Narrator Listen to part of a discussion in an astronomy class. Now get ready to answer the questions. You may use your

Professor   So far as we know, most of the matter in the universe is in the form of stars. The majority of stars, uh...including our sun, are clouds of gas and are maintained at a high temperature. Well, how can they keep at such a high temperature...
Student A  The burning of the energy liberated by the...the thermonuclear reactions in their centers.
Professor   Right! Due to the thermonuclear reactions. But do you know how we gain knowledge of the stars and get to know how they evolve? ...By studying the visible light they emit. The birth of stars, however, can’t be followed by an optical telescope, since it takes place in regions of space from which light cannot escape. To study this process, therefore, astronomers have to make use of telescopes operating at other wavelengths. The recent technical developments in radio, millimeter-wave and uh... infra-red astronomy, have turned the study of star-formation into one of the most exciting areas of astronomy. The birth of a star is a rare, slow event; all but a very few of the stars that are visible to the naked eye have existed longer than mankind. So, we must first consider the evidence that new stars are now being formed at all.
Student B  Do you mean that new stars are being formed at any time?
Professor   Yep! The energy, say, uh...radiated into space by a normal, so-called "main sequence"  star,  is...generated by the uh...the conversion of hydrogen to helium. Here, you may ask what a main sequence star is. OK...the main sequence refers to a major grouping of stars that uh... forms a relatively narrow band from the upper left to the lower right, when plotted according to luminosity, that is, how bright it is, and surface temperature, on the Hertzsprung-Russell diagram.
Student A  You just mentioned that stars are being formed at any time, but can we... by any means, know the potential lifetime of a star?
Professor   Yeah, that’s right what I’m gonna explain. If we compare the mass of hydrogen "fuel"—just now I mentioned the conversion of hydrogen to helium and thus energy is generated—the mass of hydrogen fuel in a main-sequence star with the rate at which energy is being emitted, we can estimate its potential lifetime. It is found that the lifetime of a star depends strongly on its mass—low mass stars are small, cool and long-lived, while high mass stars are large, hot and short-lived.
Student A  So, professor, can we simply say that the heavier the star is, the shorter it lives?
Professor   Well, you’ve right got it, I should say. Let’s uh...take our sun for example. Our sun is now half-way through its total lifetime of about...uh... 10 billion years, but a star with a mass thirty times greater than the sun would live for only a few million years. We know, there’re many bright stars we can see existing in the space. This, in some way, implies that star formation must have taken place over the past few million years. Since our galaxy is some ten thousand million years old, it is reasonable to assume that somewhere in the galaxy, the same process is taking place even now.
Student B  But how are these stars formed?
Professor   Ah... that’s the point. According to the observation, those hot bright stars are almost always in the nearby areas of interstellar gas clouds.
Student B  This may indicate what?
Student A  Maybe the birth of stars is related to the gas clouds.
Professor   Exactly! This leads us to conclude that it is out of such clouds that new stars condense. The internal gravity of an interstellar gas cloud tends to make it shrink, and the thermal pressure tends to make it expand. James Jeans, in 1926, showed that a cloud of a given temperature and density can collapse only if its mass is greater than a certain minimum value. It’s right a balance between these two actions, say, shrink and expand, that keeps the evolution of a gas cloud. Well, if the balance does not exist any longer, the cloud will start collapsing. Um...for example, a typical cloud with a temperature of uh... 100 000 degree centigrade and a density of 100 hydrogen atoms per cubic centimeter has to be 3 000 times more massive than the sun in order to start collapsing.
    Once the collapse process has started and the density of hydrogen atoms of the cloud has risen significantly, the fragmentation into smaller cloudlets will take place. And, subsequently, these cloudlets eventually collapse to form individual stars. Therefore, the theory of uh...gravitational condensation, predicts...well, in agreement with observation, that new stars form in clusters containing hundreds of thousands of stars rather than as isolated entities.