eso8807-en-ie — Science Release
Most Distant Star Ever Seen: Supernova Explodes 5 Billion Years Ago
9 September 1988
The most distant, individual star ever recorded was detected with a telescope at the ESO La Silla observatory on August 9, 1988. The object is an exploding star, a supernova, and is situated in an inconspicuous galaxy, itself a member of a distant cluster of galaxies. Additional observations indicate that the cluster, known as AC118, is at a distance of about 5 billion light-years (1 billion = 1000 million). Thus this supernova explosion occurred 5 billion years ago, or about the time when the Sun and the planets were born. Ever since then, the light emitted by this event has been travelling towards us, only arriving here now. It is the most distant supernova (ESO8802) observed so far.
The discovery was made by Danish astronomer Hans Ulrik Nørgaard-Nielsen, working with the Danish 1.5 m telescope at La Silla. It is the successful outcome of a dedicated search programme for supernovae in very distant galaxies, which has been carried out during the past few years by a group of astronomers with this telescope.
The explosion of a star as a supernova is an extremely powerful event and during a short time it may shine brighter than the entire galaxy in which it is located (and which consists of several hundred billion stars). But due to the enormous distance, the light we receive from this supernova event is 4 million times fainter than what can be seen with the unaided eye. It is therefore difficult to detect such an object.
The present detection technique is based on the comparison of two images of the same cluster of galaxies, obtained at different times. "Subtracting" the two images from each other in principle allows to distinguish any object which is present in one of the images, but not in the other. A supernova will show up as an additional spot of light in an image obtained soon after the light from the explosion reaches us, when compared to an earlier image. This is clearly illustrated in the photo which accompanies this Press Release.
A supernova only remains at its brightest during a short time. For a distant supernova this implies that it must be detected as soon as possible after the explosion, so that additional observations can be made before it becomes too faint to observe. By good luck, the detection of the present supernova appears to have been made less than one week after the explosion became visible. Further images were obtained until August 16, when it had faded to half of its brightness at discovery, unusually fast for a supernova. On September 6, the supernova was too faint to be seen.
Still, by concerted action, it was possible to obtain spectra with the EFOSC instrument at the ESO 3.6 m telescope and, a few days later, with the Anglo-Australian 4 m telescope at Siding Spring, NSW, Australia. These spectra are now in the process of being interpreted, a difficult task due to the faintness of the object.
The present supernova is apparently of Type I, and many supernovae of this type have been observed in nearby galaxies. (Type II supernovae are generally fainter than Type I). It is therefore of great interest to compare this very distant supernova - which we now observe as it was 5 billion years ago - with those in our neighborhood. If it behaves identically to the nearby Type I supernovae, then we may hope to use these objects for cosmological investigations, also in distant regions of the universe.
For instance, if the intrinsic brightness of all Type I supernovae is the same everywhere, then they may serve as independent distance indicators, allowing us to verify distance determinations based on the Hubble expansion. Moreover, distant supernovae may be used as "clocks" to check the time dilation predicted by the General Theory of Relativity. This implies an apparent slow-down of events observed at great distances. The fact that this supernova apparently faded more rapidly than most other known Type I supernovae is not necessarily in contradiction to relativity; it is probably inherent to the supernova itself.
By now, this supernova has become too faint to observe with existing telescopes. However, this event has convincingly demonstrated our ability to detect very distant supernovae. When the Hubble Space Telescope and the ESO Very Large Telescope become available in the 1990's, we shall be able to study these important objects in much more detail, thereby opening a new line in our investigations of the distant regions of the universe.
The group of astronomers includes Hans Ulrik Nørgaard-Nielsen (Danish Space Research Institute, Lyngby, Denmark), Leif Hansen and Henning E. Jørgensen (University Observatory, Copenhagen, Denmark), Richard S. Ellis (University of Durham, U.K.) and Warrick J. Couch (Anglo-Australian Observatory, Epping, NSW, Australia)
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