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lehpron
Topic Author
Posts: 6846
Joined: Tue Jul 10, 2001 3:42 am

Red Shift

I know it has to do with the color spectrum but how does this technique find how far star are?
The meaning of life is curiosity; we were put on this planet to explore opportunities.

DLKAPA
Posts: 7962
Joined: Wed Dec 03, 2003 10:37 am

RE: Red Shift

I think it has something to do with frequency dissipation. The lower frequency is what you see, and the higher frequency is not carried as far.

But I have no real Idea, just a guess.

DLKAPA
And all at once the crowd begins to sing: Sometimes the hardest thing and the right thing are the same

BlatantEcho
Posts: 2132
Joined: Wed Sep 27, 2000 10:11 am

RE: Red Shift

If I remember Astronomy at all, it has to do with the Doppler effect.

Blue shifted, and things are going away. The light frequency is (slowing down or speeding up, I really don't recall) towards the blue as it travels towards you from a distant object.

If you compared the white light and how much it is refracted from "true" you can calculated the shift, and then approximate the speed at which the object is moving away (or towards) your relative position.

Remember the universe is expanding, you go out 10billion light years or so, and those sysems are scooting away from us (no we aren't the center of hte earth, but again, it's all relative) at speeds that eventually close on 3x10^8 m/sec, or light speed.

George

ilikeyyc
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Joined: Tue Dec 09, 2003 8:09 am

RE: Red Shift

not sure either. From what I remeber of HS physics is that normal white light is comprised of all the colors of the spectrum. The red shift means that the star is moving away from us as opposed to a blue shift where it would be moving toward us. This is a case of visual doppler effect. The star is moving away, so the white light waves a made longer so they appear red. You know, I can't explain this with words alone because I suck at explaining things. I would only be wasting your time if I tried to write more. Basically, it is the Doppler effect applied to light. Sorry I can't explain it better.
Fighting Absurdity with Absurdity!

DLKAPA
Posts: 7962
Joined: Wed Dec 03, 2003 10:37 am

RE: Red Shift

I said I didn't really have any Idea, and now that I remember, I was right. I had no Idea whatsoever and should not have been talking.

My Apologies.

DLKAPA
And all at once the crowd begins to sing: Sometimes the hardest thing and the right thing are the same

B2707SST
Posts: 1289
Joined: Wed Apr 23, 2003 5:25 am

RE: Red Shift

Red shift is a result of the Doppler Effect, as BlatantEcho mentioned. Imagine a wave of light being fired at a mirror. If the mirror is stationary, the waves reflect back toward the source at the same frequency as they arrived.

If the mirror is moving away from the source, though, the time it takes for consecutive peaks to hit and be reflected will increase, because the mirror is in motion relative to the wave. The reflecting wave will be stretched out, producing lower-frequency radiation. In the EM spectrum, this makes yellow and bluish light appear more red (per red-orange-yellow-green-blue-indigo-violet).

The same effect occurs when the mirror is moving toward the source, but in this case, the light waves are being compressed, creating a blue shift. Radar guns use the blue shift of reflected radio waves to measure the speed of cars, airplanes, etc.

In the case of stars, the source itself is in motion; instead of being red-shifted as it is reflected off something, the light is red-shifted as it leaves a moving star. Astronomers have identified a particular class of stars called Cepheid variables, which have a strong correlation between the period of variability and the luminosity of the star.

Because we know how luminous a particular Cepheid star is by measuring its period of variability, we can calculate how far away it is by its apparent brightness; the farther away the star, the dimmer it will be. Measuring the Cepheids' red shifts has shown a very tight relationship, called the Hubble Constant, between a star's distance from us and how fast it is moving away from us.

Other celestial objects, like supernovae and quasars, can also be used to calibrate distances, which allows us to impute distances to extremely remote (~10-15 billion light years) objects whose light is so red-shifted that it is nearly infrared radiation. Residual EM energy from the Big Bang is even more red-shifted; it permeates space as low-level microwave radiation called the Cosmic Microwave Background. The CMB has been measured and matches theoretical predictions of the Big Bang theory.

--B2707SST

[Edited 2004-01-22 05:10:10]
Keynes is dead and we are living in his long run.

redngold
Posts: 6686
Joined: Wed Mar 22, 2000 12:26 pm

RE: Red Shift

So, in short, "Red Shift" doesn't really tell us how far away a star is located; rather, the amount of red shift tells us the velocity at which a star is moving away from us.

redngold

[Edited 2004-01-22 05:34:19]
Up, up and away!

B2707SST
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RE: Red Shift

Technically yes; red shift measures motion. Distance is calculated by the apparent brightness of Cepheid variables, quasars, and other objects of constant luminosity. However, the Hubble Constant (the ratio describing how more distant objects recede from us more quickly) has been proven so accurately that red shift alone is also a reasonable measurement of distance.

