Astronomy - spectroscopy - 2/3


Uploaded by rhcrcgvp on 11.07.2009

Transcript:
Kirchhoff's first law deals with

the continuous spectrum.

As opposed to emission spectra,

which appear as bright lines at

specific wavelengths, continuous

spectra are produced by an

object that gives off light at

all wavelengths.

For example, a horseshoe is
For example, a horseshoe is
For example, a horseshoe is being heated in a forge.
being heated in a forge.
being heated in a forge. As it heats up, it changes from
As it heats up, it changes from
As it heats up, it changes from dull red to brighter orange to
dull red to brighter orange to
dull red to brighter orange to brilliant yellow-white as the
brilliant yellow-white as the
brilliant yellow-white as the atoms become more excited,
atoms become more excited,
atoms become more excited, causing more collisions and
causing more collisions and
causing more collisions and radiating photons at shorter
radiating photons at shorter
radiating photons at shorter wavelengths.
wavelengths.
wavelengths. So the hotter the object is, the
So the hotter the object is, the
So the hotter the object is, the brighter it becomes and the
brighter it becomes and the
brighter it becomes and the higher energy photons it will
higher energy photons it will
higher energy photons it will emit.
emit.
emit. But it also radiates photons at
But it also radiates photons at
But it also radiates photons at all the other wavelengths,
all the other wavelengths,
all the other wavelengths, because the atoms are so crowded
because the atoms are so crowded
because the atoms are so crowded and interact so forcefully that
and interact so forcefully that
and interact so forcefully that they produce light of every
they produce light of every
they produce light of every color.
color.
color. Each atom wants to give off
Each atom wants to give off
Each atom wants to give off light at its own specific
light at its own specific
light at its own specific energies, but the energy
energies, but the energy
energies, but the energy levels are blurred.
levels are blurred.
levels are blurred. A solid, liquid, or dense gas
A solid, liquid, or dense gas
A solid, liquid, or dense gas excited to emit light will
excited to emit light will
excited to emit light will radiate at all wavelengths in
radiate at all wavelengths in
radiate at all wavelengths in this way and thus produce a
this way and thus produce a
this way and thus produce a continuous spectrum.
continuous spectrum.
continuous spectrum. The peak, or brightest color
The peak, or brightest color
The peak, or brightest color of the spectrum, indicates the
of the spectrum, indicates the
of the spectrum, indicates the temperature of the object.
temperature of the object.
temperature of the object. The value of the continuous
The value of the continuous
The value of the continuous spectrum to astronomers is that
spectrum to astronomers is that
spectrum to astronomers is that it easily reveals an approximate
it easily reveals an approximate
it easily reveals an approximate temperature of a dense gas.
temperature of a dense gas.
temperature of a dense gas. However, unlike the emission
However, unlike the emission
However, unlike the emission line spectrum, it does not
line spectrum, it does not
line spectrum, it does not reveal the composition of that
reveal the composition of that
reveal the composition of that gas.
gas.
The third type of spectrum gas.
The third type of spectrum gas.
identified by Kirchhoff is gas.
identified by Kirchhoff is gas.
the absorption or dark-line gas.
the absorption or dark-line gas.
spectrum.ption or dark-line gas.
spectrum.ption or dark-line gas.
This occurs when a cold gas is gas.
This occurs when a cold gas is gas.
between an observer and a source gas.
gas.
of continuous light.

