<p>
A ramble about stars, by a geeky chap who resides on planet Earth. This episode
is entitled a wee dot on a dark sky.
</p>
<p>
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 (<a href="https://en.wikipedia.org/wiki/Black_body">https://en.wikipedia.org/wiki/Black_body</a>)- the spectrum any object will have at a given
temperature. Astronomers and physicists prefer to measure temperature
in units of kelvin (<a href="https://en.wikipedia.org/wiki/Kelvin">https://en.wikipedia.org/wiki/Kelvin</a>),
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.
</p>
<p>
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.
</p>
<p>
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.
</p>
<p>
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: <a href="https://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_diagram">https://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_diagram</a> 
</p>
<p>
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.
</p>