Episode: 1587
Title: HPR1587: Beginner's guide to the night sky 3 - A wee dot on a dark sky
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr1587/hpr1587.mp3
Transcribed: 2025-10-18 05:24:06

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Hello and welcome to this third episode in this series especially for Hacker Public Radio HBR.
Gold beginner's guide to the night sky with me McNalloo, real name Andrew and this episode
episode three is entitled A We Dot on a Dark Sky and there's two things in that title.
There's the dark sky bit, it's quite remarkable, certainly it's worth remarking upon that
our night sky is dark but that is something for the future.
Absolutely cosmology because it's telling us something about whether the universe is infinite or
finite, about whether the universe is very full of stars, a high star density, about whether
the light from these stars have had enough time to reach us here at the earth.
As I say that is interesting but fortunate in a way that we have a dark sky otherwise we
wouldn't be able to notice the faint dots of star light.
So what we have in our real night sky is a little tiny amount of light from the stars at night
other than the sun which we can only see during the day of course by definition and these
this feeble amount of light from stars is telling us all we know about them.
We didn't learn a huge amount about stars from the sun because there's only one example of
the sun and it's a pretty normal star, very close to it but that doesn't help us because there's
only one close to one example of a star. And when we look up at the night sky all we get to see
of all these other stars at least as far as our eyes are concerned is a little dot of light,
a weird dot of light. So how do we know so much about stars? Well let me start with the fact that
well we don't just see a little dot of light. Well first of all let's look at the difference
between a star and a planet. Now the first most obvious thing about planets is that they are
wanderers. That's what they mean, that's what they mean literally from the Greek planet means
wanderer and they move position quite rapidly amongst the background of the stars but the patterns
of the stars themselves remain fixed. Now simple notions of parallax tell us that things that are
closer appear to move more quickly. So a tree that passes your car window where you're driving
down the road will appear to move more quickly than a jet fighter traveling in the horizon some miles
away even if you're looking at it through the same window. Similarly the planets appear to move
much more quickly because they're closer. And the second thing as planet light is that the dots
aren't that weak. With the naked eye you should be able to perceive that a star at a given
altitude or angle above the horizon will twinkle. Now the lower the better actually the lower the
star and the more it twinkles. A planet say Jupiter or Saturn or Mars anyone will do. At the same
altitude will twinkle a lot less in fact hardly at all. And the reason is because the dots of a
planet is not so weak it's not so small. And in fact with a small telescope in the case of
certainly Jupiter but also Mars and Saturn you'll quite easily be able to make out the the
fact that the planet has a disk. And that is why the planet doesn't twinkle as much because the
star even to the best of our optical telescopes is essentially a dot of light an unresovable dot of
light well below the optical diffraction limit of any traditional telescope that we've ever made
certainly any that I imagine myself or any listeners here will ever lay their hands upon
and amateur capacity. So we're not going to tell enough a lot about stars by looking at their
their disks other than the Sun of course. So what else would be good to go on? Well your eye can
pick up one other thing about some stars. If you look at Orion which I mentioned in the first
episode you'll notice that Rachel which is in the bottom right foot of Orion as you're looking at it
is white tissue maybe tinged with blue. I never see the blue with my eyes. Whereas Betelgeuse,
a star of compatible magnitude that is about the same right in the store eyes. At the top left
shoulder of Orion has quite a distinct orange U-rayed hue to it now. Again depending on your urban
environment, your location and your eyes and also whether you've got a fever or not, whether you're
drunk or not, your dark adaption and therefore your perception of color may differ.
So you may not see these colors quite the way I described them or at all but usually most people see
the color of Betelgeuse. For the dimmer stars you can't perceive color at all they're just two
dims. It's only the very brightest of stars or if you view stars through binoculars or a telescope
that'll allow more light into your eye and therefore give you a better idea of the color.
Now the color of a star is simply related to its temperature. It's basic physics actually. You can
do the experiment just turn on an electric element and it will start off not glowing. It will be
giving you an infrared radiation which you can confirm by holding your hand near it. You can feel
something as heating up your hand if you hold it close enough and then as it heats up it should go
red and then orange, then yellow and then white. Although if you have an electric element in a
cooker or an electric heater that goes white hot I suggest you turn it off quickly and call an
electrician. But if you let it keep going eventually it would start to emit off the blue end of
this bettrum. Get bluish tinge and be beaming out ultraviolet which of course would give you a nice
suntan but you won't get a suntan by looking at a very hot star such as a riddle or even a star
tan for that matter. It's obviously just two people. So just with your eyes you can make some
determination over the physical property of a star. It's temperature just from its color.
