Sunday, March 4, 2012

Globular Cluster Photo Series (Part 15): M79

Today I have another Messier globular cluster to show you, Messier 79. This one is located just below Orion's feet in the constellation Lepus, the Hare. M79 is fairly small on the sky, just 8.7 arcminutes across (less than a third the width of the full Moon). This is mainly due to its large distance of about 42,000 light years from us, as the cluster itself is roughly average in size at about 118 light-years across.

Messier 79 in Lepus.

There are two very interesting things about Messier 79, and they may be related to each other. First, M79 is in an unusual position for a globular cluster: it's almost directly opposite from the galactic core in the sky. By far the majority of globular clusters are located somewhere in the same hemisphere as the core (and hence are visible during the summer); M79 is one of the very few that's not.

Secondly, M79 may not be native to the Milky Way. It is possible that it (along with three other smaller, fainter globular clusters) were originally satellites of another dwarf galaxy that is in the process of being absorbed into the Milky Way. A candidate galaxy was discovered back in 2003 and is known as the Canis Major Dwarf Galaxy based on the constellation in which it was first discovered in. I say "candidate" because it is still not entirely certain if this galaxy actually exists as an independent galaxy or is simply an over-dense part of the Milky Way. This is because it lies behind the galactic disk from our vantage point, making it impossible to see in visible light and difficult to analyze.

If the Canis Major Dwarf Galaxy does exist it would be the closest external galaxy to us, being only about 25,000 light-years away, and actually closer to us than to the Milky Way's core. It seems to correspond with a structure known as the Monoceros ring, an incredibly long stream of tidally disrupted stars that stretches over 200,000 light-years long and wraps around the Milky Way three times. If M79 was originally a satellite of the CMDG it would help to explain its strange position (although it's not impossible for a true Milky Way globular to have such a position; it's just rather unlikely).

Anyway, enough about this unusual globualr cluster; tune in tomorrow to see our very first non-Messier one!

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