The gas absorbs the light that
The gas absorbs the light that
The gas absorbs the light that has the correct energies to
has the correct energies to
has the correct energies to boost its electrons to its
boost its electrons to its
boost its electrons to its various energy levels.
various energy levels.
various energy levels. All other light passes through.
All other light passes through.
By the time the light gets to All other light passes through.
By the time the light gets to All other light passes through.
the observer, those energieso All other light passes through.
the observer, those energieso All other light passes through.
absorbed by the cold gas have All other light passes through.
absorbed by the cold gas have All other light passes through.
been removed and appear asave All other light passes through.
been removed and appear asave All other light passes through.
gaps in the spectrum.ar asave All other light passes through.
gaps in the spectrum.ar asave All other light passes through.
gaps in the spectrum.ar asave All other light passes through. Instead of bright emission lines
Instead of bright emission lines
Instead of bright emission lines at specific energies, absorption
at specific energies, absorption
at specific energies, absorption spectra show dark lines where
spectra show dark lines where
spectra show dark lines where energy is being absorbed by the
energy is being absorbed by the
energy is being absorbed by the atoms.
atoms.
atoms. Stars produce absorption
Stars produce absorption
Stars produce absorption spectra.
spectra.
spectra. The surface of a star is cooler
The surface of a star is cooler
The surface of a star is cooler and thinner than the gas
and thinner than the gas
and thinner than the gas underneath.
underneath.
The light from a hot, dense gas
The light from a hot, dense gas
The light from a hot, dense gas inside a star will produce a
inside a star will produce a
inside a star will produce a continuous spectrum, and the
continuous spectrum, and the
continuous spectrum, and the cooler gas above it will absorb
cooler gas above it will absorb
cooler gas above it will absorb specific wavelengths of that
specific wavelengths of that
specific wavelengths of that light which appear as gaps in
light which appear as gaps in
light which appear as gaps in the underlying continuous
the underlying continuous
the underlying continuous spectrum.
spectrum.
spectrum. >> And those gaps occur at very
>> And those gaps occur at very
>> And those gaps occur at very strategic and specific points
strategic and specific points
strategic and specific points along the wavelength.
along the wavelength.
along the wavelength. These are indicative of the
These are indicative of the
These are indicative of the chemical elements.
chemical elements.
chemical elements. These elements block some of
These elements block some of
These elements block some of the light at these specific
the light at these specific
the light at these specific wavelengths of light.
wavelengths of light.
wavelengths of light. And by seeing where these occur,
And by seeing where these occur,
And by seeing where these occur, that's a signature of what the
that's a signature of what the
that's a signature of what the star is made of and how much of
star is made of and how much of
star is made of and how much of that element is in the star.
that element is in the star.
that element is in the star. >> narrator: So with
>> narrator: So with
>> narrator: So with spectroscopy, astronomers can
spectroscopy, astronomers can
spectroscopy, astronomers can determine what an object is made
determine what an object is made
determine what an object is made of and estimate its temperature.
of and estimate its temperature.
of and estimate its temperature. But for more precise temperature
But for more precise temperature
But for more precise temperature readings, they must take a
readings, they must take a
readings, they must take a closer look at the energy levels
closer look at the energy levels
closer look at the energy levels of atoms.
When astronomers first
When astronomers first
When astronomers first classified stars using
classified stars using
classified stars using spectroscopy, they needed to
spectroscopy, they needed to
spectroscopy, they needed to understand the role that
understand the role that
understand the role that temperature played in the
temperature played in the
temperature played in the spectra.
spectra.
spectra. Temperature is a measurement of
Temperature is a measurement of
Temperature is a measurement of the speed of atoms.
the speed of atoms.
the speed of atoms. Simply put, the faster the atoms
Simply put, the faster the atoms
Simply put, the faster the atoms move in a material, the hotter
move in a material, the hotter
move in a material, the hotter it is.
it is.
it is. In our daily lives, we measure
In our daily lives, we measure
In our daily lives, we measure temperature using the scales of
temperature using the scales of
temperature using the scales of Fahrenheit and Celsius.
Fahrenheit and Celsius.
Fahrenheit and Celsius. In our warm world these scales
In our warm world these scales
In our warm world these scales work fine.
work fine.
work fine. But in the extremely cold vacuum
But in the extremely cold vacuum
But in the extremely cold vacuum of space and for the raging heat
of space and for the raging heat
of space and for the raging heat of stars, a better scale is
of stars, a better scale is
of stars, a better scale is needed.
needed.
needed. Astronomers use the Kelvin
Astronomers use the Kelvin
Astronomers use the Kelvin temperature scale, which is
temperature scale, which is
temperature scale, which is proportional to the movement of
proportional to the movement of
proportional to the movement of atoms.
atoms.
atoms. There are no minus degrees below
There are no minus degrees below
There are no minus degrees below zero.
zero.
Instead, zero Kelvin, or zero.
Instead, zero Kelvin, or zero.
absolute zero, is the coldest zero.
absolute zero, is the coldest zero.
possible temperature, wherest zero.
possible temperature, wherest zero.
there is no motion of the atoms. zero.
there is no motion of the atoms. zero.
there is no motion of the atoms. zero. At their surfaces, stars range
At their surfaces, stars range
At their surfaces, stars range from about 25,000 to 30,000
from about 25,000 to 30,000
from about 25,000 to 30,000 Kelvins, equivalent to about
Kelvins, equivalent to about
Kelvins, equivalent to about 4,000 to 55,000 degrees
4,000 to 55,000 degrees
4,000 to 55,000 degrees Fahrenheit.
Fahrenheit.
Fahrenheit. The easiest way to measure a
The easiest way to measure a
The easiest way to measure a star's temperature precisely is
star's temperature precisely is
star's temperature precisely is by observing the absorption
by observing the absorption
by observing the absorption lines of hydrogen in the visible
lines of hydrogen in the visible
lines of hydrogen in the visible part of the spectrum.
part of the spectrum.
These are called the Balmer part of the spectrum.
These are called the Balmer part of the spectrum.
Tseries.e called the Balmer part of the spectrum.
Tseries.e called the Balmer part of the spectrum.
>> There are essentially two part of the spectrum.
>> There are essentially two part of the spectrum.
different ways of determining part of the spectrum.
different ways of determining part of the spectrum.
the temperature of a star:ing part of the spectrum.
the temperature of a star:ing part of the spectrum.
One methodature of a star:ing part of the spectrum.
part of the spectrum.
is photometrically.m.

