Astronomy Revision Notes

The Big Bang Theory

The Big Bang theory describes the Universe as originating from an extremely hot and dense state, expanding and cooling over billions of years to form the structures we observe today. Key evidence supporting it includes the expansion of the Universe (redshift of galaxies) and the existence of cosmic microwave background (CMB) radiation.

The Steady State Theory

The Steady State theory proposed that the Universe has always existed in roughly the same form, with new matter continuously forming to fill the voids created by expansion, thereby maintaining a constant density. This theory did not predict a definite beginning or end for the Universe.

Why Big Bang is Accepted

Scientific consensus overwhelmingly favors the Big Bang theory. This is because significant observational evidence, such as the cosmic microwave background radiation and the red-shift of light from distant galaxies, is difficult to explain with the Steady State theory. The accumulation of robust evidence led to the Big Bang becoming the currently accepted model for the Universe's origin.

The Doppler Effect Explained

The Doppler Effect is the apparent change in frequency and wavelength of a wave due to the relative motion between the source of the wave and the observer. It occurs for all types of waves, including sound and light.

Source Moving Towards Observer

When a wave source moves towards an observer, the wave fronts are perceived to be closer together. This 'compression' leads to a shorter observed wavelength and a higher observed frequency. For sound waves, this means a higher pitch; for light waves, it means a shift towards the blue end of the spectrum (blueshift).

Source Moving Away From Observer

When a wave source moves away from an observer, the wave fronts are perceived to be spread further apart. This 'stretching' leads to a longer observed wavelength and a lower observed frequency. For sound waves, this means a lower pitch; for light waves, it means a shift towards the red end of the spectrum (redshift).

Key Principles of the Doppler Effect

The speed of the wave through the medium generally remains constant, regardless of the source's motion. What changes is how often the wave crests reach the observer. The Doppler effect depends only on the relative velocity between the source and the observer.

Gravitational Field Strength (g)

Gravitational field strength, 'g', is the force experienced per unit mass in a gravitational field. It is measured in Newtons per kilogram (N/kg). The value of 'g' depends on the mass and radius of the celestial body. A more massive body or a body with a smaller radius (for the same mass) will generally have a stronger gravitational field strength at its surface. Earth's 'g' is approximately 9.8 N/kg, while the Moon's 'g' is about 1.6 N/kg.

Weight vs. Mass

Mass is a measure of the amount of matter in an object and is a scalar quantity, remaining constant regardless of location. Weight, however, is a force and is defined as mass multiplied by gravitational field strength (Weight = mass × g). Since 'g' varies between celestial bodies, an object's weight will also vary. An object's weight on the Moon will be about one-sixth of its weight on Earth because the Moon's 'g' is about one-sixth of Earth's 'g'.

Factors Affecting 'g'

The gravitational field strength 'g' at the surface of a celestial body is directly proportional to its mass and inversely proportional to the square of its radius. This means a larger, more massive planet will have a stronger 'g' compared to a smaller, less massive body if all other factors are constant. This is why the Moon, being less massive and smaller than Earth, has a significantly lower 'g' value.