This course is designed for anyone who is interested in learning more about modern astronomy. We will help you get up to date on the most recent astronomical discoveries while also providing support at an introductory level for those who have no background in science.
9. Spectra
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来自 University of Arizona 的课程
天文学：探索时间和空间
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Matter and Radiation
Astronomers harvest information across the electromagnetic spectrum, using spectra to measure the composition of distant objects and diagnose extreme physical conditions.
与讲师见面
Chris ImpeyDistinguished Professor
Astronomy
Some of the most important information in astronomy comes from the technique of
spectroscopy, which involves dispersing light into its constituent wavelengths,
as Newton did with his prism hundreds of years ago.
Looking at an image of this star cluster, all we see is the stars.
Maybe we can measure their colors, but there's little extra information here.
Look instead at this star cluster where we can see the spectra of
every star within it.
From these spectra, in work first attempted a hundred years ago,
we can learn not only the temperature of the stars, but
also their density, their composition, and parts of their life stories.
Spectroscopy gives us a lot more information than just taking an image.
It is one of the most powerful tools of astronomy.
It gives us remote sensing for physical conditions far away, millions or
even billions of light-years away.
To talk about spectroscopy,
we have to look at the way light interacts with matter.
Essentially everything we know in astronomy comes from
the interactions of light or other forms of electromagnetic radiation with matter.
We are using remote sensing to learn about distance objects based on
the radiation they emit or absorb.
In principle, light can be emitted, absorbed, scattered, or
reflected from material and material objects, like stars.
We use terminology like transparent or opaque.
A gas or an object that is invisible to radiation or
where radiation cannot be transmitted is considered opaque.
An object where radiation is freely transmitted without interaction is
considered transparent.
We're familiar with some of these interactions in our everyday life.
Its important to be clear about the distinction between the apparent and
intrinsic properties of something.
When you look at something like a wall or a sweater that appears to be red,
that's not because that's thermal radiation from the object reflecting its
temperature, that's because of a pigment or dye in the paint or the, or
the dye used for the cloth, and it's what that radiation from the sun or
from an overhead light reflects and absorbs.
An object that appears red in that sense, it's just reflecting the red part of
the visible or white light to you while absorbing the blue light.
That's quite different from the thermal radiation,
where a blue object is a hotter object, and a red object is a cooler object.
To dig deeper into the interaction between light and
matter, we have to look at how light interacts with individual atoms.
It was learned,
about 100 years ago that the atom does not have a continuous set of energy states.
It has a quantized set of energy states.
This is the basis of the so called quantum theory of atomic structure.
It was first understood by Bohr, Heisenberg, and
others, in the early 20th century.
In other words, atoms cannot have any energy.
And they cannot exchange energy in any amount.
They can only absorb or emit particular amounts of energy corresponding to
their particular energy levels, the quantized energy levels of an atom.
Every single unique element with a unique number of protons and
electrons has a unique set of energy levels.
Then mark it out.
It's like the fingerprint of an atom, and it of course means they
have a spectral fingerprint as well, or a set of photons that can be emitted or
absorbed that's completely unique to each chemical element.
Taking the simplest element, hydrogen, it has a set of
energy transitions corresponding to energy that it can lose or gain.
And these energies map, by the electromagnetic spectrum,
into particular wavelengths or frequencies of light.
So hydrogen, if it's excited or if it absorbs energy,
has a particular fingerprint of possible transitions, it's spectrum.
This was first seen over a hundred years ago.
We can take a Bunsen burner and heat up chemical elements and
then disperse the light that comes off, or is emitted by, those elements with a prism
and we will see these particular emission lines or sharp spectral features of
particular wavelengths and they are always unique to the element.
In these examples, we see the uniqueness of this imprint.
Hydrogen, helium, and heavier elements like neon and
mercury have particular sets of transitions.
The appearance of the spectra from these objects to the naked eye is based on
the combination of all these ingredients,
because the eye does not disperse into a spectrum.
But in the case of neon, we see that a lot of the energy comes in
two particularly intense lines in the red or orange part of the spectrum,
which gives neon signs their purity of their red characteristic.
Other elements like helium have spectral lines across the blue, red, and
yellow parts of the spectrum.
And so it made it a duller glow of yellow or pale yellow light.
Regardless of the particular characteristics,
the fact that each element has a unique spectral signature means that if you
have a combination of these gases, those elements just add.
If you have a mixture of hydrogen and
helium contained within a gassy experiment.
You'll see the lines corresponding to the sum of both the helium lines and
the hydrogen lines superimposed on each other.
