What is in the Solar System?

Our Solar System is one star — the Sun — and everything held in orbit around it. The spec expects you to know its members:

• the Sun (the central star);
• the eight planets (Mercury → Neptune);
• dwarf planets, such as Pluto;
• moons — natural satellites orbiting planets;
• asteroids — rocky bodies, mostly between Mars and Jupiter;
• comets — icy bodies on long, very stretched (highly elliptical) orbits.

Scale up: the Sun and its system sit within a galaxy (the Milky Way — billions of stars), and the universe is billions of galaxies.

Name that body

Why things orbit

Every mass sits in a gravitational field. The gravitational force this field exerts is what keeps planets, moons, comets and satellites in orbit — it constantly pulls the orbiting body towards the central body.

Analogy — ball on a string: whirl a ball on a string in a circle and the string tension pulls it inward; cut the string and it flies off in a straight line (a tangent). For a planet, gravity is the string.

Watch out: there is no "outward" force flinging the planet away. Gravity is the only force, and it provides the centripetal force toward the centre. An orbiting body is really continually falling toward the central body — it just keeps missing because it moves sideways so fast.

Which way does the force point?

How fast does it orbit?

Treat an orbit as a circle of radius r. In one full orbit the body travels the circumference, 2πr, in the time of one orbit — the orbital period T. So its orbital speed is:

Rearranged, you can also find the radius or the period:

Reasoning, not just plugging in: the further a planet is from the Sun, the weaker the Sun's gravitational pull, so the planet orbits more slowly and its period is longer. Mercury races; Neptune crawls.

Your turn — orbital speed

Your turn — rearrange for the period

Comparing orbits

The life cycle of a star

A star is born in a nebula — a giant cloud of gas (mostly hydrogen) and dust. Gravity pulls the cloud together into a protostar, which heats up until nuclear fusion of hydrogen begins. It then becomes a stable main-sequence star, like our Sun today.

A main-sequence star is a balancing act:

Misconception fix: a star is not "trying to explode" or "trying to collapse" — it sits in equilibrium between gravity pulling in and pressure pushing out. When the hydrogen fuel runs low, that balance breaks and the star changes.

Two fates — set by mass

When the fuel runs low, what happens next depends on the star's mass:

Order a massive star's life

A star much more massive than the Sun goes through these stages. Tap a stage, then tap the slot it belongs in — earliest at the top.

What keeps a star stable?

Sorting stars: the H–R diagram

The spec also asks you to describe how the brightness of a star can be shown using its absolute magnitude, and how stars are classified on a Hertzsprung–Russell (H–R) diagram — a plot of luminosity (brightness) against temperature.

• Main-sequence stars (including the Sun) form a diagonal band across the H–R diagram.
• Red giants and supergiants sit up to the top-right (cool but very bright).
• White dwarfs sit at the bottom-left (hot but dim).

Absolute magnitude is a measure of a star's true brightness on a standard scale — a smaller (or more negative) magnitude means a brighter star.

Red-shift: light from galaxies

When a wave source moves relative to an observer, the observed frequency and wavelength change — the Doppler effect. Light from almost every distant galaxy has its wavelength stretched longer, shifting toward the red end of the spectrum: this is red-shift, and it tells us the galaxy is moving away from us.

Misconception fix: red-shift is not the galaxy "turning red". It is the wavelength being stretched as space itself expands between us and the galaxy. The galaxies are not flying through space away from a centre — the space between them is growing.

Reading a red-shift

Your turn — speed of a galaxy

An expanding universe

More distant galaxies show greater red-shift, so they are receding faster. This is exactly what you'd see if the whole universe is expanding.

Run that expansion backwards in your mind: everything was once squeezed into a tiny, hot, dense point that began expanding. This is the Big Bang theory. Two key pieces of evidence support it:

• Red-shift of galaxies — the universe is expanding (8.18).
• The cosmic microwave background (CMB) — faint microwave radiation reaching us from all directions, the cooled-down "afterglow" of the early hot universe (8.14).

The debate (8.16–8.17): the older Steady State theory said the universe has always looked the same, with new matter created as it expands. The Big Bang theory says it began in a hot, dense state and has been expanding ever since. The discovery of the CMB fits the Big Bang but not Steady State, which is why the Big Bang is now the accepted model.

Evidence for the Big Bang

Section 8 in a nutshell

You've covered all of Edexcel International GCSE Physics (4PH1) Section 8 — Astrophysics. Press Finish to see your score.