HPR1587: Beginner's guide to the night sky 3 - A wee dot on a dark sky

A ramble about stars, by a geeky chap who resides on planet Earth. This episode is entitled a wee dot on a dark sky.

I comment briefly on why it's remarkable that the night sky is dark. I then go on to talk about the colour of stars, which we can just perceive with the naked eye. To learn more you need to use a prism, or, as professional astronomers prefer, a diffraction grating to obtain a spectrum of a star. I talk a little too much about the mathematics of diffraction gratings but eventually get back to talking about spectrum of the Sun which in overall shape is very close to what physicists call a black body spectrum (https://en.wikipedia.org/wiki/Black_body)- the spectrum any object will have at a given temperature. Astronomers and physicists prefer to measure temperature in units of kelvin (https://en.wikipedia.org/wiki/Kelvin), and to convert to it you only need to add 273 to the celsius temperature. Conversion from Fahrenheit is left as an exercise to the listener.

The Sun shows spectral lines, specifically dark lines on the broad spectrum called absorptions lines. This is caused by atoms in a cooler layer of gas (called the chromosphere) that's just above the bright surface of the Sun (called the photosphere). In fact, Helium is named as such because it was first discovered by its absorption lines in the solar spectrum (Helios is Greek for Sun). Many other elements can be found in the spectrum of the Sun and other stars, but most of the mass of all stars is made up of hydrogen and helium.

The temperature of a star is correlated with colour, with blue stars being hotter than red stars. This was originally measured by astronomers by something called colour or B-V (B minus V) index.

The luminosity of a star is the rate at which it emits energy as light, and can be measured in the same units as light bulbs, i.e. watts (W). But to estimate the luminosity we need to know the distance to a star which, for nearby stars, can be found by the parallax method. By plotting colour index (a proxy for temperature) against luminosity we can form a key piece of empirical evidence - the Hertzsprung Russell diagram: https://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_diagram

It turns out that our nearest star - the Sun - is quite unremarkable. It is neither very hot or cool, nor very bright or dim - it's a fairly typical star.

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