Podcaster: Richard Drumm

Title: Famous Women Astronomers – Henrietta Swan Leavitt

Organization: 365 Days Of Astronomy


Hosted by podcast editor Richard Drumm.

She was born in Lancaster, Massachusetts in 1868, to a Congregational Church minister and his wife. She first attended Oberlin College and then transferred to The Society for the Collegiate Instruction of Women at Harvard University.

This mouthful of a name became known as Radcliffe College in 1894, two years after Ms. Leavitt had graduated.

Bio: Richard Drumm is President of the Charlottesville Astronomical Society and President of 3D – Drumm Digital Design, a video production company with clients such as Kodak, Xerox and GlaxoSmithKline Pharmaceuticals. He was an observer with the UVa Parallax Program at McCormick Observatory in 1981 & 1982. He has found that his greatest passion in life is public outreach astronomy and he pursues it at every opportunity.

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This is the 365 Days of Astronomy Podcast. Today we bring you a new series. I’m going to kick off the Famous Women Astronomers series with the story of Henrietta Swan Leavitt.

She was born in Lancaster, Massachusetts in 1868, to a Congregational Church minister and his wife. She first attended Oberlin College and then transferred to The Society for the Collegiate Instruction of Women at Harvard University. 

This mouthful of a name became known as Radcliffe College in 1894, two years after Ms. Leavitt had graduated.

She worked for a time at the Harvard College Observatory, or HCO, and then traveled to Europe, taking mathematics courses at Cambridge University, as well as working at Wisconsin’s Beloit College. 

In 1903 she returned to HCO for good. 

Edward Pickering, the HCO director, first assigned her the task of determining the brightness of stars using the new technology of glass photographic plates.

Dim stars make small diameter dots on the plates and bright ones make larger images, so the task is fairly straightforward. 

But some stars refused to cooperate, they varied in brightness. Ms. Leavitt discovered hundreds of variable stars first in Orion and then thousands in the whole sky.

Pickering then assigned her to work on the variable stars in the Large Magellanic Cloud, and the Small Magellanic Cloud, or LMC & SMC as they’re known.

In these two irregular, dwarf companion galaxies of our Milky Way she found over 1,700 variable stars. She published this finding in 1908 in HCO’s Annals publication. 

She writes:

“It is worthy of note that in Table VI the brighter variables have the longer periods.”

By “variables” she meant Cepheid variable stars.

So what’s the deal with these so-called Cepheid variable stars?

When she wrote that the brighter variables have longer periods you have it right there! That’s the key!

It’s the keys to the kingdom!

Cepheid variables, are a class of pulsating star that brightens and dims over a regular period of time. Delta Cephei is the star that started it all, and gave that class of stars its name.

The farther away that any star is from us, the dimmer it’ll be. And, of course, if all stars were identical, we’d know how far away they were by simply measuring their brightness.

It’s like how a 100 watt light that is a meter away from your face is way brighter than a 100 watt light that’s a kilometer away. 

But not all stars are the same, and some are immensely brighter than others. To determine how far away from us they are we can’t use their apparent brightness as a gauge. 

We need to know more. We need to know their wattage, so-to-speak.

But for the Cepheids we CAN know that wattage. We have the period/luminosity relationship to guide us.

Once we know how many days long the star’s period is, we know what it’s real brightness is, what that wattage is.

With these Cepheids all being in the SMC, we can assume that they are all at approximately the same distance from us.

Then we can see how dim a Cepheid appears to be in our sky and we can calculate approximately how far away it is. Or we could if only we knew how far away the SMC is. That’s our starting point. 

Dang! So close!

In addition, Leavitt felt that the number of Cepheids was too small to draw a general overarching conclusion about the period & luminosities.

So she kept working on the problem for 5 more years.

In March 1912 Pickering published a followup paper in the HCO Circular with data from 8 more Cepheids in the SMC. 

The very first sentence of the paper reads: “The following statement regarding the periods of 25 variable stars in the Small Magellanic Cloud has been prepared by Miss Leavitt.”

Though Pickering signed the paper by himself he is currently credited as a co-author with Leavitt as principal author. And the period/luminosity relationship is now called the Leavitt Law.

Pickering knew how to assign credit where it’s due! Him signing the paper was just a way to steamroll any opposition it might encounter.

And with the 8 new stars adding confirming data, there was no problem about coming to the conclusion that the period/luminosity relation was real. It WAS real.

But we still didn’t know how far away the SMC was. We dodn’t know the scale. 

We’re still stuck.

In the 1912 paper Leavitt writes: “It is to be hoped, also, that the parallaxes of some variables of this type may be measured.”

She didn’t have long to wait!

In November 1913, just a year and a half after that publication, astronomer Einar Hertzsprung found 13 Cepheid variables in the Milky Way that were close enough for him to calculate their distances by the well-established parallax method.

Thus he was able to calibrate the two distance methods, the Cepheid variable stars and good old parallax trigonometry. 

He had enabled the Leavitt Law to be directly used to determine distances to the Cepheid stars. Cepheids in the Milky Way and in the SMC as well.

Parallax astronomy only works out a short distance, just out to the closest few hundred stars. I’ll be doing an episode on parallax astronomy in the future, so stay, uh, tuned.

I was a team member of the University of Virginia’s Parallax Program, so this stuff is near & dear to me.

So these few close-by Cepheids that Hertzsprung found were crucial to making the jump from measuring short distances to much longer ones.

In 1918 Harlow Shapley used the Leavitt Law to approximate the size & shape of the Milky Way and determine the position of our Sun in it.

Suddenly we had an inkling of just how big the Galaxy was.

Then Edwin Hubble in 1926 found a Cepheid variable in the Andromeda Nebula, as it was called, and, using the Leavitt Law, he determined that it wasn’t part of our galaxy at all. He called it an “extragalactic nebula” – what we’d call a galaxy.

Suddenly we had an inkling of just how big the Universe was!

And it’s big. Very big.

Thank you for listening to the 365 Days of Astronomy Podcast!

End of podcast:

365 Days of Astronomy

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