Topic 8: Space physics

Space physics asks how the objects above us are arranged, how stars are born, live and die, and how the whole universe began. By studying the light from distant galaxies and the forces that hold orbits together, physicists piece together a history stretching back billions of years.

The solar system and our place in it

Our solar system sits within the Milky Way galaxy, which is just one of billions of galaxies in the universe. At the centre of the solar system is the Sun, a fairly ordinary star. Orbiting it are eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The four inner planets are small and rocky, while the four outer ones are large gas giants. Many planets have natural satellites (moons) in orbit around them, and the solar system also contains dwarf planets such as Pluto, asteroids, and comets. The Sun formed from a cloud of dust and gas (a nebula) that was pulled together by gravity. As this material collapsed, the centre became hot and dense enough for nuclear fusion to begin, and the leftover material formed the planets. A galaxy is a huge collection of stars held together by gravity, and the universe is everything that exists, made up of all the galaxies.

The life cycle of a star

Stars form when gravity pulls together a nebula, a cloud of dust and gas. As the material is squeezed, it heats up until fusion of hydrogen into helium begins, and a stable main sequence star is born. During this long, stable phase the inward pull of gravity is balanced by the outward force from the very hot gases and radiation. When the hydrogen in the core runs low, the star swells and cools at the surface to become a red giant (for a star about the size of our Sun) or a red supergiant (for a much more massive star). A Sun-sized star then sheds its outer layers and the dense core left behind becomes a white dwarf, which slowly cools and fades. A much more massive star instead explodes in a supernova. The remaining core may collapse into an extremely dense neutron star or, if the star was massive enough, into a black hole whose gravity is so strong that not even light can escape.

Nuclear fusion and the formation of the elements

Stars are powered by nuclear fusion, in which the nuclei of light elements join to form heavier ones. In a main sequence star, hydrogen nuclei fuse to make helium, releasing enormous amounts of energy that we receive as light and heat. As a star ages and gets hotter in its core, fusion can build up successively heavier elements, but elements heavier than iron cannot be made by fusion in this normal way because doing so would no longer release energy. The very heaviest elements are created during the extreme conditions of a supernova explosion. These explosions also scatter the newly made elements out into space. This material can later become part of new nebulae, which means the atoms in our bodies and in the planets were forged inside stars that died long ago.

Orbital motion: gravity, speed and radius

Planets orbit the Sun, moons orbit planets, and artificial satellites orbit the Earth. In every case it is the force of gravity that provides the centripetal force needed to keep the object moving in a circle. Although an object in a stable circular orbit travels at a constant speed, its velocity is always changing because its direction is constantly changing. This means the object is always accelerating towards the centre of the orbit. A key idea is the link between orbital speed and orbital radius: for a stable orbit at a given radius, only one particular speed will work. If a satellite were to speed up, it would move into an orbit with a larger radius; if it slowed down, it would drop into a smaller orbit. Objects in smaller orbits must move faster to stay in orbit than objects further out.

Artificial satellites

An artificial satellite is a man-made object placed in orbit around the Earth or another body. Low orbit satellites travel quite close to the Earth and complete an orbit in a short time, which makes them useful for weather monitoring, imaging the surface and scientific observation. Geostationary satellites are placed in a high orbit directly above the equator and take exactly one day to go around, so they appear to stay fixed above the same point on Earth. This makes them ideal for communications and for relaying television and telephone signals, because a ground antenna can simply point at one spot in the sky. The orbit a satellite is given depends on the job it must do, which determines the height and therefore the orbital speed it needs.

Red-shift and the expanding universe

When we observe the light from distant galaxies, we find that the wavelengths are stretched towards the red end of the spectrum. This stretching is called red-shift, and it tells us that those galaxies are moving away from us. The further away a galaxy is, the bigger its red-shift, which means more distant galaxies are moving away faster. The best explanation for this pattern is that space itself is expanding, carrying the galaxies apart. This evidence supports the Big Bang theory, the idea that the whole universe began billions of years ago from a tiny, extremely hot and dense point and has been expanding ever since. Since around 1998, observations have suggested the expansion of the universe is actually speeding up, which scientists link to mysterious dark energy and dark matter that are still not well understood.