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…

We will have to account for:
– Filtering
– 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 factor 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*
– 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
*2m = m-2.512log(2) = 2.247 ~ 2.25
Intrinsic Brightness: Blackbody
Radiation
Stars emit light
because they are hot!
 Their color is
determined by their
temperature
 Consequently, their
brightness is
dependent on their
temperature (among
other things)

Adjustable Wein Curves (if connected)
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
Filters



Astronomers use filters to
see how bright a star is in
a certain color range
The filters are simply
colored glass that goes
over the mirror or lens of
a telescope
Astronomers say Vega
has an MV of 0, which
means Vega has an
absolute magnitude of 0
in the V (for visible--no
filters) color band
See how the red filter lets very little
green and practically no blue through?
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

Apparent Brightness: Distance
The light received
from a star is
dependent on the
inverse 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 object
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