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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.

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 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) A hot, blue star 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.


Twinkling


When stars are viewed through the Earth’s atmosphere they appear to twinkle. The atmosphere is made up of many different layers of different temperature and density, each with a different refractive index that bends 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. This distance is defined as 1 Astronomical Unit (AU). The other stars are much further away. The nearest is called Alpha Centuari, a group of three stars 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 Centauri 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.

The third measure of distance commonly used by astronomers is the parsec (parallax-second, or pc). A parsec is defined using a system of measuring distances to stars called parallax.

parallax
Parallax uses the movement of the Earth around the Sun to measure the distance to other stars. As the Earth moves round the Sun, close stars appear to move against the background of more distant stars. The distance to the star can be worked out using simple trigonometry by measuring the angle that the star appears to move. This only works for the closest stars because the angles moved are very small, and get smaller the further away the star is.

If the angle that the star moves through is 1 arcsecond (1") then the distance to the star is 1 parsec (1 pc).

1 pc = 2.093 x 105 AU = 3.086 x 1016 m

1 pc = 3.262 ly


Once the stars become too distant to use parallax, the distance to them is measured by using standard candles. These are stars for which the absolute magnitude and colour are accurately known. If the apparent magnitude is compared to the absolute magnitude, the difference between them will give the distance (see below).

Measuring the brightness of stars


The observed brightness of a star is given by its apparent magnitude. The system was first devised by an astronomer called Hipparchus. He made a catalogue of about 850 stars in the sky and classified them according to a numerical scale. 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. The magnitude scale has been shown to be logarithmic, with a difference of 5 orders of magnitude corresponding to a factor of 100 in actual brightness.

The brightness of a star is measured in terms of its radiated flux, F. This is the total amount of light energy emitted per surface area. Assuming that the star is spherical, F=L/4πr2, where L is the star’s luminosity. This equation shows that as the light moves away from the star, the flux drops as 1/r2. This is the inverse square law for light.

Also defined is the absolute magnitude of a star, M. This is the apparent magnitude a star would have if it were located ten parsecs away. Comparing apparent and absolute magnitudes leads to the equation:

m - M = 5 log10 (
r

10
)


where r is the distance to the star, measured in parsecs.

m-M is a measure of the distance to a star and is called the ‘distance modulus’.


Classification of Stars


The Hertzsprung-Russell Diagram


A Hertzsprung-Russell (H-R) diagram is a plot of absolute magnitude (luminosity) against temperature. When this is plotted for a large sample of stars, it can be seen that the majority lie in a band across the middle of the plot, known as the Main Sequence. This is where stars spend most of their lifetime, during their hydrogen-burning phase. 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 as follows:

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. See Stellar Evolution for more information about the Hertzsprung-Russell Diagram.



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