1. Astronomy (Level 2)
2. Astronomical object (Level 4)
   1. Interstellar object
   2. Substellar object
3. Astrophysics (Level 4)
4. Astronomer
5. Astrobiology
   1. Extraterrestrial life (Level 4)
   2. Extraterrestrial intelligence
   3. Search for extraterrestrial intelligence
   4. Great Filter
   5. Fermi paradox
6. Astrochemistry
   1. Cosmochemistry
   2. Hypothetical types of biochemistry
   3. List of interstellar and circumstellar molecules
7. Cosmology
8. Space physics
9. Theoretical astronomy
10. Computational astrophysics
11. Time-domain astronomy
12. Night sky
13. Amateur astronomy
    1. Astrophotography
14. Space weather

This section contains 4 articles.

1. Astrophysics Data System
2. SIMBAD
3. NASA/IPAC Extragalactic Database
4. VizieR

This section contains 6 articles.

1. Messier object
2. Henry Draper Catalogue
3. New General Catalogue
4. Uppsala General Catalogue
5. 2MASS
6. Sloan Digital Sky Survey

This section contains 40 articles.

1. Observatory (Level 4)
2. Luminosity (Level 4)
   1. Sky brightness
   2. Airglow
3. Magnitude (astronomy) (Level 4)
   1. Absolute magnitude
   2. Apparent magnitude
4. Extinction (astronomy) (Level 4)
5. Effective temperature
6. Light curve
7. Photometry (astronomy) (Level 4)
   1. Photometric system
      1. UBV photometric system
8. Cosmic distance ladder (Level 4)
   1. Stellar parallax
   2. Tip of the red-giant branch
   3. Planetary nebula luminosity function
9. Astrometry (Level 4)
   1. Proper motion
   2. Radial velocity
   3. Parallax (Level 4)
10. Polarization in astronomy
11. Astronomical spectroscopy
    1. Forbidden mechanism
    2. Fraunhofer lines
12. Infrared astronomy
13. Visible-light astronomy
14. Neutrino astronomy
15. Gravitational-wave astronomy
16. Air mass (astronomy)
17. Astronomical seeing
18. Telluric contamination
19. Twinkling
20. Luminosity distance
21. Recessional velocity
22. Spectral energy distribution
23. Surface brightness
24. Angular resolution
25. Angular diameter
26. Bortle scale

This section contains 8 articles.

1. Radio astronomy
   1. Radio spectrum pollution
2. Submillimetre astronomy
3. Astronomical radio source
4. Astrophysical maser
   1. Megamaser
5. Brightness temperature
6. Spectral index

This section contains 5 articles.

1. High-energy astronomy
2. Ultraviolet astronomy
3. X-ray astronomy
   1. Astrophysical X-ray source
4. Gamma-ray astronomy

This section contains 130 articles.

