If a galaxy contains a supernova that at its brightest has an apparent magnitude of 17, how far away is the ga

Question

if a galaxy contains a supernova that at its brightest has an apparent magnitude of 17, how far away is the galaxy?

Answer 1

You cannot tell with the information given.  Apparent magnitude and absolute magnitude are different.  Absolute magnitude is how bright an object is without taking distance and interference into effect.  

So a bright distant object can have the same absolute magnitude as a dim close object.

Answer 2

Too far to be seen with anything other than a really BIG telescope.  Mag 17 is VERY faint and can only be seen in pristine dark skies with large telescope.

Answer 3

A supernova having an apparent magnitude of 17 at maximum indicates the host galaxy is hundreds of millions of light years away. Nearby galaxies such as M-83 in Hydra and recently NGC-4490 in Canes Venatici have had supernovae that top out at magnitude 12.0 and 13.4, with distances of 10 and 50 million light years away. The one I observed recently in NGC-4490 was an exploding giant star that lost it's outer envelope, or a type 1b. Assuming that the hypothetical supernova is a type 1A, which are caused when white dwarfs get too massive to remain stable and start runaway nuclear reactions, a galaxy with a 17th magnitude 1A supernova at maximum would be more than 15 times more distant than the one that recently occurred in NGC-4490. Type 1A supernovae are "standard bombs" that reach nearly the same maximum brightness and release nearly the same amount of energy. That makes them very good yardsticks to measure truly huge distances. Type 1B, 1C, and 2 supernovae are quite variable in power, and don't make nearly as good yardsticks. In other words, the parent galaxy would be at least 750 or 800 million light years away from Earth. You won't be able to see it in any amateur telescope at all unless you have a 30-inch or bigger telescope and a very dark site, or a smaller telescope fitted with a CCD camera. The link below will take you to a site where you can find out about supernovae in other galaxies, and if there's one bright enough for your telescope to see.