INTRODUCTION
Everything covered in this lesson, in one place - useful for revision or printing.
Planet Earth and Beyond Beyond the Solar System Our galaxy, the stars and the Universe
In the Solar System lesson you met the Sun and everything that orbits it. Now we travel much farther: out of the Solar System, across our galaxy and beyond it.
In this lesson you will learn:
The Universe The Milky Way Galaxy Our Solar System Earth
Earth is one of the planets in our Solar System. But the Solar System is not alone in space.
Our Solar System is in the Milky Way Galaxy, and the Milky Way is part of the Universe.
Think of it as an address in space: Earth, in the Solar System, in the Milky Way Galaxy, in the Universe.
Galaxy A collection of stars held together by their mutual gravity
A galaxy is a collection of stars held together by their mutual gravity.
Mutual means it works both ways: every star pulls on the other stars, and they pull on it. All these pulls together hold the galaxy together.
It is the same force that keeps the planets in their orbits around the Sun.
Quick check: Which force holds the stars of a galaxy together?
Our Sun is one of billions of stars in the Milky Way
The Sun is a star, and it is only one of billions of stars in the Milky Way Galaxy.
The other stars are suns too. The Sun only looks bigger and brighter than all of them because it is so much closer to us.
Quick check: Which sentence about our Sun is true?
If you could look down on the Milky Way from far above, you would see a spiral with many arms that curl out from the centre.
Our Sun, with the whole Solar System, is located towards the edge of the galaxy, in one of the spiral arms. We are not at the centre.
This picture is a model drawing. It is not to scale.
Draw a small round centre. Draw several arms that curl out from the centre, like a pinwheel. Put a dot in one arm, towards the edge , in the outer part of the spiral. Label the dot: Our Sun (Solar System) .
Activity. On paper or in your workbook, draw the Milky Way as if you were looking down on it from above. Follow the steps on the board.
Your drawing should show two things: the spiral shape with its arms, and our Solar System's location in the outer part of the spiral, inside one of the arms. Compare it with the model on the previous step.
Quick check: Where should the dot for our Sun go in your drawing?
Seen from the side, the Milky Way is a flat disc, and we live inside it.
From Earth, when we look towards the centre of the Milky Way Galaxy, we look along the disc, through a huge number of distant stars. Their light blends into a hazy path of light across the sky. You can see it on a clear, dark night, far from city lights.
The ancient Greeks described this hazy path as spilled milk. That is where the name Milky Way comes from.
Quick check: The ancient Greeks said the hazy path of light across the sky looked like spilled what?
Galaxy: stars held together by mutual gravity Our Sun: one of billions of stars A spiral with many arms We are towards the edge, in an arm
Remember:
Quick check: When do we see the hazy path of light across the night sky?
The Sun is the nearest star to Earth
The Sun is the nearest star to Earth. It is so close that it lights up our whole sky during the day.
All the other stars are so much farther away that, at night, they look like tiny points of light.
Quick check: Which star is the nearest star to Earth?
Alpha Centauri The nearest easily visible star to the Sun 4,2 light years away from our Solar System
The star called Alpha Centauri is the nearest easily visible star to the Sun. Easily visible means you can see it with your eyes alone, without a telescope.
Alpha Centauri is 4,2 light years away from our Solar System. A light year is a unit for measuring huge distances. You will find out exactly what it means in the next part of the lesson.
The Southern Cross is a constellation: a group of stars that makes a pattern in the night sky, here a small cross. Next to it are two bright stars called the Pointers, because a line through them points to the Southern Cross.
Alpha Centauri is the brighter of the two Pointers. On the star chart, bigger dots are brighter stars. Alpha Centauri is also the Pointer farther from the cross.
From South Africa, look for the Southern Cross and the Pointers in the southern part of the sky on a clear night.
Quick check: Alpha Centauri is one of the two Pointers. Which one?
The Sun: the nearest star to Earth Alpha Centauri: the nearest easily visible star to the Sun Alpha Centauri: the brighter Pointer 4,2 light years from our Solar System
Remember:
Quick check: How many light years away from our Solar System is Alpha Centauri?
about 300 000 km every second
Distances to the stars are enormous. In kilometres the numbers have so many zeros that they are hard to read and hard to compare. So we measure them using something that travels very fast: light.
Light travels about 300 000 km every second. So the distance light travels in a set time makes a very long measuring stick for space.
