Treasures Of The SMC Part I
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By Andrew Murrell
Throughout history man has recorded what he saw in the sky. Many references and legends concern themselves with the stars and constellations. Unfortunately most of these references revolve around the Northern hemisphere. "Civilised man", and I use that term very lightly, discovered the southern stars early in the 15th Century. Portuguese navigators were the first people to report the "little clouds" and the strange constellations of the south.
The Magellanic Clouds derive their name from Ferdinand Magellan, a 15th century Portuguese Astronomer / sailor who was the first ship's captain to circumnavigate the Earth. Upon the ships return it was reported that the clouds were used as a navigational aid due to their position in relation to the south celestial pole. Magellan's original reference to the clouds were Nebecular Major and Nebecular Minor.
The first detailed observations of the Magellanic clouds were conducted by Sir John Herschel. The only other work performed at that time was done by James Dunlop from the observatory at Parramatta. Herschel's telescope an 18¼" f/13 was constructed at the Cape of Good Hope. He commenced his observations on the 22nd February, 1834. Three regions of the Southern Sky attracted his primary attentions, the Small and Large Clouds and Eta Carinae, then known as Eta Argus.
Sir John Herschel's initial study of the Small Cloud revealed 40 individual clusters and Nebulae, where as the Large Cloud boasted 278 different objects. These 40 objects are known by the prefix NGC, as they become part of the catalogue that the Herschels started, Incidentally Sir John Herschel included the SMC in it's entirety as NGC 292.
Of the clouds Herschel wrote "it is which have no analogues in our Hemisphere".
Today we know that the SMC and the LMC are external Galaxies to our own Milky Way thanks to the work performed by Miss Henrietta Leavitt.
The early 20th century saw a dramatic increase in the quality of Astronomical instruments, the most significant being the ability to take photographs. Harvard University in 1895 constructed an observatory in Peru and installed a 24" refractor that could record stars below 16th magnitude in only an hour exposure (not bad for 1895), covering a field in excess of 50 square degrees.
The initial survey of the photographic plates revealed "large numbers of star clusters and Gaseous Nebulae in confirmation of the earlier visual observations by Sir John Herschel and others" and "the appalling richness in stars which could be counted not by the hundreds but by the tens of thousands".
The observatory sent the photographic plates back to Cambridge Massachusetts where they were checked and any interesting objects catalogued.
During the studies conducted on the plates, one of the workers, Miss Henrietta Leavitt, discovered a number of variable stars. By 1906 Miss Leavitt published a catalogue of the variable stars discovered in the SMC. This list contained 969 stars and included the maximum and minimum magnitude for each star. The average mean difference between maximum and minimum was found to be about 1 magnitude, regardless of the stars apparent brightness on the plates.
It was some time after Miss Leavitt had published her catalogue of variable stars that she was asked to present the results of her work. She chose 25 stars and plotted their light curve. It was then that she discovered that the brighter stars had longer periods between successive maximums and the fainter stars showed much shorter periods. Miss Leavitt and a Colleague, Professor Pickering recognised that if the brightness and period of variation were related then the mass and size must also be related. The last piece of the puzzle was discovered by Eynar Hertzsprung when he associated the variables detailed in the SMC with the Cephied variables as a standard candle and compared them to the variables in the SMC. He discovered that the SMC was at an immense distance and must therefore be a neighbouring galaxy.
The LMC and the SMC are close satellite systems to the Milky Way. The latest distance estimates put the LMC at 170,000 light years and the SMC at 195,000 light years.
The SMC is seen nearly end on, so the side facing the Milky Way is roughly 10,000 light years across while the galaxy stretches away from us for almost 30,000 light years. The SMC is also enveloped in a cloud of cold neutral hydrogen. Radio observations of this cloud show that it is being distorted by the LMC possibly due to a close approach by the LMC nearly two million (2,000,000) years ago. Also a trail of this Neutral hydrogen envelopes the Milky Way, following the orbital path of the SMC and LMC around our galaxy. This torus of material is called the Magellanic stream.
The stellar content of the SMC is considerably different to that of the Milky Way or even the LMC. The SMC is predominantly dominated by population 1 stars, stars that are usually found in the spiral arms of galaxies. They are usually young blue giants. Spectroscopic studies of the SMC stars show that they are lacking the heavier elements of carbon, nitrogen, and oxygen that are found in our own galaxy. The chemical composition of galaxies change with each successive stellar generation. The heavier elements are synthesised inside a stars' core by the thermonuclear reactions. These elements are released during supernovae explosions where they are added into the interstellar medium to be incorporated in the next generation of stars. The relative rareness of heavier elements and the predominance of young blue giant stars (population 1 ) indicate that the SMC is a young galaxy.
