Beyond the Solar System, Grade 8
Every big number in this topic gets stated twice, and the two statements disagree.
The Milky Way is given as "hundreds of thousands of stars" on one page and 200 to 400 billion on the next. The universe is "millions of galaxies" in one place and "billions" in another. The Local Group contains the Milky Way on one page and is part of it on the page before.
You do not have to argue with any of it. In every single case one of the two versions is correct, and this page points at the place where the material agrees with itself.
This page covers the Milky Way, our nearest stars, light years, galaxies, black holes and dark matter. For the Sun and the planets, see our solar system practical. For telescopes and the Southern Cross, see looking into space.
Where this fits in the curriculum
| Subject | Natural Sciences |
|---|---|
| Grade | 8 |
| Term | 4 |
| Strand | Planet Earth and beyond |
| Topic | Beyond the solar system |
| Status | Informal activities. This grade puts all three assessed pieces in Terms 1, 2 and 3 |
| Marks | 50 on our worksheet. The topic revision section is 10 |
How many stars are in the Milky Way?
Between 200 and 400 billion.
You will read that the Milky Way consists of "hundreds of thousands of stars". The very next page says 200 to 400 billion, which is correct, and a page after that says "millions".
Three different answers in seven pages, and they differ by a factor of about a million.
Use the contradiction rather than just the fact. Ask a class to find both statements and decide which one can possibly be right. That is a better lesson in how science works than the number on its own.
The shape and size of our galaxy
| Shape | A flat spiral disc with a bulge in the middle |
|---|---|
| Diameter | About 100 000 light years |
| Thickness | About 2 000 light years through the disc |
| Stars | 200 to 400 billion |
| Where our Sun is | About 27 000 light years from the centre, out in one of the spiral arms |
| At the centre | A black hole of about four million times the Sun's mass |
Why it looks like a hazy band from Earth. We are inside the disc, looking along it, so we see the combined glow of billions of stars too distant to tell apart. To the ancient Greeks it looked like spilt milk, which is where the name comes from.
Radio and infrared telescopes are what mapped it. Dust between the stars blocks visible light, and longer wavelengths pass straight through that dust. That is how we worked out the shape of a galaxy we cannot step outside of.
One correction: you will read that these wavelengths pass through "the dust from surrounding planets". It is interstellar dust, the dust between the stars. Planets have nothing to do with it.
Practical 1: the spiral in a bucket, and why it is wrong
Free. A bucket, water, a milk frother or a spoon, and a few drops of food colouring.
- Fill the bucket about three quarters with water
- Stir steadily, or run a milk frother in it, until the water is turning smoothly
- Drop food colouring into the middle and watch a spiral form
- Compare the middle with the edge. The middle turns faster
It makes a convincing spiral in about a minute. Then ask the harder question.
THE MODEL IS WRONG IN THE MOST INTERESTING POSSIBLE WAY.
In the bucket the middle turns much faster than the edge, which is what you would expect if the gravity came only from the visible mass near the centre.
The Milky Way does not behave like that. Stars far out orbit at very nearly the same speed as stars further in. The rotation curve is flat.
That flatness is the strongest evidence we have for dark matter, which the same topic says makes up about 90 % of the galaxy. So the bucket models a galaxy with no dark matter in it, and the real one has plenty.
A class that can say why the bucket is wrong has understood dark matter better than a class that was handed the number.
The nearest star, and the nearest star system
These are two different questions, and they get mixed up constantly.
| The nearest star to Earth | The Sun. 8 minutes 20 seconds away |
|---|---|
| The nearest star system after that | Alpha Centauri, about 4,2 light years |
| The nearest individual star after the Sun | Proxima Centauri, about 4,24 light years |
"Alpha Centauri is the nearest star" is only half an answer. The Sun is a star, and it is the nearest one by an enormous margin.
Three things to fix about Alpha Centauri
1. "Alpha Centauri B is also called Hadar." It is not. Hadar is Beta Centauri, a different star about 390 light years away. The same material names the two pointers correctly in the telescopes topic: Alpha Centauri and Beta Centauri, which is very likely where the mix-up started.
2. "Alpha Centauri forms part of the Southern Cross constellation." It does not. Alpha and Beta Centauri are the two Pointers, and they sit in Centaurus. They point at the Cross; they are not part of it. This one appears in a Key concepts box, which is exactly what a learner revises from.
3. "It is now the closest planet outside our galaxy." Alpha Centauri is 4,2 light years away and our galaxy is 100 000 light years across, so it is very firmly inside it. The phrase should be outside our solar system.