A blue star is hot;
A blue star is hot;
A blue star is hot; a red star
a red star
a red star is much cooler.
is much cooler.
is much cooler. So the colors of the stars are
So the colors of the stars are
So the colors of the stars are indicative of temperatures as
indicative of temperatures as
indicative of temperatures as they are for a heated piece of
they are for a heated piece of
they are for a heated piece of metal.
metal.
metal. There's a second way of
There's a second way of
There's a second way of determining temperatures of
determining temperatures of
determining temperatures of stars which is actually much
stars which is actually much
stars which is actually much more precise and gives a lot
more precise and gives a lot
more precise and gives a lot more information, and this is
more information, and this is
more information, and this is spectroscopically.
spectroscopically.
>> narrator: Since temperature spectroscopically.
>> narrator: Since temperature spectroscopically.
is related to energy, theature spectroscopically.
is related to energy, theature spectroscopically.
temperature of the gas willure spectroscopically.
temperature of the gas willure spectroscopically.
affect its spectrum.as willure spectroscopically.
affect its spectrum.as willure spectroscopically.
Only hydrogen atoms withillure spectroscopically.
Only hydrogen atoms withillure spectroscopically.
electrons in the second energy spectroscopically.
spectroscopically.
level can absorb the photons

which create the Balmer series.

Collisions among the atoms in

stars of medium temperature

place the greatest number of

electrons in the second level,

so the Balmer lines will be

the strongest in these stars.

A very hot star will excite its

atoms so much that most of their

electrons will be higher than

the second level, so the

Balmer lines appear much weaker.

And in very cool stars where

most of the electrons are below

the second level, the Balmer

lines will also appear weak or

invisible.

So it is with these lines that

astronomers can very accurately

determine the temperature of a

star.

