ESA Science & Technology - INFO 03-1997: European astronomers' successes with the Hubble Space Telescope

Their work spans all aspects of astronomy, from the planets to the most distant galaxies and quasars, and the following examples are just a few European highlights from Hubble's second phase, 1994-96.

A Scarcity of Midget stars

Stars less massive and fainter than the Sun are much numerous in the Milky Way Galaxy than the big bright stars that catch the eye. Guido De Marchi and Francesco Paresce of the European Southern Observatory as Garching, Germany, have counted them. With the wide-field WFPC2 camera, they have taken sample censuses within six globular clusters, which are large gatherings of stars orbiting independently in the Galaxy. In every case they find that the commonest stars have an output of light that is only one-hundredth of the Sun's. They are ten times more numerous than stars like the Sun.

More significant for theories of the Universe is a scarcity of very faint stars. Some astronomers have suggested that vast numbers of such stars could account for the mysterious dark matter, which makes stars and galaxies move about more rapidly than expected from the mass of visible matter. But that would require an ever-growing count of objects at low brightnesses, and De Marchi and Paresce find the opposite to be the case - the numbers diminish. There may be a minimum size below which Nature finds starmaking difficult. The few examples of very small stars seen so far by astronomers may be, not the heralds of a multitude of dark-matter stars, but rareties.

Unchanging Habits in Starmaking

Confirmation that very small stars are scarce comes from Gerry Gilmore of the Institute of Astronomy in Cambridge (UK). He leads a European team that analyses long-exposure images in the WFPC2 camera, obtained as a by-product when another instrument is examining a selected object. The result is an almost random sample of well-observed stars and galaxies.

The most remarkable general conclusion is that the make-up of stellar populations never seems to vary. In dense or diffuse regions, in very young or very old agglomerations, in the Milky Way Galaxy or elsewhere, the relative numbers of stars of different masses are always roughly the same. Evidently Nature mass-produces quotas of large and small stars irrespective of circumstances. This discovery will assist astronomers in making sense of very distant and early galaxies. They can assume that the stars are of the most familiar kinds.

Another surprise was spotted by Rebecca Elson in Gilmore's team, in long-exposure images of the giant galaxy M87, in the nearby Virgo cluster. It possesses globular clusters of very different ages. In the Milky Way and its similar spiral neighbour, the Andromeda galaxy, globular clusters contain the oldest stars. While M87 has ancient globular clusters too, some are different in colour and much younger.

The theory is that they were manufactured during collisions of the galaxies that merged into M87, making it the egg-shaped giant seen today. If so, the absence of young globular clusters in the Milky Way may mean that our Galaxy has never suffered a major collision.

Accidents in the Galactic Traffic

Brighter than a million million suns, a quasar is the most powerful lamp in the Universe. Astronomers understand it to be powered by matter falling into a massive black hole in the heart of a galaxy. Mike Disney of the University of Wales, Cardiff, leads a European team that asks why some thousands of galaxies harbour quasars, in contrast to the billions that do not. In almost every case that he and his colleagues have investigated, using Hubble's WFPC2 camera at its highest resolution, they see the quasar's home galaxy involved in a collision with another galaxy.

"It's my opinion that almost any galaxy can be a quasar," Disney says, "if only its central black hole gets enough to eat. In the galactic traffic accidents that Hubble reveals, we can visualize fresh supplies of stars and gas being driven into the black hole's clutches. My American opposite number, John Bahcall, prefers to stress those quasar hosts that look like undisturbed galaxies. But the important thing is that we have wonderfully clear pictures to argue about. Quasar theories were mostly pure speculation before we had Hubble."

The History of the Elements

Astronomers at the Hamburger Sternwarte use the Faint Object Spectrograph to analyse ultraviolet light from distant quasars, which they also examine by visible light from the ground. They trace the origin, through cosmic time, of elements like carbon, silicon and iron, from which planets and living things can be built. On its way to Hubble, the quasar light passes through various intervening galaxies and gas clouds, like the skewer of a kebab. Each object visited absorbs some of the quasar light, depending on the local abundances of the elements. As they detect more and more objects, Dieter Reimers and his colleagues form an impression of galaxies building up their stocks of elements progressively through time, by the alchemy of successive generations of stars.

Apart from primordial hydrogen the second lightest element, helium, has also been abundant since the origin of the Universe. The first major discovery after Hubble's last refurbishment came from Peter Jakobsen of ESA's Space Science Department at Noordwijk, who detected ionized helium in the remote Universe, by the light of a very distant quasar, 0302-003.

That was in January 1994, and since then Jakobsen has looked for the ionized helium using other quasars. He now suspects that this helium is nearly all gathered in clumps, rather than scattered freely through intergalactic space. If so, it greatly increases the estimates of the total mass of ordinary matter in the Universe.

Through a Lens to the Early Universe

Natural lenses scattered through the cosmos reveal distant galaxies, and make an astronomical tool for Richard Ellis of the Institute of Astronomy, Cambridge (UK). The strong gravity of an intervening cluster of galaxies can bend the light from more distant objects, so magnifying and intensifying their images. In one spectacular case, cluster Abell 2218 creates in Hubble's WFPC2 camera more than a hundred images of galaxies lying beyond it. Without the magnifying effect of the cluster, many of these remote objects would be too faint to study in detail.

Compared with man-made optics, the gravitational lenses are complex. They produce multiple images (as many as seven or more views of the same object) and they also smear the images into arcs. Team-member Jean-Paul Kneib, who is now at Toulouse, uses the distortions as a guide to distance. The more distorted the image, the farther off a galaxy is. The galaxies imaged by Abell 2218 are 5 to 8 billion light-years away, and Kneib's estimates have been confirmed by Tim Ebbels of Cambridge using the William Herschel Telescope located on the Spanish island of La Palma. Seen as they were early in the history of the Universe, the objects seem surprisingly similar to nearer and more mature galaxies.

The Cosmic Scale

Gustav Tammann of Basel and his collaborators use the Hubble Space Telescope to measure the Hubble Constant. Both are named after Edwin Hubble who discovered, almost 70 years ago, that the galaxies are spreading apart. The Hubble Constant is the rate of expansion - and the most important number in cosmology, because it fixes the size and the maximum age of the observable Universe. Since the launch of the space telescope in 1990, two independent teams have tried to fix the constant but their answers disagree. A high expansion rate, which makes the Universe relatively young, is preferred by Wendy Freedman's team consisting largely of American astronomers. A lower value for Hubble's Constant, implying an older Universe, comes from a mainly European team led by the American astronomer Allan Sandage. Tammann belongs to the latter, "old Universe" camp and he is philosophical about the delay in reaching a consensus.

"I've been waiting nearly 20 years for this result, and I expect the arguments will go on for a while longer," Gustav Tammann says, "In 1979 I asserted that a key task for the space telescope should be to use variable stars to fix the distances to nearby galaxies in which standard supernovae have been seen. Then the supernovae become candles lighting our way far out into the Universe. Well we've done it now, with stars in seven galaxies, and their supernovae give us wonderfully consistent answers. So we're in no mood to compromise, or to split the difference with Wendy Freedman's Hubble Constant. Time will tell us who is closer to the right answer."