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Stars - An Overview


The Nature of Stars


Our Sun is an average star. It looks very different to the stars that we see at night because it is much closer. In reality, the night-time stars are also huge balls of gas held together by gravity, creating large amounts of 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 heavier elements such as oxygen, carbon, and 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. This happens to all stars, so we can only compare their sizes at similar stages in their lifetimes.

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!

White Dwarf stars

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




Colours and Temperatures


Betelgeuse, a red giant in the constellation of Orion WWhen glancing up at the night sky, stars all appear to be white in colour. In fact if you stop and look more closely, their colours are different. Some stars look redder in colour, such as Betelgeuse (right) A hot, blue star in the constellation of Orion. These are cooler stars, with surface temperatures of about 2000C. Others stars appear blue, such as Sirius, the Dog Star. Sirius has a surface temperature of around 15000C. Our Sun is an average yellow star, with a surface temperature of about 6000C.

We can see this relationship between colour and temperature even in our own homes. 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.


Twinkling


When you look up at stars, many of them will seem to twinkle as you watch. Astronauts in space do not see this because it is caused by the Earth’s atmosphere. The atmosphere is made up of many different layers of different temperature and density, all of which bend light from the stars in a different way. As these layers move and change, the light we see also changes and the stars seem to twinkle. It is similar to looking at the bottom of a swimming pool through the water – the black lines seem to move around as the surface of the water ripples.


Distances to Stars


Our closest star is the Sun, 149,700,000 km away from the Earth. Astronomers call this distance 1 Astronomical Unit (AU). The other stars are much further away. The nearest is called Alpha Centauri, at a distance of 4.2 light years. A light year is the distance travelled by light in one year – about 9,460,000,000,000 km - so the light from Alpha Centurai takes 4.2 years to reach us. The light from the Sun takes only 8 minutes. Even the very closest stars are much too far away for us to explore by spacecraft.


Measuring the Brightness of Stars


The brightness that a star appears to be is called its apparent magnitude. This was first measured by an astronomer called Hipparchus. He made a catalogue of about 850 stars in the sky. The brightest stars were given an apparent magnitude of m=1, while the dimmest stars visible to the naked eye were classified as m=6. The lower the numerical value, the brighter the star.

Another important measurement is the absolute magnitude of a star, M. This is the apparent magnitude a star would have if it were placed at a distance of ten parsecs away. Comparing apparent and absolute magnitudes gives us the distance to the star.


Classification of Stars


The Hertzsprung-Russell Diagram


A Hertzsprung-Russell (H-R) diagram is a plot of a star's absolute magnitude (luminosity) against its temperature. When this is plotted for many stars, it can be seen that most lie in a band across the middle of the graph, known as the Main Sequence. This is where stars spend most of their lifetime, while they are burning hydrogen. Hot stars are found at the upper left hand end of the Main Sequence while cooler stars are found to the lower right. Stars are all classified according to temperature and spectral type, with the hotter stars called ‘O’ type stars and the coolest called ‘M’ type stars. The order of classification is:

O-B-A-F-G-K-M

And can be remembered using the mnemonic “Oh Be A Fine Girl, Kiss Me.

During its lifetime a star will evolve on the H-R diagram depending on its mass.


Click on the links below to find out more about stars


Stars Introduction

The Sun

Structure of Stars

Stellar Evolution

Variable Stars

Objects to Observe with the Faulkes Telescope



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Authors: Carolyn Brinkworth and Claire Thomas

Last updated: July 2001