Messier 93, NGC2447, was discovered on this day in 1781. This binocular object is an open cluster about 3,600 light years away, spanning approximately 10 light years, in the constellation Puppis. It was the last open cluster to be discovered by Charles Messier himself. M93 contains a good collection of blue giant stars, as well as quite a few red giants, possibly in clusters of their own. It’s hard to say how many stars a cluster contains, mainly because they get in each other’s way when you try to count them. This one has at least 80 identifiable members, but may well turn out to be several hundred strong.
The small, densely packed globular cluster Messier 92 was discovered on this day in 1777, not by Charles Messier, but by Johann Elert Bode. Messier didn’t spot it until 1781.
M92 is a barely-naked eye object of magnitude 6.4, located in the constellation of Hercules. It is one of the oldest and brightest globular clusters, though not as bright as its neighbour in Hercules, M13, and is about 27,000 light years from Earth.
Some of the stars in M92 are about 14 billion years old, making them roughly the same age as the Universe. This established age probably led to lively debate among astronomers when the Hubble Constant was first used to put the age of the Universe at 12 billion years.