>> The strongest absorption
>> The strongest absorption
>> The strongest absorption lines in stars of intermediate
lines in stars of intermediate
lines in stars of intermediate temperature, say about 10,000
temperature, say about 10,000
temperature, say about 10,000 Kelvin, those are the Balmer
Kelvin, those are the Balmer
Kelvin, those are the Balmer lines of hydrogen.
lines of hydrogen.
lines of hydrogen. So when one obtains spectra of
So when one obtains spectra of
So when one obtains spectra of stars, one can classify those
stars, one can classify those
stars, one can classify those stars, and this was first done
stars, and this was first done
stars, and this was first done in a rather empirical way, and
in a rather empirical way, and
in a rather empirical way, and the main criteria that was used
the main criteria that was used
the main criteria that was used initially was the strength of
initially was the strength of
initially was the strength of the Balmer lines, which are very
the Balmer lines, which are very
the Balmer lines, which are very conspicuous.
conspicuous.
conspicuous. >> narrator: By the late 19th
>> narrator: By the late 19th
>> narrator: By the late 19th century, astronomers had
century, astronomers had
century, astronomers had photographed the spectra of many
photographed the spectra of many
photographed the spectra of many stars.
stars.
stars. Although they were all dark-line
Although they were all dark-line
Although they were all dark-line spectra like our sun's, they
spectra like our sun's, they
spectra like our sun's, they showed a bewildering variety of
showed a bewildering variety of
showed a bewildering variety of different patterns.
different patterns.
different patterns. It was thought at first that
It was thought at first that
It was thought at first that stars must vary widely in their
stars must vary widely in their
stars must vary widely in their chemical composition.
chemical composition.
>> There was a very famous chemical composition.
>> There was a very famous chemical composition.
group of women who worked at the chemical composition.
group of women who worked at the chemical composition.
Harvard College Observatory whoe chemical composition.
Harvard College Observatory whoe chemical composition.
were responsible for many of the chemical composition.
were responsible for many of the chemical composition.
things that we still use today.e chemical composition.
things that we still use today.e chemical composition.
things that we still use today.e chemical composition. Annie Jump Cannon, who
Annie Jump Cannon, who
Annie Jump Cannon, who classified 400,000 or more
classified 400,000 or more
classified 400,000 or more stars.
stars.
stars. I mean, that's just amazing.
I mean, that's just amazing.
I mean, that's just amazing. She did it by going through
She did it by going through
She did it by going through one by one and classifying
one by one and classifying
one by one and classifying them, and she was extremely good
them, and she was extremely good
them, and she was extremely good at it and extremely accurate.
at it and extremely accurate.
at it and extremely accurate. >> narrator: What Annie Jump
>> narrator: What Annie Jump
>> narrator: What Annie Jump Cannon had done was to build the
Cannon had done was to build the
Cannon had done was to build the first complete classification of
first complete classification of
first complete classification of the stars.
the stars.
But it would take another woman the stars.
But it would take another woman the stars.
astronomer in a field thenwoman the stars.
astronomer in a field thenwoman the stars.
dominated by men to link thisan the stars.
dominated by men to link thisan the stars.
classification to stellarthisan the stars.
classification to stellarthisan the stars.
temperature.on to stellarthisan the stars.
temperature.on to stellarthisan the stars.
temperature.on to stellarthisan the stars. She would show that stars were
She would show that stars were
She would show that stars were nearly identical in their
nearly identical in their
nearly identical in their composition, being mostly
composition, being mostly
composition, being mostly hydrogen and helium, and that
hydrogen and helium, and that
hydrogen and helium, and that the different patterns were
the different patterns were
the different patterns were actually caused by stars varying
actually caused by stars varying
actually caused by stars varying widely in surface temperature.
widely in surface temperature.
widely in surface temperature. >> Cecilia Payne Gaposchkin, who
>> Cecilia Payne Gaposchkin, who
>> Cecilia Payne Gaposchkin, who was responsible for, really,
was responsible for, really,
was responsible for, really, most of our understanding of the
most of our understanding of the
most of our understanding of the fact that when we classify
fact that when we classify
fact that when we classify stars, what we're really doing
stars, what we're really doing
stars, what we're really doing is finding a temperature
is finding a temperature
is finding a temperature sequence, that when we look
sequence, that when we look
sequence, that when we look at--originally, of course, as
at--originally, of course, as
at--originally, of course, as with any kind of taxonomy, if
with any kind of taxonomy, if
with any kind of taxonomy, if you're trying to classify
you're trying to classify
you're trying to classify things, you put them in boxes.
things, you put them in boxes.
things, you put them in boxes. And so the way it was originally
And so the way it was originally
And so the way it was originally done was to look at a spectrum
done was to look at a spectrum
done was to look at a spectrum and say, "This spectrum has very
and say, "This spectrum has very
and say, "This spectrum has very strong hydrogen lines."
strong hydrogen lines."
strong hydrogen lines." And they would put it in box A
And they would put it in box A
And they would put it in box A because it was the strongest.
because it was the strongest.
because it was the strongest. Then they would look at the
Then they would look at the
Then they would look at the next one.
next one.
