Red Supergiants

A star of 15 solar masses exhausts its hydrogen in about one-thousandth the lifetime of our sun. It proceeds through the red giant phase, but when it reaches the triple-alpha process of nuclear fusion, it continues to burn for a time and expands to an even larger volume. The much brighter, but still reddened star is called a red supergiant. Betelgeuse, at the shoulder of Orion, is the best-known example. Absolute luminosities may reach -10 magnitude compared to +5 for our sun.

Some of these supergiants are unstable and form the very important Cepheid variables. In their final stages, supergiants may explode into supernovae. The collapse of these massive stars may produce a neutron star or a black hole.

Betelgeuse

If it were positioned at the center of our sun, its radius would extend out past the radius of Mars, approaching 2 astronomical units! Most stars on the main sequence have about the same size as the sun. It has a surface temperature of about 3000 K as determined from its blackbody radiation curve. Betelgeuse is about 310 light years away from the Earth. Its celestial coordinates are RA=5 h 52 m , dec=7° 24' .

Arcturus

Arcturus is a red giant star in the Northern Hemisphere of Earth's sky. It is the brightest star in the constellation of Bootes, the fourth-brightest in the night sky, and the brightest in the northern celestial hemisphere. Together with Spica and Regulus, Arcturus is part of the Spring Triangle and, by extension, also of the Great Diamond along with the star Cor Caroli. It is positioned almost at the north galactic pole of the Milky Way.

Relatively close at 36.7 light-years from the Sun, Arcturus is a red giant of spectral type K0III - an aging star around 7.1 billion years old that has used up its core hydrogen and moved off the main sequence. It is 1.08±0.06 times as massive as the Sun, but has expanded to 25.4±0.2 times its diameter and is around 170 times as luminous.