Stars

People thought that the Earth was at the centre of the Universe. They saw that stars seemed to go round a fixed point. There is a star, Polaris, which quite by chance is that fixed point. There were also patterns of stars called constellations. These appeared fixed.

Early astronomers like Galilleo got into serious trouble for suggesting that the Earth was not at the centre of the Universe. Even when people accepted that the Earth goes round the Sun, many astronomers had a heliocentric (sun-centred) view of the Universe.

The astronomer William Herschel (who was also an outstanding musician and classical composer) made detailed maps of the night sky using his telescope outside London.

Astronomers in the Nineteenth Century used spectroscopes to examine the spectra of light given out by stars, which in turn gave clues as to the chemical composition of stars.

The physicist Huygens used an inverse square law to estimate the distance of stars. However his assumption was that they all glowed with the same original intensity. Newton tried the same method.

Stars are massively long distances away, so it took a long time to devise instruments of high enough quality to measure changes in parallax. Parallax is the reason why, when you are sitting on a train, you see close objects moving through your field of vision a lot quicker than distant objects. Parallax was first used in stellar observation in 1838. The diagram shows the idea:

Suppose the close-by star appears next to star A on December 21st. On June 21st, it appears next to B. It appears to have moved through a certain angle; from the diagram, it is 2a radians. If we know the distance of the Earth to the Sun (1.5 × 10¹¹ m), and we measure a (in radians), we can calculate the distance d using:

The closest star to the Sun has a parallax angle of 1" (1 arcsecond = 1/3600^o).

This relationship gives us the units used by professional astronomers, the parsec (derived from parallax second), defined as:

The distance of an object which has a parallax of one arcsecond when observed from Earth

The hotter the star, the whiter it is. The American astronomer A J Cannon spent her lifetime making painstaking study of about 200 000 stars. She classified them according to the following:

Surface Temperature (K)	Colour	Spectral Type
>30 000	blue	O
11 000 - 30 000	blue-white	B
7500 - 11 000	blue-white	A
6000 - 7500	white	F
5000 - 6000	yellow white	G
3500 - 5000	yellow-orange	K
<3500	red	M

Blue stars (O-type stars) are rather bigger than the Sun (a G-type star) and tend to have a short life span. M-type stars are getting old; their surface temperatures are low, but still quite hot enough to turn you into pork scratchings.

A Danish astronomer Ejnar Hertzsprung recognised patterns within stars. Independently an American, H N Russell came up with the same sort of idea which gave rise to a useful tool called the Hertzsprung-Russel Diagram. This is essentially a graph of temperature on the horizontal axis while on the vertical axis we can put the absolute magnitude or the luminosity compared with the Sun. Sun = 1.

Most stars lie along the main sequence, going from very bright blue stars to very dim red stars. The Sun is somewhere in the middle of the main sequence.

To the top right there two distinct classes of star, the red giants and the red supergiants. Although they are cool, they have to be big to achieve the luminosity. The star Betelgeuse would engulf the orbit of Jupiter.

To the bottom left we have dim stars. Spectral line analysis suggests that they are very hot, but their low luminosity suggests that the stars are very small. White Dwarfs are thought to be about the size of the Earth but with a mass similar to the Sun. They are common but hard to observe.

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