Space physics

📚The physics

Why we have days, years and Moon phases. The Earth rotates on its axis once every 24 hours, which gives day and night. The Earth orbits the Sun once every 365¼ days, which gives the year — and the quarter is why we need leap years. The Moon orbits the Earth about once a month, and we see phases because we see different amounts of its sunlit half from where we stand. The Moon is not changing shape and the Earth’s shadow is not involved; that is a lunar eclipse, which is a different and much rarer thing.

The Solar System has eight planets. In order out from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. The four inner ones are small and rocky; the four outer ones are large and gaseous, because the young Sun’s heat drove the lighter material outwards before the planets formed. Everything formed from a rotating cloud of gas and dust pulled together by gravity, which is why nearly all the orbits lie in one plane and go the same way round.

What keeps a planet in orbit is the gravitational attraction of the Sun, acting towards the centre. Orbits are ellipses rather than perfect circles, though most are close to circular. Planets further from the Sun travel more slowly and have much longer years — Neptune takes 165 Earth years to go round once.

EXTENDED For a roughly circular orbit the speed is \( v = \dfrac{2\pi r}{T} \), since \( 2\pi r \) is the circumference and \(T\) is the time for one lap.

The Sun is a star of medium size, made mostly of hydrogen and helium. It releases energy by nuclear fusion in its core, and radiates most strongly in the visible and infrared parts of the spectrum.

The life cycle of a star. All stars begin as a nebula, a cloud of gas and dust that gravity pulls together into a protostar. When the core gets hot enough for fusion, the star becomes stable — it stays that way for most of its life because the inward pull of gravity is balanced by the outward push from the hot core. What happens next depends on mass:

• A star like the Sun swells into a red giant, then sheds its outer layers as a planetary nebula and leaves a white dwarf.
• A much more massive star swells into a red supergiant, then explodes as a supernova, leaving a neutron star or, if massive enough, a black hole.

Supernovae matter beyond the exam: they are where elements heavier than iron are made, and they scatter them into space. The iron in your blood was made in a star that exploded.

The Milky Way is our galaxy: a spiral containing hundreds of billions of stars, with the Sun somewhere in one of its arms. Distances are measured in light-years — the distance light travels in a year, about \( 9.5 \times 10^{15} \) m. A light-year is a distance, not a time, and saying otherwise is a guaranteed lost mark.

Redshift and the Big Bang. Light from distant galaxies is shifted towards the red end of the spectrum, and the further away a galaxy is, the greater its redshift. That means distant galaxies are moving away from us, and faster the further they are. It does not mean we are at the centre of anything — space itself is expanding, so every observer anywhere sees the same thing. Running that expansion backwards gives the Big Bang.

EXTENDED The Hubble constant \( H_0 \) relates a galaxy’s speed to its distance, \( v = H_0 d \). Rearranged as \( d/v = 1/H_0 \), that ratio is roughly the time since everything was in the same place — an estimate of the age of the Universe.