July 4 – Happy Birthday, Henrietta Swan Leavitt

Posted on July 4, 2015

The American astronomer Henrietta Swan Leavitt was born waaayyy back on this date in 1868, and yet she is the one who discovered an important “measuring stick” that allowed us to better understand the enormity of the universe.

And this “measuring stick” was made of stars!

Did you know that some stars do not shine steadily? Instead, some stars seem to flicker or even to wildly fluctuate with brighter and then dimmer light. The changes in brightness reflect eruptions within the star itself or the in-falling of material from a nearby companion.

They are called variable stars.

Some variable stars are called Cepheid variables. They pulsate at a regular pace, changing in both size and temperature. Their brightness varies along with the size-and-temperature changes. And, it turns out, there is a strong direct relationship between a Cepheid variable's luminosity and pulsation period.

Because of this, we can discover how far away a Cepheid variable star is. We measure its brightness and its pulsation period. Then we figure out how bright it SHOULD be, if we were near it. Voila! We can now compute how far away the Cepheid variable must be to make it appear at the brightness we observe.

It is Leavitt who discovered the relationship between the luminosity and period of Cepheid variables. Her discovery gave astronomers their first way of measuring the distance between the Earth and faraway galaxies, and it was her discovery (and red shifts) that enabled Edwin Hubble to discover that the universe is expanding. And of course, once we discovered the expansion of the universe, we were able to reason that the universe must have started with a Big Bang.

Leavitt was born in Massachusetts. She attended Oberlin and Radcliffe Colleges and then traveled in America and in Europe. During those post-graduate travels, she got ill and lost her hearing. However, she was still able to get a job as a human “computer” working for Edward Pickering at the Harvard College Observatory to measure and catalog the brightness of stars in the observatory's collection of photographic plates.

How did Leavitt make her discovery? She made the assumption that all the Cepheid variable stars in the Magellanic Clouds were roughly the same distance to the Earth. That's sort of like saying that every mountain and crater on the Moon is roughly the same distance from your house. It's not completely true – some of the deepest craters are some tens of thousands of feet “farther away” from your house than some of the mountain tops on the Moon. But compared to the more than one BILLION feet away that the moon is from Earth, that distance is like nothing. (The difference between the lowest and highest spots on the Moon is only about 0.0047% of the distance between the Earth and the Moon.)

Large Magellanic Cloud -
it's really a small, relatively
nearby galaxy.
So, in the same way, we can talk about a group of stars that are very far away from the Earth – such as all the stars in another galaxy – being roughly the same distance away from us. The Large Magellanic Cloud is about 160,000 light years away from us, and the Small Magellanic Cloud is about 200,000 light years away from us. And each light year is about 1,000,000,000,000 miles or kilometers away. So that makes the Magellanic Clouds some 160,000,000,000,000,000 to 200,000,000,000,000,000 miles away!

(Pretty far!)

Interestingly enough, I discovered that recent, more exact studies of the Magellanic Clouds have resulted in fine-tuning Leavitt's assumption. We now think that, based on more exact measurements of the light of Cepheid variables, the Large Magellanic Cloud is tipped 35 degrees, so that the northeast part of the galaxy is slightly closer to our galaxy than the southwest part. That's what I love about science: we just keep getting better and better at discovering and describing reality!

I bet you have already guessed this part: Leavitt got less recognition for her very important discovery than we would like. Someone tried to nominate her for a Nobel Prize, but that prize is never awarded after someone's death, and at that point Leavitt had already died of cancer. Hubble, who enjoyed quite a bit of fame, always said that Leavitt should've gotten a Nobel for her work.

So, not too much fame. Did Leavitt earn fortune? Well, she had money because of her family, so at first she wasn't even paid for her work for the Harvard Observatory; later she earned about 30 cents an hour (woo-hoo!).

Leavitt has gotten SOME credit, now that she is long gone. There is a Leavitt asteroid and a Leavitt crater on the moon. And Leavitt's story has been shared in books and in Neil deGrasse Tyson's Cosmos. I think maybe all of us should spread the word – Henrietta Swan Leavitt made a great contribution to astronomy despite being deaf (AND female, back when that was a hurdle to overcome)!

Also on this date:

Indivisible Day

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