Looking at the sky

Everything we know about starsstar: A large object that produces its own heat and light using nuclear fusion reactions. and galaxiesgalaxy: A cluster of billions of stars, held together by gravity. has come from the light, and other radiations, that they give out. This has become more difficult to see from the Earth’s surface, as light pollution from towns and cities interferes with observations of the night sky.

Looking at the sky with the naked eye shows the Sun, Moon, stars, planets and a few cloudy patches called nebulae. When telescopes were invented and developed, astronomers could see that some of the nebulae were in fact groups of millions of stars. These are galaxies.

Parallax

Powerful telescopes allowed astronomers to answer a question which had baffled scientists since Copernicus first suggested that the Earth moved around the Sun. If the Earth moves, you would expect to see a different view of the stars at different times of the year, in the same way as the room you are in looks slightly different if you move your head to one side. That is to say everything seems to move in the opposite direction to your head, but the objects close to you seem to move more. This effect is called parallax. So if the Earth was moving, why did the stars always look the same?

The answer to the question was revealed by more powerful telescopes. These showed that nearby stars do seem to move from side to side and back every year when compared with very distant stars, but that the amount of movement is tiny.

Finding the distance of a star using parallax

The second nearest star to us is Proxima Centauri. The Sun is the nearest.

It seems to move through an angle of 1.5 seconds between January and June. As one second = 1/60 of a minute, and one minute = 1/60 of a degree, this tiny movement, which is less than a thousandth of the diameter of the Moon, needed powerful telescopes and accurate measurement to observe.

Light pollution

In the last 200 years, it has become very difficult to make astronomical observations in industrialised countries such as Britain. This is not just because of cloudy weather or air pollution. It is due to the bright lights found in cities and towns, and on roads. This light pollution means that it is hard for many people to see more than a few of the very brightest stars at night.

Telescopes

Telescopes are now placed in the few remote, dark places left on our planet, or out in orbit around the Earth.

The Very Large Telescope is part of the Paranal Observatory which is built on top of the Cerro Paranal mountain, which is 2635 metres high, in the Atacama Desert in Chile.

Hubble Space Telescope - image courtesy of NASA

Telescopes in space, such as the Hubble Space Telescope, can observe the whole sky. They are above light pollution, and dust and clouds in the atmosphere. However, they are difficult and expensive to launch and maintain. If anything goes wrong, only astronauts can fix them.