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NGC 253--Caroline's galaxy.

Many galaxies vie for the title 'best in sky' and the choice is obviously very subjective, but there is no doubting that for many observers NGC 253 would be close to the top of their list. The problem for British observers is that lying in Sculptor with a declination of -25[degrees] it never rises far above the horizon, although seen from further south it can be a truly spectacular sight. Discovered from Datchet (near Slough) it is another Herschel discovery, but this time by William's sister, Caroline, who was sweeping for comets in 1783 September with her 4.5-inch [115mm] reflector when she discovered this remarkable galaxy.

NGC 253, often called the Silver Coin or Silver Dollar galaxy, is a starburst galaxy which means that it is currently undergoing a period of intense star formation, possibly caused by a recent merger or collision with another galaxy. The newly formed stars are very bright so starburst galaxies are among the most luminous. NGC 253 lies around 11 million light years distant and with a diameter of about 70,000 light years is comparable in size to the Milky Way. It is a barred spiral and the brightest member of the Sculptor group of galaxies: the nearest group to our own Local Group.

Even when seen from more southerly declinations, Sculptor is a faint constellation whose brightest stars are only 4th magnitude, so the constellation will probably be all but invisible from the UK. For the majority of observers therefore setting circles or a Go To telescope will be necessary to find it. With an RA of 00h 47.6m and declination of -25[degrees] 17' (2000.0) the galaxy lies just under 5[degrees] NNW of alpha Sculptoris. Some observers claim it is a naked eye object, and while this may only be true under a really good sky with the galaxy overhead, it is certainly a stunning sight in any form of optical aid. Binoculars with a field of around 4[degrees] will also show the globular cluster NGC 288 in the same field, giving an interesting contrast between the small round ball of haze that is the cluster and the large cigar shape that is the galaxy.



A 150mm telescope will show the galaxy orientated NE-SW, about 20 arcmin in length and 4 min wide with highly tapered ends. Averted vision will show hints of subtle dark and light markings in the halo, along with a well condensed and extended core. A larger telescope is needed if the galaxy is to live up to the reputation it has gained among observers. Carl Knight lives in New Zealand where the galaxy culminates almost overhead. His drawing through a 12-inch (30cm) SCT at x80 was made under a magnitude 6.5 naked-eye sky and shows the galaxy almost filling the 52 arcmin eyepiece field.


From the UK the galaxy barely rises above the murk that lies around the horizon and seeing detail in it will never be easy. Dale Holt overcomes these problems by using a video camera and viewing and sketching from a computer screen. His screen drawing, obtained through a 350mm Newtonian with a Watec 120N video camera attached, shows just what a complex galaxy this is, and also how much detail can be seen, even when it is at such a low altitude.

Paul and Liz Downing's stunning image was taken from their observatory in southern Spain where the galaxy rises an important few degrees higher. They commented that with their 350mm Celestron C14 SCT operating at f/7 it proved difficult to fit this huge galaxy on to the chip of the ST10 CCD camera. Their LRGB image consists of 20, 10, 10, 10 minute exposures respectively.

Although living in the UK Peter Howard makes regular observing trips to the Arizona desert where he images with a Celestron C11 SCT coupled to a Starizona Hyperstar optical corrector. This allows his f/10 telescope to operate at a range of focal ratios to suit the size of the object being imaged. His image of NGC 253 using this system is shown here and is composed of 180s L and 30s each RGB. The complex structure in the galaxy, much of which is easily visible optically in a 50-60cm class telescope, is clearly seen.

Caroline Herschel is often thought of as William's amanuensis and not as a serious astronomer in her own right. The fact that she swept-up this galaxy just a few degrees above the Datchet horizon with a small telescope, and also discovered many other deep sky objects and comets both before and after, shows what a good observer she in fact was. It is a pity she never had the opportunity to view this galaxy high in the sky and through a large telescope, so that she could realise what a wonderful object she had discovered.


Stewart L. Moore, Director, Deep Sky Section
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Title Annotation:Observers' Forum
Author:Moore, Stewart L.
Publication:Journal of the British Astronomical Association
Date:Oct 1, 2012
Previous Article:Accounts for the year ended 2012 June 30.
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