Observing the Universe

Electromagnetic radiation: electric and magnetic fields traveling               through space.

Travels at c = 3 x 105 km/s (in vacuum)

Light a type of e-m radiation

Acts like a wave, and a particle

Wavelength: distance between two successive wavecrests. Any           value greater than 0.

Frequency (f): number of wavecrests that pass per second.

For electromagnetic radiation:

c = 3x105 km/sec

Short wavelength - high frequency

Long wavelength - low frequency

Visible light:

300-800 nm (nanometer)

1 nm = 10-9 m

Blue light 400 nm

Red light 700 nm

Spectrum: arrangement of electromagnetic radiation by wavelength

Regions of the spectrum:

Name            Wavelength

High energy

      Gamma rays       10-12 m (10-3nm)

      X-rays                 10-2nm - 10nm

Ultraviolet            10nm - 300nm

Visible                 300nm - 800, 1000 nm

Infrared                 1000 nm - 106nm (0.1 cm)

Radio                 longer than 0.1 cm (includes microwave)

Photon: particle of light

      Amount of energy depends on wavelength

      Shorter wavelengths - higher energy (Blue light, x-rays)

      Longer wavelengths - lower energy (Red light, radio waves)

Refraction: the bending of electromagnetic radiation as it passes from one transparent medium to another

Prism: different wavelengths refracted different amounts

Light spread out by wavelength (color), producing spectrum

Types of Spectra (Kirchhoff's Laws)

continuous spectrum - hot, dense body

absorption spectrum - continuous spectrum behind cool gas cloud

emission spectrum - hot, low density gas cloud

Elements identified by patterns of lines


Nucleus: 0.0000016 nm

Nucleus: protons (+)

               neutrons (0)

Electron Cloud: 0.1-0.5 nm
            electrons (-)

Bohr Model: electrons exist only in certain orbits

Orbits correspond to electron energy (energy levels)

Energy state of electron is raised if add energy.

Must add energy equal to energy difference of two energy levels

Only photons of particular energies (wavelengths) can be absorbed by atom

Each element has its own set of energy levels, absorbs only particular photons

Atoms return to lowest energy state by emitting photons corresponding to energy difference of two levels

Doppler effect: change in wavelength of emitted light because           of motion towards or away from earth.

          Motion towards earth, lines shift to shorter wavelength,                     'Blueshift'

          Motion away from earth, lines shifted to longer wavelength,     'Redshift'

          Measurement of radial velocities only


Refracting telescopes - lenses

Telescopes have 2 lenses

          Objective lens


Problems with refracting telescopes

Reflecting telescopes - use mirrors - glass coated with aluminum

          Primary mirror (objective mirror): focuses image at prime                     focus.

          Various arrangements of secondary mirrors

Modern telescopes

          a. Segmented mirrors, e.g. Keck telescope

          b. Spin-cast mirrors - honeycombed glass

          c. Floppy mirrors - thin - (computer controlled)

          d. Adaptive optics: nullify effects of atmosphere on light                               passing through it.

Seeing: blurring of light as it passes through the atmosphere

          Telescopes in space avoid problems with seeing

Adaptive optics measures bright star to determine distortion, adjusts a deformable mirror to correct for the distortion.

Telescope Parameters

Light gathering power - depends on area of objective lens or primary mirror.

Light gathering power proportional to diameter squared.

Resolving power (R.P.) - angular distance between 2 stars can just separate in telescope.

Radio Telescopes

          reflecting 'dishes' (antenna)

          Resolving power (R.P.)

Diam. = 25 meter

R.P. = 104 arcsec = 2.8 deg.

Want a large diameter!!

          E.g. 3 arcsec resolution at 21 cm requires 18 km telescope!!!

Radio Interferometer

          > 2 radio telescopes linked together

          R.P. equivalent to a telescope with diameter equal to           distance between telescopes

          Very Large Array (VLA)

                    27 telescopes

                    Simulates radio telescope >30 km in diameter

Very Long Baseline Interferometer (VLBI)

VLB Array - 10 telescopes Virgin Islands to Hawaii

Space Astronomy - Why Bother?

      1. Detect wavelengths that don't penetrate to earth's surface.

      2. Get above turbulence in earth's atmosphere

Infrared (IR) Astronomy

      Dry mountain tops

      Balloons, SOFIA - 747 - 2005

      IRAS - Infrared Astronomy Satellite

          Telescope cooled to near 0 deg.

      ISO - Infrared Space Observatory (European)

      SIRTF (Space Infrared Telescope Facility) 2003

Ultraviolet Astronomy (UV)

      < 300 nm (nm = 10-9 m) absorbed by ozone

      Must observe from space



X-ray Astronomy

      Violent events

      10 nm - 0.01 nm

      balloons, rockets, satellites

          CHANDRA - currently in orbit

Gamma-Ray Astronomy



            active galactic nuclei

Cosmic Rays

      energetic particles - protons, electrons, positrons, other             atomic nuclei

      detected from satellites

      'showers' detected on ground

      source not well-understood


      particle with tiny mass, no electric charge, travels at speeds                   very close to c

      generated in nuclear reactions in stars, at origin of universe

      Difficult to detect - don't interact with ordinary matter


            Large tanks of chlorine or very pure water