Astronomy C

[SilverNight]

Can someone help me with a question I have about spectra?
In the book I'm reading, it says that a emission line is produced when a thin gas is seen against a cooler background. I know what a continuous spectrum and an absorption line is, but I don't understand emission line. What does it mean by "a thin gas is seen against a cooler background"?
Another question about absorption spectra: when electrons absorb the photon of light of a specific wavelength and jump to a higher energy level, doesn't it fall right back down and send the photon back? ... So doesn't that mean there shouldn't be a dark line where the photon is absorbed?
... I hope my questions make sense and don't sound stupid...

[Schrodingerscat]

I think to your first question they mean a thin gas as opposed to very dense gases which emit continuous spectra when heated. I believe (but am not certain) that the emission lines are the result of thermal energy increasing electrons energy levels, and are brighter lines surrounded by the darker rest of the spectra. The answer to your second question is that the photon is emitted in a random direction, not necessarily in the same direction of the original photon. So certain photons from a background source such as a star will be scattered in random directions creating the absorption spectrum.

[Trees]

DSO's from the Invitationals (photos in link/on the wiki page http://scioly.org/wiki/Astronomy/DSOs)

Mira http://science.nasa.gov/science-news/sc ... 5aug_mira/
Carina Nebula http://scioly.org/wiki/File:CarinaNebula.jpg
Sirius B http://scioly.org/w/images/2/20/SirB.jpg
NGC 2440 http://apod.nasa.gov/apod/ap070215.html

Not sure which one of these were there too, though I THINK so (I think 2 are right):
Tycho's SNR http://scioly.org/w/images/d/d8/TychoSNR.jpg
Kepler's SNR http://scioly.org/w/images/8/80/KeplerSNR.jpg
Rosette Nebula http://scioly.org/w/images/3/35/Rosette.jpg

There was a total of 10; I forgot the first one. So that means about 4/10 are unknown. I'll look around. Here's the list from the rules manual:
Mira, SNR 0509-67.5, CH Cyg, Kepler's SNR, Tycho's SNR, M15, Carina Nebula, T Tauri, Sirius B, RR Lyrae, U Scorpii, Rosette Nebula, BP Psc, NGC 2440, RX J0806.3+1527, DEM L238 & L249


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Candidates. Not sure yet but when the Test comes out I'll let you all know for sure (or someone might beat me to the punch, ha) (Funny enough, both of these appear in Invitationals but not on the manual...)

Perseus A
NGC2623 [confirmed by http://scioly.org/w/images/9/90/Ast-sol.pdf]

On the same note: http://scioly.org/w/images/6/68/AstronomyWayzata11.pdf needs the answers, heh.

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OKAY NOW TIME FOR QUESTIONS.
We were given a problem where two stars appeared to be the same brightness; however, one was about 4 times farther (or whatever number they used). We couldn't find the formula to answer this question with...

[EastStroudsburg13]

From which invitational are these DSO's from? They really shouldn't test on DSO's from previous years, but luckily last year's list is on the wiki, so it might be useful to print it out JUST IN CASE a supervisor decides to use it.

Based on your description of the problem, the distance modulus looks like it would work. I'd link you to the wiki, but LaTeX looks like it's not working.

[Flavorflav]

It's simpler than that - inverse square law, then take the log. The more distant star must have 16 times the luminosity to be equally bright at four times the distance, so it is 1.2 magnitudes brighter absolutely.

[ichaelm]

First of all, Trees, that question isn't a question. But I'll assume that you meant to ask how many times brighter one star was than the other. In that case, the inverse square law is all you'd need. Flavorflav was assuming that it was asking for the difference in astronomical magnitudes which is also be a reasonable question, but he took the log base 10, despite the fact that magnitudes correspond to a factor of about 2.51, not 10, so you'd need to take the log base 2.51.

[salcedam]

Just letting you guys know that there are a couple typos on the formula sheet that's linked in the wiki. For the formula relating the luminosity of a star and its radius and temperature, it says L/Lsun = (R/Rsun)(T/Tsun) when it should be L/Lsun = (R/Rsun)^2(T/Tsun)^4. Also in the formula for Hubble's Law, it says that Hubble's constant is 20 km/s/Mpc. It should be 70 km/s/Mpc. And the last typo is on the page of constants. It says that the luminosity of the sun is 3.826E26 kg. Kilograms obviously does not make sense as a unit for luminosity. That should be W (or watts).

[Flavorflav]

First of all, Trees, that question isn't a question. But I'll assume that you meant to ask how many times brighter one star was than the other. In that case, the inverse square law is all you'd need. Flavorflav was assuming that it was asking for the difference in astronomical magnitudes which is also be a reasonable question, but he took the log base 10, despite the fact that magnitudes correspond to a factor of about 2.51, not 10, so you'd need to take the log base 2.51.

DoH! You're right, of course - make that about 3 magnitudes brighter.

[AlphaTauri]

Hm, does anyone have a definitive answer for the distance of Tycho's SNR? Chandra says 13,000 LY, but every other source I have says somewhere between 7500 and 10,000 LY.

[Infinity Flat]

Hm, does anyone have a definitive answer for the distance of Tycho's SNR? Chandra says 13,000 LY, but every other source I have says somewhere between 7500 and 10,000 LY.

SIMBAD has it at 2.8 +- 0.4 kpc. Which is about 9000 ly.
SIMBAD
Original Paper