Perseus

Is it any won­der that most avid stargaz­ers hate Day­light Sav­ing Time? When DST ends on Novem­ber 4, we can get a decent view of the night and still get to bed at a rea­son­able hour.

Novem­ber 4! I’m not much of a con­spir­acy the­o­rist, but does any­body remem­ber when DST was just a sum­mer thing? It seems like over the years East­ern Stan­dard Time has shrunk to just a few months. SST is now the excep­tion, not the rule.

Does Con­gress really have the right to tell us what time it is, and why does it keep chang­ing its col­lec­tive mind?

God bless EDT. By 7:30 p.m. blessed dark­ness will have fallen. The con­stel­la­tion Perseus rises beau­ti­fully in the north­east about half way up to the top of the sky. The bright­est stars of the ancient Greek hero form an upside-down “Y.”

Here, my well-rested stargaz­ing friends, there is much to see.

Start from the bright­est star of the con­stel­la­tion, located at the cen­ter of the “Y.” The star Mir­phak gets its name from the old Arab stargaz­ers, who mapped the sky with great pre­ci­sion. Mir­phak is a short­ened ver­sion of Mir­phak al Thu­rayya, the “Elbow Near­est the Many Lit­tle Ones.”

“Lit­tle Ones” there are aplenty near Mir­phak. Look at the area with binoc­u­lars and you will dis­cover the dozens of stars in the “Perseus Asso­ci­a­tion,” which pro­vides one of the best “bino” views in the sky.

Mir­phak is rel­a­tively dis­tant from Earth for a naked-eye star at some­thing like 620 light years away. To be so bright, it must be pow­er­ful, pro­duc­ing 4,000 times the energy out­put of our puny day­time star, the sun.

Up and to the right from Mir­phak is M34, vis­i­ble in a small fuzzy patch in binoc­u­lars. In a small tele­scope, it will resolve into a few dozen stars. M34 is a star clus­ter, a group of grav­i­ta­tion­ally bound stars trav­el­ing together in space.

The real “star” of Perseus is Algol, the Demon’s Head. Find it by look­ing to the right and slightly down from Mirphak.

Algol has a long and sto­ried his­tory. From early times, it has rep­re­sented the head of the snaked-haired Medusa, which Perseus holds, newly sev­ered, in his hand. One look at her face turned the unfor­tu­nate viewer to stone, so don’t look at Algol too long.

Some­time before 1672, the Ital­ian astronomer Gem­ini­ano Mon­ta­nari noticed that Algol was not steady in its bright­ness. From 1782–83, British astronomer John Goodricke care­fully stud­ied the vary­ing bright­ness of the star and noticed that it faded from bright to dark and back again over a period of two days, 20 hours, and 40 min­utes. Every 2.87 days, Algol dims abruptly for about 10 hours.

Algol was thus the first “vari­able” star to be dis­cov­ered and mea­sured accurately.

Goodricke sug­gested that Algol was really two stars. A small, bright pri­mary star has a dim­mer, larger star in orbit around it. As the dim­mer star passes in front of the brighter one, the light from the brighter star is fil­tered through the dim­mer one. As a result, the bright star dims abruptly. Such stars came to be called eclips­ing binaries.

The great­est astronomer of Goodricke’s age, William Her­schel, rejected the idea. He couldn’t see the sec­ond star in his tele­scope. Later, Her­schel dis­cov­ered other stars that had com­pan­ions in orbit around them. He changed his mind and accepted Goodricke’s find­ings whole­heart­edly, even though Agol’s com­pan­ion is too faint and too close to Algol to be seen in a telescope.

You can observe the dim­ming of Algol your­self by using an old stargazer’s trick. Variable-star observers mea­sure the chang­ing bright­ness of a star by com­par­ing it night after night with the bright­ness of a star that doesn’t vary.

The per­fect star for that pur­pose is called Delta, down and to the left of Mir­phak. Most of the time, Algol will look brighter than Delta. When Algol is in eclipse, it will look about as bright as (or even a tiny bit dim­mer than) Delta.

It might take sev­eral nights of observ­ing to see the change, but it’s worth the wait. The Demon’s Head has winked at you from the deep recesses of space. Don’t for­get to wink back.

Tom Burns is the direc­tor of Perkins Obser­va­tory. He can be reached at tlburns@owu.edu.

Posted by Tom Burns on Oct 14 2012. You can follow any responses to this entry through the RSS Feed. Comments can be made below.