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As their name indicates, supernovae (SNe) are discovered in the sky as "new stars" (-novae) of exceptionally high brightness (super-). The fact that SNe are formidable explosions completely different from, and vastly more energetic than classical novae (a) , was first recognized by Baade and Zwicky (1934). They noticed that novae during explosion become no brighter than about 1 million times (i.e. 15 mag) they are in a quiescent phase. Therefore, any historical event in our Galaxy that had reached a magnitude as bright as 0 or brighter but is not detectable at present, had to belong to a separate class of intrinsically brighter objects. And indeed, the distribution of observed magnitudes of explosive events detected in galaxies of the Local Group indicated the presence of two peaks, one at the expected brightness of classical novae and another at more than thousand times brighter luminosities.

Supernovae represent the explosive death of both low mass stars (type Ia) and moderate and high mass stars (types Ib/c and II). They are extremely bright, (roughly 109 Lsun (b) rivalling, for a few days, the combined light of the entire host galaxy. In all cases, a SN explosion injects highly metal-enriched material (at least 1 Msun) and a conspicuous amount of kinetic energy (about 1051 ergs) into the surrounding medium (see Section 2.1). In addition, the blast waves from SN explosions produce powerful sources of radio and X-ray emission - supernova remnants - that can be seen and studied many thousands of years after the event (see Section 2.2). Therefore, it is clear that SN explosions are crucial events that determine most of the aspects of the evolution of galaxies, i.e. most of the visible Universe.

Some SNe in our Milky Way galaxy have been close enough to be visible to the naked eye, and records of their occurrence can be found in ancient annals. In particular, during the past 2000 years 9 such events have been recorded. A few of these events were very bright. The supernova of 1006 AD, for example, was about 1/10 as bright as the full moon! The last supernova to be seen in our Galaxy was discovered in 1604 by the famous astronomer Kepler. On the basis of these historical records one may infer that the average rate of SN explosions in the Galaxy be of the order of 5 per millennium. However, one has to allow for the fact that most SNe are either too far or are too obscured by dark dust clouds of the galactic disk to be visible. Actually, one can estimate that only about 10% have been close enough and bright enough to be detectable by naked eye. Therefore, a more realistic SN explosion rate for our Galaxy is about one every twenty years (see also Section 2.3).

Being so bright, SNe are ideal probes of the distant Universe. And indeed studies of SNIa up to redshifts ~ 1.2 have allowed us to explicitely measure both the local expansion rate of the Universe and other cosmological parameters (see Section 2.4). The brightest supernova discovered in the last three centuries is supernova 1987A in the Large Magellanic Cloud, a small satellite galaxy to the Milky Way. Section 3 is devoted to it.

a Novae are produced by sudden nuclear ignition of a very thin layer of hydrogen near the surface of a degenerate star accreting matter from a binary companion. Back

b In Astronomy the symbol sun is used to denote the Sun. Thus, Lsun = 3.8 × 1033 erg s-1 is the solar luminosity and Msun = 2.0 × 1033 g is the solar mass. Back

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