Henrietta Leavitt and the Discovery of Stellar Distances


By Emily Levesque, University of Washington

In the 1920s, astronomers’ explanation of the distant spiral structures was very different. They weren’t sure whether they were strangely shaped nebulae, or mysterious distant objects. The question was ultimately settled thanks to an amazing discovery by Henrietta Leavitt—one of the Harvard Computers—and observations carried out by one of the most famous names in all of astronomy: Edwin Hubble.

An image of the galaxies in space
There might be more than two trillion galaxies in the universe. (Image: Kees Scherer/Public domain)

What Are Galaxies?

Galaxies are huge, gravitationally bound systems of stars, gas, dust, and dark matter. The smallest galaxies contain a few hundred million stars, while the largest can have up to 100 trillion. Modern estimates believe that the observable universe contains more than two trillion galaxies. Yes, that’s trillion, with a T

Some galaxies appear as irregular clusters of stars and gas. Others look like large, compact ellipses. Still, when most people picture a galaxy, they picture a dramatic spiral shape, with curved arms dotted with bright clusters of stars and filaments of dust. Today, entire fields of astronomy are devoted to studying the formation, evolution, chemistry, and physics of galaxies.

Henrietta Leavitt, a Harvard Computer

To understand how we know about galaxies, we need to go back to the Harvard Computers and what we learned about stars. In 1895, Henrietta Swan Leavitt began working as a Harvard Computer. She’d been born in Massachusetts and attended Oberlin and Harvard colleges, developing a focus in astronomy during her studies. 

Along with Annie Jump Cannon, she was one of the earliest Harvard Computers, and also like Cannon, she lost her hearing in her 20s due to an illness. As a Computer, Leavitt was assigned to work on astronomical images taken at the Harvard observatory in Peru and focused on the Large and Small Magellanic Clouds. 

By the time Henrietta Leavitt was tasked with studying variable stars in the Magellanic Clouds, their nature as large clusters of stars was well-known, though exactly how far away are the Clouds had remained a mystery.

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Henrietta Leavitt’s Discoveries

An image of Henrietta Swan Leavitt working at her desk
Henrietta Leavitt is regarded as a great discoverer of new variable stars, cataloging hundreds of them. (Image: Margaret Harwood/Public domain)

Leavitt proved to be a prolific discoverer of variable stars—stars whose apparent brightness fluctuates. In 1905 alone, she cataloged 900 new variable stars in the Small Magellanic Cloud. In addition to discovering new variables, Leavitt was also hard at work quantifying each star’s brightness and tracking how it changed with time. 

During this work, she noticed something surprising. A number of these variable stars belonged to a class known as Cepheid variables. Originally discovered in the late-1700s, Cepheids displayed a distinctive variation in their brightness as a function of time, commonly referred to as a star’s light curve.

As Leavitt discovered more and more Cepheid variables in the Small Magellanic Cloud, she noticed that the brightest of the Cepheids seemed to have the longest variation periods. Leavitt assumed that all of the stars inside each of the Magellanic Clouds shared a common distance from Earth; this assumption turned out to be correct. 

Because of this, the apparent brightness of these stars can be directly connected to the intrinsic brightness of the stars. The amount of light that a star emits is commonly referred to as its luminosity and represents how much energy the star is emitting from its surface.

The Parallax Technique

Eventually, other astronomers were able to measure the distance to the Magellanic Clouds using parallax—a word derived from Greek meaning alternation. Parallax is a simple technique and one that you can easily test out at home. If you close one eye, and then the other, you’ll notice that nearby objects appear to shift back and forth a great deal depending on which eye you use, while more distant objects appear to move far less. 

By taking high-precision observations at different points in the Earth’s orbit around the Sun, and using basic principles of geometry, astronomers could use this same technique to measure the distances to nearby stars depending on how much they appeared to move. They determined that the Small Magellanic Cloud was about 200,000 light-years away. 

Luminosity period of 25 variable stars
Leavitt law says that there is a relationship between variable stars’ luminosity and their periods. (Image: Leavitt, Henrietta S; Pickering, Edward C/Public domain)

Today, we know that the Large and Small Magellanic Clouds are actually small, irregular galaxies—satellite galaxies—orbiting the Milky Way, but at that time astronomers believed that both Clouds were simply large, distant clusters of stars in our own galaxy.

Leavitt’s Law: The Period-Luminosity Relationship

With this new distance measurement based on parallax, Leavitt was able to calculate the relationship between a Cepheid variable’s period and its true luminosity. Her discovery was published in a 1912 paper. In it, Leavitt noted the key fact that there was a simple relation between the luminosity of the Cepheid variables and their periods. 

This relationship, which came to be known as the period-luminosity relationship, or Leavitt’s law, was quite exciting at the time, but the true impact of this discovery had galaxy-shaking implications. Henrietta Leavitt had discovered the first example of something called a standard candle technique.

In astronomy, a light source with a known luminosity is known as a standard candle. If we have a way of determining the actual luminosity we expect from the object, we can compare that to the apparent brightness that we see and measure its distance.

Common Questions about Henrietta Leavitt and the Discovery of Stellar Distances

Q: Who was Henrietta Leavitt?

Henrietta Leavitt was one of the Harvard Computers who was able to solve the problem of distant spiral structures. She began working in 1895 as a Harvard Computer. Born in Massachusetts, Henrietta Leavitt attended Oberlin and Harvard colleges with a special focus on astronomy during her studies.

Q: What was Henrietta Leavitt’s job as a Harvard Computer? 

Henrietta Leavitt was one of the first Harvard Computers. As a Harvard Computer, Leavitt was appointed to work on astronomical images taken at the Harvard Observatory in Peru and concentrated on the Small and Large Magellanic Clouds.

Q: What did Henrietta Leavitt find out while working on the discovery of new stars?

Henrietta Leavitt was able to list 900 new variable stars in the Small Magellanic Cloud. She noticed something amazing during her work. Some of the stars she listed belonged to a class called Cepheids variables. These stars show specific changes in their luminosity as a function of time, commonly known as the starlight curve.

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