Astronomy C

[Adi1008]

Hm, that post makes me wonder if you wouldn't mind updating the onward to the edge blog if you're gonna end up making astro posts anyway...

Some comments on your notation. Most astronomers simply say "F" of some sort rather than "energy flux", E_f. E_f might be a bit confusing if you've ever done conservation of energy in physics or various other instances (it can get confusing when you speak in flux density...I don't want to get into it'd probably be confusing). Physicists also use "I" for intensity, though I personally like to think of that as current. Just a bit of (semi-important) semantics! Thankful the astro peoples on this website still exist .

Thanks for the feedback syo_astro! I changed some of the notation on the post (y)

Referring to the Chandra videos for this year:
There is usually a powerpoint that goes with the videos. Has anyone seen that for this year?

The audio seems very poor to me, I'm having a hard time understanding what she is saying.

I couldn't find anything about it too :/

Also, do zero-age main sequence stars become more luminous and hotter, or more luminous and cooler? I've seen arguments for both and they all seem pretty convincing lol

[syo_astro]

If I hear anything, or if Tad wants to, we'll try to update things as we see it. It's busy at the end of the year sadly :/ (they're more in the loop than me because well...they directly do everything).

Adi, do you mean what is the relationship between luminosity and temperature for ZAMS? If you look at a main sequence on any HR diagram really, you should see luminosity and temperature are directly related (they both increase together). I don't really see the argument going either way unless you meant some other phrasing. See, there is a mass-luminosity relation whereby increasing mass of stars (which happens when you go up the main sequence) increases L, but this also increases T and R (which basically is related to the Stefan-Boltzmann Law). It might get weird when you think of other regions of the HR diagram (giants, white dwarfs, etc). The main thing that decreases with increasing mass (and thus other increasing quantities) is main sequence lifetime, though there are many things that can change with stars (like elemental abundances, pre-main sequence v. post-main sequence evolutions, etc). Hope that helps!

[sciolymom]

I'd appreciate it too if somebody could post something

https://somewhatoverwhelmed.wordpress.c ... mperature/

Forgive me for linking to something I wrote myself, but I think it explains it well. Hope this helps.

Thank you so much, that is very helpful!

[Adi1008]

If I hear anything, or if Tad wants to, we'll try to update things as we see it. It's busy at the end of the year sadly :/ (they're more in the loop than me because well...they directly do everything).

Adi, do you mean what is the relationship between luminosity and temperature for ZAMS? If you look at a main sequence on any HR diagram really, you should see luminosity and temperature are directly related (they both increase together). I don't really see the argument going either way unless you meant some other phrasing. See, there is a mass-luminosity relation whereby increasing mass of stars (which happens when you go up the main sequence) increases L, but this also increases T and R (which basically is related to the Stefan-Boltzmann Law). It might get weird when you think of other regions of the HR diagram (giants, white dwarfs, etc). The main thing that decreases with increasing mass (and thus other increasing quantities) is main sequence lifetime, though there are many things that can change with stars (like elemental abundances, pre-main sequence v. post-main sequence evolutions, etc). Hope that helps!

Thanks for the explanation syo, that makes sense

I recently read a bit saying the following:

A star remains on the main sequence as long as there is hydrogen in its core that it can fuse into helium. So far we have assumed that a star on the main sequence maintains a constant energy output. In fact, as a main sequence star ages its luminosity increases slightly, resulting in it expanding and its outer layer cooling. This explains why the main sequence is a broad band rather than a narrow line - stars move up and to the right on this band as they age.

and saw this picture (http://imgur.com/CJzsgm1) showing that as the temperature decreased, the luminosity increased. I would think that they reason they say that the surface of the star would cool is because expanding gases drop in temperature(I think), but that doesn't seem to make sense when you consider the reasons you outlined above.

The quote is from this: http://www.atnf.csiro.au/outreach/educa ... tmain.html

https://somewhatoverwhelmed.wordpress.c ... mperature/

Forgive me for linking to something I wrote myself, but I think it explains it well. Hope this helps.

Thank you so much, that is very helpful!

No problem, glad it helped!

[syo_astro]

Yes, but we specified ZAMS, not evolution past that. You might as well be talking about the subgiant branch if anything. Of course, you don't have to know too much about main sequence evolution considering the event topic, though it always is nice to have a good base of knowledge. I think what they mean in that context is that main sequence stars are a broad band either in the sense of observations (obviously you won't get a straight line then) or just in the sense of on any HR diagram (you see a bendy curve rather than a straight line).

