Is it a star or a planet? What are stars? Star classification The brightness of the stars: The magnitude scale Temperature and colour

The sparkling jewels of the night sky have always raised the wonder and aspirations of mankind. Stars have guided our explorers, have allowed us to dream and driven us on our quest for higher knowledge.

Stars are not only very beautiful, they are scientifically truly awe inspiring. What we can see in the night sky is only a drop in the universal ocean; a guess at how many stars exist in the Universe is at best, an estimate.

As astronomers obtain better and better telescopes, small dots of light turn out to be huge galaxies or clusters of galaxies, each with millions of their own stars. In fact, through large telescopes the night sky is a blaze of light, unlike the sky we see with the naked eye, with a few bright stars on the dark background of space.

Is it a star or a planet?
Most of the points of light we see in the night sky are distant stars (although such stars are all within our own galaxy). Identifying a planet is not always easy, but the nursery rhyme "Twinkle Twinkle Little Star" can be remembered as a rough guide: planets "twinkle" far less than stars, or not at all.

Stars are much farther away from us, and so appear to us as very tiny points of light. Because of this, their light is affected far more by the turbulence of our atmosphere than is the sunlight reflected by the planets, which are closer and cover a greater "area" in the sky. An extreme example is the Moon, which does not appear to twinkle when viewed with the unaided eye because atmospheric effects are "averaged out" by its large apparent size.

What are stars?
Stars are huge balls of plasma - hot ionised gas - undergoing nuclear fusion reactions involving the combination of small hydrogen nuclei to form helium and other larger nuclei. In this process, large quantities of energy are produced in the form of light and heat from the conversion of matter into energy in the core of the star.

Star classification
Stars are classified using "The Hertzsprung-Russell diagram" according to their colour and brightness. Our own Sun is classified as a main sequence G2 Yellow Dwarf star, but there are also Red Giants, Super Giants White Dwarfs and even Neutron stars, all members of the family of stars.

The brightness of the stars: The magnitude scale
Looking up to the night sky we can see that the stars have a range of different brightness. The stars were first classed into six brightness categories, or magnitudes, with the brightest stars being of magnitude 1 and the faintest visible to the eye of magnitude 6. We still use a similar scheme today, but measure the magnitudes accurately. Some of the brightest stars actually have negative magnitudes: Sirius, the brightest star in the night sky, has a magnitude of -1.4. A star of a given magnitude is a factor of 2.512 times brighter than a star of the next fainter magnitude, so that five magnitudes correspond exactly to a brightness difference of a factor of 100 (2.512 to the fifth power). The best telescopes we have today can detect objects nearly as faint as magnitude 30.

Temperature and colour
We can tell the temperature of a star from its colour. The coolest stars appear red while blue stars at the other end of the visible spectrum are the hottest.

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