1. Feb 1, 2006

### fasterthanjoao

I've been reading over my astronomy coursework and trying some questions, and i'm coming across a few things I'm not sure about, i'd be grateful if you could provide the answers.

1/ Give three possible astrophysical reasons for periodic variations in the observed photometric signal from a star.
What is thought to be the source of the photometric variation in the H-alpha emission line signal from a Be star?

- for this question, I can only think of: rotation of the star? or pulsations (like a cepeid or something) or something passing in front of the star. I'd be happy if you could correct or expand.
For the second part, I'm sure the 'e' in Be has something to do with the H emission, and I think B stars are associated with having a disk of material - is this along the correct track?

2/
Describe the chain of events that causes stars in the instability strip to pulsate.

- Here, I'm assuming that they're looking for information such as; radiation pressure forces the layers to expand, pushing further and further out until the density is low enough that the radiation escapes more readily and the push isn't as effective, allowing gravity to take over - pulling it back in? A pushing/restoring force sort of thing.

3/ I keep seeing reference to the beta-index, as far as I can work out it's some sort of luminosity indicator, but I'm not sure what it actually is and I can't find much in my textbooks - is there an expression describing it? and how would I describe the process of beta-index photometry?

4/ Describe how a stellar wind could be inferred from a stars spectrum.

- Is this something to do with a P-Cygni profile? would I see a P-Cygni profile when I look at a spectrum from a star with a wind? And am I correct in saying that winds are more common in stars that are hot/luminous because high particle velocities, high energies and it's easier for the particles to jet off?

I realise this post is long, and I'm not asking you to read it all, but if you could give me feedback on any question, it's well appreciated.

thanks.

2. Feb 1, 2006

### Astronuc

Staff Emeritus
I think in question 1, that in addition to the physics of the star, a nearby object, e.g. another star in a binary system, or planet, will obviously decrease the apparent luminosity, and then there could be a nebula or dust cloud nearby.

Reference to beta index here "The nature of Balmer line variability in chemically peculiar stars" -

and here - http://www.univie.ac.at/tops/CoAst/archive/DSSN2/QConstant.html

and here - A sub-millimetre survey of the Galactic Plane
http://www.astro.keele.ac.uk/~ae/

and this looks interesting - http://www.edpsciences.org/articles/aa/pdf/2005/42/aa2491-04.pdf [Broken]

Seems like beta index has to do with the H$_\beta$ lines.

I'll pass on #2 and #4 - you need SpaceTiger, Cronos or Garth for those.

More useful stuff (maybe) -
http://ie.lbl.gov/education/glossary/glossarya.htm#
http://scienceworld.wolfram.com/astronomy/

Last edited by a moderator: May 2, 2017
3. Feb 1, 2006

4. Feb 2, 2006

### Chronos

Astronuc did a nice job, as usual.
1] Obviously, eclipsing binaries and intrinsic variables cover 2/3 of Q1. The third possibility is most likely polarization effects resulting from axial tilt.

2] Pretty much, if I understand the question correctly. See:
http://ams.astro.univie.ac.at/?s=pms [Broken]

The H Beta Index as an Age Indicator of Old Stellar Systems: The Effects of Horizontal-Branch Stars
http://www.arxiv.org/abs/astro-ph/0004247

4] Yes. See:
X-Ray Plasma Diagnostics of Stellar Winds in Young Massive Stars
http://www.arxiv.org/abs/astro-ph/0110035

Pretty tough questions for an undergraduate course!

Last edited by a moderator: May 2, 2017