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

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