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Observational History of The Small Magellanic Cloud (SMC)

Introduction

Throughout history man has recorded what he saw in the sky. Many legends and historical documents concern themselves with the stars and constellations.

Many tribes of Australian Aboriginals have "dreamtime stories", which they have passed down from generation to generation, to explain the universe as they perceive it. One such legend describes the clouds as the campfires of an old couple, the Jukara. The Jukara relied on other star people to supply them with fish and lily bulbs caught in the Milky Way to survive. The old couple cooked the food over their campfire, which was the star Achernar. The Large cloud represented the old man while the Small cloud was the old woman.

Detailed observations of the far southern sky were first made by Europeans in the 15th Century. In their writings, Portuguese navigators reported the "Little Clouds" and strange Constellation they had observed on their voyages:

"Manifestly twooclowdes of reasonable bygnesse movynge abowt the place of the pole continually now rysynge and now faulynge so keepynge theyr continuall course in circular movynge with a starre ever in the myddest which is turned abowt with them abowt xi degrees frome the pole"

Ferdinand Magellan

The Voyage

The Magellanic clouds are named after Ferdinand Magellan, a 15th Century Portuguese Astronomer/Sailor who was the captain of the first ship known to have circumnavigated the Earth. Having been disgraced in his native Portugal, Magellan offered his services to the King of Spain, Charles I. He proposed the search for a new trade route to the Spice Islands via the New World. The King granted Magellan a fleet of 5 ships, with crews, for a two year tenure.

On September 20th Magellan set forth on his voyage of discovery. The 5 ships were named the Trinidad, captained by Magellan himself, the San Antonio, the Concepcion, the Victoria and the Santiago. In total, 270 men set forth to explore the uncharted waters.

However, as the voyage progressed, the captains of the San Antonio, the Victoria and the Concepcion plotted mutiny and to murder Magellan.

After a brief stop at the Canary Islands to take on fresh supplies, Magellan set sail for Brazil. On this stretch of the voyage Magellan suppressed the mutiny by having Juan de Cartegena, Captain of the San Antonio, relieved of his command and placed in the brig of the Victoria.

On December 13th the fleet anchored in the place that is now the port of Rio de Janeiro. After again restocking, the fleet sailed south down the coast of South America to Patagonia (Argentina), where Magellan decided to wait out the cold winter months. During this time, a second mutiny was attempted after the captain of the Victoria released Juan de Cartegena. Magellan was able to stop the mutiny and had Captain Mendoza, of the Victoria, and Captain Quesada, of the Concepcion, executed. Cartegena suffered the fate of being marooned on Patagonia.

During that winter, Magellan sent the Santiago down the coast looking for a passage to the Pacific Ocean. Unfortunately, the ship was lost in the rough seas.

Eventually Magellan set sail again and on October 21st 1520, a year after he had started, Magellan found the strait that is now named in his honor.

However, during the passage through the 370km long strait the new captain of the San Antonio turned back and set sail for Spain the way they had came.

Once having cleared the strait by the last week of November, Magellan incorrectly estimated that the Spice Islands were probably only a weeks sailing away. He made the error of not taking on fresh supplies. In fact the next stretch of the voyage took 4 months to complete.

Finally in March the fleet reached the island of Guam where they were able to collect fresh supplies. On March 28th Magellan reached the Philippines. It was there that Magellan met his fate when he became involved in a tribal dispute and was killed by a poison arrow on the 27th of April 1521.

By this time the remaining 3 ships had a combined crew of only 115 men. After transferring its crew to the other ships, it was decided to scuttle the Concepcion.

The newly appointed Captain Sabastian del Cano sat sail on May 1st. Finally, in November, they reached the Spice Islands where they took on the precious cargo for which they had came.

To maximize the chance of at least one ship returning to Spain, it was decided that the Trinidad and the Victoria would each try and make their own way home. The two ships thus parted company. The Trinidad sailed east. Unfortunately she was intercepted by the rival Portuguese. The ship was seized and the crew slain. The Victoria was luckier and returned to Spain with its valuable cargo on September 6th 1521, three years after they had first set off. Of the original crew of 270 men, only 18 had survived.

