Stars

The Pleiades Cluster taken from Pat Murphy's Archive

Introduction

On a cloudless night, the velvety black sky seems to be filled with countless sparkling points of light - the stars. In fact, about 1000 or 2000 stars are visible to the naked eye, producing patterns which we call constellations. Too faint to be seen unaided, but revealed by the telescopes of astronomers, are millions more. The stars we see on a dark night are all members of our galaxy, the Milky Way - and this is only one of millions of galaxies which exist in the universe, each galaxy containing vast numbers of stars. What are stars? How far away are they? This section seeks to answer these and other questions.

The nature of stars

Our Sun is a star. It is relatively very close to us so it looks very different to the stars we see in the night time sky. However, this appearance is deceptive. In reality, the night time stars are also vast globes of gas held together by gravity, and creating large amounts of heat and light through nuclear fusion - converting the hydrogen which was already present in the universe at early times, into heavier elements like helium, oxygen, carbon, and so on. These elements are needed for the creation of other objects - including all living things. Stars are huge factories producing the materials which make up the universe we see around us.

Distances to the stars

The Sun is our nearest star. The Earth orbits the Sun in a path which is very nearly circular, and on average the distance between the centre of the Earth and the centre of the Sun is 149,597,870 km. Astronomers give this distance a special name: the Astronomical Unit (A.U.) The other stars are much more distant than our Sun. The nearest is called Alpha Centauri (which is in fact a group of three stars) at a distance of 4.2 light years (One light year is the distance that a beam of light would travel in one year - about 9,460,000,000,000 km !) Other stars are more distant; our own galaxy, the Milky Way is about 100,000 light years in diameter, and is filled with stars. So even the very closest stars are much too far away for us to send spacecraft to explore.

The sizes of stars

Stars form in a variety of sizes, and as they get older, different processes occur inside them causing many changes; the star evolves. The size of the star is one of the quantities which will change as it evolves. Astronomers divide stars into several different types depending on properties such as their size, age, temperature and luminosity, and so when we make a general comparison of stellar size, we must take care that we are comparing similar types. However, to gain some idea of the range of sizes, it is interesting to note that White Dwarf stars can be one thousand times smaller than our sun, whilst Red Giant stars can be over one hundred times larger than our sun. That means that stellar sizes cover a range of (approximately) 1,400 km to 1,400,000,000 km in diameter !

Colours, temperatures, twinkles

It may not be immediately obvious on a clear night, but stars have different colours. Although in the case of most stars our eyes are not sensitive enough to see the colours without aid, using binoculars or a telescope makes the effect more noticeable; photography shows the colours in their full glory. Stars can be seen in a variety of subtle shades, from the red star Aldebaran in the constellation of Taurus, to the brilliant white Vega in the constellation of Lyra, to the blue stars of the Pleiades shown in the image at the top of the page. These colours give us clues to the temperature of the star.

To see the relationship between colour and temperature, we need look no further than our own homes. Consider a poker placed in a fire and left to heat up. When the poker is first removed from the fire, it may be so hot that it glows with a bright orange - yellow colour. But as the poker cools, the colour changes, at first to bright red, then getting dimmer until eventually becoming black. If we were able to heat the poker to a higher temperature, we would see it turn "white hot", then begin to glow green, and eventually the tip would be so hot that it would turn blue. (Of course in reality the metal would have melted by this time, but the example serves as a good illustration of the effect). In stars, the same effect is observed. The very dim red stars are cooler objects - some are so cool that they are visible only in infra red which lies below visible red in the spectrum. Hotter objects have colours closer to the blue, short wavelength end of the spectrum, and very hot stars can be seen at even shorter wavelengths such as ultraviolet, or even X-ray energies.

Astronomers use the technique of spectroscopy to analyse the light from individual stars. By looking at their colour, and finding out which wavelengths of light are present or absent in the light from the star, we can gain valuable information about the star such as its temperature, pressure, and even how fast it is rotating. Using these techniques, we know that the surfaces of stars range in temperature, with the coolest stars at around 2,000K and the hottest at over 50,000K.

One of the most famous features of the stars in the night sky is the way in which they appear to "twinkle". In fact, this is nothing to do with the star at all. Instead, the effect is due to the fact that we are looking at the star through the atmosphere of the Earth. The atmosphere contains large amounts of dust, and these dust particles can block the starlight momentarily, making the star twinkle.

Stars can change their brightness or magnitude, and we call such stars variable stars.