### Section 2.3 - Stellar Evolution

#### The Lives and Deaths of Stars

Stars are born out of huge clouds of gas and dust called nebulae. Over time, clumps of this gas will attract due to gravity, collapsing inward and causing temperature and pressure to rise immensely. At around 10 million degrees Celsius, the temperature is high enough that hydrogen atoms begin to fuse, giving birth to a star. The surrounding spinning disk of material, known as an accretion disk, may give rise to planets and other bodies, similar to all the objects that fill up our solar system.

 A newborn star surrounded by an accretion disk

Once a star has achieved equilibrium and has started fusing hydrogen into helium, it can be plotted on the Hertzsprung-Russell Diagram

#### The Hertzsprung-Russell Diagram

The HR Diagram is a scatterplot of stars relating their luminosities with their surface temperatures. The above diagram also includes stellar masses and approximate lifetimes (larger stars die more quickly much more quickly)

Notice the band of stars going from top-left to bottom-right. These stars are called main sequence stars - they are all mature, middle-aged stars in the hydrogen burning stage of their lifetime. Can you find the Sun? Once you do, you'll see we orbit a very unremarkable star.

You'll also notice that main sequence stars are classified by spectral class: O, B, A, F, G, K, M (hottest to coolest). My astronomy professor at UT Austin gave us this acronym to remember: "Oh, Be A Fine Girl, Kiss Me"

Luminosity and temperature are mathematically related by the Stefan-Boltzmann Law:

$L = 4\pi R^2\sigma T^4$

L = luminosity
T = surface temperature

The stars in the top right are the red giants. They are cool, but they are still very luminous because they're so huge. The stars on the bottom left are the white dwarves. They still glow hot, but they're not very luminous because they're so tiny. More on these below.

#### Stellar Death

Once a star runs out of hydrogen to fuse, they follow one of three paths at the end of their lives, depending on their mass

• Outcome 1: White Dwarf (<8x the mass of our Sun)

Once a star runs out of hydrogen, there's no external pressure to counteract gravity, so the star briefly contracts inward. This contraction produces enough pressure to start fusing helium into carbon in what's known as the triple alpha process (more on that in the next section). The star then expands and cools, turning it red and huge (hence the term red giant).

 When the Sun becomes a red giant, it will engulf Mercury, Venus, and possibly Earth

But once even all the helium is exhausted, there's nothing left to power the star, so the outer layers of the star puff off into space, leaving the small dead core of the star, a white dwarf

 Some examples of white dwarves, to scale

• Outcome 2: Neutron Star (8-20x the mass of our Sun)

Supermassive stars will also go through the helium burning red giant phase after running out of oxygen, but they won't stop with the triple alpha process that makes carbon out of helium. There's enough mass (i.e. pressure and heat) to keep fusing the carbon into heavier and heavier elements (discussed in greater detail in the next section)

 I like to think of stars in this stage of their lives as giant flaming nuclear onions

Still, once all the fuel is used up, the external pressure will cease and the star will rapidly collapse inward, hitting the core and then violently rebounding outwards into space. This sudden and violent explosion is what's known as a supernova. Supernovae are incredible astronomical phenomena that astronomers are always on the lookout for because of their spectacular energy output
 This supernova was so powerful, producing a billion trillion times more energy than the largest man-made nuclear explosion, that it stood out from the rest of its galaxy

The force of gravity and all the stellar material rushing into the core is so great that the protons and electrons are compressed into an unbelievably dense ball of neutrons, creating a neutron star. The remaining neutron star is so dense, one teaspoon of it would weigh 900x the Great Pyramid of Giza

• Outcome 3: Black Hole (>20x the mass of our Sun)

This path follows all the same steps as the neutron star, except that after the supernova, the stellar core is compressed even dense to the point where the density is so high and the gravitational pull so intense that nothing can escape, not even light, hence the term black hole (although it term black hole is a bit misleading, it's really a sphere)

This was touched on briefly in the section about the Theory of Everything, but the study of black holes is exceptionally important because its nature as a point-mass of infinite density is analogous to the conditions of the Universe before the Big Bang, so understanding black holes will help us understand the creation of the Universe and the fundamental nature of existence

 Gargantua, from the movie Interstellar