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Log, Apr 11, 2008

No people. The week after I got back from South America, where among other things I viewed Omega Centauri [OC], Ernie Evans sent me a stunning article. OC has long thought to be a globular cluster in spite of its very great size. It turns out that OC is apparently something much more exciting - a small galaxy in a life and death struggle with the Milky Way Galaxy [MWG].

I will be paraphrasing recent findings of Eva Noyola and her team at the Max-Planck Institute for Extraterrestrial Physics to avoid any copy write infringement. I will try to be careful but any mistakes are my own.

OC is physically located about 17,000 lightyears away from us, just above the plane of MWG. This would make its core about 11,000 lightyears CLOSER to us that the center of our own MWG. OC has a large apparent diameter - about the size of the Moon. The core of the MWG is much larger but invisible because dust and gas between us and it dim light more than a trillion times.

Standard globular clusters are composed of 100,000 to perhaps a 1,000,000 stars formed all about the same time. So far, no globular cluster has been proven to have a massive central black hole, but massive central black holes are being found at the center of every elliptical or spiral type galaxy. For example, the central black hole of the MWG has between 3 and 4 million solar masses in a very compact region called Sagittarius A*. Standard globular cluster rotate at a rate consistent with the number of stars they seem to contain. Standard globular clusters are essentially spherical.

OC has always been an oddball when compared to other "globular clusters" but until recently we had no idea how odd it really is. New measurements of this object reveal many divergences from globular clusters. OC is huge - at least 10 times larger than any other MWG globular cluster. Recent examinations reveal a central compact object that contains 40,000 stellar masses. Only a black hole can be this large and this compact, but if it is a black hole, it is the first ever discovered in a globular cluster. Unlike globular clusters where all the stars seemed to have formed at the same time, OC had a wide variety of stars of differing ages. OC is flattened rather than globular. OC rotates far faster than a typical globular cluster but at speeds consistent with small galaxies.

Yet visually, OC looks a lot like a globular cluster. It is far too small to be an elliptical galaxy. Elliptical galaxies are the super sized galaxies of the universe. OC's size is more consistent with a small spiral like galaxy. If so where are the famous spiral arms of OC? The explanation seems to be that as OC passed close to the center of the MWG, all it stars in the OC arms were stripped away and incorporated in the MWG.

Omega Centauri is at right ascension: 13h27m and declination: 47.45 degrees south latitude [dsl]. Any object which is north of declination 48.63-dsl will make an appearance at FDO at least some time during the year depending on the declination and the hour angle of the object. The closer an object is to -48.63-dsl, the shorter period the object will appear and the closer it will be to the due south horizon. At 10PM when viewing should be best, OC first rises above the horizon on May 4th, reaching a maximum altitude of 1.2 degrees on May 24 before sinking below the horizon for another year on June 11th. OC barely squeezes in at FDO - below the tree line unfortunately but above the horizon at the parking lot at the end of Park lane. We are going to be making tries to see it with a portable telescope in the parking lot on Friday nights about 10PM on May 16 through June 6. Seeing will depend on a clear horizon but it should make a spectacular addition to anyone's life list.

Francine made the comment that she knew someone who saw OC in Massachusetts by climbing into a tree in the mountainous part of the state. This is an interesting example of what old time sailors called the dipping angle. When a lighthouse was first identified coming over the horizon from the crow's nest, the lookout would climb up and down the mast until the top of the light just disappeared. From the point on the mast where this happened it is possible to determine exactly how far the boat was from the light house giving a very precise ship location without the need for triangulation. The same thing happens with objects near the horizon. The higher you climb, the farther the horizon recedes. If I've done my trig correctly this means that for 1.2 mile of elevation you gain about 69.2 miles (1 degree of latitude) towards the South Pole. At a point just 70 miles north of FDO you could never see Omega Centauri at sea level. Providence (at sea level) OC will just barely skim the horizon on May 24th at 10PM. From Boston (at sea level), OC will never clear the horizon. You might just get a glimpse of OC from the top of the Hancock building in Boston, but I don't know its height so I can't be sure.

-Les Coleman

Leslie Coleman
Author:
Leslie Coleman
Entry Date:
Apr 11, 2008
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Leslie Coleman's Log
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