The European Space Agency's (ESA's) orbiting X-ray observatory XMM-Newton has enabled astronomers to re-discover an ignored celestial gem.

The object in question is the remains of a star that exploded around a thousand years ago. Its shape, age and chemical composition will allow astronomers to better understand the violent ways in which stars end their lives.

Exploding stars seed the Universe with the heavy chemical elements necessary to build planets and create life. The expanding cloud of debris that each leaves behind, known as a supernova remnant (SNR), is a bright source of X-rays and radio waves. Generally the debris is thought to appear as an expanding bubble or ring. When astronomers took the first high-resolution radio images of a source known as "G350.1-0.3" in the 1980s they saw an irregular knot of gases that did not seem to meet these expectations. So G350.1-0.3 was classified as a probable background galaxy, and was quietly forgotten.

Now, Bryan Gaensler, Anant Tanna, both of the School of Physics, and their colleagues have used the X-ray capabilities of XMM-Newton to prove that appearances can be deceptive. G350.1-0.3 is indeed the debris of an exploded star despite its misshapen configuration, and in fact turns out to be one of the youngest and brightest supernova remnants in the Milky Way.

To explain its shape, the team looked at radio surveys and discovered that G350.1-0.3 had exploded next to a dense cloud of gas about 15000 light years from Earth. The cloud prevented the blast from expanding evenly in all directions, resulting in an example of a rare kind of misshapen supernova remnant.

G350.1-0.3 is incredibly small and young in astronomical terms, only eight light years across and about 1000 years old. "Only a handful of such young supernova remnants are known. So even having one more is important," says Tanna.

That is because young supernova remnants are highly luminous and the newly formed chemical elements are glowing brightly, making them easier to study. "We are seeing these heavy elements fresh out of the oven," says Gaensler.

As well as exhibiting the newly created elements, young supernova remnants contain clues about the way the original star exploded. Such information is lost in the majority of supernova remnants because, as they expand and age, they lose their initial characteristics. "After 20,000 years, all sorts of explosions look more or less the same," says Gaensler.

Astronomers are now recognising that stars explode in many different ways. Some stars might be just big enough for an explosion to occur, others might be much more massive. There will be differences in the chemical composition of the exploding stars and some may have a companion star in orbit around them.

Gaensler and Tanna hope that further investigations of G350.1-0.3 will yield clues as to exactly what kind of star exploded. "It may turn out that many of the youngest supernova remnants have these strange shapes," says Tanna. "The hunt is now on to find more."

Despite the light from the supernova having reached Earth during the time of William the Conqueror, Gaensler thinks humans would not have seen it. "The X-ray data tell us that there is a lot of dust lying between it and the Earth. Even if you had been looking straight at it when it exploded, it would have been invisible to the naked eye," he says.

Thankfully, XMM-Newton's sensitivity and the scientific detective work by Gaensler and Tanna mean that this important celestial object will never be forgotten again.

The results will appear in the Astrophysical Journal (Letters).