Transcript Light Spectrum 2012-2013

Blackbody
Radiation
And
Spectra
Light is a form of _______.
energy
Why is this important?
With very few exceptions, the
only way we have to study
objects in Astronomy is via
the light (energy) they emit.
• The Earth’s atmosphere is __________
transparent
visible light some ________,
infrared and
to __________,
the radio
____. It is opaque to ___,
UV ______,
X-rays
and gamma
______ rays.
Blackbody Radiation
• Ex. Heat an iron rod with a torch.
–1st visible color – “____
Red Hot”
Orange
–2nd visible color – Bright _______
–3rd visible color – Bright ________
Yellow
–4th visible color – Bright ______
White
Blue
–5th visible color – Bright _____
• As you heat an object it gets _______
brighter
emits more ______________
electromagnetic
and ______
radiation
_________.
Blackbody Radiation
• The dominant color or wavelength of the
emitted radiation changes with temperature.
Cooler objects emit at _______
longer
–_______
wavelengths - Red, Infrared
–______
Hotter objects emit at _______
shorter
wavelengths - Blue, Violet, UV
• Object actually gives off ____
all
___________
wavelengths of electromagnetic
spectrum.
Blackbody Radiation
• A blackbody ________
absorbs all the
electromagnetic radiation that
strikes it (none is reflected or
scattered).
• Object is heated and then reemits
_______
different
the energy it absorbed at _______
__________ of the EM spectrum.
wavelengths
Blackbody
Curves
• A body with any
temperature radiates
thermal energy
_____________,
sometimes called
blackbody radiation.
• For a given size,
______
hotter objects give
off __________
more energy than
cooler objects, and
bluer
are _____.
Blackbody
Curves
• For a given
temperature,
larger bodies
give off more
energy than
smaller bodies,
in direct
proportion to
their surface
areas.
Visible light
• Most people can perceive ______.
color
• Different colors correspond to
different frequencies (or
wavelengths).
• The colors of the rainbow are ROY G
BIV: red orange yellow green blue
indigo violet.
Visible light
• In the visible,
wavelength
 ____
red has the longest __________,
the smallest __________,
frequency and the
energy
lowest _______.
 _____
_________,
violet has the shortest wavelength
the highest __________,
frequency and the
energy
highest ______.
The spectrum
• A graph of the
intensity of light vs.
the color (e.g. the
wavelength,
frequency, or energy)
is called a ________.
spectrum
• A spectrum is
probably the single
most useful diagnostic
tool available in
Astronomy.
Important points
• The spectrum of a star is approximately a
black body spectrum.
Hotter stars are bluer, _____
cooler stars are
 ______
redder.
 For a given temperature, ______
larger stars
give off __________
more energy than smaller stars.
 ______
Shorter the wavelength, the _______
greater
the temperature
• In the
constellation of
Orion, the
reddish star
Betelgeuse is a
relatively cool
star. The blue
star Rigel is
relatively hot.
The spectrum
• A spectrum can tell us about the temperature
and composition of an astronomical object.
• There are _____
two major types of spectra of
concern here:
Continuous spectra - the intensity varies
 _________________
smoothly from one wavelength to the next.
Line spectra (2) - there are discrete jumps
 _____________
in the intensity from one wavelength to the
next.
The Line Spectrum
• Upon closer examination, the spectra of
real stars show fine detail.
• Dark regions where there is relatively
little light are called lines.
The Line Spectrum
• Today, we rarely
photograph
spectra, but rather
plot the intensity vs
the wavelength.
Emission and Absorption
• If you view a gas against a dark
emission lines
background, you see _____________
(wavelengths at which there is an
abrupt spike in the brightness).
Emission and Absorption
• If you view a continuous spectrum
absorption
through cool gas, you see __________
lines (wavelengths where there is
______
little light).
Why is this important?
• The line spectrum tells about
what elements are present in the
star.
• Why is this?
How Light Interacts with Matter.
• Atoms are the basic
blocks of matter.
• They consist of
protons and
neutrons in the
nucleus,
surrounded by
lighter particles
called electrons.
How Light Interacts with Matter.
electron will interact with a
• An ________
photon.
• An electron that _______
absorbs a photon will
gain energy.
_____
loses energy must
• An electron that ______
______
emit a photon – giving off light.
• The ____________
(electron plus
total energy
photon) remains
______________
constant during this
process.
Atomic Fingerprints
• Hydrogen has a
specific line
spectrum.
• Each atom has
its own specific
line spectrum.
Atomic Fingerprints
• These stars have
absorption lines
with the
wavelengths
corresponding to
hydrogen!
Atomic Fingerprints.
• This cloud of
gas looks red
since its
spectrum is a
line spectrum
from hydrogen
gas.
The Doppler Shift: Measuring
Motion
• If a source of
waves is ____
not
_______,
moving then
the waves are
_______
equally
_______
spaced in all
directions.
The Doppler Shift: Measuring
Motion
• If a source of
is
waves ___
moving then
_______,
the spacing of
the wave crests
________
depends on the
direction relative
to the direction of
motion.
The Doppler Shift: Measuring
Motion
• Think of sound waves from a fastmoving car, train, plane, etc.
 The sound has a ___________
Higher pitch
(higher frequency) when the car
__________.
approaches
 The pitch is _____
lower (lower frequency)
as the car ______
passes and moves
further away.
The Doppler Shift: Measuring
Motion
• If a source of
moving
light is _______
_____,
away the
wavelengths are
increased, or
“__________”.
redshifted
The Doppler Shift: Measuring
Motion
• If a source of
moving
light is _______
_______,
closer
the
wavelengths are
shortened, or
“__________”.
blueshifted
The Doppler Shift: Measuring
Motion
• The size of the
wavelength
shift depends
on the relative
velocity of the
source and the
observer.
Using a Spectrum, we can…
• Measure a ________________
star’s temperature by
measuring the overall shape of the
spectrum (essentially its color).
chemical elements are in a
• Measure what _______________
star’s atmosphere by measuring the lines.
relative velocity of a star by
• Measure the ______________
measuring the Doppler shifts of the lines.