next one. "Well, these aren't quite as
"Well, these aren't quite as
"Well, these aren't quite as strong, so we'll put them in box
strong, so we'll put them in box
strong, so we'll put them in box B."
B."
B." And they would continue to
And they would continue to
And they would continue to classify, and they would find
classify, and they would find
classify, and they would find different spectral lines, and,
different spectral lines, and,
different spectral lines, and, "Okay, this one looks like this
"Okay, this one looks like this
"Okay, this one looks like this one; they must be the same kind.
one; they must be the same kind.
one; they must be the same kind. We'll put them in the same box."
We'll put them in the same box."
We'll put them in the same box." So they literally had a
So they literally had a
So they literally had a classification scheme which was
classification scheme which was
classification scheme which was nothing more than alphabet: A,
nothing more than alphabet: A,
nothing more than alphabet: A, B, C.
B, C.
B, C. And Cecilia Payne Gaposchkin
And Cecilia Payne Gaposchkin
And Cecilia Payne Gaposchkin came along and realized that, in
came along and realized that, in
came along and realized that, in fact, what you were looking at
fact, what you were looking at
fact, what you were looking at was a temperature sequence,
was a temperature sequence,
was a temperature sequence, because as you go from cool
because as you go from cool
because as you go from cool stars to hot stars you change
stars to hot stars you change
stars to hot stars you change the types of spectral lines that
the types of spectral lines that
the types of spectral lines that you can observe, because the
you can observe, because the
you can observe, because the atoms are in different states.
atoms are in different states.
atoms are in different states. She actually defined for us
She actually defined for us
She actually defined for us what the whole spectral
what the whole spectral
what the whole spectral classification thing was about.
classification thing was about.
classification thing was about. It really was a temperature
It really was a temperature
It really was a temperature sequence, and it gave us the
sequence, and it gave us the
sequence, and it gave us the perfect way to actually measure
perfect way to actually measure
perfect way to actually measure the temperatures of stars.
the temperatures of stars.
the temperatures of stars. And so, essentially, instead of
And so, essentially, instead of
And so, essentially, instead of being A, B, et cetera, it became
being A, B, et cetera, it became
being A, B, et cetera, it became O, B, A, F, G, K, M, which is,
O, B, A, F, G, K, M, which is,
O, B, A, F, G, K, M, which is, of course, the classification
of course, the classification
of course, the classification we use today,
we use today,
we use today, going from hottest--the O
going from hottest--the O
going from hottest--the O stars--to coolest--the M stars.
stars--to coolest--the M stars.
>> narrator: So based on the stars--to coolest--the M stars.
>> narrator: So based on the stars--to coolest--the M stars.
appearance of a star's spectrum, stars--to coolest--the M stars.
appearance of a star's spectrum, stars--to coolest--the M stars.
astronomers can place it intoum, stars--to coolest--the M stars.
astronomers can place it intoum, stars--to coolest--the M stars.
one of sevencspectral classes,m, stars--to coolest--the M stars.
one of sevencspectral classes,m, stars--to coolest--the M stars.
each one with a corresponding,m, stars--to coolest--the M stars.
each one with a corresponding,m, stars--to coolest--the M stars.
temperature.h a corresponding,m, stars--to coolest--the M stars.
stars--to coolest--the M stars.
stars--to coolest--the M stars. Astronomers have refined the
Astronomers have refined the
Astronomers have refined the seven spectral classes by
seven spectral classes by
seven spectral classes by dividing each into ten
dividing each into ten
dividing each into ten sub-classes to reveal the
sub-classes to reveal the
sub-classes to reveal the stellar temperature of the star
stellar temperature of the star
stellar temperature of the star within 5%, a much more accurate
within 5%, a much more accurate
within 5%, a much more accurate measurement of temperature than
measurement of temperature than
measurement of temperature than just observing the color of
just observing the color of
just observing the color of stars.
stars.
stars. But the spectra of stars have
But the spectra of stars have
But the spectra of stars have even more information to share.
Everything in the universe is in
Everything in the universe is in
Everything in the universe is in motion, even you.
motion, even you.
motion, even you. As you view this program, you
As you view this program, you
As you view this program, you are riding on the Earth as it
are riding on the Earth as it
are riding on the Earth as it spins on its axis, revolving
spins on its axis, revolving
spins on its axis, revolving around a star which is orbiting
around a star which is orbiting
around a star which is orbiting the center of the galaxy that is
the center of the galaxy that is
the center of the galaxy that is traveling in a universe that is
traveling in a universe that is
traveling in a universe that is expanding.
expanding.
expanding. Motion is everywhere.
Motion is everywhere.
Motion is everywhere. Because of this, motion is an
Because of this, motion is an
Because of this, motion is an important quantity that
important quantity that
important quantity that astronomers seek out in their
astronomers seek out in their
astronomers seek out in their observations.
observations.
observations. With spectroscopy, they can tell
With spectroscopy, they can tell
With spectroscopy, they can tell not only if an object is moving
not only if an object is moving
not only if an object is moving toward or away from the
toward or away from the
toward or away from the observer but if it's spinning
observer but if it's spinning
observer but if it's spinning or expanding or orbiting
or expanding or orbiting
or expanding or orbiting another object.