Topic F: Astronomy and Space Science (Elective)

This elective extends mechanics and waves to the scale of the Universe. You will learn how the night sky is organised, how astronomers measure vast distances and brightness, how gravitation governs the orbits of planets and satellites, and how stars are born, evolve and die, leading to modern ideas about the origin and fate of the cosmos.

Stars and the night sky

The night sky is filled with stars grouped by observers into constellations, which serve as a map for locating objects. The stars appear to rotate around a celestial pole because the Earth spins on its axis, and different constellations are visible in different seasons because the Earth orbits the Sun. Our Sun is an average star, one of hundreds of billions in the Milky Way galaxy, which is itself one of countless galaxies. Stars differ in colour, which indicates their surface temperature: blue and white stars are hottest, while red stars are coolest. The apparent brightness of a star depends on both its true luminosity and its distance from us.

Astrophysical measurements: distance and brightness

Because astronomical distances are enormous they are measured in special units: the astronomical unit is the average Earth-Sun distance, the light year is the distance light travels in one year, and the parsec is based on parallax. Parallax is the apparent shift of a nearby star against distant background stars as the Earth moves around the Sun; the larger the shift, the closer the star. Luminosity is the total power a star radiates, while apparent brightness is how bright it looks from Earth, which falls off with the square of distance by the inverse square law. By comparing luminosity and apparent brightness, astronomers can deduce a star's distance, and the colour of a star reveals its surface temperature.

Telescopes

Telescopes gather and focus radiation to reveal faint and distant objects. A refracting telescope uses a large converging objective lens to collect light and form an image, which is magnified by an eyepiece; a reflecting telescope uses a curved mirror instead, avoiding the colour distortion and weight problems of large lenses. A larger aperture collects more light, making faint objects visible, and gives better resolution, the ability to distinguish close objects. Astronomers also build telescopes for other parts of the electromagnetic spectrum, such as radio, infra-red and X-ray telescopes, and place some in orbit to avoid the blurring and absorption caused by the Earth's atmosphere.

Orbital motion and gravitation

Gravitation explains the orbits of planets, moons and artificial satellites. For a body in a circular orbit the gravitational force provides exactly the centripetal force needed, G M m over r squared = m v squared over r, which links the orbital speed to the radius. This leads to Kepler's third law, that the square of the orbital period is proportional to the cube of the orbital radius, so planets farther from the Sun move more slowly and take longer to orbit. The same physics fixes the height of a geostationary satellite, which must orbit with a 24-hour period to stay above one point on the equator. Escape velocity is the minimum speed needed to break free of a body's gravity entirely.

The life cycle of stars

A star forms when gravity pulls together a cloud of gas and dust until the core becomes hot and dense enough for nuclear fusion of hydrogen into helium to begin. During this long, stable main-sequence phase the outward pressure from fusion balances the inward pull of gravity. When the hydrogen fuel runs low the star swells into a red giant. A low-mass star like the Sun then sheds its outer layers and leaves a small, dense white dwarf. A high-mass star fuses heavier elements, then collapses and explodes as a supernova, leaving behind a neutron star or, if massive enough, a black hole. The heavy elements scattered by supernovae seed new stars and planets.

Cosmology and the expanding Universe

Cosmology studies the origin, structure and evolution of the Universe as a whole. When the light from distant galaxies is analysed, its spectral lines are shifted toward longer, redder wavelengths, a red shift that shows the galaxies are moving away from us. Hubble found that more distant galaxies recede faster, evidence that the whole Universe is expanding. Running this expansion backward leads to the Big Bang theory, the idea that the Universe began about 13 to 14 billion years ago from an extremely hot, dense state and has been expanding and cooling ever since. The faint cosmic microwave background radiation detected from every direction is strong supporting evidence for the Big Bang.

Spectra and the composition of stars

Much of what we know about distant stars comes from analysing their light. A hot dense source gives a continuous spectrum, but the cooler gas in a star's outer atmosphere absorbs specific wavelengths, producing dark absorption lines whose pattern acts as a fingerprint of the elements present. By matching these lines to those measured in the laboratory, astronomers identify which elements a star contains. The overall colour and the peak of the emitted spectrum reveal surface temperature, while the red or blue shift of the lines reveals motion toward or away from us. Spectroscopy therefore turns starlight into detailed information about composition, temperature and movement without ever visiting the star.