Messier 99 - St. Catherine's Wheel
Messier 99 is a beautiful spiral galaxy in the constellation "Coma Berenices", which also carries the name "St. Catherine's Wheel". With a brightness
of 9.7mag and an apparent size of 5.3'x4.6', it can be seen even with a smaller telescope. The galaxy was discovered together with Messier 98 and
Messier 100 in 1781 by P. Méchain. The spiral arms were first seen by Lord Rosse in 1846.
The distance to Messier 99 is around 50 million light-years, although the error bar still appears to be very large. With a diameter of 85,000 light-years,
the galaxy is only slightly smaller than our own Milky Way and belongs to the Hubble Type SA(s)c, which is defined by loosely wound spiral arms.
The absolute magnitude is given as M = -21.3mag, which corresponds to a luminosity of 30 billion suns.
The fact that the southern (lower) spiral arm sticks out so far and that the core of Messier 99 is not in the center of the galaxy is probably due to a close
encounter 280 million years ago with Messier 98 or NGC 4262. In addition, an increased star formation rate can be observed in Messier 99.
So far, four supernovae have been observed - the last one in 2014 with a magnitude of 15.4mag.
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Messier 99 is a very nice object in my 20" f/3 Dobsonian. The spiral arms stand out best at a magnification of 210...270x. The most striking is of course
the southern arm, which spirals very far around the core. The two northern arms are less pronounced and also fainter.
At 380x, some knots and thickenings can be seen in the arms, but only with averted vision. Noticeable are two neighboring HII regions in the southern
arm, which are also the easiest to see.
