A central assumption: The fundamental physical laws that apply on earth and can be tested in terrestrial laboratories also apply to astronomical objects. Physics can teach us about astronomy, and astronomy can teach us about physics.


Astronomy deals with very large and very small numbers, so we will frequently use scientific notation.

100 = 1
101 = 10
102 = 10x10 = 100
103 = 10x10x10 = 1000
106 = 103x103 = 1 million
109 = 103x106 = 1 billion

10-1 = 1/10 = 0.1
10-2 = 1/102 = 0.01
10-3 = 1/103 = 0.001


We observe the universe (and the everyday world) primarily with electromagnetic radiation.

A wave (e.g., in the ocean) is a propagating disturbance. An electromagnetic wave is a propagating disturbance in an electromagnetic field, which affects electrical charges and magnets.

Electromagnetic waves (a.k.a. electromagnetic radiation) can be characterized by their wavelength, `lambda', or by their frequency, `nu'=c/`lambda'.

The electromagnetic spectrum encompasses (from shortest wavelength to longest wavelength): gamma rays, X-rays, ultraviolet radiation, visible light, infrared radiation, microwaves, and radio waves.

All forms of electromagnetic radiation are carried by parcels of energy called photons. All photons travel at the speed of light:

c = 300,000 km/sec = 186,000 miles/sec.

Photons have properties of both waves and particles.

The energy of a photon is proportional to its frequency, E=h`nu'=hc/`lambda'. Gamma-ray photons have the highest energy, radio photons have the lowest energy.

Only visible light and radio waves can easily get through to the earth's surface. Most other electromagnetic radiation is absorbed by the atmosphere, and astronomers can observe it only from satellites.


The wavelengths of waves emitted by a moving source are shifted by the Doppler effect.

Source moving away -> light shifted to red (longer wavelength).
Source moving towards ->light shifted to blue (shorter wavelength).
The shift is small if speed is much smaller than c.

An astronomer can use a spectrograph (very fancy prism) to measure the spectrum of an object, the intensity of its light as a function of wavelength.

The spectrum of a star contains many narrow dark bands, where atoms and molecules in the star's atmosphere absorb light.

By comparing the wavelength at which we observe a line to the wavelength at which it was emitted, we can determine the velocity of the star relative to us.


With a few exceptions, astronomical objects do not change on human timescales.

We must build our understanding of the history of stars, galaxies, and the cosmos from ``snapshots.''

Three broad approaches:

  1. Reconstruct history via theory; test theories with observation.
  2. Infer timescale from frequency, more common -> longer lived.
  3. Look at distant objects to see back in time. Telescopes as time machines.

Unlike most sciences, astronomy is ``look but don't touch.'' We cannot experiment with astronomical objects, we can only observe them.

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Updated: 1997 January 11 [dhw]