Download Foundation 1 - Discovering Astronomy

Chapter 4 The Origin and Nature of Light
What do you think?
• How hot is a “red hot” object compared to
objects glowing with other colors?
• What color is the Sun?
What can we learn by
analyzing starlight?
• A star’s temperature
Peak color (wavelength) shifts to shorter
wavelengths as an objects is heated
increasing temperature
Peak color (wavelength)
emitted depends on an object’s
temperature
Peak color (wavelength) shifts to shorter
wavelengths as an objects is heated
hotter object
cooler object
The intensities of different emitted
colors reveal a star’s temperature
• Wien’s law
• wavelength is inversely proportional to temperature
lmax = (2.9 x 10-3) / TKelvin
wavelength is inversely proportional to temperature
lmax = (2.9 x 10-3) / TKelvin
What color is our 5800K Sun?
The Sun emits all colors
(wavelengths of
electromagnetic
radiation); however,
the colors it emits
most intensely are in
the blue-green part of
the spectrum.
What can we learn by
analyzing starlight?
• A star’s temperature
• A star’s chemical composition
Each chemical element produces its own
unique set of spectral lines when it burns
The Sun’s Spectrum
The brightness of spectral lines depend on
conditions in the spectrum’s source
Law 1 A hot object or a hot, dense gas
produces a continuous spectrum -- a
complete rainbow of colors with without
any specific spectral lines. (This is a black
body spectrum.)
The brightness of spectral lines depend on
conditions in the spectrum’s source
Law 2 A hot, rarefied gas produces an
emission line spectrum - a series of bright
spectral lines against a dark background.
The brightness of spectral lines depend on
conditions in the spectrum’s source
Law 3 A cool gas in front of a continuous
source of light produces an absorption line
spectrum - a series of dark spectral lines
among the colors of the rainbow.
Absorption Spectrum of
Hydrogen Gas
Kirchhoff’s Laws
But, where does light actually
come from?
Light comes from the
movement of electrons
in atoms
An atom
consists of a
small, dense
nucleus
surrounded by
electrons
Atomic Vocabulary
• The nucleus contains protons and neutrons
• All atoms with the same number of protons
have the same name (called an element)
• Atoms with varying numbers of neutrons
are called isotopes.
• Atoms with a varying
numbers of electrons are
called ions.
Spectral lines occur when an
electron jumps from one energy
level to another
What can we learn by analyzing starlight?
• A star’s temperature
• A star’s chemical composition
• A star’s movement
– Barnard’s Star
Spectral lines shift due to the relative motion
between the source and the observer
Doppler Shifts
• Red Shift: The distance between the
observer and the source is increasing
• Blue Shift: The distance between the
observer and the source is decreasing
The Doppler shift
allows astronomers
to measure radial
velocity
What can we learn by
analyzing starlight?
• A star’s temperature
– by peak wavelength
• A star’s chemical composition
– by spectral analysis
• A star’s radial velocity
– from Doppler shifts
What did you think?
• How hot is a “red hot” object compared to
objects glowing with other colors?
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 colors (wavelengths of
electromagnetic radiation). The colors it emits most
intensely are in the blue-green part of the spectrum.
Self-Check
1: State the Stefan-Boltzmann law and Wien’s law and explain their meaning in the
context of blackbody radiation and temperature determination.
2: Describe the evidence for the particle nature of light and indicate how the energy
per photon is related to the wavelength and frequency in the wave model.
3: State Kirchhoff’s three laws of spectral analysis and indicate what information is
derived about the nature of the light source in each case.
4: Describe the Bohr model of the atom in terms of its constituents and their
distribution. Explain how spectral lines can be produced by a low-density gas.
5: Describe how spectroscopic analysis provides information about the chemical
composition of celestial objects and indicate for which part of the object the
information is valid.
6: Indicate how the numbers of protons, neutrons, and electrons are used to define
elements, ions, and isotopes.
7: Describe the origin of line series in the hydrogen atom and explain why the Balmer
lines occur at visual wavelengths but the other line series do not.
8: Define excitation and ionization in the context of the Bohr model of atoms.
9: Describe how the Doppler shift reveals the radial motion of the stars.