How near to earth would a nebula have to be to be as bright as a full or even half moon?

Question

Presume the orion nebula (although any example would do), how near to earth would it need to be to appear as bright as a half or full moon to human eyes in the night sky? How large would it appear in such an instance (ie: faint, but spread out over a lot of the sky, or bright, but no larger than the moon or perhaps a very very large star)?

Answer 1

The apparent magnitude of Orion Nebula (from Wikipedia) is given as +3. The apparent magnitude of the Full Moon is slightly brighter than -12. A difference of 15 magnitudes.

The magnitude scale is such that a difference of 5 magnitudes is like multiplying brightness by 100. So, the Orion nebula would have to appear 100*100*100 = 1 million times brighter than it does now.

Brightness varies as the square of the distance, so its distance (given as 1,500 light-years in Wiki) would have to be divided by 1,000.

The Orion Nebula would have to be 1.5 light years away. However, even at its present distance, it is not a point source. It has apparent size.

Make it 1,000 times closer and it would appear to cover a good portion of the celestial sphere, so that talking of its "magnitude" will not make the same mathematical sense as if we were talking about stars.

PS:

According to Wiki, it has a radius of 15 light years. If the centre is only 1.5 lightyears away, then we'd be inside the thing. It would cover the entire sky (with a brighter portion towards its centre)

Answer 2

The short answer is that the Orion nebula would not look ANY brighter no matter how close you got. All that would happen is it would look bigger. So instead of a small, dim object, you would see a big dim object. Even if it filled half the sky, it wouldn't be as bright as the Moon; it would look more like the Milky Way does now; big but dim.

As has already been pointed out, the Orion nebula (or any nebula) is not a point source like a star, it has a size to it. (OK, stars are not REALLY point sources, but they are so small compared to their distances that for all practical, optical, purposes they can be treated as point sources.) So what happens is that even though there is more total light from it reaching Earth, it is spread out over more sky, so the surface brightness would be unchanged. This fact almost always escapes non-astronomers. Nebulae and galaxies are hard to see not because they are small, but because they are dim. For a dramatic example of what I am saying, look at the source. It shows how the Andromeda galaxy actually looks bigger in our sky than the Moon does. But the galaxy is so dim that your eye cannot see more than the brightest part of the core. Your eye cannot see the outer parts at all. Only long exposure photographs can do that. The picture looks fake because it is a long time exposure picture of the galaxy combined with a short snapshot of the Moon. It is a digital composite of two pictures that could never be taken in a single exposure, even if the Moon ever did pass in front of the Andromeda galaxy, which it doesn't.

Answer 3

All math aside, just picture the effect of a few hundred forming stars being closer. It would look amazing. Remember, though, just how close the Moon is and how close the Sun is, which make the Moon bright. Now think about our nearest neighbor, Vega. If you took the brightest three or four stars in the sky and put them near each other that's not even close to the Moon. You'd have to increase that be a factor of at least 5. So, at the least you would have a bright cloud taking up the size of an extended hand.

It sure would be pretty.

Answer 4

I couldn't answer your question, but I do know of an interesting site you could visit; www.spaceweather.com is a cool place to visit. I was just checking out the sun spot activity and some of the "Near Earth Asteroids". It's a really cool site. Check it out.