It was at this point that we began to leave the main sequence. The main sequence is a region on a chart that describes the different types of stars, categorizing them by temperature and luminosity. The graph below shows the main sequence, and it also shows the different paths stars take when they leave the main sequence based on their mass. This is called an H-R diagram.
Just for a little
clarification:
O stars
(farthest left) are the most massive of all types of stars - they can be
up to 40 times the mass of our Sun!!!! These stars are also the most
luminous - 5 x 10^5 times as bright as the Sun - the hottest at 40,000K,
and the largest, up to 18 times the radius (half the distance from one
side of the star to the other) of the Sun! These are really big,
really bright, really hot stars!!!
B stars come second with 16 times the mass, 2 x 10^4 the luminosity, and 7 times the size of the Sun with temperatures up to 28,000K.
Next are the A stars, which can be up to 3.3 times the mass of our Sun, 80 times as bright, 2.5 times as big, and temperatures up to 10,000K.
F stars are next on the list with 1.7 solar masses, 6 times the luminosity, 1.4 solar radii, and temperatures of 7,500K.
Our sun is a G star. These stars can be up to 1.1 solar masses, 1.3 solar luminosities, 1.1 times as big as our Sun, and have temperatures up to 6,000K.
After G stars come the K stars - stars that are only 0.8 times as massive as the Sun, 0.4 times as bright, 0.8 as big, and it only 5,000K.
The very smallest and coldest stars are the M stars. M stars are 0.4 as massive, 0.03 times as bright, and 0.6 as big as the Sun. Temperatures on M stars only reach about 3,500K.
Also shown on the graph are special types of stars that are in their last stages of stellar evolution. Giants and Supergiants are stars that could be called the "middle aged" stars. They have expanded beyond their original surface area and are more luminous than they were previously. Giants are less luminous than Supergiants, even though they can be of the same temperature. Also shown on the diagram are White Dwarfs, very hot stars that are very dim -so dim, in fact that you need a fairly large telescope to see them!!!
Not only does our diagram show the temperatures and luminosity of stars, it also shows the spectral classes that astronomers use to help classify stars. The spectral classes are given by the roman numerals beside the blue lines.
Stars of Ia
spectral classes are the brightest supergiants.
Ib stars
are less luminous supergiants.
Bright giants
are classified as II.
Regular giants
are III.
IV stars
are subgiants - between the giants and the main sequence stars.
V is the
classification given to the main sequence stars.