How Do the Properties of Light Help Us to Study Supernovae and Their Remnants?
| There are special
properties of light that we can take advantage of to understand even
objects that are millions and billions of light years away. In this
section we explore some of these properties and how we can use them to
understand our Universe. In the previous section of this unit, you were
told that superheated material created by the supernova explosion gives
off X-rays and gamma-rays. X-rays and gamma-rays are really just light
(electromagnetic radiation) that has very high energy. |
What is Electromagnetic (EM) Radiation?
Although it would seem that the human eye gives us a pretty accurate
view of the world, we are literally blind to much of what surrounds
us. A whole Universe of color exists, only a thin band of which our
eyes are able to detect; an example of this visible range of color is
the familiar rainbow (an example of a "spectrum"). The optical spectrum
ranges in color from reds and oranges up through blues and purples.
Each of these colors actually corresponds to a different energy of
light. The colors or energies of light that our eyes
cannot see also have names that are familiar to us.
We listen to radios, we eat food heated in microwaves, we have X-rays
taken of our broken bones. Yet many times we do not realize that
radio, X-ray, and microwave are really just different energies of light!
The entire range of energies of light, including both light we can see
and light we cannot see, is called the electromagnetic
spectrum. It includes, from highest energy to lowest: gamma-rays,
X-rays, ultraviolet, optical, infrared, microwaves, and radio waves.
Because light is something that is given off, or radiated from an object,
we can call it radiation. That's why we often talk about X-ray
radiation - it's the same thing as saying X-ray light. When we refer to
the whole spectrum of light, we can call it electromagnetic radiation.
Because we can see only visible light, we are put at a disadvantage
because the Universe is actively emitting light at all different
Light has different colors because it has different energies. This is
true whether we are talking about red and blue visible light, or
infrared (IR) and X-ray light. Of all the colors in the visible
spectrum, red light is the least energetic and blue is the most.
Beyond the red end of the visible part of the spectrum lie infrared
and radio light, both of which have lower energy than visible light.
Above the blue end of the visible spectrum lies the higher energies of
ultraviolet light, X-rays, and finally, gamma-rays.
What Units are Used to Characterize EM Radiation?
Light can be described not only in
terms of its energy, but also its wavelength, or its frequency. There is a
one-to-one correspondence between each of these representations.
X-rays and gamma-rays are usually described in terms of energy,
optical and infrared light in terms of wavelength, and radio in terms
of frequency. This is a scientific convention that allows the use of
the units that are the most convenient for describing whatever energy
of light you are looking at. For example, it would be inconvenient to
describe both low energy radio waves and high energy gamma-rays with
the same units because the difference in their energies is so great. A radio
wave can have an energy on the order of 4 x 10-10
eV as compared to 4 x 109 eV for gamma-rays.
That's an energy difference of 1019, or ten
million trillion eV!
Wavelength is the distance between two peaks of a wave, and it can be
measured with a base unit of meters (m) (such as centimeters, or
angstroms). Frequency is the number of cycles of a wave to pass some point
in a second. The basic unit of frequency is cycles per second, or Hertz
(Hz). Energy in astronomy is often measured in electron volts, or eV or
its multiples (such as kilo electron volts, or 1,000 eV)
Wavelength and frequency are related by the speed of light
(c), a fundamental constant. Energy
is also directly proportional to frequency (the constant of
proportionality is Planck's
constant, h) and inversely proportional to wavelength. It was Max Planck
who demonstrated that light sometimes
behaves as a particle by showing that its energy (E), divided by its
frequency (usually denoted using the Greek letter n) is a constant.
Since we know that frequency is equal to the speed of light (c)
divided by wavelength (the Greek letter l), we also know the
relationship between energy and wavelength. The energy (or wavelength or
frequency) of light can give important clues into how the light was
produced, and it is this characterization of light emission that allows us
to understand objects in the distant universe.
Since light can act like both a particle and a wave, we say that light
has a particle-wave duality. We call particles of light photons.
Low-energy photons (i.e. radio) tend to behave more like waves,
while higher energy photons (i.e. X-rays) behave more like particles.
This is an important difference because it affects the way we build
instruments to measure light (telescopes!).
You are familiar with light in many forms, like sunlight, which you see
every day. But how is this light created? Further, how can we use the
properties of light to understand objects in the Universe?
Observing Supernovae and Their Remnants at Different Energies
It pays to make multiple observations of astronomical objects bacause
they emit light of different energies. Supernovae remnants
can give off visible light, ultraviolet light, radio waves and X-rays.
Each observation of a supernovae remnant can give us different information
examine the Crab Nebula; it is unique in that it contains one of only a
pulsars that are observable at so many different energies.
The Crab Nebula's creation was witnessed in July of 1054 A.D. when
Chinese astronomers and members of the Native American Anasazi tribe
separately recorded the appearance of a new star. Although it was visible
for only a few months, it was bright enough to be seen even during the day!
In the 19th century, French comet hunter Charles Messier recorded a fuzzy
ball of light near the constellation Taurus. This fuzzy ball turned out
not to be a comet after all, but the remains of a massive star whose
explosive death had been witnessed centuries before by the Chinese and the
The location of the Crab Nebula (inset)
in the Milky Way Galaxy.
Scientists now believe the Crab Nebula is the remains of a star which
a supernova explosion. The core of the star collapsed and formed a
rapidly rotating, magnetic neutron
star, releasing energy sufficient to blast the
surface layers of the star into space with the strength of a 1028
megaton bomb or a hundred million nuclear warheads. Nestled in the nebulous
cloud of expelled gases, the rotating neutron star, or pulsar, continues to
generate strobe-like pulses that can be observed at radio, optical, and
X-ray energies. The Crab Nebula was one of the first sources of X-rays
identified in the early 1960s when the first X-ray astronomy observations
Crab Nebula in Radio
(Click on Crab
images for larger views)
wavelengths, the Crab Nebula, seen to the left,
displays two distinctive physical features. The nebulous regions hide radio
emission coming from unbound electrons spiraling around inside the nebula.
The pulsar at the heart of the Crab Nebula generates pulses at radio
frequencies roughly 60 times a second. In this image, the pulsar's flashes
are blurred together (since the image was "exposed" for much longer than
1/60 s) and it appears as the bright white spot near the middle of the nebula.
In the optical, both a web of filaments at the outer edges of the
nebula and a bluish core become apparent. The blue core is
from electrons within the nebula being deflected and accelerated by the
magnetic field of the
central neutron star. The red filaments surrounding the
edges of the nebula are the remnants of the original outer layers of the star.
Crab in optical
Crab in UV
In the ultraviolet (or UV) the nebula is slightly larger than when seen in
X-rays. Cooler electrons (responsible for the UV emission) extend out beyond
the hot electrons near the central pulsar. This supports the theory that the
central pulsar is responsible for energizing the electrons.
observations reveal a condensed core near the central pulsar,
which is the bright dot visible slightly left and below center in the image
to the right. The Crab Nebula appears smaller and more condensed in X-rays
because the electrons which are primarily responsible for the X-ray emission
exist only near the central pulsar. Scientists believe that the strong
magnetic field near the surface of the neutron star "heats up" the electrons
in it and that these "hot" electrons are responsible for the X-ray emission.
Crab in X-ray
For the Student
Using the text and any external references,
define the following terms:
visible, ultraviolet, X-rays, gamma rays,
light energy, photon, electromagnetic spectrum, electromagnetic
radiation, Hertz, wave peak, frequency, and
The EM Spectrum
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