Virat Markandeya writes:

David McComas has a favorite “astrosphere,” the environment created by a star’s stellar wind as it buffets the surrounding interstellar medium. It belongs to a star named Mira. In an image from 2006, Mira is heading to the right, at 291,000 miles an hour, five times the speed our sun ambles through its local interstellar cloud in the Milky Way. You can make out a “bow shock” forming ahead of the star, like one would ahead of a boat sailing through water. Gas there heats and mixes with the wind of the cooler hydrogen gas blowing off Mira, and then flows to the star’s rear, forming a wake. Mira’s astrosphere, trailing behind the star to the left, looks turbulent, fragmented, and stretched. “How clearly you can see it sort of fall apart from this single structure to these turbulent smaller structures,” McComas, a professor of astrophysical sciences at Princeton, said, in a video interview recently. “I think it is very beautiful.”

When McComas isn’t admiring Mira’s astrosphere, he’s spearheading efforts to understand our own, the “heliosphere,” a bubble canonically comet-shaped. He’s eager to learn about the functions it might serve. Since 2008, McComas has been the principal investigator of the Interstellar Boundary Explorer (IBEX) mission. He oversees the data the IBEX satellite collects to disclose the nature of our solar system’s edge. He’ll also be in charge of IBEX’s successor, the Interstellar Mapping and Acceleration Probe (IMAP), which is set to launch in 2024.

What might count as our solar system’s boundary? There is no definite point at which light from our sun completely fades, or where its gravity stops being felt, so neither of those could mark it. But the heliosphere can. It “moves through the galaxy, keeping our home safe,” McComas says. The sun’s solar wind, an outflow of ionized gas, or plasma, pushes out against the galactic material between stars, also called the “interstellar medium.” The interstellar medium in our very local region is a mixture of plasma, helium, and neutral hydrogen. It is formed by warm, partially ionized clouds found in the Local Bubble, a large cavity filled with plasma that was likely produced by multiple supernova explosions, along with interstellar dust and other stellar winds. The barrier separating us from this occupies a region far beyond the orbit of Pluto, one you can define and measure. [Continue reading…]