Tools of Modern Cosmology Friday, October 3 Next Planetarium Shows: Tues 7 pm, Wed 7 pm.
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Transcript of Tools of Modern Cosmology Friday, October 3 Next Planetarium Shows: Tues 7 pm, Wed 7 pm.
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Tools of Modern Cosmology
Friday, October 3Next Planetarium Shows: Tues 7 pm, Wed 7 pm
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We learn about the universe by gathering light from distant objects.
Some objects emit light (stars, Sun); others reflect light (planets, Moon).
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Our eyes are good, but not perfect, at detecting light.
Blurred on small scales. Can’t see faint sources.
Can’t see ultraviolet, infrared, etc.
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TelescopesTelescopes remedy some of our eyes’ problems.
Telescope: to see distant faint objects at
high resolution.
Microscope: to see nearby small objects
at high resolution.
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A refractingrefracting telescope uses a lenslens to gather light.
Light is bent (or “refracted”) when going from air to glass (or vice versa).
glass
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A convexconvex lens (thick in the middle) focuses light to a point:
Light from a large area is funneled into a small area.
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A reflectingreflecting telescope uses a mirrormirror to gather light.
When light reflects from a mirror, angle angle of incidenceof incidence equals angle ofangle of reflectionreflection.
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A mirror shaped like a parabolaparabola focuses light to a point:
Light from a large area is funneled into a small area.
focus
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Lenses and mirrors, if shaped correctly, produce an accurate image of an object.
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The main purposes of a telescope are to gather lightgather light and resolveresolve detaildetail.
Telescope = “light bucket”. Bigger bucket = more light.
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Amount of light collected per second is proportional to area area of the lens or mirror.
2D4
Area
D = diameter of lens/mirror
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Looking East, midnight tonight:
The Pleiades, a cluster of stars.
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Without a telescope, most people can see sixsix stars in the Pleiades.
With his small telescope, Galileo saw more than forty forty.
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With large modern telescopes, about a With large modern telescopes, about a thousandthousand stars are seen in the Pleiades. stars are seen in the Pleiades.
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A bigger lens or mirror is able to resolve finer detailresolve finer detail.
low resolution
high resolution
Two stars are resolvedresolved if they are seen as separate points.
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Smallest angle resolved is proportional to 1/D.
MagnificationMagnification is not as important: Big, blurry image is less useful
than small, sharp image.
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Looking straight up, midnight tonight:
Andromeda Galaxy.
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The Andromeda GalaxyAndromeda Galaxy, as seen by unaided eyes, is a faint oval smudge.
Star atlas of Al Sufi, AD 964.
Andromeda Andromeda Galaxy.Galaxy.
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With large modern telescopes, the With large modern telescopes, the Andromeda Galaxy looks like this….Andromeda Galaxy looks like this….
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BIGGER BIGGER IS IS
BETTER!BETTER!Larger lens or mirror
means more light, higher resolution.
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The world’s biggest telescopes are reflectors, not refractors.
What’s wrong with lenses?
◦Lenses absorb light.
◦Lenses sag.
◦Lenses have chromaticchromatic aberrationaberration: colors don’t focus at the same point.
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World’s largest refracting telescope:
Yerkes Observatory, D = 1 meter, completed 18971897.
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A modern reflectingreflecting telescope:
Large Binocular Telescope: twotwo mirrors, each with D = 8.4 meters.
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Radio telescopesRadio telescopes can detect radio waves invisible to your eyes.
Parabolic “dish” of a radio telescope acts as a mirror, reflecting radio waves to the focus.
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Radio telescopes can be huge, because they don’t have to be fantastically smooth.
Arecibo Telescope, Puerto Rico
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Why bother with radio observations?Why bother with radio observations? They give a different view of the universe.
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Turbulence in air makes stars “twinkle” and limits resolution.
City lights drown out faint stars.
Good idea: Put a telescope in orbit!
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The Hubble Space Telescope is 600 kilometers above the Earth’s surface.
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Hubble Space Telescope has great angular resolution; it’s above the
turbulent atmosphere.
Light-gathering ability? Not as great; it’s only D = 2.4 meters in diameter.
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Monday’s Lecture:
Reading:
Chapter 2
Atoms & Light