March 31, 2020

Astro Event of the Week: 24-30 June, 2008

Early morning risers next Monday are in for a treat; a rare occultation of the star cluster, the Pleiades, by the Moon.

Observers can see a waning crescent Moon seem to glide through the Pleiades star cluster. The action begins low in the northeastern sky at moonrise, which occurs at 1:52 AM EDT. However, it’ll take the Moon and the Pleiades about an hour to gain an appreciable altitude in the eastern sky; assure you have a low, clear eastern horizon to view this event. The Moon should be midway through the cluster by 3 AM local, and may present a cool photographic target. During this time, the Moon will also occult, or pass in front of, several bright stars in the cluster. This is always an inspiring event to watch as stars wink out, and then later seem to wink back into existence. Through a telescope, one can discern a feeling for the Moons’ orbital motion. These occultations also still serve a scientific purpose. If enough observers can accurately time the ingress and egress of these stars, a map of the lunar limb profile can be constructed. See a star wink out in a step wise fashion? That could be a very good sign that it’s an extremely close binary; double stars have been discovered in this fashion. This is a very good event to follow with a telescope, binoculars, or good ole’ naked eyes. The sun rises about 4:50AM EDT local; the sky will begin to lighten up about an hour before that. How long can you follow both the Moon and the Pleiades in the brightening sky? Also, don’t forget to take note of fleeting Mercury, low on the eastern horizon!

Now for some quick perspective: the Moon is about 1 light second away; that represents the current extent of manned space travel. The Pleiades is however, about 410 light years away.  That means we see them as they looked around the time the Church was considering our current calendar reform (see last weeks post!) And to think, the Pleiades are one of the closer open clusters!

This weeks’ astronomy related term is Open Cluster. While these types of star clusters may not be the sexiest observationally, they are the most prevalent. They tend to be loose associations of young stars such as the Pleiades (M42), which are “only” about 115 million years old. They are also not as dense as globular clusters, which may pack hundreds of stars in a cubic light year. For contrast, the nearest star to our solar system is Proxima Centauri, about 4.3 light years away. Several open clusters are visible in the northern hemisphere to the naked eye, including the aforementioned Pleiades, the Hyades (also in the constellation Taurus), and the beautiful Double Cluster in Perseus.   The sun itself probably started off its life in such a stellar nursery; we are currently passing a loose association of stars known as the Ursa Major Group, of which most of the stars in the Big Dipper are a part. Some controversy now exists as to whether the brightest star in the sky, Sirius, is also a member. Our Sun, however, is too old and lacks the same proper motion to be part of this cluster. Perhaps the rest of our siblings are long scattered across the galactic disk, and our progenitor cluster has yet to be indentified.


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