Download 13Oct_2014

Reading
Unit 28, Unit 29, Unit 30
Will not be covered by the first exam
As a blackbody object becomes hotter, it also
becomes ____________ and _____________
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a. more luminous, redder
b. more luminous, bluer
c. less luminous, redder
d. less luminous, bluer
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Compare two blackbody objects, one at 200K and
one at 400K. How much larger is the flux from the
400K object, compared to the flux from the 200K
object?
a. Twice as much
b. Four times as much
c. Eight times as much
d. Sixteen times as much
Star A and star B appear equally bright in the sky.
Star A is twice as far away from Earth as star B. How
do the luminosities of stars A and B compare?
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a. Star A is 4 times as luminous as star B
b. Star A is 2 times as luminous as star B
c. Star B is 2 times as luminous as star A
d. Star B is 4 times as luminous as star A
Which of the following factors does *not* directly
influence the temperature of a planet?
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a. The luminosity of the Sun
b. The distance from the planet from the Sun
c. The color of the planet
d. The size of the planet
The strangest telescope
[tele-scope (look from a distance)]
• IceCube Observatory to indirectly detect neutrinos
photomultiplier tubes
Cherenkov Effect
blue Cherenkov light in
a nuclear power reactor
charged particle traveling
in transparent medium
faster then light (in the medium)
Sonic boom
condensates moisture
Cherenkov Effect
charged particle traveling
in transparent medium
faster then light (in the medium)
neutrinos produce
energetic charged particles
that emit Cherenkov light
Sonic boom
condensates moisture
Telescopes
• Telescopes have been
used for hundreds of years
to collect light from the
sky and focus it into an
eyepiece. An astronomer
would then look through
this eyepiece at planets,
nebulae, etc.
• The human eye is not very
sensitive to dim light, and
was replaced in astronomy
by the film camera.
• Film is sensitive to only
around 10% of the
impinging light, and is
usually replaced by a…
The Charge-Coupled Device (CCD)
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The CCD, similar to those
found in commercial digital
cameras and phones, utilizes
the photoelectric effect to
collect around 75% of the
visible light that is focused
on it!
It has revolutionized
astronomy – images can be
recorded and downloaded to
a computer anywhere in the
world for analysis
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The science of developing new methods for
sensing, focusing and imaging light in
astronomy is called instrumentation
Outside the visible spectrum
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Many objects of astronomical interest are
visible only in wavelengths other than the
visible!
Much can be learned from studying a star,
planet or nebula in multiple wavelengths.
Radio telescopes can be used from the ground
to image pulsars and other bodies
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Observations in other wavelengths
require instrumentation to be lifted
above the Earth’s atmosphere.
X-ray, Gamma ray and infrared
wavelength telescopes are currently
in orbit!
Modern Telescopes
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Modern telescopes are designed to
collect as much light as possible, and
must be built to exacting standards.
Collected light is of nanometer
wavelength, so the telescopes must
be extremely precise to keep the
waves coherent for maximum
efficiency
Radio Telescopes
• Radio telescopes,
like the one in
Arecibo, Puerto
Rico, collect radio
waves from
astronomical objects
and events
Radio Telescopes
• Radio telescope
arrays to achieve
large collecting
areas
National Radio Astronomy Observatory (U.S.A.)
Size Matters!
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Aperture size is very important
when collecting light!
A large collecting area allows
astronomers to image dim and
distant objects.
For a telescope with an aperture
a distance D in diameter,
Collecting Area =
p
4
´ D2
Refraction
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Light moves at a fixed speed, c.
The value of c changes depending
on what substance, or medium, it
moves through.
The speed of light in vacuum is
around 300,000 km/s. Its speed
through glass or water, however, is
slightly slower
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If a beam of light (or light ray) enters a
new medium at an angle, light on one
side of the ray enters first, and slows.
This slowing of one part of the ray
causes the ray to change direction,
similar to driving a car from asphalt onto
sand can make a car swerve.
This bending of a light ray’s path is
called refraction
Refraction in Water
Dispersion
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The amount a light is diffracted depends on its wavelength
A prism spreads the light out, using this effect
This dispersion of light is a problem in refracting telescopes, as the focal plane will be
at a slightly different location for each wavelength of light
This leads to chromatic aberration, a blurring effect.
Lenses
• A lens is a specially
shaped piece of glass that
bends light rays passing
through it so that they
focus a particular
distance away (the focal
length) at a particular
location (the focal plane).
• A sensor such as a human
eye, a camera or CCD, if
placed in the focal plane
can image the light
Refracting Telescopes
• Telescopes that use lenses to focus
light are called refracting telescopes,
or refractors.
• Large refractors are difficult to build!
– Glass is heavy, and glass lenses must be
supported only by their rims, a difficult
engineering problem
– Glass sags under its own weight,
defocusing the light!
– Refractors suffer from chromatic
aberration, a blurring effect due to
changes in the focal plane of the lens
for different wavelengths of light
Reflecting Telescopes
• Reflecting telescopes, or
reflectors, use a curved
mirror to focus light
• Mirrors can be supported
from behind, and so can be
much larger than refractors
• Larger sizes mean that more
light can be collected and
focused, allowing
astronomers to image
dimmer or more distant
objects
• Most modern telescopes are
reflectors.
Different styles of reflectors
X-Ray reflectors
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X-rays only reflect at glancing angles,
otherwise they are absorbed or pass
through the mirror!
X-Ray mirrors are designed to gently
reflect the incoming photons, focusing
them at the end of a long tube-shaped
array of mirrors
Very Large Mirrors
• Reflectors can be made very large
if multiple mirrors are used as the
primary mirror.
• The Keck Telescope uses 36 large
mirrors to create a single huge
primary.
• The positions of the mirrors are
precisely measured by lasers, and
can be individually adjusted to
keep them perfectly aligned.