transcript of episode 5: DO CHICKENS SEE GREEN STARS? 12th March 2021

[🎶 INTRO: "Spring Swing" by Dee Yan-Kee 🎶]

welcome to the error bar: where distortion is straightened out

in this episode: discussing the claim there are no green stars, astrophysicist dr ulrike kuchner & psychophysicist dr christopher taylor describe green valleys flares & chickens

here is the brain news on the 12th March 2021:



a tweet from Jim Al-Khalili in February 2020 said that there are 'NO green stars'. this is interesting, but colour is not a property of stars, it's a property of the brain of whoever is looking at the stars. several Wikipedia articles also cover the story.

the error bar investigated by talking about stars to astrophysicist Dr Ulrike Kuchner and about the human visual system to psychophysicist Dr Christopher Taylor.

see the full discussion here, or listen to episode #5 of the podcast.


prove it!


the science was by Harre & Heller in Astronomical Notes; reported in Twitter by @jimalkhalili on 28/Feb/20, & Wikipedia on 10/Mar/21

the astrophysics of green stars

welcome to the error bar, would you like to introduce yourself?

my name is Ulrike Kuchner. i am a postdoctoral researcher at the University of Nottingham. i am an astrophysicist, and i study how galaxies evolved in the universe and how the large-scale structure in the universe came to be

i also have a background in fine arts and i make art and curate art in this intersection called art science

great to have you here. do you need a drink?

ooh yes please. we ok with a martini?

martini works

gin martini then

what brings you to the error bar?

well, i hear that really interesting questions are being discussed here, and i am a creature of curiosity

about a year ago i read this tweet from Jim Al-Khalili, a physicist and a science journalist, and he said: 'science tells us there are NO green stars'. and it puzzled me, and i thought 'oh there must be green stars' i thought that was also a psychological question rather than a physics question. i want there to be green stars, so maybe you can tell me why Jim Al-Khalili said there are no green stars?

i like that you want them to be there

well, first of all, it's such a good question, because it sounds really simple, but it's really multi-layered. for the physics layer we can back up a bit and just discuss stars, and the fact that they come in all colours, all kinds of colours, but more precisely from the outer layer of the star. and that is based on its temperature

so it's a really important feature, the colour of the star. we know that bluer stars are hotter and redder stars are colder, for example. so the temperature is related to the kinetic energy, so the motion of the particles and the mass of the star - we really know a lot from the colour

but all objects emit light at all wavelengths, and that is from radio to high energy like x-rays and gamma-rays. but the overall statistical distribution of it, that describes all that light, that's close to this black body curve, which is like a smooth curve that peaks at a certain wavelength, but has a really long tail as well

so for example cool bodies like you and i, we peak in the infra red, but most of the stars like our sun peak in the visible or the optical light. and that also means that we can actually seen them with our eyes - that's the actual issue here

now back to the stars, if we have a reddish star, that distribution of the black body curve peaks in the red area of that distribution, so we see it as reddish. if we go to hotter stars then they become orange and yellow and then white - but not green. and that is because that peak includes what's left and right of it, which is red and blue, and so it's actually a mix of all these three colours and we kind of see this mix as white. and then as it goes even hotter that curve shifts to the blue side, and we see the blueish colours of the really hot, massive stars. i think that is the mystery

well it's disappointing there's no green stars. i found an old book of mine at home and there was this picture of the spectral classes, and they'd use greeny colours to illustrate the spectral classes. do you think it's just a convenient shorthand to say that the middly-ones are sort of greenish

i think perhaps they did this out of simplicity, because this is a complex problem that involves the human biology as well as the physics, so perhaps they just wanted to stick to the one discipline in that book and not make it more complicated

one thing that i will say is that even now we don't have the exact accurate representation in our visualisations. a more precise stellar spectrum template would create a little bit different colour codes which actually shows that there aren't really really red stars, so they are more orangey, yellow, and then kind of like our sun is a white - not yellow - star, and then there's lots of different shades of blue

is it just stars or do whole galaxies or clusters or nebulae, are they green at all? is there any green out there?

