A long time coming - but worth the wait

Astronomers on a new detection programme have found a star so distant that its light has taken 5,800 million years to reach us.

Astronomers in Chile have just announced the discovery of the most remote star ever seen. The star, in the constellation of Sextans, is so distant that its light has taken 5,800 million years to reach us - which means that it set out 1,000 million years before our solar system came into existence.

This is no ordinary star. To be seen a third of the way across the universe, it must be extremely bright - and astronomers in fact picked it up when it was in its death throes, exploding as a supernova. "SN 1995K" was first detected with the four-metre telescope at the Cerro Tololo Inter-American Observatory on 30 March, during a programme to search for distant supernovae in remote galaxies. Even so, the dying star appeared extremely faint - at magnitude 22.7, it was 5 million times fainter than the dimmest stars visible to the unaided eye. Even with sensitive electronic detectors, astronomers needed an exposure time of 2.5 hours to reveal it.

The beauty of the new detection programme is that it allows astronomers to follow up the discovery immediately, using some of the world's largest telescopes. These observations split up starlight to uncover the "message" encoded within, so revealing the "redshift" of the parent galaxy - a guide to its distance - and the nature of the star that has "gone supernova".

To astronomers' delight, SN 1995K turned out to be a Type 1a supernova - the explosion of a wizened cadaver of a star called a white dwarf. Not only are Type la supernovae the brightest exploding stars (at their peak, they shine 6,000 million times brighter than our Sun), they also all reach the same maximum brilliance. If all Type la supernovae were situated at the same distance, they would appear equally bright - they are "standard candles". So the apparent faintness of a Type la supernova reveals how far away it lies. Distances to the remotest objects in the cosmos are currently uncertain by a factor of two, so stellar suicides like SN 1995K are essential to our understanding of the scale of space.

If astronomers succeed in catching a handful more of these distant Type la supernovae at maximum light, they may be able to foretell the ultimate fate of the universe. Currently, the universe is expanding - a legacy of the Big Bang that took place some 15,000 million years ago. Measurements of the distances to these remote supernovae can give us an estimate of the "deceleration parameter", q0, a measure of how quickly the expansion is slowing. If q0 is small, then there is insufficient matter in the Universe to give it effective brakes - and it will continue to expand forever. If, on the other hand, q0 turns out to be large - which means that there is enough matter to put a halt on things - then the Universe will ultimately stop expanding. Our very distant descendants may have to face the very real prospect of a final "Big Crunch".

Planets and stars

If you have an exceptionally clear north-west horizon, look for Mercury around 9 September, when it reaches greatest elongation 27 east of the Sun. However, it sets within an hour of the Sun, and will become even harder to spot as the month goes by.

Venus is also an evening star, but as it sets even closer to sunset, it will be even harder to see.

Mars, too, is plunging into the twilight, and sets about an hour after the Sun.

The planets to reserve your energies for are the two biggest gas giants, Jupiter and Saturn. Jupiter - at magnitude minus 2.0 - is by far the more conspicuous. It passes from the constellation of Scorpius to Ophiuchus this month, and makes a striking grouping with the blood-red star Antares on 20 September. However, it too is moving into line with the Sun, and sets by 9.30pm at the end of the month.

Saturn reaches opposition - its closest point to the Earth - on 14 September. It can then be seen all night as a brightish, 0.7-magnitude object in Aquarius, just below the barren Square of Pegasus.

Like the planets, the stars on view this month are distinctly lacking in lustre. The brilliant trio of Deneb, Vega and Altair - the "Summer Triangle" - is beginning to shift over to make way for the fainter constellations of autumn, such as Pegasus and the attached constellation of Andromeda. It certainly requires a stretch of the imagination to see in those bare constellations the shape of a winged horse and a maiden tied to a rock.

Diary (all times BST)

2 Sept 10.03am moon at first quarter

9 Sept 4.37am full moon

14 Sept Saturn at opposition

16 Sept 10.10pm moon at last quarter

23 Sept 1.13pm autumn equinox

24 Sept 5.55pm new moon

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