Starlight, star bright

first star I see tonight...

We all remember the old nursery rhyme, but there is a bit of science behind it. The brightest stars in the sky will always be the first stars we see in the gloaming of twilight. And the measure of a star's brightness is expressed scientifically by referring to its magnitude.

Background

In the second century BC, the Greek astronomer Hipparchus described the brightest stars in the sky as being of the first magnitude, the next brightest group were referred to as being of the second magnitude and so on until he reached stars of the sixth agnitude, which were the faintest visible to the naked eye.

In the mid 1800s, astronomers determined mathematically what old Hipparchus did visually, giving his scale a definable basis. Measurements showed that a difference of five magnitudes equalled a factor of about 100 in brightness. The magnitude scale takes some getting used to. This is because for every increase in one order of magnitude corresponds to a 2.51 increase in the apparent brightness of a star. The chart for the magnitude scale can be found below:

For example, taking a star of third magnitude and comparing it was a star of fifth magnitude would result in a difference in magnitude of two, meaning that the first star is 6.31 times as bright as the second one.

Putting it into use

Some stars are so bright that they must be assigned negative magnitude values in order for the magnitude six stars to remain a the faintest visible to the naked eye. An example of this is Sirius, which is the brightest star in the sky, shining at mag -1.4. The Moon and some planets are brighter that this. At its brightest, Venus can shine at mag -4.4. The full moon is mag -12.3 and the Sun is mag -26.8.

The following is a list of the 20 brightest stars in the sky¹: