Stars Nature

Stars - An Overview

The Nature of Stars

Our Sun is an average star. Like all stars, it is a large sphere of gas held together by gravity. Stars generate heat and light through nuclear fusion. This process combines hydrogen that was already present in the Universe at early times into helium. In more massive stars, further fusion reactions convert the helium into carbon, then oxygen, then silicon. Everything we see is made from the elements created in massive stars, including our own bodies. Stars are huge factories producing the materials that make up the Universe we see around us today.

Sizes of Stars

Our Sun is approximately 1.4 million km in diameter, but its size will change throughout its lifetime as it evolves. We can only compare stellar sizes at similar evolutionary stages.

White Dwarf stars can be one thousand times smaller than our Sun, whilst Red Giant stars can be over one hundred times larger. That means that stellar sizes cover a range of (approximately) 1,400 km to 1,400,000,000 km in diameter.

The white dwarf stars are circled. They are much smaller than the other Main Sequence stars in the image.

Colours and temperatures

When glancing up at the night sky, stars all appear to be white in colour but when viewed through binoculars or a telescope their colours vary. Redder stars such as Betelgeuse (right) in the constellation of Orion are cooler than the Sun, with surface temperatures of about 2000K.Others stars appear blue, such as Sirius, the Dog Star. Sirius has a surface temperature of around 15000K. Our Sun is an average yellow star, with a surface temperature of about 6000K.

This relationship between colour and temperature can be demonstrated in the following thought experiment:

Imagine placing a poker in a fire and leaving it to heat up. When it is first removed from the fire, it may be so hot that it glows with a bright orange - yellow colour. But as it 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 point!). This is the same effect as seen in stars. The very dim red stars are cooler objects - some are so cool that they are visible only in infrared 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.

courtesy by http://www.le.ac.uk/ph/faulkes/web/stars/r_st_overview.html