yes and no

so, i study galaxies, and galaxies are just made up of many many billions of stars, and since most of the light in a galaxy does come from this population of stars, if most of the stars in there are red then the galaxy will appear red

if we look at the many billions of galaxies out there, we see this bimodality. we have either those red or those blue galaxies, and they tell us about their evolutionary status, and that they're massive, or young or old, and it's really important. and we think that there is an evolution that goes from the blue with the young, massive stars, to the red galaxies that are older and more passive. and as they do this they transition and change their make up. and we call those the 'green valley galaxies'. but they aren't actually green, but they're called green

so the nebulae, etcetera, if you think of these beautiful images where you see a green glowing nebula, that tells us about the composition or the elements in the gas of this nebula. so if it excites in a very specific wavelength that is at the frequency where green is, then this is the actual colour of it, yes. and we tease that out of the image by putting filters on it that are just picking up that narrow wavelength range

when astrophysicists talk about colour, what situations are they really describing? are they talking about on a computer screen or with the naked eye?

we are usually talking about computer screens, or numbers. the images - or spectra for that matter - that are taken by professional astronomers, these are taken in telescopes that are placed in areas where it's very dark, where the air is very calm, no pollution nearby, so these are excellent seeing conditions. in fact, the best is of course to leave the Earth's atmosphere altogether and observe from space

we then put filters that block wavelength ranges and just take the same image through different filters, and that gives us some colour information. so they're broadband filters that reflect these broad green or broad blue light, but there are also these very narrow filters that really look at interesting individual areas where some emission might happen

so a colour is really subtracting one image through one filter by another image through another filter

you talked about atmospheric pollution and getting the best viewing conditions. is that why the stars twinkle?

not just pollution, but because the atmosphere is just not stable. so we have all these layers that are moving, and so the light that comes through it just jiggles and bounces and breaks. and so that's why the single point that should be the star is changing brightness and position very quickly, and so to us that looks like twinkling. so it's pretty annoying

yeah. i was looking at Sirius the other day through my lovely, brand new telescope, and it was definitely orange and blue at the same time. and then i de-focused the telescope, and there was definitely some green shimmering twinkly in there. that's just aberration, or it's like imperfections in everything that's just splitting up the colour, is that?

i'm not so sure about the green twinkle. yes, so i would guess that because you have the optics in your telescope as well, it's all about the imperfections

it seems that you liked it?

yeah, it was quite pretty. it was sort of shimmering and changing colour. it was very nice

there is this phenomenon, the green flash. i have been at the VLT, the very large telescope in Chile, with some staff astronomers there, who were standing outside looking at the sunset: 'is there gonna be the green flash or not?'. so that's a really, i think it's a rare, very brief moment where you have a tiny flash of green, when all the blue and reds are absorbed and not seen

i have not seen one, but i would love to see a green flash

so it's out there?

that is out there, yes. that's when the other wavelengths are blocked, by Earth or by the atmosphere

that's good news

so one problem that your colleague, Dr Meghan Grey pointed out, was that, a lot of astrophysicists are male, and colour deficiency - colour vision deficiency - is more common in men. so how do you as a community get around the problem of having quite a high proportion of people who can't see all the same colours?

yeah, that is an excellent question. it is true that there is a problem in our field, that there's an under-representation of non-males - it's about 20% female currently that have permanent professory positions. is it 10% or so of the male population who are colour blind? in fact, my phd supervisor was colour blind - well he still is colour blind - i guess i was trained to look out for that a bit

it's actually really easy to get around it, but it doesn't happen enough. you can just plot, instead of using colours, use different patterns and labels and shapes. there are wonderful websites out there to help you make your plots colour blind friendly. there are simulators where you can just see what that's like

so i would hope that it's known by now, that we have to take care of this. but it's not as much as it should

can you tell us a bit more about the interface between physics and art. do you think it gives you some advantage in trying to plot, or describe, or perceive the universe?

it's a shame that we have only a few minutes left because i love that topic

the interface between art and science, it's such a big topic, and where to even begin, because they have obviously a relationship to one another, but seem to play these very different roles in our human experience. because we are having such complex problems to solve it's really important to try such trans-disciplinary collaborations. i think important and urgent problems cannot be solved by one discipline alone. like open research is one of them. here the freedom of art and science is really on the line. so these two working together is very powerful

but you actually asked whether you think i have a unique perspective. and, i mean, don't we all have some kind of unique perspective? we all come to creating knowledge with our own historical background, and so mine just happened to be art

and i guess finally, i watched your video - your great video "Do I [dare] disturb the universe?", and you talked in there how you've used errors and mistakes from instruments and cameras to create projects. the error bar loves errors - it's why we're here to talk about mistakes, and what we can learn from them. do you have a view on the role of errors in your work?

