Archive for green

On the Colours of Stars

Posted in The Universe and Stuff with tags , , , on January 20, 2021 by telescoper

There’s an interesting paper on the arXiv by Harre & Heller with the title Digital Color Codes of Stars. Here’s the abstract:

Publications in astrophysics are nowadays mainly published and read in digitized formats. Astrophysical publications in both research and in popular outreach often use colorful representations of stars to indicate various stellar types, that is, different spectral types or effective temperatures. Computer generated and computer displayed imagery has become an integral part of stellar astrophysics communication. There is, however, no astrophysically motivated standard color palette for illustrative representations of stars and some stars are actually represented in misleading colors. We use pre-computed PHOENIX and TLUSTY stellar model spectra and convolve them with the three standard color matching functions for human color perception between 360nm and 830nm. The color matching functions represent the three sets of receptors in the eye that respond to red, green, and blue light. For a grid of main sequence stars with effective temperatures between 2300K and 55,000K of different metallicities we present the red-blue-green and hexadecimal color codes that can be used for digitized color representations of stars as if seen from space. We find significant deviations between the color codes of stars computed from stellar spectra and from a black body radiator of the same effective temperature. We illustrate the main sequence in the color wheel and demonstrate that there are no yellow, green, cyan, or purple stars. Red dwarf stars (spectral types M0V – M9V) actually look orange to the human eye. Old white dwarfs such as WD1856+534, host to a newly discovered transiting giant planet candidate, occur pale orange to the human eye, not white. Our freely available software can be used to generate color codes for any input spectrum such as those from planets, galaxies, quasars etc.

This reminded me of a post I wrote in 2011 about why you never see any green stars. They say a picture paints a thousand words so here’s an illustration from the above paper:

This shows that although there are stars in the Main Sequence whose spectra peak at wavelengths corresponding to green light, none of them look green. The paper also claims that there are no yellow stars either. The Sun can look yellow when viewed from Earth but that is to do with scattering in the atmosphere: from space, the Sun looks white.

For another discussion of the use of colour representations in cosmology, see here.


The Royal Observatory Bomb and the Rise of Unreason

Posted in History, Literature, Politics, Science Politics with tags , , , , , , , , , , on February 16, 2014 by telescoper

I missed the anniversary by a day but I thought I’d pass on a fascinating but very sad little bit of history. One hundred and twenty years ago yesterday, on February 15th 1894, a 26-year old Frenchman by the name of Martial Bourdin blew himself up near the Royal Observatory in Greenwich. His death seems to have been an accident caused by the bomb he was carrying going off prematurely. It is not really known either whether the bomb was meant for the Royal Observatory or somewhere else. Anarchist attacks involving bombs were not uncommon in the 1890s and the range of targets was very wide.


Bourdin was found alive, though very seriously injured, by people who heard the blast. Though able to speak he did not offer any explanation for what had happened. He died about half an hour later.

This sad and perplexing story inspired Joseph Conrad‘s famous novel The Secret Agent. Conrad added an “Author’s Note” to the manuscript of his book:

The attempt to blow up the Greenwich Observatory: a blood-stained inanity of so fatuous a kind that is is impossible to fathom its origin by any reasonable or even unreasonable process of thought. For perverse unreason has its own logical processes. But that outrage could not be laid hold of mentally in any sort of way, so that one remained faced by the fact of a man blown to pieces for nothing even most remotely resembling an idea, anarchistic or other. As to the outer wall of the Observatory, it did not show as much as the faintest crack.

We’ll never know what Bourdin’s motivations were; perhaps he didn’t really know himself. He is usually described as an “anarchist” although that term describes such a wide spectrum of political beliefs that it doesn’t really explain Bourdin’s actions; not all anarchists embrace violence and aggression, for example, although some – such as the members of Class War – clearly do. At one end of the anarchist spectrum there are the violent thugs who are nothing more than the mirror image of fascism and at the other there are reasonable intelligent people who simply don’t believe in hierarchical structures.

Brighton has its share of anarchists and the thing that’s most noticeable about them to an outsider like me is their conformity; the dress code is apparently very strictly enforced. The obvious irony aside, this suggests to me that much of the attraction of being an anarchist is not really the existence of a compelling political philosophy, but simply to fulfill the need to belong to something.

