John A. Graham
The Magellanic Clouds are the nearest of the external galaxies. Each is an independently evolving star system, actively forming stars at the present time but also containing some which are as old, about 15 billion years, as any that we know. Their importance is many-fold but two aspects especially stand out. First, they act as a mirror to our own Milky Way galaxy and provide a guide as to how it would appear if we could view it from a vantage point high above its dusty disk. Second, we can make use of them to tell us about other galaxies far too remote for any sort of detailed study. The Magellanic Clouds are fundamentally important for the calibration of extragalactic distance indicators. They represent one of the few opportunities we have to intercompare rare objects like the most luminous blue supergiant stars, variable stars, star clusters, and HII regions directly with common stars similar to the Sun, all at the same distance and all comparatively unobscured by interstellar dust. With firm calibrations in hand, we can then confidently proceed to more distant systems where only the very brightest objects may be identifiable.
Even with a small telescope trained on one or other of the two Magellanic Clouds, there is an immediate sense of looking into the heart of a galaxy. It is the young population which is immediately the most striking. These are the massive stars which expend energy so profusely that their nuclear fuel is used up after only a few million years. They tend to clump in close groups of associations and often illuminate the surrounding gas to form bright HII regions. These are the classic markers of Population I as defined by Walter Baade in the middle of this century. The older Population II is much less conspicuous, contributing a faint substratum of stars which have long ago arranged themselves in extended, rather uniform distributions.
Population I and Population II are very much extreme categories and we have learned, after Baade's fundamental work, that there is a continuous transition between them. Stars and clusters of all ages covering a wide range of chemical composition are found in both the Magellanic Clouds and the Galaxy. As well as discussing the two extreme groups, it is appropriate to give special mention to this intermediate population as it is comparatively so prominent in the Magellanic Clouds.
Among the representatives of Population I, the brightest stars provide a unique opportunity to study the evolution of massive stars and the upper limit to the mass that a star can have and remain stable. Stars like this are sufficiently rare and widespread that this is an impossible job to do within the Milky Way. For most of this century, it was thought that a mass of 70-80 times that of the Sun was the maximum that a star could have and remain vibrationally stable. In the last decade, largely through Magellanic Cloud research, it has been shown that stellar winds dampen incipient instabilities very effectively and that stellar masses 100-200 times that of the Sun are not only possible but probable.
All massive stars end as supernovae and we were incredibly privileged in 1987 to witness in the Large Magellanic Cloud the brightest supernova since the invention of the telescope nearly 400 years ago. Among many other things, this event founded a whole new science of extrasolar neutrino astronomy and provided indisputable observational evidence that nucleosynthesis actually occurs inside stars. SN 1987A was formerly a normal, undistinguished blue supergiant star in one of the rich Population I regions of the Large Magellanic Cloud.
Also included among Population I are the Cepheid variable stars. Cepheids have become one of the standard distance indicators for galactic and extragalactic research through their period-luminosity relation and its validity from galaxy to galaxy. Fortunately, encouraging progress is being made in removing this uncertainty by observing many of the Cepheids in the Magellanic Clouds. With new instrumentation, the accuracy of the brightness and color measurements is being refined and observations in the infrared are proving especially useful.
Neutral and molecular hydrogen gas has a close association with Population I. 21-cm radio surveys have shown that each Magellanic Cloud has an abundant supply remaining for future generations of stars. Molecular hydrogen is harder to detect as we rely mainly on measurements at mm wavelengths of carbon monoxide, a tracer molecule. Both are present in regions where luminous stars are now forming in the Magellanic Clouds.
When we come to study the older populations of the Magellanic Clouds, we look past the brilliant associations with their blue supergiants and HII regions, past the Cepheid variables, and the numerous open star clusters until we see in each Cloud only the faint amorphous background which is made up of stars and planetary nebulae a billion years or more old. Relieving the general uniformity, old globular star clusters similar in appearance to those of our own galaxy are seen but, except for the occasional nova, every star in this old population is faint. Yet it is this component which forms the structural backbone of each Cloud, accounts for most of the mass and thereby determines the internal dynamics. Of the oldest objects, those which tell us that the Magellanic Clouds have existed for as long as our galaxy, the short period RR Lyrae stars are perhaps the easiest Population II objects to discover. Most information about the chemical composition of Population II comes from observing red giants in the oldest globular clusters. As in our galaxy, there is a good correlation between age and heavy element abundance although, in neither Cloud does the heavy element enrichment reach the level that is found in the youngest galactic stars.
