Re: The Pistol Star

Hi Gary,

The "Pistol Star" is definitely a living, massive star, and not a star in the process of being destroyed. The two types of object look VERY different from each other, making it very unlikely that one could be confused with the other. The Pistol Star looks the way we expect a massive star to appear. Probably the strongest evidence is that the Pistol Star was first seen in the early '90s, and hasn't changed brightness significantly since then. A massive star which is dying as a supernove, however, will flare up enormously in brightness, and then fade fairly rapidly. Other processes by which stars might be destroyed would also lead to rapid changes in brightness.

In spiral galaxies, such as the Milky Way, most star formation does, as you believe, occur in the spiral arms. This is because star formation is greatly enhanced in areas where interstellar gas clouds are compressed, which increases the strength of their gravity, making it easier for them to collapse and form stars. In the Milky Way, the mild shocks at the leading edges of the spiral arms compress the interstellar clouds, triggering much star formation. The results can be very spectacular.

The process can occur in other places, as well. For example, inbarred spiral galaxies, the stars follow a bar-shaped distribution in the center, with the spiral arms trailing off of the ends of the bar. The motions of the stars and interstellar clouds within the bar can be very complicated, with the result that the clouds will have many collisions and generally experience a lot of compression, leading to much star formation in the bar.

There is another way in which stars can form, which is unrelated to the type of galaxy in which the interstellar gas clouds reside. If a massive star forms in a galaxy, its radiation, wind, and eventually the blastwave formed when it dies as a supernova can all act to compress nearby interstellar clouds. The compression may then trigger star formation in that nearby cloud, causing it to form stars. The process can then continue, like a row of dominoes. This kind of triggered star formation is the only efficient mechanism by which stars can form in galaxies which have no spiral arms or bars, or which aren't colliding with another galaxy.

The center of the Milky Way may have a small bar, but certainly triggered star formation is occurring there as well. As Dr. Morris indicated, we see much evidence that star formation in the center of the Milky Way tends to favor massive stars. This may be because the star formation there heats the interstellar gas clouds. Hotter clouds resist the pull of gravity more strongly, and so they will not collapse until more mass has accumulated (which increases the gravity).

As you indicated, we also see much evidence of stellar death near the center of the Milky Way. This is only to be expected. Massive stars live for only a few million years (an eyeblink in astronomical terms), and so dying/dead stars are almost always associated with newly forming massive stars. It would actually be surprising if we saw one but not the other!