July/August 1998 Table of Contents
As a child I
recall all those evenings spent outside and away from my house and
its lights. At bedtime I would wander back, all the time noticing
how lights at the house and barn would grow in intensity, their
anti-darkness pouring over and enveloping the stars.
intensity, you see, is a strong function of distance. The closer
you are to a source, the brighter, or more intense, that source
appears. Nature has even codified this effect for us as the inverse-square
law of light: The intensity of incident light-the amount of energy
received by your eye or other detector per unit area per unit time-decreases
as the square of the distance between you and the emitting source.
Floating out around Jupiter (just imagine, okay?), you'd notice
the Sun to be only 1/25 as bright as it appears from Earth; the
mighty Jovian world is just a little more than five times farther
from the Sun than Earth.
any source of light, be it a porch light or a star 10000 lightyears
away, is constrained by this dilution effect. Now, this makes things
a little dicey when you start comparing stars. The star second to
the Sun in brilliance is Sirius. It is simply impossible to ignore.
And while Sirius is larger than Sol, the primary reason behind its
brightness in our evening skies is that it's close to us.
different distances lead to different perceived brightnesses, but
there is more. Stars have different intrinsic brightnesses. A 100
watt light on my porch dumps out the same energy per time as a 100
watt bulb on my neighbor's porch a kilometer away-her's just appears
dimmer because it's farther away. But stars are not like identical
bulbs: Their varied masses and evolutionary states result in a variety
of intrinsic brightnesses.
the magnitude system, passed down to us through a hundred generations
of sky watchers, as a means of quantifying stellar brightness ("Accidental
Astrophysics," May/June, p. 9). Bright stars have small, even negative,
magnitudes; faint ones, larger. This system, as I have described
it thus far, has a severe limitation, however. Magnitudes as we've
discussed them are based on how bright the stars appear to us. Sirius
has a magnitude of †1.46, Rigel, in neighboring Orion, shines at
+0.14, but the mighty Sun dominates the sky with a magnitude of
†26.72. With this limited information, we correctly conclude that
the Sun is apparently the brightest of the three stars. Here's the
problem, though: The Sun's intrinsic brightness is by far the lowest
of the three! It is brightest to us because it is so near, but compared
to Sirius and Rigel, it is truly dimmest.
are we to do then to extricate ourselves from this confusing situation?
Think of your favorite police drama, the setting a line-up room.
Before a white wall stand six rough-looking chaps; a police officer
tells them to stand with their backs to the wall. Why does the officer
want them at the same distance from you? What, for goodness sakes,
has this to do with stars? Having them at the same distance from
you permits you to see differences between the individuals: The
effect of distance, which might make a shorter person standing close
to you appear taller, is removed. And this is what we do with stars.
We put them in a line up, all at the same distance from us. Now
we compare the stars based on their intrinsic brightnesses.
magnitude is how bright a star or quasar appears to us-what we measure
on a CCD image or photographic plate or estimate with our eyes.
As a measure of intrinsic brightness, we use absolute magnitude:
As in the police line-up, we imagine moving stars or other objects
10 parsecs away ("back against the wall, #4!") and then measuring
their brightness. The absolute magnitude is, therefore, the apparent
magnitude of an object at a distance of 10 parsecs. Recall my examples
Sirius, Rigel, and the Sun? Well, even though Rigel appears less
bright than Sirius, its absolute magnitude is actually †6.8, while
that of Sirius is only +1.4. And Rigel is more than 800 lightyears
away! It is incredible. The Sun, however, is a meager star. Its
apparent magnitude is an impressive †26.72, yet its absolute magnitude
is only +4.8. Viewing Sol from that comparison distance of 10 pc,
you'd see a star only slightly brighter than most of its neighbors
magnitude is a relatively straightforward quantity to obtain. Getting
an absolute magnitude is a battle, one that I'll discuss in future
columns. But what we have so far is wonderful! Let's say we have
a star's apparent magnitude, and, through strenuous means, we've
obtained its absolute magnitude as well. What's the difference between
these two numbers? Yes, it is a magnitude difference, but that difference
is due to the distance between us and the star. Distance, yes! Magnitudes
have led us to a means of determining distances in outer space.
C. WHITE II
is the editor of Mercury and an associate professor in the Physics
and Astronomy Department at Middle Tennessee State University. He
admits that his old brightness scale of "dim," "bright, " and "it
hurts," is too coarse for good astronomical research.