Stellar Evolution

What is a star? A self-luminous celestial body consisting of a mass of gas held together by its own gravity in which the energy generated by nuclear reactions in the interior is balanced by the outflow of energy to the surface, and the inward-directed gravitational forces are balanced by the outward-directed gas and radiation pressures.

In other words… A star is a fusion reactor driven and contained by gravity.

Stars form from clouds of gas and dust called nebulae

The material in the nebula falls into its gravitational center The material in the nebula falls into its gravitational center. This is the beginning of a protostar. A protostar is a star in the process of forming. Computer animation of gravitational collapse within a nebula

Protostar Formation As the nebula collapses, gravity compresses and increases the temperature in the core. When the temperature of the core reaches about 10 million K, hydrogen fusion begins. This process converts matter into energy and is the reason stars shine.

Hydrogen Fusion Hydrogen fusion is also the source of energy for nuclear bombs.

Once an equilibrium is established between fusion and gravity, a star is born.

The HR Diagram Astronomers use a graph of luminosity vs. temperature to understand stellar evolution

The Main Sequence Main Sequence stars are new stars undergoing hydrogen fusion. The Main Sequence represents the expected relationship between luminosity and temperature: Hot stars are bright, and Cool stars are dim. Which stars do you think live longer?

The fate of the new star is determined by its mass Big stars will be hot and bright Small stars will be cool and dim All Main Sequence stars are fusing hydrogen in their cores. Massive stars will burn through their fuel faster than small stars. Our Sun began with a 10 billion year supply of hydrogen in its core.

The Fate of our Sun The Sun fuses 600 million tons of hydrogen into helium every second It has been doing so for about 5 billion years In a few billion more years, it will run out of fuel The good news is that the Sun is big enough to then switch to helium fusion. The bad news is that when it does this, it will leave the main sequence to become a red giant. This is the end of the road for our planet. 

Bad News for Planet Earth

Red Giant Stage Red Giants are dying stars that have run out of hydrogen in their cores and are fusing heavier elements instead. Red Giants are brighter because of their size. Their red color indicates a cooler temperature. Red Giants are in the upper, right-hand part of the HR diagram

White Dwarf Stage In the end, the Sun will run out of helium in the core and will stop shining with energy from fusion. A white dwarf is the carbon core of a dead star. It will shed its outer envelope and its carbon core will glow brightly with its leftover heat From a distance it will look like a ring of glowing gas (a planetary nebula) with a small, bright center (a white dwarf). Eventually, it will stop shining altogether and become a black dwarf.

Planetary Nebulae

The Fate of Massive Stars Stars much larger than the Sun can continue to fuse small atoms into bigger atoms and continue to shine Unfortunately, this only works up to a point When the core of the star becomes a ball of iron, further fusion reactions do not convert matter into energy anymore This means that the core can no longer push back against gravity When this point is reached, the star suffers a catastrophic collapse and explodes.

Supernovae The explosion is called a supernova There are different types of supernovae, but they all have triggered by a gravitational collapse and a huge release of energy A supernova is so bright that it can outshine an entire galaxy for a few days On average, a typical galaxy will host such an explosion about once per century Because there are billions of galaxies, there is always a supernova going off somewhere in the universe The blast blows away the outer layers of the star and produces a different kind of nebula called a supernova remnant

Supernova Remnants

Neutron Stars A neutron star is the core of a massive star that exploded. It is a ball of neutrons a few kilometers across. The object is like a giant atomic nucleus, and is incredibly dense. A cubic centimeter of this material would weigh millions of tons.

Gravity Wins in the End If the neutron star is bigger than about 1.5 solar masses, gravity will crush the object to an infinitely small size. Nothing can stop this collapse. Nothing, not even light, can go fast enough to escape. A Black Hole is a collapsed star from which nothing can escape.

Black Holes The infinitely small, dead star still has mass and gravity. It is called a “singularity” If you get too close to a black hole, you’ll become part of it. The critical distance is called the Schwarzschild radius, and it defines a spherical boundary called the “event horizon”

Supermassive Black Holes Black holes come in different sizes Those formed from the death of a big star are called stellar mass black holes The centers of galaxies have black holes that have the mass of hundreds of thousand to hundreds of millions of solar masses. These supermassive black holes are what is left of the quasars in the early universe.

The Black Hole at the Center of the Milky Way

Finally… If we put the mass of the universe into the formula for a black hole’s Schwarzschild radius: Rs = GM/c2 , we get a radius close to that of the known universe. This suggests, but does not prove that the Big Bang may have resulted from something like a Black Hole singularity.

The End