The nearest star to earth (other than the sun) is proxima centauri, and is approximately 40 000 000 000 000 km. That’s forty million million kilometers!

 

To cope with these vast distances we use the light-year scale. This is the distance that a particle of light can travel in a year - about 9.4 million million kilometers. On this scale, the nearest star is 4.3 light years away.

 

Stars are not uniformly distributed throughout the universe, but are clumped together in huge formations called galaxies. Galaxies are crowds of stars held together by gravity, and come in a fascinating variety of shapes and sizes including spiral, elliptical, spheroidal and irregular  galaxies.  Our sun is part of a spiral galaxy we call the Milky-Way galaxy.  In fact, all the stars we see in the night sky are part of our Milky Way Galaxy, and none of them is more than 5000 light years away.

 

On a clear night, you can see up to 2000 stars with the naked eye.  However, our Milky-Way galaxy contains more than 100 billion stars (that’s 100 thousand million!)

 

The glow is actually caused by the billions of stars in the Galaxy, whose faint light merges to form a solid glow.  The band is not directly overhead, but seems to be at an angle; the orbit of the earth is not aligned to the Milky Way.

 

The ancients used the constellations for many purposes - as navigation aids, time-keepers and signs of the seasons.

 

The Southern Cross

This is the most familiar constellation in the southern hemisphere. This tiny constellation (the smallest in the entire sky) was once part of Centaurus, but the sight of such a brilliant cross in the sky was so compelling that it became a constellation of its own in the sixteenth century.

 

For early adventurers the constellation was an important clock; by looking at the angle it was at in the night sky, they could determine the time.

 

The principal star in the constellation is Acrux (alpha Crucis). This is not a single star, but a binary system – two stars orbiting each other. We call them gamma A and gamma B.  The two stars together make what appears to us to be one very bright star.  The stars are 320 light years away, and each is approximately one and a half to twice the size of our Sun.

 

Beta Crucis (also known as Mimosa) is the brightest single star of the group, a blue-white giant nearly five times the size of the sun.  The star is an estimated 580 light years away, and has a luminosity of nearly 8000 (ie shines 8000 times brighter than the sun). The star is a variable – it pulsates for some inexplicable reason, changing it’s apparent brightness over time.

 

Gamma Crucis (Gacrux) forms the top of the cross. This is an apparent binary, but the two stars are not gravitationally bound to each other.

 

Delta Crucis is the western arm, very similar in size and distance to alpha Crucis.

 

The cross is very helpful to any stargazer. Apart from being easily recognisable, it allows us to find true south.

·        Take the long axis of the cross.

·        Extend it downwards in the sky (towards Earth).

·        End the line when it is four times the length of the long axis.

·        Drop another line from the end point down to the horizon.

Links: Wits Planetarium