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How many galaxies are there?
Submitted by Brian Abbott on Fri, 7/10/2009, 4:26 PM
There are about 80-100 billion galaxies in the observable universe, that is, that part of the universe visible to us. There may be more in that part of the Universe beyond our observable limit, which occurs about 400,000 years after the Big Bang. Before that time, the universe remains opaque to us. It's like trying to see through a dense fog. After this time, the "fog" lifted and light could traverse the universe. This occurred because the universe expanded and cooled, allowing electrons to pair with protons and light to travel without bumping into the free electrons.
The mind blowing thing to think about is all the stars in these galaxies... Our Milky Way has roughly 200 billion stars. While we've discovered over 300 planets outside our Solar System, the galaxy must be filled with trillions of planets. Now think about each of those 100 billion galaxies in our observable universe. Each has hundreds of billions of stars and probably trillions of planets too. Do any of them harbor life? We do not know, and we may never know. But we continue to look.
Will Pluto and Neptune ever collide?
Submitted by Saavik Ford on Mon, 4/27/2009, 3:33 PM
Since Pluto spends part of its orbit closer to the Sun than Neptune, you might think that their orbits cross, and eventually that the two of them could collide. But there are two reasons why they will forever remain like two ships in the night.
1) The usual diagram of Neptune and Pluto's orbits is a "top down" view of the solar system, also called a polar view. In this view, the two orbits appear to cross. But this is a two-dimensional picture of a three-dimensional event! A "side" view shows that the orbits of Pluto and Neptune are also inclined to one another. In fact, Pluto's and Neptune's orbits never actually cross each other - at their closest, Pluto's orbit is "above" Neptune's by a few hundred million miles. This is like the difference between looking at a road map showing that two roads cross, but then driving to the "intersection" only to find out that one road is an overpass and you cannot actually get from one road to another.
2) The orbits of Pluto and Neptune around the Sun are in resonance. This means that every time Neptune orbits the Sun twice, Pluto goes around exactly three times. When one of them is near the "crossing point" (remember, they don't actually cross) the other one is somewhere else entirely on its orbit. The inclination of Pluto's orbit is not entirely stable - over time it will change and this might eventually cause a true crossing with the orbit of Neptune. But the resonance, which is extremely stable, will prevent the two of them from ever being in the same neighborhood at the same time.
If you'd like to learn about this in more detail, I recommend checking out the wikipedia page on Pluto's orbit.
