Measuring Distances to Stars and Galaxies

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I was looking in the exoplanet catalog yesterday at one of 373 listed exoplanets. What surprised me most was that the distance to its star was listed as 200 pc +/- 100 pc. Wow, that’s a very uncertain number. I guess the primary distance measuring method for stars is parallax. Our baseline for measuring is ~2 AU or 186 million miles. That seems harder than just measuring a simple change in angle. We’re measuring from a spinning Earth, so you have to take the measurement at the exact second. Making it even trickier is that a star that appears at night will be in daylight 6 months later when earth is on the other side of its orbit. Tricky calculations indeed!

Turning to galaxies, I thought about what I’ve heard as the distance to Andromeda. Estimations range from 2,000,000 LY to 3,000,000 LY, but mostly around 2.5 million LY. Still, not too specific. I suppose distances to galaxies are estimated using two methods. First, there’s the red shift, but galaxies tend to jostle around a bit and so nearby galaxies like those in the local group … red shift doesn’t have much value. At greater distances, like more than 1 billion LY, expansion would cancel out local jostling. Andromeda in fact is getting closer, so its blue shift doesn’t lend itself to that method. Another method is “standard candles” (Type II-plateau supernovae) but since a galaxy might only have one supernova in 100 years (and not necessarily a Type II-plateau supernova, it too doesn’t seem very useful in measuring distance to a particular galaxy.

My questions: are these methods (parallax, red shift, and standard candle) the only methods at our disposal; and are the examples above really the best we can do in pinning down large astronomical distances? Exactly how consistent is the brightness of Type II-plateau supernovae?


I thought is was type 1 novae that are used as standard candles, but I need to check.

In any case multiple methods are used (the redshift/blueshift is not one used to determine distance, but rather relative motion)

Parallax is not useful for galaxies.

I'll have to do kore research before answering in detail.


Yep, it's Type IA supernova that are used as standard candles.

Parallax is only useful for more "local" stars in our own galaxy.

Cepheid Variables, a pulsating star, are also used as standard candles for objects that are further. The relationship between it's pulsation period and it's luminosity is fairly precise. Polaris is a Cepheid Variable.

Type Ia standard candles are "calibrated" off of Cepheid Variables, and used for objects that can't be measured with Cepheid Variables. So, whatever uncertainty was in the Cepheid Variables measurement gets propagated to the Type Ia measurements.

Astronomy magazine just had some nice articles on Cepheids and standard candles and their uncertainties. I think it was the August issue, but not sure.


I'm not sure about this exact star - if it were at 200 pc, the parallax would be pretty darn small and, say, a ground-based parallax might produce such a large error. It's also possible that the star's distance was derived using what we call spectrophometric methods. Each star has a spectral type, which we can determine by looking at the spectrum of light it emits, and this corresponds roughly to its temperature (since hotter stars will emit more blue light, and vice versa). This in turn can (with some assumptions, namely that the star is on the main sequence) give you an estimate of its luminosity. Now, if we know how bright it is, and how bright it looks on Earth, we can figure out its distance pretty easily. It works, but there's an awful lot of intermediate steps, and it's conceivable that such errors could show up.


ramparts":3v4iapu7 said:
It works, but there's an awful lot of intermediate steps, and it's conceivable that such errors could show up.

Here is the basic principle; let's assume we have two stars, x and z, with similar measured and determined luminosities.

Star x is measured and determined to be 400 times brighter than star z. Parallax P for star x is measured and determined to be 0.125''. Hence we know that

Luminosity Lx = Lz
Intensity Fx = 400*Fz
Intensity for Fx = Lx / 4pi*r^2, where r=x
Intensity for Fz = Lz / 4pi*r^2, where r=z
Parallax for Px = 0.125''

We compute the distance for star x in parsecs, pcx = 1 / 0.125'' = 8 parsecs.

Distance for star z in parsecs,

Lx / 4pi*r^2 (where r=x) = 400*Lz / 4pi*r^2 (where r=z) =

pcz^2 = 400*Lx/Lz*pcx^2

pcz = 20pcx

Distance for star z is 20*8 parsecs = 160 pc.

As you can see, even a slight error in determining star's luminosity may yield a result in distance that is quite a bit off.


There have been recent papers (in Science or Nature, can't recall which I read last night) that some ancient galaxies are much more compact than expected. Still digesting the info (The first read through is never enough for me :) )


i think we can draw vector lines for distance to a far galaxy. Accurately.


What do you mean by "vector lines" Can you please use the language of physics????


Yes there are different but interdependent methods of measuring astronomical distances. Parallax, spectral parallax (using the know luminosity of stars via the HR diagram), Chepied variables, type I supernova, and the Hubble law, are the most commonly used. Have a search under "cosmic distance ladder" . There is a reasonable wikipedia entry and a PDF slide show by Terrance Tao (it's historically based).

200 parsecs is near the limit of the parallax method. The accuracy drops off with distance, and brightness of the star. A faint red dwarf is harder to measure than a brighter solar type star at the same distance. Check out the Hipparcos satellite mission web-page for details about the best parallax measurements. The GAIA mission will do a lot better. Most of the distances in the exoplanet encyclopedia come from Hipparcos.
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