And if you want to do better than that then you need to start splitting up the light into the
spectrum of its colors. Now the simplest device you can use for such a purpose is called a prism
it's the block of glass usually it's of triangular shape and that would do the job rather than
likely to produce a spectrum of the light. In astronomy at least in professional astronomy a prism
is very rarely used. In fact it's almost never used to produce a spectrum. For two reasons first of all
every millimeter that light has to travel through glass or a perspex or any transparent material
you'll lose some of that light because no material is perfectly transparent. So you really want to
avoid losing light by passing it through a big thick block of glass like a prism. Now you can make
a small prism yes and that would reduce the effect but even then there's all kinds of other
problems with using prisms. Not least of which is that the mathematics describing the diffractions
out of prism is rather complex. So the solution to this is to use something called the diffraction
and it works in a different way it uses the interference wave properties of light to cancel
light at certain angles leaving you with only if you're viewing light through a diffraction
grating for a particular angle you're guaranteed only to see one wavelength of light.
So it works quite differently for a prism but it has the same desired effect it will produce a
spectrum if you hold a screen on the other side of the diffraction grating so you're at your
telescope and one side the diffraction grating and another side you'd have a screen where you can
view the spectrum or you replace that with photographic film or in modern equipment of course you'd
replace that with some kind of digital camera and you get an ice image of the star spectrum.
Another reason that you want to use a diffraction grating quite apart from the light loss problem
of a prism is that the mathematics of a diffraction grating is much simpler and in fact you
can work out the angle of diffraction, the angle at which you'll find a particular wavelength,
the sign of that angle is equal to the wavelength divided by the distance between lines and
the diffraction grating which doesn't mean, think of a diffraction grating as a tiny little fence
with slits and slats and typically you'll have thousands watts per millimeter to produce the
desired effect with light giving you the spectrum and so if you can get down to thousands
of lines per millimeter that kind of level or maybe hundreds would do actually thousands
of bulletin much but certainly hundreds of lines per millimeter then you'll get your
interference effect to produce your spectrum and then you get that simple equation that just said
the sign of the angle of diffraction equals the wavelength divided by the distance between slats
and the diffraction grating. So from that you can probably guess well if you've got any maths
and any feeling for the maths that because the angle is wavelength divided by the distance between
slats and the diffraction grating then you want as I was suggesting before it a small
gap between the slats and diffraction grating as possible because that'll produce a wide range
of angles and indeed that's why you need several hundred lines per millimeter.
Anyways I'll a bit be distracted there with diffraction gratings but what you will see if you do
that is that a star has a continuous spectrum and indeed you can sort of tell that from our own
sun. The sunlight is whitish it's got a yellowish tinge to it cartoon drawings by children often
calling the sun in yellow but actually the sun if you think about it is not that yellow I mean if
you look at a white wash wall on a sunny day it doesn't look yellow does it? No it looks
decently white. So but there is it but it's got a yellowish tinge to it which is why
old fashioned tungsten filament light bulbs give us a much more pleasing light than some of the
LED or low energy fluorescent equivalent bulbs because essentially you're looking at with a
filament bulb you're looking at an object the filament glowing at at the same temperature
of the surface of the sun so by physics it produces more or less the same spectrum what in physics
you would call a black body spectrum rather confusing because it's not black at all I'm not going
to go into the physics of black bodies I already digressed off into diffraction grating so resist
that temptation but maybe I'll come back and talk about black bodies and I might even mention it
in a show notes because who knows it might put up the number of hits and search engines to the
episode anyway so yeah so you get this emission across the spectrum but if you were to look at it
carefully you would find that it isn't flat across the whole spectrum white light is emission
across the whole spectrum but you'd actually find that sunlight peaks in the green parts of the
spectrum not that our eyes can see it because the sun doesn't look green because as I say it's
it's really quite broad spectrum but it's slightly higher in the green there's more light
emitted in the green than there is say in the red of the blue and that's because the surface of our sun
is at a temperature of 5,800 Kelvin now it's probably worth me saying what a Kelvin is
it's very simple in one setting it's not like Fahrenheit in degree c a change of one Kelvin
is one degree c and the only thing is that the Kelvin scale starts at minus 273 degrees c
so if you take a temperature of if I say get caught your temperature of in Kelvin of 5,800 Kelvin
you have to subtract 273 from it so ballpark let's say the surface of the sun is 5,500 degrees c
now as far as our everyday experience of temperature goes that's just hot 5,500 5,800 you know
and isn't that quite important but in everyday intuition that's just basically hot and admits white
across the spectrum now I said that the sun spectrum peaks in in the green that that doesn't mean
