Stellar evolution

We can now explain the HR diagram.
The key property of a star is its mass M.
The mass will determine its radius, temperature, lifetime and fate.

Star formation

Many gas clouds can be found in the galaxy.
They are typically 75% hydrogen H and 23% helium He, by mass.
An event can trigger the gas cloud to start collapsing.
Gravity of the centre of the cloud draws in more mass from the outskirts.
The centre of the cloud becomes hotter and denser.

The Great Orion Nebula

Source: ESA/NASA Hubble Public Domain

Proplyds - Star formation in the Orion Nebula

Source: ESA/NASA Hubble Public Domain

Nuclear fusion

If the mass of the collapsing cloud is above about 0.1 M_☉ the density and temperature will get high enough for nuclear fusion reactions to start.
Nuclear fusion releases energy which raises the temperature still further.
The temperature reached depends on the mass.
The core temperature of the Sun is approx. 15 million K

The p-p chain

Source: Borb CC-BY SA 3.0

Nucleosynthesis

The nuclear fusion releases energy as gamma rays; these heat the core of the star.
Nuclear fusion fuses hydrogen nuclei to form helium nuclei, hence the name.
For each He nucleus, four H nuclei will be used.
The creation of a new element (or nucleus) is called nucleosynthesis.
But, over time, the hydrogen gets used up.

The Pleiades

Source: NASA, ESA, AURA/Caltech, Palomar Observatory Public Domain

Pressure vs gravity

As the temperature rises, so does the pressure.
Eventually, the pressure becomes sufficient to balance gravity; the collapse stops.
The star is now stable.
It is now a Main Sequence star.

Mass is the key

A star with larger mass will have:

a larger radius
a higher core and surface temperature
and so a large luminosity

Properties by mass

Mass	Class	Surface temperature	Radius	Luminosity
≥ 16 M☉	O	≥ 33,000 K	≥ 6.6 R☉	≥ 30,000 L☉
2.1–16 M☉	B	10,000–33,000 K	1.8–6.6 R☉	25–30,000 L☉
1.4–2.1 M☉	A	7,500–10,000 K	1.4–1.8 R☉	5–25 L☉
1.04–1.4 M☉	F	6,000–7,500 K	1.15–1.4 R☉	1.5–5 L☉
0.8–1.04 M☉	G	5,200–6,000 K	0.96–1.15 R☉	0.6–1.5 L☉
0.45–0.8 M☉	K	3,700–5,200 K	0.7–0.96 R	0.08–0.6 L☉
≤ 0.45 M☉	M	2,000–3,700 K	≤ 0.7 R☉	≤0.08 L☉

Larger luminosity, shorter lifetimes

A 2 M☉ has a luminosity about 10 L☉
It has twice the amount of hydrogen (its fuel).
But it uses it up 10 times faster because luminosity is 10 times larger.
So a two solar mass star has a much shorter lifetime than the Sun:
- Sun - 10 billion years
- 2 M☉ - 2 billion years
- 8 M☉ - 80 million years

Running out of fuel

If the core runs low on hydrogen, nuclear reactions slow.
Core temperature and pressure drop, gravity wins and collapse can occur.
Stars undergo internal re-arrangements and become unstable.
The details of what happens next depends on mass.

Low mass stars

Less than two solar masses (like our Sun):

Fusion reactions may occur outside the core.
This can push outer layers out, creating a red giant.
Shells of gas may be thrown out into space - planetary nebulas.
A tiny white dwarf star will form - up to 1.4 M☉ but a few thousand km across (size of the Earth).
Density is about 10⁶g cm^-3 so a sugar cube lump has a mass of a metric tonne.
The white dwarf is stable, but cools forever.

Red giant to scale

Source: Oona Räisänen CC-BY SA 3.0

The ring nebula - a planetary nebula

Source: NASA/ESA/Hubble Public Domain

Sirius B - a white dwarf

Sirius A is the brightest star in our night sky, but it has a binary companion, Sirius B, which is a white dwarf. Sirius B is the dot at the bottom left.

Source: NASA/ESA/Hubble/M. Barstow CC-BY SA 3.0

High mass stars

More than two solar masses:

Other fusion reactions can start in and outside the core.
Heavier elements can be created, up to iron.
But, after iron, the star suddenly has no fuel source.
Catastrophic collapse occurs - the star implodes.
The imploding material rebounds off the dense core and out into space.

Supernova

An incredible release of energy - can be billions of solar luminosities.
One star can outshine its galaxy!
But, it fades within days.
Rare nuclear reactions occur, creating elements heavier than iron.

Supernova 1987A

Source: Copyright - Anglo-Australian Telescope

Supernova 1987A- aftermath

Source: Mark McDonald CC-BY SA 3.0

1054 supernova

In 1054 a supernova was seen, and recorded by Chinese astronomers.
It was bright enough to see during the day at its peak.
This supernova was in our own galaxy (1987A was near but not in our galaxy).
We can still see the explosion today, as the Crab nebula in the constellation of Taurus.

Crab nebula