By Yuri EFREMOV, Dr. Sc. (Phys. & Math.),

and Vladimir SURDIN, Cand. Sc. (Phys. & Math.), Sternberg State Astronomy Institute of Moscow State University

Now we know that the world around us originally was filled with uniform matter, which gradually broke up into parts and consolidated into gigantic bright masses-stars and galaxies. Contrary to the great English physicist Newton's suggestion (1692), however, they are not scattered chaotically, but are strikingly organized into regular structures. To see this, look at the pictures of spiral galaxies spreading out their symmetrically extending arms consisting of sequences of star clouds. The clouds comprise associations of young stars which include dense clusters, in turn having numerous binary and multiple star systems, and so on down or up this hierarchic ladder, every single rung of which provides us with knowledge about the physical mechanisms that helped shape the Universe into the structure as we know it. Here ,we want to write up one of these rungs, major clusters of young stars and protostellar matter spread over thousands of light years. ^* Researchers at our Institute have succeeded, after years of intense studies, in identifying these mammoth aggregations and complexes which, they found, provided the building blocks of stars forming the galaxies.

Stars are born somewhere within the interior of massive interstellar gas clouds. Within their cores, basically consisting of molecular hydrogen, gravity carves up the matter into fragments that are eventually shaped into luminaries. The process has long remained a closed book to astronomers, because the clouds are impenetrable to visible light. It was only after the invention of infrared and radio telescopes that scientists could peer into the nurseries of the stars and study the way in which they are formed. How do, then, these nurseries come about? Understanding this may be promoted by studies of young stars on different spatial scales.

In our own Galaxy, young stars are clustered about its equatorial plane, exactly where the layer of interstellar gas is. In the firmament, they and the gas clouds they have heated up trace the bright trail of the Milky Way. Back at the turn of this century, astronomers understood that the visible structure of the Milky Way was due to the clots of light-impervious interstellar dust and gas girdling the Solar System. Those closest to us appear as dark patches against the background of the luminous band. Wherever dust clots thin out we see "galactic horizons" thickly studded with stars, which we take for star clouds. But it turns out that the Milky Way is not like that throughout.

There are, however, real star clouds, in which the young luminaries are clearly related to one another. A good example is the compact cloud some 50 parsecs ^** across, containing inside a much smaller and much denser star cluster Markarian 38 in the direction of the Sagittarius constellation, at a distance of 1,500 parsecs from the Sun. The stars within either of them are of the same age, 12 mln years, which means they were born in the same birth throes.

We know of other extensive groups of young stars, some of them of a considerably larger size, up to 1 kiloparsec across. Studying them, however, is no easy thing - they are too rarefied and too large, and almost do not stand out against the Galactic

^* Light year, the distance which light covers in a year, i.e., 9.46 x 10 ¹² km. - Ed.

^** Parsec, a unit of measurement used in astronomy, equal to 3.086-10 ¹⁶ m (3,263 light-years). - Ed.

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background. This was certainly the reason why the more compact groups of scattered clusters like the Pleiades were spotted and studied first. Relatively dense, with a few hundred or thousand stars each, they successfully resist the destructive force of galactic gravitation to live relatively long, an average 500 mln years.

Such clusters are frequently enveloped by rarefied coronas of stars equally young. Some coronas exist on their own, without an enclosed cluster, and are known as star associations. In the case of the Milky Way, only the most massive and brightest of their components can be made out. A few associations show signs of ongoing expansion: the most probable explanation is that massive hot stars just born heat up the surrounding gas, expelling it from their birthplace, only to follow in the great haste. Associations are short-livers - within 10 to 20 mln years they expand to a size in excess of 100 parsecs, and cannot be distinguished against the background stars. The impression is that associations are rare formations, whereas in actuality they are as frequent as the clusters, but they disintegrate more rapidly.

Associations and clusters share one feature-both are unitary groups of stars (focused on a common center) formed in a single coup that killed the original molecular cloud. Surprisingly, polycentric groups of clouds-star aggregates and complexes-have occasionally been discovered. In fact, they are the common star clouds. Major research to identify young polycentric star groups has been undertaken by one of these authors assisted by a group of his students.

He suggested the term "aggregation" to be assigned to large groups of this type, like, for instance, the association in Orion, with a diameter of nearly 150 parsecs. It takes in the superdense and very young cluster around Trapezium Orionis, surrounded with an ample corona of young flaring and variable stars. Next to them, a dense molecular cloud obscures another two clusters, and further north is a group of young hot stars in Orion's girdle, a more rarefied group in Orion's head, and a few foci of current star formation.

While an aggregation is a group of young clusters buried inside an extensive association, a complex is a much larger and more complicated system. The well-known double cluster in Perseus is surrounded by an extensive bright disk of some super-giants, and next to them there are three fainter clusters and a few cepheids. The latter are almost ten times as old as the double cluster itself, but astronomical measurements confirm that all these objects belong to a single group about 300 parsecs in diameter. North of them sits an association in Cassiopeia, which comprises a giant molecular cloud and several foci of star formation underway at present. Probably, these two groups are in some way related to the compact system of eight clusters lying a short distance away, together with the local association. As a result, the entire complex widens to 600 parsecs.

