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Chapter 10.2
Radiation Tells Us the Temperature,
Size, and Composition of Stars
We know about light
We know how light interacts with matter
But do we know where light come from?
And fort that matter, what is matter?
Properties of Matter
(then)
Democritus (470-380 B.C)
“atoms” meaning: indivisible
All matter made up of 4 elements:
fire, water, earth, air
Properties of Matter
(now)
“atoms” are divisible
All matter made up of a lot
than 4 elements
more
Niels Bohr and Ernest Rutherford
Parts of an atom:
Nucleus
proton (+ charge)
neutron (no charge)
electron (- charge)
mass of proton is
almost the same as
mass of the neutron
mass of electron is
about 1/2000 th of the
mass of the proton.
Nuclear Density
a teaspoon of material as
dense as the matter in an
atomic nucleus would weigh
~ 2 billion tons!!
If the proton and neutron were 10 cm across, the
quarks & electron would be < 0.01 cm, the entire
atom would be 10 km (1,000,000 cm) across.
http://phys.org/news/2012-09-world-atomicmicroscope-chemical-bonds.html
Actually seeing an atom!
http://newscenter.lbl.gov/newsreleases/2013/03/07/atomic-collapse-graphene/
http://techon.nikkeibp.co.jp/english/NEWS_EN/201011
05/187158/
Magnify an atom 1012 times
4.5 Football fields
.
Grape seed
• Atomic Number = # of protons
• Atomic Mass Number = # of protons + neutrons
proton
neutron
Atomic Terminology
• Isotope: same # of protons but different # of
neutrons. (4He, 3He)
Atomic Terminology
• Ion: loss of electron(s)
O loses an electron
O loses an electron
O+
O+ +
ionization
Atoms
• The kind of atom depends on the
number of protons in the nucleus.
• Most abundant: Hydrogen (H),
with 1 proton and 1 electron
• Next: Helium (He), with 2 protons
and 2 neutrons and 2 electrons
• Molecules: two or more atoms
(ex. H2O, CO2)
We know that atoms function:
Not like a mini solar system!
But we will use the orbital
model anyway:
How do atoms function? Atoms are Picky!
When a photon with
exactly the right energy
comes along, an electron
will be kicked into a
higher orbit
The photon is absorbed,
and the electron jumps to
higher (excited) state.
Electron Orbits
• Electron orbits in the electron cloud are
restricted to very specific radii and energies.
r3, E3
r2, E2
r1, E1
• These characteristic electron energies are
different for each individual element.
Energy Level Transitions (Hydrogen)
Not Allowed
Allowed
Electronic Energy States
• Electrons can only have certain energies; other
energies are not allowed.
• Each type of atom has a unique set of energies.
• Energy level diagram.
Atoms can store energy
Excited States
Ground State
Atoms absorb/emit very specific energies of photons!
Each element has a unique set of energy levels
Energy levels of Hydrogen
Each transition
corresponds to
a unique photon
energy
Now you know the
basics of atoms !
How can we use the way we know an atom works
to get information from LIGHT?
Just by analyzing the light received from a star,
astronomers can learn about a star’s
surface temp, chemical composition, total energy
output, velocity, rotation period.
Putting refraction to work:
Spectrum
(singular)
Spectra
(plural)
Different colors of visible light correspond to
different wavelengths.
Putting refraction to work:
spectrograph
a device that spreads out the light
from an object into its component
wavelengths
spectrometer records
the spectrum
Emission: an electron emits a photon and drops
to a lower energy state, losing energy.
• Emission: an electron emits a photon and
drops to a lower energy state, losing energy.
– The photon’s energy is equal to the energy
difference between the two levels.
It’s the missing information
that’s provides THE information!
Absorption: an electron absorbs the energy of a
photon to go to a higher energy level.
– The photon’s energy has to be equal to the energy
difference between the two levels.
Continuous
Spectrum
Emission
Line
Spectrum
Absorption
Line
Spectrum
• The wavelengths at which atoms emit and absorb
radiation form unique spectral fingerprints for each
atom.
• These spectral lines help determine a star’s
temperature, composition, density, pressure, and
more.
Some light leaving the star
is absorbed
by atoms or molecules in
the star’s atmosphere.
Makes
absorption
lines.
Sometimes
see
emission
lines.
Electron drops: emission lines
electrons absorb the incoming photon and the
electron jumps to a higher energy level:
absorption lines
Absorption Spectrum Dominated by
Balmer Lines
Modern spectra are usually
recorded digitally and
represented as plots of intensity
vs. wavelength
Observations of the H-Alpha (H) Line
Emission nebula, dominated
by the red Hline.
Chemical Fingerprints
hydrogen
Chemical Fingerprints
• Observing the fingerprints in a spectrum tells
us which kinds of atoms are present
Energy Levels of Molecules
• Molecules have additional energy levels
because they can vibrate and rotate
Energy Levels of Molecules
• The large numbers of vibrational and rotational
energy levels can make the spectra of
molecules very complicated
• Many of these molecular transitions are in the
infrared part of the spectrum
End of Chapter 10.2
Now you know where
Light comes from and
the basics about spectra!