Cepheid variable stars in our galaxy


Tom Burns - Stargazing



As the Internet has developed, it has become increasingly trendy to be a nerd. It has not always been so.

I speak from deep, sometimes-painful and sometimes-joyous personal experience. 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 physical skills, I find little to admire in football quarterbacks or slam-dunking 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, “just about 2.5 million light years away.” Invariably, some punk-nerd with an attitude will yell, “How do you know that.”

Good question. One light year is approximately equal to 6 trillion miles. The distance to Andromeda is in miles exactly (let’s see, hmm, carry the zero) really, really far away.

In fact, the question is complicated a bit by the structure of the galaxy and the angle at which we see it. Andromeda is a flattened spiral of about 300 billion stars. If we could see it from the side, it would look like a toothpick with a bulge in the center. If we could see with the spiral facing us, it would look round like a child’s pinwheel.

Instead we see it at about a 75-degree angle. (If t were edge on, we’d be seeing it at a 90-degree angle.) Thus, the edge closest to us is perhaps as much as 100,000 light years closer to us than its trailing edge. Since the light from the trailing edge takes at much as 100,000 years more to reach us than the close edge, we aren’t even seeing the light from Andromeda at the same time.

But its sheer distance is the real problem in determining how far away it is. 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 5 1/3 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.

About 700 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.

Our Friday night public observing sessions continue at Perkins Observatory. Call 740-363-1257 for more details and to reserve tickets.

http://aimmedianetwork.com/wp-content/uploads/sites/40/2016/10/web1_tom_burns-6.jpg

Tom Burns

Stargazing

Tom Burns is director of the Perkins Observatory in Delaware.

Tom Burns is director of the Perkins Observatory in Delaware.