The Clouds

To help navigate the unknown southern waters it is recorded that Magellan had an 'artificial star', in the way of a burning torch, placed at the bow of his ship. 'The Clouds' first became visible to them when the fleet approached the latitude of Brazil. Magellan originally referred to the clouds by the names Nubecular Major and Nebecular Minor. Upon the ships return it was reported that the clouds had been successfully used as a navigational aid owing to their position in relation to the south celestial pole.

Neuerthelesse, the Portugales of owre tyne haue sayled to the fyue and fyftie degree of the south pole. Where Coompasinge abowte the poynt thereof they myght see throughowte al the heauen about the same, certeyne shynynge whyte cloudes here and threr amonge the starres, like unto theym whiche are seene in the tracte of heaven cauled Lactea via, that is the mykle whyte waye. They say, there is noo notable starre neare about that pole lyke vuto this of owres which is common people thynke to bee the pole it selfelf.

James Dunlop - Parramatta, Australia

James Dunlop of the Parramatta Observatory was one of the first people to observe the SMC with a telescope. In 1820 he became the assistant at the new observatory and was promoted to the superintendent from 1831 to 1847. During this time he catalogued over 7835 stars and in 1826 he published a list of 621 nebulae and a catalogue of nebulae and clusters of stars in the Southern Hemisphere observed at Parramatta in New South Wales.

The Nebula Minor, to the naked eye, has very much the appearance of a small Cirrus-cloud; and through the telescope, it has very much the appearance of one of the brighter portions of the Milky Way although it is not so rich in stars of all the variety of small magnitudes, with which the brighter parts of the Milky Way in general abound, and therefore it is probably a beautiful specimen of the nebulosity of which the remote portion of that magnificent zone is composed.

John Herschel - Cape of Good Hope

Sir John Herschel conducted the only other detailed observations of the Magellanic clouds done at about this time. Herschel's telescope an 18.25" f13 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 its entirety as NGC 292.

Of the Clouds, Herschel wrote:

It is evident that the Nebeculae are to be regarded as Systems Sui Generis and which have no analogues in our Hemisphere".

And:

The two Magellanic clouds, Nubecula major and Nebecular minor, are very remarkable objects. The larger of the two is an accumulated mass of stars, and consists of clusters of stars of irregular form, either conical masses or nebulae of different magnitudes and degrees of condensation. This is interspersed with nebulous spots, not resolved into stars, but which are probably star dust, appearing only as a general radiance upon the telescopic field of a twenty-feet reflector and forming a luminous ground on which other objects of striking and indescribable form are scattered. In no other portion of the heavens are so many nebulous and stellar masses thronged together in an equally small space. Nebecula minor is much less beautiful, has more unresolved nebulous light while the stellar masses are fewer and fainter in intensity.

Cleveland Abbe

Cleveland Abbe made an early evaluation of the Magellanic Clouds in 1867 after he studied the newly published General Catalogue of Nebulae and Clusters of Stars, by Sir John Herschel. He summarized his findings thus :-

The study of the foregoing Tables may lead to the following conclusions or suggestions:

1. The clusters are members of the Via Lactea, and are nearer to us than the average of its faintest stars
2. The nebulae resolved and unresolved lie in the general without the Via Lactea, which is therefore essentially stellar
3. The visible universe is composed of systems, of which the Via Lactea, the two Nebeculae, and the Nebulae, are the individuals, and which are themselves composed (either simple multiple, or in clusters) and of gaseous bodies of both regular and irregular outlines.

It would be another 50 years before the Magellanic Clouds were accepted as individual Galaxies made up of a multitude of millions of stars.

