The Lives of the Stars:
Biography of a Star

A Star is Born...

Stars form in Giant Molecular Clouds, huge "stellar nurseries" of hydrogen and helium (with a few other chemicals mixed in).

Some of the gas starts to contract due to gravity...  
it gets smaller... (and hotter!)
and smaller... (and even hotter!)
and smaller still... (and still even hotter!)

What can stop the collapse?? ...

... Nuclear Fusion!!

Eventually the gas shrinks enough that its temperature and density become high enough, that a nuclear fusion reaction starts in its core! -- It becomes a giant Hydrogen Bomb!

The energy from fusion produces enough pressure outward to balance the inward force of gravity -- a Main Sequence Star is born!
Energy released from fusion of hydrogen into helium in the star's core balances the crushing force of gravity:
A star spends most of its life burning hydrogen into helium on the Main Sequence.

The Main Sequence

So how does a star decide where it is going to be on the Main Sequence?
---> its Mass!

High Mass Stars: they need to burn fuel (hydrogen) very fast in order to produce enough energy to keep from collapsing under gravity. So...

High Mass Stars are HOT
and have LARGE Luminosities

Low Mass Stars: they can burn their fuel very slowly because their gravitational force is "small". So...
Low Mass Stars are Cold
and have small Luminosities

Mid-life Crisis! -- Giants

Stars do not stay on the Main Sequence forever - eventually they run out of hydrogen fuel. When this happens they go through a "Mid-life Crisis"!

They start to burn hydrogen in a shell around the helium core...

In response to this change the outer parts of the star "puff out", its surface temperature drops, but its luminosity increases

---> it's on its way to become a Giant!

Stellar Old Age
and Horizontal Branches...

A star burns hydrogen in a shell for a while as a Giant, but then it goes through another change: it starts to burn helium to carbon (and oxygen) in its core.

When it does that, its temperature increases, but its luminosity stays about the same...

---> it becomes a Horizontal Branch Star!

Alas, this does not last too long. The star runs out of helium in relatively short order. Once again it becomes a Giant (or it could even become a Supergiant!)

Death Throws...
(Eventually the star completely runs out of fuel...)

The most massive stars can find additional sources of fuel from carbon, oxygen, nitrogen, silicon, ... but once they reach iron, they've reached the end of the line -- a star cannot use anything beyond iron for fuel.

Now...       THE STAR MUST DIE!!!

Massive Stars: explode in a violent Supernova!

...they leave behind their core which becomes a dense Neutron Star (or maybe even a Black Hole in a few cases) which is surrounded by the glowing remnants of the outer parts of the star (a
Supernova Remnant).

Low Mass Stars: they die less violently by simply blowing their outer parts off and leaving behind the hot core which becomes a White Dwarf.

...the outer parts that were blow off glow from the heat of the White Dwarf and become what is known as a Planetary Nebula (even though it actually has nothing to do with "planets").


White Dwarfs: the former cores of Low Mass Stars are destined to radiate all of their heat away and eventually become dark.

Neutron Stars: the former cores of High Mass Stars are also destined to radiate their heat away and eventually become cold. However, some neutron stars spin very fast (one revolution ranges from a few mili-seconds to a few tens of seconds). If they do then they "flash" signals (in radio and x-ray wavelengths) at us once per-revolution and are called pulsars. However, over time they will slow down, eventually stop spinning, and become cold.

Black Holes: not much to say, they are so dense, and their gravity is so strong at close range, that anything that gets too close can fall in and will never be heard from again (even light cannot escape a black hole!).


pretty sad ending huh? Well... maybe not... the material blown off by the dieing stars is rich with heavy elements (i.e., carbon, oxygen, nitrogen, iron, copper, gold, lead, uranium, etc...), it eventually gets recycled back into
Giant Molecular Clouds where it is used to form new stars, and possibly even planets! So, the cycle starts again...