Thursday, March 23, 2006
Parallax: Distances of Mythical Proportions
It's been a busy month. Proposal time. This is when all the astronomers and astrophysicists madly write four page justifications for why the really expensive telescope should look at their favorite source instead of somebody else's favorite source. This particular deadline was for Chandra, the satellite X-ray telescope with the resolution of an optical telescope. What this means is that it can see details at the arcsecond level, which, because of our atmosphere, is similar to what the best telescopes on the ground can see. The atmosphere completely blocks out X-rays, so X-ray telescopes need to be put on satellites. Which means they are expensive. Which means there aren't very many of them. Right now, there are four X-ray telescopes operating in the soft X-ray band (sensitive to photons less than 10 keV). Chandra is the only one with such good resolution. It is also the first with such resolution, and there won't be another for at least 15 years. I think it cost around 4 billion dollars. Around a thousand proposals are submitted each year, all of them by teams of astrophysicists with Ph.D.s from impressive universities. Maybe 150 will be accepted. These are reviewed by panels of experts in each sub-field of X-ray astronomy. About 10 people per panel. Each panel looks at about 70 proposals, and gets to recommend about 10. Then a final panel looks at all the recommendations of from all the panels and resolves conflicts if more than one proposal wants to look at the same object. There is no room for useless proposals.
While the specifics vary, every telescope has some sort of analagous process. While for many optical telescopes run by individual universities the competition isn't so tough, any national facility is highly competitive. This means all of the radio, millimeter, X-ray, and gamma-ray telescopes. And all of the large (8 meter and larger) optical telescopes, not to mention Hubble. I've written dozens of proposals, and reviewed hundreds. Believe me, if the panel thinks that a proposal won't work, it won't get accepted.
So, the day after the deadline, I was having dinner, and the guy at the table next to me is trying to impress his date. She is nodding and smiling with that half-glazed look which means that she really isn't interested, but wants him to think she is. He asks the question "how do astronomers know how far away stars are?" I wonder if I should butt in and answer. I refrain. Then he starts answering his own question, and starts talking about parallax. Parallax is simple geometry, and the most accurate way of measuring the distance to nearby stars. It is the basis of what we call the cosmological distance scale. Parallax is what your mind uses to give you depth perception. When you close one eye, and then the other, objects near you appear to move back and forth relative to more distant objects. How much they appear to move depends an the distance between your eyes and the distance to the object. Astronomers use the motion of the Earth around the Sun for the blinking of the eyes. By taking two images at opposite ends of the Earth's orbit, they maximize how much nearby stars will move relative to more distant objects. Astronomers when they speak of distances do not use light-years, but parsecs. One parsec is the distance that an object would have that the motion of the earth would cause one arcsecond, 1/1,296,000 of a circle, of apparent motion on the sky. This is 206,265 times the diameter of the Earth's orbit, or the distance light travels in 3.26 years.
Anyway, this bozo at the table next to me, after trying to explain parallax to his bored date, then claims that it is a myth. That the parallax of stars is so small that it can't be measured, and astronomers admit this among themselves but just use this explanation in textbooks to fool the gullible into thinking astronomers know the distance to things. Then he started blatehring about the redshift of stars (apparently not even bothering to notice that redshift as a distance measure is only used when talking about light from distant galaxies, not stars in our own Galaxy). That's when I realize that he is some sort of Creationist type misusing scientific terms to sound knowledgable while he makes ad-hominem attacks on scientists, and he must have heard some fundamentalist using this to "debunk" modern cosmology.
So, just in case some twit starts blathering to you about how parallax can't be measured, let me tell you about some of the amazing measurements that are being done today. Having been on review panels where people have asked for time to make parallax measurements, I have some idea of the current state of the art (by the way, many of these proposals were accepted). To measure something 10 parsecs away, you have to accurately measure a change in angle of less than 1/10 of an arcsecond. This is hard, but with care can be done reliably and repeatedly with optical telescopes on the ground, and people have been doing it for over a century. However, even the nearest star is slightly more than 1 parsec away, and for a long time getting reliable measurements to the nearby Hyades starcluster around 150 parsecs away was a major goal of people who measured parallax. The difficulty was the above mentioned limiting resolution of about one arcsecond caused by the Earth's atmosphere for optical telescopes. The launch of Hubble finally gave us a telescope with much higher resolution. But a single, very expensive one that had lots of other things to do than make alot of parallax measurements.
