How galaxies go bananas

Heather Couper and Nigel Henbest take a distorted view of the universe

Unveiled at this month's National Astronomy Meeting in Cardiff was one of the most astonishing images of the universe ever seen. Like a cosmic spider's web, dozens of glittering rings surround a central clutch of galaxies. Each ring is broken into separate arcs. Each is a galaxy, distorted to resemble a grossly elongated banana.

The image came from the far-seeing eye of the Hubble Space Telescope. It gave the impression that the telescope's newly corrected optics had been replaced by a fairground distorting mirror. But these warps were in fact introduced far away, as the light passed through a natural distorting lens in space.

The galaxies in the foreground were the culprit. Their gravity bends and focuses light like a giant lens - an effect first predicted by Albert Einstein's general theory of relativity, published in 1916. Astronomers confirmed the theory when they found that the Sun's gravity deflects passing starlight - by a tiny amount - during a total solar eclipse in 1919.

The depths of the universe provide a vast stage for testing Einstein's theory. Einstein predicted the different shapes of mirage we should see: multiple images or curved arcs. Appropriately, it was during the centenary of Einstein's birth that astronomers stumbled across the first gravitational lens. In 1979 radio astronomers from Jodrell Bank checked out the optical appearance of a radio source that they suspected was a distant quasar.

Instead, it seemed that they were seeing double; there were two quasars, almost equal in brightness. When examined with a spectroscope, the light from these two quasars turned out to be almost identical. The "twin quasar" must be two images of the same quasar.

According to Einstein's theory, the quasar's light can be focused only if it passes close by a galaxy lying right in front, as seen from Earth. And a careful analysis showed up the faint foreground galaxy, lying about one-third of the way to the quasar.

Since the discovery of that first gravitational lens, dozens of others have turned up. Some are multiple images: as well as a twin quasar, we now have a quadruply-imaged "lucky cloverleaf" quasar. Others form arcs, or a perfect circle, an "Einstein ring".

A cluster of galaxies acts as the most powerful gravitational lens; because its matter is more spread out, a cluster has a weaker focusing effect than a single galaxy - just as a hand magnifier is less powerful than a small, highly curved microscope lens. But if we look through a cluster of galaxies, we can expect to see some distortion of anything lying beyond.

The first examples were found, by accident, in 1985 and others have turned up in more systematic searches since then. The distorted images of the background galaxies indicate the strength of the gravitational focusing, and hence how the mass in the cluster is distributed. Most of this matter is not in the form of galaxies, but comprises mysterious "dark matter". The gravitational lensing has shown that the dark matter is generally spread more widely than the galaxies in a cluster. It cannot consist merely of extra ballast within each galaxy, nor "dark galaxies", distributed like the visible galaxies. Most likely, it consists of a sea of subatomic particles filling the cluster.

The new Hubble Space Telescope image of Abell 2218 is the best gravitational lens so far. It contains 120 different mirages. The Einstein lens splits seven of the background galaxies into the different arcs, on opposite sides of Abell 2218. As well as distorting the images, the gravitational lens makes them much brighter: without Abell 2218, the background galaxies would not be visible at all. The combination of the giant natural lens and Hubble's superior view is showing galaxies 50 times fainter than any ground-based telescope could hope to achieve.

We see these distant lensed galaxies as they were when the universe was one-quarter its present age. The internal structures in the images - once they have been corrected for the distorting affects of the lens - will show how star forming regions are spread within very early galaxies, providing new archaeological insights into the youth of galaxies such as our own Milky Way.

The planets

Mercury just creeps on to the star charts this month, appearring as an "evening star" in the northwest. On 12 May, it sets more than two hours after the Sun. Look for the elusive planet on 1 May, when it will be just above the thin crescent Moon.

Venus will be on show in the morning sky. However, it rises only an hour before the Sun for the whole of the month, and so will not be a spectacular sight.

Mars, in Leo, is now fading as it and Earth pull away from one another. On 24 May it passes just two moonwidths above Leo's heart, the bright star Regulus.

The giant planet Jupiter is the most prominent world this month, rising at 11pm as Mercury is setting. However, it is rather low in the sky.

Ringworld Saturn rises at about 2.30am this month. But if you look at it through a small telescope around 22 May, there will be no rings to speak of. That is because Earth passes from north to south through the plane of Saturn's rings - which "disappear" as a result.

The stars

Almost overhead these May evenings is one of the smallest constellations in the sky - but one with a tale to tell. Canes Venatici lies halfway between the bright star Arcturus (high in the south) and the familiar seven stars of the Plough, which is part of Ursa Major (the great bear).

In Greek mythology, Arcturus is in hot pursuit of the great bear. The name Arcturus in fact means "bear driver". In 1690 the Polish astronomer Johannes Hevelius decided to help him out by putting in front of him his two hunting dogs, Canes Venatici.

Oddly enough, the brightest star in the constellation bears a non-canine name. In 1725, Edmond Halley - he of comet fame - called this star Cor Caroli, meaning "Charles's heart".

According to tradition, it shone brilliantly in 1660 at the restoration of the monarchy by Charles II. Through a small telescope, it is a beautiful double star.

Diary

(all times BST)

7 10.44 pm Moon at first quarter

12 Mercury at greatest elongation

14 9.48 pm Full Moon

20 Pluto at opposition (needs large telescope to see it)

21 12.36 pm Moon at last quarter

22 Earth crosses Saturn's ring plane - rings "disappear"

29 10.27 am New Moon