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Star Wars !!!!; Spacey moods
Topic Started: May 13 2011, 06:12 AM (995 Views)
Phimi
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Mehz.
I can field this one. By difference in nm, I presume you mean a change in wavelength, represented by delta lambda. Specifically, the Doppler equations are delta lambda/lambda=v/c, so if you know the change in wavelength, divided it by the original wavelength, then times by the speed of light to get the reccession velocity. Convert the reccession velocity into km/s, then divide by 65 (Hubbles constant) to get the galaxies distance in megaparsecs. To get this into light years, times by 3261.

*Edit* Standard form fail, times by 3,261,000. I forget mega- meant 10^6.
Edited by Phimi, Aug 7 2011, 02:56 PM.
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suzumebachi
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[post goddess]

You know the normal wavelength of an excited Hydrogen is ___ and identified that the wavelength ____ in the galaxy is also the same type of Hydrogen.

can you calculate how fast the galaxy is going from that?

because if you can:

v = H0D H0 is Hubble's Constant, and D is the distance

'cause I think if I remember hubble's constant has something to do with galaxies and the universe expanding. I'm guessing it's an acceleration or something, but I can't remember my basic physics :/
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Phimi
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Mehz.
Say we're looking for light at a wavelength of 656nm, but from the spectrum taken from a galaxy, we find the line we're looking for to be at 675nm. This is a change in wavelength of 20nm, so 20nm/656nm= a Doppler shift of 0.0305. The Doppler shift is also equal to the galaxies recession velocity, v, divided by the speed of light. so 0.0305 times 300,000 (the speed of light in km/s) = 9150. The galaxy is moving away from us at 9150 km/s (this figure is frankly ridiculous, and would never actually appear, doppler shifts are usually much smaller). As you said, v=H0d, with H0 being equal to 65km/s/megaparsec. so 9150/65 gives the distance, in megaparsecs, of the galaxy, which is 140.8 (again, much too big). To convert megaparsecs into light years, you first put it into parsecs, so times by a million, then 1 parsec is equal to 3.261 light years, so times by 3.261. The galaxy in this example would be around 459 million light years away.
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suzumebachi
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[post goddess]

So. Doppler shift = change in wavelength / normal wavelength.

Velocity = Doppler shift x Speed of light (c)

Distance = Velocity / H0

so. Distance = (c[change in wavelength/original wavelength])/H0

:D ok, got it! and I guess I should probably start off with c in mpc/s? Just to skip some conversions?

Edit: I just saw Phimi's post above on the top of this page... ._. I think it's cause I had this page open for a while before actually posting...hehe
Edited by suzumebachi, Aug 7 2011, 07:50 PM.
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suzumebachi
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[post goddess]

*bumpthread*

Ok. Can anyone explain absolute luminosity or whatever it's called, and brightness...etc.?
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Phimi
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Mehz.
Absolute luminosity (henceforth referred to as magnitude, as that's how I learnt it) is the magnitude of a star when "viewed" from 10 parsecs. It's just putting a more uniform scale on the magnitude of stars.

You can switch between absolute and apparent magnitude with this equation:
m-M=5log(d/10)

Where m=apparent magnitude
M=absolute magnitude
d=distance in parsecs.


Hope this answers your question.
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suzumebachi
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[post goddess]

Basically, absolute magnitude is comparing how bright they are if they weren't at different distances.

Ok, thanks :)

Is "luminosity" just a different word?
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Phimi
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Mehz.
Pretty much, yeah.
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Darkceus
Angel
How do you guys know all this stuff? -.-
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labouka
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Ruler of Black Holes
Well it's all about interest. I was going on with space from the age of 8 and, like all people who are very nerdy on any subject from these ages, I knew extraordinary facts dispite my age. Funny thing is that somehow I thought that the whole world knew about this stuff in the first place and soon I found out the horrible truth at the age of 16 when I saw that my classmates didn't even knew the order of the planets in our Solar System. A let down indeed.

As for fellow consort and space subject consultant Phimi, it is absolutely clear that the girl's got it for sure. I don't remember if she told me on what age she started getting down on these things but it must have been somewhere close to mine. And the good thing with her is that she is observant-she likes to get the bottom of things. Maybe in the future if she follows that department she will be the top of her class. And the thing that I really like about Phimi is that she is the first person I've ever met to know things regarding those subjects and have a good talk with, overall (the second is Suzu, obviously, and all of you guys in the site. Seems I found my people here).

Hope that answers your question.
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suzumebachi
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labouka
Aug 21 2011, 06:14 AM
Funny thing is that somehow I thought that the whole world knew about this stuff in the first place and soon I found out the horrible truth at the age of 16 when I saw that my classmates didn't even knew the order of the planets in our Solar System. A let down indeed.
I find people fall on a spectrum when trying to empathize: those who judge based on the assumption that everyone's the same, and those who assume that they're unique. I prefer and fall closer to the first...so I know exactly what you mean... Sorry, random...

I kinda just got into Astronomy through Science Olympiad, though I did have a background of science interest, and did, in fact, know the order of the planets before I was incredibly interested. That was...8th grade, so around the age of 13.
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suzumebachi
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[post goddess]

/bumpthread

So. What's the difference between a type Ia and II supernova? (I found my Astronomy competition rules :D)
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Phimi
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Mehz.
A type Ia supernova happens when a white dwarf is accreting matter (often in binary systems where the white dwarf is dragging material from it's companion), until it reaches the Chandrasekhar limit (about 1.4 times the mass of the sun), and collapses suddenly. These supernovas tend to be used as standard candles to determine distances, as their luminousity is very consistent.

A type II supernova is when a large star of around 9 solar masses continues fusion past carbon and oxygen which is generally the stage normal white dwarves get to. These massive stars usually end up with a nickel and iron core, and if this exceeds the Chandrasekhar limit, then it implodes, at around 1/4 the speed of light. These kinds of supernova lose about 10% of the star's mass in a 10 second burst of neutrinos.

That answer your question?
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