At the beginning of the 20th century, the universe seemed to be a much, much smaller place. At the time, astronomers believed that the Milky Way galaxy was all there was. They didn’t know there were billions of other galaxies; they didn’t know how small we really are.
They didn’t know this because they couldn’t measure distances to distant stars. Why? There was a fairly simple problem in astronomy: a bright, distant star looks almost the same as a faint star nearby.
It’s the same here on Earth. Imagine being on the beach at night and seeing two lighthouse lights glowing in the distance, but one seems brighter than the other. If you knew both lighthouses used the same bulb, you could conclude that the fainter light is farther away. But it’s also possible that the weaker light is simply coming from a lower wattage light bulb, perhaps closer to you.
Scientists needed a way to figure out the intrinsic brightness of stars – to calculate their wattage, so to speak. That’s when Henrietta Levitt, a Massachusetts-born “computer” who worked at the Harvard College Observatory, came along. 1908, she published a discovery that may sound small, but is one of the most important in the history of astronomy. As we discuss in this week’s Unexplainable podcast (see the embed above), the universe broke open.
Flashing lights are a yardstick to measure the universe
For Henrietta Leavitt, many astronomers looked at the stars in what is known today as the Andromeda galaxy – some 2.5 million light years away – and mistakenly thought they were part of our own Milky Way Galaxy (that’s just about 100,000 light-years in diameter).
Those Andromeda stars were orders of magnitude further away. Scientists just didn’t know.
astronomers at the time had some methods to calculate distances to stars, but they only worked for stars relatively close to Earth. Leavitt’s discovery — linking the pulse of one type of star to their actual brightness, as described in the above image — was key to measuring objects moving further and further into space.
If astronomers wanted to measure distant things, Leavitt’s discovery showed, they just had to look for Cepheid variables. Her formula led astronomers to chart relative distances to stars: they could use it to compare two stars and figure out which one was closer.
It took a bit more work from other scientists to calibrate this measure, to put concrete numbers on it. But once they did and started measuring it, the cosmos grew and grew.
Leavitt paved the way for Edwin Hubble to discover galaxies beyond our own
Curtis believed that Andromeda was a separate galaxy, far, far away from the Milky Way. At the time, this was a bizarre idea. Shapley represented the more mainstream view — that Andromeda was just a hazy, cloudy region of our galaxy that he had recently estimated was there 300,000 light-years across. That was also the supposed size of the entire universe.
If Curtis was right, that would mean the universe was two or three times the size of Shapley’s estimate — anyway.
To settle the debate, Edwin Hubble – the namesake of the famous space telescope – searched for Cepheid stars in Andromeda. Evening after evening, he took pictures of Andromeda, looking for Cepheids. In October 1923, he found one, flashing in one of Andromeda’s spiral arms. Another week observations enabled him to follow Leavitt’s formula and determine the distance.
Hubble estimated it to be about a million light-years from Earth – well beyond the boundaries of Shapley’s universe. (Hubble was a bit from: Andromeda is closer to 2.5 million light years away.) After reading about the Hubble finding, Shapley reportedly said said: “Here is the letter that destroyed my universe.”
Scientists continued to build on Leavitt’s ruler to measure the universe. And as they used these measuring tools, their understanding of the universe evolved. They realized that it was much bigger than previously thought, there are billions of galaxies and it is expanding: those galaxies are moving further and further away from each other.
Astronomers also realized that the universe had a beginning. If galaxies are now moving away from each other, it means they were closer together in the past – which gave scientists the idea of the big bang.
It also led them to realize that the universe could eventually end.
This week’s episode of Inexplicablecafemadrid’s podcast about unanswered questions in science tells that story and more.