A review of main sequence (MS) stars and how they work, although much of this applies to stars in general:

1. Stars shine because they're hot and dense: emit a thermal (blackbody) radiation spectrum modified by absorption lines; this absorption line spectrum is produced at its surface (called a photosphere).

2. If core T exceeds 107 K:  4 Hydrogen nuclei fuse into 1 Helium nucleus + energy (gamma-rays) + 2 neutrinos at a rate sufficient to balance off the loss of energy in the form of light at the star's surface (i.e., it's luminosity). In our Sun, the central temperature is at present about 15.7 million K, and 613.8 million metric tons of H are fused into 609.5 million metric tons of He each second. By the way, what happens to the other 4.3 million tons of matter processed per second?
3. so...higher mass main sequence stars:
4. If pressure - gravity balance is established with central temperatures significantly less than 107 K, the full set of fusion reactions converting hydrogen into helium + energy cannot occur. The object never achieves energy balance (it radiates energy away in the form of light from its surface, but has no way to replace that lost energy), and thus this object never becomes a full-fledged star, and is instead called a brown dwarf (a failed or `still-born' star). This occurs for objects2 whose masses lie below 0.08 Msun. There are now thousands of "brown dwarfs" known, and they are suspected to be nearly as numerous as all normal stars in the Milky Way Galaxy combined.

Remember, you can compare the structures of only those stars that have the same energy source in their cores. For main sequence stars, this is the fusion of hydrogen into helium in their central most regions.  Can you see how the many known observed facts of main sequence stars are logically and causally linked together by the laws of nature?

1Strictly speaking, this is true when averaged over the characteristic time required to transport the energy through the star. If for any reason the star isn't in energy balance, you can bet that it is adjusting its structure (expanding/cooling or contracting/heating) until it is.
2By this time these objects' densities have become so high that a weird quantum mechanical pressure known as "electron degeneracy pressure" becomes important. Since this kind of pressure does not depend on temperature, it can balance against the force of gravity even though the object continues to radiate energy away as light from its surface. As a consequence the object does not contract, and without contraction no more gravitational potential energy can be released to raise the temperature, and so the temperature slowly falls over the eons of time.

Kirk Korista
Professor of Astronomy
Department of Physics
Western Michigan University