The brightness of a star as seen from earth?

Answer 1

Absolute magnitude. Two stars of the same absolute magnitude usually do not have the same apparent magnitude because one may be much farther from us than the other. The other that is farther away will appear dimmer. To compare absolute brightness, astronomers determine what magnitude the stars would have if they were at a standard distance of about 32.6 light years. The sun has an apparent magnitude of -26.7, if located at a distance of 32.6 light years, have an absolute magnitude of 5. Stars with absolute magnitude values lower than 5 are brighter than the sun. Because of their distance, however, they appear much dimmer.
A lot brighter than you think actually.

Answer 2

This is a large subject. [We know of no other larger!]

Briefly, once it was hypothesized that all stars were equally bright, and that their apparent brightness was due to their distance from us. These studies soon led to classifications of stars that shared similar characteristics; colour, brightness, etc; and this led to a broader understanding of a better model.

In particular, a group of stars that periodically varied in brightness, the Cepheid variables, were discovered, and in this group there was a definite correlation between their brightness and their variability. This gave us a 'standard candle' for measuring the distance of stars, and from the resultant data, Hubble and others conceived the idea of an expanding universe, which in more recent times has led to the Big Bang theory for the origin of the Universe some 14 billion years ago.

Of course, many of what we think of as stars, are in fact complete Galaxies in their own right. There have been many attempts to classify the stars and galaxies.

Answer 3

Technically the Sun is a star - so the Sun.

The brightest star seen from Earth is Sirius A (alpha Canis Majoris), located 8.64 light-years from Earth, is the brightest star in the night sky, with an apparent magnitude of -1.46 million miles (2.33 million km), a mass 2.14 times that of the Sun, and is visually 24 times brighter than the sun.

Answer 4

Commonly called "magnitude" but more accurate is "apparent magnitude" because a very bright object could be so far away it is difficult to see it.

Answer 5

That's called the star's "apparent magnitude".

Answer 6

The apparent magnitude.

Answer 7

Apparent Magnitude