Astronomy - spectroscopy - 3/3


Uploaded by rhcrcgvp on 11.07.2009

Transcript:
>> Once we take a spectrum of an
>> Once we take a spectrum of an
>> Once we take a spectrum of an astronomical object, besides
astronomical object, besides
astronomical object, besides disentangling these patterns,
disentangling these patterns,
disentangling these patterns, figuring out what the object is
figuring out what the object is
figuring out what the object is made out of, there's also a lot
made out of, there's also a lot
made out of, there's also a lot of useful information because of
of useful information because of
of useful information because of the Doppler effect.
the Doppler effect.
>> narrator: The Doppler effect the Doppler effect.
>> narrator: The Doppler effect the Doppler effect.
is the apparent change ineffect the Doppler effect.
is the apparent change ineffect the Doppler effect.
wavelength of a wave, depending the Doppler effect.
wavelength of a wave, depending the Doppler effect.
on the motion of the sourceding the Doppler effect.
on the motion of the sourceding the Doppler effect.
toward or away from an observer. the Doppler effect.
toward or away from an observer. the Doppler effect.
toward or away from an observer. the Doppler effect. Astronomers observe this effect
Astronomers observe this effect
Astronomers observe this effect with wavelengths of light, but
with wavelengths of light, but
with wavelengths of light, but we experience it with sound as
we experience it with sound as
we experience it with sound as well.
well.
>> If you've ever stood there on well.
>> If you've ever stood there on well.
a street corner, and you'vere on well.
a street corner, and you'vere on well.
heard an ambulance go by, the on well.
heard an ambulance go by, the on well.
first thing you hear is a veryon well.
first thing you hear is a veryon well.
high-pitched noise as it'sveryon well.
high-pitched noise as it'sveryon well.
coming from your left andsveryon well.
well.
passing by you, and as it passes

by you, what you hear are lower

frequencies, a very low-pitched

sound.

So what is happening in that

instance is, as the ambulance is

coming toward you, the sound

waves that are getting

transmitted by the horn on the

ambulance--the sound waves are

compressed.

As it goes away from you, those

sound fronts, you could say, get

stretched out, so it corresponds

to a longer wavelength and a

lower pitch.

So you hear a lower sound.

The same principle can be

applied to light.

If you have something coming

towards you, you will see the

spectrum get shifted but toward

shorter wavelengths, and those

shorter wavelengths are mainly

blue wavelengths, so they get

shifted, they say, toward the

blue.

If something--and this happens

often in astronomy--if something

is going away from you, it's

going to get shifted toward

longer wavelengths, and that's

mostly toward the red.

>> narrator: These shifts are

classified by the direction they

move within the spectrum.

Objects moving away from us

exhibit shifts to the longer

wavelengths.

We call them redshifts.

Objects moving toward us

exhibit shifts to the shorter

wavelengths and are called

blueshifts.

