July 20 – Star Clusters


M13 in the constellation Hercules

Evening Highlights:
  • M13 – First discovered by Edmond Halley (of Halley’s Comet fame) in 1714, and one of the oldest and most densely populated globular clusters with nearly one million stars.
  • M15 – 32000 light years distant, may have a 4000 solar mass black hole at its core.
  • M3 – First discovered by Charles Messier in May 1764.  Thought to contain over 500,000 stars, and is 190 light years across.
  • M80 – Several hundred thousand stars, 32000 light years distant, and one of the densest clusters.
  • Three planets are visible in the sky tonight- Mercury, Jupiter and Saturn (Mercury sets very early, shortly after dusk).
  • Unlike most deep sky objects, all of the globular clusters listed above are visible through both telescopes and  binoculars.  Please feel free to bring a personal pair of binoculars to tonight’s workshop.
  • If you arrive by car, please park by 8:10 PM.  Bring a chair if you can.  Our presentation will start about 8:30, and observing at 9:15.

Please note that this event is held “weather permitting”, and is cancelled if it is raining or excessively windy; announcement of a cancellation will be posted both on this page as well as facebook.com/sdbhas.  We will begin observing by 9:00 P.M.  Please arrive well before dusk so car lights do not damage night vision.

Globular clusters, part of the more general stellar cluster classification which includes open clusters, are a close collection of stars often arranged in a sphere that is over 100 light years in diameter, and range in number from hundreds of thousands to over one million stars.  These pearls of the night sky, over 150 of which orbit the Galactic center, are some of the oldest objects in the Milky Way and increase in concentration toward the Galactic center near Sagittarius and Scorpius.  Globular clusters form from molecular clouds, which are clouds of gas and dust that collapse due to gravity and eventually form stars. It’s generally believed that when the universe – and thus the Milky Way Galaxy – was very young, hydrogen and helium were essentially the only elements present and, as a result, stars formed during this period were very metal-poor (the term metal is used for any element heavier than helium).  The lives of stars vary greatly in length, but as they near the end of their lives they begin to generate elements heavier that helium (i.e. metals) such as lithium, carbon and nitrogen, and either gradually shrug these elements into space as they age (some eventually becoming planetary nebulae), or expel them much more violently if the star goes supernova.  Thus, each succeeding generation of stars generally contains a higher proportion of metals leading to the conclusion that metal-poor stars are much older, possibly over 12 billion years old.  Globular cluster formation, age and composition continues to be a very active area of research.

Tonight’s Images

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