Time for another blog.
If anyone is interested in my blogs please
comment or I may think I am speaking to
myself.:eek::eek:
Anyway Wolf-Rayet stars - what are they
what do their spectra look like and
moreover why are they interesting?
Wolf-Rayet stars are extemely hot stars
(surface temperatures 25000 to 50000
celsius compared to the suns cool 6000
celsius) which have evolved from hot O
stars - O stars are the ones that look blue
when you look through the telescope.
First the O star expands to become a red
giant as it starts to run out of hydrogen
and then it evolves to become a Wolf-
Rayet star when it burns heavier elements
in its core.
So O stars as they start to die (run out of
hydrogen to burn) evolve into Wolf-Rayet
stars. The stars then spend 10% of their
lives in this Wolf-Rayet state before they
finally go bang - and I mean BANG!!! -
when we end up with a
supernova/gamma ray burst. Apart from
the big bang thats near the biggest bang
you can get.
Another thing about Wolf-Rayet stars is
that they are rare - approximately 300
found in our galaxy so far and they are
not bright so spotting them is difficult
In the northern hemisphere the brightest
are just under magnitude 7 which means
unless you have a very dark site then
you'll need a telescope just to look at
them.
Because they are dim and as they are rare
it wasn't until Charles Wolf and Georges
Rayet discovered some of them in Paris in
1867 - thats under 150 years ago.
Thats enough history - the thing that
excites people like me is their spectra.
These stars instead of having the usual thin
dark absorption lines that our more
common stars have, have extremely wide
and bright emission lines!
The reason for this is that the stars are
suffering from extreme mass loss through
the large stellar winds they have (500-
2500km per second) - these winds are
powered from the burning taking place in
the core of the star.
There are two main types of Wolf-Rayet
star the Nitrogen type called WN
characterised by it helium and nitrogen
emission lines and the carbon type called
WC characterised by its strong carbon,
helium and oxygen emission lines.
The type of the star really depends on
what the burning process is that is going
on in the core of the star.
For the WN type there is still some
Hydrogen left in the core and it combines
with carbon to eventually produce Helium
and Nitrogen. I'll save you from all the
equations but this is called the CNO
process
For the WC type there ain't the hydrogen
there so we have full on Helium burning in
the star so the Heliums fuse together to
form eventually Carbon. Depending on the
conditions in the core of the star we can
sometimes get Carbon fusing with Helium
to get Oxygen. This is called the triple
alpha process (an alpha particle being a
helium nucleus hence the name).
Ok now for the star I took the spectra of.
For all of you out there who like beautiful
images here is a link to a picture of WR
136 taken by the Isaac Newton telescope
showing the star and its surrounding
nebula
APOD: 2009 September 15 - NGC 6888: The Crescent Nebula
Ok now time for the spectra - this star is
mag 7.5 so its a pretty challenging target
I took a couple of half hour exposures to
get this spectra which was not like any I
had taken before:
And the line profile from the spectra is
here:
Ao we have a couple of very broad
emission lines. So what are they?
Fortunately several professional
astronomers have done extensive
examinations of Wolf-Rayet stars in
general and this star in particular. One
called Hamann in 1993 produced a model
for the star showing that it was mostly
Helium with the remainder being 12.5%
hydrogen and 1.5% nitrogen.
So the two emission lines predicted from
the model are for the bright one a mixture
of Helium and Hydrogen (mainly helium)
and for the second line Helium alone.
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