Introductory Astronomy: Mid Mass Stars

After a mid mass star has converted all of the hydrogen in its core into helium, it stops being a main sequence star and begins the internal changes that will make it a Red Giant star, placing it on the Red Giant Branch on the HR diagram..

When it begins burning helium, the star's internal changes place it on the Horizontal Branch. Eventually the core will exhaust its supply of helium and fusion will again stop. As before, the core will collapse, heating the outer layers, and eventually igniting a helium fusion shell, which will heat the layers above it and create a hydrogen fusion shell. During this stage, the inside of the star consists of a core of carbon (where no fusion is taking place), surrounded by a shell of fusing helium, surrounded by a shell of fusing hydrogen, surrounded by the envelope of the star. The envelope is only loosely bound to the star and can be blown off by winds. This results in a Planetary Nebula which surrounds the star. The remaining star (at the center of the planetary nebula) is only the hot core of the original star. The temperature of this remnant can be as high as 200,000 degrees Kelvin.

In mid mass stars, the core temperature will never become high enough to cause carbon fusion (temperatures of 5-8x108K are required). Instead, it will keep contracting due to gravity until electrons themselves begin to resist the contraction, exerting what is known as Electron Degeneracy Pressure. This electron pressure opposes gravity and stops the collapse of the star. The star is then known as a White Dwarf.

Mid mass stars are similar to the Sun. Stars of one solar mass have a main sequence lifetime of about 10 billion years.