Astronomers have at last found definitive evidence that the universe's first dust - the celestial stuff that seeded future generations of stars and planets - was forged in the explosions of massive stars.
The findings, made with NASA's Spitzer Space Telescope, are the most significant clue yet in the longstanding mystery of where the dust in our very young universe came from. Scientists had suspected that exploding stars, or supernovae, were the primary source, but nobody had been able to demonstrate that they can create copious amounts of dust - until now. Spitzer's sensitive infrared detectors have found 10,000 Earth masses worth of dust in the blown-out remains of the well-known supernova remnant Cassiopeia A.
Space dust is everywhere in the cosmos, in our own neck of the universe and all the way back billions of light-years away in our infant universe. Developing stars need dust to cool down enough to collapse and ignite, while planets and living creatures consist of the powdery substance. In our nearby universe, dust is pumped out by dying stars like our sun. But back when the universe was young, sun-like stars hadn't been around long enough to die and leave dust.
That's where supernovae come in. These violent explosions occur when the most massive stars in the universe die. Because massive stars don't live very long, theorists reasoned that the very first exploding massive stars could be the suppliers of the unaccounted-for dust. These first stars, called Population III, are the only stars that formed without any dust.
The atomistic principle rendered a very great service in the development of a picture of gases which could account for such properties as pressure. Newton’s mechanics, which contributed so much toward our knowledge of the stellar universe, finds a beautiful application to the molecules of a gas. Although the conception of the idea of the kinetic theory of gases can be traced further back, Clausius ( 1857) really founded it after Joule (185 I) had attempted to calculate the velocity of a hydrogen molecule. The calculations were inexact, but they mark the beginning of an actual atomic view of matter as distinguished from the mere symbolic view of the chemist up to that time.
The molecules of a solid are considered to be bound to certain positions which result in equilibrium, but this is not the case in gases. The molecules of a gas are continually in motion, excepting at absolute zero, about 491 degrees Fahrenheit below the melting-point of ice. At first they were likened to elastic spheres shooting in all directions and colliding with each other. At ordinary temperatures and pressures a gas-molecule will suffer several billion collisions each second. Although the modern ideas of the atom are not fully developed in relation to the kinetic theory of gases, it is likely that the molecules do not actually collide in the ordinary sense. The modern conception is that atoms are tiny planetary systems of electrons revolving around a positive nucleus. Doubtless they are surrounded by electric forces so that instead of a collision we have a repulsion of the electric forces. At any rate the true picture is of little consequence here because it will not alter the actual facts.
The pressure on the sides of a vessel containing a gas is due to the billions of collisions of the molecules with the sides. The force of an impact is determined by the mass and the velocity. The pressure is the result of the number of impacts per second. If the density of the gas is doubled there will be twice as many impacts in a second on a given area of the vessel and, therefore, the pressure will be doubled. This relation of density and number of molecules was suggested by Boyle (1627-1691) which gave rise to Boyle’s law. This law states that, for a given temperature, the product of the pressure and the volume is constant.
The temperature of a gas is determined by the mean energy of motion of a molecule. Here again Newton’s mechanics enter, for the energy of motion of a molecule is proportional to its mass and to the square of its velocity. As the temperature decreases, the energy of motion (kinetic energy) of a molecule decreases until at absolute zero there is no motion. Hence, at absolute zero the pressure of any gas is zero. It may be of interest to know that absolute zero has not quite been reached in the laboratory, but it has been approached to within less than a degree Centigrade.
The atomistic idea has not been limited to the structure of matter by any means. It is associated with several properties or characteristics of matter such as heat, gas-pressure, and light or radiant energy. The early idea of heat was that it was a fluid. Count Rumford (1753-1814) a little more than a century ago was one of the first to relegate this idea to the discard by experiments which suggested that heat was a mode of molecular motion. It is true that Roger Bacon (12141294) suggested heat was a matter of agitated particles such as molecules, but as in other phases of science, experimental data were lacking. Mayer and Joule (1842) discovered that mechanical work was converted into heat and that the amount of heat was always exactly proportional to the amount of work and vice versa.
It had long been recognized as a direct consequence of Newton’s laws of motion, that energy could not be destroyed. This is the well established law of conservation of energy. For example, if a moving body collided with another body, the resultant mechanical energy remained equal to the total before the collision. The total mechanical energy required to produce a certain amount of heat (agitation of the molecules of the body in which the heat was generated) would remain unaltered. The molecules of the body now possessed so much more mechanical energy; in other words, they were sufficiently more agitated so that their excess mechanical energy was just equal to the mechanical energy required to produce the amount of heat that the body received.
Thus we see not only the atomistic idea of matter used to account for a change in temperature (due to the production of a certain quantity of heat), but also Newton’s laws of motion applied to the molecules of the body. It is a beautiful picture of unification. For example, suppose a large iron ball is dropped from a height and it strikes a thick iron plate of great mass. The mechanical energy of the falling ball at the moment before striking the plate is proportional to its mass and the square of its velocity. Actually the ball will bounce a few times, but let us assume it always hits in the same place and comes to rest there. The energy of the ball is now distributed among myriads of molecules of iron both in the plate and in the ball. Owing to the minuteness of the agitated molecules we cannot see their increased movements, but we can feel the result of the increase. The two bodies are of a higher temperature at first only near the points of contact, but the heating effect rapidly spreads and equalizes. This is what we term conduction of heat and is really a communication of the increased agitation to other molecules. It should be noted that the molecules of a body are always in a state of agitation (excepting at absolute zero), the degree of agitation being indicated by the temperature.
The atomistic principle rendered a very great service in the development of a picture of gases which could account for such properties as pressure. Newton’s mechanics, which contributed so much toward our knowledge of the stellar universe, finds a beautiful application to the molecules of a gas. Although the conception of the idea of the kinetic theory of gases can be traced further back, Clausius ( 1857) really founded it after Joule (185 I) had attempted to calculate the velocity of a hydrogen molecule. The calculations were inexact, but they mark the beginning of an actual atomic view of matter as distinguished from the mere symbolic view of the chemist up to that time.