Enter Hipparcos. This was an European satellite that operated between 1989 and 1993. By getting above the Earth's atmosphere, it was able to measure the parallax to about 120,000 stars, out to distances of several hundred parsecs. This dataset is now the very solid first rung of the cosmological distance ladder. Each stage of the ladder is based on the previous rung. The next stage is by comparing properties of stars in a distant cluster to ones in a nearby cluster that the distance is known to (hence the importance of the Hyades). The next after that is having enough reliable measurements to clusters that collectively contain enough of a particular type of variable star called Cepheids to calibrate the relationship between their period and luminosity. It turns out, with some massive stars in a certain stage of life, they go through a stage of heating up, which causes expansion, which causes cooling, which causes contraction, which causes heating, which causes expansion and so on and so on. The length of time it takes to go through such a cycle is correlated to the intrinsic brightness of these stars. Hence, if you can measure the period of the star, you know its intrinsic brightness. Then, by measuring how bright it appears to us, you can infer the distance. It was by using Cepheids that Edwin Hubble first measured distances to nearby galaxies and noticed that the further away the galaxy, on average the faster it seemed to be moving away from us (as inferred from the average redshift of the light coming from the galaxy). Once the Hubble relationship is accurately determined (by determining the Cepheid P-L relationship by measuring the distances to far away clusters by measuring the distance to nearby clusters by measuring the distance to stars in the cluster by using parallax) we can use a measurement of the redshift of a far-away galaxy to determine its distance.
By the way, there are a whole bunch of other ways of measuring distances to individual objects, some better, some worse. Typically, the distance to a particular object in our Galaxy can only be determined to within 50% or so. Often not better than to within a factor of 2. That's because our Galaxy is some 30,000 parsecs across, and so not many objects are amenable to classical parallax measurements.
Unless they are radio sources.
By using a combination of telescopes placed thousands of miles apart (a process called very long baseline interferometry), radio astronomers can achieve a resolution of about a thousandth of an arcsecond, and determine positions to about a tenth of that. Accurate distance measurements to pulsars up to 2000 parsecs away are routinely being made with the VLBA, a system of 10 radio antennas spread out across North America. Water masers in the circumstellar shells surrounding certin types of massive stars are also being used for parallax measurements out to a few kiloprsecs.
Parallax is a fundamental astronomical method that serves as the basis of all interstellar distance estimates. It is simple in principle, although difficult in practice. And the robust, repeatable results are almost impossible to deny. That is why those who want the Universe to be less than 6000 years old (i.e. nothing we see is more than 1800 parsecs away), hate parallax measurements. They are the basis of the evidence that the Universe is much larger than allowed by their limited mythology.
While the specifics vary, every telescope has some sort of analagous process. While for many optical telescopes run by individual universities the competition isn't so tough, any national facility is highly competitive. This means all of the radio, millimeter, X-ray, and gamma-ray telescopes. And all of the large (8 meter and larger) optical telescopes, not to mention Hubble. I've written dozens of proposals, and reviewed hundreds. Believe me, if the panel thinks that a proposal won't work, it won't get accepted.
So, the day after the deadline, I was having dinner, and the guy at the table next to me is trying to impress his date. She is nodding and smiling with that half-glazed look which means that she really isn't interested, but wants him to think she is. He asks the question "how do astronomers know how far away stars are?" I wonder if I should butt in and answer. I refrain. Then he starts answering his own question, and starts talking about parallax. Parallax is simple geometry, and the most accurate way of measuring the distance to nearby stars. It is the basis of what we call the cosmological distance scale. Parallax is what your mind uses to give you depth perception. When you close one eye, and then the other, objects near you appear to move back and forth relative to more distant objects. How much they appear to move depends an the distance between your eyes and the distance to the object. Astronomers use the motion of the Earth around the Sun for the blinking of the eyes. By taking two images at opposite ends of the Earth's orbit, they maximize how much nearby stars will move relative to more distant objects. Astronomers when they speak of distances do not use light-years, but parsecs. One parsec is the distance that an object would have that the motion of the earth would cause one arcsecond, 1/1,296,000 of a circle, of apparent motion on the sky. This is 206,265 times the diameter of the Earth's orbit, or the distance light travels in 3.26 years.