The typical star complexes may include the Local group containing the majority of massive stars within 400 parsecs from the Sun, the Orion aggregation, and another aggregation in Scorpius and Centaurus, as well as a few faint and relatively young clusters (the Pleiades among them) and a dozen super-giants. The mass of the stars in the Local group is put at 500,000 solar masses and that of the interstellar gas, more than a million solar masses, the age of the Local group being estimated at some 50 mln years.

Besides, the complexes contain numerous molecular clouds immersed in a common shell of neutral gas. As one of these authors and the American astronomer B. Elmengreen have determined, the masses of such complexes are equal to millions of solar masses. The giant systems of this type, detected in our own Galaxy and in its neighbors, provide the basic cells to sustain star formation. How important is their role in the life of galaxies?

As we already have said, stars are bom out of interstellar gas near the galactic plane. Like the gas clouds that have spawed them, the young luminaries continue to move along circular orbits for some time after birth. Gradually, however, their paths are deformed, now carrying the stars towards the galactic center and then hurtling them away, causing them to bob up high-

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er and higher above the galactic plane. What physical mechanism forces the star orbits to shift?

The motion of the stars is influenced by the gravitational field of the Galaxy, which induces them to move around the galactic center. Although stars appear never to collide as they close in on or more away from one another, their pull of attraction strongly disturbs the motion of their closest neighbors altering velocities of the latter and, in the end, leveling out kinetic energies or temperatures.

The young stars just out of their Galactic womb have a very low temperature, its components almost stationary relative to each other. With the much older stars scampering about in the vicinity and nudging the newboms this way and that, the youngsters start to move faster and in a more chaotic pattern than they did at birth. Gradually, the younger stars reach the velocity of their older neighbors, and soon the kinetic temperatures of the whole lot are equalized.

This process tends to play a critical role at times. Occasionally, the luminaries in a cluster behave so boisterously that some of them are pushed out of the circle in the scuffle. Losing one star after another, the clusters are growing leaner with millennia, and may fade out of existence within a few hundred million years.

An affirmative answer was received in the mid-70s, when American radio astronomers detected some previously unknown, extremely cold interstellar clouds of molecular hydrogen. On closer observation, the biggest of them were estimated to have the mass of a million Suns. On top of that, their total number in the Galaxy is nearly 5,000; they "stir" stellar orbits in the Galactic disk.

The discovery of giant molecular clouds (GMC) turned around the astronomers' attitudes to gas as a dynamic factor with a major role in the evolution of the Galaxy. It had been held previously that it was sparse and evenly spreaded around the stars, and was, therefore, assigned a minor role in the interplay of the galactic forces. Now the interstellar gas has been found to account for 5 percent of the total mass of the Galaxy, while in places where its concentration is the highest (near the Galactic plane) its share in the mean density of matter shoots up to 35 percent. What is most important, however, is that the gas is gathered in massive clouds capable of dramatically modifying star orbits.

Quite a spate of papers have been published in recent years on studies carried out into interactions between giant molecular clouds and stars. It has been hypothesized, for instance, that should the Solar System approach a GMC even once, and the comets roaming at its periphery would alter their paths and some of them would be swept into the region occupied by the larger planets. According to one hypothesis, the periodic biospheric catastrophes that have visited upon the Earth, and in particular, the extinction of dinosaurs 65 min years ago, were triggered by a shower of comets hitting the Earth after the Solar System had a glancing flyby past a giant molecular cloud.

The question now is: Are the GMS the only force that plays havoc with star movements? Powerful as they are, the giant molecular clouds are too weak to cause an appreciable reshuffle among the orbits of the young stars. Looks like an impasse.

The deadlock was broken by Japanese theorists. By simulating star interactions with GMCs in a computer, they lumped the clouds together in groups of 10 to 12, which made the gravitational field of the galactic disk even more irregular, so much so that it now could madly distort star movement, exactly as they are observed.

The idea of GMCs combining to form long-living groups has been confirmed by studies made into star-gas complexes by researchers at our institute. Definitely, these complexes are the giant ladles that are perennially stirring the matter within the galactic disk.

We would not assert, of course, that the origin of star complexes has been settled now, once and for all. So far, a coherent theory is still missing. According to some calculations, the interstellar gas in the galactic disk tends to spontaneously split up into fragments about a kiloparsec in size. In fact, such super-clouds have been detected in the disk of our Galaxy and in those of its neighbors. Deep within them, conditions are created for the emergence of molecular hydrogen clouds, the star nurseries. A massive super- cloud is not, however, prone to immediately release molecular clouds and the newbom luminaries from its gravitational grip-they hang on as star complexes for tens of millions of years.

The scenario is based on the assumption that the complexes originate spontaneously. Their appearance, however, may be stimulated as an older generation helps the next one come to life. In fact, every large group of newly bom stars has a few massive members which have an exceptionally strong radiation and can heat up and shatter the parent cloud and, therefore, terminate the star forming process in their region. The fragments of the shattered cloud are now free to expand through space, scooping up, in the bulldozer blade fashion, and compacting the rarefied gas into new dense clouds. The process then comes full circle - stars are born within the clouds and, after gaining strength, they stimulate the formation of new clouds. Astronomers in Europe and one of these authors have made a quantitative analysis of the process in which star complexes and gas clouds are sequentially formed, obtaining results that agree well with the observable parameters of these aggregations in our own Galaxy and in the neighboring galaxies.

It is hard to tell now, though, which of the proposed scenarios is the more plausible. Nature is full of surprises, and the galactic world is so diverse that any sound evolutionary scenario may be matched by conditions in which it will be played out.

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