A Bubble in the Local Fluff

Every once in a while, it’s a good idea to appreciate our somewhat-lucky place in the galaxy. How lucky? Well, it turns out the Sun and solar system move through space encased in a bubble. That “Local Bubble” is about a thousand light-years across, and the Sun happens to be sitting right in the middle. That gives astronomers a good vantage point in all directions.

A story last week about the Local Bubble caught my attention. That’s because we—and the bubble—are moving through a larger area of the galaxy called the “Local Interstellar Cloud”. It’s also known as the “Local Fluff”. This is a region where the density of hydrogen is a bit higher than the rest of nearby space. Some have suggested it’s more like a “void” compared to Earth’s upper atmosphere, for example, but it’s still a distinct structure with its own temperature and densities.

The Sun has been transiting this “Fluff” for about 10,000 years and may be about to leave it. There’s been a lot of scientific speculation about what things will be like when we do finish moving through it, in maybe a couple of thousand years. Will we be subject to higher rates of cosmic rays? What else? Good questions for which there are incomplete answers.

Astronomers have known about the Fluff and the Bubble for some decades now. Until recently, they didn’t have enough data to completely understand their nature. Some years ago, Hubble Space Telescope was used to study the Fluff and characterize its size and density. Astronomers have also used measurements made by the Voyager spacecraft to help probe this region of space. More recently, the Gaia spacecraft has been used to give astronomers a 3D look at the immediate neighborhood around the Sun.

Starbirth to Stardeath

The Bubble is a fascinating part of the local galactic structure. Think of it as a cavity in the interstellar medium. That cavity is a low-density, high-temperature plasma held in place by an outer “shell” of cold, neutral gas. It began forming about 14 million years ago thanks to a burst of starbirth activities. Among those newborn stars were massive ones with short lives. They died in supernova outbursts that pushed hot gases and other elements out into the surrounding interstellar medium. That created the cold gas shell which now forms the outer boundary of the bubble. So, think of it like blowing hot air into a balloon, and then watching the expansion the balloon makes into the colder outside air.

Interestingly enough, nearly all the active molecular clouds where starbirth is taking place closest to the solar system are located on the surface of the expanding bubble. The ongoing expansion of the bubble pushed those clouds together. That caused them to condense and collapse, which begin the process of star formation.

Finding the Bubble’s Characteristics

To understand the bubble and related star formation action, astronomers used data from the Gaia mission. It’s is mapping the Milky Way, giving a “3D view” of the stars in the galaxy. This helped them figure out the dimensions of the Local Bubble and the regions of starbirth on its surface. It’s providing an interesting look at the mechanics of supernova-driven star formation in our local neighborhood.

If you want to read more about the Local Bubble, there are papers and illustrations available with more details.

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