Stars and the Main Sequence

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 conbination 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.

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