Half Moon
How near to earth would a nebula have to be to be as bright as a full or even half moon?
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)?
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)
Janis Joplin – Half Moon
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