This video is sponsored by Brilliant. The Universe is peppered with a zoo of breathtaking
astronomical objects, everything from quasars to icy moons, from tidal streams to magnetars. Time and time again, nature reminds us that
her imagination can easily exceed our own. Yet amongst this plethora of phenomena, there
is one simple type of object which is bizarrely missing, a case where our imagination indeed
wins out - a green star. From physics, we learn that green is one of
the three primary additive colours, along with red and blue. Mix them together to make any other colour
you want. When we first learn about stars, it’s perhaps
not remotely surprising that some stars are red and some stars are blue. And yet there are no green stars. At first, you might think that perhaps the
answer to this is that there are in fact green stars out there somewhere, they’re just
incredibly rare. After all, in infinite space, surely all possibilities
eventually happen and it’s just a question of traveling far enough to find one. Yet the absence of green stars isn’t merely
an issue of insufficient data, it turns out to be an intrinsic rule of our universe. Green stars simply cannot exist. It’s as if someone wrote into the rules
of the cosmos that we’re allowed red stars and blue stars, yes, but our universe doesn’t
deserve to enjoy the restful pale light of a tranquil green star. So what gives? Well to understand that we have to quickly
cover what gives a star it’s color. Stars are essentially nuclear engines converting
lighter elements, primarily hydrogen, into heavier elements, primarily helium deep within
their core. The thermal energy released keeps the inner
core incredibly hot, tens of millions degrees Kelvin, and the outer inert layers absorb
some of this heat too through radiation and convection, with the outermost layer of the
Sun being about 6000 degrees Kelvin. Almost all of the atoms in a star are ionised
because of the high temperatures, which means that the protons and electrons have been separated
into a plasma soup. Photons strongly interact with ionised plasmas,
so much so that plasma are opaque to light. So that means we can’t actually see the
inner layers of the Sun, the outer layer blocks our view and has no transparency. Thus, when we look at a star, it’s appearance,
including color, is completely governed by the outermost layer - known as the photosphere. So really rather than saying there are no
green stars, what we really mean is that there are no green photospheres. Since that’s the only part we can actually
see. OK fine, but what gives the photosphere its
color then? Put another way, in what ways does light get
released from this layer? There’s three basic ways in which light
can be released from a given strata of matter. Transparency, reflection and emission. We’ve already discussed how the stars are
mostly a plasma which means that they have negligible transparency and reflection. Light just gets absorbed by the ionised material. So that leaves us with emission only. Molecules can emit light in sorts of interesting
ways, such as changes between different vibrational modes. But a star is essentially too hot for molecules,
they just get ripped apart by the extreme heat. Atomic transitions can still occur, but the
vast majority of a star’s energy gets released through the most basic process, thermal radiation. To an excellent approximation, stars really
emit in the same way that an idealised blackbody would. That means the distribution of how much high
energy and low energy photons are produced, known as the emission spectrum, is conveniently
governed by a single parameter - it’s temperature. This curve, known as the Planck or blackbody
function, is the sort of thing our physics students grapple with in their freshman year. It’s an incredibly important physics concept
but one we don’t have time to explore deeply today. A great way to learn more about blackbody
radiation is through our sponsor, Brilliant. Brilliant is an online learning platform that
promotes education by problem solving and practical examples, rather than memorisation. The
astrophysics course on Brilliant includes sections on Radiation and the blackbody function,
as well as sections on Atomic Spectra, and how energy is produced inside stars - all
things I’ve only been able to touch on briefly in this video. And let me add that Brilliant has many courses
beyond Astrophysics too, such as engineering, software development, coding and finance. Brilliant have poured a lot of energy into
the optimisation of education here, making learning a richly enjoyable experience. You can subscribe, and help support us out,
by using the link brilliant.org/CoolWorlds and the first 200 of you to use that link
will get 20% off the annual Premium subscription. For our purposes, the most important thing
about the Planck function is that no matter what temperature we choose, it always has
the same basic shape, it starts from zero, rises up to some peak, and then slowly drops
back down again. This immediately tells us that blackbodies,
and thus photospheres, and thus stars, do not emit light at just one wavelength, one
color of light - no, they emit at many colours simultaneously. And, yet more, most of the wavelengths are
completely invisible to us, our eyes can’t see x-rays and ultraviolet, nor can they see
infrared and radio waves. What we see, the color, is governed by the
shape of the blackbody curve within this little slither corresponding to the visible band. The basic shape is always the same, except
that it shifts over to higher energies as the object’s temperature rises. This is actually what the disk of the star
looks like as we change the stellar temperature, and on the right I’m showing the location
of the star’s color on a RGB color wheel. As you can see, the increasing temperature
makes the star changes from red to white to blue, but gracefully avoids green. With what we’ve discussed so far, we’re
now ready to actually get some understanding as to why green stars don’t exist, indeed
why they can’t exist. The visible part of the spectrum can be further
sub-divided into three colors, red at the low energy end, then green, then blue. Now in this visible region, the blackbody
spectrum can really just do one of three things. If we have a very hot star, then the spectrum
is shifted over to the high energy side and thus within the visible region we are looking
at the declining tail part of the Planck function. What does that mean? Well it means that we have a little bit of
red, a bit more green, and lots of blue. So, hot stars tend to look more blue than
anything else. Alternatively, take a very cool star, now
the peak occurs in the low energy region and the part of the Planck function falling within
the visible band is the rising portion. So this means we have lots of red, some green,
and hardly any blue. Thus, overall, red wins and we get an reddish
looking star. Ok, two possibilities and no green star. But perhaps there’s some middle ground you
might be thinking, where we can tune this to yield a harmonious green hue. Let’s try setting the peak of this curve
right in the middle of the green band. That would correspond to a star whose photosphere
