Week 4: Astrophysics in a Nusthell Chapter 2.3

Chapter 2.3 of Astrophysics in a Nutshell details the Hertzsprung-Russell diagram, which is used to classify and understand stars. The H-R diagram is a plot of luminosity vs. surface temperature:

Annoyingly, temperature is conventionally shown to decrease to the right.

Stars can be classified into three main groups: the main sequence, red giants, and white dwarfs. The main sequence, which is the diagonal line in the middle of the diagram, is where most stars spend the majority of their time. Our Sun lies on the main sequence, along with stars with radii anywhere from 1/4 to 25 that of the Sun. Initially it was believed that stars moved along the main sequence from hot to cool as they aged, but we've learned that that's not the case. Instead, old stars tend to become red giants.

A rare photo of a cluster of 7 red giants.

Red giants are larger, redder, and more luminous than stars on the main sequence, so they form their own cluster above the main sequence line. They continue to die, shrinking in size and increasing in temperature as they lose their outer layers, finally becoming white dwarf stars. Since white dwarfs are less luminous than the main sequence stars, they lie below the main sequence on the H-R diagram. Eventually, these stars will radiate away all of their energy and become a theorized type of star called a black dwarf. 

If the dying star is really massive, though, it might explode as it collapses under its own gravity. The resulting supernova will create either a black hole or a neutron star--extremely dense stars with very little luminosity. Neutron stars can pack 1-3 times the mass of the Sun into a sphere of radius 7 miles. That means that over 17,000 neutron stars could fit between the Earth and the Moon, or that I could drive past two neutron stars going from my home to my high school. It probably wouldn't be great for the commute, but I guess it'd be better than having red giants (or slow drivers) in the way. 

Sources

Astrophysics in a Nutshell, Dan Moaz

https://upload.wikimedia.org/wikipedia/commons/1/17/Hertzsprung-Russel_StarData.png

http://cdn11.ne.be/movies/10527/31736.jpg

https://en.wikipedia.org/wiki/Black_dwarf

https://en.wikipedia.org/wiki/Neutron_star