Starry Skies Above Santa Monica
April 26 - May 2, 2000

Mirek Plavec

Emeritus Professor of Astronomy, UCLA

Meet Another Zodiacal Constellation: Virgo

   During this week, the Moon will be rising very late: the Last Quarter comes on April 26, and after that, the Moon will be an ever-shrinking crescent shining in the morning hours. Thus, if no more rain comes, this is a good time to look at some more constellations of the evening sky. 
   I hope that some of you found the Lion I described in the previous issue. If you go from Leo to the south-east, you will find Virgo. There is also a celestial round-trip you can take, if you start with the Big Dipper in the north-east. If you extend the handle southward, you will find a bright orange star called Arcturus. Our chart projects the celestial sphere onto a plane, and distorts the directions somewhat. Actually, in the sky, the direction from the Big Dipper to Arcturus could be described as a “straight line”, in reality, a segment of a great circle. And if you continue past Arcturus to the south, you will find another first-magnitude star, Spica – and you can then locate the entire constellation of Virgo, between Spica and the Lion.
   Arcturus is the brightest star of the constellation Bootes, which could translated as “The Herdsman”. Actually, the constellation deserves the name The Big Celestial Question Mark!
   Similarly, I would rename Virgo into “The Athlete”, since I see in it a girl athlete, running to the southeast, and perhaps jumping a hurdle. In doing so, I would avoid the sexual connotation that the official constellation name carries, and which was really absent when the name of the constellation took roots. This large (and, under dark sky, fairly conspicuous) constellation rises before the Sun in the morning sky in the early fall. It was the time of vintage, and young girls were much more skillful in picking the grapes. To make the work less monotonous, they used to sing. The sun sign of Virgo is usually associated with art, beauty, and diligent work: here is the origin of that association!

Another Anniversary…

   80 years ago, on April 26, 1920, a famous cosmological debate took place before National Academy of Sciences in Washington, DC. Two prominent astronomers proposed and defended their general picture of the universe: Harlow Shapley, then an astronomer at the Mount Wilson Observatory (above Pasadena), and Heber Curtis, who was director of the Allegheny Observatory near Pittsburgh.
   By that time, it was well known that we live in a very large stellar system, called the Milky Way Galaxy. (The word “galaxy” is derived from the Greek name for the Milky Way, so that “Milky Way Galaxy” may be read to mean “Milky Way Milky Way”). We usually simply write only Galaxy with a capital G when we mean our home stellar system. 
   Actually, I have just now indicated the main point of controversy. With a good amateur telescope, and even more with the largest professional telescopes, we observe many faint objects that had been called “spiral nebulae” by their early observers. The question was: are they huge (and therefore distant) stellar systems quite similar to our Galaxy, or are they much more modest subsystems located somewhere at the outskirts of the Galaxy? 
   Shapley argued that the Milky Way Galaxy is very large and fills the entire universe. Curtis was of the opinion that our Galaxy is not as large as Shapley believed, and that the universe is full of large stellar systems essentially similar to our Galaxy.
   The telescopes available in 1920 did not allow the astronomers to be sure if those “nebulae” consisted of stars, and there also was no safe way to determine their distances and sizes. And even if some astronomers believed that they could see “stars” in some of these “nebulae”, there was so very little understood about the real nature of stars that it was possible to believe that they are some kind of “starlets”, much smaller and fainter than the stars around us.
   The debate between Curtis and Shapley was conducted on a highly specialized level. I will present here just three of the hotly debated points.
   (1) Many “spiral nebulae” were already known at the time of the debate, and it was known that they seem to avoid the silvery band of the Milky Way. That band forms a great circle around the entire sky, and even a small telescope shows that it is formed by millions of stars. You also find in it very many star clusters and gaseous nebulae. Some of these nebulae are irregular in shape, others show small but fairly symmetrical disks (such a “planetary nebula” is actually a detached, expanding atmosphere of a star). However, you can’t see any “spiral nebulae” within the Milky Way band. Shapley: These spiral nebulae somehow know about the orientation of our Galaxy, therefore they are part of it. Curtis: Some of the spirals seem to be filled by material that obscures starlight. If our Galaxy has such an obscuring band, and if our Sun lies inside it, then that band would prevent us to see the spirals behind it. – So it seemed that Curtis had to invent two new, unproved assumptions to explain the phenomenon; Shapley’s explanation seemed simpler.
   (2) From time to time, a bright “new star” temporarily appears in our sky, and is sometimes easily visible with the naked eye. In 1885, such an object appeared inside the nearest spiral nebula (M31 in Andromeda), and was just about visible to the naked eye (5th magnitude). Shapley: The Andromeda spiral must be relatively nearby, for that nova to be so bright. Curtis, rather meekly: Perhaps the star of 1885 was a new type of phenomenon. 
   (3) There existed observations by a few astronomers indicating that some of the spirals were rotating fairly fast. If they were huge stellar systems, as Curtis proposed, their speed of rotation would be incredibly (or even impossibly) high. Curtis could only argue that these observations must be wrong…
   Thus, Shapley seemed to be the winner, and might have enjoyed this pleasant feeling for about five more years. At the end of 1924, Edwin Hubble, working with the 100-inch telescope at Mt. Wilson, discovered several variable stars of the “delta Cepheid” type in the Andromeda spiral. These variable stars pulsate regularly, and their period of pulsations is the longer the more luminous the star is. Using this relation, Hubble could determine the distance to the Andromeda spiral and established beyond doubt that this is a very distant and very large stellar system, actually even bigger than our own Galaxy. Thus, contrary to the immediate impression from the 1920 debate, Curtis won! He was right that the bright object in the Andromeda spiral seen in 1885 was not an ordinary nova, but a much more conspicuous phenomenon, a supernova – a real explosion of a star. And he was also right in saying that, along the central plane of our Galaxy, there is a belt of dark clouds that prevent us from seeing external galaxies in that belt. And the observations of the rapid rotation of certain spirals were indeed completely wrong!