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Posted on: June 17, 2009 - 9:23am
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I am still studying and learning about the big bang. But I have a question that I hope someone could help me with. If a star is thousands of light years away, how can we tell how far away it is. What is used to determine the distance of stars?
"Take all the heads of the people
and hang them up before the Lord
against the sun.” -- Numbers 25:4
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BigUniverse wrote,
"Well the things that happen less often are more likely to be the result of the supper natural. A thing like loosing my keys in the morning is not likely supper natural, but finding a thousand dollars or meeting a celebrity might be."
Not quite, that measures velocity, specifically, how fast it is moving away from us.
To measure Distance, we generally use Paralax, which is pretty much just big ass triangulation using the Earth's Orbit as the base line
Or at least that is what I remember
A rough estimate of distance can also be obtained by plotting the star's observed luminosity vs its expected absolute luminosity based on color and using the difference between the two to determine how mcuh the star's light has difused. Again, if I remember correctly, which I wouldn't expect.
You would probably be better off looking here
or googling 'determine distance astronomical objects'.
When you say it like that you make it sound so Sinister...
I'm not mathematician and I'm certainly no astrophysicist, but if you can tell the redshift of an object can't you extrapolate its distance relative to us because the redshift is relative to us? (That's right, right? That the closer you are to an object that appears extremely redshifted from Earth's perspective, the less redshifted it will be relative to the closer vantage point?)
There's also this article.
BigUniverse wrote,
"Well the things that happen less often are more likely to be the result of the supper natural. A thing like loosing my keys in the morning is not likely supper natural, but finding a thousand dollars or meeting a celebrity might be."
ok, I understand the triangulation.
There's lots of ways, but the preferred method has always been parallax. Redshifting works, gravitational lensing, you can observe the spectra through a telescope which will tell you the degree of ionization, which gives you the temperature, which gives you the distance. You can measure the magnitude of a supernova to create distance spots, you can measure the luminosity, the HI-line, fundamental plane theorm (sort of like parallax but the three points to be plotted are luminosity, dispersion, and the radius). The newest method is called light echos which is when they basically echo off of nearby nebula (I believe) It all depends on what type of galaxy the star is in and the conditions you're working with (like whether you're waiting for a cosmological event to occur).
There are several ways that astronomists use to determine distance. I think they started with parallax which works well on nearer objects. But the further and further away from Earth an object is the more they use other methods which are more accurate. Universe published by DK is a really good book. If you buy it from Amazon, consider doing it through RRS's Amazon portal so they can get a tiny kick back to help keep thier site and work going.
Respectfully,
Lenny
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Madmen fed on fear and lies, To beat and burn and kill"
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A very useful method of estimating distance to other galaxies is to look for a particular type of star called a 'Cepheid variable', which pulses in brightness in a very regular way. From observing close examples of such stars, whose distance can be estimated by other means, we have found that there is a very close relation between the rate of pulsation and their inherent brightness. So by measuring the pulsation frequency, we can make a good estimate of how inherently bright they are, and comparing this to their apparent brightness, we can work out how far away they are, and therefore the distance to other stars that seem to be in the same group, cluster or galaxy.
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Actually, I have been working on a fairly long post on this very subject to put in the blogging category here. As it is not ready to go, let me add a bit here anyway.
First off, the triangulation that you already know about is accurate far enough out to include quite a large number of stars. From that, we are able to work out the general properties of various different type of stars, including how bright they would be at a standard distance.
From that, we can look at many stars that are too far away to use triangulation as a reliable method of distance measuring and compare them to stars for which we do know the absolute brightness of. From that, we estimate the distance.
Further out, we need other ways of measurement called “standard candles”. Each of the various standard candles are useful at different ranges of distance.
That being said, BobSpence1 is slightly off with his use of cephid variable stars. The technical description is accurate and valid but he is using it in the wrong range. We use cephid variables to estimate the distance to globular clusters, which are sort of like tiny galaxies comprising several thousand closely spaced stars but embedded within our own (and presumably every other substantial) galaxy.
To measure the distance of galaxies, there are two standard candles.
For the nearest several thousand galaxies, we look for what is known as a type 1a supernova. The properties of type 1a supernova are very well worked out and they have a standard brightness that never changes. When one happens, for a couple of weeks, it will be brighter than the galaxy in which it is embedded.
Red shift is the most reliable way to measure galaxies that are so far off that they are notably not affected by the gravitation of the galaxies in our immediate area. Thus the expansion of the universe is the dominant effect on their motion away from us.
So different methods are used for different ranges of distance. Each is valid but critically, the ranges at which each one may be usefully employed has a large overlap with the next range, so we have a high degree of confidence in each of them.
For example, there are enough cephid variables which are close enough to use parallax to confirm what we know about them. Some of the closest galaxies have cephid variables that we can monitor (so yes, bob is technically correct for a few galaxies). Those galaxies also have occasional type 1a supernovae that confirm the cephid variable measurements. Finally, there are no small number of galaxies far enough away to measure the red shift that have type 1a supernovae.
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