Possibly similar to what our own Milky Way looks like, Messier 100
[1] is a grand design spiral galaxy that presents an intricate
structure, with a bright core and two prominent arms, showing
numerous young and hot massive stars as well as extremely hot knots
(HII regions). Two smaller arms are also seen starting from the inner
part and reaching towards the larger spiral arms.

The galaxy, located 60 million light-years away, is slightly larger
than the Milky Way, with a diameter of about 120 000 light-years.

The galaxy was the target of the FORS1 multi-mode instrument on ESO’s
Very Large Telescope, following the request of ESO astronomers
Dietrich Baade and Ferdinando Patat, who, with their colleagues Lifan
Wang (Lawrence Berkeley National Laboratory, US) and Craig Wheeler
(University of Texas, Austin, US), performed detailed observations of
the newly found supernova SN 2006X [2].

SN 2006X was independently discovered early February by Japanese
amateur astronomer Shoji Suzuki and Italian astronomer Marco
Migliardi. Found on 4 February as the 24th supernova of the year, it
had a magnitude 17, meaning it was 1000 times fainter than the
galaxy. It was soon established that this was another example of a
Type-Ia supernova [3], observed before it reached its maximum
brightness. The supernova indeed brightened up by a factor 25 in
about two weeks.

Since SN 2006X became so bright and since it is located inside the
very much studied Messier 100 galaxy, there is no doubt that a great
wealth of information will be collected on this supernova and,
possibly, on the system that exploded. As such, SN 2006X may prove an
important milestone in the study of Type Ia supernovae. This is
particularly important as these objects are used to measure the
expansion of the universe because they all have about the same
intrinsic luminosity.

This is not the first supernova ever found in Messier 100. Indeed,
this is one of the most prolific galaxies as far as supernovae are
concerned. Since 1900, four others have been discovered in it: SN
1901B, SN 1914A, SN 1959E, and SN 1979C. Recent observations with
ESA’s XMM-Newton space observatory have shown quite surprisingly that
SN 1979C is still as bright in X-ray light as it was 25 years ago. In
visible light, however, SN 1979C has since then faded by a factor
250. SN 1979C belongs to the class of Type II supernovae and is the
result of the explosion of a star that was 18 times more massive than
our Sun.

Notes

[1] Messier 100, the 100th entry in Charles Messier catalogue, was
discovered in 1781 by French astronomer Pierre Méchain. Located in
the Coma Berenices (Berenice’s Hair) constellation, slightly north of
the celestial equator, it is one of the brightest members of the
Virgo-Coma cluster of galaxies, about 60 million light-years away.
Its visual magnitude is about 9, meaning it is about 15 times fainter
than what the unaided eye can see. Messier 100 is one of the more
distant galaxies where accurate measurements of Cepheid variables
have been made, as can be seen in the ESO/ESA astronomy exercises
(see ESO PR 29/01).

[2] Other Type Ia supernovae studied by Dietrich Baade and his
colleagues can be found in ESO PR Photo 26/05 and ESO PR 23/03.

[3] Such a supernova is thought to be the result of the explosion
of a small and dense star – a white dwarf – inside a binary system.
As its companion was continuously spilling matter onto the white
dwarf, the white dwarf reached a critical mass, leading to a fatal
instability and the supernova.