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!)|
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:|
|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...
Stars do not stay on the Main Sequence forever - eventually they run
out of hydrogen fuel. When this happens they go through a
They start to burn hydrogen in a shell around the helium core...
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.
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.
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!).