Download The Electromagnetic Spectrum

The Electromagnetic
Spectrum
All black bodies when
heated start to glow,
emitting radiation, some
we can see, some
invisible.
The Visible Spectrum
• White light (from a light
bulb or from the sun) is
actually a mixture of many
different wavelengths.
• When separated, a rainbow
or a continuous visible
spectrum is revealed.
• However, there are many
other wavelengths that
human sight cannot detect.
This is the electromagnetic
spectrum.
Short wave, high
energy
Long wave, low
energy
Wavelength
• Electromagnetic energy
is wave energy, similar to
waves in water.
• The distance between
two crests is called the
wavelength ().
• The number of waves
per second is the
frequency (f).
• All wavelengths of light
travel at 186,000 mi/sec
(known as c).
Gases that glow produce
emission spectra.
This is what
you see when
you point a
spectroscope
toward a neon
light.
Energy Levels
• Electrons when energized jump to a higher level.
• When they drop down, they emit a specific wavelength of
light.
• In a spectroscope, this is seen as a line in an emission
spectrum.
• Hydrogen has the simplest emission spectrum, since it only
has one electron.
Emission spectra are like
fingerprints…each element is
different!
Helium tube
Argon tube
A hot radiating body
surrounded by cool gas
produces an
absorption spectra.
HOT
COOL
This is the situation for the
Sun and most stars.
The Hydrogen absorption
spectrum for the Sun.
It’s a negative image of
the emission spectra.
SUMMARY: Three Spectra
• Stars produce continuous spectra, with no gaps
between the colors.
• Thin gases emit emission spectra.
• When light passes through cool gas, colors are
removed, forming an absorption spectrum.
The Effect of Temperature
• Temperature can be
measured in Fahrenheit,
Celsius, or Kelvin scales.
• In the Celsius scale, 0 °C
is freezing and 100 °C is
boiling.
• The Kelvin scale is
measured from absolute
zero, the coldest possible
temperature (– 273 °C).
• The higher the temperature
the more energy is emitted
from a glowing object.
Spectral Graph for the Sun
Most of the
sun’s energy is
in the visible
part of the
spectrum as
well as the
infrared.
This indicates a
temperature of
5800 °K.
Other Stars,
Different Temperatures
• The hotter the star, the
more energy it emits into
space.
• The peak of its spectrum
shifts toward the violet
and even the ultraviolet.
• Cool stars have peaks in
the red.
• Planets (like Earth) peak
in the IR part of the
spectrum.
Wien’s Law
• Max Plank determined
that black bodies have a
wavelength of maximum
energy.
• It drops off in both
directions from that point.
• Wien’s Law states the
higher the temperature,
the shorter that maximum
wavelength will be and
the more intense its light
will be.
Using Wien’s Law…
• If we know the
temperature of a star, we
can determine the
wavelength of its
maximum intensity.
• Use the formula on the
right.
• Temperature must be in
°K and the wavelength
will be in nanometers
(one billionth of a meter).
 max = 2,900,000/12000 =
242 nm (blue-white star)
Picturing a Nanometer…
• The average width
of a human hair is 1
mm or 100,000
nanometers.
• A chlorine atom is
around 0.2 nm
across, so a
nanometer is equal
to 5 Cl atoms.
0.2nm
Finding Temperature…
• This red supergiant star
has a max of 950 nm.
What is its surface
temperature?
• T = 2,900,000 ÷ 950 =
3053 °K
• This version of Wien’s
Law is much more
practical, since we can
directly measure
wavelength in a
spectroscope.
The peak of the sun’s
radiation is in the middle of
the visible spectrum, so…
The Sun is
yellow. Its peak
radiation is about
500nm, so its
temperature is
5800 °K.
Introducing the Doppler Effect
• The train has a higher pitch
whistle when approaching
you.
• The train has a lower pitch
when moving away from
you.
• This Doppler Effect is caused
by compression or stretching
of sound waves.
• The same phenomenon
occurs with light, only the
object must be moving very
fast to detect it.
Blue Shifts, Red Shifts
• Light waves moving away
from an observer are
stretched.
• They shift toward the red
end of the spectrum.
• Those waves moving
toward an observer are
compressed.
• They shift toward the blue
end of the spectrum.
• They larger the shift, the
faster that object is
moving.
Detecting Rotation…
• If a galaxy is
rotating, then one
end should show a
blue shift and the
other a red shift.
• This principle
applies to stars with
planets. They show
a spectral wobble.
Magnitude
• Stars differ by brightness, which is
measured by magnitude.
• The lower the magnitude, the brighter
the star.
• The Sun has a magnitude of -27, by far
the brightest object in the sky.
• Without telescopes you cannot objects
with magnitudes over 6.
If a photograph is
taken of the sky,
the stars appear
as dots. The
larger the dot, the
brighter the star
(the lower the
magnitude).
When you
decrease the
magnitude by 1,
the brightness is
2.5 stronger.
1
2
Star Names
• Bright stars have traditional
names, many from Arabic:
Betelgeuse, Aldebaran, Sirius,
Arcturus, etc.
• Today, astronomers name stars
using Greek letters ()
followed by the constellation
name with a Latinized ending.
• Therefore, Arcturus is also
known as Alpha ( Bootis.
EXAMPLE:
The brightest star in the
constellation Ursa Minor is
Polaris. In modern astronomy it
is also called Alpha ( Ursa
Minoris.
The second brightest star
Kocab (on the “bowl” itself) is
known as Ursa Minoris
If the Greek alphabet is used
up, then Roman letters are
used.
Polaris
Quick Quiz!
• Short wave radiation is produced by objects
with…?
• What is an emission spectrum?
• What is an absorption spectrum?
• Why do we know that Hydrogen and Helium are
found on most stars?
• How do we know that the Sun’s surface
temperature is 5800 °K?
• What is the Doppler Effect? What does it tell us?
• How would we name the fourth brightest star in the
constellation Orion?