Astrosat Picture of the Month

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Image of the Month

The AstroSat Picture of the Month for November 2017 are the Near-UV (left) and Far-UV (right) images of the Globular Cluster NGC 1851, taken by UVIT on board the ASTROSAT. The FUV image shows only the hottest stars in the cluster. All colours are artificial.

(Picture Credits: Annapurni Subramaniam and collaborators )

To download a high resolution image, see link.

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“AstroSat Picture of the Month” is an initiative of the Public Outreach and Education Committee of the Astronomical Society of India and the AstroSat Training and Outreach Team.

NGC 1851: Two star families in one Globular Cluster

NGC 1851 is a Globular Cluster which is almost 40000 light years away from us, in the southern constellation of Columba, near Canis Major. A Globular Cluster is a group of stars tightly bound together by their own gravity. All the stars in these spherical clusters orbit around the centre of our galaxy together. NGC 1851, or Caldwell 73, is one such cluster, visible in a moderate telescope at a magnitude of 7.3, with a size that is a third of the full moon. It was discovered by James Dunlop from Australia in 1826.

The stars in a Globular Cluster are usually born together, and hence share similar properties. However, NGC 1851 is one of the few clusters where two distinct types of stars with different properties seem to co-exist! Many individual stars in this object have been studied before with the HST, but good ultraviolet images were needed to understand this mystery better. This prompted a group of 18 astronomers, including 12 from India, to use the UVIT on board the AstroSat. They imaged this cluster in the Near and Far ultraviolet wavebands far better than earlier attempts with other telescopes.

The superior resolution of AstroSat allowed them, for the first time, to measure the ultraviolet properties of individual stars in the inner crowded region of the cluster. Using this data, they could show that NGC 1851 does indeed have two distinct families of stars within it, which still retain their separate histories. This tells us that NGC 1851 was probably formed when two smaller clusters merged together some time in the past!

The paper describing these results can be downloaded from here.


AstroSat, India's first dedicated multi-wavelength space observatory, was launched by ISRO on 28 September, 2015. It has five instruments on board – the Ultra Violet Imaging Telescope, the Soft X-ray Telescope, the Large Area X-ray Proportional Counter, the Cadmium-Zinc-Telluride Imager and the Scanning Sky Monitor.

Get answers to your common queries about ASTROSAT in English, in हिंदी, and in मराठी.


  1. Pale Blue Dot (EARTH): Look again at that dot. That’s here. That’s home. That’s us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. The aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love, every mother and father, hopeful child, inventor and explorer, every teacher of morals, every corrupt politician, every “superstar,” every “supreme leader,” every saint and sinner in the history of our species lived there-on a mote of dust suspended in a sunbeam.

    The Earth is a very small stage in a vast cosmic arena. Think of the rivers of blood spilled by all those generals and emperors so that, in glory and triumph, they could become the momentary masters of a fraction of a dot. Think of the endless cruelties visited by the inhabitants of one corner of this pixel on the scarcely distinguishable inhabitants of some other corner, how frequent their misunderstandings, how eager they are to kill one another, how fervent their hatreds.

    Our posturings, our imagined self-importance, the delusion that we have some privileged position in the Universe, are challenged by this point of pale light. Our planet is a lonely speck in the great enveloping cosmic dark. In our obscurity, in all this vastness, there is no hint that help will come from elsewhere to save us from ourselves.

    The Earth is the only world known so far to harbor life. There is nowhere else, at least in the near future, to which our species could migrate. Visit, yes. Settle, not yet. Like it or not, for the moment the Earth is where we make our stand.

    It has been said that astronomy is a humbling and character-building experience. There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world. To me, it underscores our responsibility to deal more kindly with one another, and to preserve and cherish the pale blue dot, the only home we’ve ever known.


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