Stellar Distances

Vekkypoo's bud writes: "Researchers at the Palomar Observatory have measured the most accurate value of a stellar distance to date. Using the Palomar Testbed Interferometer, the distance to the star Zeta Geminorum in the constellation Gemini is 1100 lightyears. That's accurate to about 13%. While that seems like a large error margin, it's actually three times more accurate than the best previous measurement. Check it out here."

Comments

Still need better

[Sea]

OK, the've gotten the error smaller, but at those figures it's *still* 143 lightyears!! I don't want to have to be the one that tells spaceship crews "You'll be there is 1100 years, give or take 140."

--Maarken

Re:Still need better

[markbark]

143 light years is approx. 1.352 x 10^15 kilometres

To paraphrase an old senator: "A billion miles here, a billion miles there... pretty soon, it adds up to REAL distances"

Re:How does this affect the Hubble Constant?

[efuseekay]

Hipparcos has measured parallaxes to the nearest cepheids at about 100pc.

Re:How does this affect the Hubble Constant?

[mu_cow]

Firstly, just to be picky, this was about a Cepheid variable star. It is too far away to measure by parallax (which only works out to about 50 parsecs).

As to your main point, I think it is just a matter of standard error statistics. If you take n independant samples of something, then you reduce the standard error by root(n). The Hubble constant is derived from the values taken from many galaxies, so its value is more precisely known than the distance of any one star.

This point is illustrated well on this image. As you can see if you pick one star/galaxy at random you could get a value for the hubble constant in a range of about 50-100. However, just looking at the graph, it is fairly clear that if H0 takes just one value, it would be sensible to give it a smaller range than that.

Wait a second.

[empesey]

So, if I read this article correctly, they're saying that all the distances they spout off are just estimations? I'm not talking about estimations like 96 million miles (no one believes the sun is exactly 96 million miles away, but apparently they could be way off on some of these estimations.

On the other hand, astrologers will have to go back and redo their all those horoscopes.

--

Re:Wait a second.

[mu_cow]

Anyone who has done a physics course will know that measured physical quantities always come with an uncertainty attached. It just so happens that the uncertainties with these measurements are very large. Proper papers will always quote those uncertainties. By the time it filters to the general press they have probably been stripped as 'the public wouldn't understand'.

Unless you have a particular interest in the subject, the important thing is to look at the order of magnitude of the result. They are able to say that the star is nearer 1,000 ly than 10,000 ly. That is still very useful. Do you think an engineer can tell you exactly what his bridge can hold? Of course he can't, but he'll estimate, as you put it, a capacity from an approximate model. Then you'll know you can put 100 cars on it, but not 1000.

As for you second point, horoscopes are based on positions of constellations at certain times; the distance of the stars from the earth is irrelevant. Incidentally, the constellations are no longer in the same positions in the sky as they were when the star signs were invented. But I won't start an astrology rant...

How does this affect the Hubble Constant?

[A nonymous Coward]

I follow astronomy just enough to know that the accepted Hubble constant range has been narrowing from 50-100 down to, say, 70-80. I had assumed that parallax measurements were similarly close, so this surprises me. How can measurements off by 39% yield Hubble constants closer than that? Or have I been misunderstanding all this time :-)

--

Re:Not quite that simple...

[efuseekay]

You mus tbe refering to Cepheids Pop I and II.

Which is confusing terminology, so people call the shorter period Cepheid (Pop II I think) W Virginis stars instead.

It is believed (note the disclaimer), that Long-Period cepheids pulsates at the fundamental mode, because of the location of the "unstable" layer (more jargon : partial ionization layers) deep inside. Shorter-period WW Virg stars are believed to pulsate at higher overtones. The location of the unstable layer depends on the mass/luminosity of the star. Cepheids are more massive which translate (after lots of maths) into longer period etc.

Yes, astronomers are REALLY lousy at giving names.

The article I think refer to the "Classical" (long period") Cepheids, which are brighter than the W Virg types.

Getting

Correction

[mu_cow]

"Researchers at the Palomar Observatory have measured the most accurate value of a stellar distance to date"

This is not true; as the article says, they found the most accurate value of a stellar distance for a Cepheid variable. To do this they had to use clever interferometric techniques. However, a lot of closer stars can be measured more accurately using a parallax method. This involves observing the difference in position of the star over 6 month interval.

Not quite that simple...

[tesserae]

From the article:

Cepheid variables are stars that have very predictable relationships between their absolute brightness and the frequency with which they brighten up. A Cepheid is useful for measuring distances because, if it is known how bright the star really is, then it is a simple task to measure how bright it appears on Earth and then calculate the distance.

While this is mostly true, there's not simply a "very predictable relationship" between luminosity and distance -- as demonstrated by new information on Polaris (the North Star, a Ceppheid variable). In particular, check this graphic, which shows two distinct period-luminosity relationships (differing in the zero-point offset), depending on whether the Cepheid is a fundamental-mode or first overtone pulsator (and note that there's arguably some evidence of other pulsation modes in the clustering of points off the main trends, in the lower right-hand corner of the plot).

Just knowing the brightness of the Cepheid variable won't tell you how far away it is; you also have to know the pulsation mode, which isn't always easy. But then, astronomy hasn't ever been really easy... :) This data on Polaris comes from ground-based interferometry, by the way -- just a different instrument than that mentioned in the article -- as well as measurements from the Hipparcos satellite.

[I hope everyone can get to those URLs without having to pay -- I can't check that from here, 'cause I'm already logged into Science Online by default. Some info's available to everyone, some requires a subscription.]

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