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.
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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