INFO 04-1995: A Year of Hubble Results
6 February 1995After a year the refurbished Hubble Space Telescope is looking further into space with unprecedented clarity than any other instrument and things are not quite as astronomers had expected.
"We are beginning to understand that because of these observations we are going to have to change the way we look at the Universe," said ESA's Dr Duccio Macchetto, Associate Director for Science Programs at the Space Telescope Science Institute (STScI), Baltimore, Maryland, USA.
The European Space Agency plays a major role in the Hubble Space Telescope programme. The Agency provided one of the telescope's four major instruments, called the Faint Object Camera, and two sets of electricity-generating solar arrays. In addition, 15 ESA scientific and technical staff work at the STScI. In return for this contribution, European astronomers are entitled to 15 percent of the telescope's observing time, although currently they account for 20 percent of all observations.
"This is a testimony to the quality of the European science community," said Dr Roger Bonnet, Director of Science at ESA. "We are only guaranteed 15 percent of the telescope's use, but consistently receive much more than that."
Astronomers from universities, observatories and research institutes across Europe lead more than 60 investigations planned for the telescope's fifth observing cycle, which begins this summer. Many more Europeans contribute to teams led by other astronomers.
Looking back to the very start of time
European astronomer Dr Peter Jakobsen used ESA's Faint Object Camera to confirm that helium was present in the early Universe. Astronomers had long predicted that 90 percent of the newly born Universe consisted of hydrogen, with helium making up the remainder. Before the refurbished Hubble came along, it was easy to detect the hydrogen, but the primordial helium remained elusive. The ultraviolet capabilities of the telescope, combined with the improvement in spatial resolution following the repair, made it possible for Dr Jakobsen to obtain an image of a quasar close to the edge of the known Universe. A spectral analysis of this picture revealed the quasar's light, which took 13 billion years to reach the telescope, had indeed passed through helium, and not only that, the helium was of just the right variety to match the established theory.
Dr Jakobsen has spent more than 20 years working on this subject. His recent efforts concentrated on seeking out a quasar unobscured by clouds of hydrogen, which block the tell-tale signature of helium. His search drew him to the Space Telescope project and during the telescope's early years in orbit he studied 25 likely quasars and found one promising candidate.
Dr Jacobsen then had to wait for the telescope's new optics before he could get the quality of data he needed to prove the existence of helium.
"We were looking for a break in the cloud cover, so to speak," the astronomer said. "We had a tantalising glimpse of the quasar with the aberrated telescope but it was only after we fixed it that we could really get a clear answer. One of the first things that we did once we had the corrective optics in place was look at this object and it was exactly as we'd hoped."
Getting the Universe to measure up
When it comes to studying the expansion of the Universe, however, the telescope has raised morn; questions than answers. By determining how fast the Universe is expanding astronomers will be able to calculate its age and size. It may then become possible to discover what is the ultimate fate of the Universe; will it simply continue to expand until it evaporates? Will the expansion come to a complete stop? Or will the Universe stop expanding, start contracting and end in a big crunch?
The rate at which the Universe expands is known as the Hubble Constant or H0. To measure this value, astronomers need to calculate how far away a galaxy is and how fast it is moving away from us. The former is difficult to determine because reliable distance indicators, sometimes known as cosmic yardsticks, such as variable stars and supernovae, must be found in the galaxies.
An international team of astronomers recently used the Hubble Space Telescope to make accurate measurements of the distance of M100, a far away galaxy located in the Virgo cluster of galaxies. To do this they used a number of Cepheid variable stars, rare objects that change in brightness over a regular period. Because astronomers know that there is a direct link between the period of the Cepheid's pulsation and its actual brightness, they can do a simple calculation to work out the distance to the object by comparing its actual brightness with how bright it appears to Hubble. The astronomers had to study more than 40 000 stars before finding the 20 Cepheids they used for their calculations.
The results revealed that M100 lies 56 million light years from Earth. With this, astronomers calculated that the Universe is expanding at a rate of 80 kilometres per second per megaparsec (1 megaparsec = 3 261 600 light years). This is much faster than astronomers had expected.
On the basis of this value for the Hubble Constant, the Universe must be aged somewhere between eight and twelve billion years old. But, this flies in the face of established facts. We know there are stars in our Universe that are 16 billion years old - how can they be older than the Universe in which they exist? It could be that the theory that explains the evolution of stars or the Big Bang theory are wrong. Or perhaps it is the observations that are incorrect.
Hubble astronomers soon hope to solve this riddle by taking further measurements to refine their figures. "This is a programme that we know is going to give us some really firm answers in the next three to five years," said Dr Macchetto, one of the 15 European astronomers at the Space Telescope Science Institute. "It will take that time to collect and analyse enough data."
Part of the Universe is missing !
Once astronomers know the expansion rate of the Universe they will be one step away from determining its fate. But it will be a big step - for astronomers will have to work out the mass of the Universe and according to current theories some of our Universe is missing!
There just isn't enough visible matter in our Universe to account for known gravitational effects, such as the rotation of galaxies. As much as 90 percent of our Universe could be invisible to astronomers. The only other explanation is that our understanding of gravity is seriously wrong.
"It's quite an embarrassing situation for scientists. We like to tell people that we know what we're doing but we can't find a good part of the mass in the Universe," said Dr Francesco Paresce, an ESA astronomer based at the STScI. "This is perhaps one of the most fundamental issues today in astronomy. The amount of matter determines almost everything about the Universe."
The Hubble Space Telescope has joined the search for the dark (or missing matter) and so far it has eliminated one likely theory. Two groups of astronomers, one of which is led by Dr Paresce, have determined that the missing matter is not contained in dim stars called red dwarfs, which - before Hubble - were thought to be widespread. Astronomers expected to find a large number of these faint objects but instead they found, relatively speaking, only a handful.
"This throws a big spanner into the whole subject, because all of a sudden you're saying that it can't be the simplest explanation that we all had. It's going to get a lot more complicated from now on," said Dr Paresce.
Astronomers will now have to find another explanation for the missing mass. One possibility is that this elusive matter is not matter as we know it, but is actually mysterious exotic particles.
Are we alone ?
Hubble's work is also raising questions about the probability of life elsewhere in the Universe. Observations of young stars in the Orion Nebula have revealed that more than half are surrounded by discs of dust and gas, material that may be the building blocks of planets. Before, astronomers were aware of only a few stars with these so-called proto-planetary discs. The apparent abundance of these discs means that many more stars than originally thought could have planets.
"This is going to change the way we look at a number of things, including star formation and perhaps the more remote question: is there life in the Universe?" said Dr Macchetto.