Stellar Evolution by RockonBen (Ben Shapiro)

An HR Diagram stands for Hertzsprung-Russell Diagram. It shows the luminosity of stars compared to their temperature. The sun, our own star, is at 1 luminosity. As you go left or right along the scatterplot, you are determining the temperature. If you go up or down, you are determining the luminosity. The stars are also color-coded based on the temperature of it – as you can see, if it is lighter, it is hotter, if it is darker, it is cooler. This also surprised me because regular small-size stars, like our sun, range from a large variety of brightness and temperatures. The white dwarfs also are darker, but the temperature is usually quite high. Finally, giants and supergiants are larger and emit more light (obviously), but they are at the low-end of the temperature scale. 

 
 

All stars begin their lives in a stellar cloud with protostars, as seen above. It either becomes a small star or gains enough mass to become a large mass star. I’ll start with a small star. A whopping 90% of the activity going on inside the core is fusion of elements, because gravity squeezes the core so intensely. Due to this, energy from fusion balances out the squeeze of gravity. Fusion with hydrogen turns to helium (combine H (1 proton, 1 neutron) plus another one gives you He (2 protons, 2 neutrons)). This sinks to the center to form a helium inner core. Then at 100,000,000K, helium fusion starts, with 3 helium combining to make 1 carbon. This is now a red giant. Now there is an outer layer of hydrogen fusion, an inner layer of helium fusion, and finally an inner core of solid carbon. But now, the temperature cannot get high enough for carbon fusion which causes the core to compact. However, electrons prevent further compaction of the core. The outer layers slowly puff away, and as it does, the inner parts of it leave behind a planetary nebula, and eventually it becomes a white dwarf. This leads me into my next section.

A white dwarf is the last stage of a low-mass star’s life. These are very dense, but not as dense as a Neutron star. They are about as dense as the sun, and as big as the Earth. They are composed of electron-degenerate matter, which means it burns off the matter as time goes on. Over a very long time, it will grow darker and darker until it is no longer visible, or a black dwarf.

Large mass stars work in a completely different function, such as red giants. H fuel in the star becomes low very quickly, making the star red and swollen, hence the word “red” in “red giant”. He fusion begins extremely quickly and builds up a layer of solid carbon. The outer layers expand like in a low-mass star, and compress the core until electrons try to resist. However, this is where large-mass stars stand out. It bypasses the resistance of the electrons, compressing the core even further. Finally, it compresses the core so hard that carbon fusion starts which forms oxygen. Then suddenly, a chain reaction fusion process uses up the previous fuel incredibly quickly to make multiple different layers. (To be specific, O to Ne to Mg to Si to Fe, and Fe (iron) is the last product made. Having this be the last product made, gravity squeezes the core. Instantaneously, in a fraction of a second, this core (about the size of Earth) is reduced to the size of Brooklyn, NY. The weak force instantly turns protons into neutrons (hence the term neutron star) and releases neutrinos. Then, in the next thousandth of a second, the neutrons crystallize, the core explodes at 20% the speed of light and slams into the outer layers, creating a supernova that creates a massive shockwave which rips the outer layers to shreds. In the explosion, much heavier elements such as silver, gold even uranium are made by the fusion. The cloud from the explosion does not dissipate for thousands of years, but when it does, the new neutron star is revealed.

A Neutron Star might be one of the strangest stars in the universe. It forms from a supernova of a star that has a solar mass of 1.5-3. (In other words, it doesn’t form from the small mass stars but not from the biggest high-mass stars.) As it collapses, it gains so much mass because the supernova turns the whole star into one incredibly dense ball. Its mass is typically 500,000 times more than the mass of the Earth, with the diameter only the length of my dad’s birth town – Brooklyn, NY. These also have insanely powerful magnetic fields – charged particles can spiral down to the surface through it. They spin rapidly, up to 1000 rotations a second.

 

A Pulsar is a neutron star that happens to emit radio waves that cross Earth’s path. This name really comes from the word “pulse” because it emits electromagnetic radiation in short bursts. In order to see it from Earth, the radio waves would have to be pointing at Earth. This is like a lighthouse at night – you can only see it when the light is facing towards you. The link below is a movie about what this is.

http://www.youtube.com/watch?v=G08RsjkR_28

Thanks for reading my post - RockonBen