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Next: 1.3 Globular Clusters Up: 1. The Universe in Previous: 1.1 Gravity

1.2 Galactic Suburbia

The sun is a star like any other among the hundred billion or so stars in our galaxy. It is unremarkable in its properties. Its mass is in the mid range of what is normal for stars: there are others more than ten times more massive, and there are also stars more ten times less massive, but the vast majority of stars have a mass within a factor ten of that of the sun. Our home star is also unremarkable in its location, at a distance of some thirty thousand light years from the center of the galaxy. Again, the number of stars closer to the center and further away from the center are comparable. Our closest neighbor, Proxima Centauri, lies at a distance of a bit more than four light years.

This distance is typical for separations between stars in our neck of the woods. A light year is ten million times larger than the diameter of the sun (a million km, or three light seconds). In a scale model, if we would represent each star as a cherry, an inch across, the separation between the stars would be many hundreds of miles. It is clear from these numbers that collisions between stars in the solar neighborhood must be very rare. Although the stars follow random orbits without any traffic control, they present such tiny targets that we have to wait very long indeed in order to witness two of them crashing into each other. A quick estimate tells us that the sun has a chance of hitting another star of less than $10^{-18}$ per year. In other words, we would have to wait at least $10^{18}$ years to have an appreciable chance to witness such a collision. Given that the sun is less than five billion years old, it is no surprise that it does not show any signs of a past collision: the chance that that would have happened was less than one in a hundred million. Life in our galactic suburbs is really quite safe for a star.

There are other places in our galaxy that are far more crowded, and consequently are a lot more dangerous to venture into. We will have a brief look at four types of crowded neighborhoods.


next up previous contents
Next: 1.3 Globular Clusters Up: 1. The Universe in Previous: 1.1 Gravity
Jun Makino
平成18年10月10日