Astrophysics: Earth & the Universe

The Solar System

Our Solar System is held together by the gravity of the Sun, a star at its centre. The Sun contains over 99% of all the mass in the Solar System, which is why everything else orbits around it.

Orbiting the Sun are eight planets. In order of increasing distance from the Sun they are:

1. Mercury
2. Venus
3. Earth
4. Mars
5. Jupiter
6. Saturn
7. Uranus
8. Neptune

The four inner planets (Mercury to Mars) are small, dense and rocky. The four outer planets (Jupiter to Neptune) are much larger gas giants.

Besides the planets, the Solar System also contains:

Dwarf planets — bodies such as Pluto and Ceres that orbit the Sun but are not large enough to have cleared other objects from their orbital path.
Moons (natural satellites) — objects that orbit a planet, such as our Moon orbiting Earth.
Asteroids — small rocky bodies, most found in the asteroid belt between Mars and Jupiter.
Comets — bodies of ice and dust that travel on long, highly elliptical orbits. As a comet nears the Sun, ice vaporises and forms a glowing tail.

Key terms

Star — a hot ball of gas (mostly hydrogen) that gives out its own light from nuclear fusion.

Planet — a large body orbiting a star that has cleared its orbital path; it does not produce its own light.

Natural satellite (moon) — a body that orbits a planet.

Gravity and Orbits

Every object orbits because of the gravitational force pulling it towards the larger body. The Sun's gravity pulls on the planets; a planet's gravity pulls on its moons.

For an object moving in a circle at steady speed, there must be a resultant force directed towards the centre of the circle. This is called the centripetal force. In space, gravity provides this centripetal force.

Notice that the gravitational force (centripetal force) always points towards the centre, while the planet's velocity points along the orbit, at right angles to the force. The force constantly changes the direction of motion without changing the speed, so the planet keeps moving in a circle.

Orbital Speed

For an object in a circular orbit, in one complete orbit it travels the circumference of the circle, $2\pi r$, in a time equal to its orbital period $T$. So its orbital speed is:

$$v = \frac{2\pi r}{T}$$

where $v$ is the orbital speed (m/s), $r$ is the orbital radius (m) and $T$ is the orbital period (s).

Worked example

A satellite orbits Earth at a radius of $7.0 \times 10^{6}$ m with an orbital period of $5800$ s. Calculate its orbital speed.

Write the equation:

$v = \dfrac{2\pi r}{T}$

Substitute the values:

$v = \dfrac{2 \times \pi \times 7.0 \times 10^{6}}{5800}$

$v = \dfrac{4.40 \times 10^{7}}{5800}$

$v \approx 7600 \text{ m/s}$

The satellite travels at about $7.6$ km/s.

Exam tip

Keep everything in SI units: radius in metres and period in seconds. If a period is given in days or hours, convert to seconds first (1 day = $86\,400$ s).

Why Period Increases with Distance

The further a planet is from the Sun, the longer its orbital period. There are two reasons:

The orbit is larger, so there is a greater distance ($2\pi r$) to travel.
Gravity is weaker further out, so the orbital speed $v$ is slower.

A larger path travelled at a slower speed means the period $T$ is much longer. This is why Mercury takes only 88 days to orbit the Sun, while Neptune takes about 165 Earth years.

Stars, Galaxies and the Universe

These three terms describe very different scales:

A star is a single hot ball of gas releasing energy by nuclear fusion (our Sun is a star).
A galaxy is an enormous collection of billions of stars, held together by gravity. Our galaxy is the Milky Way.
The universe is everything that exists — all the galaxies, and the space between them. It contains billions of galaxies.

Key terms

Galaxy — a huge group of billions of stars bound together by gravity.

Universe — the whole of space and everything in it, made up of billions of galaxies.

The Life Cycle of a Star

Stars form, live and eventually die. What happens depends on the mass of the star.

Both types of star begin in the same way:

1. A star forms from a nebula — a giant cloud of gas (mostly hydrogen) and dust. Gravity pulls the cloud together until it becomes hot and dense enough for nuclear fusion to begin.
2. The star then spends most of its life as a stable main sequence star, like the Sun is now. The outward pressure from fusion balances the inward pull of gravity, keeping the star a steady size.

A star like the Sun (low mass):

1. When its hydrogen runs low, the star swells and cools into a red giant.
2. The outer layers drift away (forming a planetary nebula), leaving a hot, dense core called a white dwarf, which slowly cools.

A much more massive star (high mass):

1. It swells into an even larger red supergiant.
2. It then explodes in a supernova.
3. The remaining core collapses into a neutron star, or — if the star was massive enough — into a black hole.

Exam tip

The first two stages (nebula → main sequence) are the same for all stars. Make sure you can state which mass of star ends as a white dwarf and which ends as a neutron star or black hole.

Red Shift and the Expanding Universe

When astronomers study the light from distant galaxies, they find that the light is shifted towards the red (longer wavelength) end of the spectrum. This is called red shift.

Red shift tells us that distant galaxies are moving away from us. Key observations are:

Light from almost all galaxies is red-shifted.
The further away a galaxy is, the greater its red shift, so the faster it is moving away.

This is strong evidence that the whole universe is expanding. If the universe is expanding now, then in the past everything must have been closer together. Running this backwards points to a single starting point — the Big Bang, the explosion of space from which the universe began.

Key terms

Red shift — the increase in the wavelength of light from a galaxy that is moving away from us.

Big Bang — the theory that the universe began from a single point and has been expanding ever since.

Studying Space with Light and Other EM Waves

We cannot travel to the stars, so almost everything we know about space comes from the electromagnetic waves they emit. Different waves reveal different things:

Visible light and radio waves are used by telescopes to map stars and galaxies.
Studying the spectrum of light (and its red shift) tells us how fast galaxies are moving and what they are made of.
Other waves such as infrared, X-rays and microwaves reveal hot gas, distant objects and the faint background radiation left over from the Big Bang.

Because light travels at a finite speed, light from very distant galaxies has taken billions of years to reach us. So when we look far out into space, we are also looking far back in time — seeing objects as they were long ago.

Real world

Space telescopes are placed above the atmosphere so the air cannot absorb or blur the waves. They capture infrared, ultraviolet and X-rays that would otherwise never reach the ground, letting astronomers see the birth of stars and the most distant galaxies in the universe.