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Favourite Things 3

This blog entry continues my series on my favourite things. I want to do another with a technology, space-based theme. Last time it was the Voyager spacecraft, this time it is the Hubble Space Telescope.

The HST is nearing the end of its life and it could so easily have been a total failure from the beginning, yet it has provided some of the greatest pictures ever of space, from relatively nearby planets in our Solar System to galaxies on the very edge of the observable Universe.

These pictures have not only been of great scientific value but they have been both beautiful and awe inspiring to many non-scientists. They have increased our knowledge of the Universe and increased the enthusiasm the public have for both astronomy in general and the increasingly beleaguered space program.

So let’s start at the beginning. The facility is called the “Hubble Space Telescope” so who or what is Hubble?

Edwin Hubble was a great American astronomer who mainly worked around the 1920s. He made several incredibly important discoveries: first, confirming that the Milky Way galaxy isn’t the whole Universe (yes, that knowledge is less than 100 years old); and second, discovering that the more distant a galaxy is the more quickly it is receding from our galaxy (the Hubble Law), in other words that the Universe is expanding (up until then it had been assumed that it was static).

He was a brilliant and dedicated observer rather than a theorist but in astronomy (and most other sciences) observation both leads and confirms theory and the two branches are equally important in increasing our understanding of the universe.

Some of Hubble’s work was best described as gruelling. At the time a lot of information was gathered by taking spectra of distant galaxies onto photographic plates. But the plates had limited sensitivity, the galaxies were very faint because of their distance, and the process of creating a spectrum made the light even fainter. So it was necessary to expose the plates for a long time to get a good image.

Because of the Earth’s rotation the stars move across the sky and the telescope’s drive (which moves the scope to track the stars) needed constant correction by the observer. So the astronomer had to sit in the cold looking through an eyepiece and correcting the tracking for long periods of time.

Hubble took some photos with exposures much longer than could be done in one night. He actually closed the shutter before sunrise and pointed the scope back at exactly the same place the next night and continued the same exposure on the same plate. In fact some exposures might have taken a week of all night guiding to get a single plate! It is an extraordinary example of skill and patience that he succeeded so well with these observations.

The big discoveries Hubble made settled some major questions at the time. There was a debate about whether our galaxy was the whole universe, because many people thought other galaxies were just smaller localised nebulae embedded in our galaxy. Hubble’s detailed observations showed the nebulae were really galaxies as big, and often bigger, than our own. So the size of the universe expanded by a factor of billions virtually over night.

His discovery of the expansion of the universe caused another even greater revolution. The spectrum for any bright object has a number of colours visible which show the elements it is made from (this explanation isn’t completely technically correct because I have simplified it here). Hubble found the pattern expected (because stars everywhere produce light through the same process) but in the wrong part of the spectrum. His conclusion was that the light was “red shifted” which is caused by an object moving away from the observer and “stretching” the colour of the light into the red.

He expected that he would find about the same number of galaxies red shifted (moving away from us) and blue shifted (moving towards us) but it soon turned out that almost every galaxy was moving away. Not only that, but the further away the galaxy was (determining distances in space is another complex and fascinating subject) the faster it was moving away.

The only logical conclusion is that the whole fabric of space is expanding. Note that any observer in any galaxy would see the same expansion pattern so this doesn’t mean that we are in a special place. The few close galaxies which are moving towards us are explained by the fact that galaxies also move around randomly but only with the closer ones, where the relative universe expansion is smaller, can the overall expansion be overcome by this smaller local motion.

And if the universe if getting bigger then in the past it must have been smaller, and go back far enough and it would have been zero size. So the potential for a beginning, which eventually lead to the Big Bang Theory, was then understood.

Even Albert Einstein had put a special factor into his equations for relativity to make the Universe static. Initially his work indicated it should be expanding but few people believed this at the time. So Einstein added the “cosmological constant” to balance gravity and make the universe static.

A few years later when Hubble showed that it was expanding Einstein admitted the constant had been “the biggest blunder of his career”. Ironically in the last few years it has become obvious that there is something very similar to the cosmological constant at work in the universe (so called “dark energy”) causing the rate of expansion to increase (it was previously thought it would decrease due to the effect of gravity) so maybe Einstein was right even when he thought he was wrong!

So you can see that the observations Hubble made really did revolutionise our understanding of the universe. Earlier in his life he was considering a career in law. What a waste that would have been!

I have just noticed that I have written a blog entry about Edwin Hubble, not the Hubble Space Telescope as I originally intended. It’s great the way one subject can lead to another, isn’t it? Since this entry is probably long enough already I will continue my thoughts on the HST in my next blog entry.

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