Transcript astronomy ch4 - Fort Thomas Independent Schools

CHAPTER 4:
Visible Light
and Other
Electromagnetic
Radiation
WHAT DO YOU THINK?
How hot is a “red hot” object?
 What color is the Sun?
 How can we determine the age of space
debris found on Earth?

You will discover…
•the origins of electromagnetic radiation
•the structure of atoms
•that stars with different surface temperatures emit
different intensities of electromagnetic radiation
•that astronomers can determine the chemical
compositions of stars and interstellar clouds by
studying the wavelengths of electromagnetic radiation
that they absorb or emit
•how to tell whether an object in space is moving
toward or away from Earth
BLACKBODY RADIATION
A heated iron poker will begin to glow emitting photons. This is different from a burning
process because no chemical change is involved. The amount and wavelength of the
radiation changes with temperature.
•As the object heats up, it gets brighter, emitting more photons of all colors
(wavelengths).
•The brightest color (most intense wavelength) of the radiation changes with
temperature.
WHEN FIRST HEATED THE
POKER GLOWS DIMMLY
AND IS RED
AS THE TEMPERATURE
RISES, THE POKER
BECOMES BRIGHTER
AND GLOWS ORANGE
AT HIGHER TEMPERATURES
THE POKER BECOMES EVEN
BRIGHTER AND GLOWS
YELLOW
Shown is a plot of
intensity versus
wavelength for
blackbodies at different
temperatures. At
higher temperatures the
most intense
wavelengths are
shorter. Since the
observed color
depends on these
emitted wavelengths,
blackbodies at different
temperatures will
appear different colors.
Stellar surfaces emit light that is close to an ideal blackbody. We
can estimate the surface temperature of a star by examining the
intensity of emitted light across a wide range of wavelengths.
A spectroscope is used to examine the
wavelengths of light emitted from a source
When a chemical is
burned, the light
produced is made of
only specific
wavelengths.
Different chemical
elements have their
own series of
wavelengths.
Elements are arranged in order of increasing number of protons (atomic
number) and their properties. The elements in each column have similar
chemical properties.
The combination of lines
from a stellar spectrum
allow us to determine
which chemicals are
present and in what
quantities.
For example, by matching the spectrum of iron to the absorption lines
from the Sun, we see that there is iron present in the Sun’s atmosphere.
A grating spectrograph
separates light from a
telescope into different
colors by passing it
through a grating of tiny
parallel grooves.
Peacock feathers are natural gratings.
THE SPECTRUM OF HYDROGEN GAS
ABSORPTION SPECTRUM
Signature wavelengths appear as dark
lines on an otherwise continuous
rainbow.
Lines appear as dips in the intensity
versus wavelength graph.
EMISSION SPECTRUM
Signature wavelengths appear as bright
lines on an otherwise black background.
Lines appear as peaks in the intensity
versus wavelength graph.
Different types of spectra are produced depending on how a
how blackbody and a cloud of gases are observed
ATOMIC STRUCTURE
At the center of an atom is a dense nucleus which contains positively-charged
particles, called protons, and particles with no charge, called neutrons. This
nucleus is surrounded by a cloud of negatively-charged particles called electrons.
Most of the mass of an atom is contained within its nucleus. The neutron and
proton in the nucleus are each about 1800 times more massive than the electrons
in the surrounding cloud.
However, most of the space of an atom is occupied by the electron cloud. The
nucleus of an atom is 100,000 times smaller than the atom itself. The remaining
space is filled by the electron cloud.
NUCLEUS
TINY AND MASSIVE
CENTER CONTAINING
PROTONS AND
NEUTRONS
THE ELECTRON CLOUD
EXTENDS FAR FROM THE
NUCLEUS
The number of protons contained in the nucleus, called the atomic number,
determines the element of the atom. Elements containing the same
number of protons in their nucleus (and are thus the same chemical
element) but have different numbers of neutrons are called isotopes.
Because two protons of like charge repel each other, there must be another
force which holds the nucleus of an atom together. We call this force the
strong nuclear force, and it is the strongest of the four fundamental forces.
However, it only has a range of effect inside the atomic nuclei.
The electrons in an atom can only
exist in certain allowed orbits with
specific energies. The lines seen
from the chemicals are made when
an electron moves from one energy
level to another.
When an electron moves from a lower
energy level to a higher one, a photon
is absorbed. When an electron
moves from a higher energy level to a
lower energy one, a photon is emitted.
The energy of the photon, and thus its
wavelength, are determined by the
energy difference between the two
energy levels.
The visible light portion of the spectrum of a hydrogen
atom shows the photons representing transitions to and
from the n = 2 energy level to a higher energy level,
forming a series of lines, the Balmer Series. This
spectrograph shows the hydrogen Balmer absorption
lines from 13-40.
Emitted photons sent out in all directions will cause the gas
surrounding a star to glow different colors, depending on
which gases are abundant.
HYDROGEN RICH
CLOUDS GLOW RED.
OXYGEN RICH CLOUDS
GLOW GREEN.
Radial Velocity
The proper motion of a star is its
motion perpendicular to our line of sight
across the celestial sphere. This is so
small that it can only be measured for
the closest stars.
The radial velocity of a star is
its motion along our line of
sight either toward or away
from us. Using the spectrum,
we can measure this for
nearly every object in space.
Recall that the wavelength of
light, and therefore the
wavelength of the photons that
light contains, is slightly shifted
when the source is traveling
toward or away from the
observer—the Doppler Effect.
Stars moving toward us show spectral lines that are shifted to blue.
Stars moving away from us show spectral lines that are shifted to red.
The amount of the shift increases with the radial speed.
The Balmer series lines from the spectrum of the star Vega are all shifted toward the
blue side by the same amount. From this we determine that Vega is moving toward us
(blueshift) with a speed of 14km/s (determined from the amount of the shift).
We can examine the proper
motion of nearby stars over long
periods of time.
This picture is made from three
overlapping photographs taken
over a four-year period.
The three dots in a row are
Barnard’s Star seen moving
over the four-year period.
WHAT DID YOU THINK?






How hot is a “red hot” object?
Of all objects that glow from heat stored or
generated inside them, those that glow red are
the coolest.
What color is the Sun?
The Sun emits all wavelengths of
electromagnetic radiation, with blue-green most
intense.
How can we determine the age of space debris
found on Earth?
We measure how much long-lived radioactive
elements have decayed in the object.
Key Terms
absorption line
absorption line spectrum
atomic number
blackbody
blackbody curve
continuous spectrum
diffraction grating
element
emission line
emission line spectrum
energy flux
excited state
ground state ion
ionization
isotope
Kirchhoff’s laws
luminosity
quantum mechanics
radial velocity
radioactive
spectral analysis
spectrograph
Stefan-Boltzmann law
strong nuclear force
transition (of an electron)
virtual particles
Wien’s law
molecule
nucleus (of an atom)
periodic table
Planck’s law
proper motion