Transcript chapter2lecture

Investigating Astronomy
Timothy F. Slater, Roger A. Freedman
Chapter 2
Decoding the
Hidden Messages
in Starlight
Light Takes Time to Travel
• When close to Jupiter,
the moons appeared to
eclipse “too early.”
• When far from Jupiter,
the moons appeared to
eclipse “too late.”
• Light takes time to
travel the extra
distance!
• c = 300,000 km/s
Light Travel Time To Earth
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Moon
1.5 seconds
Sun
8.3 minutes
Jupiter
43.2 minutes
Saturn
79.3 minutes
Pluto
5.5 hours
1 light-year ≈ 6 trillion miles ≈ 10 trillion km
Proxima Centauri
4.3 years
Andromeda Galaxy
2.5 million years
Furthest known galaxy 13.2+ billion years
ConceptCheck
• Why has the speed of light been
historically so difficult to measure?
Glowing objects, like stars, emit an
entire spectrum of light.
• The Sun emits
energy that:
– Your eyes can see
– Your skin can feel
– Can burn your skin
Sunlight Is a Mixture of All Colors
• Prisms don’t “add” colors
to the sunlight.
• Each color light “bends” as
it passes through the
material.
Light Travels in Waves
• Water waves
show diffraction,
addition, and
canceling.
• So does light!
A wave!
Our Eyes See Only Some of
the Spectrum of Light
Half of this image was
taken with a “visible light”
camera, the other half was
taken with a “UV
camera.”
Bees can see
designs on the
petals!
As Frequency Increases,
Wavelength Decreases
• f is the symbol for
frequency
• Hertz = 1 wave per
second
• λ is the symbol for
wavelength
• λf = c, or
• f = c / λ, or
• λ=c/f
Shorter wavelength
Higher frequency
Higher energy
More “particle-like”
Light has
properties
of both
waves and
particles
Longer wavelength
Lower frequency
Lower energy
More “wave-like”
Electromagnetic Radiation
• A “field disturbance”
• Both electric and magnetic properties
• A spectrum of waves, varying in wavelength
and frequency
ConceptCheck
• If you cover a white light with a specially
designed green plastic gel so that only the
green light passes through, which color
plastic cover gel do you need to add to the
pure green light to make it change to red?
• Do light waves move up and down or back
and forth as they move through space?
• Which form of electromagnetic radiation
has a wavelength similar to the diameter
of your finger?
ConceptCheck
• How do the frequencies of the longest
wavelengths of light compare to the
frequencies of the shortest
wavelengths of light?
• What is the wavelength of radio
waves from your favorite FM radio
station?
• If a photon’s wavelength is measured
to be longer than the wavelength of a
green photon, will it have a greater or
lower energy than a green photon?
Infrared light can pass through interstellar
clouds that visible light cannot.
If our eyes can only see some parts of
the spectrum, there must be things
we can’t see.
Infrared light can pass through clouds
of dust and gas.
Objects emit specific amounts of light,
revealing their temperatures.
Wien’s Law: The higher the
temperature, the more intense
the light and the shorter the
wavelength….
How much energy a star emits is determined
by both temperature and surface area.
As temperature
increases, the
energy released
by the object
increases.
ConceptCheck
• Which form of light is being emitted most
intensely by a frozen ice cube at 0° Celsius?
• What single piece of information do
astronomers need to determine if a star is
hotter than our Sun?
• Which wavelength of light would our Sun
emit most if its temperature were twice its
current temperature of 5800 K?
• If astronomers observe a red star and a blue star
in the sky, how do they distinguish which star is
at a higher temperature?
Identifying Chemical Substances
Using Spectral Lines
The light from a
burning chemical
makes a special,
unique pattern
when it passes
through a prism.
Electrons Occupy Specific
Orbits within Atoms
• Each orbit is a specific
energy state.
• Electrons “leap”
between orbits.
• Electrons “leap” when they absorb the perfect
amount of energy.
• Electrons “fall” and emit that same specific
amount of energy.
Kirchhoff’s Laws
Law 1: A hot, opaque body or a hot, dense gas
produces a continuous spectrum—a complete
rainbow of colors without any spectral lines.
Kirchhoff’s Laws
Law 2: A hot, transparent gas produces an
emission line spectrum—a series of bright
spectral lines against a dark background.
Kirchhoff’s Laws
Law 3: A cool, transparent gas in front of a source
of a continuous spectrum produces an absorption
line spectrum —a series of dark spectral lines
among the colors of the continuous spectrum.
Kirchhoff’s Laws
The wavelengths absorbed by the gas exactly
match the wavelengths emitted by the gas.
ConceptCheck
• What type of spectra would result
from a glowing field of hot, dense
lava as viewed by an orbiting
satellite through Earth’s
atmosphere?
Spectra Also Reveal Motion
An object’s motion through space is revealed by the
precise wavelength positions of its spectrum of light.
The Doppler Effect
Exploiting the Doppler Effect
The wavelength
we observe
The wavelength
we “should” observe
=
The velocity of the object,
toward or away from us
The speed of light
ConceptCheck
• How is the spectrum changed when
looking at an emission spectrum
from an approaching cloud of
interstellar gas as compared to a
stationary cloud?
• How fast and in what direction is a
star moving if it has a line that
shifts from 486.2 nm to 486.3 nm?
Telescopes Gather Light
Telescopes aren’t primarily used to magnify stars.
Light-gathering
power is directly
related to the
size of its
objective lens―
the gathering
area.
Refracting Telescopes
• Use a lens to concentrate incoming light at a
focal point
• Magnifies near objects
Reflecting Telescopes
• Use a curved mirror to concentrate incoming
light at a focal point.
• More durable, and can be made bigger and less
expensive.
Adaptive Optics
Computers compensate
for turbulence in the
atmosphere.
Telescopes in Orbit
• Detect light that
does not
penetrate the
atmosphere
Looking toward
the center of the
Milky Way using
the best of Earthbased and space
telescopes
Charge-coupled devices record
very fine image details.
ConceptCheck
• In large sizes, which type of telescope can
be made lightest and most inexpensively?
• Look at Figure 2-28, which shows the
transparency of Earth’s atmosphere.
Would astronomers most prefer to have
a new ground-based telescope
constructed that is most sensitive in the
X-ray region, the ultraviolet wavelength
region, or in the microwave region?
• What is the primary advantage of an
orbiting space telescope, compared to a
ground-based telescope?
• Why can CCDs more efficiently observe
faint stars than photographic film or
photographic plates?