Asymptotic Giant Branch Stars

This is a shortened version of material found elsehwere in this course under the heading of Cool Red Giants. I have placed this information here because I refer to it so frequently in the discussion of White Dwarf stars, binary systems, and cataclysmic variables. So much information in addition to this page and those on the previously mentioned topics can be found at the AAVSO webpage.

In the fall of 2004, as I was teaching a unit on White Dwarf stars, things that I had been learning finally became connected in a way that acutally caused me to lose sleep over the excitement. I know this sounds pretty weird, but it was really cool to me and this page is an outgrowth of what finally makes sense.

Uncle Bill and Janet Mattei

My Uncle Bill is pictured above with Janet Mattei, the former director of the AAVSO. Janet passed away in March, 2004 and her long and illustrious career came to a sad close. Click on the picture to learn more about her.

Meanwhile, Bill Albrecht has studied variable stars for over 40 years. During that time he has personally made over 90,000 individual observations of stars in varying levels of brightness. At times I have even wondered if he fully understands how important this work is for he seems more interested in the joy of making the observation than in the science behind the stars that are changing so. As I attended the semi-annual meeting of the AAVSO in the summer of 2002 as well as participating in the High Energy Astrophysics Symposium, I dutifully sat and wrote copious notes on a subject that seemed beyond the realm of my comprehension. Bill Albrecht would go outside, look through his telescope for a particular star, and then measure its apparent magnitude relative to stars in the same field of view whose apparent magnitudes were constant and known. He would note the date and time for the star and pencil his numbers in a notebook. These observations were then sent off to the AAVSO where Janet Mattei would compile the numbers from Bill and hundreds of other observers worldwide. What is created from all of these observations is a light curve like you see below. This chart is the light curve for a star called SS Cygni, and the thousands of individual points are from observations collected at the office of Janet Mattei.

Then came that October night when everything suddenly clicked.

AGB Stars and Binary Systems

Stars whose mass is 1-8 solar masses all appear to follow a similar pattern of death. After the main sequence star has exhausted is core supply of hydrogen fuel, having converted it into helium, there is no longer a source of fuel for nuclear fusion. With a sudden loss of outward pressure, the core collapses and shrinks to very high density. This increase in internal pressure raises the energy levels of the core to a point where helium can be fused into carbon. A new source of nuclear fusion results in extreme outward pressure that pushes the outer envelope of the star to great size. Adding to this outward pressure in a second shell of hydrogen gas fusing into helium just outside the core. The same amount of stellar material pushed out to an increased volume causes the outer photosphere of the star to experience a great drop in temperature and the star becomes a Red Giant.

After the core supply of helium is exhausted, there is another shut down of fusion and gravity collapses the core further. In stars like our own Sun (those with a mass between 1 and 8 solar masses) the collapse will raise the core temperatures higher, but not enough to sustain carbon fusion. The core is essentially a dead object, unable to ignite fusion. With no outward pressure, gravity begins to take over in the star and shrink it. Here is where it gets a bit confusing. It is believed (in a grossly simplified manner) that helium and hydrogen in the outer layers of the expanded Red Giant star will fall in toward the tiny core and experience sufficiently high temperatures to ignite nuclear fusion (He becomes C, and H becomes He). The collapsing Red Giant suddenly gets pushed back out. This outward pressure is so sudden that the outer atmosphere of the star gets pushed out into space, creating a shell of swelling gas. A planetary nebula is formed! Below is such a planetary nebula, the Helix Nebula. To the right is a close-up showing the outward pressure of the expanding gas on globules of stellar material that used to be part of the original star.

Below are a set of four images of the star V838Monocerotis taken by the Hubble Space Telescope on successive dates of May 5, September 2, October 28, and December 17, 2002. You can see the changes in this system that holds an AGB star in the center that is blowing gas away from itself.

Click on the last image to see a big photo of this beautiful star and its gas shell.

Inside the planetary nebula, the core fuses the infalling gas and fusion stops again. Gravity takes over again, but newly infalling matter reignites and pushes the star outward again, and more material escapes into space. This pulsating is seen on the HR Diagram as a back-and-forth movement between areas of the M or K spectral class without really brightening too much. The star has reached the asymptotic limit of its upward crawl on the HR Diagram, as it an Asymptotic Giant star. Stars in this area of the HR Diagram are collectively called Asymptotic Giant Branch (AGB) stars.

In piston-like action, the planetary nebula expands as more and more mass is blown away from the pulsating star hiding in the gas. Eventually, all that remains is the White Dwarf core. Members of the AAVSO study the brightening and dimming of some of these AGB stars, adding to our understanding of how they die.

AGB's in Binary Systems

What is far more interesing is what happens in a binary star system when 2 stars are orbiting each other and one of them is in the AGB stage of its death while the other is a White Dwarf. This is the stuff that is really interesting. To learn more about these systems, please move to Cataclysmic Variables.

You could also go back to the Hottest Stars, or back to Main Sequence Dwarfs, or the Introduction to Stars, or the Syllabus.

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