Astronomy found that its mass determines nearly every aspect of a star’s physics; in the end, mass matters. The star’s place on the HR diagram is primarily determined primarily by its mass. The star’s position on the HR diagram determines its absolute brightness and color.
The problem was that astronomers did not understand why this was so at the beginning of the twentieth century. First, they had thought that maybe younger stars would burn brighter, but this was not necessarily so. Then astronomers hypothesized that larger stars would be brighter, which is also not necessarily so.
The reason why so much physics depends upon the balance between gravity and thermal pressure
Gravity and Thermal Pressure Desplan Why Mass Matters
There are large clouds of interstellar gas composed primarily of hydrogen and helium with trace amounts of other elements. These gas clouds have a temperature that approaches absolute zero in many areas, but nearby stars heat some gas clouds. The clouds have mass and therefore have a gravitational force that concentrates the mass toward its center. Heat within the gas cloud tends to counter its gravitational force until the cloud reaches an equilibrium. An equilibrium is established when expansionary pressure from heat is balanced by contraction pressure from gravity.
Some gas clouds are relatively cool, so gravity gradually concentrates the gas into smaller and smaller areas. Initially, the gas cloud does not heat up very much, so gravity exerts greater influence. Heat mas a more difficult time radiating away from the center of this contracting mass because the concentrated gas becomes less transparent. The concentrated gas then becomes a protostar. This is an area where an actual star is likely to form.
The heat from a protostar is radiated away from the surface even as gravity pulls the mass into a smaller and smaller volume producing more and more hear in the center. While the heat at the surface of the protostar will radiate away, heat becomes trapped in its center. The forming star cannot generate heat fast enough to counter gravity as its surface continues to radiate away heat.
Larger Protostars Turn Into Stars
The increasing heat at the center of a protostar will eventually reach the amount necessary to cause nuclear fusion to begin. At this point, the protostar will become an actual star as hydrogen is fused into helium to release tremendous amounts of energy. At this point, the star will likely expand due to the tremendous heat from fusion. A new equilibrium will be reached between gravitational force and heat, with this new equilibrium lasting for millions if not billions of years. The star now has a stable core temperature fusing hydrogen into helium, constant surface temperature, luminosity, and color. It is now a main sequence “dwarf” star and will main in this state for as long as conditions which the balance between gravity and hear continue.
For a protostar to turn into an actual star, it needs to have enough mass so that the gravitational pull inward will heat its core to a high enough temperature for fusion to occur. The protostar needs to have at least 8 percent of the sun’s mass for this to occur. Otherwise, it will never generate enough heat for nuclear fusion proton0proton chain reaction.
A Brown Dwarf Star
Protostars which are about 1 – 8 percent of the mass of the sun, follow another pathway. They are not big enough for the fusion of hydrogen into helium.