NOTE: Even if stars do move up and to the right as they age...they'd pretty much all do this. Your diagram shows this well, and so it still holds for the main sequence about the relationships I mentioned (because really it does come from mass). Of course, that initial mass changes how they evolve pre-main seq and post-main seq, which is where you get other interesting things.

Summary: I think you might be worried about the subgiant branch, so you might want to look into that if your curious. Otherwise, don't worry and keep at it!

[Adi1008]

Yes, but we specified ZAMS, not evolution past that. You might as well be talking about the subgiant branch if anything. Of course, you don't have to know too much about main sequence evolution considering the event topic, though it always is nice to have a good base of knowledge. I think what they mean in that context is that main sequence stars are a broad band either in the sense of observations (obviously you won't get a straight line then) or just in the sense of on any HR diagram (you see a bendy curve rather than a straight line).

NOTE: Even if stars do move up and to the right as they age...they'd pretty much all do this. Your diagram shows this well, and so it still holds for the main sequence about the relationships I mentioned (because really it does come from mass). Of course, that initial mass changes how they evolve pre-main seq and post-main seq, which is where you get other interesting things.

Summary: I think you might be worried about the subgiant branch, so you might want to look into that if your curious. Otherwise, don't worry and keep at it!

I feel as if we're addressing different things here - I should have specified that I meant the part only between the solid and dashed pink lines (so while the star is still on the main sequence), which is still going up and to the right (or is that what you were talking about the whole time and it went over my head?), not the entire HR diagram. Essentially what I am asking about is the change in a star's luminosity and temperature while on the main sequence, not the entire HR diagram.

In both the quote and the HR diaigram it seemed to imply to me that while on the main sequence, the star would get more luminous, yet also cooler. Sorry for not being clearer

Anyways, thanks syo!!

[syo_astro]

Well, on ZAMS they obviously don't expand because hydrostatic equilibrium. Past that you build up Helium -> higher fusion rates nearby the core (which I can explain if you want) -> higher L + expansion (more expansion happens than L inc) -> lower SURFACE T (as you suggest from the expansion). I guess they are saying that after ZAMS you still get the He build-up happening (which is true, just it's not a pure, inert He core until it hits the RGB).

I think the wording from that webpage is a bit funny, though maybe it is better (I guess it's perspective on how we should simplify things). Unless someone else has better input...either way, though. Your main question was along the main sequence what happens, and I think there are two points that. One is what happens when you look at the whole main sequence by mass, which follows from what was already mentioned. The other is what happens for an individual star, one would become cooler and have L changes etc etc. I do hope nobody tests in such a way about confusing terms like this, though...especially considering the topic is on exoplanets and pre-main sequence stuff anyway >.>.

Just to explain myself in case you were confused, I kept mentioning the sub-giant branch because if you look on your HR diagram and compare it to other stellar evo tracks, it seems a lot like the sub-giant branch...which makes me think of it like the core building up to that pure inert He core we know and love of red giants, but then this gets confusing (like wiki says sub-giants all have pure He at their core). I'd say be aware that the SGB is a thing, and just think of the main sequence like the ZAMS unless having to think of otherwise.

In astro you have to look at multiple things at once, but I hope I distinguished the concerns here (unless there's something I missed...).

[tad_k_22]

The webinar slides have now been posted: https://www.soinc.org/astronomy_c.

Thanks for catching that, /u/sciolymom!

[Hankdaddy]

on the answer key to the 2014 Pennsylvania state finals exam in astronomy, the correct answer to number 59-according to the key is 5.35 E36, this number seems incorrect, did anyone else notice this?

[syo_astro]

on the answer key to the 2014 Pennsylvania state finals exam in astronomy, the correct answer to number 59-according to the key is 5.35 E36, this number seems incorrect, did anyone else notice this?

I believe you mean the 2015 one, but nonetheless. Also, note the negative sign is very important. I would apply something called the virial theorem for this question. I tried it quickly and got slightly off, but it might just be choices of conversion factors (I'm going to check last year's posts and edit this one if anything comes up). If you haven't heard of this or tried it and got something extremely off, I suspect it might be a calculation/number error on your part.

I found this out about 5 minutes after my post, but nontheless.

Edit: http://scioly.org/phpBB3/viewtopic.php? ... 05#p274468 <- refer to this