Luminosity Classes of Stars

Stars are not only described by their temperatures, but also by their brightness, or luminosity. These luminosity classes can tell us what kind of star the star is and what its past was like and what its future may hold.


Luminosity Class Ia and Ib - Supergiants:

• Typically, Supergiants are stars that had have evolved off the main sequence. These stars are very bright because they have a large radius. There may be some that are blue, but the majority of them are red and lie in the upper right corner of the HR Diagram. Betelgeuse in Orion is an example of a supergiant. They come from massive stars that can no longer fuse hydrogen in their core and will one day supernova, leaving behind either a neutron star or a black hole.

Luminosity Class II - Bright Giants:

• Giants that are brighter than "normal" giants, these are evolved medium mass stars (7 times or more heavier than the Sun). When they go supernova, they will most likely leave behind a neutron star.

Luminosity Class III - Giants:

• Evolved average mass stars (like the Sun). They will burn until they no longer can fuse elements in the core into heavier material. The Sun will become a giant once it stops fusing hydrogen in the core. These stars will have radii on the order of the Venus' orbit (0.7 AU) to Mars' orbit (1.4 AU). They become white dwarfs once fusion ends.

Luminosity Class IV - Subgiants:

• Likely the next stage of evolution for low mass stars (M type dwarf stars). Too small to burn anything heavier than helium in their cores and may be too cool to even fuse helium. Probably become low mass white dwarfs.

Luminosity Class V - Dwarfs or Main Sequence Stars:

• The luminosity class with the highest population. The majority of stars we see in the sky are class V. These are stars that fuse hydrogen into helium in their cores via the proton-proton chain or the CNO cycle (using carbon, nitrogen, and oxygen as catalysts to fuse hydrogen into helium). Depending on their masses, these stars can spend a few million years in this stage or billions of years (higher the mass, shorter the main sequence stage). Class V stars will evolve into the above classes.

Luminosity Class VI - Subdwarfs:

• Stars that are too small to fuse hydrogen into helium. Probably brown dwarfs, but are still hot from formation.

Luminosity Class D - White Dwarfs (do not confuse with Dwarfs):

• The final stage of a low mass star. No fusion occurs. The star is just slowly cooling off. These stars are found in the lower left corner. Very hot, but very due to being very small. This topic will be expanded on.

There are two other types of stellar remnants as mentioned in the supergiant category: neutron stars and black holes. These two topics will be explored more in detail in future posts.

Magnitudes

To help understand how stars relate to each other, besides the HR Diagram, astronomers also describe the stars in terms of magnitude. There are basically two types of magnitudes associated with stars: absolute magnitude and apparent magnitude. Magnitude is just a measure of how bright a star is and can be measured at different wavelength bands in the electromagnetic spectrum. For our purposes, we are only concerned with the visible magnitude of a star in this post.


Apparent magnitude (m) is how bright a star appears as seen from Earth. In general, for stars of the same type and class, the farther away the star is from Earth, the dimmer it appears. Using the apparent magnitude, a dimmer star will have a larger apparent magnitude.

• Vega was originally chosen as the zero point for apparent magnitude. After more measurements, Vega has an apparent magnitude of +0.3.
• A difference in magnitudes of 5 is equal to a difference in brightness by a factor of 100
• A star with m=1.0 is 100 times brighter than a star with m=6.0
• Then for a difference in magnitude of 1, a star with m=1.0 is 2.512 times brighter than a star with m=2.0 (2.512 is the fifth root of 100)
• Apparent magnitudes (as seen from Earth) for some common objects
• the Sun: m=-26.74 (obviously the brightest thing in the sky)
• Sirius (the brightest non-Sun star in the sky): m=-1.47
• Proxima Centauri (the closest star to the Sun): m=5.35 (too faint to see without the aid of a telescope or binoculars)
• Alpha Centauri A (the brightest star in the triple star system Alpha Centauri (including Proxima Centauri): -0.01
• Betelgeuse (Red Supergiant in Orion): 0.42
• Rigel (Blue Supergiant in Orion): 0.13
• Venus (the brightest planet at maximum): -4.89 
• Full Moon (at perigee): -12.92

Absolute magnitude (M) is the standard for all stars. It is how bright a star would be at 10 parsecs (approximately 32.7 light years from the Sun). It is measured the same way as apparent magnitude with a difference of 5 equating to a factor of 100 in brightness.

• Absolute Magnitudes for some common stars:
• the Sun: M=4.83
• Rigel: M=-7.0
• Deneb (in Cygnus): M=-7.2
• Betelgeuse: M=-5.6
• Sirius: 1.4
• Absolute magnitudes are a measure of a stars luminosity as well, and can be used on the y-axis of the HR Diagram.