Frozen Fires

The center of the Milky Way in multiple wavelengths. Low X-ray bands in blue,
medium bands in green, and high bands in red. Credit: NASA/UMA/D.Wang et al.


Nov 02, 2012

The Chandra X-ray Telescope has puzzled astronomers with the discovery of abnormally high temperatures at the core of the Milky Way.

A news release announcing this image of the center of the Milky Way stated that the X-ray spectrum of the gases is consistent with a cloud of “hot gas” that varies from 10 million degrees Celsius to as much as 100 million degrees. This result was unexpected and difficult to explain.

According to consensus opinions, shock waves from supernova explosions are the most likely explanation for heating the 10 million degree gas, but no one can explain how the 100 million degree gas is heated. “Ordinary” supernova explosions are not sufficiently powerful, and heating by high-energy particles produces the wrong X-ray spectrum.

Another Chandra image of the star cluster Trumpler 14 shows about 1600 stars and a diffuse glow from hot multimillion degree X-ray producing gas. Any material that has a temperature of millions of degrees is not a gas—it is a plasma.

After more than a century, laboratory investigations have established the fact that plasma has electrical properties, and can conduct electricity. The flow of electricity through a plasma forms Birkeland filaments, double layers, and electric current instabilities. Each formation is capable of accelerating charged particles, releasing X-rays. In fact, they can accomplish that feat without having a million degree temperature, just a strong electric field.

The region within 900 light-years of the Milky Way Galaxy’s core is shown in other images threaded through with glowing filaments more than 100 light-years long. The latest radio telescope probes of this region show that the filaments are associated with pockets of star-formation. The exact mechanism for creating the filaments remains to be discovered, but modern astronomers suggest that one possibility is the collision of winds blown off by individual stars.

Plasma cosmologists expect temperature discrepancies in our galaxy (among others), because laboratory plasma experiments indicated that they should exist. Hannes Alfvén, in the introduction to his book, Cosmic Plasma, points out examples of plasma behavior in his lab that astronomers were not aware of:

“The plasma exhibited striations, double layers, and an assortment of oscillations and instabilities. The electron temperature was often found to be one or two orders of magnitude larger than the gas temperature, with the ion temperature intermediate.”

What Chandra found at the core of the Milky Way is plasma that behaves exactly the way it behaves in experiments on Earth. Some laboratory measurements show temperatures ten to a hundred times higher than simple kinetic effects can produce (“wind collisions” and shock waves). If astronomers had known of the lab results, and taken them as seriously as they take theories of hot gas, they would not have been surprised.

Stephen Smith


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