Located in the southern constellation of Fornax, today’s Astronomy Picture of the Day features the diverse and varied Fornax galaxy cluster, one of the closest clusters of galaxies to the Local Group, the galaxy cluster that contains our Milky Way and the Great Galaxy in Andromeda. The distance to the center of mass of any such cluster is said to be the distance to that cluster and, in the case of the Fornax cluster, that distance is about 62 million light years or about 25 times the distance to the closest major galaxy to the Milky Way, the great galaxy in Andromeda. Most of the galaxies in this cluster are elliptical galaxies with one notable exception, the beautiful barred-spiral galaxy, NGC-1365, the center of which contains an active, rapidly-rotating, super-massive Black Hole.
To get a sense of how many stars are contained in just this image alone, each galaxy contains anywhere from 50 billion to 250 billion stars in the case of a large galaxy such as the Milky Way or the Great Galaxy in Andromeda. There are between about 100 and 1000 galaxies in a cluster so, a quick calculation yields about 250 trillion solar masses in a galaxy cluster such as the Fornax cluster! There are an estimated 1E+11 (100 billion) stars in each galaxy and there are an estimated 1E+11 galaxies in the observable universe, yielding an astounding 1E+22 stars (1 with 22 zeros) in the observable universe!
A typical galaxy cluster is between 5 and 30 million light-years across. To get a sense of size and scale, the field of view of the above image is about 2 degrees from one side to the other or about 4 times the diameter of the full moon. It is also interesting to note, the Fornax cluster is 10 percent farther than the better known and more populated Virgo Galaxy Cluster.
In the case of the Virgo and Fornax Clusters, the preponderance of the galaxies are elliptical with a paucity of spirals or barred-spirals. Elliptical galaxies are among the most massive, coherent structures in the universe, completely devoid of gas and dust, the raw materials from which new stars and planetary systems form. Thus, all star formation has ceased with the stellar population consisting of mostly old, evolved stars whose ages date back to the very earliest epochs in the universe’s history.
In the case of spiral galaxies, any gas and dust is located in the spiral arms, the regions where we observe new star formation in the leading edges, powered by the spiral arm’s leading-edge rotational compression waves. In the case of elliptical and spiral galaxies, the nuclei are devoid of gas and dust, indicating that the nuclear stellar population consists of mostly old, evolved stars whose ages and similarities are consistent with the stellar populations of elliptical galaxies. The overall hue of the region suggests the composition of their respective stellar populations. In the case of spiral and barred-spiral galaxies, the galactic disc and spiral arms of those galaxies, the hue is generally blue and yellow-blue, suggesting the stellar population consists of relatively hot, luminous, newly-formed stars while the nuclei are generally warmer, more yellow-red, suggesting the stellar population consists of older, more evolved stars.
Imagination is more important than knowledge
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