| Astrophysics (Index) | About |
These are astronomical quantities such as distances and counts to hint at the scales of things in astronomy and give some sense regarding a quantity such as 100 Mpc. For more such quantities, see "Wavelengths table" for electromagnetic radiation band ranges and "Distances table" for distances and cosmological redshifts. See "cosmological redshift" for relation between distance and cosmological redshift. See "spectral class" and "luminosity class" for stellar classifications and "extra-solar planet" for various types of planets. See "Lambda-CDM model" for its parameters. Regarding distance scales:
| light-year (ly) | 63241 AU | 9.461×1015m or 5.879×1012 miles |
| parsec | 3.262 ly | 206,265 AU, 3×1016m or 1.917×1013 miles |
| speed of light | 186,000 miles/sec | 300,000 km/sec |
| Earth radius | 3959 miles | 6371 km |
| Earth mass | 5.972×1024 kg | |
| Earth orbital speed | 30 km/sec | 67000 MPH |
| Moon distance | 238,900 miles | 384,400 km, 60 Earth radii, 0.55 solar radius or 0.0025 AU |
| Moon radius | 1080 miles | 1737 km or 0.27 Earth radius (1/4) |
| Moon mass | 7.3×1022 kg | 0.012 Earth mass (1.2%) |
| Moon orbital speed | 1 km/s | 2300 MPH |
| Sun distance | 93 million miles | 150 million km, 215 solar radii or 1 AU |
| Sun radius | 432,288 miles | 696,340 km, 109 Earth radii or 0.0047 AU |
| Sun mass | 2×1030 kg | 333,000 Earth masses |
| Jupiter orbit radius | 483.7 million miles | 778 million km or 5.2 AU |
| Jupiter radius | 43441 miles | 143,000 km, 11 Earth radii or 1/10 solar radius |
| Jupiter mass | 1.898×1027 kg | 318 Earth masses or 0.001 solar mass (0.1%) |
| Neptune orbit radius | 2.8 billion miles | 4.5 billion km, 30 AU, 0.0005 ly or 4hr10min × c |
| Neptune radius | 15299 miles | 24,764 km, 3.85 Earth radii or 0.035 solar radius (3.5%) |
| Neptune mass | 1026 kg | 17 Earth masses or 0.00005 solar mass (0.005%) |
| Alpha Centauri distance | 4.344 ly | 1.33 pc or 9100 × Neptune's orbit radius |
| galactic center distance | 26000 ly | 8 kpc or 6000 × distance to Alpha Centauri |
| Milky Way number of stars | 100-400 billion | |
| Milky Way visible diameter | 100,000 ly | 30 kpc |
| Milky Way visible depth | 10,000 ly | 3 kpc |
| Milky Way galactic halo diameter | 300,000-400,000 ly | 100 kpc |
| Canis Major Dwarf Galaxy distance (claimed nearest galaxy) | 25000 ly | 8 kpc |
| Sagittarius Dwarf Elliptical Galaxy (accepted nearest galaxy) | 65000 ly | 20 kpc |
| Andromeda distance (nearest similar galaxy) | 2.5 million ly | 765 kpc, 0.8 Mpc or 25 Milky Way diameters |
| Local Group diameter | 10 Mly | 3 Mpc or 100 Milky Way diameters |
| Local Group galaxy count | ~100 | |
| Virgo Cluster (nearest galaxy cluster) distance | 54 Mly | 17 Mpc |
| Virgo Cluster galaxy count | 1000-2000 | |
| Virgo Supercluster diameter | 110 Mly | 33 Mpc |
| Virgo Supercluster cluster/group count | 100 | |
| Virgo Supercluster galaxy count | 50000 | |
| Laniakea Supercluster diameter | 520 Mly | 160 Mpc or distance to z = 0.04 |
| Laniakea cluster count | 300-500 | |
| Laniakea galaxy count | 100,000 | |
| typical stars per galaxy | thousands to 100 trillion | |
| superclusters in observable universe | estimated 10 million | |
| galaxies in observable universe | estimated 170 billion | |
| stars in the observable universe | estimated 1024 (septillion) | |
| age of universe | 13.8 gigayears | |
| diameter of observable universe | 93 Gly | 28.5 Gpc or 180 × Laniakea diameter |
| mass of observable universe | estimated 1053kg | |
| visible light wavelengths | 4000-7000 angstroms | 4×10-7 to 7×10-7 meters |
| atoms in a gram of hydrogen (Avogadro's number) | 6.022×1023 | |
| atoms in the universe | estimated 1078 to 1082 | |
| photons in the universe | estimated 1089 | |
| neutrino density | 340/cm3 | |
| neutrinos in the universe | estimated 1.2×1089 | |
| atomic density of the universe (baryonic mass density) | estimated 0.2-0.25 hydrogen-atom masses/meter3 | (basically, daltons) |
| critical density of the universe | estimated 5 hydrogen-atom masses/meter3 | threshold between eventual expansion versus collapse |
| density including dark matter and dark energy | roughly same | very close to critical density |
Some of the counts have varying estimates ranging over an order-of-magnitude or more, and are subject to substantial improvement over time, in which case the above numbers are no more than example estimates. This is true of some of the distances and masses beyond those within the solar system, and the item counts for galaxies, galaxy groups, clusters, superclusters, and the entire observable universe.