the sun looks green as I said and I don't think it's related to the fact that plants are green
because of the chlorophyll in them I don't think that's true I I feel it's just the number of times
I've never quite got to the bottom of it but then to be honest I haven't seriously looked
and I you know I'll look again and maybe in the next episode I'll tell you what I find but if you
know if the fact that what why most plants are green most leaves the green do let me know in
the comments to the show anyway another decoration now superimposed upon that continuous spectrum
are parts of the spectrum that appear a little bit darker and these are called absorption lines
so if you look at a spectrum of the sun you'd see it going as you'd expect a rainbow as a rainbow
you go sort of red, blue, green and then off through the blues eventually I'm going to violet color
and then those all the colors that you see with the eye and of course one end you've got infrared
and no further end you've got ultraviolet but we can't see them anyway on top of this sort of rainbow
spectrum you would see these dark lines called absorption lines and these are caused because
atoms in the atmosphere of the sun so above that surface of the sun I referred to called
the photosphere that's what it's called a bright surface that bright ball that we see that's
if you like what we think of as the sun but there's an atmosphere above that called the
chromosphere which we can't see except junior eclipses and in that there's some actually cooler
gas and in that you'll find atoms and those atoms are stopping some of the light at very
particular wavelengths that correspond to energy levels in the atom so again I have to go
to wee bit of degradation and to physics here so you've got an atom and let's say it's mainly hydrogen
there's other things in there but a hydrogen atom which is what the sun and most stars are made of
is a positively charged proton one particle in the middle called the nucleus and around that
is a single negatively charged electron now just like you can find planets in the solar system
orbiting different distances from the sun electrons in an atom can be found at different
distances from the proton so electrons orbiting the atom can be different distances from the proton
but since we're now in the realm of the weird and wonderful quantum effects
the terms of the electron doesn't really occupy any old orbit it is constrained to be in one of a
set of allowable orbits quantum orbits if you like and each of those corresponds to a different
energy level and when a light from the sun comes in and the particle of light is called a photon
as opposed to a proton which was the positive charge thing in the center of the atom when a photon
photon particle of light arrives at the atom if that has just the right amount of energy
if it's just the right wavelength it will be able to bump the electron from its current energy
level up to a higher one and then that photon will disappear and it won't come to us here at the
earth and it won't make it through your direction grating in your telescope etc so you'll get a dark
light but only at that wavelength now hydrogen atom has many different energy levels so it can
be used a number of lines and these dark lines historically go by the name of fronhofer lines
also you will find the lines corresponding to other elements and what's particularly interesting
is that the element helium which may be in a balloon near you is named after the sun helium
heliose Greek word for the sun because that is where it was discovered it wasn't discovered
here in earth it's very rare it doesn't bond with anything so it's helium only unlike hydrogen
which is plentiful in water and other places helium can only really be found by itself
it's not bond with anything it's no bill it's called a no-bill gas it doesn't react
so it was never found here in earth and they noticed the strange pattern of spectral lines
these absorption lines in the sun that they'd never seen anywhere else it concluded there must
be another element called helium and eventually it was found here in the earth in tiny quantities
I then bagged up and sold as balloons but we need to stop doing that otherwise well we might run
out of helium for medical purposes again another story so once it was realized that you could
associate these spectral lines that you'd find in the spectrum of the sun with light from atoms
that you could analyze in the lab so you could take a hydrogen sample in the lab pass them white
light through it and create your own absorption lines and match those up with what you saw in the sun
and then in the reverse direction helium was discovered in the sun and then we found it here in the
earth and a lot of other elements that we know about which is just carbon for instance were identified
here in earth in their spectral signature and then we found them in the sun and it turns out
that although the light from other stars is relatively feeble it's still possible with the
telescope to produce a spectrum and find these spectral lines quite easily even in the feeble
list of light from star light was to equipment so very quickly we were able to determine the
only what temperature the surface of a star was from as broad spectrum but by hunting for spectral
lines these absorption lines in particular we could start to to make measurements of what elements
were present in in the stars and what we found is that the sun most stars in fact most of the
universe is predominantly hydrogen and a small amount of helium and only a tiny amount of everything
else so roughly speaking the universe is about 75% hydrogen 20 something percent helium and then
probably generally less than 1% everything else and this varies slightly from star to star Jupiter
shows for example the gas giant it shows a similar makeup it does vary from star to star and
from different places in the universe if you look at different places you'll find slightly
