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owlofcreamcheese 01-24-2003 01:00 PM

How do we know things about quasars
This question applys to quasars and pulsars and all the other stuff we have discovered in space, honestly it applys to stars as well, but somehow it seems diffrent (mabey because we DO have a local example) my question is: how do we know anything about quasars?

what was the base assumption that allows us to determine the size or brightness? or the distance? it seems like you have to know something about them in advance to determine anything about them.

like say we found a new object in space called a blurch... we looked at it, how would we decide that it was a million miles across and a billion miles away, and not 1 mile across and 50 miles away. (well thats exageration, at that point paralax would tell us pretty quick).

same with red shift and dopler effect, what is the base assumption, if we took the blurch and saw the light was tinted red, what did we originally know how red it 'should be'.

what are the base assumptions we use when we discover a totally unknown object? it seems like alot of what we use is relitive to some other property of the object, we find size by knowing distance, we know speed by observeing color. something farther away looks smaller, something closer looks bigger, don't we have to know the size to find the distance or the distance to find the size?

and after a certain distance, we can't use paralax to see how fast it moves relitive to something else, I mean the stars haven't moved in my lifetime any nodiceable amount relitive to the earth.

I don't really understand, how do we find out ANYTHING about the objects we see in telescopes?

Grey 01-24-2003 01:16 PM

For distances we used parallax initially. Stellar positions shift very slightly depending where the earth is in its orbit. From the angular shift we use some trig and we arrive at a distance. Then we discovered that certain stars had intrinsic luminosity that periodically varied. These Cepheid variable stars could then be used as a yardstick. You ID 1 star with a specific brightness at a specific distance, from there the next star with the same period but fainter is assumed to have the same surface brightness and is only dim because its further away. You can figure that distance based solely on how dim it is compared to your initial star. Now we use supernovas in a similar manner.

But as for understanding their make up, we work from the assumption that what we see on earth is applicable across the universe. If hydrogen, when heated, emits a specific spectrum here in the lab, then hydrogen 2 million light years away must emit the same spectrum. Now when you go and look, the spectrum is the same but shifted slightly to the red end of the spectrum. Why? Well turns about that cosmic expansion acts shifts light arriving at earth by a measure proportional to the distance it has traveled. And were back to how we estimate the initial distance.

Kind of rambling, sorry about that. Hope it helps.

ftg 01-24-2003 03:33 PM

Following up Grey's post. To determine the distance to an object well outside our Local Cluster of galaxies, you need two pieces of info: it's red shift and the Hubble constant. The red shift is easy to determine if you have a powerful enough telescope and a long enough exposure time. It's the later number that's the big bugaboo. It has been remeasured, recalculated, etc. many, many times. But given the current constant and the red shift, distance is an easy calculation.

To determine size, if the object is not a point source, then distance and the arc angle of the image tells you it's size. For point sources, there are some other tricks to use. For instance, one thing that tells us quasars are fairly small, is that some have varied in brightness in short periods of time. There is nothing that can change the brightness of a galaxy in a year, let along days. Speed of light and all that.

So, if you discover new very far off blurch objects, get a spectrum and measure the red shift. Watch a whole bunch of them over time and see if they vary in brightness.

Chronos 01-24-2003 04:40 PM

May I toot my own horn for a bit?
Astronomical distances

As for the redshift, suppose that we know that the spectrum of hydrogen looks something like this

Red ..................|..|.....||...|....|.. Blue

And now suppose that we see something with a spectrum that looks like this

Red ...|..|.....||...|....|................. Blue

It's safe to say that that's probably the same spectrum, but shifted towards the red. Note, by the way, that this is not shifted "slightly", like Grey said, in the case of a quasar, but it is definitely shifted, and it's still recognizable.

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