Carpe Datum

Geminga

I decided to do a more complete update for the record since this is a neat topic, and I'll link to this from my Gemini post. To set the scene, I would like to talk about our local Galactic environment. Here we are on Earth in the solar system, but what is nearby around our solar system? What's space like in the vicinity of our local neighborhood?

Well, Earth's local environment within the solar system is dominated by the effects of the Sun, namely the solar wind. This is a superhot ionized stream of particles coming off the Sun. This wind pushes against the surrounding interstellar medium, creating a hot bubble of gas within which the entire solar system rests. This is the heliosphere, sort of like the extended atmosphere of the Sun, and I mean it is REALLY extended.

We use Astronomical Units to measure distances in the solar system. One AU is 93 million miles which is the average Earth-Sun distance. Neptune is about 40 AU away from the Sun and marks the practical boundary of the main solar system. There is a flat belt of icy debris out beyond Neptune that extends from about 40 AU to perhaps 400 AU or more. We call this the Kuiper Belt, and it is the source of many short-period comets such as Comet Halley. For scaling, the nearest star (Alpha Centauri) is about 500,000 AU away.

The heliopause is somewhere in the Kuiper Belt, on the order of 100 AU from the Sun. This is the limit of the hot bubble of the Sun's influence, where the outward-pushing pressure of the solar wind finally runs out of gas while pushing against the vast interstellar medium in which we are immersed. Our own little solar bubble is itself immersed in a hotter-than-average bubble in the interstellar medium. We call this enormous surrounding bubble the local bubble, and just as our own hot bubble is "blown up" by the Sun, something must have cleared out the local bubble.

And this brings us to the object I forgot, a little pulsar in the constellation Gemini that we call Geminga. Geminga is a rapidly spinning neutron star, likely the remnant of a Type II supernova explosion, about 330 light years from Earth (making it the closest neutron star to us). Based on the rate at which it is slowing (and having a good guess as to its initial rotation rate), we estimate Geminga is about 300,000 years old, so presumably what happened here is that Geminga went off fairly close to Earth back then, maybe close enough to cause us some problems according to the fossil record. The supernova cleared out a space that we now call the Local Bubble.

There are other bubbles nearby, some cleared by explosions, some cleared by young, hot clusters with enormously powerful stellar winds such as the Sco-Cen association, the nearest such cluster to us. The rest of the interstellar medium in our galaxy is pretty complicated, as you might imagine, and it is tough to study. I'll talk about how we do it and what we've learned some other time.

Posted by Observer at January 29, 2008 10:31 PM