Plasma Storms

The center of the Milky Way. Credit: NASA/CXC/UMass/D. Wang et al. Click to enlarge.

Dec 6, 2019

Astronomers are puzzled by abnormally high temperatures in the Milky Way’s core.

In the image at the top of the page, an X-ray spectrum of the gases in the center of the Milky Way is consistent with temperatures from 10 million Celsius to as much as 100 million Celsius. 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 gas, but no one can explain how it is heated. “Ordinary” supernova explosions are not sufficiently powerful, and heating by high-energy particles produces the wrong X-ray spectrum. Any substance with that kind of temperature is not a gas—it is a plasma.

Over the last century, laboratory investigations established 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’s core is 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 and double-layers, the electron distribution was non-Maxwellian, there were all sorts of oscillations and instabilities. In short, it was a field which was not at all suited for mathematically elegant theories.”

The Milky Way is a plasma phenomenon 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

The Thunderbolts Picture of the Day is generously supported by the Mainwaring Archive Foundation.

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