1. Solar System (Level 2)
   1. Formation and evolution of the Solar System (Level 4)
      1. Nebular hypothesis (Level 4)
      2. Nice model
   2. Interplanetary medium (Level 4)
      1. Gegenschein
      2. Interplanetary dust cloud
2. Sun (Level 2)
   1. Solar wind (Level 4)
   2. Solar core
   3. Solar cycle
   4. Photosphere
   5. Chromosphere
   6. Stellar corona
   7. Heliosphere
   8. Solar neutrino
   9. Coronal mass ejection
   10. Solar phenomena
   11. Solar flare
   12. Solar storm
   13. Sunspot
3. Mercury (planet) (Level 3)
   1. Atmosphere of Mercury
   2. Geology of Mercury
4. Venus (Level 3)
   1. Atmosphere of Venus
   2. Geology of Venus
5. Earth (Level 1)
   1. Earth's orbit
   2. Earth's shadow
   3. Earthlight (astronomy)
   4. Location of Earth
   5. Future of Earth
6. Moon (Level 2)
   1. Origin of the Moon
      1. Giant-impact hypothesis
      2. Theia (planet)
   2. Orbit of the Moon
   3. Selenography
      1. Near side of the Moon
      2. Far side of the Moon
      3. Lunar south pole
   4. Geology of the Moon
   5. Internal structure of the Moon
7. Near-Earth object
8. Mars (Level 3)
   1. Atmosphere of Mars
   2. Geology of Mars
      1. Geological history of Mars
      2. Olympus Mons
      3. Martian polar ice caps
   3. Life on Mars
   4. Moons of Mars
      1. Phobos (moon)
      2. Deimos (moon)
9. Asteroid belt (Level 4)
   1. Ceres (dwarf planet) (Level 4)
   2. 2 Pallas
   3. 3 Juno
   4. 4 Vesta
   5. 10 Hygiea
   6. 243 Ida
   7. 433 Eros
10. Jupiter (Level 3)
    1. Atmosphere of Jupiter
       1. Great Red Spot
    2. Magnetosphere of Jupiter
    3. Rings of Jupiter
    4. Moons of Jupiter
       1. Galilean moons
       2. Io (moon) (Level 4)
       3. Europa (moon) (Level 4)
       4. Ganymede (moon) (Level 4)
       5. Callisto (moon) (Level 4)
    5. Jupiter trojan
11. Saturn (Level 3)
    1. Magnetosphere of Saturn
    2. Rings of Saturn
    3. Saturn's hexagon
    4. Moons of Saturn
       1. Mimas
       2. Enceladus (Level 4)
       3. Tethys (moon)
       4. Dione (moon)
       5. Rhea (moon)
       6. Titan (moon) (Level 4)
          1. Lakes of Titan
       7. Iapetus (moon)
12. 2060 Chiron
13. Uranus (Level 3)
    1. Atmosphere of Uranus
    2. Rings of Uranus
    3. Moons of Uranus
       1. Ariel (moon)
       2. Umbriel
       3. Titania (moon)
       4. Oberon (moon)
       5. Miranda (moon)
14. Neptune (Level 3)
    1. Rings of Neptune
    2. Moons of Neptune
       1. Triton (moon) (Level 4)
    3. Great Dark Spot
15. Trans-Neptunian object
16. Kuiper belt (Level 4)
    1. Pluto (Level 4)
       1. Atmosphere of Pluto
       2. Moons of Pluto
          1. Charon (moon)
    2. Orcus (dwarf planet)
    3. Haumea
       1. Moons of Haumea
    4. Makemake
    5. Gonggong (dwarf planet)
    6. Quaoar
    7. 486958 Arrokoth
17. Scattered disc
    1. Eris (dwarf planet) (Level 4)
       1. Dysnomia (moon)
18. Oort cloud (Level 4)
    1. Hills cloud
    2. Sedna (dwarf planet)
    3. Sednoid
19. Planets beyond Neptune
    1. Planet Nine

This section contains 5 articles.

1. Halley's Comet (Level 4)
2. Comet Hyakutake
3. Comet Hale–Bopp
4. Comet Shoemaker–Levy 9
5. Kreutz sungrazer

This section contains 47 articles.

1. Achernar
2. Aldebaran
3. Algol
4. Alpha Centauri (Level 4)
   1. Proxima Centauri
5. Acrux
6. Altair
7. Antares
8. Arcturus (Level 4)
9. Barnard's Star
10. Beta Centauri
11. Beta Pictoris
12. Betelgeuse (Level 4)
13. Canopus (Level 4)
14. Capella (Level 4)
15. Castor (star)
16. Crab Pulsar
17. CW Leonis
18. Cygnus X-1 (Level 4)
19. Deneb
20. Epsilon Canis Majoris
21. Epsilon Eridani
22. Eta Carinae
23. Fomalhaut
24. Gamma Cassiopeiae
25. LBV 1806−20
26. Mimosa (star)
27. Mira
28. Polaris (Level 4)
29. Pi Puppis
30. Pollux (star)
31. Procyon (Level 4)
32. PSR B1937+21
33. Regulus
34. Rigel (Level 4)
35. Scorpius X-1
36. Sirius (Level 4)
37. Spica
38. Tabby's Star
39. Tau Ceti
40. TRAPPIST-1
41. TW Hydrae
42. UY Scuti
43. Vega (Level 4)
44. VY Canis Majoris
45. WISE 0855−0714
46. Zeta Ophiuchi

This section contains 11 articles.