The age of the SMC is difficult to estimate but the percentage of heavier elements in the galaxy lead to an approximate age of about 2 billion (2,000,000,000) years. Almost half the age of the earth.
The classification of the SMC has also been difficult to determine. The galaxy was originally classed as an irregular and was used as the yardstick to classify other irregular galaxies. The latest research on the SMC has revised this classification to a Barred Spiral Galaxy of low surface brightness with some peculiar aspects or SBsm Pec. The peculiar nature is derived from tidal interactions caused by the LMC and the Milky Way. Any attempt to classify the SMC is fraught with trouble as the galaxy is disturbed by these interactions.
The visual brightness of the SMC is 2.2 magnitude considering the galaxy's distance of 195,000 light years the Absolute Magnitude derived is -17. This may seem bright but if you compare the Absolute Magnitude of the Milky Way at -21 you can conclude that the SMC is quite small as galaxies go. The estimated mass of the SMC ranges between six hundred million and five billion (600,000,000 and 5,000,000,000) solar masses. The latest results of "weighing" the SMC puts this figure at almost 2 billion (2,000,000,000) solar masses. The low mass and the low Absolute magnitude of the SMC classes it as a Dwarf Galaxy.
To assist the amateur observer in looking at the SMC, several very useful books and observing aids exist to help. Star atlases that detail deep sky objects in the SMC include Uranometria 2000, Herald Bobroff Astroatlas, Mati Morel's SMC Charts and the Hodge Atlas of the Small Magellanic Cloud.
Uranometria 2000 was the first atlas that showed individual deep sky objects in the cloud. The scale in the atlas is small and only the deep sky objects belonging to the NGC and IC catalogue are included. Stars to 9th mag are plotted so navigating around the cloud with this atlas is difficult.
Herald Bobroff's Astroatlas would be the most detailed atlas that is generally available off the book shelves. The atlas gives 3 separated views of the SMC. The "B" series chart is a low power view similar to Norton's star atlas. The "C" series gives a medium power view similar to Uranometria showing stars to 9th magnitude, and the brighter deep sky objects. The "E" series chart is the most detailed showing the main body of the cloud with most of the deep sky objects visible in 12" telescopes. Stars are marked to 14th magnitude which makes moving from object to object fairly easy. The scale on the "E" chart is still quite small.
In 1989 an Australian observer created on observing guide that is second to none, when it comes to dealing with the SMC. The Mati Morel SMC Charts are fairly large scale 30" per mm and detail stars down to 13th magnitude. A considerable proportion of the deep sky objects visible through a 12" telescope are included. The charts are categorised into 3 types. The "A" charts detail the deep sky objects, which are labelled for easy identification. The "B" charts show the same areas, but the variable stars and some selected stars showing estimated magnitudes are marked. Finally a set of 10 charts in larger scale 5" per mm of selected areas of interest are included. These charts show stars marked below 16th magnitude, with even more deep sky objects identified. The only drawback is that the charts are made for Epoch 1950.
The last and definitely the most comprehensive work to detail with the content of SMC is the Paul Hodge Frances Wright photographic atlas of the Small Magellanic Cloud. Published by the University of Washington press, the atlas is comprises of over 200 photographic charts detailing all the known variable stars and the deep sky objects. The charts are grouped into V and B for photos taken in Yellow and Blue light respectively. The limiting magnitude on the charts is about 17.5 for the B series. The scale is large at 16" per mm, for the main set of charts. The main body of the SMC is given an even more detailed review with an even larger scale chart to identify the crowded central region. The deep sky objects are again labelled for easy identification. The variable stars of the SMC are also marked. Just to make this atlas even better a reference book is supplied to tie all of the information together.
Both of these last references mentioned are available from Mati Morel while the Hodge Wright atlas of the SMC can be ordered through a book importer or Library service agent such as James Bennett library service.
Armed with any of these sources the intrepid observer may wish to start observing the Small Magellanic Cloud. My advice is to go out and observe the SMC today (tonight!), but if you're waiting for my description of the deep sky objects contained in the cloud you will have to have a little bit of patience. A detailed description of the 75 or so objects marked on Mati Morel's "A" charts will follow over the next 3 to 4 months with a map showing the positions of each object. These deep sky objects should be visible in a 10"-12" telescope from a good dark sky like Bowen Mountain or Wiruna. At the end of each instalment, there will be a link to a table, listing, if known, the relevant data for each object. Enjoy the SMC!
