Download 03 Starlight and Atoms

Starlight and Atoms
Electromagnetic Radiation
Waves
Spectrum
Black Body Radiation
Atoms and Atomic Transitions
Kirchoff’s Laws of Radiation
The Amazing Power of Starlight
Just by analyzing the light
received from a star,
astronomers can retrieve
information about a star’s
1. Total energy output
2. Surface temperature
3. Radius
4. Chemical composition
5. Velocity relative to Earth
6. Rotation period
Information from the Skies
Electromagnetic Radiation: Transmission of
energy through space using traveling waves
of electric and magnetic fields
Examples:
•Radio
•Infrared
•Visible Light
•Ultraviolet
•X-rays
•Gamma rays
Waves
Wave motion: transmits energy without the
physical transport of material. Sound, water
and light, for example, transmit energy with
wave motion.
Example: Water Waves
■
Water
moves up
and down
locally
■
Wave
travels and
transmits
energy
Properties of Waves
Frequency (f or n): number of wave crests that pass a
given point per second
Period (P): time between passage of successive crests
Relationship: Frequency = 1 / Period
f = ν = 1/P
Wavelength (l): distance between successive crests
Velocity (c): speed at which crests move
Relationship: Velocity = Wavelength / Period
c = l/P = ln = l f
l
Electromagnetic Waves: Oscillating electric
and magnetic fields. Changing electric field
creates a magnetic field, and vice versa.
Speed of electromagnetic waves: c = 3.0 x 108 m/s
The Wave Nature of Radiation
Diffraction is purely a wave phenomenon. If
light were made of particles, we would see a
spot the size of the hole, with no fuzziness.
Light as Particles
• Light can also appear as particles, called
photons (explains, e.g., photoelectric
effect).
• A photon has a specific energy E,
proportional to the frequency n :
E=hn=hf
h = 6.626x10-34 J s
is the Planck constant.
The energy of a photon does not depend on the
intensity of the light!!! The higher the intensity, the
greater the number of photons
The Electromagnetic Spectrum
Wavelength
Frequency
Need satellites
to observe
High
flying air
planes or
satellites
Visible Spectrum
Color and Temperature
Stars appear in
different colors,
from blue (like Rigel)
Orion
Betelgeuse
via green / yellow
(white) (like our Sun)
to red (like
Betelgeuse).
Rigel
These colors tell
us about the star’s
temperature.
Temperature
Temperature is
a measure of
average
internal kinetic
energy
The Kelvin Temperature Scale
• All thermal
motion ceases at
0K
•
Water freezes at
273 K and boils
at 373 K
Heat Transport
Heat is transported by 3 physical mechanisms:
•
Conduction
•
Radiation
•
Convection
Thermal Radiation
The light from a star is usually
concentrated in a rather narrow
range of wavelengths.
The spectrum of a star’s light is
approximately a thermal
spectrum called a black body
spectrum.
A perfect black body emitter
would not reflect any radiation,
thus the name “black body.”
The mathematical formula was
derived by Max Planck (1900)
and is also called Planck
radiation
Laws of Thermal Radiation
1. The hotter an object is, the
more luminous it is. Stefan’s
Law:
F =  TK 4
(where F (flux) is power/unit
area, σ = 5.67 x 10-8 W/m2·K4 ,
TK is the temperature in Kelvin).
2. The peak of the black body
spectrum shifts towards
shorter wavelengths when the
temperature increases. Wien’s
displacement law:
l max ≈ 2,900,000 nm /TK
Note how the hottest star looks blue
and the coolest star looks red
Light and Matter
Spectra of stars are
more complicated than
pure black body spectra.
They have characteristic
lines, called absorption
lines.
To understand
these lines, we
need to understand
atomic structure
and the interactions
between light and
atoms.
Different Kinds of Atoms
• The kind of atom
depends on the
number of protons
in the nucleus.
• Most abundant:
Hydrogen (H),
with one proton
(+ 1 electron).
• Next: Helium (He),
with 2 protons (and
2 neutrons + 2 el.).
Helium 4
Different
numbers of
neutrons 
different
isotopes
Emission Energies correspond to energy
differences between allowed energy levels.
The quantum theory speaks of an electron
“cloud” representing the probability of electron
position rather than an electron orbit.
Emission Lines of Hydrogen
Most prominent lines in
many astronomical
objects: Balmer lines of
hydrogen
Atomic Transitions
 An electron can be
kicked into a higher
orbit when it absorbs a
photon with exactly
the right energy.
 The photon is absorbed,
and the electron is in an
excited state.
Eph = E3 – E1
Eph = E4 – E1
Wrong energy
(Remember that Eph = h n)
• All other photons pass by the atom unabsorbed.
The Formation of Spectral Lines
(a) Direct decay
Absorption
(b) Cascade
Emission
Absorption
Emission
Spectral Lines
Emission lines from single elements. When there are multiple
elements, spectral lines of each element are superimposed in the
spectrum and are used to identify all of the elements present.
H
Na
He
Ne
Hg
Absorption spectra and emission spectra can be
used to identify elements. These are emission
and absorption spectra of sodium:
Kirchhoff’s Laws of Radiation
1. A solid, liquid, or dense gas
excited by heat to emit light will
radiate at all wavelengths and
produce a continuous spectrum.
Kirchhoff’s Laws of Radiation
2.
If a continuous spectrum of light passes through a
cool, low-density gas exciting its electrons to higher
states, the result will be an absorption spectrum.
Light excites electrons in
atoms to higher energy states
Spectral lines correspond to the transition energies
that are absorbed from the continuous spectrum.
Kirchhoff’s Laws of Radiation
3. A low-density gas excited to emit light will do
so at specific wavelengths and thus produce
an emission spectrum.
Light excites electrons in
atoms to higher energy states
Electron transitions back to lower energy
states emits light at specific frequencies
The Spectra of Stars
Inner, dense layers of a
star produce a continuous
(black body) spectrum.
Cooler surface layers absorb light at specific frequencies.
This implies that the spectra of stars are absorption
spectra.
Absorption Spectrum Dominated by
Balmer Lines
Modern spectra are usually recorded
digitally and represented as plots of
intensity vs. wavelength
Emission nebula, dominated by the red Ha line.