1. 51 Pegasi b
2. 55 Cancri e
3. Gliese 581d
4. HD 189733 b
5. HD 209458 b
6. K2-18b
7. Kepler-22b
8. Kepler-186f
9. Kepler-442b
10. Kepler-452b
11. Proxima Centauri b

This section contains 7 articles.

1. Milky Way (Level 3)
2. Galactic Center (Level 4)
   1. Sagittarius A
      1. Sagittarius A* (Level 4)
3. Orion Arm
4. Perseus Arm
5. Scutum–Centaurus Arm

This section contains 35 articles.

1. Local Group (Level 4)
   1. Andromeda Galaxy (Level 4)
   2. Satellite galaxies of the Milky Way
      1. Magellanic Clouds
      2. Large Magellanic Cloud (Level 4)
      3. Small Magellanic Cloud (Level 4)
      4. Sagittarius Dwarf Spheroidal Galaxy
   3. Triangulum Galaxy (Level 4)
   4. Virgocentric flow
2. 3C 273 (Level 4)
3. Antennae Galaxies
4. Arp 220
5. BL Lacertae
6. Cartwheel Galaxy
7. Centaurus A (Level 4)
8. Cygnus A
9. Hoag's Object
10. IC 1101
11. Markarian 421
12. Markarian 501
13. Messier 49
14. Messier 77
15. Messier 81
16. Messier 82
17. Messier 83
18. Messier 87 (Level 4)
19. Messier 106
20. NGC 1275
21. NGC 4151
22. NGC 5548
23. NGC 6946
24. Pinwheel Galaxy (Level 4)
25. Sculptor Galaxy
26. Sombrero Galaxy
27. Whirlpool Galaxy (Level 4)

This section contains 21 articles.

1. Carina Nebula (Level 4)
2. Cassiopeia A
3. Cat's Eye Nebula
4. Coalsack Nebula
5. Crab Nebula (Level 4)
6. Cygnus Loop
7. Dumbbell Nebula
8. Eagle Nebula (Level 4)
9. Horsehead Nebula (Level 4)
10. Lagoon Nebula
11. NGC 6302
12. NGC 7027
13. Omega Nebula
14. Orion Nebula (Level 4)
15. Rho Ophiuchi cloud complex
16. Ring Nebula (Level 4)
17. Rosette Nebula
18. Sagittarius B2
19. Tarantula Nebula
20. Trifid Nebula
21. Westerhout 40

This section contains 13 articles.

1. 47 Tucanae
2. Beehive Cluster
3. Double Cluster
4. Hyades (star cluster) (Level 4)
5. Messier 4
6. Messier 7
7. Messier 13
8. Messier 15
9. Messier 22
10. Omega Centauri (Level 4)
11. Pleiades (Level 4)
12. R136
13. Trapezium Cluster

This section contains 14 articles.

1. Virgo Supercluster (Level 4)
   1. Virgo Cluster (Level 4)
2. Laniakea Supercluster
3. Centaurus A/M83 Group
4. Coma Cluster
5. Fornax Cluster
6. IC 342/Maffei Group
7. M81 Group
8. Perseus Cluster
9. Great Attractor (Level 4)
10. Bullet Cluster
11. Shapley Supercluster
12. Pisces–Cetus Supercluster Complex
13. Local Sheet

This section contains 7 articles.

1. Kepler's Supernova
2. SN 1006
3. SN 1054
4. SN 1572
5. SN 1972E
6. SN 1987A
7. GW170817

This section contains 14 articles.

1. Local Bubble
   1. Local Interstellar Cloud
   2. G-Cloud
2. Hubble Deep Field
3. Hubble Ultra-Deep Field
4. Local Hole
5. Zone of Avoidance
6. Boötes Void
7. CMB cold spot
8. Local Void
9. Giant Void
10. South Pole Wall
11. Huge-LQG
12. BOSS Great Wall

This section contains 4 articles.

1. ʻOumuamua
2. 2I/Borisov
3. Magellanic Stream
4. Dipole repeller

1. Celestial mechanics (Level 4)
2. Orbit (Level 3)
   1. Barycenter (astronomy) (Level 4)
   2. Tidal locking (Level 4)
   3. Orbital eccentricity
   4. Orbital elements
   5. Orbital inclination
3. Eclipse (Level 4)
   1. Eclipse cycle
   2. Lunar eclipse (Level 4)
   3. Saros (astronomy)
   4. Solar eclipse (Level 4)
   5. Umbra, penumbra and antumbra
4. Astronomical transit
   1. Transit of Mercury
   2. Transit of Venus
5. Kepler's laws of planetary motion (Level 4)
6. Lagrange point (Level 4)
7. Libration
8. Sidereal time
   1. Daylight
   2. Dawn
   3. Twilight
   4. Sunrise
   5. Morning
   6. Daytime
   7. Noon
   8. Afternoon
   9. Evening
   10. Sunset
   11. Dusk
   12. Night (Level 4)
   13. Midnight
   14. Midnight sun
   15. Polar night
   16. Zodiacal light
   17. Belt of Venus
   18. Blue hour
   19. Golden hour (photography)
   20. Terminator (solar)
9. Lunar phase
   1. Full moon
   2. New moon
   3. Supermoon
10. Apsis
11. Gravitational collapse
12. Axial precession
13. Accretion (astrophysics)
14. Velocity dispersion
15. Roche limit
16. Tidal acceleration

This section contains 21 articles.

1. Orbital mechanics (Level 4)
   1. Space rendezvous
   2. Orbital resonance
   3. Orbital station-keeping
2. Escape velocity (Level 4)
   1. Bi-elliptic transfer
   2. Hohmann transfer orbit
3. Geostationary orbit (Level 4)
4. Geosynchronous orbit (Level 4)
   1. Molniya orbit
5. Gravity assist (Level 4)
   1. Oberth effect
6. Low Earth orbit (Level 4)
   1. Frozen orbit
   2. Sun-synchronous orbit
   3. Polar orbit
7. Halo orbit
8. Lissajous orbit
9. Co-orbital configuration
   1. Horseshoe orbit
10. Graveyard orbit

This section contains 29 articles.

1. Celestial sphere (Level 4)
2. Celestial cartography
3. Constellation (Level 4)
4. Asterism (astronomy)
5. Spherical astronomy
6. Epoch (astronomy)
7. Astronomical coordinate systems (Level 4)
   1. Equatorial coordinate system (Level 4)
   2. Equinox (celestial coordinates)
      1. Right ascension
      2. Declination
   3. Galactic coordinate system
8. List of Chinese star names
9. Equinox (Level 4)
   1. March equinox
   2. September equinox
10. Ecliptic (Level 4)
11. Horizon (Level 4)
12. Solstice (Level 4)
    1. Summer solstice
    2. Winter solstice
13. Hour angle
14. Position angle
15. Meridian (astronomy)
16. Nadir
17. Zenith
18. Celestial equator
19. Tropical year
20. Zodiac (Level 4)

This section contains 4 articles.

1. Argo Navis
2. Big Dipper
3. Orion's Belt
4. Quadrans Muralis

This section contains 88 articles.

This section contains 12 articles.

1. Aquarius (constellation)
2. Aries (constellation)
3. Cancer (constellation)
4. Capricornus
5. Gemini (constellation)
6. Leo (constellation)
7. Libra (constellation)
8. Pisces (constellation)
9. Sagittarius (constellation)
10. Scorpius
11. Taurus (constellation)
12. Virgo (constellation)

This section contains 76 articles.

