In this video, we present three favorite star clusters of summer for observers in the Northern Hemisphere. We open looking due south at twilight for mid-northern latitudes and note brilliant Venus setting as it follows the sun and Scorpio on the meridian with Sagittarius due east.
Scorpio with red Supergiant Antares, the heart of the Scorpion, is on the meridian at 9:20 PM, EDT with Sagittarius due east and prominently placed by midnight.
In this edition, we’d like to highlight 3 favorites of the summer sky, 2 globular clusters and one galactic cluster:
Messier-4
Messier-22 (The Crackerjack Cluster)
Messier-11 (The Wild Duck Cluster)
Featured Image
Globular Star Cluster NGC-1866 imaged with the Hubble Space Telescope’s Wide Field Camera 3 (WFC3). As Globular Clusters go, NGC-1866 is surprisingly young. Although there is still debate over how globular clusters form, observations such as this reveal most of their stars are old with a low metallicity. In astronomy, ‘metals’ are any elements heavier than hydrogen and helium; since stars synthesize heavier elements within their cores to produce energy, a low metallicity indicates a star is very old. Stars within globular clusters are so old that they were actually some of the very first to form after the Big Bang. The stellar population of NGC 1866 is diverse and different populations of stars are thought to coexist within the cluster.
Included in the video and located in the disk of the Large Magellanic Cloud is NGC-1866, a Globular star Cluster in the southern constellation Dorado.
In this Hubble Space Telescope view we note the striking red star in the upper quadrant of the image. This is a “Carbon” star, an evolved, red giant that is producing mostly carbon, some oxygen and a lot of energy through helium fusion in its core. These late, highly-evolved stars (along with the other red giants and supergiants in the cluster) represent some of the oldest stars in the universe, some dating back to the stellar epoch following “Recombination” about 13 billion years ago. This is the point in the universe’s evolution, about 400,000 years after the “Big Bang”, where “Barionic” (regular) matter was constituted into the form it is today.
Carbon Stars
Carbon stars are not that common and represent highly-evolved (“late”) stars in their helium-burning stage of evolution. Our sun will become a helium-burning star in about 4.5 billion years. Not all helium-burning stars evolve to become Carbon (C) stars but all Carbon stars are helium burners.
Helium burning stars evolve to become Carbon stars when strong internal convection cycles dredge up freshly synthesized carbon from the core up to the star’s photosphere. The light thus emitted is attenuated by the high carbon content of the photosphere with almost nothing but red light remaining, and the star thus appears quite red.
Stars do not have a solid surface but a photosphere, the physical balance point between gravity and internal gas/radiation pressure.
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