HOW DO WE KNOW?

Wanganui Chronicle, 11 May 1948, Page 6

HOW DO WE KNOW?

(By

‘Sky Pilot”)

These distances are measured in terms of the “astronomical unit”—the distance of the earth from the sun. This we have to determine from the earth. We have to determine the diameter of the earth. This is worked out by geometry from the surveyed measurements of the earth's size and shape. Using this as a baseline, the distance to the sun is then figured out by triangulation in a similar way to that employed by a surveyor in measuring distances on the earth's surface. Since the sun's brilliance along with other factors prevent accurate results by this method, another is now used. One of the tiny members of the sun’s family approaches very near to the earth. It is easy to measure by triangulation the distance of this star-like point of light in miles, and then to find how much farther away from the sun it is than the earth. It is then an easy matter to find the earth’s distance from the sun in miles. The speed the earth travels in its orbit is reckoned from the observation of light. With this knowlege in hand, and the time it takes to complete its revolution it is possible to work out its orbit, and from this the radius of its orbit, that is, the distance of the earth from the sun. With the astronomical unit in hand it is possible by very careful working, to determine in terms of that unit all other distances in the solar system. STAR DISTANCES. The method of triangulation was tried for the distance of a star too, but no baseline on the earth was long enough. The diameter of the earth’s orbit was therefore used. The star, 61 Cygni in the constellation of Cygnus, was the star chosen for measuring the distance. Observations were taken one night and six months later were taken again, when the earth was on the other side of its orbit—that is, at the opposite end of a baseline 186 million miles long. "The apparent shift of the star on the celestial sphere, caused by the change in the earth’s position, was measured, the mathematical triangle drawn and the distance calculated.” Bessil’s original measurement in 1838 has agreed pretty well with measurements made later with more accurate instruments. The star was observed to have a paraller which is the term used for the apparent shift, of .30 seconds of arc which means that its distance in miles is 60,000,000,000,000 which does not convey much meanting to us other that the fact that it seems a long, long way away. And so the earthly unit of the mile is useless for such vast measurements, and “the astronominal unit” of 93,000,000 miles is also too small. Astronomers therefore turned to the “light year” (light travels at the rate of 186,000 miles per second —the light year is 6 million million miles) as the standard. With this standard we can express the distance of 61 Cygni in understanding terms: 10.9 light years which means that the light we see now left that star that many years ago. .Even this unit has not proved satisfactory and so a new one is used, the parole which is 3.258 light years and is some 18,000,000,000,000 miles. REACHED ITS LIMIT

But this trigonometrical method quickly reached its limit. Further soundings of the depths of space must be made in some other way. This is done from a consideration of the magnitude of a star is the result of two things the real or intrinsic brightness and its distant from us. The term absolute magnitude is used to denote its intrinsic brightness. The absolute magnitude of a star is graded by what its apparent magnitude would be at the distance of 10 parsecs. The sun’s absolute magnitude on this grading is + 4.7 which means that if it were removed to the distance of 10 parsecs then it would appear as a star of about fifth magnitude. The intensity of a stars spectrum lines has a close relation, too, to its absolute magnitude, so close that it is now used as "a measure of absolute magnitude.” So the spectrum of a star indicates really how luminous it is. Once the absolute and apparent magnitude of a star are known the way is clear to figure out its distance. I hope this is not to technical for you to grasp but it is about as simple as it can be put and helps the reader to understand how we know the distance of very, very distant stars and that quite accurately. It is not guess work.

To arrive at the very great distances of star clusters and the spiral nebulae the relation of absolute and apparent magnitudes is also used but differently. Miss Leanith of Harvard, some years ago abserved that Cepheid Variable stars with shorter periods of light variation were the fainter and those with longer periods were the brighter. These differences in magnitude it was discovered are absolute as well as apparent, "that is a Cepheid with a period of five days is actually intrinsically brighter than one with a period of one day.” When Cepheids are discovered in clusters and nebulae "the period of the star tells us how bright it really is. We compare the light we receive from it with the amount that would reach us if the star were at a known measured distance, and an easy problem gives the real distance. It is like measuring how much farther away one lamp is than another if both are known to give exactly the same amount of light. The Cepheid Variable is our “standard candle.” Olcott and Pulnam. From what has been said it is clear that measuring the distances of the stars is both intricate and fascinating. It is really marvellous how the mind of man has applied itself to this exacting task and the result is not without a great thrill. Mathematics has served him well in the venture. For those who do not have to do the job it is a great adventure which deserves our fullest appreciation because it has depthed the universe for us, given us a grand conception of the vastness of space in which the great Suns find their place and in which they have ample scope to move as is so characteristic of our > own sun.