Galaxies
Galaxies
- Unit 1: Particles & Pressure
- Unit 2: Forces & Motion
- Unit 3: Energy & Heat
- Unit 4: Electricity
- Unit 5: Magnetism & Electromagnetism
- Unit 6: Waves: Sound & Light
- Unit 7: Scientific Investigations
In this topic you will:
- discover that galaxies contain dust, gas, stars and other solar systems.
Key words
- elliptical
- galaxy
- irregular
- spiral
- stellar dust
- Universe
The Milky Way
If you look at the sky on a clear night, far away from any lights, you can see a milky band across the sky. This milky band is part of the galaxy where we live, called the Milky Way. The photograph shows what this looks like through a camera set for very low light conditions.
The word ‘galaxy’ comes from a Greek word for ‘milky’.
Shapes of galaxies
The reason why our own galaxy looks like a band across the sky is because of the shape of the galaxy. The Milky Way is a spiral galaxy. If you were to look at the Milky Way from far away, it would appear as a spiral.
Because we live in a spiral galaxy, we can only see one ‘arm’ of the spiral, which is that band across the sky. In fact, most of the stars we see at night are in our own galaxy. There are an estimated 250 000 000 stars in the Milky Way, including our Sun.
There are other galaxies in the Universe besides our own. The word Universe is used to describe all of space and everything in it.
These other galaxies have different shapes, and they are classified according to shape. They are called elliptical galaxies or irregular galaxies.
Common Mistake
It’s easy to think we see the whole Milky Way at night, but we actually only see a small part of it. The rest of the galaxy is not visible because of its spiral shape and our position within one of its arms.
The closest known galaxy to the Milky Way is called the Canis Major Dwarf Galaxy. It is elliptical in shape and a distance of 240 000 million million km away from us.
What are galaxies made of?
Galaxies are made from stellar dust, gas, stars and solar systems held together by gravity. Stellar dust is the dust found in space. The Earth is travelling through a cloud of stellar dust that is estimated to contain one dust particle in every one million m³ of space!
The gravity holding a galaxy together is strong because galaxies have very large mass. Even if you could travel at the speed of light (300 000 km/s), it would take you more than 100 000 years to cross from one side of the Milky Way to the other! Scientists estimate the mass of the Milky Way to be 1 500 000 000 000 times the mass of the Sun.
How many galaxies are there?
Scientists have counted the galaxies in one part of space. The scientists then multiplied this number up to estimate the number of galaxies in the Universe. The answer they got was 100 000 000 000 galaxies!
Estimates such as this may not be accurate. There could be more or fewer galaxies in the part of space that the scientists counted compared with the rest of the Universe. Also, the scientists may not know the total volume of the Universe accurately.
Quick Fact
If the Milky Way were scaled down so it was only 1 meter wide, our entire solar system would fit inside a grain of sand — that’s how massive galaxies are compared to individual star systems.
Questions
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Spiral, elliptical, and irregular.
Choose all that are correct from the list.
planet star Universe stellar gas
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Star, stellar, gas, and planet are found in galaxies. "Universe" is not a part of a galaxy — it contains galaxies.
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Gravity.
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Most stars we see are in the Milky Way because it is our galaxy and much closer to us than other galaxies. Stars in other galaxies are too far away to see clearly with the naked eye.
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Because the Milky Way is so large and contains dust and gas that obscure parts of it, scientists cannot see or count every star directly. They estimate the number based on models and observations of parts of the galaxy.
Think Like a Scientist
Estimating large numbers
In this investigation, you will use an analogy for estimating the number of stars in a galaxy. You will work independently.
You will need:
- one large cup or other container filled with coarse sand or fine gravel
- hand lens (magnifying glass)
- piece of white paper
- small container
- access to measuring equipment (mass or volume)
- calculator
Scientists cannot count the number of stars in a galaxy because there are too many. However, they can estimate this number using indirect methods.
In this activity, you will estimate the number of grains of sand in a container — too many to count one by one — as an analogy for how scientists estimate stars.
Method
- Put a small quantity of sand from the large container onto the white paper and separate the grains.
- Count the grains, record the number, and put the grains into the small container.
- Repeat Step 2 a few times, each time adding to the small container.
- Write down the total number of grains placed in the small container.
- Decide whether to work in mass or volume for the remaining steps.
- Measure the total mass or volume of the small container contents.
- Put the grains back into the large container and measure the total mass or volume of the large container.
Questions
Call this value G for grains.
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Answers will vary — based on the classroom activity.
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Measured in grams or millilitres depending on method.
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Measured using the same units as S (mass or volume).
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This gives an estimate for the total number of grains in the large container.
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It saves time and makes it feasible to estimate very large numbers quickly.
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Sampling error, inaccurate measurements, or uneven grain size can affect accuracy.
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Repeat the sampling process several times and take an average. Use precise measuring equipment.
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Astronomers collaborate and share data to estimate galactic properties just as a class shares measurements to estimate grain counts.
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Multiply the time taken for the small sample by the estimated scale factor (L ÷ S).
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Sample a known volume of sand, estimate grains per volume, and multiply by the estimated total beach volume.
Self-assessment
Decide how confident you are about:
• understanding how this method of estimating works
• whether you could apply this method to estimating some other large quantity