Light year The distance that light travels in one year
A light year is the distance that light travels in one year.
People use light years to measure distances to stars and other objects beyond the Solar System, because those distances are far too big for kilometres to be useful.
A distance A light year IS a distance, like a kilometre Yes A time A light year is NOT a length of time No
Careful: the word year tricks many people. A light year is not a time. It is a distance, just like a kilometre, only far bigger.
The time is hidden inside the unit: a light year is how far light gets in one year.
So Alpha Centauri is 4,2 light years away tells you how far away it is. It also means its light needs about 4,2 years to reach us.
Quick check: Thandi says: "A light year is a measure of time, because it has the word year in it." Is she right?
One light year is about 10 trillion km 10 000 000 000 000 km
One light year is equal to about 10 trillion kilometres.
Written out in full, that is 10 000 000 000 000 km. Try reading that number aloud, and you will see why scientists prefer to say one light year.
Distance to Alpha Centauri 4,2 light years One light year about 10 trillion km Distance in km 4,2 × 10 trillion km = ?
Now you can change light years into kilometres.
Alpha Centauri is 4,2 light years away, and each light year is about 10 trillion km. So multiply the number of light years by 10 trillion km.
Quick check: Work out 4,2 × 10. About how many trillion km away is Alpha Centauri?
Light: about 300 000 km every second Light year: the distance light travels in one year A light year is a distance, not a time One light year: about 10 trillion km Alpha Centauri: 4,2 × 10 trillion = 42 trillion km
Remember:
Quick check: What do people use light years to measure?
Light hour The distance that light travels in one hour Our Solar System: about 13 light hours across
A light hour is the distance that light travels in one hour. It is much shorter than a light year, but still enormous.
Our Solar System has a diameter of about 13 light hours. The diameter is the distance straight across, through the centre. So light would need about 13 hours to cross the whole Solar System.
Quick check: Our Solar System has a diameter of about 13 light hours. What does that tell you?
Light minute The distance that light travels in one minute Earth to the Sun: about 8 light minutes
A light minute is the distance that light travels in one minute.
Earth is about 8 light minutes away from the Sun. So the sunlight reaching you right now left the Sun about 8 minutes ago.
Quick check: Imagine the Sun suddenly stopped shining. About how many minutes would pass before we saw it go dark on Earth?
about 8 light minutes from the Sun to Earth much farther about 13 light hours across our Solar System much farther 4,2 light years to Alpha Centauri much farther about 28 billion light years the size of the observable Universe Not to scale Each rung is far, far bigger than the one before, so these distances cannot be drawn to scale on one line. Read it as a list, from smallest to largest.
Here are the light distances in this lesson, from smallest to largest.
Choose the unit that fits the distance: light minutes inside the Solar System, light hours across it, and light years for stars and everything beyond.
The ladder is not to scale: each rung is so much bigger than the one before that they cannot fit on one line.
Light minute: the distance light travels in one minute Sun to Earth: about 8 light minutes Light hour: the distance light travels in one hour Solar System: about 13 light hours across
Remember:
Quick check: Which of these is the greatest distance?
The Milky Way is only one of billions of galaxies
The Milky Way is enormous, but it is only one of billions of galaxies scattered across the Universe.
Each of those galaxies is also a collection of stars held together by their mutual gravity.
Quick check: How many galaxies are there in the Universe?
The observable Universe is estimated to be about 28 billion light years in size
The observable Universe is the part of the Universe that we are able to observe. Its size is estimated to be about 28 billion light years. It is an estimate because it is far too big to measure directly.
Remember that a light year is a distance. Alpha Centauri, only 4,2 light years away, is already 42 trillion km from us. The observable Universe is far, far bigger.
Quick check: The size of the observable Universe is estimated to be about how many billion light years?
Galaxies have various shapes and sizes. Three shapes you should know:
Quick check: A galaxy that is a smooth oval ball of stars with no arms is:
The Milky Way: one of billions of galaxies Observable Universe: about 28 billion light years Galaxy shapes: spiral, elliptical, irregular
Remember:
Quick check: What shape is our own galaxy, the Milky Way?
Our Solar System is in the Milky Way Galaxy A spiral; our Sun is towards the edge, in an arm Alpha Centauri: 4,2 light years away A light year is a distance, not a time Billions of galaxies; various shapes
Beyond the Solar System: full summary