A better view with adaptive optics into the heart of a globular cluster

May 01, 2013
Globular cluster NGC 6496 observed with SAM. The image is about 3 arc minutes across. The enlarged sections of the cluster show the image with SOAR adaptive optics (AO) on and off. (Image Credit: NOAO/AURA/NSF)

(Phys.org) —Astronomers at the Southern Observatory for Astrophysical Research (SOAR) and the Cerro Tololo Inter-American Observatory (CTIO) have demonstrated the significant difference that sharp stellar images can make in our understanding of the properties of stars. They have observed the globular cluster NGC 6496 using a new instrument dubbed SAM, for SOAR Adaptive Module, which creates an artificial laser guide star. SAM, built by CTIO/NOAO-S, is mounted on the SOAR 4.1 meter telescope.

From the surface of the earth, stars twinkle as their image wobbles around due to the effects of the Earth's atmosphere, rather like observing a penny on the bottom of a swimming pool. By removing this wobble, using an that utilizes a laser guide star, the stellar images are sharpened, and fainter stars appear. The accompanying figure shows this globular cluster, and the difference between the image of NGC 6496 with the artificial laser-produced guide star turned on and off. Turning on the artificial guide star allows the effect of the atmosphere to be determined so that the adaptive optical system can sharpens the image.

Time exposure of SOAR telescope observing at Cerro Pachon in Chile. (Image Credit: Daniel Maturana/NOAO/AURA/NSF)

The resulting stellar images allow astronomers to make more precise measures of the colors of the stars, and for a globular cluster, this translates into a better measurement of distance, age, and what astronomers call metallicity: how much the stars are enriched with elements that are heavier than hydrogen and helium. This, in turn, allows for better understanding of the of the stars in these dense clusters.

There are only about 150 known globular clusters in , important because they represent some of the oldest objects in the Galaxy. Because NGC 6496 is on the other side of the , it is seen through a thick layer of dust. The position of this globular cluster has made it difficult to determine its basic properties. For example, previous measurements of its distance do not agree well with each other.

Luciano Fraga, Andrea Kunder and Andrei Tokovinin (in a paper accepted for publication by the Astronomical Journal) used the capabilities of SAM to sharpen star images to peer deep into this crowded cluster, obtaining more accurate results than done previously from the ground. The authors find a distance of 32,600 light years, an age of 10.5 billion years, and a value for metallicity that is much higher than in most globular clusters. To do this, they measured over 7000 stars in the cluster. Then they plotted the colors and brightness of each star, resulting in a diagram referred to as a color-magnitude diagram. This diagram immediately tells astronomers a great deal about the evolutionary phase of the in the cluster.

SAM works in the visible spectral region and can cover a field of 3 arc minutes, about one tenth the size of the full moon. It compensates for the lower atmospheric turbulence by using an artificial created by a powerful ultra-violet laser. While this technique, called , has been used on other telescopes before, SAM covers a wider field of view and shorter wavelengths. Information about the SAM can be found at: http://www.ctio.noao.edu/new/Telescopes/SOAR/Instruments/SAM/

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User comments : 10

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gwrede
not rated yet May 01, 2013
Surprisingly small difference in the on-off pictures. I wonder if the weather happened to be extra calm that night.
GSwift7
not rated yet May 01, 2013
Surprisingly small difference in the on-off pictures. I wonder if the weather happened to be extra calm that night


The reproduction of the images on your computer screen may not be a good representation of the true difference between the images.
Tuxford
1 / 5 (2) May 01, 2013
"The authors find a distance of 32,600 light years, an age of 10.5 billion years, and a value for metallicity that is much higher than in most globular clusters."

Higher metallicity in an early cluster? Say it ain't so!

Fleetfoot
5 / 5 (1) May 01, 2013
"The authors find a distance of 32,600 light years, an age of 10.5 billion years, and a value for metallicity that is much higher than in most globular clusters."

Higher metallicity in an early cluster? Say it ain't so!


That would suggest it might be in the Oosterhoff type I class.

http://en.wikiped..._content
GSwift7
not rated yet May 02, 2013
Higher metallicity in an early cluster? Say it ain't so!


No, this is a very old cluster.

It has had 10 billion years to evolve heavy elements. This cluster is not located 10 billion years away from us, and the light we are seeing is not 10 billion years old. It is part of the Milkyway galaxy, and is only 32 thousand LY away. The stars in this cluster are many generations old. If this cluster did not have hight metallicity, something would be very wrong. The cluster is old, but the stars in it are not all old. The large hot stars that should have been there 10 billion years ago are long gone, exploded into supernovae and reformed into second, third, fourth and fifth generation stars we see there today.
Fleetfoot
not rated yet May 02, 2013
Higher metallicity in an early cluster? Say it ain't so!


No, this is a very old cluster.

It has had 10 billion years to evolve heavy elements. This cluster is not located 10 billion years away from us, and the light we are seeing is not 10 billion years old. It is part of the Milkyway galaxy, and is only 32 thousand LY away. The stars in this cluster are many generations old. If this cluster did not have hight metallicity, something would be very wrong. The cluster is old, but the stars in it are not all old. The large hot stars that should have been there 10 billion years ago are long gone, exploded into supernovae and reformed into second, third, fourth and fifth generation stars we see there today.


I think you need to revise the subject. In globular clusters, almost all the stars formed at the same time and there have been virtually no new stars since then.
GSwift7
not rated yet May 03, 2013
I think you need to revise the subject. In globular clusters, almost all the stars formed at the same time and there have been virtually no new stars since then


There are many types of globular clusters. Some are very low in metal and are composed of very old first and second generation stars. Some globular clusters have two or three distinct generations of stars which formed at different times. Some have relatively higher metalicity (these tend to be seen near the center of the Milkyway, like the one above). GC with high metalicity are called Type 1, though when they say a GC has high metalicity, they are only talking relative to other GC. It's still not as metallic as our solar system.

It's not correct to assume the above GC is composed of first generation stars though.
Fleetfoot
5 / 5 (1) May 03, 2013
I think you need to revise the subject. In globular clusters, almost all the stars formed at the same time and there have been virtually no new stars since then


There are many types of globular clusters. Some are very low in metal and are composed of very old first and second generation stars. Some globular clusters have two or three distinct generations of stars which formed at different times. Some have relatively higher metalicity (these tend to be seen near the center of the Milkyway, like the one above). GC with high metalicity are called Type 1, though when they say a GC has high metalicity, they are only talking relative to other GC. It's still not as metallic as our solar system.

It's not correct to assume the above GC is composed of first generation stars though.


Agreed, but even the higher metalicity clusters formed the bulk of their stars over a short period, albeit perhaps in a different environment.
yyz
5 / 5 (1) May 03, 2013
In the case of NGC 6496 the color magnitude diagram identifies only one stellar population with an age of 10.5 plus or minus 0.5 Gyr: http://arxiv.org/abs/1304.4880
VendicarE
not rated yet May 03, 2013
"Say it ain't so!" - Tuxford

It ain't so. You are confusing age with distance.