# How many stars are there?

If you look up into a clear night sky in the heart of the country well away from town lights, you will be able to see about 2,000 stars. In the entire sky, there are between 5,000 and 6,000 stars visible to the naked eye.

But there are millions upon millions of stars to be seen through a powerful telescope. The table shows the numbers of starts of different magnitudes or brightness.

Image given below will give a clear cut idea regarding the age of stars.

The numbers of stars of different magnitudes. Bright stars are of first magnitude. Second magnitude stars are 2.5 times less bright, and so on. Stars of 25th magnitude are the faintest we can detect. They are 10,000 million times less bright than first magnitude stars.

## How hot is a star?

Stars vary in temperature greatly. The hottest stars have surface temperatures of as much as 35,000°C or more and shine a brilliant blue-white. The less hot stars are, the redder they appear to be.

The table shows the surface temperatures and colors of five kinds of stars together with an example of each kind. Stars are classed into various groups according to their surface temperature. The sun is in a group of medium temperature.

## Do stars have planets?

The sun is surrounded by no less than nine planets, so it is very likely that other stars also have planets. However, other stars are so far away from us that any planets they possess are far too small for us to be able to see, even with the most powerful telescopes.

Nevertheless, it is possible to detect the presence of very large Nevertheless; it is possible to detect the presence of very large planets near other stars. If a star has a massive planet, the planet’s gravity produces a wobble in the star’s motion.

In 1944, a wobble was observed in the motion of a star in the constellation Cygnus (the Swan) called 61 Cygni B. By measuring this wobble, astronomers calculated that the planet was detected around a nearby star called Barnard’s Star.

The planet has a mass half as big again as Jupiter and revolves once around Barnard’s Star in 24 years. This planet is probably very similar to the large planets which are part of our own solar system.

An impression of the surface of a planet near a red giant star.

If a star has a very large planet, the centre of gravity of the two bodies lies between them. Both bodies move around the centre of gravity (right) and this movement produces a wobble in the star’s general motion (left)

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