i'm thinking a lot about how we respond to errors and uncertainties, and how we deal with mistakes and weaknesses. and also all these images that we create and i talked about a little bit, we try to remove of course the errors. and that makes a lot of sense

but i love the idea of just keeping them as they are. keeping those imperfections and reflect on those. and we try to always find always patterns in the noise and make sense, even if it's just complete, random noise. but if there's a little thing in there that's not supposed to be there, we immediately look at that and 'ooh that is interesting, and that's a little dead pixel in there.' i think it's just giving some space to reflect. and that's what i, that's my favourite kind of art. it's art that invites you to meditate and reflect

but if we know that even the best telescopes and the best instruments make mistakes - and forgive them - maybe we can understand ourselves a little bit better

[🎶 "Sloshed," by Dee Yan-Key 🎶]

any personal errors or mistakes you want to confess to?

oh, loads!

what's your biggest mistake maybe, or your most interesting one?

i don't think i have a good answer for that to get in a podcast

i'm reading and listening to people like, i think it was Neil Gaiman who said: 'make more mistakes, brave, big ones, and embrace them'. i really would like to just stand on a platform and say look at that! i did that

I'm not quite there yet

the psychophysics of green stars

welcome to the error bar. would you like to introduce yourself?

hi i'm Christopher Taylor, from the New England College of Optometry located in Boston's Back Bay. i work chiefly in the field of emmetropization, or how you grow and eyeball to match the power of the optics, so you don't need things like glasses.

and that's what i'm working on now, but i've worked on things such as ageing and vision, and spatial vision, and temporal vision, going on twenty years now, since i was an undergrad. so, i'm really feeling it. makes me feel like i need a drink

what can i get you?

hearing those dulcet British tones makes me really yearn for a real ale

this will do?

this will do, yes, yes, definitely

it's a real coincidence you've dropped in, because we were just talking about colour perception with an astrophysicist, so i think i could have a few questions for you


in a sentence, how do we perceive colour?

well, it starts with a photon, usually more than one photon, that will excite one of two classes of detectors known as photoreceptors either rods and cones - this is way more than a sentence - but, that's the easy bit

the brain is continually taking that physical property of wavelength, and using it to determine not only the relative colours of objects but to take out the colour of the illumination. and the brain's trying to give us the best interpretation of the wavelengths that are coming off an individual surface to give us the colour of an object

in a sentence is really complicated but requires mechanisms all the way up from the retina to the visual cortex. it's a huge matter of interpreting wavelength into even words and language which influences colour perception. colour is simple yet complicated sometime

so this is why it's good to have you here, because i've been speaking to astrophysicists, and for them there are no green stars, or yellow or cyan or purple. and that for me as a psychologist just sounded a bit too neat. so any initial thoughts on the colour of stars?

yeah. that they're confused by the trickery of psychology by using colour names instead of the precision of wavelength, which is understandable. it's a really common error, i think, just to believe that colour is just out there, rather than the brain constructing the world. the whole factoid that there are no green stars is a really neat one, and does say something special and particular about the universe. but it's also an interesting point about how scientific communication can delight, yet mislead at the same time

colour perception is complex. i think we get that. but let's break it down. so, i'm thinking of star gazing and astronomers are going out at night, you know, with a red torch, and then looking at the stars. what happens when it gets dark?

in the first eight minutes the cones will adapt, so that one type of photoreceptor that we see detail with and we see colour chiefly with. and once those are adapted after eight minutes, they aren't sensitive enough to catch light efficiently any more. and then the rods, that have bigger apertures and more sensitive photopigments will start to dominate

at that point, the rods will continually get more and more sensitive. in the central vision, where we tend to look and have our most detailed vision, doesn't contain any rods. so, when going out into the night it's better to look slightly off from any faint star object that you're looking to see