Anyway, this bozo at the table next to me, after trying to explain parallax to his bored date, then claims that it is a myth. That the parallax of stars is so small that it can't be measured, and astronomers admit this among themselves but just use this explanation in textbooks to fool the gullible into thinking astronomers know the distance to things. Then he started blatehring about the redshift of stars (apparently not even bothering to notice that redshift as a distance measure is only used when talking about light from distant galaxies, not stars in our own Galaxy). That's when I realize that he is some sort of Creationist type misusing scientific terms to sound knowledgable while he makes ad-hominem attacks on scientists, and he must have heard some fundamentalist using this to "debunk" modern cosmology.
So, just in case some twit starts blathering to you about how parallax can't be measured, let me tell you about some of the amazing measurements that are being done today. Having been on review panels where people have asked for time to make parallax measurements, I have some idea of the current state of the art (by the way, many of these proposals were accepted). To measure something 10 parsecs away, you have to accurately measure a change in angle of less than 1/10 of an arcsecond. This is hard, but with care can be done reliably and repeatedly with optical telescopes on the ground, and people have been doing it for over a century. However, even the nearest star is slightly more than 1 parsec away, and for a long time getting reliable measurements to the nearby Hyades starcluster around 150 parsecs away was a major goal of people who measured parallax. The difficulty was the above mentioned limiting resolution of about one arcsecond caused by the Earth's atmosphere for optical telescopes. The launch of Hubble finally gave us a telescope with much higher resolution. But a single, very expensive one that had lots of other things to do than make alot of parallax measurements.
Enter Hipparcos. This was an European satellite that operated between 1989 and 1993. By getting above the Earth's atmosphere, it was able to measure the parallax to about 120,000 stars, out to distances of several hundred parsecs. This dataset is now the very solid first rung of the cosmological distance ladder. Each stage of the ladder is based on the previous rung. The next stage is by comparing properties of stars in a distant cluster to ones in a nearby cluster that the distance is known to (hence the importance of the Hyades). The next after that is having enough reliable measurements to clusters that collectively contain enough of a particular type of variable star called Cepheids to calibrate the relationship between their period and luminosity. It turns out, with some massive stars in a certain stage of life, they go through a stage of heating up, which causes expansion, which causes cooling, which causes contraction, which causes heating, which causes expansion and so on and so on. The length of time it takes to go through such a cycle is correlated to the intrinsic brightness of these stars. Hence, if you can measure the period of the star, you know its intrinsic brightness. Then, by measuring how bright it appears to us, you can infer the distance. It was by using Cepheids that Edwin Hubble first measured distances to nearby galaxies and noticed that the further away the galaxy, on average the faster it seemed to be moving away from us (as inferred from the average redshift of the light coming from the galaxy). Once the Hubble relationship is accurately determined (by determining the Cepheid P-L relationship by measuring the distances to far away clusters by measuring the distance to nearby clusters by measuring the distance to stars in the cluster by using parallax) we can use a measurement of the redshift of a far-away galaxy to determine its distance.
By the way, there are a whole bunch of other ways of measuring distances to individual objects, some better, some worse. Typically, the distance to a particular object in our Galaxy can only be determined to within 50% or so. Often not better than to within a factor of 2. That's because our Galaxy is some 30,000 parsecs across, and so not many objects are amenable to classical parallax measurements.
Unless they are radio sources.
By using a combination of telescopes placed thousands of miles apart (a process called very long baseline interferometry), radio astronomers can achieve a resolution of about a thousandth of an arcsecond, and determine positions to about a tenth of that. Accurate distance measurements to pulsars up to 2000 parsecs away are routinely being made with the VLBA, a system of 10 radio antennas spread out across North America. Water masers in the circumstellar shells surrounding certin types of massive stars are also being used for parallax measurements out to a few kiloprsecs.
Parallax is a fundamental astronomical method that serves as the basis of all interstellar distance estimates. It is simple in principle, although difficult in practice. And the robust, repeatable results are almost impossible to deny. That is why those who want the Universe to be less than 6000 years old (i.e. nothing we see is more than 1800 parsecs away), hate parallax measurements. They are the basis of the evidence that the Universe is much larger than allowed by their limited mythology.
