Download The Brightness of Stars

The Brightness of Stars
The Simple Answer to: How Bright?
Quantifying the brightness of stars started with
Hipparchus (2nd C. BC) and his magnitude scale
 He designated the brightest star he could see as
a “1” magnitude and the dimmest a “6”
magnitude
 Astronomers still labor under a more quantified
version of this system
 One tragic consequence is that objects brighter
than the brightest star have negative
magnitudes!

However…

Eventually we will have to account for:
– Distance
– Reddening
– Extinction

But first things first…
Apparent vs Absolute
The Apparent Magnitude of a star is how bright
it appears to the naked eye, disregarding any
interfering factors
 On our Hipparchian scale, the Sun would have
an apparent magnitude of -26, the Moon -11,
and Venus -3
 The Absolute Magnitude is how bright a star (or
other object) would appear at a distance of 10
parsecs ~ 32.6LY
 The Sun’s absolute magnitude is 4.83

The Difference

Consider a 100W light bulb; 100W is its intrinsic
brightness
– It emits 100W of light no matter how far away it is; at
the specified distance of 10 parsecs it would have
some ( ) absolute magnitude
very tiny

However, since a 100W bulb in your face seems
much brighter than a 100W bulb 10 parsecs
away, its apparent magnitude would depend on
how close or far away it is
Rule of Thumb
You can’t add magnitudes, absolute or apparent,
directly because they are calculated with base
ten logarithms
 A difference of 1 magnitude means a difference
of 2.512 in brightness
 So, if you ask how bright two 3-magnitude stars
are together, it’s not 6, it’s not 5.048, it’s 2.25
(Yikes! Math!)
 That is, it’s 75% brighter that a single 3magnitude, and remember, smaller numbers are
brighter!

Don’t worry, you won’t have to calculate
the summed brightness of multiple stars
 But you do have to know that you can’t
just add magnitudes
 And you must realize that smaller
numbers, even negative numbers, mean
brighter objects

Blackbody Radiation
Stars emit light
because they are hot!
 Their color is
determined by their
temperature
 Consequently, their
flux is dependent on
their temperature
(among other things)



Stars that are cool, ~3500K, will be reddish; stars
that are hot, ~10,000K, will be white
White light is a combination of all colors, so a hot
star will appear brighter than a red star, all other
things being equal, because not all light from a star
is visible to the human eye
– This fact obscures a star’s intrinsic brightness
Size
The surface area of a star is another factor in
the brightness of a star
 Two stars of the same temperature will have
different magnitudes, depending on their size
 A red supergiant can emit vastly more light than
a red dwarf

Distance
The flux received from a star is dependent on
the square of its distance from us.
 Knowing this helps astronomers find its distance
using a method known as standard candles

Standard candles works this way: say you know
the intrinsic brightness of a star and its
magnitude;
 If you see an identical star but with a different
magnitude, you can use the inverse square law
to find the distance

Reddening


One of several “seeing” problems
The dust in the disk of the galaxy
absorbs the blue component of a stars
light, making it seem redder than it is.
Extinction
Another “seeing” problem
 Anything in the light path from a star,
nebula, or galaxy absorbs or scatters
light
 This attenuation is called extinction

Summary
The brightness of a star or other celestial is
quantified by its magnitude
 Factors that determine the light output of a star:

– Temperature  color
– Size

Factors that determine its perceived brightness:
–
–
–
–
Color
Distance
Reddening
Extinction