Lois & Clark Forums Forums General Discussion Off Topic The Big Bang and the Universe, Part 3: Enter Henrietta and Hubble!

Okay, the last part was slightly boring, wasn't it? It was all about the stars, not about the universe. But if you don't understand the stars, you can't know about the universe.

I called this part "Enter Henrietta and Hubble", but we've got to start with William, William Herschel (1738-1822), composer turned astronomer.



Born in Germany, Herschel moved to Britain, where he took up a career in astronomy along with his sister Caroline. William and Caroline were both good at using the telescope and good at making discoveries with it, and in 1781, William became the first person since antiquity to discover a new planet. Herschel saw Uranus in his telescope and realized that he was looking at a planet, not a star. Not bad!



Aqua-colored Uranus displays a featureless disk.

But for the purpose of my article here, Herschel is more important because he tried to determine the position of our own solar system in relationship to the universe as a whole. Herschel had no idea that there were galaxies in the universe, let alone that there was more than one! But he set out to find the position of the solar system among all the stars in the heavens by counting all the stars he could see in every direction. After he was done counting, he had produced a map of the starry sky which looked like this:



The bright dot near the middle is the Sun. Based on this map, Herschel concluded that the Sun and the Earth are situated close to the center of the universe! The reason why he made this conclusion was that he thought that he saw a more or less equal number of stars in every direction of the sky as he looked at it from the position of the Earth, and therefore the Earth (and the Sun) ought to be pretty much in the middle. But Herschel didn't realize that there are huge quantities of dust that hides much of our galaxy from view.



William, you couldn't see the galaxy for all the dust it contains!

Like I said, back in Herschel's days nobody knew that galaxies existed in the first place. The unawareness of the existence of galaxies was, in a way, like the unawareness of the true character of the Earth back when people had no way of getting an overview of the Earth:

Flat Earth

The discovery that the Earth was not at the center of the universe was made by a young American named Harlow Shapley:



Shapley was interested in the largest stellar clusters known, the globular clusters:



Shapley believed that we live in a huge collection of stars of some kind (a galaxy), and that the distribution of the globular clusters in the sky might tell us something about the shape of the galaxy that we live in and our own place in it. In order to determine the distribution of globular clusters in the sky, Shapley needed to measure the distance to them. But how on Earth could he do that? Even today we are completely unable to use triangulation and parallax to measure the distance to the globulars.

Okay, enter Henrietta!

Henrietta Leavitt, an underappreciated heroine of astronomy! Apparently only two photos exist of her, and this is one of them:



This isn't exactly what you'd call a heroic portrait. She looks rather bashful, probably because she was made to feel that she was the sort of person who really ought to stay in the background. During her lifetime she was mostly ignored. No heroic portraits were made of her. But her discoveries were important enough that even the Hubble Telescope has scoured other galaxies for the crucially important distance-indicating stars that she discovered.



The Hubble Telescope determined the distance to galaxy M 100 by measuring the brightness of the galaxy's population of Cepheid variables, the crucially important variable stars that Henrietta Leavitt discovered.

Leavitt, 1861-1921, was hired by the astronomy department of the Harvard University to do the menial task of counting stars on photographic plates taken by other astronomers. (Women were not allowed to operate telescopes at Harvard in Henrietta's days.) Nobody expected Henrietta Leavitt to do any thinking or reflection of her own. She might as well have been an unthinking monkey or a computer - and actually she was hired to be a "computer", which in those day simply meant a person doing the menial task of counting a large pile of boring and unintelligible data!

Among the boring things that Henrietta Leavitt was supposed to count was the number of stars on photographic plates of the Milky Way's satellite galaxy, the Small Magellanic Cloud:



At Harvard, astronomers took several pictures of the same parts of the Small Magellanic Cloud only a few days apart. The sharp-eyed Henrietta discovered that there were stars that varied in brightness from one photographic plate to another taken just a few days later. On the first plate the star might be bright, then on the next plate it was faint, then on the next plate it was bright again. Henrietta Leavitt had discovered variable stars in the Small Magellanic Cloud.