-B2707SST
Keynes is dead and we are living in his long run.

lehpron
Topic Author
Posts: 6846
Joined: Tue Jul 10, 2001 3:42 am

RE: Red Shift

What exactly is considered accurate in space when taking the vastness of it into account?

I.e., when is the line drawn or what are the sig figs? 3 can't apply with light years and thousands of them when each is 6 trillion miles or 10 trillion km, or 10000 Terameters. Hmm, that doesn't seem like much.

10 Exameters sounds close by.
The meaning of life is curiosity; we were put on this planet to explore opportunities.

bobrayner
Posts: 2038
Joined: Sun Apr 27, 2003 8:03 am

RE: Red Shift

Distance is calculated by the apparent brightness of Cepheid variables, quasars, and other objects of constant luminosity. However, the Hubble Constant (the ratio describing how more distant objects recede from us more quickly) has been proven so accurately that red shift alone is also a reasonable measurement of distance.

I don't want to detract from your excellent explanations... but shouldn't we include parallax too? A very limited method, perhaps, but it's one of the least ambiguous ways of measuring distance.
Cunning linguist

POSITIVE RATE
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RE: Red Shift

Parallax is only useful if the object in question is located very close by- within a few light years. Anything more than that and it's inaccurate.

delta-flyer
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RE: Red Shift

....the Hubble Constant (the ratio describing how more distant objects recede from us more quickly) has been proven so accurately that red shift alone is also a reasonable measurement of distance

If this phenomenon is really true, it is a consequence of the "big bang" theory. If all matter started from a single point and radiated outward, then those pieces farthest away would be moving the fastest. But from our perspective, being in the middle of this radiating pack of matter, those pieces that are behind us are falling behind, and those ahead of us are increasing their distance. Thus, everything in the universe appears to be moving away from us.

That's why everything exhibits a red shift.

Pete
"In God we trust, everyone else bring data"

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RE: Red Shift

Thus, everything in the universe appears to be moving away from us.

Not quite everything. The Andromeda Galaxy which is located 2 million light years away is actually headed straight toward our Milky Way Galaxy at like 70 km/s and will actually collide with our galaxy in a few billion years time. It'll certainly be a sight to behold for those(if anyone?) on Earth who sees it. Our night sky will gradually get brighter and brighter with more and more stars until eventually the whole night sky will literally be filled with stars.

VirginFlyer
Posts: 5571
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RE: Red Shift

Delta-flyer - that's not really true. All it is showing is that everything is moving apart. What caused that move apart is still a matter of conjecture. While the Big Bang is regarded as the most plausible theory, it is only that, a theory. Perhaps the most difficult part to explain is the fact that, based on the best observations we have so far, the speed of expansion of the universe is increasing.

For those trying to picture an expanding universe, the best way to think of it is as a giant balloon. If you put enough air in a balloon just for it to hold its shape, and draw a number of points on it, then as you continue to blow up the balloon, the points move further apart. From any point, every other point appears to be moving away, and the further they are, the faster they are moving.

Which of course brings us to the two great unknowns:

1) As said above, what caused the expansion. Where did it come from. Indeed, where did the universe come from.

2) Where is this expansion going. Will it eventually slow down and then reverse? Will it keep accelerating forever?

In the absence of any empirical evidence, question one is probably best answered by faith (be it a belief in some form of intelligent creator, or a belief in some form of non-intelligent creation event, or a belief in some form of flatulence from another dimension - really, take your pick), and question two, given the same lack of any real evidence to show us where we are go, is probably best answered exactly the same way. Perhaps in time what we choose to believe in will be proven wrong, just as was the believe in a flat Earth, or an Earth centred universe. Until then, it is up to you to study the evidence, explore the opinions of experts, and come to your own conclusion.

V/F
It is not for him to pride himself who loveth his own country, but rather for him who loveth the whole world. The earth is but one country, and mankind its citizens. —Bahá'u'lláh

POSITIVE RATE
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RE: Red Shift

2) Where is this expansion going. Will it eventually slow down and then reverse? Will it keep accelerating forever?

That's not so hard to answer. If there is enough matter in the universe then eventually the expansion should slow and in fact start retracting toward a "Big Crunch". If there isn't sufficient matter out there for this to happen then it will continue to expand forever.

VirginFlyer
Posts: 5571
Joined: Sun Sep 10, 2000 12:27 pm

RE: Red Shift

Positive rate - yes, but lets have an answer to that one (particularly in light of the evidence which says the expansion appears to be accelerating)

V/F
It is not for him to pride himself who loveth his own country, but rather for him who loveth the whole world. The earth is but one country, and mankind its citizens. —Bahá'u'lláh

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