was 5400 Kelvin. Now as we can see here, it peaks in the the
green but it’s still overall fairly flat. That means there’s still quite a bit of
red and blue light mixed in here. In fact, with a bit of math we can calculate
that the star produces 15% green light, but 13% red and 13% blue. And if you add roughly equal amounts of red,
green and blue together, you get white light. In fact, this is exactly the case for our
Sun, it peaks close to the middle here but still appears white - at least if your in
space - because of the contributions of the red and blue. The only way we could make a star green would
be if we could somehow keep the peak in the middle of the green band, but squish the Planck
function inwards, compressing around the peak more sharply at green wavelengths. But, remember that the Plank function just
has one controlling parameter, temperature. There’s nothing else for us to change here
to possibly produce the desired effect. Remarkably, it turns out that this is true
even if we try and change the constants of the universe. In the Planck function, we have the following
constants that we could consider varying. In 1893, Willhelm Wien showed that the peak
of the Planck function occurs at this wavelength, so we let’s impose that this equals 530nm,
the centre of the green band. The width of the Plank function is characterised
by it’s standard deviation. We want to try and shrink this down to something
like 10 nm or smaller to create our vivid green star. With a bit of math, we find that the standard
deviation of the Planck function is this, which recall we’re going to try and set
to be below 10 nm. Don’t worry about the details here, the
important thing is that the second term looks familiar because it’s already in our other
equation for the peak, so we can do a little substitution here to conveniently remove this. Finally, we can multiply through these numbers
here to get our final simplified inequality. Remember that this represents our requirement
for a universe to create a green star. It states that we need 900 nanometers to be
smaller than 10 nanometers. But of course it’s not, it’s almost 100
times bigger. And I didn’t even choose constants of the
universe here, they all just cancelled out. Staggeringly, there aren’t just no green
stars in our universe, there are no stars in even a multiverse of differing physical
constants. Is there anyway for a green star to appear? Changing the constants of nature clearly doesn’t
work. Nor in fact does adding a blue and red star
together, that just makes a whiteish star again. There is one way to create a green star, but
its really only an optical illusion created when viewing stars through an atmosphere. Our Sun provides a clear example of this through
the famous green flash effect. As the Sun sets on the horizon, light has
to travel through more atmosphere than when it’s overhead. The atmosphere preferentially scatters blue
light, hence why the sky is blue, and so at sunset and sunrise, the Sun’s blue light
component gets removed. So this leaves with us a reddish looking Sun. Now to get green, we combine this scattering
effect with refraction. As the Sun dips below the horizon, light from
the Sun can bend through the air via refraction and thus continue to reach our eyes. But, crucially, red light doesn’t bend as
much as green, so at just the right angle the red light gets blocked off by the horizon,
leaving now just the green light. The Sun dips behind the horizon pretty fast
so this effect doesn’t last very long, often just seconds, hence the name green flash. In the same way, a bright Sun-like star would
also appear green through our atmosphere via the same effect - but it would only be a temporary
illusion. In space, dust clouds can sometimes interact
with light too, but it’s difficult to imagine how such clouds could conspire to create the
same effect in a persistent way, and so it really does seem as if nature prohibits green
stars to grace our skies. Now, in a way, this actually presents an interesting
opportunity. To the best of our knowledge, nature simply
cannot produce green stars. So if we ever did see a green star, that would
seemingly imply it was not natural. An artificial effect. It’s really not that technically hard to
create a green star artificially, just grab a green filter and put around your star. It would be like enveloping a star within
a giant bubble of semi-transparent material. Of course, building a filter this large would
be an outrageous engineering project, one only possible by a highly advanced civilisation. But in principle this bubble could just sit
around the star for billions of years, making the star appear green, and thus artificial. For billions of years, anyone looking up at
that star would immediately be able to tell something was very wrong with it, that some*one*
must have done that deliberately. Why? Perhaps simply as a cosmic art piece to some
highly advanced civilisation that thinks nothing of wrapping stars up in bubbles as they see
fit. Or perhaps as a deliberate beacon that a civilisation
exists in this stellar neighbourhood. After all, the effect is non-natural, persistent
for potentially billions of years and is essentially a passive beacon requiring no active power
source. Whenever astronomers come across possible
non-natural phenomena like this, we call it a technosignature. Narrowband radio communication is perhaps
the most famous example. What makes this green star effect an interesting
technosognature is that it’s easy to look for, we can trivially measure the color of
stars and indeed regularly do so. Now, I bet, when you started this video, a
video asking why we don’t see green stars, you didn’t expect it to end up in the arena
of technosignatures. But I think this little journey illuminates
an example of how astronomers come up with technosignatures. You start with a simple question about the
universe, you work out if there’s possible natural exceptions, you consider how easy
it is to measure and in the end your left with something potentially interesting to
go after. Whether a civilisation would actually go to
the enormous trouble of doing this, who knows, I have to admit I’m rather skeptical about
that. But on the other hand, xenopsychology is complete
speculation, so perhaps so. The real point is that this is incredibly
easy for us to check out, many stars have already had their colors measured and catalogued
online through services like SIMBAD. And if we find a green star and it turns out
to not be an alien civilisation after all, you’ve just discovered the very first example
of a completely new phenomenon anyway. So there you have it, green stars can’t
naturally exist, not in our universe or indeed even within a broader multiverse of varying
physical constants. But, perhaps they are out there as beacons
of artificial constructs, monuments to a civilisation’s mastery of the cosmos. More importantly, sometimes asking simple
questions like this, the kind of question a child might pose, can lead to fascinating
conclusions. Never dismiss the simple questions, pull on
those threads and see what unravels, because sometimes,
the thread might just pull down the very sense of our place in the universe. So until next time, stay thoughtful, and stay
curious.
