Star Types

There are 4 fundamental forces of nature:

Strong nuclear - this keeps protons close together
Weak nuclear a responsible for radioactive decay
Electromagnetism - light, electricity, magnetism, etc
Gravity - weakest of all, but furthest reaching



A star (Latin root stella-) is essentially a ball of gas powered by nuclear reactions, held together by gravity.

Stars may appear white, but their color is a combination of many colors (and non visible e-m waves like UV).

Spectral types are listed in order of decreasing temperature:

O B A F G K M

with a temperature range from 60,000 K down to under 3500 K.

There are further subdivisions (C and S stars under M).

You can learn a lot about a star from where it lies on the Hertzsprung-Russel diagram.

The H-R diagram plots magnitude (brightness, from dim to bright) vs. temperature (high to low, usually as spectral type).

Hottest stars are on the left if the graph - they are normally brighter than cooler stars.

Most stars fall on along a diagonal band from upper left to lower right on the H-R diagram. We call this the Main Sequence, and the stars there are main sequence stars or dwarfs (which is a misleading term).

Stars above and to the right of the MS are giants (including supergiants).

Faint hit objects (white dwarfs) are below and to the left of the MS.

Our own Sun is a G2 star, somewhere close to 4.6 billion years old.

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