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How Bright is that Star?
Part 1
Magnitude
Magnitude
The Greek astronomer Hipparchus devised the first
system for rating the brightness of Stars.
The brightest stars like Betelgeuse, Rigel, & Deneb he
called 1st magnitude.
Not quite so bright, like Mizar in the Big Dipper’s
handle, were 2nd magnitude, and so on…down to the
dimmest, 6th magnitude.
(Notice that the lower the number is the brighter the
star is.)
Modern astronomy still uses this system, but has
refined it.
Decimal and Negative Magnitudes
Using more accurate measures of brightness stars are
now rated more precisely.
A 1st magnitude star is 100x brighter than a “6th ”
Each order of magnitude is therefore
2.15 times brighter than the one below it.
Magnitude is now given in decimal form. Deneb now
rates a 1.26, and Betelgeuse rates .87.
Hipparchus underestimated how bright the brightest
were, so now we even use negative numbers for the
very brightest,
Sirius rates -1.44.
Absolute and Apparent Magnitude
Hipparchus thought all stars were at the same distance,
on the celestial sphere, they are not.
Thus how bright a star appears is affected by the
inverse square law and depends on distance.
The Magnitude a star appears to have is now called its
“Apparent Magnitude”
“Absolute Magnitude” is a truer measure of how
bright the star really is.
It is what magnitude the star would have if it was
placed 10 Parsecs away.
Sirius rates 1.4…. Deneb rates –8.7!