1. Andromeda (constellation)
2. Antlia
3. Apus
4. Aquila (constellation)
5. Ara (constellation)
6. Auriga
7. Boötes
8. Caelum
9. Camelopardalis
10. Canes Venatici
11. Canis Major (Level 4)
12. Canis Minor
13. Carina (constellation)
14. Cassiopeia (constellation) (Level 4)
15. Centaurus (Level 4)
16. Cepheus (constellation)
17. Cetus
18. Chamaeleon
19. Circinus
20. Columba (constellation)
21. Coma Berenices
22. Corona Australis
23. Corona Borealis
24. Corvus (constellation)
25. Crater (constellation)
26. Crux (Level 4)
27. Cygnus (constellation)
28. Delphinus
29. Dorado
30. Draco (constellation)
31. Equuleus
32. Eridanus (constellation)
33. Fornax
34. Grus (constellation)
35. Hercules (constellation)
36. Horologium (constellation)
37. Hydra (constellation)
38. Hydrus
39. Indus (constellation)
40. Lacerta
41. Leo Minor
42. Lepus (constellation)
43. Lupus (constellation)
44. Lynx (constellation)
45. Lyra
46. Mensa (constellation)
47. Microscopium
48. Monoceros
49. Musca
50. Norma (constellation)
51. Octans
52. Ophiuchus
53. Orion (constellation) (Level 4)
54. Pavo (constellation)
55. Pegasus (constellation)
56. Perseus (constellation)
57. Phoenix (constellation)
58. Pictor
59. Piscis Austrinus
60. Puppis
61. Pyxis
62. Reticulum
63. Sagitta
64. Sculptor (constellation)
65. Scutum (constellation)
66. Serpens
67. Sextans
68. Telescopium
69. Triangulum
70. Triangulum Australe
71. Tucana
72. Ursa Major (Level 4)
73. Ursa Minor (Level 4)
74. Vela (constellation)
75. Volans
76. Vulpecula

1. Planetary science
2. Planet (Level 3)
   1. Definition of planet
      1. IAU definition of planet
         1. Clearing the neighbourhood
   2. Terrestrial planet (Level 4)
      1. Carbon planet
      2. Iron planet
   3. Giant planet (Level 4)
      1. Gas giant
      2. Ice giant
      3. Chthonian planet
   4. Exoplanet (Level 4)
      1. Rogue planet
      2. Hot Jupiter
      3. Super-Earth
      4. Earth analog
3. Atmosphere (Level 4)
   1. Extraterrestrial vortex
4. Magnetosphere
5. Ring system (Level 4)
6. Minor planet (Level 4)
   1. Asteroid (Level 3)
      1. Asteroid family
   2. Dwarf planet (Level 4)
   3. Minor-planet moon
7. Natural satellite (Level 3)
   1. Satellite system (astronomy)
   2. Exomoon
8. Comet (Level 3)
   1. Sungrazing comet
9. Meteoroid (Level 4)
   1. Bolide
   2. Meteor shower (Level 4)
10. Impact event (Level 4)
11. Trojan (celestial body)
12. Centaur (small Solar System body)
13. Axial tilt
14. Planetary differentiation
15. Planetary habitability
    1. Habitable zone
    2. Galactic habitable zone
    3. Drake equation
    4. Rare Earth hypothesis
16. Planetesimal
17. Protoplanet
18. Albedo (Level 4)
    1. Geometric albedo
19. Methods of detecting exoplanets
20. Planetary core
21. Planetary mass