>> I think the Doppler effect is
>> I think the Doppler effect is
>> I think the Doppler effect is one of the most important things
one of the most important things
one of the most important things in astronomy, because it
in astronomy, because it
in astronomy, because it enables you to say something
enables you to say something
enables you to say something about the radiant motion of
about the radiant motion of
about the radiant motion of celestial objects, whether they
celestial objects, whether they
celestial objects, whether they move towards you or away from
move towards you or away from
move towards you or away from you, and the basic concept
you, and the basic concept
you, and the basic concept behind it, I believe, is very
behind it, I believe, is very
behind it, I believe, is very simple.
simple.
simple. It has to do with the fact that
It has to do with the fact that
It has to do with the fact that light is electromagnetic waves.
light is electromagnetic waves.
light is electromagnetic waves. Now, with a specific wavelength
Now, with a specific wavelength
Now, with a specific wavelength and a color of the light
and a color of the light
and a color of the light corresponds to the wavelengths
corresponds to the wavelengths
corresponds to the wavelengths of these waves.
of these waves.
of these waves. So a red light, for instance,
So a red light, for instance,
So a red light, for instance, has very long wavelengths; blue
has very long wavelengths; blue
has very long wavelengths; blue light has short wavelengths.
light has short wavelengths.
light has short wavelengths. Now, the spectra lines, as I
Now, the spectra lines, as I
Now, the spectra lines, as I said, are in characteristic
said, are in characteristic
said, are in characteristic locations in the spectrum.
locations in the spectrum.
locations in the spectrum. That means they have exact
That means they have exact
That means they have exact characteristic wavelengths.
characteristic wavelengths.
characteristic wavelengths. So if a body is not in motion, I
So if a body is not in motion, I
So if a body is not in motion, I find these spectrum lines at
find these spectrum lines at
find these spectrum lines at exact locations in the
exact locations in the
exact locations in the spectrum, which corresponds to
spectrum, which corresponds to
spectrum, which corresponds to the laboratory wavelengths of
the laboratory wavelengths of
the laboratory wavelengths of those element spectra lines.
those element spectra lines.
those element spectra lines. Now, wavelengths of light,
Now, wavelengths of light,
Now, wavelengths of light, emitted light, can change if a
emitted light, can change if a
emitted light, can change if a body moves relative to me.
body moves relative to me.
body moves relative to me. For instance, the wavelengths
For instance, the wavelengths
For instance, the wavelengths become longer if the body moves
become longer if the body moves
become longer if the body moves away from me.
away from me.
away from me. And they become shorter if the
And they become shorter if the
And they become shorter if the radiating body comes towards me.
radiating body comes towards me.
>> narrator: An object that is radiating body comes towards me.
>> narrator: An object that is radiating body comes towards me.
rotating exhibits yet anothers radiating body comes towards me.
rotating exhibits yet anothers radiating body comes towards me.
effect.g exhibits yet anothers radiating body comes towards me.
effect.g exhibits yet anothers radiating body comes towards me.
As it spins, one part isothers radiating body comes towards me.
As it spins, one part isothers radiating body comes towards me.
moving toward the observer,ers radiating body comes towards me.
moving toward the observer,ers radiating body comes towards me.
while another part of that same radiating body comes towards me.
radiating body comes towards me.
object is moving away.owards me.

A single spectral line will get

both redshifted and blueshifted,

which will broaden it, smearing

it out in wavelengths.

>> If that absorption line is

smeared out, you know that that

star must be rapidly spinning.

You have a Doppler effect that
You have a Doppler effect that
You have a Doppler effect that shifts some of that light into
shifts some of that light into
shifts some of that light into the blue, some of that light to
the blue, some of that light to
the blue, some of that light to the red and effectively smears
the red and effectively smears
the red and effectively smears out that absorption line.
out that absorption line.
out that absorption line. The Doppler effect also allows
The Doppler effect also allows
The Doppler effect also allows us to detect and identify binary
us to detect and identify binary
us to detect and identify binary stars: two stars physically
stars: two stars physically
stars: two stars physically bound together by their mutual
bound together by their mutual
bound together by their mutual gravitation.
gravitation.
gravitation. What you see when you observe
What you see when you observe
What you see when you observe it--you might not see both
it--you might not see both
it--you might not see both objects.
objects.
objects. They're so far away that you
They're so far away that you
They're so far away that you can't resolve them.
can't resolve them.
can't resolve them. But you see them as one object.
But you see them as one object.
But you see them as one object. But when you take a spectrum of
But when you take a spectrum of
But when you take a spectrum of that light, you see, now,
that light, you see, now,
that light, you see, now, there's actually two.
there's actually two.
there's actually two. You see a hydrogen line that's
You see a hydrogen line that's
You see a hydrogen line that's shifting back and forth.
shifting back and forth.
shifting back and forth. In fact, you see two hydrogen
In fact, you see two hydrogen
In fact, you see two hydrogen lines--one from each star--and
lines--one from each star--and
lines--one from each star--and they're moving back and forth in
they're moving back and forth in
they're moving back and forth in a very periodic fashion because
a very periodic fashion because
a very periodic fashion because of the Doppler effect.
of the Doppler effect.
of the Doppler effect. And from that, you can deduce
And from that, you can deduce
And from that, you can deduce that that's not a single object;
that that's not a single object;
that that's not a single object; it's actually two objects: a
it's actually two objects: a
it's actually two objects: a spectroscopic binary.
spectroscopic binary.
spectroscopic binary. >> narrator: The Doppler effect
>> narrator: The Doppler effect
>> narrator: The Doppler effect is also helping us find other
is also helping us find other
is also helping us find other worlds around distant stars.
worlds around distant stars.
>> My collaborator Paul Butler worlds around distant stars.
>> My collaborator Paul Butler worlds around distant stars.
and I dreamed up the idea ofer worlds around distant stars.
and I dreamed up the idea ofer worlds around distant stars.
discovering planets around other worlds around distant stars.
discovering planets around other worlds around distant stars.
stars back in the mid-1980sother worlds around distant stars.
stars back in the mid-1980sother worlds around distant stars.
before any planets were known.er worlds around distant stars.
before any planets were known.er worlds around distant stars.
>> narrator: What Butler andn.er worlds around distant stars.
worlds around distant stars.
Marcy had realized is that the

gravitational tugs that planets

have on their stars would be

revealed in the star's spectra

in the form of Doppler shifts.

