Modern Research 2006
Hello, and welcome to the course website of Modern Research of 2006. This is an astronomy course at the University of Leiden and is given by Dr. Bernhard Brandl. There are six students who have followed this course.
In the beginning of the course, students have discussed a list of current interesting topics. They have chosen one of these as the main topic of the course. This year the main topic is "Stellar Evolution".
The goals of this course are learning how to:
For people who did not follow the course, it may be a bit difficult to understand much about the subject, because the PowerPoint slides are only to aid the speaker. Therefore, you can only fully understand the subject if you have also heard the spoken part.
Early stages of Evolution
Every star begins as a molecular cloud. Due to some distortion, like exceeding the Jeans mass or a supernova explosion occurred nearby, the cloud begins to collapse under its own gravitation. All the matter in the cloud starts swirling inward and after a some time, a protostar, such as a T-Tauri star or Herbig Ae/Be star, will form in the center. Some spectacular things can occur in these stages, such as Herbig Haro objects.
Because of the angular momentum of the infalling matter, a nebula-like circumstellar disk will form around the protostar. As time progresses the protostar will move toward the main-sequence line on the HR- diagram or, when it doesn't have enough mass, it will become a brown dwarf.
This pre-main sequence stage is much more interesting than the main sequence, because there is no equilibrium between two processes that keeps the star "stable" but one process rules. This makes it an interesting phase to dig deeper into.
Late stages of Evolution
At some point a star will move from the main sequence line, because the processes keeping the star in balance are distorted. This could be because the star has burned up all its hydrogen in its core. The star then moves up the Red Giant branch.
Because the inner processes are no longer in equilibrium, the star could have variations in magnitude such as the luminous blue variable stars, or variations in radius such as the Cepheids.
A strong stellar wind will create a substantial mass loss. If it is a low mass star, it will throw off its outer envelope which you can see as beautiful planetary nebulae.
But if the original star had enough mass, it could blow away its entire envelope of hydrogen, becoming a Wolf-Rayet star and burning helium and eventually heavier elements in its core.
If these massive stars eventually have an iron core, there will be no more source of energy through fusion. The core collapses and the star will go supernova. It will outshine its entire galaxy for a few days.
If this core again is heavy enough, it could immediately create a black hole which could create a gamma-ray burst, which is even brighter than a supernova.
There are several ways a main sequence star could end; a white dwarf, a neutron star or a black hole. All the processes leading to one of these are very interesting and many are still open for debate. Therefore, the post-main-sequence stage of a star is also a good topic to learn more about.