1. Star (Level 3)
2. Variable star (Level 4)
   1. Cataclysmic variable star
      1. Nova (Level 4)
   2. Cepheid variable (Level 4)
      1. Classical Cepheid variable
      2. RR Lyrae variable
      3. Type II Cepheid
   3. Flare star
   4. Luminous blue variable
   5. Mira variable
   6. T Tauri star
   7. Herbig Ae/Be star
   8. Instability strip
3. Star system (Level 4)
   1. Planetary system (Level 4)
   2. Binary star (Level 4)
      1. Binary pulsar
      2. Common envelope
      3. X-ray binary
      4. Roche lobe
4. Star cluster (Level 4)
   1. Stellar association
   2. Open cluster
   3. Globular cluster
      1. Blue straggler
   4. Super star cluster
5. Compact object
   1. White dwarf (Level 4)
      1. Black dwarf
      2. Chandrasekhar limit
   2. Neutron star (Level 4)
      1. Pulsar (Level 4)
      2. X-ray pulsar
      3. Magnetar
      4. Radio-quiet neutron star
   3. Exotic star
      1. Quark star
6. Intergalactic star
7. Stellar classification (Level 4)
   1. Main sequence (Level 4)
      1. Red dwarf
   2. Subdwarf
   3. Subgiant
   4. Giant star (Level 4)
      1. Blue giant
      2. Red giant (Level 4)
   5. Carbon star
   6. S-type star
   7. Supergiant (Level 4)
      1. Blue supergiant
      2. Yellow supergiant
      3. Red supergiant
   8. Hypergiant
   9. Wolf–Rayet star (Level 4)
   10. Be star
   11. B[e] star
8. Stellar evolution (Level 4)
   1. Red-giant branch
   2. Helium flash
   3. Asymptotic giant branch
   4. Horizontal branch
9. Star formation (Level 4)
   1. Starburst galaxy (Level 4)
   2. Starburst region
   3. Jeans instability
   4. Pre-stellar core
   5. Protostar (Level 4)
   6. Hayashi track
   7. Pre-main-sequence star
   8. Young stellar object
   9. Herbig–Haro object
10. Stellar kinematics (Level 4)
11. Stellar magnetic field (Level 4)
12. Stellar nucleosynthesis
    1. Proton–proton chain
    2. CNO cycle
    3. Triple-alpha process
    4. Carbon-burning process
    5. s-process
    6. r-process
13. Stellar structure (Level 4)
    1. Stellar core
    2. Radiative zone
    3. Convection zone
    4. Stellar atmosphere
14. Stellar rotation
15. Stellar mass
    1. Initial mass function
16. Stellar wind
17. Metallicity (Level 4)
    1. Chemically peculiar star
18. Circumstellar disc
    1. Debris disk
    2. Protoplanetary disk
    3. Proplyd
19. Brown dwarf (Level 4)
    1. Sub-brown dwarf
20. Hertzsprung–Russell diagram (Level 4)
21. Double star
22. Protoplanetary nebula
23. Planetary nebula (Level 4)
24. Supernova (Level 3)
    1. Hypernova
    2. Type Ia supernova
    3. Type Ib and Ic supernovae
    4. Type II supernova
    5. Superluminous supernova
    6. Pair-instability supernova
    7. Supernova nucleosynthesis
    8. Supernova remnant
25. Gamma-ray burst (Level 4)
    1. Gamma-ray burst emission mechanisms
    2. Gamma-ray burst progenitors
26. Fast radio burst
27. Kilonova
28. Asteroseismology
29. Mass–luminosity relation

This section contains 56 articles.

1. Galactic astronomy
2. Extragalactic astronomy
3. Outer space (Level 3)
4. Galaxy (Level 3)
   1. Galaxy formation and evolution (Level 4)
5. Astrophysical jet
6. Cosmic dust
7. Black hole (Level 3)
   1. Supermassive black hole (Level 4)
   2. Stellar black hole
   3. Intermediate-mass black hole
   4. Micro black hole
   5. Kugelblitz (astrophysics)
   6. Accretion disk
   7. Event horizon (Level 4)
      1. Schwarzschild radius
8. Interstellar medium (Level 4)
   1. Interstellar cloud
   2. Interstellar ice
   3. Nebula (Level 4)
   4. Dark nebula (Level 4)
      1. Bok globule
   5. Reflection nebula
   6. Emission nebula
   7. Molecular cloud (Level 4)
   8. Superbubble
   9. H I region
   10. H II region (Level 4)
9. High-velocity cloud
10. Cosmic ray (Level 4)
    1. Ultra-high-energy cosmic ray
11. Galactic plane
    1. Thin disk
    2. Thick disk
12. Galactic bulge
13. Galaxy rotation curve
14. Galactic halo
    1. Dark matter halo
15. Interacting galaxy
    1. Galaxy merger
       1. Andromeda–Milky Way collision
    2. Galactic tide
16. Satellite galaxy
17. Galaxy groups and clusters (Level 4)
    1. Galaxy cluster
    2. Galaxy group
    3. Supercluster (Level 4)
18. Gravitational lens
    1. Strong gravitational lensing
    2. Weak gravitational lensing
19. Intergalactic dust
    1. Intracluster medium
20. Warm–hot intergalactic medium
21. Field galaxy
22. Dark flow
23. Wormhole

