Chapters 5 & 6 – Electromagnetic Radiation
Almost all the information we have about the universe comes to us in the form of
electromagnetic radiation. Electromagnetic (EM) radiation that the human eye can
see is called light. Visible light is just a small part of the entire EM spectrum.
Imagine EM radiation as a wave, like a radio wave, or a microwave, or a light wave. The
wavelength determines what kind of wave it is.
Gamma rays (far left) have the shortest wavelength, which makes them the highest
energy. Radio waves (far right) have the longest wavelength, which makes them the
lowest energy. Shorter wavelengths have higher energies.
All EM waves travel at the same speed: the speed of light. EM waves from the sun
take 8 minutes to reach Earth. EM waves from the nearest star (Proxima Centauri) take
4.24 years to reach Earth, so Proxima Centauri is 4.24 light-years away. When we look
at the sky, we are looking into the past.
Our eyes are a kind of very small telescope which can only see visible light. The first
real telescopes – the kind Galileo built – were like having a very big eye. The lens of a
telescope focuses all the EM waves that hit it down to a very small focal point.
This focal point is where you usually put your eye, but modern telescopes put a
computer detector (called a CCD) at the focus. The CCD can look at many
wavelengths; it is not limited to visible light. You should know the difference between
refracting and reflecting telescopes; see page 78 for a nice diagram. Refracting
telescope lenses are expensive and show chromatic aberration.
All modern
telescopes are reflecting.
Because radio waves are so much longer than visible light waves, radio telescopes must
be much larger to achieve the same resolving power. (Compare the 1000-meter
Aricebo telescope to the 2.4-meter Hubble!) Scientists can link many telescopes
together to create one very large telescope; this is called interferometry. (Radio
telescopes “listen” to EM radio waves, not sound.)
Unfortunately, Earth’s atmosphere blocks most wavelengths from ever reaching Earth’s
surface. Only visible light and radio waves pass easily through our atmosphere. If
you want to study gamma rays, or microwaves, then you have to put your telescope high
on a mountaintop (above most of the atmosphere), or in space. Even visible light gets
mucked up by the atmosphere; the two Keck telescopes on Mauna Kea are 10-meters
each, but Hubble’s single 2.4-meter reflector gives clearer photos!
So modern telescopes are like giant eyes that can see all different kinds of EM radiation!
But what is out there to see? What information do we get from EM radiation?
Everything radiates EM waves; this is called blackbody radiation. Humans, at about
300 degrees Kelvin, radiate mostly infared waves. The Sun, at about 5800 degrees
Kelvin, radiates mostly visible light. (It is no surprise that our eyes evolved to see the
most common EM waves on Earth!) A 20,000 degree star would radiate mostly UV
waves. Wien’s Law says that hotter objects radiate higher energy waves, and the
Stefan-Boltzmann Law says hotter objects radiate more total energy.
But wait, what are all those dark bands in our spectrum? To know this, we need to learn
about atoms, and how they interact with photons (those waves of light).
An atom consists of electrons orbiting a central nucleus. Just like a hotel where you can
sleep on the ground floor or the fourth floor, an electron can exist at the ground orbital,
or the next orbital up, or the next orbital after that. But an electron cannot exist between
orbitals. Only an EM wave with the exactly perfect amount of energy can jump an
electron “up” an orbital. When the electron falls “down” an orbital, it releases an exact
amount of energy as an EM wave. (This is quantum mechanics!)
A spectrogram of a star will show all the EM waves, except a few will be missing. These
are called absorption lines because the missing waves have been absorbed by atoms.
By looking at the absorption lines, we can figure out which atoms are out there!
Similarly, sometimes cool clouds of gas will be very bright in a few, exact EM waves.
These emission spectra (emitted by electrons as they fall down from excited states)
also tell us what atoms are in the cloud. See pages 108-109 for nice diagrams.
Because hydrogen is so common, we have studied its absorption lines in detail.
Hydrogen absorption lines are called Balmer lines, and their intensity varies with the
temperature of the star (because of blackbody radiation). So the Balmer lines can tell us
how hot a star is; this is called the Balmer thermometer.
Many many stars’ EM spectra have been looked at, and stars are categorized by their
spectral class. From hottest to coldest, the classes are O, B, A, F, G. K, M. The Sun is
a G2 star, and is 5800 degrees Kelvin. O stars can be 50,000 degrees Kelvin!
If something emitting yellow light is moving towards you very fast, the light will appear to
be higher frequency (more blue-ish), and will be blue-shifted. If the object is moving
away, its emitted light appears lower frequency, and is red-shifted. This shifting of EM
waves from moving objects is called the Doppler Effect. The Doppler shift of a star can
tell you how fast it is moving towards or away from you. (It turns out everything we look
at is red-shifted, indicating that the entire universe is expanding! This is where the Big
Bang theory comes from.)
Homework
Read chapters 5 and 6
Page 97, review questions 3, 4, 10, 12, 13, 15, 17
Page 116-117, review questions 1, 8, 13, 15, 16