April 18, 2008

In Today's One Minute Astronomer...

Variable Stars

The ancient Greek philosopher Aristotle declared the heavens were perfect and unchanging, and his view went unchallenged for nearly 2,000 years. But keen-eyed renaissance astronomers like Tycho Brahe discovered stars that varied in brightness. The science of variable stars was born.

The Basics

• There are dozens of types of variable stars. Some vary by hundreds or thousands of times over the course of days, weeks, or months. Others have short, shallow brightness oscillations. We'll discuss many types of variables in later issues.

• Our Sun (fortunately for us) changes brightness by only 1 part in 1000 every 11 years or so, keeping the Earth at a pleasant temperature, more or less.

• Some stars change their true, or intrinsic, brightness. They are (drum-roll please) intrinsic variable stars. The pulsating variable star Mira, which you can find in the constellation Cetus, is a famous example.

• Other stars change their apparent brightness because of external influences, like rotation or eclipses by another star. These are “extrinsic variables”. Algol, the “demon star” in Perseus is the most famous example.

• Some variables change brightness periodically, every few hours or days or weeks. Others like Mira change in a semi-regular fashion. And some variables brighten unpredictably for a short period before fading away.

A Deeper Look

• Supernovae are the ultimate intrinsic variable star, causing an explosion so catastrophic, it produces more energy in a few weeks than our sun produces in 10 billion years.

• By comparing computer models to observations of variable stars, astronomers get a better understanding of the structure and processes inside the star.

• If you have a small telescope and a little practice, you can make your own scientifically useful measurements of variable stars with the help of the American Association of Variable Star Observers (AAVSO).

A Bit of History

David Fabricius, a Lutheran pastor and astronomer from Osteel, Germany discovered the first pulsating variable star in 1596, which he named Mira (also called omicron Ceti).

He and his son Johannes were amateur astronomers of some repute. They corresponded with Johannes Kepler, and they were the first to notice the movement of sunspots across the sun's surface, correctly deducing the sun's rotation.

Reverend Fabricius met an ignonimous end, alas, when he was hit on the head with a shovel by a peasant whom he had accused of stealing a goose.

Personal View

As a “one-minute astronomer”, I don't have the time to measure variable stars, sadly. But there is no question… variable stars offer you, as an amateur, a rich opportunity to make a true contribution to science.