A star is a luminous sphere of plasma held together by its own gravity. The nearest star to Earth is the sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, the brightest of which were given mythological or zodiacal names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the stars in the universe, including all stars outside our galaxy, the Milky Way, are invisible to the naked eye from Earth. Indeed, most are invisible from Earth even through the most powerful telescopes.
A star's engine uses the nuclear fusion of hydrogen into helium to create radiant energy, and this makes them cosmic energy generators. In the core of a star, gravity produces high density and high temperature. The density of gas in the core of our sun is 160 g/cm3, much higher than the densest metal, and the temperature is 15,000,000 K. At this temperature, the hydrogen and helium gases become a plasma. The fusion of hydrogen nuclei uses up hydrogen to produce helium and energy. Hydrogen is the fuel for the process. As the hydrogen is used up, the core of the star condenses and heats up even more. This promotes the fusion of heavier and heavier elements, ultimately forming all the elements up to iron.
Type II or beyond use stellar engines to gather their energy, or even move them. The artificial creation of a star is called stellification.
An evolutionary life cycle of a star can generally follow three paths (where M = 1 original solar mass):
- Massive star (>8M) --> red supergiant --> supernova --> black hole / neutron star
- Low-mass star (<8M) --> red giant --> nova or white dwarf --> black dwarf
- Failed star (<0.08M) --> brown dwarf
Therefore our own Sun would become a red giant and never go supernova.
More precisely, a collapsar is a star which has undergone gravitational collapse:
- If it is less than 1.4 resultant solar masses, the star will become a white dwarf
- Less than 3 solar masses, it will become a neutron star
- Above 3 solar masses, the star will become a black hole
Spectral types:[]
| Type | Color | Example |
|---|---|---|
| O | Blue | 10 Lacerta |
| B | Blue | Rigel |
| A | Blue | Sirius |
| F | Blue/White | Procyon |
| G | White/Yellow | Sun |
| K | Orange/Red | Arcturus |
| M | Red | Betelgeuse |