Astronomy C

[AlphaTauri]

AlphaTauri,

Why did you say for problem 2b, it was 70 arcseconds instead 90 arcseconds (the image says 90).

Well, the entire image is a 90" square, but V838 doesn't extend all the way to the edges, so both I and the key guesstimated that it was about 70" in diameter.

Also, wouldn't there be an easier way of just using LY or parsecs to put your answer in (I'm not talking about converting once you get your solution at the very end, but rather converting at the very beginning).

And yes, you could easily answer for distance in LY or pc if you had the linear diameter in LY/pc as well, but I used meters because it was the most convenient unit (since I calculated the expansion time in seconds, and I know the speed of light in m/s).

Also, for the problem 2d , do you know why the instructor squared the radius and temperature?

I think you meant 1d? It's a LRT (Luminosity-Radius-Temperature) problem; the equation is . I believe it's derived from the Stefan-Boltzmann Law and luminosity equation - but the powers are just part of LRT.

Finally, how do use Cepheids to calculate distance.

Okay, so the period of a Cepheid varstar is related to its luminosity (more luminous Cepheids have longer periods). The exact luminosity of a Cepheid can be calculated by the Period-Luminosity Relationship. Once you know the luminosity of the star, you can work out its absolute magnitude - or, some forms of the P-L Relationship directly spit out absolute magnitude. Once you know absolute magnitude, you can take the star's apparent magnitude from your observations and use the distance modulus to work out how far it must be.

Hope that helps! Feel free to ask if you have any more questions :]

[astro124]

Thank you!...again.

Okay, so relating to the cepheids, once you find the Period-Luminosity Relationship using either one of the equations(listed below), you would have to use the ever so infamous distance modulus (at least to me it is infamous), correct?

Population I: Mv = -2.81 logP - 1.43
Population II: Mv = -2.81 logP + 0.15

Now, I'm not sure which distance modulus you use (I use m-M=5log(d)-5). However, I don't think it should matter seeing that any distance modulus should give about the same answer. Also, how could use 'your observations' to find out apparent magnitude.

Secondly, I was researching stellar evolution (specifically higher mass stars) and I stumbled upon your blog. So it's my understanding that once a high mass star leaves the main sequence it can either become a red supergiant or blue supergiant, but the has the ability to switch back and forth between BSGs and RSGs, right? Also, can a star start off as BSG?

[syo_astro]

Heh, the distance modulus isn't so infamous. I was the one to write the apparent/abs magnitude post, so I had to hammer in all the parts of that derivation just to try to write that logically...it's actually not so bad after that XD. I would recommend knowing that derivation and at least the beginning and end parts since they are quite important, and it pulls together: distance, luminosity/brightness/flux, apparent magnitudes, absolute magnitudes, AND reddening. But just in case I have variants solving for distance (as in d=...), and the other stuff to in my notes. Also, what do you mean by your observations (is this from Northview, sorry I wasn't fully looking >.<).

For massive stellar evolution it's a bit different from our good ol' normal track (heh, mass matters!). If you have googled the massive tracks you should actually be able to see it go across the track to the RSG stage, but some do become BSGs and have all those other variants we mentioned. Any parts that really confuse you with that? Why it happens you mean (did we say that)? If I recall supergiants are generally evolved stars, but I'll check on that.

[astro124]

I was asking AlphaTauri what he meant by 'your observations' in order to find the apparent magnitude, so I'm just as confused as you. on this.

As for the the stellar evolution, I was confused as to why some stars become red supergaints and others blue supergiants. When I checked on Chandra, the diagram had protostars becoming blue supergiants as opposed to high mass main sequence becoming supergiants, which I didn't think was possible.


....I feel bad for people taking Forestry that have stumbled into this thread. They wouldn't understand anything.

[AlphaTauri]

I believe you're referring to this, yeah?

Finally, how do use Cepheids to calculate distance.

Okay, so the period of a Cepheid varstar is related to its luminosity (more luminous Cepheids have longer periods). The exact luminosity of a Cepheid can be calculated by the Period-Luminosity Relationship. Once you know the luminosity of the star, you can work out its absolute magnitude - or, some forms of the P-L Relationship directly spit out absolute magnitude. Once you know absolute magnitude, you can take the star's apparent magnitude from your observations and use the distance modulus to work out how far it must be.

On a test they would have to give you the apparent magnitude, but if you were a real astronomer (like Henrietta Swan Leavitt ), app. mag would come from actually measuring the brightness of the star you're studying.

As for stellar evolution, low-mass stars like our sun only swell up into RGs once, because they can't achieve high enough temperatures to ignite carbon fusion. But high mass stars that can fuse all the way up to iron wander back and forth over the HR diagram, from BSG to RSG and back, as successive fusion stages ignite in their cores - these are called "blue loops". During shell burning with an inert core, the faster rate of fusion in the shell increases radiation pressure, causing the star to puff out and cool off, become redder. When the core reaches a high enough temperature to ignite the next fusion step, the star becomes bluer (probably due to a decrease in radiation pressure? I'm not sure on this) and starts building up another inert core...and this cycle repeats until the star gets to iron and then it goes BANG.

High-mass protostars do become large blue stars, but they quickly evolve off the main sequence to become a variety of things, depending (as always in stellar evolution) on mass. Lower-mass high-mass stars tend to do the whole thing with blue loops, while really massive high-mass stars become LBVs or WR stars instead.

By the way, I'm female...don't worry, it's hard to tell over the internet.

[astro124]

Thanks for the information, this site and those on it (like you) have been really helpful :]. Also, sorry for the gender confusion.

One last thing (well....maybe), is a month too late to get serious for Astronomy? From what I've seen it seems like Astronomy is a broad, tenuous subject.

I've started the overdrive research mode about a 1 week and half ago, and the comp. is in early March. Is it still possible to do well (I'm still going to research and study like crazy regardless of the answer).

[syo_astro]

Thanks for the information, this site and those on it (like you) have been really helpful :]. Also, sorry for the gender confusion.

One last thing (well....maybe), is a month too late to get serious for Astronomy? From what I've seen it seems like Astronomy is a broad, tenuous subject.

I've started the overdrive research mode about a 1 week and half ago, and the comp. is in early March. Is it still possible to do well (I'm still going to research and study like crazy regardless of the answer).

For this I would say it completely depends on you and your time for research. It can be a tough event, that's why they allow a laptop and a textbook (...I mean binder)! But it is certainly fun and interesting (and let's you talk basically in another language with other astro people)! If there's nobody else on your team who can do it you're probably going to have to do it either way...I would say just keep up with the DSO research, have both you and your partner learn the general stuff, and one of you should especially master the math (like seriously know all the equations and be able to get it up in the notes really fast).

Also, make sure you are working with your partner, that's very important. Decide which notes you're using, stack them up, split up the work, get to know eachother, it's very important. Lastly, of all the events YOU CANNOT leave astronomy to the last minute. So just get working, and hope you have fun! Any questions of course ask and whatnot.

[identicalgamer]

What follows is my current astronomy packet. I am not sure what the rules are in regard to posting stuff like this but I feel if we get a discussion going based upon this packet then everyone will benefit.


https://docs.google.com/document/d/1ZI9 ... sp=sharing

[foreverphysics]

Well...I mean, I can see several things right off the bat that I could add to, but it's your packet, right?

[identicalgamer]

Yeah, I have made it all by myself. Ideas to add to this would be greatly appreciated.