A field guide to the cosmos.
Astronomy studies everything beyond Earth's atmosphere: nearby worlds, the life cycles of stars, the structure of galaxies, and the large-scale history of the universe. These notes collect useful facts and scale comparisons in one place.
Planets
Our Solar System has eight recognized planets, from rocky Mercury to distant ice giant Neptune.
Earth–Sun distance
One astronomical unit is about 149.6 million kilometers and is a convenient unit for Solar System distances.
Sunlight travel time
Light from the Sun takes roughly eight minutes and twenty seconds to reach Earth.
Nearest star system
Alpha Centauri is the nearest stellar system to the Sun; Proxima Centauri is its closest known member.
Milky Way width
The Milky Way's stellar disk spans on the order of one hundred thousand light-years.
Cosmic age
The universe is estimated to be about 13.8 billion years old based on modern cosmological measurements.
Speed of light
Light in vacuum travels at 299,792 kilometers per second, setting the universe's fundamental causal speed limit.
Solar photosphere
The Sun's visible surface, the photosphere, has a temperature of roughly 5,500 degrees Celsius.
Earth day
A mean solar day is approximately twenty-four hours, while Earth's sidereal rotation period is slightly shorter.
Moon orbit
The Moon completes one orbit relative to the distant stars in about 27.3 days.
White-dwarf limit
The Chandrasekhar limit is about 1.4 solar masses, above which an unsupported white dwarf cannot remain stable.
Cosmic background
The cosmic microwave background has a temperature of about 2.7 kelvin and is relic radiation from the early universe.
Useful distance units
| Unit | Approximate scale | Typical use |
|---|---|---|
| Kilometer | 1,000 meters | Planetary sizes and local distances |
| Astronomical unit | 149.6 million km | Solar System distances |
| Light-year | 9.46 trillion km | Distances between stars |
| Parsec | 3.26 light-years | Professional stellar and galactic astronomy |
How stars change
Formation
Stars form when dense regions of molecular clouds collapse under gravity and heat until sustained nuclear fusion begins.
Main sequence
For most of a star's lifetime, hydrogen fusion in its core supplies the energy that balances gravitational contraction.
Late stages
A star's initial mass strongly influences whether its remnant becomes a white dwarf, neutron star, or another compact object.