Many Watch Night Sky In Summer

Press, Volume CIV, Issue 30947, 31 December 1965, Page 11

Many Watch Night Sky In Summer

This is the time of the year when many people look at the night sky. This is because it is so relaxing, while on holiday, to sit in the cool of the evening and watch the stars appear. As a result the summer skies are better known to most people than those of winter. One of the best-known star groups, called constellations, is Orion, the mighty hunter.

As the long evening twilight fades, the first star to appear, just a little north east of the point overhead, is Sirius, the brightest star in the sky. A little to the north of Sirius four bright stars can be seen forming a large rectangle. These mark Orion. At the northern corner of the rectangle is the brightest of the four stars. This is Rigel, whose blue colour makes a sharp contrast with the red star at the opposite bottom corner of the rectangle. The red star is Betelgeuse. The contrast in colour of these two stars has a very real significance. Red stars are cool, whereas the blue and white stars are hot. You get the same effect, on a very minor scale, if you heat a poker in a hot fire. After a time it will glow red-hot, but as the temperature becomes higher it becomes white. Betelgeuse has a surface temperature of around 6000 degrees Fahrenheit, which is cool for a star. By contrast Rigel is 22,000 degrees. The contrast extends further because Betelgeuse is a much older and larger star. Although it lies at a distance of some 600 light years from us, its actual diameter has been measured. A light year is just a convenient method of expressing distances to save using huge figures. It is the distance travelled by light in one year and is around six million, million miles. The diameter of Betelgeuse is so large that if its centre was placed where the sun is, then it would extend almost to the orbit of Jupiter. All the planets closer to the sun than Jupiter would be swallowed up inside the star. Its actual

size is about 400 million miles.

Rigel is about' 60,000 times as bright as our sun. It only appears fainter because it is so far away. Stars as bright as this could not keep shining at the rate they do for very long. They must therefore be young stars. If you look again at the rectangle forming Orion, you will see in its centre three fairly bright stars in a row. These form what is known as the belt <xf Orion. Slightly higher in the sky and just above the southern star of the belt you will just see, on a moonless night, three much fainter stars. These form the sword of Orion. Because the ancients who wove their legends into the sky were in the Northern Hemisphere we see the sword above the belt from which they imagined it hung. Even with the naked eye, the middle star of the sword has a hazy appearance. This marks, the Orion Nebula, a diffuse cloud of isolated atoms and minute grains of dust The nebula shines because the atoms absorb light from some bright hot stars embedded in the nebula and then re-emit the light In many parts of Orion this diffuse gas is found shining because of the bright stars in it. But in other places it is dark as there are no stars embedded in the gas. It then acts as a veil hiding the more distant stars.

These diffuse nebulae are probably the birth-place of the stars. For it is in them that are found the youngest stars. These stars have formed from the dust and atoms of the nebula in recent times’, that is, on the celestial time scale. By the peculiar manner in which these stars change in brightness they tel! astronomers something of their birth. In geons of time they will run through their stellar life and die. As you gaze at Orion you will be watching a portion of the heavens that has yielded to astronomers many of the clues to the evolution of the stars.