Chapter 4-6

"Empty Space":
The Birthplace of Stars

photo of expanding nebula
A bubble in space.
Massive, rapid winds pouring out over several million years from three bright blue stars formed a large, hot gas stellar space, its periphery bubble is then seen as a series of irregular arcs around the much smaller "planetary" nebulae expelled by two of the stars, now shrunk into hot, blue stars called subdwarfs. On one side of the bubble the boundary arcs (which represent the interface between the bubble gas and the cooler interstellar material) are not seen, perhaps due to overlying dust clouds or because the bubble has merged there with a larger one not readily discerned in the photograph. (Photograph by the U.K. Schmidt Telescope Unit, copyright 1978, Royal Observatory, Edinburgh; used by permission.)

Our first exploration of the deep space beyond the solar system is approaching. Our spacecraft - Voyagers 1 and 2, Pioneers 10 and 11, and still others to come - are moving out toward the space between the stars. Thanks to Space Age astronomy, and especially to observations made in ultraviolet wavelengths from satellites, we already know something about what to expect there.

Our view of the interstellar space that they are approaching has changed completely since the Pioneers and Voyagers were launched for solar system studies several years ago. For one thing, we now know that "empty" space isn't empty and quiet. True, you can count on your fingers the number of atoms in a cubic inch of interstellar space, but these atoms are subjected to violent processes. Some are heated to more than a million degrees, while others may cool to within fifty degrees of absolute zero. Within this thin gas, winds blow and bubbles are formed, expanding and sometimes popping out of our galaxy altogether.

As the spacecraft move out, they will first enter a region of warm gas that surrounds the solar system to at least 10 light years in all directions from the Sun and which contains our nearest neighbor stars such as Alpha Centauri and Sirius. We have been able to measure the temperature of the cloud (about 12,000 C) and its density, about 100,000 atoms per cubic meter ( 1.6 atoms per cubic inch). Since the Sun is moving through the cloud, we observe the gas streaming through the solar system. It will take Voyager 1 more than 200,000 years to leave this cloud, but when it does it will enter a region of much higher temperature and much lower density, a bubble of very fine vacuum in space. The bubble extends at least 100 light years in all directions around the smaller "warm" cloud. It will be millions of years until Voyager leaves the bubble and enters another zone of denser interstellar gas. The high temperatures in the bubble - more than 10,000 C - pose no threat to the spacecraft (which will have ceased operating several hundred thousand years earlier) because the gas is an almost-perfect vacuum. There are less than 10 atoms per cubic meter (1 atom in 6100 cubic inches) in the bubble. Even if the tem perature of the gas is a million degrees, the energy the gas conveys to the spacecraft will be quickly radiated into space. The greatest danger to the spacecraft in interstellar space will be that of getting too cold.

The recent discovery of large bubbles of hot, very thin gas has pro duced a radically new view of what's going on in the space between the stars. In the old view, gas in interstellar space was either warm or cold. Now, with X-rays and ultraviolet light, we find that most of the space between the stars is much hotter than we had suspected. In the old view, pressures of thin, warm gas were balanced against those of denser cold gas, and the situation was stable. Our telescopes in space now tell us that this is not true; pressures are out of balance. There is a great deal of interstellar pushing and shoving going on. Matter gathers and cools in some places because matter elsewhere is heated and dispersed. Besides discovering the very hot gas, the orbiting telescopes discovered two sources of the gas: the intense stellar winds that boil off hot stars, and the rarer but more violent blasts of matter from exploding supernovae.

The hot bubbles of very thin gas pile up colder matter at their boundaries as they spread out into space, much as a soap bubble collects a film of dust from the air. All over the galaxy, these bubbles have piled up mat ter and swept it into clouds. Some of these cold clouds are enormous and contain enough matter to make millions of suns. Our own solar system formed out ofjust such a cloud. We have even found evidence in meteorites telling us that the solar system itself formed only a few million years after the very first stars had formed in the cloud that was to be our Sun's nursery.

Although infrared astronomy research satellites have not yet been launched, infrared astronomy conducted from balloons, airplanes, and mountaintop observatories has given us important new information about the cool interstellar clouds. It has been discovered that the tiny interstellar grains contain silicon compounds similar to clay minerals. These particles can survive temperatures up to 1500 C and may possibly come from the atmospheres of red giant stars. The origin of the interstellar grains is one of the great questions of astrophysics that we can investigate by space astronomy. Very recently, ultraviolet telescopes in orbit found evidence for a covering of graphite (a form of carbon) on the silicate grains. Our best picture of inter stellar space is that half its matter (other than the predominant hydrogen gas) is in the form of sooty sand grains the size of tiny smoke particles. It is possible that comets are made of accreted interstellar material, and so we hope that someday grains like this will be brought back from a comet sample-return mission.

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