1B11 Foundations of Astronomy The Electromagnetic Spectrum

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1B11 Foundations of AstronomyThe
Electromagnetic Spectrum

• Liz Puchnarewicz
• emp_at_mssl.ucl.ac.uk
• www.ucl.ac.uk/webct
• www.mssl.ucl.ac.uk/

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1B11 The electromagnetic spectrum
When an electric charge is accelerated,
electromagnetic energy is produced. This energy
can be thought of as propagating as a wave or,
equally as a particle. The waves are usually
referred to as light waves or radiation. The
particles are known as photons.
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1B11 EM waves
Electromagnetic waves are transverse sine waves.
For a wave travelling in the x direction, the
electric field at time t is given by
c speed of light l wavelength
E
E0
x
0
l
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1B11 The electromagnetic spectrum
Travelling at right angles to the electric field
but in the same direction, is the magnetic field,
B.
electric field
magnetic field
EM waves are self-propagating, ie they need no
medium.
c speed of light in m s-1 l wavelength in
m n frequency in Hz
c2.998x108 ms-1
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1B11 Wave-like properties of light

• Refraction the direction of travel of light
  changes when light crosses the boundary from one
  medium to another.
• Diffraction where light waves bend when they
  strike the end of a barrier.
• Interference complex pattern forms when two or
  more wave systems combine
• Polarization where the planes in which the
  waves vibrate lie preferentially in one direction
• Doppler Effect where the observed wavelength,
  l0, is different from the emitted wavelength, l,
  due to the velocity of the emitter, v.

l obs wavelength l0rest wavelength vvelocity
of source
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1B11 Quantum nature of light
Alternatively, light can be thought of as packets
(or quanta) of energy called photons.
Photon energy, E
n frequency (Hz) h Plancks constant
(6.63x10-34 Js)

• high frequency
• short wavelength
• high energy

• Examples of particle nature can be seen in
• Photo-electric effect
• Atomic spectra

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1B11 Units
Wavelength SI units metre, m Optical/UV
Angstrom, A 1A 10-10 m 10-8 cm 0.1nm
nanometre, nm 1nm 10-9
m Infra-red micron, mm 1mm 10-6
m
Frequency SI units Hertz, Hz Radio
Gigahertz, GHz 1GHz 109 Hz
Energy SI units Joules, J X-ray electron
volts, eV 1eV 1.6x10-19 J
1keV 1.6x10-16 J
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1B11 Map of the EM spectrum
Log frequency (Hz)
24
20
18
16
14
12
10
8
6
4
2
22
g-rays
X-rays
UV
IR
microwave
radio
-16
-12
-10
-8
-6
-4
-2
0
2
4
6
-14
Log wavelength (m)
Atmosphere transparent
Opaque with narrow windows
Atmosphere opaque
visible
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1B11 Spectroscopy
Spectra were first seen due to the effects of
refraction ie the bending of light as it passes
through a transparent medium, which is
wavelength-dependent.
White light
prism
Blue light is bent the most, red light the least.
Spectroscopy is the astrophysical technique
which is key to our understanding of astronomical
objects.
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1B11 Kirchoffs Rules (1824-1887)

