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 firstname.lastname@example.org.