Reading

1
Reading
Unit 26, 27, 28, 29, 30
2
The Michelson-Morley Experiment

• Two scientists devised an experiment to detect
  the motion of the Earth through the aether
• Light should move slower in the direction of the
  Earths motion through space
• Detected no difference in speed!
• No aether, and the speed of light seemed to be a
  constant!

3
Einsteins Insights

• Albert Einstein started from the assumption that
  the speed of light was a constant, and worked out
  the consequences
• Length does indeed contract in the direction of
  motion, by a fraction equal to the Lorentz factor
• Time stretches as well, also by the Lorentz
  factor
• Moving clocks run slow
• Moving objects reduce their length in the
  direction of motion

4
Special Relativity

• Time dilation and length contraction depend on
  the observer!
• To an observer on Earth, the spacecrafts clock
  appears to run slow, and the ship looks shorter
• To an observer on the ship, the Earth appears to
  be moving in slow-motion, and its shape is
  distorted.
• The passage of time and space are relative!

5
Possibilities for Space Travel

• Example A spacecraft leaves Earth, heading for
  a star 70 light-years away, traveling at .99c
• To an observer on Earth, it takes the spacecraft
  140 years to get to the star, and back again
• To passengers on the ship, it only takes 20 years
  for the round-trip!
• This means that high speed travel to the stars is
  possible, but comes at the cost of friends and
  family

6
General Relativity Mass Warps Space

• Mass warps space in its vicinity
• The larger the mass, the bigger dent it makes
  in space
• Objects gravitationally attracted to these
  objects can be seen as rolling downhill towards
  them
• If the mass is large enough, space can be so
  warped that objects entering it can never leave
  a black hole is formed.

7
Another underappreciated female in science?
Mileva Maric
Einstein Nobel Prize was for photoeffect, which
was the theme of diploma work by Mileva. To what
extend she inspired Einstein or collaborated with
him is a subject of debates.
8
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

9
The Charge-Coupled Device (CCD)

• 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

• The science of developing new methods for
  sensing, focusing and imaging light in astronomy
  is called instrumentation

10
Outside the visible spectrum

• Observations in other wavelengths require
  instrumentation to be lifted above the Earths
  atmosphere.
• X-ray, Gamma ray and infrared wavelength
  telescopes are currently in orbit!

• 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

11
Modern Telescopes

• 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

12
Radio Telescopes

• Radio telescopes, like the one in Arecibo, Puerto
  Rico, collect radio waves from astronomical
  objects and events

13
Radio Telescopes

• Radio telescope arrays to achieve large
  collecting areas

National Radio Astronomy Observatory (U.S.A.)
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Size Matters!

• 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,

15
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

16
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.

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Different styles of reflectors
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X-Ray reflectors

• 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

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Chandra
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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.

21
Diffraction and Resolution

• Some stars that appear to be single bodies to the
  unaided eye are, when viewed through a telescope,
  found to be two separate stars.
• The telescope is able to separate the two stars,
  while the human eye is not.
• The telescope, then, has better resolution than
  the human eye.
• The telescopes resolution is better because it
  has a larger aperture, and light is diffracted
  less as it passes through it.

• Diffraction is a rippling effect due to the
  finite size of an aperture.
• Light waves approach the aperture as flat plane
  waves, similar to the straight water waves seen
  above.
• As the waves pass through the aperture, the waves
  become curved.

22
Diffraction Effects

• Diffracted light waves can interfere with, or
  cancel, each other.
• This results in a diffraction pattern, a blurring
  of the image as it passes through the telescope.
• Larger apertures have less diffraction, and
  therefore higher resolution than smaller
  apertures.
• For observing light of wavelength ?nm, the
  smallest separation angle ?arcsec a telescope can
  resolve is related to the telescope aperture Dcm
  by

23
Interferometers

• To counter diffraction effects (and build
  telescopes with higher resolution), astronomers
  use interferometers.
• Signals from these arrays of widely-separated
  telescopes are added together to create images
  with very high resolution.
• In fact, the resolution is equivalent to that of
  a single telescope with an aperture as large as
  the separation in the array!

24
Before and After

• Before
• What looks like a single star

• After
• is actually two stars!

25
Atmospheric Absorption

• The Earths atmosphere absorbs most of the
  radiation incident on it from space
• This is a good thing for life high energy
  photons would sterilize the planet!
• This is not a good thing for astronomy, however!

• Visible, radio and some infrared wavelengths are
  not absorbed readily by the atmosphere
• Optical and radio telescopes work well from the
  ground
• Gamma Rays, X-rays, and UV photons are absorbed
• Observatories for these wavelengths must be kept
  above the Earths atmosphere!

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Ground- and Space-based Observatories
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Light Pollution

• Ambient light from cities are a real problem for
  optical astronomy.
• This light pollution washes out images in
  telescopes.
• Research telescopes are built far from cities to
  reduce the effects of light pollution
• It is getting harder to find good locations for
  telescopes!

28
Atmospheric Effects

• Air refracts light just like glass or water, but
  to a lesser degree.
• Cool air refracts light more than warm air
• Pockets of cool air in the atmosphere create
  moving lenses in the sky, shifting the light rays
  randomly
• This causes a twinkling effect, called
  scintillation.
• A stable atmosphere causes less scintillation
• We say the seeing is good.

29
Adaptive Optics

• Some observatories measure the amount of
  atmospheric turbulence with lasers, and then
  adjust the mirrors in their telescopes with tiny
  motors to eliminate the effect
• This technique is called adaptive optics

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Spitzer Space Telescope
James Web Space telescope
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Edwin Hubble
Hubble Telescope
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Observatories in Space