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.
2. Atoms
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From the course by University of Arizona
Astronomy: Exploring Time and Space
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From the lesson
Matter and Radiation
Astronomers harvest information across the electromagnetic spectrum, using spectra to measure the composition of distant objects and diagnose extreme physical conditions.
Meet the Instructors
Chris ImpeyDistinguished Professor
Astronomy
Matter is made of atoms.
This was a speculation as we've heard, by Democritus, 2,500 years ago,
but he didn't have any data or observation or experiment to support the hypothesis.
In the early 20th century, elegant experiments by Rutherford and
others showed that there were tiny, indivisible particles that we call atoms.
The structure of atoms was only unveiled by high energy physics experiments through
the middle part of the 20th century.
And in fact, if you asked the question, when was the first individual atom imaged,
that only happened within the last 20 years, but the evidence for
atoms is compelling.
Atoms are real and they compose everything in the universe.
In this series of zoomed images, we go in,
in orders of magnitude, starting with a familiar situation, grains of sand.
An individual grain of sand may be a millimeter,
down to a few tenths of a millimeter.
At 10 times zoom, we can see the individual grains of sand.
If we increase the zoom to a factor of a few hundred.
Now, the individual grains of sand start to take up the entire view.
And we're looking at the details of the mineral that compose sand.
We're still not able to see individual molecules, but large numbers of molecules.
Going in another order of magnitude, to a factor of 10 or 100,000.
The individual crystals now devolve into planes that correspond to
the molecular layers within the sand, which is silicon dioxide.
Another order of magnitude or
so and those individual molecular layers are now visible.
And going into the final level of zoom,
which is 100 million times, individual molecules are now visible.
Composed, each one, of three atoms.
This is the method that's only been developed and perfected in
the last few decade, that allows us to inspect matter at the atomic level.
What is the basic structure of matter, or structure of an atom?
In the 19th century, it was thought that atoms might be billiard balls or
bb's, tiny, indivisible particles, hard-edged.
But experiments by Rutherford and
others in the 20th century show that, that is not the case.
Rutherford's elegant experiment shows that atoms are mostly empty space.
When he fired charge particles through a thin gold foil, much to his surprise, most
of those charge particles pass straight through, as if the atoms weren't there.
And we know from his experiments that atoms are 99.999999999999% empty space.
Occasionally, a charged particle would violently recoil from
the nucleus of an atom.
And from that violent recoil, in the same beautiful set of experiments, Ruther con,
Rutherford concluded that the atomic nucleus is small,
dense, and charged from the protons within it.
The combined picture we get from these experiments is of a tiny atomic nucleus
housing 99.9% of the mass of every atom, and a diffuse cloud of
negatively charged electrons, look, occupying most of the volume.
Atoms are incredibly small.
If you put them edge to edge, a trillion would fit across the head of a pin.
A scale model that gives us a sense of the scale within the atom,
would be a football stadium, where a baseball sits at the 50 yard line.
That's the atomic nucleus.
And the electrons orbit in a cloud at the outer edge of
the stadium in the upper part of the stands.
That shows how much is empty space in the atom.
Where a tiny percentage of the mass occupies the large volume.
Most of the mass is in that baseball at the 50 yard line.
And then the next atom would be a football stadium away.
This is the emptiness of normal matter.
The fact that matter feels solid when you run into a wall or
step your foot on the ground is an illusion due to
the electrical force that keeps atoms apart in normal matter.
Other terminology that we need in terms of atoms
is the distinction between atomic number and atomic mass.
Each unique atomic number corresponds to an element, and
that is the number of protons in the nucleus.
One for hydrogen, two for helium, six for carbon and so on.
Atomic mass, however, involves the entire mass of the nucleus,
which is a combination of neutrons and protons.
Neutrons and
protons have about the same mass, but neutrons have no electrical charge.
The uniqueness of an element comes from an equal number of protons in the nucleus and
electrons surrounding it.
Neutrons do not play into the electrical force because they have no
electric charge.
We define an isotope of an element as a version of
that element with a different number of neutrons.
Remember, the varying number of neutrons does not affect the chemical behavior and
so it is same element, but it's an isotope because it has a different mass.
Some isotopes, with large numbers of neutrons relative to protons,
can be radioactive and decay.
Molecules are one or more atoms held together by the electrical force.
There are many types of molecules.
In the universe, over 120 different molecules have been detected,
mostly in cool, interstellar, space.
On Earth, because of our technology, tens of thousands,
perhaps millions of different molecules have been created in the lab.
A compound is a mixture of molecules not naturally forming.
Basically a mixture, whether or not held together by their electrical forces.
Organic compounds or
organic materials, are those that specifically involve the element carbon.
We've seen that the solidity of matter is essentially an illusion of
the electrical force.
These experiments, of course, were done over 100 years ago.
More recently with high energy physics experiments and
accelerators, we can pick apart atoms and show in detail how they're made.
The nucleus of an atom in particular,
is an impenetrable fortress held together by the electrical force,
one of the strongest forces of nature, far stronger than gravity.
So it takes extraordinary energies to break apart an atomic nucleus.
Much smaller amounts of energy required to take electrons from an atom, and
ionize it.
Since were dealing with extremely large numbers in this class, we'll use orders of
magnitude to characterize the number of atoms in different familiar situations.
Let's start with our grain of sand.
A grain of sand made of silicon dioxide contains roughly 10 to the 19 atoms.
That's 10 billion billion.
A human contains roughly a billion times more, the equivalent of a billion grains
of sand and most of humans are composed of water, H2O plus some carbon and nitrogen.
That total number of atoms is 10 to the power of 28.
A one with 28 zeros after it.
Taking the Sun as a typical star,
the number of atoms in a star is 10 to the power 57, a one with 57 zeros.
And in the case of stars like the Sun,
almost all of those atoms are the simplest two elements, hydrogen and helium.
Going up to the next scale of structure in the universe, we have galaxies.
A galaxy will typically contain about 100 billion stars.
And that corresponds to about 10 to the 69 atoms.
A phenomenal number, one with 69 zeros.
And perhaps the largest pure number in science,
is the number of atoms in the universe.
Composed of about 100 billion galaxies in the observable Universe.
That number is 10 to the 80, a one with 80 zeros after it.
Now, obviously, no one has ever counted that number of atoms, much less the number
of stars in any one galaxy, nor the exact number of galaxies in the universe.
This is how astronomers use estimation or projection or
extrapolation to derive overall numbers about the universe we live in.
To summarize atoms, the evidence for
atoms has become extraordinary in the last few decades.
We've been able to inspect within normal matter and see individual atoms and
make images of them.
There is no doubt that atoms exist.
We can understand the universe in term of the atoms it contains,
many of which are single atoms and some of which are composed into molecules.
Most of the universe is made of the simplest two possible atoms,
hydrogen and helium.
We can characterize the typical number of atoms in terrestrial everyday objects,
like human beings, or in stars, galaxies, and the universe itself.
Leading to one of the largest numbers in science, 10 to the power 80,
the number of atoms in the visible universe.
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