Star death

• We went through the description (according to stellar models) of the evolution of a core collapse supernova.
• We started to study the discovery of Supernova 1987A. We need to complete this.
• We can then turn to neutron stars . These are often seen as "pulsars." These objects emit pulses of radio waves (and sometimes visible light). We will spend ten minutes on an abbreviated description of how this could happen. The most important thing is to see that there is evidence that supernovae leave behind neutron stars.

The end stages of stellar evolution are discussed in CM Sec. 20.3 (white dwarfs); 22.1 and 22.2 (neutron stars); and 22.5 and 22.8 (black holes).

I hope that we have time to go over some of the main things that we have learned.

First, a main goal of the class has been to improve everyone's critical thinking skills. The "critical" part is demanding that ideas (theories) be supported by evidence and recognizing what evidence supports a theory or falsifies it. The "thinking" part is using available information plus logic to reach a conclusion, often with the help of a calculation. This is not so easy, but over the course of a four year university education, it is possible to get a lot better at it.

We have learned a lot about stars. Looking at the night sky, everything seems unchanging and peaceful. But it is not so. The time scale for changes is mostly longer than a human lifetime, but every star has a history. Furthermore, sometimes big things happen in just one second.

• Stars are born in clouds of gas, pulled together by gravity.
• We can tie things together a bit now that we have studied the whole lifetime of a star: sometimes the initial push to make a gas cloud collapse can come from a nearby supernova explosion. Furthermore, the heavy elements that are available to make nice comfy planets come from supernovae. (Some of the not so heavy elements can also come from planetary nebulae made by lighter stars.)
• Nuclear fusion provides the energy source of stars. The predominant reaction is 4 H -> He.
• Stars are stable as long as they are burning hydrogen to make helium at the center of the star. (Then they are on the "main sequence.")
• For light stars, this lasts many billions of years, while the lifetime of heavy stars is much less.
• The universe is "only" about 13 billion years old (take ASTR 123 to see the evidence for this), so the lightest stars have remained in the main sequence stage.
• A not too heavy star eventually becomes unstable and its outer parts get blown into space. The core becomes a white dwarf. This is one of the possible stable endpoints for a star.
• A big star eventually forms an iron core that, if it grows big enough, collapses catastrophically to make a "core collapse supernova".
• The remaining core becomes a neutron star. This is one of the possible stable endpoints for a star.
• Possibly, the core could become a black hole. This is the third possible stable endpoint for a star. It is not so clear how often this happens.

1. Most of the matter in the universe is "dark matter" that creates a gravitational force but is otherwise invisible. We don't know what the dark matter is.
2. There also seems to be "dark energy" throughout the universe. The dark energy is causing the expansion of the universe to accelerate. We don't understand the nature of the dark energy.

ASTR 122 course home page

Updated 19 November 2007