Variable Stars

by Kevin Shank | May 1, 2024 | 0 comments

Betelgeuse, the yellow star in the lower left corner, is a pulsating variable star.

Aside from their twinkling, stars seem to always shine at a fixed brightness. Things aren’t always as they seem though. If you watch some stars closely, you’ll find they brighten and dim over time. These are called variable stars.

There’s a fairly large community of amateur astronomers who find these stars fascinating enough to spend hours tracking their brightness changes, reporting what they find on a regular basis. In fact, instead of professional astronomers, amateur astronomers are the ones who are dedicating the most time to variable star observations and data gathering.

Variable stars are very useful in the science of astronomy. They can be used to estimate distances across the universe. Cepheid variables are stars that dim and brighten on a regular basis as a result of pulsing—they literally get bigger and smaller over time. They’re huge stars; even at their smallest they are hundreds to thousands of times bigger than the sun. Astronomers have found that the rate of their pulsing corresponds to a specific brightness level. Since the actual brightness can be determined by observing the rate of pulsing, the apparent brightness can show its distance from us.

Betelgeuse in Orion is a pulsating variable star. Its gravity causes an enormous amount of pressure on the core as the mass of this supergiant star crushes inward. This causes an acceleration of the ongoing explosive nuclear force in the core. This, in turn, causes the star’s inward contraction to reverse course and expand outward until the core pressure is decreased to the point the nuclear force slows and the star collapses inward again. It continuously cycles through this pattern like a hyperventilating puffer fish as its diameter yo-yos back and forth by about 50%. Photographs of it usually show it has an asymmetric (lopsided) shape as giant billowing plumes of gases boil off the star.

A cataclysmic variable is one that is locked in a tight orbit with another star. The star with the dominant gravity pulls star material away from its companion and onto itself. This influx of fuel and pressure starts a runaway nuclear reaction surge within the star, sometimes forming spectacular novae explosions.

All of the variable stars we’ve discussed so far are intrinsic variables. This means the variability comes from processes within the star itself. Extrinsic variables are stars that vary in brightness because of external factors.

An eclipsing binary star system is an example of an extrinsic variable. In this case, two stars are orbiting each other and situated so that, from our point of view, one crosses in front of another, dimming the total output of the stars on a regular basis. Sometimes dimming stars also give a telltale sign of a planet orbiting the star, blocking some of its light periodically. The Kepler spacecraft has found thousands of planets by observing this regular dimming.

A pulsar is a rapidly spinning star that sends out a concentrated beam of energy similar to a lighthouse. Whenever the beam passes over the Earth, we see it as a flash of light from the star. It’s called an extrinsic variable since the flashing only appears to the outside universe because of the star’s rotation, not because of an actual brightening and dimming within the star.

Do you want to see a variable star for yourself? Look for Algol in the constellation Perseus. It dips from magnitude 2.1 to 3.4 every 2.87 days. The dimming lasts for about ten hours each time as its companion star blocks some of its light.

Variable stars declare their Creator’s creativity and glory, but He is much greater than their feeble declaration. With Him, there is “No variableness, neither shadow of turning.” (James 1:18)

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