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Astronomy 1020-H
Stellar Astronomy
Spring_2016
Day-12
Course Announcements
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1st Quarter observing night: TONIGHT @ 7:30pm
READ through the “Format for the Report” BEFORE
you come.
N
5
Mean
73.3
Std. Dev.
14.2
Median
79.2
Mode
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Min.
56
Max.
87.2
Curve
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The Origin and Nature of Light
Lab This Week
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The Spectrometer
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What you need to know:
You get to visualize the spectra from various
sources.
Reading ahead in Chapter 5 will help.
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Lab This Week
 The Spectrometer
 Almost all
knowledge of the
universe beyond
Earth comes from
light.
 Light can tell us
about objects in
space: temperature,
composition,
speeds, and more.
What Light Can Tell Us.
 Photometry – Study of Brightness:
 Luminosity, Est. of Distance, Rough Temp. of Star
 Spectroscopy – Study of the EM Spectrum:
 Composition, Radial Velocity, Temp., Surface-gravity
 Mass for compact objects
 Astrometry – Study of Positions:
 Tangential Motion, Distance
 Polarimetry – Study of Polarization of Light:
 Magnetic Field
Spectrum of the Sun
But, what is light?
 Newton, Remember Newton? He did more than just
calculus and mechanics.
 In the 17th Century, Isaac Newton argued that light was
composed of little particles while Christian Huygens
suggested that light travels in the form of waves.
 In the 19th and 20th Century Maxwell, Young, Einstein and
others were able to show that Light behaves both like a
particle and a wave depending on how you observe it.
Particle Nature
Thomas Young’s interference experiment
Wave Nature
But, where does light actually
come from?
Accelerating charges (think electrons
and protons) produce light –
electromagnetic radiation!
Photons
 Light also behaves as a particle.
 Photon: particle of light.
 Photons carry energy and can have
different amounts of energy.
 Photons with high energy = light with high
frequency.
 Photons with low energy = light with low
frequency.
 Atoms can absorb or emit photons.
Einstein (Nobel Prize)
In 1905 Einstein
calculated the energy of
a particle of light
(photon) and proposed
the photoelectric effect.
photon
Ephoton = hc/ = hn
e-
 Light moves at 300,000 km/s in a vacuum.
 First measured by Rømer when observing
Jupiter’s moons.
 Speed is slower in other materials, e.g.,
glass.
 Wavelength ():
length between
crests.
 Amplitude: height.
 Frequency (f):
number of waves
that pass by each
second.
 Period (P): time to
complete one cycle.
 A long wavelength
means low
frequency.
 A short wavelength
means high
frequency.
 The speed of light, c,
is constant.
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 Light is a wave of combined electricity and
magnetism, called an electromagnetic wave.
 Changing electric and magnetic fields create a
self-sustaining electromagnetic wave.
Maxwell – EM Field Theory
Scottish physicist James Clerk
Maxwell showed mathematically
in the 1860s that light must be a
combination of electric and
magnetic fields.
MATH TOOLS 5.1
 Knowing the speed of light and one other
variable, either the wavelength or frequency
of the light in question, you can find the
remaining quantity.
 Example: Find the wavelength of the light
wave coming from a radio station
broadcasting on 770 AM:
EM Spectrum
Like the flavors
of Ice cream –
they each
provide us with
different
information.
But what do
you get when
you put all the
flavors (light)
together?