h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Aren't the absorption and emission lines in the same place?

Also used in calculating doppler shifts: the position of the lines move if approching/retreating

Sodium D lines; the D stands for Doublet, as there are two lines. Who said physicists were inventive

Yes absorption and emission lines do appear in the same place, in the spectrum, as the same energy change is occuring. As mentioned in the article, though, as the beam of light goes through the gas, the light at those lines is absorbed and then re-emited in all directions. Thus the light coming towards you is less bright at those frequencies. Also, the energy gained by absorption an be lost in other ways, heat, vibration, excetera, which further reduces the brightness of the re-emited light.

As to the doppler shift, in the same way that a police siren seems to change frequency as it moves past you, so does the frequency of light. So a star rushing towards you will have a different frequency to one going away. Hence they look a slightly different colour, allowing astronomers to tell their relative speeds and directions. So, your Sodium D lines from a star moving towards you are a slightly different colour, but the same distance apart in frequency, to the lines seen from a star going away from you.

Have I lost you yet?

Nope... I've just done A-level Physics

Tells me everything I knew already Munchkin
Wasn't it the presence of unknown absorbtion lines in sunlight which lead to the discovery and isolation of helium as there isn't too much of it on the earth.

Just burst my ego why don't you

Anyway, as to helium, I think you are right, but can't remember any details, my education is rapidly slipping away from me

I'd never burst your ego Munchkin. What you wrote was a very good basic inroduction to measuring distances on astronomical scales.

I can't remember any more details than what I posted. I was kind of hoping someone more knowledgable than myself (someone like the nice researcher Munchkin perhaps ) would fill in the other details.
Perhaps Joanna knows as she's still doing that studying stuff.

Phil you're right.

I think (and this is done from my memory, so I apologise for all the mistakes I'm about to make) that first of all when taking a spectrum, someone, I think it may have been Fraunhoffer observed the absorbtion lines from the sun. He gave them all letters- D for the doublet as Munchkin says, which happened to be the Sodium lines, although they didn't know that yet.
Then someone else (I imagine it was a different person, don't recall who though) observed the emmission lines of sodium, and matched them up to the D lines that Fraunhoffer had labelled. (she thinks)
Therefore it was reasonable to assume that all the other lines were due to some element or other, hence helium was discovered - the other lines in the spectrum could be attributed to elements here on earth which had been studied, however that line still remained.
Of course helium comes from helios the greek word for the sun.

It does me good to have my Ego burst every once in a while

I suppose the other obvious example of emission lines is from monochromatic lasers. Sadly, I can't remember any pithy facts about them off the top of my head. Why did I spend sooooo long at Uni?

Monochromatic lasers, my mate used to build them. Then he finished his PhD.

You can use them to get good patterns through a diffraction grating but *don't look into them*

is my sentence of the day concerning lasers.

Rubidium also springs to mind; they may or may not excite it...

Joanna (passed her prelims yay!, got another 2 years to look forward to...)

I've remembered why I was at Uni now, it was for the beer. Which probably explains my failure to remember the rest of it very well

Warning on laser lab (apparently real)

"Do not look into laser with remaining good eye"

LOL

OK. About astronomical scales...

A couple of years ago, an astronomer by name of Hubble figured out a few things. One was that stars are pretty much the same, so they emit pretty much the same emission lines. So when the emission spectrum got 'spread out' or redshifted as compared to our own sun, that means that the star was moving away from us (due to the Doppler effect).

The second thing he realised was that the further the star was, the more redshifted it appears and the faster it was moving away from us.

So he figured out that the universe as a whole was expanding.

Now, it's time for someone else to lay the smack down on my ego

Isn't Hubble's constant the one that keeps changing?
i.e. It determines whither the Universe will expand for ever, or collapse in on itself and they keep recalculating it as they learn more, hence gerring a different collapse/expand answer

Yes I think 'cause its the gradient of a curve, which over small timescales like hours seems like a straight line.
But even though it is constant for us at the moment, errors in astronomy are such that estimates range from 50 to 100 kilometres per second per megaparsec, although someone in our astronomy department says a small number like 25 would be good... I think he only pretends to believe that though, I'm not sure....

A small number is 'good' because it creates an interesting, closed universe (ie: ' Big Crunch ' which should produce a new Big Bang, according to some wackos... er.. cosmologists.. )

Heck yes, people keep trying to figure it out But in the language of astronomy, anything within a factor of two is good enough anyway. (Except in exams.)

The factor that determines whether the universe will expand forever (or collapse in on itself) is the entire mass of the universe. More mass equals more gravity, if there's enough mass then gravity will overcome the expansion and the universe collapses. Hubble's constant might be a clue to working it all out.

So we're trying to figure out the mass of the universe. It's a problem because there's a lot of dark matter out there and we don't know exactly how much...