How bright are the stars?

Every star is giving out vast amounts of light, and also radio waves and heat and lots of other forms of energy. Today’s professional astronomers have huge telescopes and other instruments which allow them to measure all of these, however far away the star is from the Earth.

But the only thing that mattered to the ancient astronomers, and today the only thing which matters to most people just looking at the Night Sky, and to Sam and his friends, is what today’s astronomers call their apparent visual magnitude, how bright they appear to be to people just looking at the sky without a telescope.

In the same way that football teams are put into Leagues and Divisions, stars are put into magnitudes: a magnitude 1 star is brighter than a magnitude 2 star, and so on. Without a telescope, on a clear moonless night most people can see stars down to magnitude 6. Most ancient astronomers had realised that there were probably many more stars that were not bright enough for them to see. With his telescope Sam can see many more stars.

Today’s astronomers also rank stars within each magnitude, for example magnitude 3.7 - but they are actually using what mathematicians call a logarithmic scale. Sam does not understand logarithms, as he has not yet started learning about them at school, he just accepts that a magnitude 5.7 star is brighter than a magnitude 6.3 star, how much brighter does not worry him. But a logarithmic scale means we can have stars and planets with magnitudes of zero, or even negative magnitudes. Sam can cope with this, he just remembers that on a number line -2 is smaller than -1 and the lower the magnitude the brighter the star.

The brightest star in the sky is Sirius, with a magnitude of -1.46. Logarithmetic scales are discussed on another Page FTPO.

Some stars are variable, that is, they become brighter and dimmer in a regular cycle. This is often because if you look at them through a telescope they are actually a double star, a brighter star and a dimmer star circling round each other, so sometimes you see one and sometimes the other. Other stars have changed their magnitude since the earliest astronomers were looking at them. One of the most beautiful objects in the Night Sky is the Pleiades (pronounced PLY adees), not a star or a constellation but a star cluster. The Greeks named it after the seven daughters of Atlas and Pleione, hence its common name The Seven Sisters.

The best way of finding it in the sky is to use Orion to point the way.

Two thousand years ago most people could see seven but today most people can only see six. With a telescope you can see lots more.

The planets are not always the same distance from the Earth so they change their magnitude. Venus at its brightest has a magnitude of -4.4, bright enough to cast a shadow, and is the brightest object in the sky, after the Sun and Moon of course.

Sam once looked up the magnitudes of the Sun and Moon in one of the books his Granddad had given him, and wished he hadn’t, the logarithmic figures were quite meaningless to him. But you do not need a book to tell you that the Sun is much brighter than the Moon, and that the Moon is much brighter than anything else in the sky.

Novae and Supernovae

Sam has never seen a Nova or Supernova and probably never will, but he is still interested in them.

Stars are formed inside galaxies and cannot exist outside them. There are millions of galaxies in the Universe and millions of stars in each galaxy, but from the surface of the Earth without a telescope we can only see about five thousand of the stars in our galaxy. This is explained more fully on the Page on The Milky Way.

Stars have their own life cycles, a beginning a middle and an ending. During its lifetime a star turns the chemical elements in it into different elements, at the end of its life these elements are released and will eventually form new stars.

During these lifecycle changes a star may start to give out more light. What this means is that sometimes a star not normally bright enough to be seen without a telescope becomes a naked eye star. So one night you see a star which was not there before - a New Star, or Nova, from the Latin for new.

Until the first use of the telescope to look at the sky, in about 1609 CE, Novae were very exciting and very very rare. With today’s telescopes astronomers have realised that these changes in brightness are actually very common and have all sorts of causes, so they use the term supernova for only the really spectacular self-destruct of a star at the very end of its life, leaving a dreadful mess behind.

The Ancient Astronomers, particularly the Chinese, kept stunningly accurate records of what they saw in the sky. Since these records began there have only been seven Novae visible from the surface of the Earth without a telescope, and when today's astronomers use their telescopes to look at the sky in the places where these were observed, in every case they see the remains of a supernova.

The earliest Nova (that is, a supernova visible from the Earth without a telescope) for which records still exist was in 185 CE, and Kepler observed one in 1604 CE with an estimated magnitude of -2.5. The brightest Nova ever was visible even during the day for eight months in 1006 CE, with an estimated magnitude of -7.5.

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© Barry Gray October 2003