Henrietta Swan Leavitt

Henrietta Swan Leavitt was a US astronomer, famous for her contribution to solving what is known in Astronomy as ‘The Problem of Distances’. December 12th 2021, marks the 100-year anniversary of her death. In honor of her work, we explain here how her discovery revolutionized Astronomy.

Astronomers have since long struggled to measure the distance to the stars. That’s because we see stars on a plane in the sky, with no information with regards to their depth in it. When a star is brighter to our eyes (i.e. it has a high ‘apparent brightness’) it can either be because it’s brighter than the rest (i.e. it has a high ‘intrinsic brightness’) or because it’s closer to us than the rest of stars. Without any additional information, we cannot distinguish the two scenarios.

Astronomers us a method known as ‘parallax’ to measure the distance to nearby stars. It consists in measuring changes to the star’s background as we (the observers on Earth) orbit around the Sun. If the star is closer to us than the stars in the background, this change of perspective allows us to measure an apparent motion. Meaning, it looks as if the star has moved! The more a star apparently moves, the closer to us it is. So this method can be used to measure the distance to nearby stars.

However, faraway stars are ‘the background’ and we cannot measure a distance to them. In fact, without parallax, we cannot measure distances to anything outside of our own galaxy. And therefore, we cannot even be sure that those objects are in fact outside of our own Galaxy!

In fact, nobody knew if nebulous objects observed were other galaxies (or Island Universes, as they were called), or if they were strange objects in our own galaxy. With the resolution of the telescopes at that time, other galaxies and local nebulae looked exactly the same. So this problem of distances inspired the famous Great Debate in 1920! It was not solved until the distance to another Galaxy was measured, and published in 1925, by the famous Edwin Hubble.

And here is where Henrietta comes into place! She discovered that a particular type of stars (Cepheids) changed their apparent brightness depending on their real brightness. Using her measurements, she calibrated the distance to those stars using parallax measurements. Thanks to her calibration, it is possible to measure the distance to any Cepheid just by observing a variation on its apparent brightness!

Sadly, Leavitt’s discovery remained unnoticed for over a decade, and well until after her death. It was Edwin Hubble, in 1925, who detected a Cepheid star in Andromeda and used Henrietta’s relation to measure its distance from us. And, surprise, surprise… it was about 50 times further away than the radius of our own Galaxy! This demonstrated that Andromeda was not a cluster of gas and stars within our own Galaxy, but rather its own Galaxy, thus settling the Great Debate forever.

This ability to measure distances to other galaxies would eventually force astronomers to stop assuming the Milky Way is at the center of the Universe, and paved the way to understanding the scale and structure of the Universe, enabling us to establish that the Universe is currently expanding!

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