different makeups but it's always around that sort of 75 25 hydrogen helium ratio I should point
that that's measured by the mass of the atoms if you're going on counting atoms then that figure
is more like 90% 10% because and that's because helium is roughly four times more massive than
the hydrogen anyway another degradation so from only a tiny dot of light if you can get that spectrum
you can determine awful lot to build the stars and then once you've looked at a number of stars
you can start to build up correlations and the first correlation that you can build up is that
the surface temperature of a star is correlated with how much light is pouring out of it now the
story of how you do that is that you need some number that measures the temperature of a star now
it could be the temperature itself but in the olden days you didn't do that you took a B filter a
blue filter looked at how bright the star appeared through that through your telescope and then
you took a V filter V for visible and looked at how bright your star appeared through that through
a telescope and of course if you had a blue star then it would look brighter through the blue filter
so you could then take the difference of B minus V the values you got for the brightness of the
star and the system used back then was called magnitude where the brightest stars in the sky
have a magnitude of zero and the damage that you could see with your naked eye were about six
so you could work up the magnitude through the B and the V filter and then the B magnitude minus
the V visible magnitude would give you what's called the color index and that would give you a number
which actually is very well tied very well closely connected to the temperature of the star
surface temperature of the star and you can plot that along with axis and you can draw another
axis up the way usually and on that you can plot how bright the star actually is that is
removing the effect of the fact that different stars have different distances now for the closest
stars you can use the parallax effect which I think I mentioned in the first episode the fact that
as the sun sorry as the earth orbits the sun six months apart the earth will be
on opposite sides of the sun and a nearby star due to this parallax effect will appear to move
back and forth as the earth orbits the sun and you'll see nearby stars moving back and forth
and you can measure how far but at the angle by which they move back and forth you know you can
with a bit of simple geometry, trigonometry you can calculate the distance the star and by doing
that then you can calculate what's called the absolute magnitude of a star an absolute magnitude
of the star is how bright it would appear to be if you moved it to a standard distance which is
10 parts x which is 32.6 light years I could quote that at you in meters but it's a very large
distance indeed it's fast to see one light year is the distance that light travels in a year
and light travels 300,000 kilometers every second so it's a very large distance but it's
the standard anyway so what you end up with is a scatter plot of values along with horizontal
axis which you call the B minus V index which is essentially mentioned the surface temperature
of the star and on the vertical axis you can plot them absolute magnitude which is essentially
measuring how much light is pouring out the surface of the star and as I said before you find
the two are very strongly correlated for more stars and in fact on the famous diagram it's
called the heart's broadened rustle diagram what you see is that that red stars tend to be
intrinsically rather dimmed what's called low luminosities luminosity is the amount of energy
pouring over star per second we actually could measure it in the same units we use for light bulbs
so like there's a light bulb next to me it's about 100 watts I think
and the sun which I can't see because it's Scotland and it's all cloudy and if it was a light bulb
it would be rated at about 4.6 times 10 to the 26 watts and that's 4.6 and then another 23
25 0s watts so rather more powerful than a light bulb and anyway I've again done my
degradation thing so what you end up with is a correlation you've got red dim that is low
luminosity stars and blue bright intrinsically bright high luminosity stars most stars will
this a straight line going from red and dim up to blue and luminous and that line is
astronomy called the main sequence because you find most stars on that and the sun sits in the middle
not particularly hotter cool surface temperature not particularly high luminosity not very low
luminosity either neither is it red nor is it blue it's typical I don't say average because
more star that average star would be in the red end more stars are small faint and low surface
temperature the sun is I would say a typical star does nothing special about what it sits
in the diagram and you see other stars you see these strange luminous
but red things and they're called red giants and you might sometimes pick up these rather
odd little times called white dwarfs which are surprisingly high temperature but very very dim
and so it's not quite as simple as just a simple main sequence there are other things going on in
this in this plot compared to brown russell diagram and I'll provide a link to one in the show
nodes so I think what I'll do is I'll bring this episode to a close here because I think I've
actually packed quite a lot in rather more than I intend to if it was too much please let me know
if there are things you feel like skated over or gave short shrift to let me know what that is
well and I'll never to address that in future episodes um well thank you very much again
for listening and thanks to all those uh on hpr who made this a great community and I
shall hopefully do another beginner's guide to the night sky episode four in the not too
distant future thanks bye
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