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.
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 |
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.
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.
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.