14 DAY TRIAL // The nature of the interstellar medium has received the attention of astronomers and scientists over the centuries. It is not at all a void; on the contrary, there are many particles floating in space.
The interstellar medium (or ISM) is the name astronomers give to the gas and dust that pervade interstellar space. While the ISM refers to the matter that exists between the stars within a galaxy, the energy, in the form of electromagnetic radiation that occupies the same volume is called the interstellar radiation field.
The ISM consists of an extremely dilute (by terrestrial standards) mixture of ions, atoms, molecules, larger dust grains, cosmic rays, and magnetic fields. The matter consists of about 99% gas and 1% dust by mass. It fills interstellar space, and blends smoothly into the surrounding intergalactic medium. The ISM is usually extremely tenuous, with densities ranging from a few thousand to a few hundred million particles per cubic meter, and an average value in the Milky Way Galaxy of a million particles per cubic meter. As a result, of primordial nucleosynthesis, the gas is roughly 90% hydrogen and 10% helium by number of nuclei, with additional heavier elements ("metals" in astronomical parlance) present in trace amounts.
Some of this debris was thought to lurk within a few light-years of the sun and generate x-rays that have been detected by space probes and ground-based observations. Researchers suspected that these emissions originated in thin, sizzling clouds of interstellar gas.
But a team of astronomers looking for hot gas between the sun and its galactic neighbors has come up empty. The gas's absence perpetuates an x-ray mystery and could be linked to the reason why Earth remains congenial for life.
Astrophysicist Martin Barstow of the University of Leicester in the U.K. and colleagues used the Far Ultraviolet Spectroscopic Explorer (FUSE) spacecraft, which studies cosmic rays and similar phenomena, to gauge the density of clouds located out to about 300 light-years from the sun. FUSE can detect intervening gas by analyzing the incoming light from nearby stars. But at the Royal Astronomical Society's National Astronomy Meeting in Preston, U.K., yesterday, Barstow said the FUSE data indicate the gas just isn't there.
"This region of interstellar space is empty", he said. According to a summary of Barstow's talk released by the Royal Astronomical Society, the most probable explanation for the missing gas is that the area was swept clear of material by a nearby star that went supernova within the last few million years.
The gas's absence requires a new explanation for the x-rays, said Barstow during his talk. One possibility, he noted, is the radiation is caused by interactions between charged particles at the boundary between the sun's magnetic field and interstellar space.
This new picture of the galactic neighborhood containing extremely little hot gas, and correspondingly little background radiation, will change our understanding of the environment of the sun.