Henrietta Leavitt

Today we know that the SMC and the LMC are external galaxies to our own Milky Way, thanks largely to the work performed by 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 1890 constructed an observatory in Peru and installed a 24-inch refractor that could record stars below 16th magnitude in only an hour exposure, covering a field in excess of 50 square degrees. The initial survey of the photographic plates revealed :-

"Large numbers of star clusters."
"Gaseous Nebulae in conformation of the earlier visual observations by Sir John Herschel and others." "The appalling 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, Henrietta Leavitt, discovered a number of variable stars. By 1906 Leavitt published a catalogue of the variable stars discovered in the SMC. This list contained 969 stars, and included the maximum and minimum was found to be about 1 magnitude, regardless of the stars apparent brightness on the plates. Included in the Harvard Circular Number 173 (1912) Henreitta Leavitt quoted:

"Since the variables are nearly the same distance from the Earth, their periods are apparently associated with their actual emission of light, as determined by their mass, density, and surface brightness.... A number of brighter variables have similar light curves, as UY Cygni, and should repay careful study. The class of spectrum ought to be determined for as many objects as possible. It is hoped, also, that the parallaxes of some variables of this type may be measured.

It was some time after 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. Leavitt and a colleague, Professor Pickering, recognized that if the brightness and period of variation were related then the mass and size must also be related.

Eynar Hertzsprung

The last piece of the puzzle was discovered by Eynar Hertzsprung, when he associated the variables detailed in the SMC with the Cepheid variables in the Milky Way. Hertzsprung used the Milky Way's Cepheid 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 neighboring galaxy. Unfortunately there was an error in the calculations and the estimated distance was only 30,000 light years. A later estimate made by Harlow Shapley rendered a distance of 60,000 light years, defiantly putting the SMC outside the Milky Way.

Harlow Shapley

Shapley devoted considerable effort to unraveling the secret of the Clouds.

He realized the significance of the SMC in establishing our place in the universe, and considered the most important contributions that the Magellanic Clouds could offer were:

1. The discovery of hundreds of giant cepheid variables in both clouds
2. Measurement of high positive radial velocities for emission-line objects associated with the clouds, suggesting their independence of the Milky Way.
3. Discovery and development of the period-luminosity relation for classical cepheids
4. Detection with radio telescopes of neutral hydrogen in and around the clouds, and the measurement of it’s distribution.
5. The finding of the maximum luminosity's reached by stars of many spectral types; for example, by the novae and the variable stars of all principal classes, by open and globular clusters and by planetaries and other bright line nebula.
6. The demonstration of a peculiar frequency distribution of the periods of the classical cepheids in the Small Cloud, with evidence of a marked dependence of period on stellar density.
7. The derivation of a colour-magnitude array for supergiant stars, brighter than Mp = -4, in the Small Cloud.
8. Discovery of numerous bluish supergiant-eclipsing stars of the Beta Lyrae type, many of which are associated with the great 30 Doradus nebula.
9. Presentation of preliminary evidences of rotation and of internal turbulence in the Large Cloud.
10. The deduction that the clouds of Magellan are not dwarf associates of the Milky Way but full size galaxies brighter and more massive than the average galaxy in our part of the Universe.
11. Conclusion that at least two types of stellar population occur in both clouds: the primitive stars, with their presence indicated by the associated globular clusters, and the recently evolved stars, represented by the blue supergiants and suggested by the abundant bright and dark nebulosity.
12. The deduction that the 30 Dorodus bright nebulosity in the large cloud is more radiant by a hundred times than the brightest globular known anywhere, and in fact is more luminous intrinsically than any of the nearby dwarf galaxies with their millions of stars.
13. Discovery of a large "wing" of the Small Cloud, and recently acquired evidence from the occurrence therein of only long-period supergiant cepheids that the wing may be, in a sense, a structure distinct from the Small Cloud; but the distance of the two must now be the same since the wing’s variables fit the Small Cloud’s period-magnitude relation.

Shapley went on to say "altogether these thirteen items emphasize the usefulness of the shynynge whyte cloudes for our understanding of the sidereal universe".

Shapley was not wrong, as the Clouds of Magellan have been used as a testing ground for many astronomical theories. During the course of the years the Small Cloud has been studied more than most astronomical objects. This continuos study has resulted in many revisions in our understanding of the Clouds.

A page this size requires many acknowledgements which are too numerous to list here.