>> People, indeed, thought we
>> People, indeed, thought we
>> People, indeed, thought we were crazy to imagine that we
were crazy to imagine that we
were crazy to imagine that we could discover planets around
could discover planets around
could discover planets around other stars.
other stars.
other stars. If we could just detect the
If we could just detect the
If we could just detect the wobbles of stars precisely
wobbles of stars precisely
wobbles of stars precisely enough, then we could see the
enough, then we could see the
enough, then we could see the effects on those stars due to
effects on those stars due to
effects on those stars due to the planets.
the planets.
the planets. It wasn't good enough just to
It wasn't good enough just to
It wasn't good enough just to measure the Doppler shift of a
measure the Doppler shift of a
measure the Doppler shift of a star.
star.
star. We had to measure the speed of a
We had to measure the speed of a
We had to measure the speed of a star to within about plus or
star to within about plus or
star to within about plus or minus 3 meters per second.
minus 3 meters per second.
minus 3 meters per second. If you think about 3 meters,
If you think about 3 meters,
If you think about 3 meters, that's about three good
that's about three good
that's about three good strides that you can walk, and
strides that you can walk, and
strides that you can walk, and that's eventually what
that's eventually what
that's eventually what we accomplished, by the end of
we accomplished, by the end of
we accomplished, by the end of '95.
'95.
'95. >> narrator: The simple act of
>> narrator: The simple act of
>> narrator: The simple act of letting Sunlight pass through
letting Sunlight pass through
letting Sunlight pass through a prism reveals a lovely array
a prism reveals a lovely array
a prism reveals a lovely array of colors.
of colors.
of colors. This same process is what
This same process is what
This same process is what creates rainbows.
creates rainbows.
creates rainbows. Underneath that beauty lies some
Underneath that beauty lies some
Underneath that beauty lies some of the most fundamentally
of the most fundamentally
of the most fundamentally important processes in all of
important processes in all of
important processes in all of physics.
physics.
physics. The spectrum of colors does more
The spectrum of colors does more
The spectrum of colors does more than just appeal to our
than just appeal to our
than just appeal to our aesthetic sense; it also tells
aesthetic sense; it also tells
aesthetic sense; it also tells us about the nature of our star,
us about the nature of our star,
us about the nature of our star, the Sun.
the Sun.
the Sun. By breaking up the light from
By breaking up the light from
By breaking up the light from distant stars and clouds of gas,
distant stars and clouds of gas,
distant stars and clouds of gas, astronomers can determine their
astronomers can determine their
astronomers can determine their temperature, their density,
temperature, their density,
temperature, their density, their velocity, and even their
their velocity, and even their
their velocity, and even their composition, all without ever
composition, all without ever
composition, all without ever once coming in direct contact.
once coming in direct contact.
once coming in direct contact. A spectrum can yield all that
A spectrum can yield all that
A spectrum can yield all that information and more.
information and more.
information and more. It is a simple irony of nature
It is a simple irony of nature
It is a simple irony of nature that we can learn so much from
that we can learn so much from
that we can learn so much from so little.
so little.
so little. It is a further irony that we
It is a further irony that we
It is a further irony that we learn about some of the largest
learn about some of the largest
learn about some of the largest objects in our universe by
objects in our universe by
objects in our universe by studying the smallest.
studying the smallest.
studying the smallest. But each atom sends us a
But each atom sends us a
But each atom sends us a signal, a tiny pulse of light,
signal, a tiny pulse of light,
signal, a tiny pulse of light, and the study of that light
and the study of that light
and the study of that light reveals the nature of that
reveals the nature of that
reveals the nature of that atom and the planet, star, gas,
atom and the planet, star, gas,
atom and the planet, star, gas, and even the galaxy in which it
and even the galaxy in which it
and even the galaxy in which it resides.
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