Re: How is our galaxy expanding?

Emily,

The short answer to your question is that our galaxy is not expanding. Many people get confused by the fact that the Universe is expanding and then assume that the galaxy does too. You've stumbled into a problem that had Albert Einstein stumped for a while. Edwin Hubble (they named the telescope after him) discovered a simple linear relationship between the distances to distant galaxies and the shifting of the wavelength of their light to the red end of the visible spectrum. Einstein realized that this shift could be explained if space itself was expanding. Imagine the Universe is a balloon and the galaxies are little dots pasted to the surface. As you blow the balloon up the dots will get further and further apart. Now imagine light travels from one of the dots towards another. Because light always has to travel at the same speed (that was Einstein's big discovery), as the space between the dots increases something has to change. The thing that can change is the wavelength of the light.

Now galaxies form due to the effect of gravity acting on the matter in a particular region of space. Gravity pulls the matter together and when enough matter forms into large enough clumps, stars form. By its nature then a galaxy is described by a contraction of matter. Because a galaxy is held together by gravity it turns out that the space within the galaxy does not expand, just as the dots on the balloon don't expand. This is actually good for us, can you imagine yourself getting bigger all the time along with the galaxy.

When a galaxy forms, because the materials that make it up are all moving, they keep on moving. This is what causes the Milky Way to revolve once every 200 million years or so. As stars burn, emitting light and energy they are constantly using up the mass that makes up the galaxy. It would seem that after a while the galaxy would start to spread out just to conserve momentum. I don't know if anyone has done research on this, to see if they can detect this spreading out, but you have to remember that the visable matter astronomers have found accounts for only about 10% of the matter in the galaxy. This would mean the spreading out of the galaxy may be so small it may be very difficult to detect, if it is happening at all.

Hope this helps.

Mike Francis

http://www.gis.net/~mtf/sm.htm