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Rastermon2
2005-Jun-06, 05:19 PM
I've been asked by a couple of people - how to we *really* know that this nebula is x lightyears away, or that galaxy is a billion ly away?
From what I understand, its calculated by the brightness of type 1a supernovae - they always shine at certain brightness, and knowing that light's brightness is the inverse-square of the distance. I also thought that EVRYTHING in the universe is moving away from us. but then I read (I think on this site) that some nebula are moving towards us due to gravity. that was new to me.
Is red-shift really an accurate indication of distance, if some object are not receeding?

umop ap!sdn
2005-Jun-06, 05:58 PM
For galaxies, we have Cepheid variables: their brightness is related to their variability period, so by measuring the period and the observed magnitude of a Cepheid in a nother galaxy we can estimate the galaxy's distance. Nebulae exist within our own galaxy; I would imagine parallax would be useful here just as with relatively nearby stars. Redshift works for galaxies that are too far away for other methods of estimating distance.

Saluki
2005-Jun-06, 06:02 PM
I've been asked by a couple of people - how to we *really* know that this nebula is x lightyears away, or that galaxy is a billion ly away?

It all depends on the distance. Some objects are close enough that we can take direct measurements of the "parallax". From there, it is a simple matter of geometry. For more distant objects, other methods need to be used. Examples include cephid variables (luminosity is a function of period), redshift (distance proportional to shift in frequency), etc.

From what I understand, its calculated by the brightness of type 1a supernovae - they always shine at certain brightness, and knowing that light's brightness is the inverse-square of the distance.

This is another example. Usually, astronomers try to use at least two independent methods. If both methods give roughly the same answer, then we can be fairly certain of the results.

I also thought that EVRYTHING in the universe is moving away from us.

No. The universe is expanding, but this is not equivilant to saying that everything is moving away from us.

but then I read (I think on this site) that some nebula are moving towards us due to gravity. that was new to me.

Everything is moving in its own gravitationally-determined orbit. In some cases, this orbit is in our direction, in other cases, it is away from us. This motion is seperate from the general expansion of the universe.

Is red-shift really an accurate indication of distance, if some object are not receeding?

It is only a rough estimate at best because of the uncertainty of the Hubble constant. Howver, as distance increases, the effect on redshift of this relative motion decreases wrt the relative expansion of the universe. So, at great distances, the only significant uncertainty is the Hubble constant.

Hamlet
2005-Jun-06, 06:04 PM
I've been asked by a couple of people - how to we *really* know that this nebula is x lightyears away, or that galaxy is a billion ly away?
From what I understand, its calculated by the brightness of type 1a supernovae - they always shine at certain brightness, and knowing that light's brightness is the inverse-square of the distance. I also thought that EVRYTHING in the universe is moving away from us. but then I read (I think on this site) that some nebula are moving towards us due to gravity. that was new to me.
Is red-shift really an accurate indication of distance, if some object are not receeding?

One of the oldest ways to measure stellar distances is using parallax (http://en.wikipedia.org/wiki/Parallax). The disadavantage with parallax is that we are only able to see changes in the positions of stars that are relatively close to us. The Hipparcos (http://en.wikipedia.org/wiki/Hipparcos) probe was developed to measure these distances more accurately.

Type Ia supernovae have been used as to calibrate distances since it is thought that they all explode with the same intrinsic brightness. You can then use the inverse-square law (http://en.wikipedia.org/wiki/Inverse_square_law) for luminosity to calculate the distance.

Red-shift can be used for distance, but it relies on having a good value for the Hubble parameter (http://en.wikipedia.org/wiki/Hubble_constant). As our ability to measure the Hubble parameter gets better, our distance calculations based on red-shift will get better.

There are some objects, like the Andromeda galaxy, with which we are gravitationally bound and moving toward. For the most part though, everything is moving away from us and we are moving away from them ( or more correctly the space between us and everything else is expanding).

Edited for clarity.

jfribrg
2005-Jun-06, 06:14 PM
EVRYTHING in the universe is moving away from us.

True, the universe is expanding and the farther away an object is the faster it is receding, but the expansion of the universe is not the only factor. As others have said, the gravitational pull of nearby galaxies can cause them to move closer to each other. The farther away a galaxy is, the more the expansion of the universe dominates the relative motion of the two galaxies, so that all galaxies except those within say 100 million light years are receding from us.

2005-Jun-06, 09:57 PM
We have various means of determining how many light years away some object is from us. At the base of the system is radar, effective out to about 100 astronomical units. We time how long it takes a light beam to bounce off a nearby planet, giving us the distance to it. Using Keplar's laws we can then determine our distance from the Sun.

