Introduction to the H-R Diagram (con't.)
The Giant Branch: Red giants are luminous, cool giant stars in spectral classes F, G, K, and M located in the middle right portion of the H-R diagram, above the main sequence. As the central core of a main sequence star with a mass from ~0.8 to 8 solar masses runs out of hydrogen, radiation pressure no longer balances gravity and the star begins to collapse. The core hydrogen has been converted to helium; however, there is still hydrogen in the outer layers surrounding the helium core of the star. As the star begins to contract, the core gets hot enough to start a thin shell of hydrogen fusion around the helium core. The increase in radiation pressure causes the stars outer atmospheric layers to expand. As the surface of the star increases, so does its apparent brightness. As the surface (photosphere) increases, it becomes cooler, and the color of the star becomes redder. Eventually the hydrogen in the shell becomes depleted and the star begins to contract once again, and this time the temperature becomes hot enough to start helium fusion. The outer layers expand even further, becoming cooler and redder. Giant stars fuse elements up to carbon. Most of these stars go through a Mira variable instability stage with a periodicity of ~80 - 1000 days. Stars that have evolved to the giant branch are commonly referred to as red giants. Eventually these red giants will shrug off a planetary nebula and leave a white dwarf core remnant. There is no relationship between mass and luminosity for stars on the giant or any other branch of the H-R diagram – only the main sequence.
The Supergiant Branch: Stars greater than ~8 solar masses evolve horizontally onto the supergiant branch, located in the extreme upper right corner of the H-R diagram. These red supergiants are extremely luminous and cool, due to their expanded size. They begin as spectral types O and B (white and blue in color) on the main sequence and evolve to class M (red), as they finish their transition to the supergiant branch. NOTE: The O and B stars on the main sequence are sometimes referred to as blue supergiants, not to be confused with the highly evolved and aging red supergiants located on the supergiant branch. Because of the mass of these stars, the fusion of heavier and heavier elements continues through neon, magnesium, silicon, sulfur, iron and nickel. Each time a new element is created the star becomes larger and redder. (Some stars with a mass of ~8 solar masses move through the Cepheid variable instability stage and pulsate with a period of 1 - 70 days). Eventually massive stars reach the supergiant branch and undergo a core collapse in a Type II supernova event, leaving behind a pulsar, neutron star, magnetar or black hole and a supernova remnant. Some hyper-massive stars collapse into back holes without a supernova event, and some undergo a supernova event that leaves no core remnant behind – such as SN2006GY.
The White Dwarf Branch: The white dwarf branch is located in the lower left corner of the H-R diagram. This branch consists of the end products of stellar evolution for mid-sized stars with an initial mass of ~0.8 to 8 solar masses. All white dwarfs are extremely hot; however they have a very low absolute magnitude because they are very small. They have a size that does not exceed 1.4 solar masses - the Chandrashekar limit. Spectral types for white dwarfs range from O to G as they slowly radiate away their energy.
NOTE: This is only a brief introduction to stellar evolution on the H-R diagram. There are exceptions to some of the evolutionary sequences, and the associated masses are “ballpark” numbers only. There are more stellar classifications than the ones mentioned above. A more in-depth description of the evolutionary tracks of stars across the H-R diagram – especially the transition stages involving variable stars – is located at: http://chandra.harvard.edu/edu/formal/variable_stars/bg_info.html