Cepheid variable stars


By Tom Burns - Stargazing



As the internet develops, 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-slinging quarterbacks or slam-dunking basketball players. Sorry.

Culturally, we must begin to appreciate the inner nerdishness that creates the true movers and shakers of civilization. They are the folks who bury themselves in a single set of ideas to the extent that they discover something extraordinary that moves their culture in a positive direction. They are the scientists and artists who see not just the deeper meaning but also the beauty of the world around them.

We can find such people by looking at the sky and the nerdish characters who have loved it and be inspired by them.

A good place to start is the now-obscure constellation Cepheus, the King, and a single star in it, the gloriously red point of light called the Garnet Star (or Mu Cephei to the astronerds among us).

Look for Cepheus around 8:15 p.m. or so. You’ll find him high in the north, just above the North Star, Polaris.

To our modern eyes, Cepheus looks like a head wearing a well-deserved dunce cap. His fitting punishment for his cowardly betrayal of his daughter, as we shall see, is that he spends most of the year hanging upside down.

Cepheus is featured in the mythologies of many ancient cultures. He is sadly unfamiliar to most people these days because his relatively faint stars barely shine through the glow of urban nighttime lighting.

The constellation goes back at least to the Chaldeans of 2300 BCE. To the later Greeks and Romans, Cepheus, king of the Ethiopians, was father to Andromeda, a constellation to the east and husband to Cassiopeia to the northeast.

He is best known for a weakness of character so great that he was willing to sacrifice his daughter to the jaws of Cetus, the Sea Monster, who happened to be ravaging the coast of his kingdom at the time.

After the fall of the western Roman Empire, ancient Arabic astronerds systematically and carefully named the brighter stars of the constellations, and we still use those names. They eventually accepted the Greek name for Cepheus, but earlier Arabs called the constellation Al Aghnam, “The Sheep.”

The change of designation created an interesting mixture of stellar names. The brightest star of the constellation is called Alderamin, a western corruption of the Arab Al Dira al Yamin, “The Right Arm,” presumably of the king. The second-brightest star is Alfirk, “The Flock,” presumably of sheep. Marking the peak of the dunce cap is Alrai, “The Shepherd” watching over the flock.

A true astronerd will know the stellar names and their derivations. However, my favorite kind of nerd is the kid we often used to get at our public programs up at Perkins Observatory.

I’d 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.”

What a glorious question! One light-year is approximately equal to six trillion miles. The distance to Andromeda is thus in miles exactly (let’s see, hmm, carry the zero) really, really far away.

How is it possible to determine such enormous distances? It’s not like we can pull out a tape measure.

To discover the answer, we must look again to the constellation Cepheus. 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 repeated period of 5 1/3 days. A whole class of stars called Cepheid variables does pretty much the same thing.

In 1912, Harvard astronomer Henrietta Leavitt discovered a startling characteristic of Cepheid variables.

Leavitt was the epitome of world-changing nerdishness. Her work as a “computer” might seem tedious to most of us. She was paid $10.50 per week to examine thousands of photographic plates to determine and carefully catalog the brightness of stars.

She began to notice a pattern in the variation in 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.

Her work gave humanity a way of measuring the 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.

Delta is scientifically interesting because it pulsates. However, some stars are simply beautiful — although some folks would suggest that you have to be a nerd to appreciate that beauty.

Buried in the constellation Cepheus is a true gem, just visible to the unaided eye from our ever-decreasing dark, rural skies. Above and half way between the top two stars of the constellation is the Garnet Star. It looks as if the upside-down Cepheus is wearing a Garnet necklace.

As the name suggests, the Garnet Star is red. In fact, it’s the reddest star visible to the unaided eye. Binoculars or a small telescope show its redness the best. The star looks increasingly orange in larger and larger telescopes, and it looks distinctly yellow-orange in an observatory-sized telescope like the one at Perkins. We get our best views of it in smaller telescopes set up on the lawn by the Columbus Astronomical Society. Ask one of them to show it to you.

The star’s color tells us much about it. The Garnet Star is a red hypergiant, an enormous star that has now reached a premature old age in a similar way to the more famous Betelgeuse in the constellation Orion.

Stars cool down considerably when they reach their decrepitude. The Garnet star is a rather tepid 6,000 degrees Fahrenheit. That fact doesn’t stop it from shining a startling 283,000 times brighter than the sun.

Stars tend to puff up when they reach old age. The Garnet Star is well over 1,000 the diameter of the sun, large enough that if the sun were removed from our solar system and replaced with the Garnet Star, Earth would be well inside the star. The star would engulf Jupiter and make it part of the way to Saturn.

It will not remain that way for long. In their youth and middle age, stars fuse hydrogen into helium in a hydrogen-bomb explosion that can last billions of years. Stars like the Garnet Star expend their hydrogen very quickly. The star is now fusing its helium into carbon and its carbon into iron. In a few million years, the Garnet Star’s hydrogen-bomb reaction will cease, the star’s core will collapse to a black hole, and its outer shell will expand explosively in a supernova explosion that will light up Earth’s distant sky.

Most stars don’t show their colors very well to human eyes. That makes the Garnet Star extraordinarily beautiful and rare and, yes, even inspiring.

Leavitt and Hubble rank among the iron-pumpin’, brain-expandin’ boss nerds of our century. The telescopists on the lawn at Perkins, amateurs all, are there to show you the wonder and majesty of the universe you live in. Who would you rather admire — somebody who can throw an inflated pig bladder or those intrepid geeks who gave us the universe? What would you rather love — a violent collision of bodies in the symbolic war for territorial control or the gentle soul who shows you the beauty of a rusty-red star in its death throes?

That’s right. I’m a nerd and proud of it.

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By Tom Burns

Stargazing

Tom Burns is the former director of the Perkins Observatory in Delaware.

Tom Burns is the former director of the Perkins Observatory in Delaware.