Stars, Page 6

Stars, Page 6: [Previous Page] [Next Page] [Class Notes] Textbook pages 217-233

Luminosity Classes and Stellar Evolution

If we display the luminosity of a star versus its mass, Dwarf Stars on the Main Sequence, again, show an interesting relationship. All the stars have something in common: They are all fueling their luminosity via Hydrogen Fusion into Helium in their central core.

Larger masses show larger luminosities. The conditions within a stars core is dependent on the gravitational pressure exerted by the star.

Higher mass leads to higher core temperatures and densities. This leads to much higher reactions rates (which are a strong function of T and density), and ultimately, a much much higher Luminosity.

About 10% of stars are found NOT to lie on the Main Sequence (not luminosity class V) are NOT currently burning Hydrogen in their cores. They exist in an advanced (later) stage of stellar evolution.

Luminosity Classes of Evolved Stars

Evolved stars are seen as luminosity classes: IV, III, II and I, which are also called sub-giants, giants, extreme (or bright) giants and supergiants.

Spectroscopic Parallax
This is not a parallax at all, but is a means of estimating the distance to a star from its spectral type (OBAFGKM) and luminosity class (V,VI,III,II,I).

If we know a stars spectral type and luminosity class, then we know in principle (see above) what their absolute Visual magnitude should be. Comparing that to their apparent visual magnitude tells us the stars distance!