the brain with colour is always doing that trick of comparing the physical input among its sensors. it's a really big trick with cone and colour perception because if you look at the response of any of the different - instead of red, green and blue, let's call them long, medium and short-wavelength cones - they have very wide ranges of responses to different wavelengths of light. so, if you hit a medium or green cone with red light, it's going to respond, and respond quite well. so narrow-band light, aside from some of the shortest wavelengths, will stimulate all the short, medium and [long] wavelength cones

and given that we're talking about adaptation and different times of day and night, the time around dawn and dusk, you can get to light levels where both the rods and cones are active. and this is well known in the film and television industry as 'the golden hour'. but that's when humans can see both the rod vision and the cone vision, so we're functionally tetra - have four receptors - instead of trichromatic. so, everyone often comments on how beautiful things look, but it's probably because of that mere addition of the rods playing a role in perception

you're unweaving the rainbow for me here

oh yeah, and then speaking of rainbow, the whole issue of - there's no purple stars - things like that, if that's said, purple is a mixture of short and long wavelengths. so it's what's known as a non-spectral hue. so, one of those colours that, you can't create if you have a tune-able light, let's say, if you vary the wavelength of it, you can't get purple, you need both red and blue

because a star is like a point of light billions of miles away, could that point just stimulate say a green cone, then a red cone, then a blue one, and you would see - would you see, green and then red and then blue?

so if you hit a green cone with a red light, it will just signal 'i have caught some photons', so people who use systems with mirrors that are flexible called adaptive optics, can do what's known as single cone psychophysics, and stimulate single cones

and having been in one of those machines as a subject, you can stimulate a green cone with red light, and because you're just activating that one cone, there's not that ability to compare the activity across photoreceptors. so my experience of that was of a whitish light. kind of, it really hammers home that point that it's the entire system, not the receptors themselves, that are driving the percept of colour

to get that vivid sensation of green, to get the red cones deactivated with green light, with the green cone activated with green light in what's known as a centre-surround organisation. where one type of cone is being inhibited versus the cone that is being excited. it's that contrast, that comparison, that's really generating that colour signal

the trick with the mirrors is to stablise the light, right? because the eye's always jiggling around and moving, and presumably the optics of everyday life are not very good - there's scratches on your glasses, and everything's wobbling. is that the problem that - or not the problem, maybe it's adaptive that, when things wobble around you actually see a lot better?

yeah, i would say so. the whole system's tuned for things being in constant motion. yeah, and those adaptive optics systems will take into account any imperfections in the eye, the eye shaking about a little bit. those same systems can take pictures of people's photoreceptor arrays. and one thing that came out of that work, done by David Williams at Rochester and Austin Roorda at Berkeley, is how individually variable people's photoreceptor mosaics are

one person can have a retina with many many more medium wavelength cones than long wavelength cones, and another completely the reverse. yet, if you drop them into a behavioural experiment they'll be entirely consistent in what they call red between one another. so that photoreceptor mosaic, somehow despite massive individual differences, and i remember seeing those pictures for the first time, thinking: 'how does the brain even make sense of seeing at all', just is able to wire itself up and extract those signals in a consistent way? obviously there are some behavioural individual differences, but the anatomy, when it's visualised and imaged, is just astoundingly different. the engineers, whoever built those, must have had one too many real ales in those times

i've seen some of those mosaic pictures and yeah, i just thought it was, it couldn't be true. are these unique individuals, how much variability is there between the normal population?

[pffff], i don't have a quantitative number at my fingertips, but those initial images that were published fifteen to twenty years ago - are not atypical in their first small sample of subjects. so, having seen one observer with a green dominated retina and another with a red cone dominated retina. yeah, there are massive individual differences in the cone mediation in the retina

what kind of saves the issue in the end, is if you look at the green and the red cones' responses to wavelengths of light is that they are very close. their sensitivities overlap greatly, and their peak sensitivity is only separated by about thirty nanometres, which is very small. so, perhaps the system can utilise that there's something slightly interchangeable about what they are, and that we're just signalling differences in the spectral output, rather than using those sensor signals to signal wavelength

does the same apply to the rare cases of people with four cone receptors?