And a planet claim worth checking. A planet said to circle Alpha Centauri B every 3,2 days was announced in 2012 and withdrawn in 2015 when it turned out to be an artefact of the analysis. The real nearby planet is Proxima Centauri b, found in 2016.
Why you cannot see the nearest star of all
You may be asked to find Proxima Centauri in the night sky with the naked eye. You cannot.
Proxima is a faint red dwarf at about magnitude 11. It needs a telescope. Alpha Centauri A and B are brilliant and easy to find; Proxima is not visible at all.
Do the activity, but change the target. Find the two Pointers and the Southern Cross, and tell the class that the closest star to the Sun is sitting right there and is still far too faint to see. That is a better lesson than a failed hunt on a cold night.
What a light year is
A light year is a distance, not a time. It is how far light travels in a year, at 300 000 km per second.
| Unit | Distance in km | How it is worked out |
|---|---|---|
| 1 light second | 300 000 | The speed of light |
| 1 light minute | 18 000 000 | 300 000 x 60 |
| 1 light hour | 1 080 000 000 | 18 000 000 x 60 |
| 1 light year | about 9 460 000 000 000 | Often rounded to ten trillion |
THE ROW THAT IS PRINTED WRONG, AND EVERY CALCULATION RUNS THROUGH IT.
Published versions of this table give the light hour as 1 080 000 000 000 km. That is a thousand times too large.
You can disprove it from the row directly above it. A light hour is sixty light minutes, and 60 x 18 000 000 = 1 080 000 000. The light second, light minute and light year rows are all correct. Only the light hour is wrong.
Have the class find it rather than telling them. Spotting that one row contradicts the row above it is the whole skill this topic is for.
Distances worth knowing
| From | To | Light travel time |
|---|---|---|
| Earth | The Sun | 8 minutes 20 seconds |
| The Sun | Neptune | About 4 hours |
| The Sun | Proxima Centauri | 4,24 years |
| One side of the Milky Way | The other | 100 000 years |
| The Milky Way | Andromeda | 2,5 million years |
One question you may meet that cannot be answered: "calculate the distance between Alpha Centauri and the Milky Way". There is no such distance, because Alpha Centauri is in the Milky Way. Ask how far it is from the centre of the galaxy instead, which has a real answer and teaches the same arithmetic.
Practical 2: looking back in time
Free. A calculator and one very good example.
In 1054 a new star appeared in the sky, bright enough to be seen in daylight. It was a star exploding about 6 300 light years away.
- The explosion actually happened about 6 300 years before 1054, so around 5 250 BCE
- What we see at that spot today left about 6 300 years ago, so we are looking at about 4 300 BCE
- What is there now is the Crab Nebula, the expanding wreckage of that explosion
It is the Crab Nebula, not the Crab Constellation. You will see it called a constellation. A constellation is a pattern of stars; a nebula is a cloud of gas and dust. The Crab Nebula sits inside the constellation Taurus, which is probably how the two got confused.
The point for a class: every time you look up you are looking at the past, and the further away something is, the older the news.
The Local Group, the right way round
"The Local Group is a group of 45 galaxies that forms part of the Milky Way."
It is the other way round. The Milky Way is one member of the Local Group, and the same material says so correctly on the very next page. A group cannot be part of one of its own members.
| The Local Group | A group of galaxies that includes the Milky Way |
|---|---|
| Members | 45 was the older count. Over 80 are known now |
| The two big ones | The Milky Way and Andromeda |
| The Andromeda Galaxy | 2,5 million light years away, and the nearest large spiral |
The shapes galaxies come in
| Shape | What it looks like |
|---|---|
| Spiral | A flat disc with arms. Ours |
| Barred spiral | A spiral with a straight bar across the middle |
| Elliptical | A featureless ball or oval. The most numerous kind |
| Irregular | No clear shape, often after a collision |
You may meet an activity that asks learners to match galaxy photographs to invented nicknames such as "Busy Galaxy" or "Fast Exit". There is no way to work those out and nothing is learnt.
Use the four shapes above instead. Show photographs and sort them into spiral, barred spiral, elliptical and irregular. That is the real classification, it is markable, and it takes the same ten minutes.
And one claim to handle carefully: you may read that "about 400 galaxies have planets in their orbits". Planets orbit stars, not galaxies, and essentially every confirmed planet outside our solar system is in our own galaxy. We expect planets to be ordinary everywhere; we simply cannot detect them in other galaxies yet.
What a black hole actually is
"A black hole is an infinite mass of nothingness with immensely powerful gravity, able to suck up everything in its path."
