Cepheid variable stars

Foot­ball sea­son is in full swing, and it tends to bring out my most nerdish behav­ior. Okay, so I’m a nerd, I’ve always been a nerd, and I’ll always be one. I pray my last words will be, “Yes … nerd … (gasp) … but with an atti­tude.” Despite their con­sid­er­able skills, I find lit­tle to admire in foot­ball quar­ter­backs or slam-dunkin’ bas­ket­ball play­ers. Sorry.

In fact, my favorite kind of kid is the one we get often at our pub­lic pro­grams up at Perkins Observatory.

I’ll be expound­ing, say, on the dis­tance to the Androm­eda Galaxy, a cigar-shaped “fuzzy” vis­i­ble to the unaided eye low in the east just after dark. “It is,” I will say, “around 2.5 mil­lion light years away.”

One light year is approx­i­mately equal to 6 tril­lion miles. The dis­tance to Androm­eda is there­fore in miles exactly (let’s see, hmm, carry the zero) really, really far away. Weirdly, the galaxy is tilted on its side with respect to us. Thus, the close edge is some­thing like 2.35 mil­lion light years away. The far edge is 2.65 mil­lion light years. That means that we don’t see the light from the galaxy all at the same time. We see the light from the front 300,000 years before we see the light from the back.

Invari­ably, some punk-nerd with an atti­tude will yell, “How do you KNOW that?”

Good ques­tion. After all, it’s not like we can string a tape measure.

To dis­cover the answer, we must look to the con­stel­la­tion Cepheus, which is vis­i­ble high in the north just after dark. At the upper right of the con­stel­la­tion is a small tri­an­gle of stars.

The bot­tom right star of the tri­an­gle has the unas­sum­ing name Delta.

Delta pul­sates from bright to dim to bright again over a period of five and a third days. A whole class of stars, called Cepheid vari­ables, does the same thing.

In 1912, Har­vard astronomer Hen­ri­etta Leav­itt dis­cov­ered a star­tling char­ac­ter­is­tic of Cepheid vari­ables. The longer their period of pul­sa­tion, the more energy they pro­duce, mak­ing them brighter. A low-energy star pulses faster. A high energy-star pulses more slowly.

By com­par­ing how bright a star really is with how bright it looks to us, we can tell how far away it is. If, for exam­ple, we look at a 60-watt light bulb, we can tell how far away it is by notic­ing how dim it is com­pared to a nearby bulb. The trick is know­ing that it’s a 60-watt bulb in the first place.

Hun­dreds of Cepheid vari­ables have been dis­cov­ered in our Milky Way galaxy. Their peri­ods of pul­sa­tion range from one day for very dim stars to 100 days for extremely lumi­nous ones.

Leavitt’s work gave human­ity a way of mea­sur­ing dis­tance to stars in our galaxy. For exam­ple, if we com­pare Delta’s appar­ent bright­ness with its 5.3-day pul­sa­tion period, we dis­cover that it’s about 1,500 light years away from us.

But what of the “spi­ral neb­u­lae,” the mys­te­ri­ous whirlpools of light we now call galax­ies? By the early 20th cen­tury, tele­scopes had got­ten big enough to begin to resolve a few stars in them, but astronomers were still con­fused about what they actu­ally were. Some believed they were small and close — inside the bound­aries of our Milky Way. Oth­ers claimed they were as large as the Milky Way and extremely far from it.

In 1924, Edwin Hub­ble used the largest tele­scope in the world to dis­cover Cepheid vari­able stars in the Androm­eda Galaxy. They looked dim, but their long peri­ods of pul­sa­tions indi­cated that they were very bright indeed. They were so lumi­nous that the “neb­ula” they were in had to be over 2 mil­lion light years away. Leav­itt and Hub­ble had dis­cov­ered the galax­ies and increased the size of the uni­verse to mind-melting proportions.

Leav­itt and Hub­ble rank among the iron-pumping, brain-expanding, boss nerds of our cen­tury. Who would you rather admire, some­body who can throw an inflated pig blad­der or those intre­pid geeks who gave us the uni­verse? That’s right. I’m a nerd and proud of it.

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

Posted by Tom Burns on Sep 23 2012. You can follow any responses to this entry through the RSS Feed. Comments can be made below.