Beyond the Solar System

Beyond our solar system

Our solar system — the Sun plus all the objects orbiting it — is enormous. The distance from the Sun to Pluto is about 6 billion km. Yet this is tiny compared to the distances between stars. The nearest star system to our Sun is Alpha Centauri, about 4.3 light years away.

The light year

Distances in space are so vast that kilometres become impractical. Scientists use the light year (ly) as a unit of distance:

1 light year = the distance light travels in one year in a vacuum = approximately 9.46 × 10¹² km (about 9.5 trillion kilometres)

Light travels at 300 000 km/s. In one year it covers almost 10 trillion km. When you look at Alpha Centauri, you are seeing light that left it 4.3 years ago — you are looking into the past.

Object	Distance from Earth
Moon	1.3 light seconds
Sun	8.3 light minutes
Alpha Centauri (nearest star system)	4.3 light years
Centre of Milky Way	~26 000 light years
Andromeda Galaxy (nearest large galaxy)	~2.5 million light years
Observable universe (radius)	~46 billion light years

Our galaxy — the Milky Way

A galaxy is a massive system of millions or billions of stars, gas, dust and dark matter held together by gravity. Our solar system belongs to the Milky Way galaxy:

Contains approximately 200–400 billion stars
Diameter: about 100 000 light years
Shape: a barred spiral galaxy
Our solar system is located in one of the spiral arms, about 26 000 light years from the galactic centre
On a clear dark night you can see the Milky Way as a bright band across the sky — you are looking through the disc of our galaxy edge-on

Types of galaxies

Spiral galaxies — flat disc with spiral arms; example: Milky Way, Andromeda
Elliptical galaxies — ball-shaped, no spiral arms
Irregular galaxies — no regular shape; example: the Magellanic Clouds visible from South Africa

South African connection: The Magellanic Clouds — the Large and Small Magellanic Clouds (LMC and SMC) — are two small irregular galaxies visible to the naked eye from the Southern Hemisphere. They are satellite galaxies of the Milky Way, approximately 160 000 and 200 000 light years away respectively.

The universe

The universe contains everything that exists — all matter, energy, space and time. The observable universe contains an estimated 2 trillion (2 000 000 000 000) galaxies. The universe began approximately 13.8 billion years ago in an event called the Big Bang.

Size hierarchy

It helps to think in levels of scale:

Planet → Planetary system (solar system) → Star cluster → Galaxy → Galaxy cluster → Observable Universe

Write your answers in the spaces below. Use full sentences.

1. What is a light year? Why do scientists use light years instead of kilometres when measuring distances in space?

2. The nearest star to our Sun is Alpha Centauri, about 4.3 light years away. What does it mean to say you are "looking back in time" when you observe a distant star?

3. Describe the Milky Way galaxy. Where is our solar system located within it?

4. Name the three main types of galaxies and briefly describe the shape of each. Give an example of each type.

5. Write the size hierarchy from smallest to largest, starting with "planet" and ending with "observable universe". Name at least 6 levels.

6. What are the Magellanic Clouds and why are they significant to observers in South Africa? How far away are they?

7. The table below gives the approximate distance of four objects from Earth in light years.

Object	Distance (light years)
Alpha Centauri	4.3
Sirius	8.6
Betelgeuse	640
Deneb	2 600

(a) Calculate how many times farther Deneb is from Earth than Alpha Centauri is (round to the nearest whole number). Show your working.
(b) Human writing systems developed roughly 5 000 years ago. Using the table, explain whether the light you see from Deneb tonight left before or after writing was invented, and by roughly how many years.
(c) Explain why the brightness of a star in the night sky cannot, on its own, tell you how far away it is.