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 42 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)
   1. Zone of Avoidance
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
    1. First observation of gravitational waves
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 134 articles.

This section contains 8 articles.

1. Solar System (Level 2)
2. Formation and evolution of the Solar System (Level 4)
   1. Nebular hypothesis (Level 4)
   2. Nice model
3. Interplanetary medium (Level 4)
   1. Gegenschein
   2. Interplanetary dust cloud
      1. Zodiacal light

This section contains 14 articles.

1. Sun (Level 2)
2. Chromosphere
3. Coronal mass ejection
4. Heliosphere
5. Photosphere
6. Solar wind (Level 4)
7. Solar core
8. Solar cycle
9. Solar flare
10. Solar neutrino
11. Solar phenomena
12. Solar storm
13. Stellar corona
14. Sunspot

This section contains 42 articles.

1. Mercury (planet) (Level 3)
   1. Atmosphere of Mercury
   2. Geology of Mercury
2. Venus (Level 3)
   1. Atmosphere of Venus
   2. Geology of Venus
3. Earth (Level 1)
   1. Earth's orbit
   2. Earth's shadow
   3. Earthlight
   4. Future of Earth
   5. Location of Earth
   6. Moon (Level 2)
      1. Geology of the Moon
      2. Internal structure of the Moon
      3. Orbit of the Moon
      4. Origin of the Moon
         1. Giant-impact hypothesis
         2. Theia (hypothetical planet)
      5. Selenography
         1. Near side of the Moon
         2. Far side of the Moon
         3. Lunar south pole
4. Near-Earth object
   1. 433 Eros
5. Mars (Level 3)
   1. Atmosphere of Mars
   2. Geology of Mars
      1. Geological history of Mars
      2. Martian polar ice caps
      3. Olympus Mons
   3. Life on Mars
   4. Moons of Mars
      1. Phobos (moon)
      2. Deimos (moon)
6. 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

This section contains 41 articles.

1. Jupiter (Level 3)
   1. Atmosphere of Jupiter
      1. Great Red Spot
   2. Jupiter trojan
   3. Magnetosphere of Jupiter
   4. Moons of Jupiter
      1. Galilean moons
         1. Io (moon) (Level 4)
         2. Europa (moon) (Level 4)
         3. Ganymede (moon) (Level 4)
         4. Callisto (moon) (Level 4)
   5. Rings of Jupiter
2. Saturn (Level 3)
   1. Magnetosphere of Saturn
   2. 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 and rivers of Titan
      7. Iapetus (moon)
   3. Rings of Saturn
   4. Saturn's hexagon
3. Uranus (Level 3)
   1. Atmosphere of Uranus
   2. Moons of Uranus
      1. Miranda (moon)
      2. Ariel (moon)
      3. Umbriel
      4. Titania (moon)
      5. Oberon (moon)
   3. Rings of Uranus
4. Neptune (Level 3)
   1. Great Dark Spot
   2. Moons of Neptune
      1. Triton (moon) (Level 4)
   3. Rings of Neptune
5. Centaur (small Solar System body)
   1. 2060 Chiron

This section contains 22 articles.

1. Trans-Neptunian object
2. 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
3. Scattered disc
   1. Eris (dwarf planet) (Level 4)
      1. Dysnomia (moon)
4. Oort cloud (Level 4)
   1. Hills cloud
   2. Sednoid
      1. Sedna (dwarf planet)
5. 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 2 articles.

1. 1I/ʻOumuamua
2. 2I/Borisov

This section contains 104 articles.

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 Ae
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 20 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. Trifid Nebula
20. Westerhout 40

This section contains 12 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. Trapezium Cluster

This section contains 4 articles.

1. Kepler's Supernova
2. SN 1006
3. SN 1054
4. SN 1572

This section contains 10 articles.

1. Milky Way (Level 3)
2. Galactic Center (Level 4)
   1. Sagittarius A
      1. Sagittarius A* (Level 4)
3. Local Bubble
   1. Local Interstellar Cloud
   2. G-Cloud
4. Orion Arm
5. Perseus Arm
6. Scutum–Centaurus Arm

This section contains 63 articles.

This section contains 35 articles.

This section contains 9 articles.

1. Andromeda Galaxy (Level 4)
2. Triangulum Galaxy (Level 4)

This section contains 7 articles.

1. Magellanic Clouds
   1. Large Magellanic Cloud (Level 4)
      1. Tarantula Nebula
      2. R136
   2. Small Magellanic Cloud (Level 4)
   3. Magellanic Stream
2. Sagittarius Dwarf Spheroidal Galaxy

This section contains 26 articles.

1. 3C 273 (Level 4)
2. Antennae Galaxies
3. Arp 220
4. BL Lacertae
5. Cartwheel Galaxy
6. Centaurus A (Level 4)
7. Cygnus A
8. Hoag's Object
9. IC 1101
10. Markarian 421
11. Markarian 501
12. Messier 49
13. Messier 77
14. Messier 81
15. Messier 82
16. Messier 83
17. Messier 87 (Level 4)
18. Messier 106
19. NGC 1275
20. NGC 4151
21. NGC 5548
22. NGC 6946
23. Pinwheel Galaxy (Level 4)
24. Sculptor Galaxy
25. Sombrero Galaxy
26. Whirlpool Galaxy (Level 4)

This section contains 20 articles.

