29:50 Modern Astronomy
Fall 2002
Lecture 18 ...October 16, 2002
Stellar Life after the Main Sequence
After the Main Sequence
What happens to a star after its time on the main sequence? Answer: it gets big, and
red, and blows up.
Now let's fill in some of the details.
Main sequence lifetimes of stars: Sun 
years. A Main sequence
star with 3 solar masses has a lifetime of 500 million years. A star with 15 solar masses
has a main sequence lifetime of 15 million years. What happens next?
Recall the structure of a Main Sequence star. A MS star is fusing hydrogen in its core to
form helium.
Draw structure of MS star.
In a Main Sequence star, the condition of the surface is a good indicator of
conditions in the deep interior.
At the end of the Main Sequence lifetime, the star has converted all its core hydrogen to helium, and has ``run out of gas''. It has lost its source of internal pressure and the interior begins to contract.
After a while, the density and temperature in a shell surrounding the inert core
are high enough for the proton-proton cycle to begin.
Diagram of interior structure of Giant star.
For reasons that are far from intuitively obvious, a star responds to this new
structure by expanding and having its surface temperature drop. We observe the star
become much more luminous than the Sun, and cooler. It has become a red giant, and we
refer to this process as ``ascending the giant branch''.
Evolution of red giant on HR diagram.
Meanwhile, back at the ranch... during this time the core has continued to contract, gain mass, and become hotter. When it has contracted to a couple of tens of thousands of kilometers in size, the temperatures are high enough for a new fusion reaction to occur, in which Helium fuses to form Carbon and Oxygen. This is called the Triple Alpha Process:
Calculations show that in a star like the Sun, this occur in a a pop lasting probably of the orders of minutes. It is referred to as the Helium Flash.
Following the Helium Flash, the interior structure of the star resembles a Viennese pastry, beginning with the inert Carbon and Oxygen core, going past a Triple Alpha shell, an inert Helium shell, a proton-proton shell, and finally the non-fusing envelope.
At this time, the star develops a powerful wind, in which it throws off its outer layers.
A typical mass loss rate is 
per year. Many of these stars
also become variable, changing in brightness by several magnitudes on time scales of
hundreds of days. The archetype of this type of star is Omicron Ceti, or Mira. The stars
become extremely luminous, up to several thousand times the brightness of the Sun.
The technical term for these is asymptotic giant branch stars.
These stars lose so much mass that they eventually uncover the very hot core of the star.
The temperature of the core is 50,000 to 100,000K, so it emits much ultraviolet light,
which then photoionizes the wind. This produces a common and beautiful type of
astronomical object called a planetary nebula.
Image of Ring Nebula
Image of Planetary Nebula
Young Planetary Nebula
At the end of this process, the Carbon and Oxygen core is left isolated in outer space.
It sits there and cools off over a period of billions and billions of years. Although it
contains the mass of the original Main Sequence core of the star, it is only the size of
the Earth.
Transparency with white dwarf.
White Dwarfs in M4
Hundreds of white dwarf stars are cataloged. The bright stars Sirius and Procyon have
white dwarf companions.
The validity of this picture of stellar evolution is born out by the fact that
we observe all of the phases of stellar evolution described here.
Link to image of red supergiant.
