Hey Captain Astro

Tom’s been out stargaz­ing 12 nights in a row, so he’s too tired to write his col­umn this week. Instead, he has asked the intre­pid CAPTAIN ASTRO to answer some of the emails that have been pour­ing in from readers.

Hey Cap­tain Astro,

I’ve tried to use the star to find the con­stel­la­tions. I can see the stars okay, but I never seem to be able to see the lines that con­nect the stars together. What gives?

— Timmy John­son, age 9

Well, Timmy, you can’t see the lines from inside the city. You’ll have to go out to a dark, rural place to see them.

Hey Cap­tain Astro,

Come on now. Really?

— Timmy

Ha, ha, old Cap­tain Astro was just pulling your leg. Of course, the lines are just imag­i­nary con­nec­tions to help group the stars into arti­fi­cial col­lec­tions called constellations.

Ancient peo­ple saw the sky as a great bowl upon which were placed points of light called stars. They thought all the stars were the same dis­tance away.

It was nat­ural for them to see pat­terns in the stars, and they gave those pat­terns the names of the most impor­tant things around them — the heroes of their sto­ries and the ani­mals they depended upon for their survival.

The ancients who stud­ied the sky more sys­tem­at­i­cally, the astrologers and astronomers, sim­ply sliced up the sky accord­ing to those even more ancient divi­sions. By 150 A.D., the great Alexan­drian astronomer Ptolemy listed the 48 con­stel­la­tions of the north­ern sky that we still use, give or take a few, today.

It was more than 1,400 years before any new con­stel­la­tions were added by astronomers in the 17th and 18th cen­turies, and most of the new ones are vis­i­ble only from the south­ern hemisphere.

By the begin­ning of the 20th cen­tury, astronomers needed a fixed def­i­n­i­tion of the parts of the sky, so the Inter­na­tional Astro­nom­i­cal Union decided in 1928 to par­cel up the sky into 88 con­stel­la­tions, includ­ing the ones vis­i­ble from the south­ern hemi­sphere. But these con­stel­la­tions are not exactly the stick fig­ures that the ancients invented, even though they bear most of the same names.

The mod­ern con­stel­la­tions are arbi­trary divi­sions of the sky, sort of like the way the world is divided into coun­tries. They are used to des­ig­nate the loca­tions of stars, neb­u­las and galax­ies so that astronomers can have an exact idea of their loca­tion. Thus, the stars that make up the stick fig­ure of Orion are IN the patch of sky we call Orion, but they don’t define the con­stel­la­tion the way they used to.

The stars in a con­stel­la­tion like Orion, the Hunter, vis­i­ble high in the south­ern sky right now, aren’t even close to each other, although they appear to be so from our van­tage point on earth. We imag­ine that the stars are pro­jected on a large black bowl we call the celes­tial sphere, kind of like a plan­e­tar­ium dome.

That makes the stars in Orion look like they are close together, but they really aren’t. In fact, Betel­geuse, at the upper left cor­ner of Orion, is 310 light years away. (One light year is about six tril­lion miles. You mul­ti­ply it out. Old Cap­tain Astro is too tired.) Rigel, at the lower right cor­ner, is about 900 light years away. Bel­la­trix is about 470 light years away, and so on.

Seen from prac­ti­cally any other angle in space, these stars wouldn’t look any­thing like the con­stel­la­tion we see from earth.

Stars that really are close to each other in space gen­er­ally look much closer together from our van­tage point, and we call them star clus­ters. They are close together because they were formed out of the same cloud of hydro­gen gas.

An exam­ple is the Pleiades, a small col­lec­tion of six stars in the con­stel­la­tion Tau­rus, the bull. It is straight over­head in the early evening, and looks like a small dip­per of stars. It isn’t a con­stel­la­tion because the stars are too close together to make it a use­ful sky divi­sion. But the stars are actu­ally close enough together to be grav­i­ta­tion­ally attracted to each other.

Hey Cap­tain Astro,

But I don’t get it. They don’t look much like the ani­mals and heroes they’re sup­posed to represent.

— Timmy

You’re right Timmy, for the most part. The ancients had a tremen­dous imag­i­na­tion. They saw shapes in the sky the way we some­times see shapes in the clouds on a sum­mer day. Also, it’s pos­si­ble that the ancients didn’t really believe that the con­stel­la­tions really were peo­ple or ani­mals. They may have been look­ing for a way to name the parts of the sky, and called those parts by the names of impor­tant peo­ple and ani­mals in their legends.

The old heroes after whom the con­stel­la­tions are named are almost forgotten.

But their names remain as a legacy to our ancient fore­bears, who looked up to the sky in awe and wonderment.

Hey Cap­tain Astro,

I have to write a report on Nep­tune for school. Please tell me every­thing you know about Neptune.

— Timmy

Er, ah, sorry kid, I left some lentils cook­ing on the stove. Gotta go. Bye.

Tom Burns is direc­tor of Ohio Wes­leyan University’s Perkins Obser­va­tory in Delaware. Con­tact him at tlburns@owu.edu.

Posted by Tom Burns on Jan 30 2012. You can follow any responses to this entry through the RSS Feed. Comments can be made below.