November Sky School – Cetus
In another article, I gave you the myth of Andromeda, who was chained to a seashore cliff as a sacrifice to Poseidon. She was to be consumed by the sea monster, now seen in the stars as the Whale, Cetus. The Whale is kept a bit distant from Andromeda, sensibly, I think. For that reason, I did not include a drawing of the constellation of the Whale. Besides, it’s a large, faint constellation; I have trouble tracing it in the sky.
Cetus is well-placed for viewing in the Fall. It is due South in November, near midnight, or around 10 p.m. in December. Remember that constellations rise four minutes later each night, making about two hours later each month.
To locate Cetus, you will have to be in a dark area with a clear view South. If you start at Pegasus and trace a straight line backwards from the line across the top of his triangular wing, you will come to Cetus. You can also start with Taurus, the Bull; Cetus is just behind it (i.e. to the West). Bull and Whale are tail-to-tail
The star Mira was named “miracle” by the ancients. They believed the stars were unchangeable, yet here was one which faded, disappeared and returned on a regular basis. We now know it as a variable star. In about a year, its brightness changes from third magnitude, about the same as the stars in the Whale’s tail, to tenth magnitude, which is far too faint to see without a good telescope. Every star must survive a constant tug-of-war between the crush of its own gravity and the pressure of its heat. The balance shifts, and the brightness can change, but usually this is too subtle or takes too long for us to notice. In some of the stars we see as variable, like Mira, this balancing act is more exciting, and they can flare up and dim down over several magnitudes in a period of months or weeks.
While Mira is spectacular for popping in and out, another variable is famous for providing a way to measure the distance to nearby galaxies.
Delta Cepheus is the nose star of King Cepheus, Andromeda’s dad. It was the first to be recognized in a family of variable stars – the cepheids – which share a very useful property, discovered by astronomer Henrietta Leavitt, published in 1912. Put simply, measuring their period of variation also tells how bright they really are. Since we know how bright they seem to be, we can estimate how far away they are. Edwin Hubble’s discovery of cepheid variables in the Andromeda nebula led to the conclusion that it is tremendously distant. Astronomers then had to agree that such nebulae were so far away that they had to be independent galaxies, outside the Milky Way. That discovery completely changed our understanding of the Universe, yet it was made less than a hundred years ago! Even now, the Hubble Space Telescope, named for Edwin Hubble, and the new generation of ground-based telescopes are measuring cepheid variables and other luminous yardsticks in extremely distant galaxies, and helping to determine the age of the Universe.