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Night Watch

Night Watch


Mankind has always been fascinated by the night sky. Indeed it seems that some of the earliest recorded observations from nature were astronomical. For example, in the mid nineteenth century, English archaeologists uncovered a huge library of clay tablets in the palace of Assyrian Emperor Sennecherib of Nineveh, who lived about seven hundred years B.C. Among other clay tablets were detailed records concerning the planet Venus. It’s interesting that the pattern of appearances and absences from the sky is different from what we see today. It’s hard to say if the observations were accurate, but we do know that many of these ancients took their studies very seriously. The wise men in the gospels are a good example. Moreover an earlier document, the book of Job, mentions constellations such as the Pleiades.

Even today the Pleiades and the Hyades (open star clusters in the constellation Taurus or the Bull) are interesting to astronomers. The Hyades are considered to be the nearest moderately rich star cluster. As a result, these stars have been assigned a central role in calibrating a measuring stick in space. The procedure has been to compare objects of unknown distance with an object of known distance. By means of mathematical equations the unknown distance can generally be calculated. For example, if star A is a known distance away, then it is easy to calculate how far it is to an equally energetic star that appears to be dimmer. The distance to the latter star is proportional to the reduced light that we see from that star. The problem is however that astronomers do not actually know how energetic (bright) a star is, if they do not know its distance.

It is obvious that we must know the distance to a source of light before that we can estimate how energetically that body is releasing light or in other words how bright it actually is. For example, a flashlight, an airplane and a star may all look equally bright to an observer. If they were all located an equal distance away from the observer however, dramatic differences would be apparent. Indeed, as astronomer Michael Perryman remarks: “Almost everything in astronomy depends in some way on knowing star distances. This is particularly true of the cosmic distance scale extending out to the farthest galaxies and quasars. And the cosmic distance scale determines how well we know the true sizes, brightness, and energy outputs of nearly everything in the universe.” (Sky and Telescope June 1999 p. 42) In view of the importance of an accurate measuring stick, one would hope that astronomers have based their calculations on very reliable numbers for the distance to the Hyades, their base point. This however has not been the case. As Dr. Perryman confides: “Many creative methods have been brought to bear on the Hyades distance problem over the last 100 years – with tantalizingly discordant results. This has been quite frustrating for a cluster so close.” (p. 45) Yet William J. Kaufmann II wrote in the 1994 edition of his text: “Because the distance to the Hyades cluster is the most accurately determined of all stellar distances, it provides the basis upon which all other astronomical distances are determined.” (Universe, Fourth Edition, p. 341) Dr. Kaufmann felt complacent enough, at the time, to assure us concerning the state of astronomy, that “In recent years, a remarkably complete picture has emerged, offering insight into our relationship with the universe as a whole and our place in the cosmic scope of space and time” (p. 337). Perhaps Dr. Kaufmann should have been more cautious.

The complacency of astronomers has however been somewhat shaken by data released in 1997. In 1989 the European Space Agency (ESA) had launched a satellite called Hipparcos (an acronym for High Precision Parallax Collecting Satellite). This device was designed to use trigonometry to directly measure distances to the closest stars. The data have proved very interesting and there have been plenty of surprises. The good news is that accurate distances (to within ten percent of the true value) have been achieved for more than 22,000 stars. Previously, such results were possible only for several hundred stars. These stars all lie within three hundred light years of Earth. Another 30,000 stars have been measured to within twenty percent of their true value. Such numbers represent a cornucopia of information.

The measurements made by the Hipparcos satellite are based on trigonometry. Just as it is possible to measure distances on earth by means of imaginary triangles, astronomers achieve similar results in space. Their triangle needs a very long base so that the angles at the corners will be large enough to measure. The base of the triangle is taken to be the diameter of Earth’s orbit at its maximum extent. Since the orbit is an ellipse, the diameter changes throughout the year. We use the maximum distance. The astronomer photographs a star on two occasions, six months apart. In this way, observations are made from opposite sides of Earth’s orbit. The angles of the triangle are then calculated by comparing the star’s shift in position compared to a backdrop of more distant stars. The length of one side of the triangle is the distance from Earth to the star. Prior to Hipparcos, astronomers were able to obtain good results only up to 65 light-years away. The closest one is Alpha Centauri, a mere 4.3 light-years from us. Now however with the European satellite, accurate measurements of much smaller angles are possible. This has astronomically expanded the number of accurately measured objects in the sky. The time had now come to compare previous estimates with the new numbers.

Hipparcos was the first space mission specifically designed to measure star positions. Data were collected for four years. This was followed by a further three years in which the results were analyzed. Among unexpected findings, two hundred relatively close but dim stars were discovered. In addition, many well known stars turned out to be much farther away (and thus more energetic) than previously believed. As a result, fewer nearby stars could be identified as “main-sequence stars” and there were only half as many giants as previously estimated. “Main-sequence stars” are identified according to the Hertzsprung-Russell diagram which plots stars’ rate of light production against their temperature. Temperature is estimated from colour but estimates of light or energy production are highly dependent on distance. The significance of the Hertzsprung-Russell sequence is that it has traditionally been interpreted as reflecting the evolution of stars. In the light of Hipparcos data, however, astronomers have come to suspect that their previous conclusions were “too simplistic. Something else sees to be going on” (Perryman p. 47). Particularly surprising are the values obtained for the Pleiades cluster. At 375 light-years, this group of stars seems to be located 15% closer than previous estimates. The result, says Dr. Perryman, is that the stars in the Pleiades cluster can “no longer easily be accommodated into existing pictures of star formation or evolution” (p. 47). In other words these stars no longer qualify as main sequence stars. The Hyades, on the other hand, were located considerably further away than expected.

Many people may wonder why we should care about the Hipparcos data. The point is that these numbers are reliable because the calculations include only values which are established by direct observation. Beyond 200 or 300 light-years however, nearly all other measuring techniques are indirect. Consequently the calculated results are only as dependable as the assumptions upon which they are based. This situation can have far reaching consequences. Particularly in astronomy, small numbers can be extrapolated into huge conclusions. This does not mean that we should ignore astronomy. It merely means that we should be aware of the uncertainties.

Margaret Helder
December 1999

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