This section contains 3 articles.

1. Galaxy filament
2. Large quasar group
3. Void (astronomy) (Level 4)

This section contains 24 articles.

1. Galaxy morphological classification (Level 4)
2. Hubble sequence
3. Spiral galaxy (Level 4)
   1. Unbarred spiral galaxy
   2. Barred spiral galaxy (Level 4)
   3. Grand design spiral galaxy
   4. Flocculent spiral galaxy
   5. Magellanic spiral
   6. Density wave theory
   7. SSPSF model
   8. Tully–Fisher relation
4. Elliptical galaxy (Level 4)
   1. Dwarf elliptical galaxy
   2. Type-cD galaxy
   3. Faber–Jackson relation
5. Lenticular galaxy (Level 4)
6. Irregular galaxy (Level 4)
7. Dwarf galaxy (Level 4)
8. Dark galaxy
9. Brightest cluster galaxy
10. Ring galaxy
11. Low surface brightness galaxy
12. Peculiar galaxy
13. Lyman-alpha emitter

This section contains 10 articles.

1. Active galactic nucleus (Level 4)
2. Eddington luminosity
3. Low-ionization nuclear emission-line region
4. Seyfert galaxy (Level 4)
5. Luminous infrared galaxy
6. Radio galaxy (Level 4)
7. Quasar (Level 4)
8. Blazar (Level 4)
   1. BL Lacertae object
9. Lyman-alpha forest

1. Physical cosmology (Level 3)
   1. Observational cosmology
2. Universe (Level 2)
   1. Observable universe (Level 4)
      1. Cosmological horizon
         1. Particle horizon
      2. List of the most distant astronomical objects
   2. World
3. Chronology of the universe (Level 4)
   1. Timeline of the universe
   2. Age of the universe
   3. Big Bang (Level 3)
   4. Grand unification epoch
   5. Cosmic inflation (Level 4)
   6. Electroweak epoch
   7. Quark epoch
   8. Hadron era
   9. Lepton epoch
   10. Recombination (cosmology)
   11. Photon epoch
   12. Reionization
4. Structure formation
5. Shape of the universe
6. Ultimate fate of the universe (Level 4)
   1. Heat death of the universe
   2. Big Rip
   3. Big Crunch
   4. Cyclic model
7. Cosmic microwave background (Level 4)
   1. Discovery of cosmic microwave background radiation
8. Cosmic neutrino background
9. Big Bang nucleosynthesis
   1. Alpher–Bethe–Gamow paper
   2. Missing baryon problem
10. Baryogenesis
11. Expansion of the universe
    1. Accelerating expansion of the universe
    2. Scale factor (cosmology)
12. Baryon acoustic oscillations
13. Primordial fluctuations
14. Dark matter (Level 4)
15. Dark energy (Level 4)
    1. Cosmological constant (Level 4)
    2. Lambda-CDM model
    3. Quintessence (physics)
16. Redshift (Level 4)
    1. Hubble's law (Level 4)
       1. Hubble volume
17. Comoving and proper distances
18. Anthropic principle
19. Flatness problem
20. Horizon problem
21. Steady-state model
22. Multiverse