jsandse

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.

Hi all, For some reason the link to my spectra in the previous blog idid not work :(:( so here is an updated link to it Stargazers Lounge - jsandse's Album: Spectra - Picture cheers John

Hi all, In my last blog I mentioned that you can generate spectra from "man-made" stellar models. In this blog I will talk a little bit more about my experience with these models. Just to rewind a bit say you have gone to the bother of capturing a stellar spectra. So you have this spectra with a number of lines in it. So what? what does it mean its just a bunch of lines - not very exciting...or is it? An example of the spectra you actually capture looks like the ones I took of Mizar in my album http://stargazerslounge.com/members/jsandse-albums-spectra.html That picture actually has 6 spectra in it which are aligned one on top of the other. But you get what I mean what can I do with this spectra? Well you can convert it into a line profile using some software (I use vspec) and then calibrate it so that you know what the wavelengths of the line in the spectra are. This calibration can be done using a spectra with a reference lamp with known lines in it. Ok so you have a line profile of a spectra with a number of dips in it corresponding to the dark lines (also called absorption lines) in the original spectra. An example of this is the one in my album of epsilon auriga What is causing these lines and what does this tell me about the nature of the star I am looking at? I'll just answer the first part of the question in this blog. Leave the second part for another time. What is causing these lines are atoms on the surface of the star absorbing some of the light before it escapes from the star. This part of the star is called the photosphere. It's thin the sun's photosphere is only 1000km thick. Each type of atom (element) absorbs light of a unique characteristic wavelength. Anyway what this means is that if I know the wavelength of a dark line in my spectrum then I can in principle find out the atom responsible. So back to stellar models. Given three basic properties of a star: a) temperature b) logG - surface gravity of the star - this is how heavy things are on the surface of the star (eg the surface gravity on the moon is less than that onthe earth - just look how long those astronauts can jump and hit a golf ball!) c) metallicity - ratio of amount of metals (to astronomers this means any elements that are not hydrogen or helium - crazy isn't it?) to amount of hydrogen in the star a stellar model can calculate for most stars the physical properties of the star (pressure, temperature at different depths in the star) and generate a stellar spectra for it. I won't go into how we find out what the temperature, surface gravity and metallicty of the star is here - thats another blog... I will just say that you need these three properties to tell you what types of atoms are in the stellar photosphere (mainly comes from metallicity) and how they behave - which depends on temperature of course and the pressure they are under ( this is related to surface gravity) Right so I have captured my spectra of Vega, I know what its temperature, surface gravity and metallicity is. I can then use some software (I use Atlas) to generate a model for it and to produce the spectra from it. Now comes the exciting part of comparing the spectra I took with the model In my album please look at the picture at this link: http://stargazerslounge.com/members/jsandse-albums-spectra-picture8198-comparison-detail-spectra-vega-i-took-against-synthetic-spectra-produced-atlas-stellar-model-vega-similarity-between-two-spec So in the picture I have placed two line profiles next to each other. One in red which was generated from the model. And the other in black I captured myself from Vega. It is remarkable how similar they are - virtually all the absorption lines in the model correspond with those that I had captured from the real star. The benefit of using the model is that it gives me the atoms that cause the absorption lines. I have annotated the picture with details of some of these elements for the more prominent lines - includign ones caused by Hydrogen (H), Iron (Fe), Magnesium (Mg) and Titanium (Ti). How cool is that! I can decode the spectrum of a star that is 150,000,000,000,000.000 miles away into the types of atoms that are absorbing the light at its surface.