1. A hot, dense object emits a continuous spectrum

2. A hot, transparent gas produces a spectrum of
emission lines. The lines depend on the elements
in the gas.
3. If a continuous spectrum passes through a
transparent gas at a lower temperature, the low-T
gas will superimpose dark absorption lines on the
spectrum.
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1B11 Continuous spectra
Hot, dense objects emit a continuous blackbody
spectrum. Surfaces of stars, for example, are
very good blackbodies. Planck (1900) showed
that the intensity of radiation emitted by a
blackbody is Units W m-2 (unit wavelength)
-1 sr-1 h - Plancks constant k
Boltzmanns constant T temp in Kelvin
c speed of light sr steradian (unit
of solid angle, 4p sr in a sphere)
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1B11 Blackbody curves
Log Il
15000K
10000K
5000K
3000K
200
500
1000
l (nm)
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1B11 Key BB relations
Wiens Law Wavelength of peak intensity, lmax
2.898 x 10-3 / T m
or Stefan-Boltzmann Law Total flux
emitted by a blackbody, F sT4 W m-2 where s
Stefans constant 5.67 x 10-8 W m-2 K-4 For
astronomers Colour Index, (B-V) -0.71 7090 /
T where T is in Kelvin
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1B11 Absorption and emission lines
Where do absorption and emission lines come from?
The production or absorption of energy
when an electron in an atom changes its level.
Taking the simple case of a hydrogen atom. It has
one proton in the nucleus and one orbiting
electron. In its stable state, the electron
orbits in level 1 (the ground state). There are
an infinite number of discrete levels, converging
to n , the ionization potential.
4
3
2
1
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1B11 Transitions
Electron moves up from 2 to 3
n
n6
n5
n4
n3
6562A
n2
n1
Absorption line at 6562A
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1B11 Energy levels
Atoms have an infinite number of energy levels,
converging to a finite value (the ionization
potential). If an electron gains more energy than
the ionization potential then it is no longer
bound to the atom. Only the lowest level (the
ground state) is generally stable. Excited states
(when an electron is in level 2 or higher) have
lifetimes of 10-8 seconds.
n
n6
n5
n4
Excited states
n3
Ionization potential
n2
Ground state
n1
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1B11 Transitions
Only certain discrete energy levels are allowed
for electrons in atoms. Transitions between
levels are accompanied by the emission or
absorption of photons. The photon energy (emitted
or absorbed) corresponds to the energy lost or
gained in the transition.
Eb
Ea
Photon energy, hn Eb - Ea
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1B11 Emission lines

• To produce emission lines, an excited state must
  first be populated when the electron in an
  excited state falls by one or more levels, an
  emission line is produced.
• To populate the excited levels
• Collisional excitation
• Photo-excitation
• Recombination
• These all produce emission lines and explain
  Kirchoffs 2nd rule.

I(l)
wavelength, l
Kirchoffs Rules
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1B11 Collisional excitation
n
n5
n4
n3
n2
n1
Collisions with electrons/ions/atoms can knock
electrons into higher energy levels. The energy
comes from the kinetic energy of the colliding
particle. The electron falls back to lower levels
and this energy is radiated away.
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1B11 Photo-excitation
n
n5
n4
n3
n2
n1
If a photon with exactly the right energy
interacts with an atom or ion, an electron can be
moved up to a higher level for a short while,
before it falls back down to the ground state.
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1B11 Ionization recombination
n
OR
n1
If a photon or particle with sufficient energy
interacts with an atom so that an electron is
stripped away completely, it is said to be
ionized. A free electron can recombine with an
ion, falling into an excited state it will then
cascade down to ground level producing line
emission at it falls.
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1B11 Absorption lines
When atoms/ions in a gas are illuminated, they
will absorb those photons at wavelengths which
will move electrons in the atoms/ions from one
level to another.
Fl
l
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1B11 Absorption lines (cont.)
Atoms or ions in a gas will absorb photons whose
energy corresponds exactly to the energy that an
electron in that atom/ion needs to move into a
higher level. After about 10-8 seconds, the
electron will fall back down to the most stable
state, emitting a photon with an energy
corresponding to the difference between the
levels, but in a random direction. So if you look
through the gas at a source, you will see much
few photons at that energy because these are
being re-emitted in random directions. This
produces an absorption line and explains
Kirchoffs 3rd rule.
Kirchoffs rules
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1B11 Spectrum of the hydrogen atom
13.6eV
n4
n3
12.1eV
n2
10.2eV
0eV
n1
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1B11 Astrophysical applications

• Chemical composition different atoms/molecules
  have different lines line strengths indicate
  abundances
• Ionization state different atoms have
  different ionization potentials different ions
  have different spectra
• Temperature and density collisional excitation
  in high densities lines broadened in high-T gas
• Pressure high pressure broadens lines
• State of motion Doppler effect
• Magnetic fields in a high-B field, energy
  levels split due to the Zeeman effect.

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1B11 Sources of absorption lines
outer layers absorb
Stellar atmospheres
blackbody emission from star
star
ISM cloud
Interstellar gas
star
Intergalactic Lya systems
quasar
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1B11 Sources of emission lines
Hot ionized nebulae
eg HII regions
Active galactic nuclei
quasar