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Stars and constellations

Perseus, Andromeda, Pegasus and Cassiopeia

If you look at the sky at about 11 pm in October then you will see the stars in this map.



The Milky Way goes through Perseus and Cassiopeia and a telescope will show you thousands of faint stars there. Some of them are grouped in clusters and a well-known cluster is called the Double Cluster (or Sword Handle Cluster) because it is really two clusters of stars close together. It's worth looking at with a telescope.



The Double Cluster

Figure 2
 

The Andromeda Nebula is the only galaxy that you can see without a telescope. On a clear night with no Moon you can just see it as a faint fuzzy patch, but it is really a huge number of stars about two MILLION light years away.

Orion and Taurus


If you look at the night sky at about 11 pm in early January then you will see these stars to the south. You can't miss them because Orion is a very obvious group and Sirius is the brightest star that can be seen in the northern sky.




There are three other important things to be seen.
1. The Pleiades is a small but bright cluster of stars that look very good in binoculars.
2. The Crab nebula can't be seen without a telescope, but it is the remains of a huge explosion when a star blew itself to pieces nearly a thousand years ago.
3. The Orion nebula can just be seen in the country on a clear night as a tiny fuzzy patch in Orion's dagger and is really good if you use a telescope. It is a huge cloud of gas that is glowing by reflected light from the nearby stars.

The following three photographs show how these objects appear when observed with my 12" reflecting telescope. (Figure 5)

Into deep space

Imagine that we could now leave the Solar System and actually take a journey out into deep space. The Universe is so vast that the best way of appreciating the scale of distances is to think about this in terms of the time it takes light to travel those distances.

Imagine now that you could travel as fast as light, that is 300,000 km a SECOND, it would still take you just over a minutes to get to the Sun and 4.3 years to get to the nearest star.

We call the distance that light can travel in a year a LIGHT YEAR. It is a very long way, about 9,000,000,000,000 km (6 million million miles).

The nearest star is over 4.3 light years away and the star Vega is 25.5 light years away!
So unless we can find another way of travelling we will never be able to reach the stars.

The following table shows you some important distances in the Universe.

Object Distance at light speed
Moon to the Earth 1.25 s
Sun to the Earth 8.3 min
Jupiter (minimum) 35 min
Nearest star to the Earth 4.2 min
Sirius to the Earth 8.6 years
Diameter of our galaxy 100 000 years
Andromeda galxy to the Earth 2 300 000 years
Virgo galactic cluster to the Earth 65 million years
Radius of the observable Universe 13 700 million years

The stars

What is a star? The first thing to remember is that the Sun is a star and quite an ordinary one at that. The only reason that it looks so bright is that is so close to us the next nearest star is more than 250 000 times further away. We now know that the Sun is just an average sort of star. It only looks bright because it is much closer to us then any of the others. It is actually a huge ball of gas nearly a million miles (1.5 million km) across and about 93 million miles (150 million km) from the Earth.

A star is really a huge cloud of very hot gas that gives out a lot of energy. It keeps shining by the energy released from nuclear fusion reactions in its core. The 'gas' in the middle has a 'squashed' to a very high density due to the enormous forces of pressure and gravitational attraction

The surfaces of some stars are fairly cool (about 2000 oC) while others are very hot like Rigel (nearly 30,000 oC). The Sun is quite average around 6000 oC at the surface. All the stars are much hotter in the middle of the core (over 2 million degrees) where the fusion reactions are producing energy.




The bright star in Figure 6 is Altair in the constellation of Aquila. It has a surface temperature of about 8500 oC and is ten times as bright as the Sun. The reason it looks so much fainter is because it is 16.6 light years away from us.

Some stars are no bigger than the Earth but some are so vast that it would take 200 years to fly once round them in our supersonic jet!



Variable Stars

Other stars don't shine steadily like the Sun. They wink on and off rather like a lighthouse, these stars are called Variable Stars.

You wont really notice stars doing this if you look at the sky; you really need to watch them very carefully from night to night.

Astronomers use these variable stars a lot, they can tell us a lot about the stars and how far away they are. An important variable star that is used to help measure distance is Delta Cephei.


Double Stars

Some stars are Double, that means there are two stars very close to each other, they go round and round each other like two people dancing.

Exploding Stars

Sometimes the nuclear furnace that powers a star gets out of control. When this happens the star blows up.
Now a star is a huge thing and so when it blows up you get a huge explosion. In fact it makes the star so bright that you can sometimes see it in daylight.

This is what happened in 1054 when the Chinese saw what they thought was a new star, it was really one that had got much brighter because it had blown up.

This explosion was so vast that we can still see the remains of it today. The huge cloud of gas that it produced is still expanding and it is called the Crab Nebula.

The picture shows the Crab Nebula taken through my 12" reflector, and the position of the nebula is marked on the map on page 2.

Astronomers call these exploding stars a NOVA, or if the explosion is even bigger a SUPERNOVA. Don't expect to see one in the sky, they don't happen very often, and sometimes the explosion is not big enough for us to see from the Earth.



 
A VERSION IN WORD IS AVAILABLE ON THE SCHOOLPHYSICS CD
 
 
 
© Keith Gibbs 2020