the milky way - moore public schools · • wavelengths of light are measured in units of...
TRANSCRIPT
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Foundations of Astronomy | 13e
Seeds
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Chapter 6
Light and Telescopes
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Guidepost
• In this chapter, you will consider the
techniques astronomers use to study the
Universe
– What is light?
– How do telescopes work?
– What are the powers and limitations of
telescopes?
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Guideposts (cont’d.)
– What kind of instruments do astronomers use
to record and analyze light gathered by
telescopes?
– Why are some telescopes located in space?
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6-1 Radiation: Information from Space
• In astronomy, we cannot perform experiments
with our objects
– Stars, galaxies, etc.
• The only way to investigate them is by
analyzing the light (and other radiation) which
we observe from them
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Common Misconception
• Misconception: we must be wary of the
word radiation
– Truth: Radiation is anything that radiates
away from a source and not all radiation
involves dangerous high-energy particles
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Light as Waves
• Light waves are characterized by:
– Wavelength
– Frequency
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Light as Waves (cont’d.)
• Wavelengths of light are measured in units
of nanometers (nm) or Ångström (Å)
• Visible light has wavelengths between
4000 Å and 7000 Å (= 400 – 700 nm)
1 nm = 10-9 m
1 Å = 10-10 m = 0.1 nm
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Wavelengths and Colors
• Different colors of visible light correspond
to different wavelengths
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Light as Particles
• Light can also appear as particles, called
photons (e.g., photoelectric effect)
• A photon has a specific energy E,
proportional to the frequency f
• The energy of a photon does not depend
on the intensity of the light
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The Electromagnetic Spectrum
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Common Misconception
• Misconception: Radio waves are related to
sound
– Truth: Radio waves are a type of light that
your radio receiver transforms into sound
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6-2 Telescopes
• Astronomers use telescopes to gather
more light from astronomical objects
– The larger the telescope, the more light it
gathers
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• Refracting telescope:
lens focuses
light onto the focal
plane
• Reflecting telescope:
concave mirror
focuses light onto
the focal plane
Refracting and Reflecting Telescopes
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Secondary Optics
• Secondary mirror: redirects the light path
towards the back or side of the incoming
light path
• Eyepiece: used to view and enlarge the
small image produced in the focal plane of
the primary optics.
Focal length
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The Powers and Limitations of Telescopes
• Chromatic aberration: different
wavelengths are focused at different focal
lengths (prism effect)
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The Powers and Limitations of Telescopes
(cont’d.)
• Light-gathering power: depends on the
surface area (A) of the primary lens or
mirror, proportional to diameter squared
A = p (D/2)2
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Resolving Power
• Minimum angular distance amin between
two objects that can be separated:
• For optical wavelengths, this gives
amin = 1.22 (l/D)
amin = 11.6 arcsec / D[cm]
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Seeing
• Weather conditions
and turbulence in
the atmosphere set
further limits to the
quality of
astronomical
images
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Magnifying Power
• Ability of the telescope to make the image
appear bigger
• Depends on the ratio of focal lengths of
the primary mirror or lens (Fp) and the
eyepiece (Fe):
• A larger magnification does not improve
the resolving power of the telescope!
M = Fp/Fe
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Common Misconception
• Misconception: The purpose of an
astronomical telescope is to magnify
images
– Truth: Very high magnification does not
necessarily show more detail. Generally, the
amount of detail that a telescope can discern
is limited by its resolving power or the seeing
conditions
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How Do We Know? 6-1
• Resolution and precision
– The precision of measurements is limited by
the resolution of the measurement technique
– The practical size of a pixel is set by the
resolution limit, and affected by:
• Atmospheric seeing
• Telescope optical quality and diffraction
– You can’t see details smaller than the pixel
size, so there is unavoidable uncertainty in all
scientific measurements
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The Best Locations for a Telescope
• Far away from civilization – to avoid light
pollution
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• On high mountain-tops – to avoid
atmospheric turbulence and other weather
effects
The Best Locations for a Telescope
(cont’d.)
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Modern Optical Telescopes
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Modern Optical Telescopes (cont’d.)
