SCIENCE: OUR PLACE IN THE UNIVERSE

Published March 31, 2024
A view of the Andromeda Galaxy. The lower arrow points to the historic picture taken by Edwin Hubble in 1923. The upper arrow points to the close-up of the Andromeda Galaxy taken by the Hubble Space Telescope
A view of the Andromeda Galaxy. The lower arrow points to the historic picture taken by Edwin Hubble in 1923. The upper arrow points to the close-up of the Andromeda Galaxy taken by the Hubble Space Telescope

Blame the star.

Humans had just started to recover from the realisation that the Earth moves around the Sun and not the other way around. The Polish priest Nicolaus Copernicus (1473-1543), building on the observations and models of other mediaeval scholars, like the Persian scholar and astronomer Nasir al-Din al-Tusi (1201-1274) and the Arab astronomer and mathematician Ibn al-Shatir (1304-1375), had taken the audacious step of making the Earth move around the Sun.

He knew what he was doing to the self-esteem of fellow human beings. He announced the demotion of the Earth from his deathbed in 1543.

But this demotion also meant that our Sun was just like other twinkling pinpoints of light. It just happened to be close to us, which made it appear big and extremely bright, especially during the daytime.

We took the demotion on the chin. We still thought that the Sun was at the centre of the universe. Even though we were stuck in the gravitational well of our star, this was still not a bad deal for us. We could still be important, riding on the Sun’s coattails.

How a remarkable discovery 100 years ago about stars that varied in brightness helped upend humanity’s view of the known universe

This was working well. But then this all changed almost exactly 100 years ago.

A REMARKABLE DISCOVERY

In October 1923, American astronomer Edwin Hubble used the 100-inch telescope on the Mount Wilson Observatory in California to detect a star that changed its brightness in a fuzzy cloud, known at the time as the Andromeda Nebula.

You can see for yourself this fuzzy Andromeda. In fact, Persian astronomer Abd al-Rahman al-Sufi (903-986) had noted this fuzzy patch in his beautifully illustrated The Book of Fixed Stars (Kitb suwar al-kawkib al-bita), written around 964.

Later, telescopes revealed that this was a collection of stars. And there were others like it, but too faint to be visible to the naked eye. But how big and how far away were these “spiral nebulae”?

Harvard astronomer Harlow Shapley, and many others, believed that these were all part of the Milky Way and that the Milky Way was the entire universe. Some, like Edwin Hubble, suspected that these “spiral nebulae” were “island universes”, similar to the Milky Way itself. They looked small and fuzzy because they were at incredibly large distances — distances that were unimaginable at that time.

These were two very different views of the universe. One would turn out to be right and the other wrong. Science can be cruel that way.

The larger of the two fish is overlapping the body of Andromeda in this illustration from al-Sufi’s Book of the Fixed Stars. The cluster of black dots at the mouth of the fish represent the Andromeda Galaxy — the first known record of it | University of Oxford
The larger of the two fish is overlapping the body of Andromeda in this illustration from al-Sufi’s Book of the Fixed Stars. The cluster of black dots at the mouth of the fish represent the Andromeda Galaxy — the first known record of it | University of Oxford

THE DECISIVE FACTOR

It all came down to one star and the incredible work of a Henrietta Leavitt.

Leavitt was a ‘computer’ at Harvard College Observatory in the early 1900s. ‘Harvard computers’ were a group of skilled women who analysed astronomical data taken from photographic plates on various telescopes. Henrietta Leavitt studied stars that varied in brightness in a predictable way.

She identified a particular group of stars, called Cepheid variables. She studied 1,777 such stars. Of particular interest were 25 stars that could be seen from the southern hemisphere. These were part of a collection of stars called the Small Magellanic Cloud.

Crucially, this meant that all 25 stars were located roughly at the same distance from us, allowing her to identify their brightness relative to each other.

She discovered that not only do these stars vary in brightness over a period of time, but their brightness is related to the intervals. For example, a star that varies in brightness every five days is a little fainter than the one that varies every 12 days. This came to be known as the period-luminosity relation or Leavitt’s Law.

A BRIGHT FLAME IN THE UNIVERSE

This observational insight turned out to be one of the most important in astronomy. Leavitt had discovered a ‘standard candle’ that would help measure distances to far-off stars.

The foundation of this is relatively simple. You can think of lighted candles during load-shedding at night. If the candle is close to you, it will look brighter. Dimmer, if it is farther away. Since we intuitively know the brightness of a flame, we can guess the distance to the candle, based on how bright it appears.

In Cepheids, Henrietta Leavitt had found a candle in the night sky. If you can measure the brightness cycle of a Cepheid star, you can figure out how bright it should be. If it appears fainter in the sky, then that means that it is farther away.

We also know that if you double the distance, the light fades away four times. If you triple the distance, the light fades away nine times (for science nerds, this is called inverse square law). Crucially, this allows astronomers to calculate distances to stars.

A Cepheid variable star could then resolve the debate over the nature of spiral nebulae.

Almost a hundred years ago, in October 1923, Edwin Hubble found the first Cepheid star in Andromeda. The cycle of brightening and dimming revealed that this Cepheid was at an enormous distance, far away from the Milky Way.

Therefore, Andromeda was a galaxy just like our own. It only looks faint and small because of its enormous distance. Today, we know that it is over two million light years away!

Hubble had fundamentally altered the view of our universe. We were just recovering from the demotion of the Earth from the centre of the universe. Now, we discovered that the universe was also unimaginably big.

When the news of Edwin Hubble’s new distance estimate to Andromeda reached Harlow Shapley, he is known to have commented that it “destroyed my universe”, underlining the fact that the universe does not necessarily care about our cherished ideas.

Henrietta Leavitt surely deserved a Nobel prize for making one of the most important discoveries in astronomy. Edwin Hubble even nominated her for the award in 1925. Unfortunately, she had already passed away in 1921, at a relatively young age of 53, and the award is not given posthumously.

Today, we know that there are more than a hundred billion galaxies in the known universe. But, so what? The Milky Way is still our galaxy, and we must surely be still be at its centre.

But it isn’t so and we have to pin some of the blame for that on Harlow Shapley.

OUR PLACE IN THE UNIVERSE

Like a good scientist, Shapley picked up the pieces of his destroyed universe and went on to use Cepheids (and other types of variable stars) to help determine the shape of the Milky Way and our position in it.

He found that, instead of being at the centre of the Milky Way, the solar system is floating on the outskirts of the galaxy.

Today, we know that the Sun is part of an inconspicuous spiral arm, located about 30,000 light years from the Milky Way centre.

We cannot, yet, go outside of the Milky Way to take a selfie. However, we can look at the spiral arms of the Andromeda galaxy and imagine a Sun-like star there, with orbiting planets and, perhaps, with lifeforms thinking about their place in the universe.

In the close-up, we only see noise — dots spread throughout the image. But each dot here is a star. We cannot see it, but we expect planetary worlds orbiting around each of these dots. And some of those worlds may even have species that can figure out their own insignificance in this vast cosmos.

Homo sapiens, located on a rocky world in the outskirts of the Milky Way, are beginning to understand the immensity of our universe. For this, they have to thank a type of star that varies in brightness, and a brilliant woman who figured out how to use this variation to measure distances to faraway galaxies.

Viva homo sapiens, viva curiosity!

The writer is professor of Integrated Science & Humanities at Hampshire College, US. He is also an astronomer affiliated with the Five College Astronomy Department in Massachusetts and hosts an Urdu language YouTube channel, Kainaat Astronomy

Published in Dawn, EOS, March 31st, 2024

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