Transcript Linking Asteroids and Meteorites through Reflectance Spectroscopy

Survey of the Universe
Tom Burbine
[email protected]
Quiz
• Next Wednesday
• Covers up to today’s material
• Absorption lines on the continuous spectrum of
the Sun
Emission Lines
How can you
determine velocities?
• Doppler Shift – The wavelength of light changes
as the source moves towards or away from you
• Since you know the wavelength position of
emission or absorption features
• If the positions of the features move in
wavelength position, you know the source is
moving
• http://www.youtube.com/watch?v=V5KaTfCnDrM
So
• Source moving towards you, wavelength decreases
- blueshift
• Source moving away from you, wavelength
increases - redshift
• Radial velocity – component of motion along the
observer’s line of sight
– Will produce Doppler Shift
• Motion perpendicular to the line of sight will not
produce a Doppler shift
Formulas
• vrad/c = λshift – λrest
λrest
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Rest wavelength of a line of hydrogen is at 656.285 nm
You observe this line at 656.255 nm for a star
What is the velocity of this star?
vrad/c = (656.255 -656.285)/656.285 nm
vrad/c = -4.5712 x 10-5
vrad = -4.5712 x 10-5 *3 x 108 m/s = -13,700 m/s = -13.7 km/s
It is blue-shifted so the motion is toward us
Telescopes
• Why do we use telescopes?
Initially
• Everybody observed with their eyes
Why are Telescopes better
than your eyes?
• They can observe light in different wavelength
regions (eyes can only see visible light)
• They can collect more light than eyes
• They can be built to compensate for the distorting
effects of the atmosphere
Telescopes
• Telescopes allow astronomers to observe objects
that are not visible to the naked eye either because
they are too dim or because they emit radiation
outside the visible range of the electromagnetic
spectrum
Atmosphere
• Atmosphere absorbs some wavelengths of light
and alters the path of light to degrade the quality
of images
• Space telescopes do not have to worry about the
effects of the atmosphere
Aperture
• The aperture is the opening in a telescope
• The larger the aperture, the more light that can be
collected
• You can see fainter , more distant objects
• Resolve more detail
http://www.astro.virginia.edu/class/oconnell/astr121/guide14.html
To measure light
• In the past (until the 1800s to the 1980s), they
used photographic plates
• Now they use CCDs (charge-coupled devices)
• CCD are electronic detectors
• CCDs are chips of silicons
1842 image of the Sun
Figure 7.5
CCDs
• CCDs can collect ~90% of photons that strike them
• Photographic plates can only collect ~10% of the
photons
• CCDs are split into squares called pixels
• Data is in electronic form and can be processed easily
by a computer
• CCDs are resuable
• Record images must faster
Photoelectric Effect
• Photon strikes the semiconductor surface of the CCD
• Photon that has sufficient energy causes an electron to
become unbound
• Electron is then free to move around
• Electrical voltages on the surface pull the freed
electrons into the nearest pixel
• The number of freed electrons per pixel is proportional
to the number of photons hitting it
• An electronic device connected to a computer counts
the number of electrons and generates an image
• Albert Einstein won the Nobel prize for
explaining the Photoelectric Effect
• Described light as composed of discrete quanta,
now called photons, rather than continuous
waves.
Collecting Area
• Collecting Area = π/4 * D2
• D is the diameter of the collecting area
Large telescopes are expensive to build
• Largest telescopes are usually national or
international facilities
• You need to apply for time to use them
Largest Optical Telescope
• Gran Telescopio Canarias (GTC) in the Canary
Islands
• Mirrors are 10.4 meter diameter
• Made up of 36 mirror segments
W. M. Keck Observatory
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Two telescopes
Mirrors are 10 meters in diameter
Each made of 36 segments
Located on Mauna Kea,
Hawaii
Largest telescopes
• Radio telescopes
• Must collect very large numbers of the lowestenergy photons
Arecibo Radio Telescope
• Located in Arecibo, Puerto Rico
• More than 300 meters in diamater
• Fixed in a natural basin
Filters
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U- ultraviolet – transmits strongest at 370 nm
B – blue - transmits strongest at 420 nm
V – visual - transmits strongest at 530 nm
R – red - transmits strongest at 600 nm
I – infrared - transmits strongest at 800 nm
I
First telescopes
• Used lenses to focus the images
• Pioneered by Galileo
Focus
• Point in an optical system in which light rays are
brought together
• The location where an image forms in such
systems
Refracting telescope
Figure 7.6
Refraction
• Light is bent when it enters a denser material
• Light’s speed slows
• Different wavelengths of light travel at different
speeds
• Refracted by different amounts
Largest Refractor
• Great Refractor at Yerkes Observatory (run by the
University of Chicago)
• Completed in 1897
• 1.02 m aperture
Mirrors
• Mirrors can also bring light into a focus
• Isaac Newton is given credit for working out a
practical design for this
Reflecting Telescope
Reflecting Telescopes
Resulting image inverted
All large modern telescopes are reflectors
• Since light passes through the lens of a refracting
telescope,
• You need to make the lens from clear, highquality glass with precisely shaped surfaces
• It is easier to make a high-quality mirror than a
lens
Also,
• Large lenses are extremely heavy
Also
• Lens focuses red and blue light slightly differently
• Called chromatic aberration
http://en.wikipedia.org/wiki/File:Lens6a.svg
Also
• Light can be absorbed by the glass as it passes
through the glass
• Minor problem for visible, but severe for
ultraviolet and infrared light
Dispersion
• Glass causes light to split into different colors
• Use a prism to perform spectroscopy (study
spectral lines)
Any Questions?