the revolution of edwin hubble -- 9/23/20
Today's selection -- from Galaxies: Inside the Universe's Star Cities by David J. Eicher. The revolution of Edwin Hubble, the man for whom the Hubble Space Telescope is named:
"On October 4, 1923, in ... [a Southern California] paradise, a brash young astronomer left his Pasadena house and trekked up to the Mount Wilson Observatory, not far from Los Angeles itself, to the 100-inch Hooker Telescope, at the time the largest telescope in the world. Originally from Missouri, Edwin Powell Hubble had moved to Illinois, graduated from the University of Chicago, and then earned a master's degree as a Rhodes Scholar at Oxford University. He embarked on a career in astronomy only after returning to school at the age of twenty-five to pursue a PhD. Hubble was now in his fourth year as a staff astronomer at Mount Wilson. He relished using the 100-inch Hooker Telescope to study his favorite subject: the fuzzy nebulae -- mysterious, glowing gas clouds -- that appear scattered across the sky.
"No one fully understood these nebulae, although they were suspected to be the birthplaces of stars. The adventurous amateur astronomer William Parsons, Third Earl of Rosse, had first sketched nebulae with spiral structures that looked like faintly glowing whirlpool patterns, using his mammoth telescope in rural Ireland in the mid-nineteenth century. But even nearly a century later, little more was known about them. Hubble was interested in cracking the code of nebulae, particularly spiral nebulae. His PhD work had centered on the topic. These nebulae's spiral shapes suggested that they are rotating, but otherwise they mystified Hubble and other astronomers.
|Sharpest ever view of the Andromeda Galaxy. Click on image to learn more.
"On the night of October 4, 1923, Hubble used the 100-inch Hooker Telescope to take a forty-minute exposure of one of his favorite nebulae, the Great Nebula in the Andromeda constellation. This spiral-shaped cloud was large, bright, and faintly visible to the naked eye as a fuzzy smear of light for those located away from the city lights of Los Angeles. The night had very poor 'seeing' when he took the exposure because Earth's atmosphere was relatively turbulent that night, and so the star images were not perfectly small dots. Nevertheless, Hubble's examination of the photographic plate he had made revealed a suspected nova: an exploding star. It was exciting to record such a relatively rare event inside one of the spiral nebulae.
"Hubble photographed the Andromeda nebula again the next night, hoping for a better-quality image of the suspected nova. The resulting photographic glass plate, exposed the night of October 5-6 and designated H335H, would become one of the most celebrated in all of scientific history. On it, Hubble successfully recorded the nova again. But before he could analyze it further, his periodic observing run on the 100-inch telescope ended and he had to leave to accommodate other observers. Early in the morning, he left the mountain and returned to Pasadena.
"At his office down in Pasadena, away from the mountaintop observatory, he continued studying earlier images of the Andromeda nebula region taken by others. And then he made an unusual discovery. A nova brightens dramatically and then fades into oblivion. But the star he recorded appeared on older plates, brightening and fading regularly over a period of thirty-one days. This star was not a nova, then. It had to be some other kind of star inside the Andromeda nebula.
"Suddenly, Hubble came upon the solution. He realized that he had made an image of a type of star similar to a well-known one in the constellation Cepheus. On his photographic plate H335H, he crossed out 'N,' for nova, and wrote 'VAR!,' denoting a variable star. Moreover, this star was a special type of variable that brightened and faded in a precise way. Astronomers had long studied this kind of star, which came to be known as a Cepheid variable (after a star in the constellation Cepheus), and they knew how intrinsically bright it was. By knowing how bright the star really was and measuring how bright it appeared to be in the sky, Hubble could use the star as a guidepost to gauge the distance to it.
"This was a monumental realization. Hubble calculated that, owing to the star's faint light, it must lie a million light-years away -- and so must the entire nebula that surrounds it. This meant that the universe stretches across a distance at least three times larger than most astronomers then believed. With his photographic plate, Edwin Hubble had single-handedly reset the size of the cosmos.
"Hubble's discovery set off a firestorm of activity among astronomers researching other spiral nebulae. Countless observations followed, and follow-up studies rolled on for many months as bickering and soul-searching lit up the world of professional astronomy. Adding fuel to the fire was a debate staged several years earlier, in 1920, between two prominent astronomers of the day, Harlow Shapley of Princeton University and Heber Curtis of the Allegheny Observatory. Shapley believed that the Milky Way Galaxy constitutes the entire universe, while Curtis speculated that spiral nebulae are separate galaxies from the Milky Way Galaxy -- essentially 'island universes.' Though not everyone would concede it yet, Hubble's discovery seemed to prove that Curtis was right.
"Hubble continued imaging Cepheid variables in other spiral nebulae, such as M33 in Triangulum, demonstrating that they, like Andromeda, are so far away that they must be distant galaxies. Hubble's observations indicated that galaxies are the basic units of stars, gas, and dust in the universe, and that they exist on a fantastic scale. He had many doubters, chief among them Shapley, but Hubble pushed on. The findings of the confident thirty-five-year-old were subsequently splashed onto the front page of the New York Times by November 1924. Egged on by supporters, he sent a paper summarizing the results to be read at the winter meeting of the American Astronomical Society, the professional organization of astronomers, on New Year's Day 1925. After the distinguished professor Henry Norris Russell of Princeton University read the paper aloud at the gathering, galaxies were on their way to becoming widely accepted. ...
"Hubble's work, building on the earlier studies of Slipher and the astronomer Milton Humason, showed that, generally speaking, all galaxies are moving away from each other over time. Hubble also found that redshifts can be used to calculate distances to galaxies.
"This research led to a monumental realization. In 1929, Hubble, with a helpful assist from the Belgian astronomer Georges Lemaitre, suggested that the new data he collected about galaxies supported the theory that if traced backward in time, the paths of all of the galaxies led to a small, dense point at which the whole universe began -- a 'Big Bang' billions of years ago. This Big Bang commenced the expansion that is causing all galaxies to move apart from one another more quickly in space. The whole universe seems to be flying apart.
"Hubble analyzed forty-six galaxies and proposed what came to be known as the Hubble Constant, the rate of expansion of the cosmos. He fixed this number as 500 kilometers per second per megaparsec of space, a higher value than what we know is correct today.
"Hubble's credibility skyrocketed following the determination of an expanding universe. This was big stuff: Hubble had piled on lots of supporting evidence for the ideas of the great physicist Albert Einstein, who had proposed in the previous generation that time and space are expanding and that the cosmos is almost unimaginably large."