20 Jul 2000
KPD1930+2752 is actually two stars, one hot bright star and a faint, dense star known as a white dwarf. The hot star whirls around the white dwarf, just like the Earth revolves around the Sun, but travelling at over one million kilometers per hour, it takes only 137 minutes to complete one trip around its companion.
But KPD1930+2752 is doomed. Energy is being sucked away by 'gravitational waves' -- a type of energy predicted by Einstein's Theory of General Relativity -- so the stars will collide within the next 200 million years. What makes KPD1930+2752 special is that the star will then be too massive and dense to survive. The star will be ripped apart in a vast thermonuclear explosion bright enough to be seen from the other side of the Universe.
These explosions, known as Type Ia Supernovae, hurl their metallic debris into space, particularly iron, nickel and cobalt. Almost all the iron on the Earth comes from Type Ia Supernovae which exploded billions of years ago, including the iron in our blood.
Type Ia Supernovae seen in nearby galaxies all reach the same brightness when they explode, so astronomers can look for Type Ia Supernovae in very distant galaxies and use them to measure the distance -- the fainter the supernova looks, the further away the galaxy is. By using this method, astronomers have found that the Universe is not only expanding, but that the expansion appears to be speeding up and will prevent the Universe collapsing in a 'Big Crunch'.
But this method only works if Type Ia Supernovae in distant galaxies are the same as the ones nearby. Astronomers have worried about this problem for several years because they have not been sure what causes Type Ia Supernovae.
Now that KPD1930+2752 has been found it can be studied in detail so that astronomers can work out how Type Ia Supernovae in distant galaxies might behave and so, perhaps, determine the fate of the Universe itself.
Additional information The discovery of KPD1930+2752 will be published in 'Monthly Notices of The Royal Astronomical Society'.
The Royal Observatory Greenwich has published a leaflet 'What is a supernova?'
There are many supernova related sites on the internet, Marcos J. Monets maintains a listing
The key observations of KPD1930+2752 were obtained in April using the Isaac Newton Telescope. The Isaac Newton Telescope is operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias.
This research was supported by the Particle Physics and Astronomy Research Council.
KPD1930+2752 is a subdwarf B star. It is about one fifth the size of the Sun and is about half as massive. Unlike normal stars, which are composed almost entirely of hydrogen, KPD1930+2752 is made of helium. It is not entirely clear how stars like KPD1930+2752 are made, but recent work suggests they are the remains of stars like the Sun which lose half their mass just before they complete the end of the red giant phase of their evolution. Only some small fraction of stars evolve this way and this is thought to be related to the fact that most subdwarf B stars are binary stars.
We observed KPD1930+2752 as part of a programme to study subdwarf B stars to understand how they are formed. We measured the Doppler shift of the star over the course of one orbit. The Doppler shifts shows that the star is orbiting an unseen companion every 137 minutes at a speed of 350km/s. We used Kepler's laws to show that the unseen companion has almost the same mass as the Sun, but it is much smaller and fainter. The unseen companion star could be a neutron star or black hole, but is much more likely to be a white dwarf star. Many subdwarf B stars have been found with white dwarf companions.
When binary stars have orbital periods as short as two hours, they produce 'gravitational waves' which drain energy from the orbit, so the stars gradually spiral in towards each other. This effect was predicted by Einstein's theory of General Relativity and was first seen directly by Hulse and Taylor in the pulsar PSR 1913+16 and earned them the Nobel prize.
KPD1930+2752 will merge within 200 million years. The white dwarf will then gain extra mass from the subdwarf B star and will exceed a critical mass, called the Chandresekhar limit. This is thought to lead to a Type Ia supernova explosion.
There has been some debate among astronomers over the cause of Type Ia supernova explosions. Most agree that they are caused by the explosion of a white dwarf star which gains too much mass but what is not clear is how it gains this mass. There are many white dwarfs which are known to be gaining mass from a normal star, but these are made of hydrogen which causes a series of small explosions before the Chandresekhar limit is reached. This is what causes a nova explosion. To make a supernova, the white dwarf has to be supplied with helium, which explodes less easily but releases much more energy. KPD1930+2752 is the first star found where helium will be dumped onto a white dwarf star which will then exceed the Chandresekhar limit.