a short summary of stellar evolution

out in interstellar space, some clouds of gas and dust
begin to contract under their own gravity...

the collapse may be triggered either becaue they cool off
(by radiating molecular rotational energy)
or become quickly denser when compressed by a shock wave
(perhaps due to the impact of supernova remnant)

       and 
giant molecular clouds  and  Bok globules

the collapse eventually forms a flattened, rapidly rotating disk...
the infall of matter releases angular momentum and energy in jets along the rotation axis of the disk

   and    and 

formation of jets and disks
 

eventually the jets disappear, leaving behind only disks surrounding a central protostar

  and 
an edge-on disk (hiding the protostar) and a face-on disk (with central protostar)

the star evolves towards life on the main sequence and the disk may form planets

like this disk surrounding the star b Pictoris

more than 90 sun-like stars have been found with at least one planet

 a star spends the large majority of its life in the main sequence phase,
converting hydrogen to helium in its hot and dense core

the sun is an example of typical main sequence star

  it looks like bigsunspot.jpg (20228 bytes)  or diagrammatically like sunlayers.gif (81684 bytes)

  when the core runs out of hydrogen, the star leaves the main sequence and becomes either


 
a giant star
 or a supergiant star like Betelgeuse
                                                                           betelgeuse.jpg (14876 bytes)
stars that have
initial stellar mass < 8 solar masses

take this path
stars that have
initial stellar mass > 8 solar masses

take this path
wpe1.jpg (978 bytes)
wpe1.jpg (978 bytes)
wpe1.jpg (978 bytes)

low mass stars die as white dwarfs surrounded (briefly) by planetary nebulae 

high mass stars undergo a core collapse 
that generally results in a supernova explosion...
some supernova remnants look like the Crab nebula

crabblue.jpg (43253 bytes)

or like Cas A

casa.jpg (31937 bytes)

or like Supernova 1987A

sn1987a.jpg (49978 bytes)

 
wpe1.jpg (978 bytes)
future evolution depends on whether the mass of the core after the collapse is larger or smaller than about 3 solar masses
planneb1.gif (4320 bytes)
 
 

and
 
 

 

arrowl.jpg (1829 bytes)
if the core mass is less than 
3 solar masses, 
a neutron star is left behind

wpe1.jpg (978 bytes)

 
arrowr.jpg (1677 bytes)
if the core mass is more than 
3 solar masses, 
a black hole is left behind

wpe1.jpg (978 bytes)
some neutron stars are oriented so that
they appear to us as pulsars

crab.jpg (26762 bytes)

 
blackhole.jpg (53142 bytes)

of course, black holes aren't visible, 
so these are just diagrams, not images
white dwarfs eventually lose their surrounding nebulas 

whitedwarf.jpg (43648 bytes)
eventually the supernova remnant evaporates away, leaving 
a bare neutron star
nslonely.jpg (47325 bytes)