Hi all,

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.