Working this logic backwards, that means if we had an unknown gas and
disbursed it's light into a spectrum, we could diagnose the chemical ingredients.
Spectroscopy actually allows us to go further.
If we have good enough spectroscopy, we could actually look at the profile or
shape of those sharp spectral features and infer the temperature or
the density of that hot gas.
And that's how we learn about distant stars,
the spectral fingerprint is an extraordinary sensitive technique.
In astrophysicists hands, we can detect the presence of trace elements in a star
at constitution of one part in the trillion, relative to hydrogen or
helium, extraordinary trace elements.
All this information can be combined to say what the universe is made of in terms
of constitutes in the periodic table, and their abundance relative to hydrogen and
helium, the most abundant elements.
In astronomy, we tend to see three different types of spectra.
One is the spectrum emitted by a hot gas, such as a discharge tube.
This is where the gas is excited and emits the narrow,
sharp spectral features, corresponding to it's element.
This is called an emission spectrum.
We can also see the smooth spectrum that just results from the random motions of
atoms or molecules in a gas, and this is the smooth thermal spectrum,
which would peak at optical wavelengths for a hot object like the sun, and
at infrared wavelengths for a cool object like a planet.
The third type of spectrum we can see is when a hot source has a cooler envelope of
gas outside it.
In this case, the high excited transitions are absorbed by the cooler gas, but
again with the characteristic fingerprint of the element that gas is made of.
This is actually what the sun spectrum looks like.
Because the hot fusion core is surrounded by cooler outer layers.
These three types of spectra to astronomers are called
a continuous spectrum, the smooth thermal spectrum of a hot object,
an emission spectrum from a hot excited, rarefied gas, and an absorption spectrum
where the hot excited gas is surrounded by cooler material.
The rules by which these three spectra are produced are called Kirchoff's laws.
Another very important attribute of radiation and
astronomy that's diagnosed by spectroscopy is the Doppler Effect.
The Doppler Effect is familiar to us from sound ways, but
it applies to any form of wave.
In a Doppler Effect a source of sound waves,
as those waves compressed in the direction of its motion.
When the ambulance approaches you, it's catching up with the waves that it emits,
compressing their frequency and raising the pitch.
And so you hear the rising pitch of the ambulance.
When it's receding from you, it's moving away from its waves, stretching out their
wavelength or pitch or lowering it, and you hear the lowering tone.
That's a very characteristic phenomena for sound waves, but
the same thing happens with light waves or any electromagnetic wave, as well.
And so in terms of light waves, the Doppler effect corresponds to
the fact that if a source of radiation is moving away from us,
the wavelength is shifted towards the red compared to that source being stationary.
And if that source if moving towards us the waves are squashed or
compressed or shortened compared to that source being at rest.
So we have red shift for
objects moving away from us and blue shift for them moving towards us.
The percentage of the shift equals the velocity of
the motion relative to the speed of light.
Since the speed of light is a very large number, 300,000 kilometers per second, and
the motions of most astronomical objects are much smaller, maybe tens or
hundreds kilometers per second,
a Doppler shift is usually a rather small amount, small fractions of a percent.
Even though it's a small effect, spectroscopy is such a powerful technique
that it's easy to detect tiny fractional shifts corresponding to this effect.
So we can easily map the motions of stars, or
even entire galaxies, using the Doppler effect.
Notice that the Doppler effect cannot tell us about the transverse motion of
an object when it's just tracking on the sky in parallel to us.
It only talks about the component of its motion towards or away from us.
Therefore, to reconstruct the full three dimensional space motion of an object,
we don't have sufficient information.
All we are getting is one component or vector of the information towards or
away from us.
Almost everything we know about the Universe comes from
the interactions between radiation, or light, and matter.
Light can be emitted, absorbed, scattered or reflected.
Spectroscopy is the powerful technique that astronomers use to
diagnose the distant Universe based on dispersing light into a spectrum, and
looking at the details of what's going on in the emission from an object.
Hot astronomical objects show characteristic spectral fingerprints which
tell us their chemical constituents, what ingredients they're made of.
This technique is extraordinarily powerful and sensitive.
We can detect the traces of high,
heavy elements at a level of one part in a trillion.
Relative to the hydrogen and
helium that are most abundant elements in the universe.
We can even use the shape of the spectral lines to talk about the temperature and
the density of a distant star, or even a galaxy.
Remote sensing, or the use of spectroscopy to diagnose a psychical conditions of
the universe is perhaps the most powerful technique in astronomy.
We can also measure the Doppler shift or
the relative space motion of an object based on its ways of being compressed or
rarefied due to its relative motion with its respect to us.
