Transcript Spectrum Presentation

ATOMS AND STARLIGHT
TYPES OF SPECTRA
KIRCHOFF'S LAWS
• CONTINUOUS SPECTRUM
• EMISSION (BRIGHT LINE)
SPECTRUM
• ABSORPTION (DARK LINE)
SPECTRUM
CONTINUOUS SPECTRUM
• Shows a blending of
colors like the
rainbow
• Formed by a glowing
solid, liquid, or dense
gas
• Glows at all
wavelengths - visible
and invisible
EMISSION SPECTRUM
• Shows only discrete
wavelengths (colors)
of light
• Formed by glowing
diffuse (low density)
gas
ABSORPTION SPECTRUM
• Shows dark lines
superimposed on a
bright continuous
spectrum
background
• Formed when a
continuous radiation
passes through a
diffuse gas
THE BOHR MODEL OF THE
ATOM
• In order to explain the formation of
spectra, the structure of the atom must be
understood.
• The model of the atom first conceived by
Niels Bohr early in the 20th century does a
good job of explaining how atoms radiate
light.
PROPERTIES OF THE ATOM
-1
• The nucleus of the atom contains protons (+)
and neutrons (0).
• Electrons (-) "orbit" the nucleus somewhat like
a miniature Solar System.
• Nuclear force binds the protons and neutrons
together.
• Electromagnetic force binds the electrons to the
nucleus.
• Atoms are mostly empty space.
• If the nucleus is the size of a marble, the
electron would be 1 mile away.
PROPERTIES OF THE ATOM
-2
• Every atom (element) has its own unique
number of protons in the nucleus.
• A neutral atom has the same number of protons
and electrons.
• There are only certain allowable electron orbits
(like rungs on a ladder).
• An atom must absorb energy to move an
electron to a higher (excited) energy orbit.
• An atom must emit energy when an electron
moves to a lower energy level.
ABSORPTION AND
EMISSION
PROPERTIES OF THE ATOM
-3
• An atom can only emit as much energy as
it has absorbed (conservation of energy).
• The ground state is the lowest energy level
in an atom.
• An ionized atom has completely lost one
or more electrons.
BLACK BODY (THERMAL)
RADIATION
• The actual distribution of light from a hot
glowing solid, liquid, or dense gas can not be
fully described unless the particle model of
light is used.
• This was first accomplished by Max Planck
(1900), and these light curves are now called
Planck curves.
• Every temperature has its unique distribution
of energy which determines the color that the
glowing object appears.
1. The Planck radiation law assumes that the object observed is a
perfect radiator and absorber of energy (black body).
2. Stars, although not perfect black bodies, are close enough so that
Planck curves are useful descriptions of their radiation.
STAR COLORS
STAR
TEMP (K)
COLOR
Betelgeuse
3,000
Red/Orange
Capella
6,000
Yellow/White
Sun
6,000
Yellow/White
Sirius
12,000
White
Rigel
18,000
Blue/White
WIENS’S LAW
• This law can be derived from Planck's Law.
• It states that the radiation peak on the Planck
curve varies inversely with the temperature.
• Red stars are relatively cool, but blue stars are
hot.
• Maximum Peak Wavelength = constant / T
STEFAN-BOLTZMANN LAW
• This law can also be derived from Planck's law.
• It states that the total energy from a radiating
object (like a star) at all wavelengths is directly
proportional to the 4th power of the
temperature.
• Therefore, a small change in temperature
results in a large change in the energy output.
E = (constant) T4
FORMATION OF STELLAR
SPECTRA
• ABSORPTION SPECTRUM
• EMISSION SPECTRUM
• CONTINUOUS SPECTRUM
STELLAR ABSORPTION
SPECTRUM
• Electrons absorb energy and re-emit it.
• Light is emitted in random directions.
• Dark lines are formed against a
continuous background.
• Most stars have an absorption spectrum.
STELLAR CONTINUOUS
SPECTRUM
• Atoms are packed together so tightly that their outer
electrons are influenced by neighbor atoms.
• Orbit separations which determine how the electron
can jump can no longer follow definite laws.
• With no definite orbits, an atom is no longer confined
to radiating a definite set of wavelengths.
• It can radiate any one of a variety of wavelengths
because a variety of orbits are possible.
• At any given moment, billions of atoms in a solid are
emitting billions of different wavelengths.
• Hence the solid, liquid, or high pressure gas radiates a
continuous spectrum.
STELLAR EMISSION
SPRCTRUM
• Electrons are excited into higher energy levels when the
atom absorbs outside energy.
• Electrons tend to drop back down to the ground state
very rapidly.
• They emit bursts of energy (light) when they drop back
to lower energy levels.
• The spectroscope uses a narrow slit to form these light
emissions into "lines".
• Each element's atom has its own unique set of spectral
lines.
• Gaseous nebulas and hot stellar atmospheres show
bright line spectra.
SPECTRUM OF THE
HYDROGEN ATOM
EMISSION NEBULA
H – ALPHA LIGHT
STELLAR CLASSIFICATION
• This pioneering work was done by Annie J.
Cannon in the early part of this century.
• The letter classification scheme actually
expresses temperature classes.
• Subclasses (0-9) further define very detailed
spectral features.
• The Sun is a G2 star.
INFORMATION FROM
STELLAR SPECTRA
• TEMPERATURE
• CHEMICAL COMPOSITION
• MOTION
STELLAR TEMPERATURE
• COLOR TEMPERATURE
• EXCITATION TEMPERATURE
• IONIZATION TEMPERATURE
COLOR TEMPERATURE
• The temperature of a star can be determined
by measuring the relative amounts radiation
being emitted at different wavelengths (colors)
by the star.
• Very hot stars emit more light from the blue
end of the spectrum, so they appear somewhat
blue in color.
• Relatively cool stars emit more light from the
red end of the spectrum, so they appear
somewhat red in color.
EXCITATION
TEMPERATURE
• SPECTRAL LINE FORMATION IS
DETERMINED BY TEMPERATURE.
• O-B-A-F-G-K-M
IONIZATION
TEMPERATURE
• THE AMOUNT OF IONIZATION OF
GASES IS DETERMINED BY
TEMPERATURE.
• STELLAR CORONAS HAVE HIGHLY
IONIZED GASES, THEREFORE VERY
HIGH TEMPERATURES.
STELLAR
CHEMICAL COMPOSITION
• The chemical composition of a star can be
determined from spectral line analysis, because
every atom has it own unique set of spectral
lines.
• In order to accurately determine chemical
composition a star's temperature must also be
known, because temperature also affects the
kinds and strengths of spectral lines seen.
STELLAR MOTION
• Motion either toward or away from an
observer will cause a shift in wavelength of an
emitting object (water, sound, light).
• The Doppler Effect.
• The motion measured in this manner is only
that part of the star's motion that is directly
toward or away from the observer (radial
velocity).
RED SHIFT
Spectral absorption (or emission) lines are
seen to be shifted toward the red end of
the spectrum (red shift) if the motion is
away from the observer.
BLUE SHIFT
Spectral absorption (or emission) lines are
seen to be shifted toward the blue end of
the spectrum (blue shift) if the motion is
toward the observer.