New Insights into Ancient Clusters Orbiting the Milky Way
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May 09, 2012

New Insights into Ancient Clusters Orbiting the Milky Way

 

           Eso1220a

 

About 160 globular clusters have been spotted encircling our galaxy, the Milky Way, mostly toward itscentral bulge.  These clusters are among the oldest objects in the Universe. And since the stars within a globular cluster formed from the same cloud of interstellar matter at roughly the same time — typically over 10 billion years ago — they are all low-mass stars, as lightweights burn their hydrogen fuel supply much more slowly than stellar behemoths.

Globular clusters formed during the earliest stages in the formation of their host galaxies and therefore studying these objects can give significant insights into how galaxies, and their component stars, evolve.

This striking view of the globular star cluster Messier 55 in the constellation of Sagittarius (The Archer) was obtained in infrared light with the VISTA survey telescope at ESO's Paranal Observatory in Chile. This vast ball of ancient stars is located at a distance of about 17,000 light-years from Earth. 

A new image of Messier 55 from ESO's VISTA infrared survey telescope shows tens of thousands of stars crowded together like a swarm of bees. Besides being packed into a relatively small space, these stars are also among the oldest in the Universe. Astronomers study Messier 55 and other ancient objects like it, called globular clusters, to learn how galaxies evolve and stars age.

Globular clusters are held together in a tight spherical shape by gravity. In Messier 55, the stars certainly do keep close company: approximately one hundred thousand stars are packed within a sphere with a diameter of only about 25 times the distance between the Sun and the nearest star system, Alpha Centauri.

Observations of globular clusters' stars reveal that they originated around the same time — more than 10 billion years ago — and from the same cloud of gas. As this formative period was just a few billion years after the Big Bang, nearly all of the gas on hand was the simplest, lightest and most common in the cosmos: hydrogen, along with some helium and much smaller amounts of heavier chemical elements such as oxygen and nitrogen.

Being made mostly from hydrogen distinguishes globular cluster residents from stars born in later eras, like our Sun, that are infused with heavier elements created in earlier generations of stars. The Sun lit up some 4.6 billion years ago, making it only about half as old as the elderly stars in most globular clusters.

The chemical makeup of the cloud from which the Sun formed is reflected in the abundances of elements found throughout the Solar System — in asteroids, in the planets and in our own bodies.
Sky watchers can find Messier 55 in the constellation of Sagittarius (The Archer). The notably large cluster appears nearly two-thirds the width of the full Moon, and is not at all difficult to see in a small telescope, even though it is located at a distance of about 17 000 light-years from Earth.

The French astronomer Nicolas Louis de Lacaille first documented the stellar grouping around 1752, and some 26 years later another French astronomer, Charles Messier, included the cluster as the 55th entry in his famous astronomical catalogue. The object is also cross-listed as NGC 6809 in the New General Catalogue, an often-cited and more extensive astronomical catalogue created in the late nineteenth century.

The new image was obtained in infrared light by the 4.1-metre Visible and Infrared Survey Telescope for Astronomy (VISTA, eso0949) at ESO's Paranal Observatory in northern Chile.As well as the stars of Messier 55, this VISTA image also records many galaxies lying far beyond the cluster. A particularly prominent edge-on spiral galaxy appears to the upper right of the centre of the picture.

This stunning new image of Messier 107 below--was captured by the Wide Field Imager on the 2.2-metre telescope at ESO’s La Silla Observatory in Chile. Messier 107, also known as NGC 6171, a compact and ancient family of stars that lies about 21 000 light-years away, is a bustling metropolis: thousands of stars in globular clusters like this one are concentrated into a space that is only about twenty times the distance between our Sun and its nearest stellar neighbour, Alpha Centauri. A significant number of these stars have already evolved into red giants, one of the last stages of a star’s life, and have a yellowish colour in the image.

Messier 107 has undergone intensive observations, being one of the 160 stellar fields that was selected for the Pre-FLAMES Survey — a preliminary survey conducted between 1999 and 2002 using the 2.2-metre telescope at ESO’s La Silla Observatory in Chile, to find suitable stars for follow-up observations with the VLT’s spectroscopic instrument FLAMES [1]. Using FLAMES, it is possible to observe up to 130 targets at the same time, making it particularly well suited to the spectroscopic study of densely populated stellar fields, such as globular clusters.

M107 is not visible to the naked eye, but, with an apparent magnitude of about eight, it can easily be observed from a dark site with binoculars or a small telescope. The globular cluster is about 13 arcminutes across, which corresponds to about 80 light-years at its distance, and it is found in the constellation of Ophiuchus, north of the pincers of Scorpius.

Roughly half of the Milky Way’s known globular clusters are actually found in the constellations of Sagittarius, Scorpius and Ophiuchus, in the general direction of the centre of the Milky Way. This is because they are all in elongated orbits around the central region and are on average most likely to be seen in this direction.

 

           6a00d8341bf7f753ef0147e0a8acbe970b-800wi

The Daily Galaxy via ESO

Image credit: ESO/J. Emerson/VISTA. Acknowledgment: Cambridge Astronomical Survey Unit.

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