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View Full Version here: : I found a Wolf-Rayet star! (Sort of)


julianh72
30-09-2016, 07:34 AM
OK, I should admit right from the start that while I feel as though I have “discovered” a Wolf-Rayet star, it turns out to be very well-known to science, so I don’t think there is any chance that I will be invited to give it a name, less yet have it named after me! :ashamed:

I wanted to capture a single spectral image with a group of several stars of comparable brightness but varying spectral classes, to illustrate the different spectral “signatures” side-by-side. I decided to go for a small open cluster - "small" because of the small capture area of my set-up (about 0.3 degrees wide x 0.24 degrees), and "open" so that I might capture perhaps 10 comparably bright stars with their spectra, spread over the frame. I selected NGC 6231 (a.k.a. Caldwell 76) as a candidate - approximately 0.25 degrees across, and with a good spread of stars with Magnitude 6 to 7, well within my capability to capture.

So I aimed my telescope (with QHY5 mono camera and Star Analyser SA100 grating), and captured the attached spectral image.

At a quick glance, you can see about 12 stars on the left side of frame, with their spectra on the right of frame. (And even more stars and spectra can be extracted with a bit of post-processing and enhancement.) As I was hoping, you can see a few different stellar “signatures” - some spectra are longer than others, some show some strong absorption lines, and so on - perfect, just what I was hoping for!

But then I noticed the spectrum of the bright star at the mid-height of the frame - it looks very different to the others. I saw that it has several intense bright emission lines - what could this mean?

So I loaded my image into RSpec, calibrated the wavelength scale, and corrected for instrument response, and yes, those bright points that I saw in the original image have come through as strong emission lines, at wavelengths of about 4650, 5690, 5810, 6560 and 6730 Angstroms.

And when I superimposed the characteristic lines of a Class WC Wolf-Rayet star, I saw excellent correlation with 4 of the classic WC emission lines - the bright emission lines that I spotted in the original image correspond to Carbon emission at 4650, 5696 and 5805 Angstroms, and Hydrogen-alpha emission at 6563 Angstroms.

Mystery solved - Open Cluster NGC 6231 (C 76) contains a Wolf-Rayet star, with Spectral Class WC! :thumbsup:

This was an exciting “discovery” for me (even more so than my recent “discovery” of methane on Neptune), so I started doing a bit of research - and discovered that NGC 6231 actually contains at least three Wolf-Rayet stars: HD 151932 (Mag. 6.48), HD 152270 (Mag. 6.61), and HD 152408 (Mag. 5.82).
https://en.wikipedia.org/wiki/NGC_6231

Sure enough, my “discovery” turns out to be the well-documented HD 152270 (a.k.a. HR 6265).

Oh well - I guess my first real discovery will have to wait for another day!
Nevertheless, I hope this little story expresses some of the genuine thrill that can be experienced in doing “real science” in your backyard!

Merlin66
30-09-2016, 07:46 AM
Julian,
Great write-up, well done!
It shows the capability of the grating very well....
A survey of the southern sky to identify all the "emission" stars (Be/ WR) would be a good idea (I have an objective prism set-up ready for such a task - pity to doesn't work through cloud!)
It is very satisfying to be able to record, see a different spectrum profile, and then identify the star.
Great stuff.

julianh72
30-09-2016, 08:18 AM
Challenge accepted!

Let's see - my set-up can capture an area about 0.3 degrees wide x 0.24 degrees high, but because the spectra smear over the right side of frame, the "useful" capture area for spectroscopy is about 0.1 degrees x 0.24 degrees per capture, say.

The image I used for this little exercise was a stack made from a 5-minute video (30 MB) of 10-second exposures, and it has captured useful spectra down to about 8th Magnitude. (I can go "deeper" with longer frame exposures, and / or longer video captures, but let's use that as a starting point for planning a Southern Skies Spectroscopic Survey.)

Okay - to capture the southern hemisphere (180 degrees x 360 degrees), I'll need about (180 / 0.24) x (360 / 0.1) = 750 x 3600 = 2,700,000 captures. At 5 minutes each, that's about 225,000 hours (25 years, or 50 years of nights!) of telescope capture time (not including the set-up time, cloud, etc). :eyepop:

The good news is that at 30 MB per video, I "only" need about 81 terabytes of disk space.

Okay - I'm gonna need some help on this project! :lol:

lazjen
30-09-2016, 08:36 AM
You just need more scopes, etc and run them all in parallel. After that, the disk space will be trivial. :)

Nice write up though - was an interesting read.

Merlin66
30-09-2016, 08:38 AM
Julian,
Are you using the grating as a objective grating?
Janet S did a survey of Cygnus with her objective grating and Canon 350D, 85mm lens and tracked exposures of >30s to record the spectra in sufficient detail for "survey" - obviously if any "suspects" are found then you could have a more detailed look.
(Grating Spectroscopes-How to use them, p27)
Probably be a bit quicker - say 5 years or so;)

julianh72
30-09-2016, 09:14 AM
No, I'm using it at the "pointy end" of my telescope. Apart from a couple of very "quick-and-dirty" shots, I haven't got around to trying it as an objective grating on my DSLR for wide-field captures, but I've got the necessary adapter, so I might give it a try. (But I don't want to rush this all-sky survey project - I'd hate to complete the survey in "only" 5 years, because I wouldn't have anything left to do after that! :lol: )

Merlin66
30-09-2016, 09:22 AM
Don't worry mate!
When we get you totally hooked on spectroscopy - there's PLENTY of other work just crying out for attention!

Southskyscience
30-09-2016, 10:47 AM
Hello Julian,

That's really great! Thank you for sharing it.

It would be a big contribution to astronomy if all of us can get interested and join force to explore the southern sky objects and/or respond to survey calls or campaigns.

One that aims for a 'moderate' resolution is https://funnel-web.wikispaces.com/The+FunnelWeb+Survey
where [Quote]The FunnelWeb will target S/N>100 spectroscopy for 3 million bright (I<12) stars south of +10 degrees (|b|>10 degrees), plus all M-dwarf candidates down to I=14, over a 3 year period starting late 2016.[Unquote]

Such was one inspiration earlier this year for having more of us start with SA100/200. Another relates to the noticing of a BIG HOLE in amateur astronomy -why so very little spectroscopy when it is such a foundation!?

It was not about an end goal but the enjoyable and knowledge-advancing journey we can all take (why? it's not really known, there's perhaps an innate part about curiosity and the love of learning in humans). We're promoting or hoping for non-competitiveness, not winner-takes-all, and specifically teaming up within the process, for each target, of research + imaging + processing + interpreting + research + reporting.

And we each can chase any favourite class: WRs, Carbon stars, green stars, planetary nebulae, comets? Where we can't do (yet) we can do (later). Or look at results of others who can.

If enough of us are interested we'll find somewhere to collate the data. It has been of great interest that the AAVSO has been interested in setting up something for spectroscopy! Great news.

Team Southskyscience

robin_astro
01-10-2016, 05:49 AM
A proof of concept targeted B star survey run in the northern hemisphere using a couple of automated ALPY spectrographs with confirmation followup using high resolution spectrographs was a good fun and turned up a few new Be star candidates.
http://www.astrosurf.com/aras/be_candidate/auto-be-candidate.html

Cheers
Robin

xelasnave
03-10-2016, 02:00 PM
Good on you Julian.
Reminds me of when at 11 years I invented the electric motor.
I did not know about them.
My father was surprised that I worked it out.
He told me there could be no perpetual motion the motor idea was my attempt as a child to prove him wrong.
A most interesting account thanks for posting.
Alex