• The 4-m Mayall
Telescope at Kitt
Peak National
Observatory
(Arizona)
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Advances in Modern Telescope Design
• Lighter mirrors
with lighter support
structures, to be
controlled
dynamically by
computers
– Floppy mirror
– Segmented mirror
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Advances in Modern Telescope Design
(cont’d.)
• Simpler, stronger mountings (“Alt-azimuth
mountings”) to be controlled by computers
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Examples of Modern Telescopic Design
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Examples of Modern Telescopic Design (cont’d.)
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Radio Astronomy
• Recall: radio waves of l ~ 1 cm – 1 m also
penetrate Earth’s atmosphere and can be
observed from the ground
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Modern Radio Telescopes
• Large dish focuses the
energy of radio waves
onto a small receiver
(antenna)
• Amplified signals are
stored in computers
and converted into
images, spectra, etc.
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6-4 Airborne and Space Telescopes
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Airborne Telescopes
• Infrared cameras need to be cooled to
very low temperatures, usually using liquid
nitrogen.
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Space Telescopes
• The Hubble Space Telescope
• Launched in 1990
• Maintained and upgraded by several space
shuttle service missions throughout the 1990s
and early 2000s
• Avoids turbulence in Earth’s atmosphere
• Extends imaging and spectroscopy to infrared
and ultraviolet
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The Hubble Space Telescope
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Space Telescopes (cont’d.)
• HST successors
• James Webb Space Telescope
– Will be in solar orbit ~1 million miles from
Earth
• Herschel Space Observatory (2009)
– Carried a 3-m mirror and instruments cooled
almost to absolute zero
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Space Telescopes (cont’d.)
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High Energy Astronomy
• Telescopes observing gamma-rays, X-
rays, and ultraviolet sources must be
located high in Earth’s atmosphere or in
space
– General-purpose telescopes: e.g., Chandra
– Single-subject telescopes: e.g., Hindode
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Chandra X-Ray Observatory
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6-5 Astronomical Instruments and
Techniques
• Cameras and photometers
– Photographic plate (record image)
• Long exposure detect faint objects
• Brightness of objects not measured very precisely
– Photometers (measure intensity of the light)
• Sensitive light meter measures brightness of
objects very precisely
– Charge-coupled devices (CCDs)
• Records image and measures the brightness
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CCD Imaging
• More sensitive than photographic plates
– Data can be read directly into computer
memory, allowing easy electronic
manipulations
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Spectrographs
• Spectral lines in a spectrum tell us about
the chemical composition and other
properties of the observed object
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Adaptive Optics
• Computer-controlled mirror support
adjusts the mirror surface (many times per
second) to compensate for distortions by
atmospheric turbulence
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Interferometry
• Combine the
signals from
several smaller
telescopes to
simulate one
big mirror
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Radio Maps and Interferometry
• Colors in a radio map can indicate
different intensities of the radio emission
from different locations on the sky
• Radio waves are much longer than visible
light – use interferometry to improve
resolution!
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Radio Maps (cont’d.)
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The Very Large Array (VLA)
• 27 dishes combined to simulate a large
dish of 36 km in diameter
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6-6 Non-Electromagnetic Astronomy
• Radiation from space does not only come
in the form of electromagnetic radiation
– Particle astronomy
• Earth is constantly bombarded cosmic rays –
highly energetic subatomic particles traveling
through space at high velocities
– Gravity wave astronomy
• Gravity waves predicted to be produced by an
mass that accelerates, but would be extremely
weak and difficult to detect
• Inferred, but not yet detected
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Discussion Questions
• Why would you not include sound waves
in the electromagnetic spectrum?
– Hint: See Figure 6-2. Do sound travel at the
speed of light? What about through a
vacuum?
• Why do optical astronomers often put their
telescopes at the tops of mountains,
whereas radio astronomers sometimes put
their telescopes in deep valleys?
– Hint: See Figure 6-3
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Discussion Questions (cont’d.)
• Why does the wavelength response of the
human eye match the visual window of
Earth’s atmosphere so well? Why not the
radio window?
– Hint: The maximum energy of the Sun is ~500
nm (green)