Beyond the Milky Way

How do galaxies stay together if the stars and planets are so far away from each other? Gravity is what keeps them together. Every object in the universe exerts a gravitational force on all other objects. Although the attraction is weak, this is what keeps planets in orbit and galaxies together.

ACTIVITY LAPTOPS- Classifying galaxies

Astronomers now have so many photos of galaxies that they simply do not have the time to classify them all. A group of astronomers has set up a crowd-sourced astronomy project called Galaxy Zoo that allows anyone to take part in some real science. You can try this too.

Search for Zooniverse Galaxy Zoo on the internet and you can have a go at classifying galaxies for yourself. You will be shown a galaxy and asked a series of questions about what you can see and the shape of the galaxy. Do not be afraid that you will get it wrong – the galaxies are shown about 30 different times to make sure that the responses are consistent.

Parallax

If you hold your index finger in front of you at arm’s length and close one eye, then swap eyes and close the other one, your finger will appear to move because you are essentially observing your finger from a different location. 

The same thing happens when we look at the stars in the night sky from a different position in space. As Earth travels around the Sun, our position in space changes and we see the stars arranged slightly differently. 

We can measure the angle between the apparent location of a nearby star and a ‘fixed’ distant star. This is called the parallax angle. Using the distance between Earth and the Sun (the astronomical unit or AU) as the baseline and some trigonometry, we can work out how far the star is from Earth.

This technique enables us to measure distant stars in parsecs (pc): one parsec is equal to 3.26 light years or 30 trillion kilometres.

Using parsecs to measure distance makes it easier to calculate distances with the following formula: 

The further away a star is, the less it appears to move due to parallax, and the smaller the parallax angle becomes. This gives a limit to the distance that we can measure using the parallax method, because when stars are too far away, their parallax angle is too small to measure. 

Figure 6.19 Astronomers can work out the distance to nearby stars by comparing their apparent location relative to the background of distant stars (which are unaffected by parallax). 

The Doppler effect

When you hear an ambulance travelling past you, the pitch of the siren changes as it passes. When the ambulance is travelling towards you, the sound wave is a little more squashed than usual and results in a higher frequency wave. You hear it as a higher pitch. 

When the ambulance is travelling away from you, the sound wave is a little more stretched than usual and results in a lower frequency wave. You hear it as a lower pitch. This phenomenon is called the Doppler effect.

The same phenomenon happens with light waves if an object is travelling fast enough. When the object is travelling towards you, the light it emits is of a higher frequency, which translates to the light appearing more blue. This is called blue shift. When the object is travelling away from you, the light it emits is of a lower frequency, which translates to the light appearing more red. This is called red shift.

When astronomers look at spectra from distant galaxies, they find that the emission lines are almost always shifted towards the red part of the spectrum. This tells us that the vast majority of galaxies in the universe are moving away from us.

Figure 6.20 Sound waves are squashed together as the ambulance travels into them, so we hear a higher pitch. Sound waves are spread out as the ambulance moves away from them, so we hear a lower pitch.