Yard sticks are very useful around the house when you need a measurement.

Yard sticks are very useful around the house when you need a measurement.

(I have also used them to fish out toys the cat knocked under a low piece of furniture.)

They are standards by which we can compare the apparent size of something, whether it be how high your child stands or the length of a box needed to store an item away. It is obviously more tricky to hold up a yard stick to the sky and try and figure how far away that star is from your eyes.

Astronomers do have a 'celestial yard stick' which has been since the early 20th century to tell the distances of star clusters and nearby galaxies, and thereby gauge the immensity of the Universe before us every time the clouds part at night.

They use a certain type of star that varies periodically in its light output.

They are called Cepheid Variables, after its most well known example, a star you can find the next clear night in the northern sky. This is Delta Cephei, a star marking one corner of the constellation Cepheus the King, on the other side of the North Star from the Big Dipper. Delta Cephei varies regularly from magnitude 3.6 to 4.3, every 5.3 days. You can track the brightness change quite easily by comparing its light with nearby stars of similar magnitude. If you have a reasonably dark sky, you won’t even need binoculars, although they will help.

So how can this type star serve as a 'yard stick'?

There are numerous types of variable stars, with different reasons why they change brightness. A Cepheid Variable fluctuates every one to 50 days; from minimum light a Cepheid will double in brightness to maximum light.

Astronomers found a relation between the time period and the star’s luminosity. By watching how long it takes the star to regularly dim and brighten, its luminosity is found.

In the early 20th century, American astronomer, Henreitta Leavitt, was studying numerous Cepheid variable stars in one of our satellite galaxies, the Small Magellanic Cloud. She determined the relationship between the time period and the luminosity, and reported it in 1912.

One of the first of these variable stars discovered was Delta Cephei, which as was mentioned, is so bright it can be tracked with unaided eyes.

Because Delta Cephei is relatively close to the sun, astronomers already were able to measure the distance to this star by the parallax method. Through extremely close inspection of Delta Cephei’s location among other stars in the background, astronomers could detect a minute shift back and forth as the observers looked from varying angles as the Earth moved from one side of the sun to the other.

Delta Cephei was found to be 891 light years away.

Having clamped down the distance to Delta Cephei and judging how bright it looks at that distance, the distance to farther Cepheid variable stars could be deduced by noting how much dimmer they appear to us.

Cepheid variable stars are believed to be large, hot stars five to 20 times as massive as the sun. They vary in light output by pulsating; the star contracts and expands.

If you’d like to read more about variable stars and how to observe them, look on-line at www.aavso.org or find a good astronomy book at your public library.

You can see Delta Cephei high in the northern sky on October evenings. Look for the upside-down “house shape” of the constellation Cepheus the King. Delta is at the top right corner, forming a tight triangle with two other stars nearby, to the right. You can see this pattern with eyes alone, but binoculars will help.

New moon is on Monday, October 8. Watch for the crescent Moon low in the southwest/ west on evenings this week following New Moon.

Keep looking up!

Peter Becker is Managing Editor at The News Eagle in Hawley, PA. Notes are welcome at news@neagle.com. Please mention in what newspaper or web site you read this column.a