The words ‘ Twinkle twinkle little star, how I wonder what you are ’ may have seeded the dreams of many children of becoming astronomers, but the twinkling of a star is ultimately a bane for astronomers. Stars twinkle because of disturbances in our atmosphere, and this effect blurs the images of celestial objects. From the time astronomers began to use photographs, they had wished they could place a telescope in space, in order to gaze upon the universe unhindered by the veil of the atmosphere.
And that wish came true twenty-five years ago with the launch of the Hubble Space Telescope (HST). With its 2.4m diameter mirror, this telescope has observed the universe in a range of wavelengths, from ultraviolet, visible to infrared, and has changed the way astronomers view the universe.
One of the first achievements of HST was to determine the rate at which our universe has been expanding. This requires an accurate measurement of the distances of far away galaxies, which is almost impossible with ground-based telescopes. This was one of the key projects of the telescope, and it was aptly named after Edwin Hubble, the discoverer of the expansion of the universe. The observations with HST settled the rate of expansion within ten per cent accuracy, ushering a new age in astronomy a quarter century ago.
The clear images taken with HST also helped to pinpoint distant supernovae of a special type that astronomers use to measure distances. This allowed the astronomers to measure the distances of even more distant galaxies, or equivalently peer back further in the history of the universe. This led to the momentous discovery that our universe is not only expanding, but also accelerating. No one knows yet why it does so, and it is a discovery that will haunt astronomers for some time to come.
The crash landing of the comet Shoemaker-Levy 9 on Jupiter gave a unique photo-op for HST in 1994. It was the first time a collision of a comet with Jupiter was studied in detail, a process that is supposed to occur once every few centuries. Another interesting discovery made by HST (last month, in March 2015) in our solar system is that of a subsurface ocean in Ganymede, a Jovian satellite.
Then in 1996, it was used to take a long exposure photograph of the universe through a hole in our Galaxy (Milky Way). The HST was pointed at a direction in the constellation of Great Bear for ten days at a stretch, and produced a photograph of numerous galaxies, among which there were some that existed in the very first billion years of the universe.
It told us how the process of star formation in galaxies first increased and then waned in the last few billion years or so. Astronomers are still trying to understand this evolution. Why did not the galaxies form stars in one bright display and then plunge into darkness? Why did the stars form in a ‘regulated’ manner over a long stretch of time?
Another revolutionary discovery in astronomy that HST helped to make was that almost every galaxy contains a black hole in its centre. It also appears that the black hole mass is intimately connected to the galaxy mass. The question that lurks behind this connection is this: does the black hole form first and fix the mass of the galaxy that should form around it, or is it the other way around? Or perhaps the black hole and the galaxy grow together, although no one has any detailed idea how that could happen. Observations with HST were crucial in this discovery because of its ability to measure the speed of gas near the centres of galaxies.
Coming back to the twinkling stars, one of the holy grails of modern astronomy is to understand the processes of the birth of stars. In this regard, the clarity of HST images and its ability to image in infrared wavelengths has helped astronomers to discover some crucial phases in newly born stars. It appears that the birth of stars is marked by a lot of violence, of gas being thrown out at great speed, sometimes in the form of jets. Technology has advanced in the last quarter century with which ground based telescopes can compete with HST in some aspects, although not all. But HST has created a legacy in astronomy, and science in general, that would be hard to beat even in the near future.
(The author is from Raman Research Institute, Bengaluru)