V. Tools in Astronomy and Astrophysics

So far used light and our eyes to gain information about the sky:

Limitations:

• Sensitivity
• Resolution of our eyes
• Range of "Colors"
• Problems with optical trickery

To improve we need:

• knowledge about light
• provide better tools

1. Light etc.

A) Changes of the light path

a) Reflection:

• light rays bounce off a plane like an elastic ball (mirror reflection)

incoming angle = exit angle

b) Refraction

• light is bent at the interface of two different materials
• entering "denser" material (glass) -> towards surface normal
• exiting "denser" material (glass) -> away from surface normal
• explained by light as a wave (wavefront changes direction with speed change)

B) Light is a wave

• Definitions:
  • Wavelength:distance between 2 wavecrests
  • Frequency:number of wavecrests passing by in 1 second
  • -> Wave Speed = Wavelength * Frequency
• Speed of light is finite, but incredibly fast: c = 300,000 km/sec
  • measured by Ole Roemer from orbits of Jupiter's moons
  • (information on moon's position comes later when Jupiter is on the far side of the sun)

2. Optical telescopes

A) Lenses and curved mirrors

• Collect and concentrate light
• Produce images of distant objects at one focal length away

B) Design of Telescopes:

• objective -> to gather light and produce an image of the object
• eyepiece -> to serve as magnifying glass to look at the image
• Use of lenses for objective and eyepiece->Refractor
• Use of a mirror for the objective, lense as eyepiece->Reflector

C) Powers and Limitations of Telescopes:

a) Light gathering:

• the larger the objective the better: scales with area (diameter(objective)²)

b) Magnification of image (apparent (angular) size DQ of object):

DQ_eye/DQ_obj = f_objective/f_eyepiece (f = focal length of the lenses)

c) Resolution of neighboring objects

the larger the objective the better: (objective >>>> wavelength!)

The Limitation is diffraction

• Narrow slit does not produce a sharp slit image:
• a new wave starts at an obstacle when hit by a lightwave and is bent around
• (like the circular waves starting from a stick in the water, when hit by a wave)

D) Problems and Limitations:

a) Lens errors

• chromatic aberration (due to dispersion)
• use mirrors (reflector telescope)
• other aberrations -> use correcting lenses

b) Support of weight of the objective

• distortion of mirrors or lenses by gravity
• ways out:
  • make individual pieces(controlled by a computer)
  • go into space (weightless)

c) Motion of the air above observatory

• blurred image
• ways out:
  • go on high mountains
  • go into space
  • use computer-controlled optics

3. Light is Electromagnetic Waves and/or Particles

A) Spectrum of Light

a) Dispersion

• refraction for blue light stronger than for red light
  • prism produces "rainbow"
• Consequences:White light consists of all colors
• Frequency or wavelength = "color"
• Definition: Spectrum of light: the amount of energy at each wavelength

b) Selective Reflection (produces colors)

• Colored object reflects only its own colors:
  • selective reflection on solid surfaces -> info on material on planets/moons
  • red surface of Mars -> rusty iron
  • yellowish parts of Io -> sulfur

c) Temperature (produces colors)

• Hot objects emit light:
  • color (wavelength) of maximum intensity depends on temperature
  • color of star -> info on temperature of star
  • hotter -> blue -> shorter wavelength and
  • hotter -> shines brighter
• There is "light" even beyond red (infra red) and beyond blue and violet (ultra violet)

B) Light is Electromagnetic Waves

• Radio and TV transmission waves are similar -> electromagnetic waves
• (all the same "stuff", only at different wavelength)

radio

microwave

IR

red

blue

UV

X

gamma.

low frequency	high frequency
long wavelength	short wavelength
low energy photons	high energy photons

For all waves:

speed = c (the speed of light) (All electromagnetic radiation moves at c)

C) Light is Particles

• Some measurements tell us light is particles (particle nature = "photons").
• (electrons are knocked off by light, as if hit by a particle)
  • light is particles
  • light acts like both: waves and particles
• Frequency or wavelength equivalent to photon energy; both equivalent to "color"

4. Radiotelescopes

A) Uses in astronomy

1. all stars also emit radio waves (the sun is very noisy)
2. gas (cold stuff) radiates in radio regime
3. remainders of supernovae emit radio waves
4. developed as spin-off of radar technology

B) Radiotelescope consists of

1. antenna dish (collector for radio waves) and
2. receiver (radio)(like satellite TV communication)

C) Powers

• Signal collection: the larger the dish the better
• Resolution: dish larger than optical telescopes, but wavelength much longer!
  • resolution worse than optical telescopes
• But telescope can be put together in pieces:
  • With radio telescopes all over the Earth -> fake telescope as large as the Earth
  • resolution better than optical telescopes

If you want to know more, get the Radioastronomy Tutorial from the Haystack Radio Observatory

5. Space Astronomy

A) The atmosphere blocks radiation

• only visible light and radio waves reach the Earth's surface
• (absorption by atmosphere, otherwise get fried by UV and X-rays)
  • observe rest of spectrum from space

B) IR light:

• telescopes similar optical telescopes
• uses -> observe cool stars, star formation

C) UV light:

• telescopes similar optical telescopes
• uses -> observe hot stars, local interstellar gas

D) X-rays

a) Image with X-rays?

• wavelength m6; atom size-> mirror not smooth any more
• but rough surface looks shiny under flat angle -> Wolter telescope
• X-rays are reflected under flat angle (grazing incidence)
• like skipping of a flat stone on a water surface under a flat angle

You can also go to the class movie on X-ray reflection.

X-ray telescopes are world champions in mechanical accuracy (Guiness Book)

b) Which eyes for X-Rays?

• array of Geiger counters

c) Uses in astronomy-> observe violent processes, star death

• normal stars (like sun) when active
• supernova remnants
• galaxies and clusters of galaxies (very hot gas)

E) Gamma-rays

a) Image from gamma rays

• use collision geometry of photons with electrons to get arrival direction
• (as if playing billiard with electrons and photons)

(Compton Telescope, built at UNH and MPE)

b) uses-> hottest and most energetic sites in the universe

• black holes
• active galaxies

F) Particles from celestial objects

a) Parameters to determine:

1. Mass (M) -> composition, i.e, source of particles (oxygen from Earth, Helium from Sun)
2. Energy (E) -> acceleration, heating
• best particle accelerators are in space
3. Charge (Q)-> temperature at origin
   • hot: -> atoms loose many electrons

Combine several methods to achieve this:

1. Deflection in electric field-> stronger for higher Charge (Q)
2. Measure time-of-flight-> heavier (higher mass (M)) is slower
3. Detector-> more energy (E) -> larger signal

Bring instruments beyond Earth's atmosphere

• near Earth, near other planets, rest of the solar system

b) Which particles do we find in space?

1. cosmic rays (energetic particles) from the sun and other places in the galaxy
2. solar wind from the sun
3. plasmas around planets (magnetospheres)
4. dust particles and meteorites

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