The main thing that occurred to me yesterday while I was reading about the Greenwich Observatory bomb plot concerns the implications of the location. If the Royal Observatory was the intended target then why was it so? The simple answer is that a core belief for most varieties of anarchist is their opposition to “the State”. A powerful symbol of the British state in 1894 was the Royal Navy; it was Britain’s maritime traditions that led to the founding of the Royal Observatory in the first place and most of the work carried out there involved accurate positional measurements designed to help with navigation. Or maybe it was to do with the role of the Observatory in defining the time? Insofar as acts like this make any sense at all, these seem reasonable interpretations. 

I’m tempted to suggest that the adoption of Greenwich as the Prime Meridian in 1884 may have given a young Frenchman additional grounds for resentment..

A different answer from the suggestion that it was an anti-establishment gesture stems from  the conflict between anarchism and the nature of scientific knowledge. Anarchists usually express their beliefs in terms of the desire to make society more “equal” and “democratic”, so that decisions should be made collectively for the common good. I’m happy with that line of argument, and agree that we should all enjoy equal rights versus the government and other institutions, and in relation to one another. However, having equal rights does not mean having equal knowledge and it doesn’t mean that any person’s opinion about anything is as good as anyone else’s. What I mean is that there are scientific experts, and the knowledge they possess has demonstrable value.

The approach of some to this challenge is simply to deny the value of scientific knowledge, and assert instead that it’s just a social construct like anything else. I am aware of a number of so-called social scientists at the University of Sussex and elsewhere who hold this view; my usual response is to ask them whether they regard witchcraft or crystal healing as equal to orthodox medicine.

CLARIFICATION: Please note I do not mean to imply that all social scientists hold the opinions described above. I’m fully aware that they are fringe views. The phrase “so-called social scientists” does not refer to all social scientists, just the fringe in much the same way I’d use “so-called geographers” to describe the Flat Earth Society.

I’m not trying to suggest that members of the Department of Sociology are plotting to blow up the Astronomy Centre! What I do think that while we should always strive to be as democratic as possible there are always limits, not just because of what is practically possible but also what is socially desirable. Any organization in which everyone votes about every decision that has to be made would struggle to function at all. We have to find ways of working that make best use of the different skills and knowledge we all possess.

A constructive approach is to argue that if we are to build  a more democratic society it is first necessary to greatly increase the level of scientific literacy in the population, so that more people can make informed decisions about the big issues facing the future, such as how we fulfill our energy requirements for the next 30 years and how we cope with global warming. That will not be an easy thing to do given the dearth of scientists in Parliament and in the media, but that’s not an argument for not trying.

Symptomatic of the widespread rejection of science among the politically disaffected is the lamentable state of Green politics in the United Kingdom. In my opinion there is huge potential for a scientifically-informed political movement focussed on environmental issues. Unfortunately the current Green Party is anti-science to the core, which would doom it to perpetual marginalization even without the loss of credibility stemming from the childish antics of the only Green MP, Caroline Lucas. I know that many will argue with me about whether the Green Party should be included in “The Left”, but since both Labour and Conservative parties now belong to the Centre-Right it seems a sensible classification to me.

It hasn’t always been like this. As Alice Rose Bell pointed out in a Guardian piece some time ago, there have been examples of constructive engagement between science and left-wing politics. This seems to me to have largely evaporated. I don’t think that’s so much because scientists have rejected the left. It’s more that the left has rejected science.

Why’s the Sun not Green?

Posted in The Universe and Stuff with tags , , , , on March 11, 2011 by telescoper

It’s Friday afternoon and time for a mildly frivolous post.

I’ve been recently been teaching first-year astrophysics students (and others) about the radiation emitted by stars, and how stellar spectra can be used to diagnose their physical properties.

Received wisdom is that the continuous spectrum of light emitted by stars like the Sun is roughly of black-body form, with a peak wavelength inversely proportional to the surface temperature of the star. Here are some examples of black-body curves to illustrate the point.

The Sun has a surface temperature of about 6000 K – actually, more like 5800 K but we won’t quibble. The peak wavelength for the Sun’s spectrum  therefore corresponds to bluey-green light, which is why the Sun appears … er… yellow.

Anyone care to offer an explanation as to why the Sun isn’t green? Answers on a postcard or, preferably, through the comments box.

And while you’re at it, you might want to comment on why,  if the Sun produces so much green light, chlorophyll is actually green?