Why do stars twinkle and planets do not meritnation?

Why do stars twinkle?

It’s a wonderful experience to go into the countryside on a pitch black night, far from the glare of city lights, and stare upwards at the awesome array of stars in the heavens. Alas, places where you can do that are becoming fewer. No wonder, then, that more people who have discovered the joy of star gazing are travelling sometimes great distances to enjoy this magical pleasure.

The night sky is filled with stars of many different ages, but all have life spans of many millions of years. Astronomers can identify stars at various stages of development at their birth, in the prime of their lives and amid the throes of death.

Stars are born when a cloud of gas, mainly hydrogen, is compressed under the force of its own gravity. That’s how our Sun, which is also a star, came into being. If you have ever blown up a bicycle tyre with a hand pump, you will know that the pump gets hotter as the gas is compressed. So it is when a gas cloud draws itself into a tighter and tighter blob. If the temperature at the centre of the condensed cloud rises to 10 million°C (18 million°F), its hydrogen explodes like an H-bomb. This reaction turns the hydrogen into helium, which adds fuel to the fire, and a star is born.

By studying our Sun, we get a guide to the nature of other stars. We know that the heaviest stars burn more furiously, and use up their fuel more quickly. Lightweight stars don’t burn so brightly, but their life might be 100 times that of a star with the weight of our Sun.

The British astronomer Edmond Halley first discovered early in the eighteenth century that the stars moved, and then deduced that even the closest were many millions of miles away. For a speck of light to be visible on Earth from that distance meant, he argued, that it must come from a source as large as our Sun.

If Sirius, the brightest star, had a fire as big as the Sun’s, Halley said it must be 19 million million km (about 12 million million miles) distant. His calculation, like Sirius itself, was way out, but Halley had no way of telling the relative brightness of the two stars. Sirius is in fact more than four times as far away - 82 million million km (52 million million miles).

The light from the Sun doesn’t twinkle, because our star is so close to us. Nor does the light from the planets, which is steady because they reflect light from the Sun. Stars make their own light, and it twinkles because it travels so far. Also, because it is weakened by distance, starlight doesn't pass so easily through the Earth’s atmosphere, which adds to the twinkling effect.

The light from individual stars varies in intensity. But whether they shine brightly or dimly, their remoteness and resultant lack of power at that range still give them that nursery rhyme twinkle.