Popcorn Supernovas

Reconstructing the history of near-Earth supernovas is difficult because old supernovas are elusive. Their glowing shells fade to invisibility in not much more than a million years. Neutron stars, the collapsed cores of supernova progenitors, last longer, but they are flung across the galaxy by asymmetries in the explosion. Unusual isotopes of iron, like the ones that coincide with the marine extinction, are difficult to find buried under millions of years of sediments.

There is, however, one obvious relic: "All those explosions blew an enormous bubble in the interstellar medium," says Hurwitz, "and we're inside it."

Astronomers call it "the Local Bubble." It's peanut-shaped, about 300 light years long, and filled with almost nothing. Gas inside the bubble is very thin (0.001 atoms per cubic centimeter) and very hot (a million degrees) - that's 1000 times less dense and 100 to 100,000 times hotter than ordinary interstellar material.

The Local Bubble was discovered gradually in the 1970's and 1980's. Optical and radio astronomers looked carefully for interstellar gas in our part of the galaxy, but couldn't find much in Earth's neighborhood. Furthermore, there seemed to be a pileup of gas - like the shell of a bubble - about 150 light years away. Meanwhile, x-ray astronomers were getting their first look at the sky using orbiting satellites, which revealed a million-degree x-ray glow coming from all directions. "We eventually realized that the solar system was inside a hot, vacuous bubble," says Hurwitz.

In response, NASA launched a satellite in January 2003 - the Cosmic Hot Interstellar Plasma Spectrometer, or "CHIPS" - to study the Local Bubble. "There's a great deal we don't know about it," says Hurwitz, who is the mission's chief scientist. How old is the bubble? What is its internal geography? How fast is it cooling? Data from CHIPS is helping to answer these questions.

The CHIPS Logo

CHIPS orbits Earth and peers into the bubble using an ultraviolet (UV) telescope. "The gas in the bubble is very bright at extreme UV wavelengths around 170 Å," explains Hurwitz. Other satellites have examined such UV light from the bubble, but CHIPS is better. It has a spectrometer on board with 100 channels ranging from 90 Å to 260 Å. "The spectrometer is the key," he says.