Star light, stars very bright

During clear evenings at this time of year, almost overhead you'll see the bright, bluish star Vega.

Now, scan westward to find the bright, orange star Arcturus. About halfway between the two stars, there is a circlet or tiara of fainter stars making up the constellation of Corona Borealis, The Northern Crown.

About halfway between this constellation and Vega, you'll find a tombstone or keystone shaped group of four stars. These mark the body of Hercules, the mythic hero.

Assume he is facing us, and Vega is near his right shoulder. Now, grab a pair of binoculars and scan slowly down his right side. Maybe a third of the way down you will see a fuzzy blob, appearing very different from the nearby stars.

This object is Messier 13, a globular cluster of stars, a spherical collection with more than 300,000 members. M13 lies about 22,200 light years away, which means its light takes roughly 22,200 years to get here.

A light year is just under 10 million million (10 trillion) kilometres, so that is a very great distance. It has a diameter of some 145 light years, which means on average the stars lie a mere two light years apart. However, the centre of the cluster is so crowded, our telescopes just show a bright glow.

As we look outward from the centre, the stars get far enough apart for us to see them as individuals. The stars near the centre have to be much closer than two light years. Compare this with our situation here on Earth, where the nearest star to us after the sun is 4.3 light years away, and the next closest significantly further.

What would it be like to be an inhabitant of a planet orbiting a star that is a member of a globular cluster? After sunset the sky would be filled with really bright, nearby stars. It would probably be brighter than full moonlight on Earth, and you would not need streetlights.

Life would be hard for any astronomers on the planet. The glare from those numerous bright, nearby stars would make it difficult to observe what lies beyond the cluster.

Most galaxies have globular clusters orbiting around them, although we have no idea why. So far, astronomers have found around 150 orbiting our galaxy, although there could be a few we have not yet found.

The Andromeda galaxy, which is rather larger than the Milky Way, has maybe 500, and giant galaxies, such as Messier 87 in the constellation of Virgo, may have 10,000 or more.

When we look closely at the stars in globular clusters, we see they have two things in common. They are old, and there is little gas or dust from which new cluster members can form.

Stars generate energy by converting hydrogen into other elements, such as silicon, oxygen, carbon and sulphur. When they die, they eject all these waste products out into space, where they mix with the dust and gas clouds from which new stars form.

So following generations contain traces of material from stars that went before. All the gas and dust in globular clusters has been used up in making stars, so there is nothing to make more. In addition, the cluster members contain little or no traces of material from earlier generations.

This suggests they are among the oldest and earliest generations of stars, dating back to the youth of the Milky Way or even of the universe itself. This makes globular clusters really interesting to astronomers.

Although we have found many globular clusters, as yet we have no good ideas as to how or why they form. Since most galaxies have them, the processes involved in creating these fascinating objects can't be that unusual.

Although you can see Messier 13 and some other globular clusters with a pair of binoculars, to really enjoy them you need a telescope.

Any moderate sized backyard telescope will capture a good number. Globular clusters are among the most popular targets for amateur observers.

• Brilliant Jupiter lies in the southwest after dark
• Saturn is low in the south.
• The moon will be full on the 15th

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.