Three things are wrong with that sentence, and it appears twice: once in the topic and again in the glossary.
| The claim | What is actually true |
|---|---|
| "Infinite mass" | A black hole has an ordinary, finite mass. The one at the centre of our galaxy is about four million times the Sun's. What is modelled as infinite is the density at the centre, not the mass |
| "Of nothingness" | It is the opposite of nothing. It is a lot of matter squeezed into a very small space |
| "Sucks up everything in its path" | No. At a given distance a black hole pulls exactly as hard as any other object of the same mass |
THE QUESTION THAT SETTLES IT.
If the Sun were replaced right now by a black hole of exactly the same mass, what would happen to Earth's orbit?
Nothing. Earth would carry on in the same orbit. It would be dark and desperately cold, but nothing would be pulled in, because gravity depends on mass and distance and neither has changed.
A class that can answer that has stopped thinking a black hole is a vacuum cleaner.
One more correction: the black hole at our galaxy's centre is four million times the mass of the Sun, not four million times its size. Its radius is roughly seventeen times the Sun's.
Dark matter, and why anyone believes in it
Dark matter gives off no light at all. We know it is there because of what it does to things we can see.
The evidence is the flat rotation curve from Practical 1. Stars at the edge of a galaxy orbit far faster than the visible matter can account for. Either gravity works differently out there, or there is a great deal of matter we cannot see.
It makes up roughly 90 % of the mass of the Milky Way, in a halo around the disc.
The expanding universe
| Age | About 13,8 billion years |
|---|---|
| Made of | Ordinary atoms, dark matter and dark energy |
| Expanding? | Yes, and speeding up, which was established in 1998 |
| Does it have a centre? | No. Every galaxy sees the others moving away from it |
You may see the age given as 13,7 billion years. That was the best value from the WMAP satellite; the later Planck results give 13,8. Either is defensible and 13,8 is current.
You may also see the observable universe given as 28 billion light years across. That is roughly the age doubled, which ignores the fact that space itself expanded while the light was travelling. The accepted figure is about 93 billion light years. At this grade, "tens of billions of light years, and larger than you would guess" is enough.
Safety
- Never look at the Sun. Not directly, not through binoculars, a telescope, a lens or a pinhole. Damage to the retina is permanent and painless while it happens
- Stargazing happens at night, outdoors. Go in a group, with an adult, and tell someone where you are
- The bucket practical uses food colouring, which stains clothes
If it does not work
| Problem | Cause | Fix |
|---|---|---|
| No spiral forms in the bucket | Stirring too fast, or not steadily | Steady and gentle, and let the turn settle before adding colour |
| The colour just mixes away | Too much colouring at once | Two or three drops |
| The light hour answers are a thousand times out | You used the printed table | 1 080 000 000 km. Check it against the light minute row |
| The class cannot find Proxima | It is not visible to the naked eye | Find the Pointers and the Cross instead |
| A question asks for the distance from Alpha Centauri to the Milky Way | The question has no answer | Ask for its distance from the galaxy's centre |
| The class thinks a black hole would swallow Earth | The printed definition | Ask the replacement question above |
How the 50 marks are made up
| Part | Marks |
|---|---|
| The Milky Way | 10 |
| Our nearest stars | 10 |
| Light years and distance | 12 |
| Galaxies and the Local Group | 10 |
| Black holes, dark matter and the expanding universe | 8 |
Nothing in this topic is prescribed for formal assessment. The topic revision section is 10 marks. Our worksheet is 50 and the split above is ours.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 50 marks, including the light hour that is printed wrong, the 1054 supernova sum and the black hole question
- Marking memorandum, with what to accept for each answer and the eleven answers that look right and score nothing
Related practicals
- The Solar System, Grade 8. The Sun, the planets, and a scale model walked out on the school field
- Looking into Space, Grade 8. Telescopes, SALT, the Southern Cross and African star lore
- Visible Light, Grade 8. Where the 8 minute 20 second sum comes from, plus the spectrum and the eye
What you need to run it
Nothing you have to buy. Both practicals on this page use a bucket, water, food colouring and a calculator, and the rest of the topic is a night sky that is free to everyone.
That is worth saying plainly, because it is the cheapest topic in the Grade 8 year and a school with no budget can teach all of it properly.
What does help is something on the wall. A chart lets a class see the shape of a galaxy they are standing inside.
| Item | Price | What it is for |
|---|---|---|
| The universe wall chart | R85 | Scale, from the solar system outwards |
| The stars wall chart | R85 | Star types and life cycles |
| Introduction to astronomy wall chart | R85 | A general overview for the classroom wall |
One honest note on the charts. We hold the astronomy charts in ones and twos, not in depth. If you want a set rather than a single chart, ask us first and we will tell you what we can actually send this week.
The models and charts are in astronomy, and the full chart range is in wall charts.