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Stuff Every Astronomer Should Know

Light, telescopes, stars, galaxies, the solar system and the sky: how astronomers know what they know.

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  1. Which kind of light has the shortest wavelength, under 0.01 nanometer?
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    Gamma rays. The spectrum runs from long radio waves to short gamma rays. Shorter wavelength means more energy per photon, so gamma rays come from the most violent events, like dying stars.

  2. Roughly what wavelength range is visible light?
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    About 400 to 700 nanometers. Our eyes see waves from about 400 nm (violet) to 700 nm (red). That band passes easily through the air, which is no coincidence: eyes evolved to use the light the Sun delivers best.

  3. Who discovered infrared light in 1800 by putting a thermometer past the red end of a spectrum?
    Show the answer

    William Herschel. Herschel spread sunlight into a rainbow and measured each color's heat. A thermometer beyond the red still warmed up, revealing invisible light. It was the first hint of light we cannot see.

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  1. Which kind of light has the shortest wavelength, under 0.01 nanometer?
  2. Roughly what wavelength range is visible light?
  3. Who discovered infrared light in 1800 by putting a thermometer past the red end of a spectrum?
  4. Why is infrared astronomy best done from mountaintops, airplanes or space?
  5. Why must X-ray telescopes like Chandra orbit above Earth's atmosphere?
  6. A 4-meter telescope mirror collects how much more light than a 1-meter mirror?
  7. With the main optic fixed, what can you swap to change a telescope's magnification?
  8. What lens flaw makes each color focus at a slightly different spot in a refracting telescope?
  9. Who built the first successful reflecting telescope, in 1668?
  10. How big is the largest working refracting telescope, at Yerkes Observatory?
  11. How does adaptive optics undo the blurring caused by Earth's turbulent air?
  12. How does each 10-meter Keck telescope make its giant mirror?
  13. What three things make the best sites for big optical telescopes?
  14. Why can a single giant radio dish see less detail than a small optical telescope?
  15. At a fixed wavelength, what sets how fine a detail an interferometer can resolve?
  16. The Very Long Baseline Array links 10 radio dishes stretching between which places?
  17. Karl Jansky discovered the first cosmic radio waves in the early 1930s. Where were they coming from?
  18. In 1815, Joseph Fraunhofer counted about how many dark lines in the Sun's spectrum?
  19. Which element was found in the Sun's spectrum before it was found on Earth?
  20. What kind of spectrum shows only bright lines at certain wavelengths?
  21. A star moves sideways across our line of sight, not toward or away. What Doppler shift do we see?
  22. A galaxy's spectral lines are shifted toward shorter wavelengths. What does that tell you?
  23. A spectral line shifts by 1/1000 of its wavelength. How fast is the source moving along our line of sight?
  24. As a star gets hotter, its peak emission moves which way?
  25. If you move twice as far from a star, how much of its light reaches you?
  26. Hubble's law says a galaxy's recession speed is proportional to what?
  27. One parsec, the distance at which a star shows 1 arcsecond of parallax, equals about how many light-years?
  28. A star has a parallax of 0.1 arcsecond. How far away is it?
  29. In what year was stellar parallax first successfully measured?
  30. How long does sunlight take to cross one astronomical unit, the average Earth to Sun distance?
  31. About how far is one light-year?
  32. How far into the past are you seeing the Moon?
  33. Who discovered the cepheid period-luminosity relation in 1908?
  34. For cepheid variable stars, how is the pulsation period linked to luminosity?
  35. What explodes to make a Type Ia supernova, the standard candle for far galaxies?
  36. A difference of 5 magnitudes means what brightness ratio?
  37. On the magnitude scale, what does a smaller (or negative) number mean?
  38. Absolute magnitude is how bright an object would look from what distance?
  39. Hubble launched in 1990 with a flawed mirror. When did astronauts fix it?
  40. Where does the James Webb Space Telescope operate?
  41. Ordinary mirrors do not focus X-rays. How does Chandra focus them?
  42. The first image of a black hole, released in 2019, showed the black hole in which galaxy?
  43. What is the Vera C. Rubin Observatory's main job?
  44. What is the celestial sphere in modern astronomy?
  45. How many degrees equal one hour of right ascension?
  46. Which Earth coordinate is declination most like?
  47. What does the ecliptic trace on the celestial sphere?
  48. What primarily causes Earth's seasons?
  49. Why does a higher summer Sun warm the ground more effectively?
  50. What causes the Moon's phases?
  51. About how long is one complete lunar phase cycle?
  52. Why does a solar eclipse not happen at every new Moon?
  53. Which alignment produces a lunar eclipse?
  54. When do spring tides produce the largest average tidal ranges?
  55. Why is a solar day longer than a sidereal day?
  56. About how long is Earth's axial precession cycle?
  57. Why will Polaris not always be the North Star?
  58. Where is the Sun in a planet's elliptical orbit?
  59. Where does a planet move fastest in its elliptical orbit?
  60. For solar orbits in years and AU, what is Kepler's third-law relation?
  61. If separation doubles, how does Newtonian gravitational force change?
  62. How does Newtonian gravitational force depend on two masses?
  63. About what is escape velocity from Earth's surface, ignoring air resistance?
  64. What is a Lagrange point for two bodies like the Sun and Earth?
  65. Which Lagrange points can be stable in the ideal system?
  66. Why is Venus hotter at the surface than Mercury?
  67. Which world has the solar system's largest volcano, Olympus Mons?
  68. What is Jupiter's Great Red Spot?
  69. What are Saturn's bright main rings primarily made of?
  70. Approximately how far is Uranus's rotation axis tilted?
