On
the diagram, SG = Super Giant; RG = Red Giant, MS = Main Sequence, and WD =
White Dwarf. Our sun is a main sequence star and its location is indicated
by the S.
When we examine stars we see an enormous range of stellar brightness and colors from red to blue. A nice example of stellar colors is found in the bright stars of Orion. Rigel and Betelgeuse are a pretty blue and red respectively.
Understanding how stars evolve will help us to understand this diversity. The (intrinsic) brightness of a star is given by the equation:
L = AT4
L stands for the brightness or 'luminosity' of the star, A is its surface area, and T is its surface temperature. In order to be highly luminous, a star must have a large area, A, or high temperature, T, (or both). Let's examine the kinds of stars we find in space. We start with an HR Diagram on which we plot the (intrinsic) brightness of a star on the vertical axis and surface color on the horizontal axis.
On
the diagram, SG = Super Giant; RG = Red Giant, MS = Main Sequence, and WD =
White Dwarf. Our sun is a main sequence star and its location is indicated
by the S.
Bright stars are at the top and faint at the bottom. Hot blue stars are to the left and cool red stars to the right. The colored circles indicate the relative sizes of the stars in different parts of the diagram.
Note that there are two ways for a star to be enormously bright (at the top of the diagram). In can be hot (blue/white) and pretty big or it can be cool (red) and enormously big (the red super giants at the top right of the diagram). Similarly there are two ways for a star to be faint. It can be small and red or very small and blue/white.
So we have two questions: 'Where do these different kinds of stars come from?' and 'If this diagram shows stars of different ages (it does) what will our sun look like in the future?'
So let's look at the life cycle of stars like the sun (mass = 40% of sun up to 8x sun).
All stars are born in nebula from the accretion of material in interstellar clouds of gas. We often mark the birth of a star from the time that hydrogen fusion begins in the core. This generates energy in stars like our sun by the process:
4 H > 1 He + Energy.
Stars 'burning' (thermonuclear fusion) hydrogen in their core are 'adult stars' or in astronomy speak, Main Sequence stars. Our sun will last about ten billion years on the main sequence before it consumes its core hydrogen. When exhausted, our sun will turn into a red giant. Red giants are much larger in size than the sun, but cooler, giving them a red color. Because of their enormous size, they are much brighter than the sun. In over simplified terms, the red giant phase of a star's life is that phase during which it uses Helium as a thermonuclear fuel. This process is
3 He > C + Energy.
Examples of well know red giant stars are Aldebaran in Taurus the Bull and Arcturus in Bootes.
After a few billion years as a red giant, our sun will consume its available helium and die, throw off its outer atmosphere producing a planetary nebula, and turn into a white dwarf. White dwarfs are very small, about the size of the earth, and very hot. Because of their small size they are very faint. For instance, see this picture of Sirius and its white dwarf companion.
The life cycle of the sun is indicated by the blue line. Our sun is currently on the Main Sequence at the position marked by the S.
Massive stars (mass = 8 to 25 x sun's mass) follow a different life cycle after they leave the main sequence. These massive stars leave the main sequence when they exhaust their core hydrogen and move over to the super giant region of the diagram. During this stage they burn helium and heavier elements all the way up to but not including iron, e.g. they burn He, C, Mg, Si, and Ne. Betelgeuse is a fine example of a red super giant. Eventually the star develops a large iron core which, because no fusion is occurring, collapses suddenly and catastrophically for the star. The resulting shock waves blow off the outer layers of the star producing what we call a supernova. Supernova shine with a brilliance of one billion suns, but can only sustain this brightness for weeks or months. The energy to power the supernova come from gravitational energy released in the collapse of the iron core of the star. The most famous recent super nova is SN1987A (which was not visible to northern observers in the US). The gas thrown into space by a supernova produces a supernova remnant. The most famous is the Crab Nebula in Taurus.
The collapsed iron core which starts the explosion may become either a neutron star/pulsar or a stellar black hole. The diagram below depicts the life cycle of a massive star, ending in a supernova explosion.
fwk iupui 2008
