Collision course?

Recent observations apparently suggest that our home Galaxy, the Milky Way, is considerably more massive than had been thought.  The actual measurements are of the orbital speeds of objects in the Galaxy, but the speed gives an estimate of the mass, and mass is more interesting than speed, so that’s what people seem to talk about.

It’s interesting that we’re still relatively ignorant about our own Galaxy: orbital speeds of objects in other galaxies  are measured much more accurately than those in our own. I guess it’s all a matter of perspective: it’s harder to tell what’s going on from our vantage point in the middle of our Galaxy.

Some news stories have  drawn attention to the fact that this means that our Galaxy will collide with the nearby Andromeda galaxy (M31 to its friends) sooner than had been previously estimated.  Supposedly, that collision is going to happen in a mere5-10 billion years.  I’ve never understood why some people say with confidence that a collision is going to happen, though.  It’s true that the two galaxies are getting closer, but as far as I know there’s no way to measure their transverse velocity, so we don’t know if they’re heading straight at each other or will move sideways past each other.  It seems quite likely to me that the galaxies are actually orbiting, not plunging straight at each other.  If anyone knows whether there’s any evidence one way or the other on this, I’d be interested.

One technical note: The new rotation speed measurements are about 15% larger than the previously accepted values.  Science News says that that results in a 50% increase in the estimated mass.  That would make sense if the mass scales as the cube of the speed, but naively it just scales as the square.  If the revised speed measurements go along with a revised length scale for the Galaxy, then that might explain it.  I suppose if I dig up the actual scientific paper rather than the news accounts, I could find out the answer, but that sounds like work.

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Ted Bunn

I am chair of the physics department at the University of Richmond. In addition to teaching a variety of undergraduate physics courses, I work on a variety of research projects in cosmology, the study of the origin, structure, and evolution of the Universe. University of Richmond undergraduates are involved in all aspects of this research. If you want to know more about my research, ask me!