  71. Which planet has measured winds near 2,000 kilometers per hour?
  72. What orbital criterion separates an IAU planet from a dwarf planet?
  73. In what year did the IAU reclassify Pluto as a dwarf planet?
  74. Where is the main asteroid belt?
  75. Where does the Kuiper Belt's main region begin?
  76. How is the distant Oort Cloud thought to be shaped?
  77. Which direction do a comet's tails generally point?
  78. What is a space-rock fragment called after it reaches the ground?
  79. What likely lies beneath Europa's icy shell?
  80. What do Enceladus's south-polar jets spray into space?
  81. What gas makes up most of Titan's thick atmosphere?
  82. From what did the solar system begin forming about 4.6 billion years ago?
  83. Why did rocky planets form mainly near the young Sun?
  84. About how hot is the Sun's core, its hottest part?
  85. Roughly how long does energy from fusion take to work its way from the Sun's center to its surface?
  86. What causes the rice-grain pattern called granulation on the Sun's photosphere?
  87. What is the Sun's photosphere?
  88. How hot is the Sun's corona, its outer atmosphere?
  89. About how fast does the solar wind flow out from the Sun?
  90. Where on the Sun does most of the solar wind come from?
  91. On average, how many years pass between sunspot maximums?
  92. How long does the Sun's full magnetic cycle take to return to the same polarity?
  93. In the Sun's proton-proton chain, how many hydrogen nuclei fuse to make one helium nucleus?
  94. When hydrogen fuses into helium, about what share of the mass turns into energy?
  95. How long do neutrinos made in the Sun's core take to escape the Sun?
  96. What balance keeps a stable star like the Sun from collapsing or flying apart?
  97. What does a star's color mainly tell you?
  98. What is the order of the main stellar spectral classes from hottest to coolest?
  99. Why do the spectra of different stars look so different?
  100. Below about what mass can an object never get hot enough to fuse ordinary hydrogen?
  101. The Hertzsprung-Russell diagram plots stars by which two properties?
  102. Why are about 90% of stars found on the main sequence?
  103. What single property mostly sets where a star sits along the main sequence?
  104. Which kind of star stays on the main sequence longer?
  105. What powers a star as it swells into a red giant after its core hydrogen runs out?
  106. About what core temperature is needed for helium to fuse into carbon?
  107. What makes a planetary nebula glow?
  108. What is the most mass a white dwarf can have, the Chandrasekhar limit?
  109. What holds up a white dwarf against gravity?
  110. Why does fusion stop once a massive star builds an iron core?
  111. When a massive star's iron core collapses into a neutron star, about how big does it end up?
  112. During some supernovae, how can nuclei grow heavier than iron?
  113. Who discovered pulsars in 1967 while a graduate student?
  114. What are pulsars, according to the lighthouse model?
  115. What kind of star explodes in a type Ia supernova?
  116. If the Sun could become a black hole, about what would its Schwarzschild radius be?
  117. What is a black hole's event horizon?
  118. About how massive is the black hole at the center of our Milky Way?
  119. How do astronomers measure the masses of stars most directly?
  120. What does the transit method of finding exoplanets tell us about the planet?
  121. Which star hosted the first exoplanet found around a regular Sun-like star, in 1995?
  122. What is a star's habitable zone?
  123. What was the Carrington Event of 1859?
  124. What broad shape best describes the Milky Way?
  125. Where is the Sun within the Milky Way's spiral pattern?
  126. About how far is the Sun from the Milky Way's center?
  127. What do the two prongs of Hubble's tuning fork represent?
  128. In Hubble's scheme, which elliptical class looks most elongated?
  129. Where do spiral arms usually begin in a barred spiral galaxy?
  130. Which galaxy category lacks a regular spiral or elliptical shape?
  131. About how far away is the Andromeda Galaxy?
  132. Which galactic neighborhood contains the Milky Way and over 50 other galaxies?
  133. How many galaxies does a galaxy group generally contain?
  134. Roughly how many galaxies can a galaxy cluster contain?
  135. Are superclusters usually gravitationally bound as single systems?
  136. What do unexpectedly flat galaxy rotation curves imply?
  137. How can gravitational lensing reveal dark matter in a cluster?
  138. About what fraction of cosmic critical density is dark matter?
  139. What observed cosmic behavior is attributed to dark energy?
  140. Approximately what share of the universe is dark energy?
  141. What relationship does Hubble's law express?
  142. What units are commonly used for the Hubble constant?
  143. Did the Big Bang occur at one location inside preexisting space?
  144. About how old is the universe?
  145. When was the cosmic microwave background released?
  146. What is the present average temperature of the CMB?
  147. About how large is the observable universe's present radius?
  148. What central engine defines an active galactic nucleus?
  149. What is a quasar?
  150. Along which direction do AGN jets escape?
  151. What produced LIGO's first direct gravitational-wave detection in 2015?
  152. How long is each perpendicular arm of an Advanced LIGO detector?
  153. What does gravitational-wave strain measure?
  154. Why can one background object appear as several lensed images?
  155. What alignment can turn a lensed galaxy into an Einstein ring?
  156. What happens when a foreground star microlenses a background star?
  157. Which geometric method anchors the nearby cosmic distance ladder?
  158. What Cepheid property reveals its intrinsic luminosity?
  159. Why are Type Ia supernovae useful on the distance ladder?
  160. Which nuclei formed in appreciable amounts during Big Bang nucleosynthesis?
  161. Where did elements heavier than primordial lithium mainly form?
  162. Which flashlight color best preserves an astronomer's dark adaptation?
  163. Why does city light pollution hide faint stars?
  164. Why are binoculars often better than a telescope for a beginner?
  165. Where does most visible AGN power originate?