Observing Variable Stars

We shall not cease from exploration And the end of all our exploring Will be to arrive where we started And know the place for the first time.

T.S. Eliot

Set free your imagination and consider one of the remarkable curiosities of nature.

A star located 600 light-years from Earth is about to do something cataclysmic. In truth, it did so 600 years ago. However, as if looking through a time machine, you can see it happen tonight. It took 600 years for the light record of this amazing event to reach us, even traveling at 300,000 kilometers per second. Upon close examination of this star you will discover that it is really two stars, a binary system, but to your naked eye it appears as a single star because of its immense distance. As a coincidence of nature, one of the stars of this binary system is much like our Sun, a relatively small yellow star. However, the second star of this system is a much smaller white dwarf, only about as big as the Earth and it is bright blue. A million Earth-sized planets would fit into our Sun so compared to its larger companion this planet-sized dwarf is as a flea to a dog.

Remarkably, the two stars orbit each other in just a few hours. They are very close to each other; the second reason that the two stars appear as one star. Another amazing fact is that although the smaller star is only the size of the Earth it is as heavy as our Sun. If you could bring a piece of this star that was the size of a sugar cube to Earth, it would weigh 16 tons (tonnes). Even more impressive, because gravity is so much stronger

Figure 1.1. Artist's conception of a dwarf nova showing the tendril of hydrogen forming an accretion disk around the white dwarf star. Copyright: Gerry A. Good.

on the surface of the massive white dwarf, this sugar cube sized piece of star stuff weighs 470,000 times more at this star's surface than it would on the Earth's surface; an unbelievable 7.5 million tons!

As a result of the larger star being so close to the smaller star, as well as the massive nature of the dwarf star, gravity allows the smaller star to steal hydrogen from the larger one at an impressive rate. A tremendous amount of hydrogen is constantly being pulled from the larger star forming a gigantic tendril of streaming hot gas leading to a disk of stolen fuel that is now encircling the smaller star like a giant donut (Figure 1.1). Hydrogen is the fuel of stars and it is the primary source of energy that powers most of them. In this situation, the hydrogen will fuel a catastrophic explosion and the larcenous nature of the smaller star will be responsible for the eventual death of this binary system! But that will be in the far future.

Tonight, in a spectacular demonstration of these chaotic circumstances and every two weeks hence, after the disk of hydrogen gas has again grown to a sufficient size, the smaller star will proclaim its theft by

Figure 1.1. Artist's conception of a dwarf nova showing the tendril of hydrogen forming an accretion disk around the white dwarf star. Copyright: Gerry A. Good.

producing an explosion within the disk that has the power of millions of nuclear bombs. As a result of the enormous explosion, this binary system becomes a hundred times brighter for a few hours.

Possibly a gift of nature, a reward for proper preparation, planning and patience, this dramatic sequence of events will be witnessed by a variable star observer somewhere on Earth. Maybe the observer will be you. To assist you in the preparation and planning for the observation of phenomena such as the one just described, this book will explain the basic nature of the various types of variable stars and suggest how best to observe them.

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