yes. those colour anomalous individuals with four - they're pretty rare. and even those individuals who we think of as red-green colour blind, aren't usually missing a photoreceptor type, it's just, instead of that thirty nanometre separation, there's a bit less of a separation between the medium and long wavelength cone

well, there are some extremely rare individuals who do lack the gene for the short wavelength cones, or one of the others. colour anomalous, and individuals lacking a photoreceptor, will have those cones closer together, and those tetrachromats will have what the anomalous individual has, plus the normal versions. so they have those photoreceptors packed in really tightly all around that range of that 530-ish nanometres

whether those tetrachromatic observers, whether that has any functional use, or anything other than something you can observe in the lab, is a question that i don't think is quite resolved, or it's something that's probably not harmful enough for evolution to select out, either in the deleterious case which is generally males, or the tetrachromatic case, which is generally females

i have to mention my friend the chicken, who has a UV cone. so chickens are used in vision research, particularly those questions surrounding colour and eye growth. and they literally, they do have a fourth cone which sees into the UV. this is true for many birds that often used it for sexual selection and mating. and they also have a cone type that has both the green and red photopigment together in one photoreceptor unit that senses luminance overall

so, despite the photopigments being preserved pretty well molecularly across evolution all the way back to the avian branch of the tree, the solutions that nervous systems and organisms have come up with have been really interesting

i feel like you need a philosopher to go along with colour perception research. why is it that massive differences in cone types or cone densities can not lead to any change behaviour?

i guess i would reach for the fact that there are so many pressures on behaving the right way, that if these small differences in anatomy truly mattered they'd be selected out. and you'd probably need a linguist too to distinguish all this, because there's the cross-cultural research where, i believe the Italians have greater distinctions in the short wavelengths, they have two commonly words for blues, or a green-blue and a blue. it's the frequencies at which these words are used vary from language to language

and linguists will analyse, well how many unique colours can folks name. and there are far more than one would predict just based on the anatomy i think

so that brings us right back to the start of this, where you sort of implied that to say that a star is green is almost like a linguistic mistake. it's just a helpful rule-of-thumb for people to communicate with each other about patterns that they observe. is that too philosophical - what am i asking?

yeah, i think so. i think that's worthwhile. colour at once is so fundamental and so common, yet you dig into it, psychophysically, anatomically, or physiologically, we're at once super-sensitive to it, yet we get to this point where we can just name a few shades, and that's our rainbow. it's at once this crucial property that our system is exquisitely sensitive to, but at the same time we categorise and really need to make quick decisions about - is that berry red, is that berry blue?

saying 'there are no green stars' seems striking to say at first, but when you dig into the details about black body radiation and things like that, it's saying something quite different from what we mean when we're talking about colour in a human context

[🎶 "Sloshed," by Dee Yan-Key 🎶]

we can offer absolution for any of your science sins. is there anything you want to own up to?

oh sins. i'm not an expert, but sloth as far as i can tell. sloth in writing up data. that's, err, i don't know if i want to be absolved from that, but maybe you can assign me a few 'hail Marys'?

i think a few bloody Mary's should probably should probably do it

much better, much better. that's the worst one. perhaps it's a virtue in the end, maybe?

green stars: a conclusion

well, the guests have gone. so what have we learnt?

the statement 'there are no green stars' sounds simple, but it's astrophysically and psychophysically complex.

star colour relates to temperature and composition, and while there are green nebulae, green flashes, and maybe even green twinkle, a star viewed on its own out in space, or using sophisticated adaptive optics, will not look green.

but this is not how most people see stars. starlight is filtered by the atmosphere, by pollution, by our eyes; and all colour is created by the brain, comparing signals from different receptors, places and objects.

so what sounds like a statement about stars, is actually a statement about the human visual system. and humans don't have identical visual systems - our eyes, retinas and brains differ, sometimes massively. and we use different words to describe colours. it's not clear, at least to me, that no-one has ever seen a green star.

conclusion? maybe the Italians have words for green stars? or maybe chickens can see green stars? i still believe there are green stars. and if you've seen one, email the truth is out there.

[🎶 OUTRO: "Cosmopolitan - Margarita - Bellini" by Dee Yan-Kee 🎶]

it's closing time at the error bar, but do drop in next time for more brain news, fact-checking & neuro-opinions. take care.

the error bar was devised & produced by Dr Nick Holmes from the University of Nottingham. the music by Dee Yan-Kee is available from the free music archive. find us at the error bar dot com, on twitter at bar error, or email talk at the error bar dot com