|
What is Spectra? |
Spectra is the collection
of radiation either absorbed or emitted by an object. In
astronomy, the objects from which
scientists collect this data are stars, planets, or other galaxies. The radiation that these objects emit is
called black body radiation.1
Black
body radiation refers to the spectra emitted by a glowing or heated
object. This spectrum is modeled after a
perfect black body which is one that absorbs and
emits wavelengths at all frequencies in equilibrium.
In other words, the amount of energy absorbed
will always equal the amount of energy re-emitted in a black body and
it would
in addition always appear black at room temperature. 1 Although
this spectrum is modeled after an ideal concept, scientists use it to
measure
objects that “emit radiation approximately as if they were black
bodies”.
1 For instance,
all objects
that exist at temperatures above absolute zero (~ -273 Kelvin) will
emit some
type of radiation based on the idea that they contain some thermal
energy due
to the motion of their particles. 1 Since
the stars, planets, and other distant galaxies glow brightly we know
they exist
at high temperatures. The disparity in
temperature between the glowing objects in space allow for different
black body
radiation curves, thus allowing astronomers to differentiate between
the bodies
in our universe.
Surprisingly both “hot” and “cold”
objects (relative to room temperature) emit black body radiation. However,
the extent
to which each emits energy varies with temperature. As
an object gets warmer, its black body
radiation curve shifts. Depicted in Figure 1 below is a typical black
body radiation curve in which an object is heated from 3000 K to 6000
K. Notice that as the object gets warmer both the peak
rises and shifts toward the visible and UV sections of the
electromagnetic spectrum. The sharper the peak becomes, the more
intense the radiation is. As the peak shifts to the left, so too
does the color of the radiation it is emitting. The color of the
black body curve explains a lot about the temperature of the
object. This is essential knowledge for astronomers because its
indicates how far away or how close an object is moving in relation to
Earth.
Figure 1 -illustrating the black body radiation curve of three bodies at varying temperatures2
Black body radiation is founded on two very
critical laws. Both the Steffan-Boltzmann and Wien Laws are used
to help scientists make use of the black body radiation curves they
acquire from objects in the universe. Figure 2 illustrates how
astronomers use these two laws mathematically.3
The Stefan-Boltzmann law explains that the radiation given
off by an object equals the temperature raised to the fourth
power. In other words, a star twice as hot as the sun would
radiate 24 or sixteen times more energy.1
One can see that a minor change in temperature yields a considerable
difference in energy. The second law of radiation allows
astronomers to use the wavelength at which a celestial body radiates
the most energy to calculate its temperature. From this equation,
astronomers can predict how hot a star is simply from its color.1
Using black body radiation allows astronomers to gather
substantial information about objects we have seen only
superficially as glittering specks. Today's astrophysicists
use spectra to predict both the temperature of a body in space and the
amount of energy it gives off.
|
If
a star's surface temperature is 6,000 K where would you find it
on the electromagnetic spectrum?
Why does the wavelength at which a star radiates the most energy depend on temperature? |
References
1. Seeds, M.A. Foundations of Astronomy; Thomson
Brooks/Cole:
3. University of Tennessee Astrophysics Wiki Homepage. http://csep10.phys.utk.edu/astr162/lect/light/radiation.html (accessed April 8th 2008).