Apparent Visual Magnitudes
Object Apparent Visual Magnitude
Sirius (brightest star) -1.5
Venus (at brightest) -4.4
Full Moon -12.6
The Sun -26.8
Faintest naked eye stars 6-7
Faintest star visible from Earth telescopes ~28
Faintest star visible from Hubble Space Telescope ~30

Apparent Magnitude

There are several ways in which we could specify the brightness and this leads to several different magnitudes that astronomers define.
"What You See" Magnitude
One important distinction is between whether we are talking about the apparent brightness of an object or its "true" brightness (its luminosity, which we discussed in conjunction with the Sun). The former is a mixture of the true brightness and the effect of distance on the observed brightness, because the intensity of light from a source decreases as the square of the distance. The apparent magnitude of an object is the "what you see is what you get" magnitude. It is determined using the apparent brightness as observed, with no consideration given to how distance is influencing the observation.

Magnitudes and Golf Scores

Notice the important point that, because of their definition, magnitudes are like golf scores: lower is "better". That is, the smaller the magnitude, the brighter the object. Notice also that negative numbers are smaller than positive numbers, and that the more negative the number is, the smaller it is. For example, a magnitude of negative 26.8 is brighter than a magnitude of negative 4.4 which is in turn brighter than a magnitude of positive 4.4.

Obviously the apparent magnitude is easy to determine because we only need measure the apparent brightness and convert it to a magnitude using the equations given earlier, with no further thought given to the matter. However, the apparent magnitude is not so useful because it mixes up the intrinsic brightness of the star (which is related to its internal energy production) and the effect of distance (which has nothing to do with the intrinsic structure of the star).

The apparent magnitudes for several objects determined using light from the visible part of the spectrum are given in the top right table. (The adjective "visual" in this table refers to the magnitude for the visible part of the spectrum. We shall discuss the dependence of magnitudes on wavelength in a later section of this module.)

The Brightest Stars
The twenty brightest stars in the sky are listed in this table and here is a more extensive list of the 314 stars brighter than apparent magnitude 3.55 in both hemispheres. The table listing the 20 brightest stars is ordered according to decreasing apparent brightness (that is, increasing apparent magnitude). The more extensive table listing all stars brighter than apparent magnitude 3.55 is instead ordered according to the position of the stars (right ascension and declination). Quantities in these tables such as proper motion, radial velocity, and spectral type will be explained in later portions of this chapter.