Study of the old populations is important for investigating the origin of the Magellanic Clouds and the differences between them and the Galaxy at the earliest epochs. The distribution of faint red stars on photographs taken with wide field Schmidt telescopes is a guide to the mass distribution within each body. Magellanic Cloud novae occur two or three times a year and, as the list lengthens, these too are giving us a better idea about where most of the matter in each Cloud is located.
Both Magellanic Clouds have been found to have a major component of intermediate age which spans the two extremes of Population I and II. It is much more developed than the analogous age group in the Milky Way. One example is the large number of rich populous star clusters. In our own galaxy, such clusters are invariably old with ages in excess of 10 billion years. In the Magellanic Clouds, similarly aged clusters exist but they are outnumbered by a strong representation of populous clusters in the 1-10 billion year age range. Partly through their dynamical history, but more directly through their star forming history, the Magellanic Clouds have been able to create and to maintain massive clusters like this at all times. In the Milky Way, populous, globular clusters were only made at the earliest epoch. Similar intermediate age clusters either never formed or have long since been destroyed.
The distribution of the more numerous open clusters that appear at all times in all three systems gives us some hints. Recent work comparing age distributions of complete samples show that the Cloud clusters can survive to much longer lifetimes as they are evidently not subject to disrupting forces as strong as those that exist in the Galaxy.
Independent of such effects, we find that even in the general field, the representation of intermediate age stars is proportionally much larger than in the Milky Way. Apparently bursts of star formation have been occurring throughout the lifetime of each Magellanic Cloud which are much greater than anything we find in the Milky Way. It is from the study of intermediate populations more than from any other that we can observe the long-term effects of differing star-forming histories and apply the knowledge gained to more distant stellar systems which cannot be resolved into individual stars.
A fundamental difference between the Magellanic Clouds and our own galaxy is that neither Cloud has a strong central concentration of stars which can maintain dynamical order in the rest of the system. Both Magellanic Clouds are very vulnerable, not only to gravitational interaction with the Galaxy but also to gravitational interaction with each other.
The Large Magellanic Cloud rotates in a fairly regular manner. The best measurements come from young supergiants and from planetary nebulae as well as from neutral hydrogen. To reconcile the observed rotation (line of nodes) with the distribution (major axis orientation) of the old populations, a transverse motion of the Large Cloud amounting to about 300 km s-1 is required. This is sufficiently large to be directly measurable now from the proper motions of Cloud stars with reference to background galaxies. However, detailed interpretation of the rotation curve is hampered by the mutual gravitational interactions mentioned above.
Observation constrains the two clouds to form a bound system which is currently orbiting our galaxy. At their last approach , approximately 200 million years ago, substantial damage was done to the Small Magellanic Cloud which is still apparent in its very disordered structure today. A long stream of gaseous material was drawn out of the Small Cloud by the interaction which is observed in neutral hydrogen radiation and has been called the Magellanic Stream. It has no associated stellar component. This close approach and others which have taken place at earlier epochs may have given rise to the bursts of star formation in both clouds which we now observe as the intermediate age population. The calculation for this intercloud orbit is remarkably explicit and it tells us that there have been several such close encounters in the last 10 billion years. However, there is still some doubt as to whether the orbit of the Small Cloud around the Galaxy is bound or unbound. The fact that the Magellanic Clouds have survived for the last 10 billion years indicates that there can have been no approaches much closer than the one we are experiencing now.
As independently evolving galaxies, the Large and the Small Magellanic Clouds are being gradually enriched with heavy elements. A finite step in this direction is taking place before our eyes as we observe the remnant of the 1987 supernova dispersing into interstellar space. Dynamically, the Small Cloud may be breaking up as a result of its last encounter with the Large Magellanic Cloud. Never held together very tightly, it is now strung out mostly along the line of sight over a distance of about 20 kpc. This is of the same order as the current distance between it and the Large Cloud. The Large Magellanic Cloud remains stable as shown by its well-defined rotation curve. Up until the present, the evolution of both Clouds has differed greatly from that of our galaxy. As low-mass galaxies from the beginning, neither underwent a major rapid collapse with a concurrent burst of star formation. This is shown by the lack of a strong central concentration of RR Lyrae stars, novae, and planetary nebulae to the degree that we see in the Galaxy. Instead star formation, and consequently heavy element enrichment, has proceeded at a much more gradual pace, with star bursts irregularly occurring every 10 million years or so wherever and whenever there is enough raw material. These are punctuated by system-wide star forming events whenever the two Clouds approach closely to each other. Through the work done over the past decade, the history of the Magellanic Clouds has become a lot clearer and we have been able to see how they relate to more massive star systems like our own galaxy.
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Adapted from The Astronomy and Astophysics Encyclopedia, ed. Stephen P. Maran