August 24 – Star Clusters


M13 in the constellation Hercules

Evening Highlights:

Star clusters are our galaxy’s oldest inhabitants and formed first as the galaxy was forming.

  • 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.
  • M5 – Some amateur observers swear that M5 is the finest globular cluster north of the celestial equator for small telescopes – even better than the celebrated M13, the Great Hercules cluster.  Spanning 165 light-years in diameter, M5 is one of the largest globular clusters known.  It contains more than 100,000 stars, as many as 500,000 according to some estimates.
  • M9 – is a globular cluster in the constellation of Ophiuchus.  It is positioned in the southern part of the constellation to the southwest of Eta Ophiuchi, and lies atop a dark cloud of dust designated Barnard 64.  It is one of the nearer globular clusters to the center of the Milky Way Galaxy with a separation of around 5,500 light-years from the Galactic Core.  Its distance from Earth is 25,800 light-years.
  • The might planets Jupiter and the rings of Saturn will also be available in the night sky this evening.
  • The presentation, free and open to the public, will start about 8:00 pm, and observing at 8:30 pm.  Available seating is limited.  You are welcome to bring a chair.
  • Follow this link to tips for having a great observing night!

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 arrive well before the presentation is to begin so car lights do not damage night vision.

The Life of a Globular Cluster

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.

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