The CFHT Open Cluster Survey : NGC 2099

The CFHT Open Cluster Survey : NGC 2099
From CFHT's Astronomy Picture of the Week
Interstellar space is mostly empty. Every now and then however, a gigantic cloud of dust and gas, a so-called Giant Molecular Cloud (GMC), is formed, usually in a spiral arm of the Galaxy. These GMC's contain enough material to build thousands of stars like our Sun. Through a poorly understood process, parts of the GMC collapse as gravity overwhelms the internal pressures. The GMC fragments into ever smaller pieces culminating in the formation of individual stars which may remain loosely bound to one another by gravity. These structures appear to us as Open Star Clusters. The fate of these clusters is to be torn apart by the tidal pull of other large GMCs as they orbit about the center of the Galaxy. Although a few of them will survive as long as a few billion years,  most of them will be dispersed faster. Nevertheless, they survive long enough for the more massive stars to complete their thermonuclear evolution, passing into the stellar graveyard of white dwarfs.
Open clusters are important since they provide a tool to study the formation and evolution of Galactic stellar populations. The stars within a single cluster formed at the same time and under the same conditions, allowing the astronomer to reliably determine the cluster age. Since clusters are both forming and dissolving at present , it is possible to use the population of clusters to develop a fairly complete picture of the life-cycle of the clusters and the stars they contain. An analogy is to consider the human population in a large school complex offering classes from kindergarten to 12th grade, where even a brief study would be sufficient for a trained scientist to fully characterize the development of the human species from early childhood through to adulthood.
This image presents NGC 2099 (Messier 37) a rich open cluster located about 5000 light years away, in the constellation Auriga. The cluster, containing about 2000 stars, was imaged with the CFH12K mosaic camera. It is a member of a carefully selected sample of open clusters, spanning a range of ages from about 1 to 5 billion years old. This sample was selected so that the Astronomers could study the white dwarf population. White dwarfs are the most common kind of star in the Galaxy. All stars whose initial mass is up to about 5 times that of the sun will end up as white dwarfs once the nuclear energy stores are depleted. It is the fate of the sun. However, many chapters of the white dwarf story remain to be written, because these earth-sized, very dense balls of carbon and oxygen are exceedingly faint and therefore difficult to study.
This two-color picture of NGC 2099 was created from separate five minute exposures at blue and yellow wavelengths using the CFHT12K camera at the Canada-France-Hawaii Telescope. The complete field of view is 28 by 42 minutes of arc.
Image Title: The CFHT Open Cluster Survey : NGC 2099
Image courtesy of Harvey Richer, Patrick Durrell, Gregory Fahlman, J. Kalirai, F. D'Antona & G. Marconi
Text based on accompanying on-line materials.

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 Last Modified On: Saturday, December 16, 2000