Ladies of the Night

© 2000 by Robert C. Moler

In the constellation of Taurus the bull are two famous clusters of stars. The stars in them are, in mythology, sisters. They are half sisters of each other. In the photograph below one of the clusters is quite obvious to the upper right. It is the more famous of the two, the Pleiades or Seven Sisters. Below left is the larger, more sparse, and more important astronomically, the Hyades. The Hyades and the bright star Aldebaran make up the V shaped face of Taurus.

From antiquity the Pleiades was thought to contain seven stars. Their names are Alcyone, Merope, Celaeno, Taygeta, Sterope, Electra, and Maia. However, one of them is decidedly dimmer than the rest, the storied Lost Pliead. Was it once as bright as the rest? No one knows. Anyway Electra is here name, and according to myth she married a mortal rather than a god as did the others. The way the stars are named today Electra is actually the third brightest Pliead. Asterope is the dimmest, actually a close pair of stars. Also according to myth they were daughters of Atlas and Pleione which are named today. Zeus turned them into doves and placed them in the sky because they were being pursued by the hunter Orion. As can be seen he chases them still.

According to myth the Hyades were the daughters of Atlas and Aethra, the half sisters of the Pleiades, and were seven in number also. There is some question as to what their names were.

There is no contest as to which of the two clusters is the most beautiful. The Pleiades wins hands down. Its brightest stars are type B, brilliant with a tinge of blue. There are at least 250 stars in the cluster. Astronomers think the Plieades is quite young, perhaps 100 million years old. If so, they were born during the age of dinosaurs on the earth. The Plieades lies some 375 light years away. Compared to the sun's 4.6 billion year age, they're the new kids on the block.

The Hyades are truly the more important of the two. The Hyades is the closest star cluster to us. It is also barely within the range of earth based triangulation or parallax methods of determining stellar distances. That range has recently been extended by the Hipparchos satellite.

The Hyades lie at a distance of 151 light years. It does not include Aldebaran, which is about half the distance of the cluster. The Hyades also show a proper motion. Proper motion is the apparent movement of stars across the sky, after we subtract out the motion caused by the moving earth. The Hyades is moving to the east among the stars and receding. The vanishing point of this motion in perspective is to the upper left of the star Betelgeuse in Orion.

The Hyades contains about 132 stars. The cluster is perhaps 400 million years old, four times that of the Plieades, but still less than a tenth the age of the sun.

The Hyades importance in determining the scale of the universe cannot be overstated. Without interfering matter to block or dim light, two stars of equal actual brightness, one being twice as far away as the other will be only a quarter as bright. That is the inverse square law of light. The problem is in making sure the actual brightness is known. But there is a way.

A graph plotting the brightness of stars against their color or surface temperature, called an HR diagram shows that stars are not randomly distributed on it. For just about all of open star clusters, there appears a diagonal bar on the chart from dim and cool to bright and hot. That line is called the Main Sequence, where stars spend the bulk of their lives. If we plot stars of another star cluster and find the differences in the magnitude of the main sequence stars from the Hyades, we can calculate the distance to the cluster in question.

By this method the Hyades has leveraged our parallax method to great distances. This was needed, because Cepheid variable stars, which are very bright and can be used to measure distances to the nearer galaxies are not close enough to measure even one by parallax.

So we have two star clusters: Beauty and Utility. Both ladies of the night.

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Uploaded: 10/31/00