ATOMS AND STARLIGHT

1
ATOMS AND STARLIGHT
2
TYPES OF SPECTRA KIRCHOFF'S LAWS

• CONTINUOUS SPECTRUM
• EMISSION (BRIGHT LINE) SPECTRUM
• ABSORPTION (DARK LINE) SPECTRUM

3
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

4
(No Transcript)
5
EMISSION SPECTRUM

• Shows only discrete wavelengths (colors) of light
• Formed by glowing diffuse (low density) gas

6
(No Transcript)
7
ABSORPTION SPECTRUM

• Shows dark lines superimposed on a bright
  continuous spectrum background
• Formed when a continuous radiation passes through
  a diffuse gas

8
(No Transcript)
9
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.

10
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.

11
(No Transcript)
12
(No Transcript)
13
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.

14
(No Transcript)
15
(No Transcript)
16
(No Transcript)
17
ABSORPTION AND EMISSION
18
(No Transcript)
19
(No Transcript)
20
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.

21
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.

22

• The Planck radiation law assumes that the object
  observed is a perfect radiator and absorber of
  energy (black body).
• Stars, although not perfect black bodies, are
  close enough so that Planck curves are useful
  descriptions of their radiation.

23
(No Transcript)
24
STAR COLORS
25
(No Transcript)
26
WIENSS 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

27
(No Transcript)
28
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

29
(No Transcript)
30
FORMATION OF STELLAR SPECTRA

• ABSORPTION SPECTRUM
• EMISSION SPECTRUM
• CONTINUOUS SPECTRUM

31
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.

32
(No Transcript)
33
(No Transcript)
34
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.

35
(No Transcript)
36
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.

37
(No Transcript)
38
SPECTRUM OF THE HYDROGEN ATOM
39
(No Transcript)
40
(No Transcript)
41
EMISSION NEBULAH ALPHA LIGHT
42
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.

43
(No Transcript)
44
(No Transcript)
45
(No Transcript)
46
(No Transcript)
47
(No Transcript)
48
(No Transcript)
49
(No Transcript)
50
INFORMATION FROM STELLAR SPECTRA

• TEMPERATURE
• CHEMICAL COMPOSITION
• MOTION

51
STELLAR TEMPERATURE

• COLOR TEMPERATURE
• EXCITATION TEMPERATURE
• IONIZATION TEMPERATURE

52
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.

53
EXCITATION TEMPERATURE

• SPECTRAL LINE FORMATION IS DETERMINED BY
  TEMPERATURE.
• O-B-A-F-G-K-M

54
IONIZATION TEMPERATURE

• THE AMOUNT OF IONIZATION OF GASES IS DETERMINED
  BY TEMPERATURE.
• STELLAR CORONAS HAVE HIGHLY IONIZED GASES,
  THEREFORE VERY HIGH TEMPERATURES.

55
STELLARCHEMICAL 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.

56
(No Transcript)
57
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).

58
(No Transcript)
59
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.

60
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.

61
(No Transcript)