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Look north on a late August evening to see the Big Dipper on the lower left, and Cepheus on the upper right, of the North Star. Note the small triangle of stars. Delta Cephei varies in brightness from a maximum of +3.6 to a low of +4.3 every 5.3 days. Compare it to the stars Zeta (magnitude +3.6) and Epsilon (+4.2).

Watch a star and measure the universe

August 21, 2009 at 01:45 PM

GATEHOUSE NEWS SERVICE

Yardsticks are very useful around the house when you need a measurement. 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. It is obviously more tricky to hold up a yardstick to the sky and try and figure how far away that star is from your eyes.

Astronomers do have a “celestial yardstick” that has been used 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 yardstick?

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 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.

Study of Cepheid variables helped astronomers interpret the cosmos. There were different opinions about the “faint fuzzy spots” that today we know to be distant galaxies, separate from our own Milky Way Galaxy. In the 19th century and early 20th xentury, a common hypothesis was that these hazy objects, some which could be seen as grand spirals, were other systems of planets in the midst of formation around their host star. In 1924, Edwin Hubble discovered Cepheid variable stars in the “Andromeda Nebula,” showing that it was a distinct galaxy as some had theorized.

They have also been useful as place markers to measure the apparent expansion of the universe.

The North Star (Polaris) is also a Cepheid variable star but changes brightness only slightly.

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

New moon is on Aug. 20. If you can get away from city or town lights, now is a good chance to witness the summer Milky Way in the evening sky.

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