Topic 6: Space physics

Space physics applies the same rules of motion and gravity that you have already met to objects on enormous scales, from moons orbiting planets to galaxies drifting apart. In this topic you will see how the solar system is arranged, how stars are born and die, and how the light arriving from distant galaxies tells us that the whole universe began with the Big Bang.

The Earth and its motions

The Earth spins once on its own axis roughly every 24 hours. This rotation is what produces day and night: the side facing the Sun is in daylight, while the side turned away is in darkness. The Earth also travels around the Sun in a near-circular orbit, completing one full revolution in about 365.25 days, which we call a year. Because the axis of rotation is tilted relative to the plane of the orbit, different parts of the planet receive more direct sunlight at different times of the year, giving rise to the seasons. The Moon, in turn, orbits the Earth roughly once every 27 days; because we always see the same face of the Moon, its rotation period matches its orbital period.

The Sun and the solar system

The Sun lies at the centre of the solar system and contains almost all of its mass. Eight planets orbit the Sun: in order outward they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The four inner planets are small and rocky, while the four outer planets are large and made mostly of gas and ice. Minor bodies such as dwarf planets, asteroids and comets also orbit the Sun. Planets nearer the Sun travel faster and take less time to complete an orbit than planets further out. The strength of the Sun's gravitational field decreases with distance, so the planets feel a weaker pull the further away they are.

Orbits and orbital speed

Planets stay in orbit because the Sun's gravity provides a force that continually pulls them towards the centre, bending their path into a closed loop. The same idea explains moons orbiting planets and artificial satellites orbiting the Earth. For a body moving in an approximately circular orbit, the orbital speed can be calculated from the distance travelled in one orbit divided by the time taken. The circumference of the orbit is 2 pi r, where r is the orbital radius, and the time for one complete orbit is the orbital period T. This gives the equation v = 2 pi r / T. Comets are an interesting case: they follow highly elongated, elliptical orbits, so they speed up when close to the Sun and slow down when far away.

The Sun as a star

The Sun is an ordinary, medium-sized star. Like all stars on the main sequence, it generates its enormous output of energy by nuclear fusion in its core. In this process hydrogen nuclei join together to form helium nuclei, releasing energy that eventually leaves the star as light and heat. The Sun emits radiation across the electromagnetic spectrum, but most strongly as visible light and infrared. A star stays stable for billions of years because the outward push from this fusion energy balances the inward pull of its own gravity.

Galaxies and the light-year

Stars are not spread evenly through space but are gathered into huge collections called galaxies, each containing many billions of stars held together by gravity. Our Sun is just one star in the Milky Way galaxy. Distances in space are so vast that ordinary units like kilometres become clumsy, so astronomers use the light-year. A light-year is the distance that light travels in one year through space. Because light moves at about 300,000 km per second, one light-year is roughly 9.5 million million kilometres. Light from distant galaxies may have set out millions of years ago, so we see these objects as they were in the distant past.

The life cycle of a star

A star begins life inside a nebula, a cloud of dust and gas. Gravity pulls the material together, and as it collapses it heats up to form a protostar; once the core is hot enough, fusion begins and a stable main-sequence star is born. The later stages depend on the star's mass. A star like the Sun eventually swells into a red giant, then sheds its outer layers as a planetary nebula and leaves behind a hot, dense white dwarf that slowly cools. A much more massive star becomes a red supergiant, then explodes as a supernova. The remnant left behind is either a neutron star or, if the original star was extremely massive, a black hole whose gravity is so strong that not even light can escape.

The Big Bang and the expanding universe

The Big Bang theory states that the universe began about 14 billion years ago from a single point of extremely high temperature and density, and has been expanding ever since. The main evidence comes from the light of distant galaxies. When this light is examined, the lines in its spectrum are shifted towards the red, longer-wavelength end. This redshift shows that the galaxies are moving away from us, and the more distant a galaxy is, the greater its redshift and the faster it is receding. This is exactly what we expect if the whole of space is expanding and carrying the galaxies apart.

The Hubble constant (Supplement)

The link between a galaxy's distance and its speed of recession can be written using the Hubble constant. The relationship states that recession speed divided by distance from Earth is approximately constant for galaxies across the universe; this ratio is the Hubble constant, H0. Rearranging gives an estimate for the age of the universe, because dividing distance by speed gives roughly the time the galaxies have been travelling apart. Taking the value of the Hubble constant as about 2.2 times 10 to the power minus 18 per second leads to an estimated age for the universe of around 14 billion years, which agrees well with other evidence.