Knowing the distance to the Sun allows us to use a paralax measurement for the nearby stars. We check the position of a star today, and then again six months from now. This gives us two observations that are 300 million kilometers apart, the distance across Earth's orbit around the Sun. It's like looking at your finger in front of your nose from first one eye and then the other. The finger looks like it shifts side to side slightly, depending on which eye you're using. In the same way, the nearby stars appear to shift side to side slightly after six months. We measure how much they appear to shift and claculate how far away they are.

There are about two dozen more ways of determining how far away stars are. Many of these methods make use of the color of a star. We know that the the color of a star determines it's intrinsic brightness. We then measure how bright it appears, and apply the inverse square law to say how far away it is.

These various methods overlap. Usually any particular object will have more than one method available to determine its distance. If you measure the distance with more than one method, and get the same number, then you have a high confidence that this is a good number.

Rastermon2
2005-Jun-06, 11:45 PM

peter eldergill
2005-Jun-06, 11:53 PM
For galaxies, we have Cepheid variables

How can you tell that there is a cepheid variable in a galaxy really far away. One star about of billions in a structure millions of light years away...I smell conspiracy! Ha! Caught you guys in the act again!!!!!!! (exclamation points to emphasize that I must be right)

Seriously. HOw can it be done?

Later

Pete

PS I understand the comparison of the cepheids..at least I used to !

jfribrg
2005-Jun-07, 01:50 AM
As I understand it, the cephids are used to calibrate the distance to nearby galaxies. After that you can make some assumptions about the luminosity of galaxies. If you are lucky enough to observe a type 1a supernova in the galaxy that you are interested in, you can use that to measure the distance. Once you get far enough away, you can use the redshift to measure the distance. All of these involve assumptions, so the margin of error increases as the distance increases. As has been mentioned, sometimes there is more than one way to measure the distance, and this has been used to calibrate the different measurements. In the end, you simply have to state your measurements and calculate the margin of error, although the margin of error is rarely mentioned in the mainstream press. M31 for instance is always reported as being 2 million light years away, although the margin of error for this is probably in the range of +- 250000 light years in addition to the fact that it is a 3 dimensional spheriod 100000 to 200000 ly in diameter.

AGN Fuel
2005-Jun-07, 02:05 AM
In the end, you simply have to state your measurements and calculate the margin of error, although the margin of error is rarely mentioned in the mainstream press. M31 for instance is always reported as being 2 million light years away, although the margin of error for this is probably in the range of +- 250000 light years in addition to the fact that it is a 3 dimensional spheriod 100000 to 200000 ly in diameter.

Not according to this article (http://smh.com.au/articles/2005/06/06/1117910243497.html?oneclick=true) in the SMH today! (my emphasis)

Geraint Lewis, of the University of Sydney, said his team used optical telescopes to photographically survey Andromeda, a galaxy of about 200 billion stars 2 billion light years away.

umop ap!sdn
2005-Jun-07, 05:18 AM
That's gotta be either a typo or a locale specific difference in the usage. I'm pretty sure M31 is 2.3 x 10^6 light years away, but I don't know the exact figure for a billion in Australia. In America it's 10^9; in Britain it's 10^12 - but I can't see any way that 10^6 would be a billion.

Must be a typo then. :-?

AGN Fuel
2005-Jun-07, 05:39 AM
That's gotta be either a typo or a locale specific difference in the usage. I'm pretty sure M31 is 2.3 x 10^6 light years away, but I don't know the exact figure for a billion in Australia. In America it's 10^9; in Britain it's 10^12 - but I can't see any way that 10^6 would be a billion.

Must be a typo then. :-?

Yeah, it's a typo. Although Australians used to use the British 10^12, the American 10^9 is more generally used today (I suspect our press barons Rupert &amp; Kerry Packer looked at the 3 extra zeroes required before their ascension to billionairehood &amp; thought "stuff that, Jack").

In astronomy terms, the a billion is 10^9.

TriangleMan
2005-Jun-07, 11:19 AM
Has anyone posted a link to The ABC's of Distances (http://www.astro.ucla.edu/~wright/distance.htm) by Ned Wright? It has 26 methods used for determining interstellar distances.

die Nullte
2005-Jun-07, 03:33 PM
Rastermon2: You might want to read the book Coming of Age in the Milky Way by Timothy Ferris. Each step in the cosmological distance ladder is built on an earlier step, e.g. size of the Earth, then distance to the Moon, then distance to the Sun, etc., and it's helpful to understand this.

John Kierein
2005-Jun-07, 03:51 PM
Here's another way.

http://www.spaceref.com/news/viewpr.html?pid=520