Stellar Evolution -- Christina Houde

http://www.physics.howard.edu/students/Beth/figure6.gif

http://aspire.cosmic-ray.org/labs/star_life/images/hr_static.jpg

These two diagrams above are called HR diagrams or Hertzsprung–Russell diagrams. This type of diagram is a scatter plot that shows the relationship between the luminosity and the surface temperature of stars.  The HR diagrams show different things in each part of the graph. The x-axis of the diagram shows the temperature of the star in Kelvins (°K). One Kelvin is equal to 274.15 degrees Celsius. The y-axis of the diagram shows absolute magnitude or the luminosity of the star. This is the measured brightness of an object in space as it’s viewed from a distance of 10 parsecs, or 32.6 light-years. The third thing that this diagram shows is a little bit of the relative size of the stars at different points in their life. Here’s a link to and animation that shows the transition between stages and how stars move on the HR.

http://aspire.cosmic-ray.org/labs/star_life/support/HR_animated.swf

http://www.factmonster.com/images/ESCI168NEBULA002.jpg

There are many stages that stars can change and go through, but throughout their life stars go through different sequences depending on what kind of star they are. A low mass star forms in the dense core of a large molecular gas cloud out of relatively cool gas and dust. Low mass stars turn into red giants as time passes and the star glows red and expands as it cools. Then it will change into a planetary nebula which has an outer layer of gas that’s being pushed off and a hot core. Once most of the gas has gone the star is left as a white dwarf. As the white dwarf cools it reddens and stops glowing so a black dwarf is made.  

High mass stars form out of gas and dust when gravitational force overcomes the internal pressure in molecular gas clouds. After expanding, cooling, and turning red this type of star turns into a supergiant. In the next phase of this star’s life, the supergiant explodes forming a supernova. A supernova can form into two things then, depending on what happens to it. If the supernova’s core collapses and becomes very dense it will become a neutron star. However, if the supernova’s core collapses and completely vanishes it will become a black hole.

A white dwarf, like I mentioned when I talked about low mass stars, is a star that is at the end of its life. Compared to other stars, these stars are incredibly dense. Just a teaspoonful of white dwarf matter would weigh as much on Earth as an elephant does—5.5 tons. However these stars typically have a radius just .01 times that of our own sun, but have about the same mass. Unlike our sun that runs on hydrogen fusion, these stars have burned up all of the hydrogen they once used as nuclear fuel.

Simulation:

http://www.passmyexams.co.uk/GCSE/physics/death-of-low-mass-stars.html

This animation shows the formation of a white dwarf star

This diagram shows the formation of a white dwarf in the right hand sequence and a neutron star in the left sequence.

A neutron star, like I mentioned when I talked about high mass stars, is one of the ancient remains of a high mass star. After the supernova exploded, the stars outside layers were forced off into space and the core remains, but doesn’t produce nuclear fusion. In spite of the small diameters of these stars, about 12.5 miles (20km), neutron stars have nearly 1.5 times the mass of our sun, and are thus incredibly dense. A single sugar cube of neutron star matter would weigh about one hundred million tons if it were put on Earth.

The extreme density of this star causes protons and electrons to combine into neutrons, which is what gives these stars their name. Because of this density neutron stars have a super strong gravitational pull, much greater than Earth's. The strength of this gravity is so abnormal because of the stars' small size.

Simulation:

http://www.nasa.gov/mpg/69478main_classic_supernova.mpg

This is an animation of the formation of a pulsar and neutron star.

A pulsar, like the simulation shows, is a rapidly spinning neutron star. Usually neutron stars slow down over time, but stars that are still spinning rapidly may emit radiation. To us on Earth this looks like the star is blinking on and off or pulsing as the star spins like the beam of light from a turning lighthouse. That’s why these still-spinning neutron stars are called pulsars. Over time these pulsars will become normal neutron stars as their spinning slows although few known existing neutron stars are pulsars.

Thanks for reading my blog :)