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Q & A: Orion Nebula Questions

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Most recent answer: 10/22/2007
Q:
First question, how far is the orion of nebula from our galaxy? second question, Where is the Orion of Nebula located from our galaxy, is it north side, west, south, or east? Third our galaxy is moving through space so what keeps our galaxy from not crashing to other galaxies, if there is such a thing? Fourth question, is the Orion of Nebula moving too, is everything in the space moving faraway from each other? Thank you very much for your help.
- Ronaldo Pereira (age 16)
Greater New York Academy
A:
Hi Ronaldo,

1) The Orion Nebula is about 1500 light-years away from us (a mere 8.8 trillion miles), which makes it relatively close on an astronomical scale, allowing us to study in detail its structure. The Milky Way galaxy is about 100,000 light-years across, and so not only is Orion in our galaxy, it is in our section of the galaxy.

2) The Orion Nebula is located in the sky very close to the ecliptic (the plane containing the sun and the Earth's orbit), and very close to the zero-hour marker on a sky chart (the "right-ascention"). Like everything else in the sky, it rises in the east and sets in the west, and so it doesn't make much sense to ask if it is in an easterly direction or a westerly direction because these directions change as the earth turns on its axis. My guess is that the zero of right ascention is a meridian over Greenwich observatory in England at midnight on January 1, but I could be wrong. It's arbitary.

The center of the galaxy is estimated to be about 30,000 light-years away in the direction of Sagittarius (-25 degrees -- south, and at a right ascention of 19 hours. 24 hours is a full circle). The Orion nebula is in our arm of the galaxy.

3) Well, in our own frame of reference, the galaxy isn't really going anywhere (aside from the fact that we are slowly orbiting its center). In another galaxy's frame of reference, our galaxy may be moving quite quickly. Nothing prevents galaxies from colliding into each other, and in fact because they attract each other gravitationally, it appears that galaxies have collided. Here's a nice web site with pictures of some colliding galaxies:

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In case the site goes away, I'll include a very nice picture from it:



4) The Orion Nebula almost certainly is moving with respect to us, but I cannot easily find a good estimate of its speed. Because it is so close, it probably isn't moving very fast at all as we look at it, because it is orbiting the center of the galaxy at more or less the same speed as we are. (Quantities like this are never exactly equal, however).

You may be asking about the Hubble law, which describes how the average velocity of objects far away from an observer increases with the distance. All objects in our galaxy are so close to us that their motion around the center of the galaxy is the most important contribution to their speed in our frame of reference. Nearby galaxies also have their own speeds relative to each other and may collide, as we have seen. Galaxies very far away tend to recede at velocities which are larger for farther away galaxies, but there is also a bit of random motion in addition to that, as galaxies move within their own local groups.

Tom

(published on 10/22/2007)

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