1. Virgo Supercluster (Level 4)
   1. Local Group (Level 4)
      1. Satellite galaxies of the Milky Way
      2. Virgocentric flow
   2. Virgo Cluster (Level 4)
   3. Centaurus A/M83 Group
   4. IC 342/Maffei Group
   5. M81 Group
   6. Local Sheet
2. Laniakea Supercluster
3. Coma Cluster
4. Fornax Cluster
5. Perseus Cluster
6. Great Attractor (Level 4)
7. Bullet Cluster
8. Shapley Supercluster
9. Pisces–Cetus Supercluster Complex
10. South Pole Wall
11. Huge-LQG
12. BOSS Great Wall

This section contains 5 articles.

1. Local Hole
2. Boötes Void
3. Local Void
4. Giant Void
5. Dipole repeller

This section contains 3 articles.

1. SN 1972E
2. SN 1987A
3. GW170817

This section contains 3 articles.

1. Hubble Deep Field
2. Hubble Ultra-Deep Field
3. CMB cold spot

1. Celestial mechanics (Level 4)
2. Orbit (Level 4)
   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
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 19 articles.

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. Belt of Venus
17. Blue hour
18. Golden hour (photography)
19. Terminator (solar)

This section contains 22 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. Geosynchronous orbit (Level 4)
   1. Geostationary orbit (Level 4)
4. Molniya orbit
5. Gravitational acceleration (Level 4)
6. Gravity assist (Level 4)
   1. Oberth effect
7. Low Earth orbit (Level 4)
   1. Frozen orbit
   2. Sun-synchronous orbit
   3. Polar orbit
8. Halo orbit
9. Lissajous orbit
10. Co-orbital configuration
    1. Horseshoe orbit
11. 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 92 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 78 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 (constellation)
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)
    1. Orion's Belt
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)
    1. Big Dipper
73. Ursa Minor (Level 4)
74. Vela (constellation)
75. Volans
76. Vulpecula

This section contains 2 articles.

1. Argo Navis
2. Quadrans Muralis

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

1. Star (Level 3)
2. 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
3. Star cluster (Level 4)
   1. Stellar association
   2. Open cluster
   3. Globular cluster
      1. Blue straggler
   4. Super star cluster
4. 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
5. Intergalactic star
6. Stellar evolution (Level 4)
   1. Red-giant branch
   2. Helium flash
   3. Asymptotic giant branch
   4. Horizontal branch
7. 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
8. Stellar kinematics (Level 4)
9. Stellar magnetic field (Level 4)
10. Stellar nucleosynthesis
    1. Proton–proton chain
    2. CNO cycle
    3. Triple-alpha process
    4. Carbon-burning process
    5. s-process
    6. r-process
11. Stellar structure (Level 4)
    1. Stellar core
    2. Radiative zone
    3. Convection zone
    4. Stellar atmosphere
12. Stellar rotation
13. Stellar mass
    1. Initial mass function
14. Stellar wind
15. Metallicity (Level 4)
    1. Chemically peculiar star
16. Circumstellar disc
    1. Debris disk
    2. Protoplanetary disk
    3. Proplyd
17. Brown dwarf (Level 4)
    1. Sub-brown dwarf
18. Hertzsprung–Russell diagram (Level 4)
19. Double star
20. Protoplanetary nebula
21. Planetary nebula (Level 4)
22. 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
23. Gamma-ray burst (Level 4)
    1. Gamma-ray burst emission mechanisms
    2. Gamma-ray burst progenitors
24. Fast radio burst
25. Kilonova
26. Asteroseismology
27. Mass–luminosity relation

This section contains 18 articles.

1. Stellar classification (Level 4)
2. Main sequence (Level 4)
   1. Red dwarf
3. Subdwarf
4. Subgiant
5. Giant star (Level 4)
   1. Blue giant
   2. Red giant (Level 4)
6. Carbon star
7. S-type star
8. Supergiant (Level 4)
   1. Blue supergiant
   2. Yellow supergiant
   3. Red supergiant
9. Hypergiant
10. Wolf–Rayet star (Level 4)
11. Be star
12. B[e] star

This section contains 13 articles.

1. Variable star (Level 4)
2. Cataclysmic variable star
   1. Nova (Level 4)
3. Cepheid variable (Level 4)
   1. Classical Cepheid variable
   2. RR Lyrae variable
   3. Type II Cepheid
4. Flare star
5. Luminous blue variable
6. Mira variable
7. T Tauri star
8. Herbig Ae/Be star
9. Instability strip

This section contains 55 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 (Level 4)
   3. Intermediate-mass black hole
   4. Micro black hole
   5. Accretion disk
   6. 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. Age of the universe
   2. Big Bang (Level 3)
   3. Grand unification epoch
   4. Cosmic inflation (Level 4)
   5. Electroweak epoch
   6. Quark epoch
   7. Hadron era
   8. Lepton epoch
   9. Recombination (cosmology)
   10. Photon epoch
   11. 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