, can escape; its boundary is the event horizon

    Stellar black holes form when the cores of very massive stars collapse.
25. 25
    Supermassive black hole at the center of the Milky Way
    Sagittarius A* (“A-star”), about 4 million times the mass of the Sun

    The Event Horizon Telescope imaged it in 2022. Its first black hole image, released in 2019, showed the one in galaxy M87.
26. 26
    Brightest star in the night sky
    Sirius, in Canis Major, about 8.6 light-years away

    It has a faint white dwarf companion, Sirius B.
27. 27
    Nearest star to the Sun
    Proxima Centauri, about 4.2 light-years away

    It belongs to the Alpha Centauri system and has at least one known planet, Proxima b.
28. 28
    Polaris
    The North Star: it sits almost directly above Earth’s North Pole, so it stays nearly still while the other stars circle around it

    Follow the two pointer stars at the end of the Big Dipper’s bowl to find it. It isn’t especially bright: dozens of stars outshine it.
29. 29
    Betelgeuse
    A red supergiant at Orion’s shoulder, expected to explode as a supernova within the next 100,000 years or so

    Its sharp dimming in 2019–2020, the “Great Dimming”, was caused mainly by a cloud of dust it had thrown off.
30. 30
    Constellation with Betelgeuse, Rigel and a belt of three stars
    Orion

    The Orion Nebula, a nearby star-forming region, lies in the sword that hangs below the belt.
31. 31
    How many constellations are officially recognized?
    88, with boundaries set by the International Astronomical Union

    Together they cover the whole sky, like countries on a map.
32. 32
    Is the Big Dipper a constellation?
    No. It is an asterism, a pattern of seven bright stars within the constellation Ursa Major
33. 33
    What kind of galaxy is the Milky Way?
    A barred spiral galaxy about 100,000 light-years across, with a few hundred billion stars

    The Sun lies about 26,000 light-years from the center, in a minor spiral arm called the Orion Spur.
34. 34
    Nearest large spiral galaxy to the Milky Way
    The Andromeda Galaxy (M31), about 2.5 million light-years away

    From a dark site it is visible to the naked eye as a faint smudge.
35. 35
    Main types of galaxy
    Spiral, elliptical and irregular

    Edwin Hubble’s classification also includes barred spirals and lenticular galaxies.
36. 36
    Hubble’s law
    Galaxies recede from us at a speed proportional to their distance: $v = H_0 d$

    It shows the universe is expanding. Measurements of the Hubble constant range from about 67 to 73 km/s per megaparsec; the disagreement is called the Hubble tension.
37. 37
    Age of the universe
    About 13.8 billion years
38. 38
    Astronomical unit (AU)
    The average distance between Earth and the Sun, about 150 million km

    Since 2012 it has been defined as exactly 149,597,870,700 m. Sunlight crosses it in about 8 minutes 20 seconds.
39. 39
    Light-year
    The distance light travels in one year, about 9.46 trillion km

    It is a unit of distance, not of time.
40. 40
    Parsec
    The distance at which a star shows a parallax of one arcsecond, about 3.26 light-years

    Distance in parsecs is $d = 1/p$, with the parallax $p$ in arcseconds.
41. 41
    What causes the seasons?
    Earth’s axis is tilted about 23.4°, so each hemisphere gets more direct sunlight for half the year; the distance from the Sun has little to do with it

    Earth is actually closest to the Sun in early January.
42. 42
    Solstice vs equinox
    At a solstice the Sun reaches its farthest north or south, giving the longest or shortest day (around 21 June and 21 December). At an equinox day and night are nearly equal everywhere (around 20 March and 22 September)
43. 43
    Phases of the Moon, in order
    New moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, waning crescent

    A full cycle takes about 29.5 days.
44. 44
    Why do we always see the same side of the Moon?
    It is tidally locked: it spins exactly once per orbit, about every 27.3 days
45. 45
    Solar eclipse
    The Moon passes between the Sun and Earth and its shadow falls on Earth; it can only happen at new moon
46. 46
    Lunar eclipse
    Earth passes between the Sun and the Moon, and the Moon moves into Earth’s shadow; it can only happen at full moon

    The Moon turns red because Earth’s atmosphere bends reddened sunlight into the shadow.
47. 47
    Why isn’t there an eclipse every month?
    The Moon’s orbit is tilted about 5° to Earth’s, so at most new and full moons the shadows miss
48. 48
    Exoplanet
    A planet orbiting a star other than the Sun; more than 6,000 have been confirmed

    The first one found around a Sun-like star was 51 Pegasi b, in 1995.
49. 49
    Transit method
    Finds a planet from the small, regular dip in a star’s brightness each time the planet passes in front of it

    The Kepler and TESS space telescopes found thousands of planets this way.
50. 50
    Habitable zone
    The range of distances from a star at which a planet could have liquid water on its surface