Globular clusters with hundreds of stars spotted in space

In summer, the constellation Hercules, named after the hero in Greek myths, is high in the sky.

The constellation looks like a broad-shouldered, headless man doing a crazy dance. To find it, follow the curve of the Dipper's handle to the bright, orange star, Arcturus. Almost overhead find Vega, a bright, bluish-white star. Hercules lies about halfway between those stars.

If the night is dark and clear, just below Hercules' left shoulder (assuming he is facing us), there is a faint, fuzzy dot. Binoculars will show it well and a nice big telescope will show it spectacularly. The object is a ball of stars, so crowded in the central regions the stars blur together in a continuous glow. Then, as we look further out toward the edges, the stars become more and more sparse.

The object is a globular cluster, about 150 light years in diameter, containing several hundred thousand stars. This globular cluster, known as M13, is one of at least 160 orbiting around the outside of our galaxy.

Almost all other galaxies seem to have their own collections of these compact clusters of stars.

Very few stars lie within 10 light years of us, and in our skies, only a few of them look really bright. In the core of a globular cluster the stars are a fraction of a light year apart.

People on planets in these crowded neighbourhoods would need no streetlights. However, so far no such planets have been found. Maybe they are rarer than elsewhere in our galaxy because there is little material available to make them.

Actually, there is another reason why, as yet, we may not have detected any planets in globular clusters. Because the stars are so close together, any planet orbiting a star in a globular cluster will have lots of nearby stars tugging at it. This continual tweaking can grossly change a planet's orbit, or even send it off into space all by itself.

Since the way we search for planets is to look for the minute dimmings as they pass in front of their respective stars, this massive orbital interference may make detecting planets unlikely, if they are there to be found.

The stars in globular clusters are old, with little material around to form new ones. It is relatively easy to see clouds of cosmic gas and dust, but how can we determine the age of a star?

Actually, this is not very difficult. Our universe started off with two elements, mainly hydrogen with some helium mixed in. Stars formed from this mixture, and obtained energy through nuclear fusion, turning hydrogen into all the other elements. When these stars exploded at the end of their lives, these by-products got added to the surrounding clouds. When new stars form from clouds containing these by-product elements, we can detect them.

There is no circulation between the core, where energy production is going on, and the surface. Therefore, anything in the surface material in addition to hydrogen and helium was inherited from earlier generations of stars. Stars formed back in the youth of the universe contain little if anything other than hydrogen and helium in their surface layers. Younger stars, like the sun, contain a good mixture of heavier elements.

Globular clusters are among the oldest objects in their galaxies and maybe in the universe. During their long lives they went through a number of episodes of star formation, but that all stopped long ago. There seems to be no new star formation going on now.

These mysterious cosmic fossils look spectacular through large telescopes, and are very common; almost every galaxy has some. They are favourite targets for amateur astronomers' telescopes.

We still trying to work out how they formed, and when that happened. However, we believe they formed when the universe was quite young.

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Venus shines very brightly in the west after sunset, with Mars, higher, in the southwest.

Saturn lies very low in the dawn glow.

The Moon will reach its first quarter on April 27.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory near Penticton, B.C.

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