But that was not all. The sharp-eyed and intelligent Henrietta soon noticed a remarkable pattern among the variable stars she had discovered: the brighter they were, the more slowly they varied! Their variations were also very precise, so that these stars could be recognized wherever they were found thanks to their well-determined variations. Thanks to Henrietta, astronomy had found its first "standard candle", a light source whose intrinsic brightness could be known even when the star was very far away.

There was of course a problem, because back then there was no way of knowing the true distance to the Small Magellanic Cloud, and there was no way of knowing the true, intrinsic brightness of Henrietta's variables. But even so, her discovery soon proved extremely useful for probing both the Milky Way and the galaxies beyond it!

Before I leave this post and open another to talk about how her discovery was used to do groundbreaking astronomy, I need to point out how ignored she was in her lifetime and a long time afterwards. The kind of variable stars she discovered were named "Cepheids", which was a name that had nothing to do with Henrietta herself or the Small Magellanic Cloud where she had found her variables. Instead, the stars were named after the discovery made by a deaf young English boy, John Goodricke (1764-1786), who discovered and described the variable star Delta Cephei, found in the constellation of Cepheus. Not that this poor boy, dead at 21, didn't deserve this honor.



John Goodricke.



Constellation Cepheus. Delta Cephei, which is the same kind of star that Henrietta Leavitt discovered a batch of in the Small Magellanic Cloud, is the whitish star on the left.

It was right that astronomy honored John Goodricke, but they should have honored Henrietta Leavitt, too!

Oh, and by the way - take a look at this link , which is so easy to understand and informative that you won't need me yammering away at you!

I should add that modern astronomy knows quite well how bright Cepheid variables really are. The very slowest, very brightest of them are about 30,000 times as bright as the Sun. Most of those detected in other galaxies are a few thousand times as bright as the Sun. But the thing is that we can know, pretty much, exactly how bright they really are just by measuring how fast they pulsate. And if we know how bright a Cehpeid variable is in another galaxy, then we also know how far away that Cepheid variable is by measuring how faint it appears to be, because the relationship between known actual brightness, measured apparent faintness and true distance is an easy thing to calculate (at least usually). And when we know how far away the Cepheid variable is, then we also know how far away the galaxy is in which the Cepheid variable was found. It's amazing, isn't it? And it's all thanks to Henrietta!

Ann

Well, we need to get back to Harlow Shapley in a hurry, the young American astronomer who discovered the Earth's true place in the Milky Way. Shapley realized that the huge collection of stars called globular clusters contain so much mass that they ought to orbit the center of the Milky Way. Shapley needed a way to measure the distance to the globular clusters to determine their distribution in the sky.



The Milky Way seen edge on, orbited by globulars. If the Earth and the Sun are close to the Milky Way's center, then the globulars ought to seem to be orbiting us.

There was no way for Shapley to measure the distance to the globulars directly, but he had heard of Henriett Leavitt's discovery of variable stars in the Small Magellanic Cloud, and he started looking for such stars in the globulars. Because if he could find such variables in the globulars, then he could determine the relative, if not the absolute, distance to the globulars, and he could place them all in the correct relative distance from each other and the Earth.

And Shapley did indeed find variable stars in the globulars! He found short-period variables, relatively faint stars, but bright enough to be discovered, and he found them in almost every globular cluster he looked at. Thanks to these variables (called RR Lyra stars, but really discovered by Henrietta Leavitt) he could determine the relative distances to the globular clusters. And they did not orbit the Solar system at all! This was the distribution of the globular clusters as seen from the Earth:



The "x" in the green circle represents the position of the Earth, and the black dots and circles represent the globulars. Almost all the globulars seemed to be way off on one side! Shapley found that the globulars orbited not the position of the Earth, but the constellation of Sagittarius instead.

Gobular clusters orbiting the constellation Sagittarius

Shapley realized that the center of the Milky Way was in the constellation Sagittarius. The reason why William Herschel, for example, had not discovered the center of the Milky Way in Sagittarius, is that the center is deeply hidden in dust, at least from our point of view!



The center of the Milky Way is in Sagittarius, but it is hidden by dust.

So Shapley determined the position of the Earth in the Milky Way, which is quite far from the center.



But Shapley still thought that the Milky Way was the universe, and that everything that we could see in the sky either belonged to or orbited the Milky Way. It fell to Edwin Hubble, the great Sherlock Holmes of astronomy, to discover that it was not so!

Ann

Edwin Hubble, the Sherlock Holmes of astronomy!



Hubble.



Holmes.

They look much the same, don't they?

Okay! The first question that Hubble, this cosmic sleuth, tried to answer was this one: Is the Milky Way and its satellites (such as the Magellanic Clouds) all there is in the universe? Or is there more out there?

Is that all there is?

No, Hubble suspected that the Milky Way isn't all there is. Along wiht Hamlet, he suspected that there are more things in heaven and earth, Horatio, than are dreamt of in your philosophy!

The first thing Hubble set out to know about was the nature of a mysterious object known as the Andromeda Nebula, which in Hubble's days looked pretty much like this through a telescope:



Not too impressive, is it? It's not what the Andromeda galaxy looks like in pictures taken with modern instruments (this picture here is nothing special):

[img]http://cs.astronomy.com/asycs/photos/galaxies/images/426011/500x330.aspx[/img]

Back in Hubble's days astronomers disagreed about what the Andromeda Nebula, as it was called, really was. It was well known that there were luminous gas clouds in the Milky Way, which were called nebulae. The most famous nebula in the Milky Way is the Orion Nebula:



There are stars in and around the Orion Nebula, but most of the light from it comes from the glowing gas in itself. Many astronomers in Hubble's day argued that the "Andromeda Nebula" was like the Orion Nebula - it was a glowing gas cloud associated with a few stars, and it was well inside the Milky Way. Some people who held this belief argued that the Andromeda Nebula was in fact a rotating gas cloud in the process of giving birth to a star. If we just kept observing the Andromeda Nebula, we might see a star appear in the middle of it, they said.

How ironic that these "star formation" proponents turned out to be catastrophically right, although they were completely and utterly wrong! Because, yes, latter-day astronomy has indeed proved that stars are born out of rotating gas clouds:



A star being born in the middle of a rotating gas cloud.

As the worst possible astronomical bad luck would have it, in 1885 a supernova went off right next to the center of the Andromeda galaxy, and the "star formation" proponents were proved right, or so it seemed! They had predicted that a star would appear at the center of the Andromeda Nebula, and lo and behold, there was the star! Not even the fact that the supernova faded after a while could prove the "star formation proponents" wrong.

Well, Hubble wasn't satisfied, so he started photographing the Andromeda Galaxy, trying to learn the nature of this system from the stars his photos would reveal. But in those days it was hard to photograph Andromeda. It was comparatively easy to photograph the Small Magellanic Cloud, because this galaxy is relatively nearby (about 170,000 light years), it contains very little dust and not very many stars, but many of the stars it does contain are relatively bright and stand out well in photographs. The Andromeda Galaxy, by contrast, is about two million light years away, contains a lot of dust and billions upon billions of stars, is seen pretty much edgewise, and contains mostly old red stars which don't stand out very well in photographs, particularly not in Hubble's days, when the photographic plates were most sensitive to blue light.

But Hubble persevered. And guess what he found in the end as he kept photographing the Andromeda Galaxy?



This is a photograph made from a photographic plate taken by Hubble. And guess what, Hubble has discovered a Cepheid variable in Andromeda! You can see what Hubble himself has written on the photograph: 6 Oct 1923, the date of his discovery, and the abbreviation "Var" for variable star, followed by an exclamation point. Hubble was excited, because now he had found the kind of star that he could use to prove that the Andromeda galaxy was very far away, much farther away than the globular clusters of the Milky Way and much farther away than the Magellanic Clouds, too. Because Hubble had found one of "Henrietta Leavitt's stars" in the Andromeda galaxy. He had found a Cepheid star in it. And thus he proved that the Andromeda Nebula was in fact the Andromeda Galaxy, a huge, huge collection of stars in its own right, independent of the Milky Way.

And like I said in one of my posts above, even today the space telescope that carries Hubble's name is looking for Cepheids, Henrietta Leavitt's stars, in distant galaxies in order to determine the distance to them!

Ann