The Hubble Space Telescope (Hubble) will be 25 years old this week. Hubble is not the biggest telescope nor does it have the most advanced detector technology, yet it is arguably the most important observatory. Why is this the case?
In short the advantage of Hubble is that it is in space. Hubble is situated above Earth’s atmosphere away from the turbulent air and bright city lights, and as a result we get a much crisper view of the universe.
When we do have the rare opportunity to look up at the night sky with our own eyes to marvel at the panoply of stars, we often note the ’twinkling’ aspect as being noteworthy and even special. What is the cause of this twinkling?
Many stars do vary in brightness but it is at such a low level as not to be detectable by the unaided eye. The twinkling is caused by the light passing through different layers of air on its way to our eyes.
The effect is similar to what happens when a straight stick is placed in a tub of water. We note that stick will appear to us to be bent but when we take the stick back out of the water to examine it we find that the stick is still straight.
This is just an optical illusion which we call refraction. Refraction happens to light too. As a beam of light from a star traverses different layers of Earth’s atmosphere, the ‘stick of light’ is bent in a different direction by each layer, with the sum total being that the exact position of the star appears to change about every 0.15 seconds.
Put another way, the star appears to be in an ever so slightly different location on the sky every small fraction of a second, but the star is not actually dancing on the sky. As with the stick, this is just an optical illusion. Basically, the greater the amount of the twinkling the worse the weather is above our heads.
Needless to say astronomers do not find stellar twinkling to be at all amusing! When we take long exposures of a star through a telescope on the ground, the net effect is a big blob on our
photograph in place of where a tiny pinpoint of a starlight ought to have been.
This view changes completely when we look at a star with Hubble. At higher spatial resolution we are able to view a star which looks much close to its true self. As a result we are able to study objects
in space in much more detail than we can manage in most cases from the ground. Even to this day when I see a new picture coming in from Hubble I am so awestruck that I nearly fall out of my chair!
No one knows how much longer Hubble will continue to operate, and when it fails there will not be a chance to repair it. Fortunately, we can look forward to Hubble’s successor, the James Webb Space Telescope, which will be larger and more powerful than Hubble and will launch in 2018.