Green is an interesting color in astronomy. Our eyes are more sensitive to green than any other color, and so it is a color that is often seen in the night sky. There are green comets, the momentary green brilliance of a meteor, the faint green glow of northern aurora, and even a green glow to some distant galaxies. These objects can have other colors as well, but green is a common color of the night sky. 

While all these objects can share a common color, the mechanism behind their greenish glow can vary widely. Comets, for example take on a green hue because of the gas tail that forms as they approach the Sun. Most of the gas consists of hydrogen, but other compounds such as cyanide (CN₂) and carbon (C₂) can be contained in the tail as well. These molecules emit light at green wavelengths, and can be bright enough to be seen with the naked eye. In fact, cyanide (specifically cyanogen) was the first in Halley’s comet in 1910, and even led to a baseless panic that Earth might be poisoned by Halley’s tail.

The green glow of comet Lovejoy. Credit: Paul Stewart.

Meteors are green for a completely different mechanism. As a meteor enters Earth’s atmosphere, it is heated to the point where its outer layer is vaporized. The metals in the meteor glow with particular colors. Green comes from nickel. The most common metallic meteors are iron-nickel, so green is a common color. This glow tends to be brightest when meteors hit the atmosphere at high speed. For example, fast moving Leonid meteors can often have a green glow.

Galaxies that glow green are known as green pea galaxies. They are compact, and have a strong green emission line from oxygen, making them look like a green pea. The oxygen surrounding the galaxies glows green when it is ionized by the galaxy’s starlight. In order for the oxygen to be bright enough to give the galaxy a green color, there has to be a lot of ionized oxygen, and thus a lot of ultraviolet light produced by young stars. So green pea galaxies are young galaxies where lots of stars are forming, and may have played a role in the reionization of the early universe.

The solar radiation spectrum. Credit: Robert A. Rohde (CC BY-SA 3.0)

Since we’re most sensitive to green light, why are there no green stars? They can be red, yellow, blue, and white, but not green. It all has to do with the way stars produce light, which is very different from other astronomical objects. Comets, meteors, and the gas around galaxies all give off color when particular atoms or molecules give of light. The electrons in these atoms jump from one energy state to another, emitting a particular wavelength of light. But the Sun and other stars produce light from internal heat. Rather than emitting specific colors, stars emit a range of colors known as a thermal blackbody. The brightest color of a star depends upon its temperature. For cooler stars, the brightest color is red, and thus a star appears reddish. Hotter stars are brightest in the blue range, and so appear blue. But a star that peaks in the green is also bright in red and blue. Thus we see red, green, and blue light from the star, which our eyes interpret as white.  Since a thermal blackbody can’t peak at green without being bright in the entire visible spectrum, a green star simply isn’t possible.

But since green is so common in the sky, its absence from the stars is not a big loss.

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Comet Lovejoy is still visible in the night sky, so if you get a chance you should check it out.  If you have seen it, you might have noticed a distinctly green color to it. That isn’t an optical illusion, because Lovejoy is really green.

Structure of a comet. Credit: NMM London

Usually when you observe a comet directly, it will appear white, or slightly bluish. The white is due to sunlight reflecting off the dusty tail of the comet, and the blue is from the gas tail of the comet. Photographs can typically reveal these colors in rich detail, compared to the limited color perception of our eyes. But sometimes the gas emitted can have a greenish hue. Many comets have an abundance of compounds such as cyanide (CN₂) and carbon (C₂). When ionized these compounds emit light in the green spectrum. The light they emit is much brighter than things like hydrogen (even though hydrogen is more common), and that combined with our sensitivity to green light means that a comet can appear green.

Green comets are fairly common, but make for a rare color in the night sky since there are no green stars.

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In the late 1970s the Voyager missions made their flyby of Jupiter. It was the first time truly detailed images were gathered of the planet (even more detailed than the Pioneer images) and they created quite a stir. These detailed images appeared all over the media, but were perhaps most widely seen as full page color photographs in National Geographic. But rather than using true-color images, the photos in National Geographic had boosted colors and depth. It made for great imagery, but wasn’t a true representation of how Jupiter looks. When this color enhancement was pointed out, it was sometimes referred to as the National Geographic effect.

You can see this effect in the image above. On the left is a Voyager II image of Jupiter’s great red spot as it appeared in NatGeo and elsewhere. On the right is the same image in its more true-color form. You can see why the colors were boosted. The true-color image lacks much of the depth and richness we like to see in images.

There are some who would argue that enhanced color images misrepresent reality in a way that runs counter to scientific accuracy. We should be honest and strive for accurate images rather than color-hyped images that are more art than science. That view has gained significant traction since the 1970s, and today you can easily find true-color images of many celestial objects.

A more true-color image of the Moon. Credit: NASA/GSFC/Arizona State University/J. Major

But on the other hand, in some ways a color-hyped image is more accurate to what we perceive, even if it isn’t as accurate to reality. Take for example, the color of the Moon. If you asked people the color of the Moon, most would say it is white or pale gray. They would say this based upon their own observation of the Moon. It appears pale gray when you look at it in the night sky. In reality, the moon is a much darker shade, with the “white” regions more the color of gunpowder, and the darker regions the more color of asphalt. A similar effect occurs with Mars. We see it in the sky as pale red, but it is actually more the color of butterscotch or cocoa powder.

Images of Enceladus, the Earth, the Moon, and Comet 67P/C-G, with their relative albedos scaled approximately correctly. Credit: ESA’s Rosetta Blog

Part of the reason for this discrepancy is that we don’t perceive colors as absolute, but rather relative to surrounding colors. Against the background of black sky, colors appear brighter and more pale.  The actual brightness of an object is related to its albedo, or the fraction of light striking an object that reflects off its surface. We normally adjust for albedo in photographs, so we don’t notice the variation. For example, Saturn’s moon Enceladus is a brilliant white body that reflects nearly 100% of the light striking it. The comet 67P/Churyumov–Gerasimenko is also photographed as a bright white object, but has an albedo of only 5%. Compared to Enceladus, 67P is as dark as a lump of coal. But even a lump of coal would be brighter than the dark of space, so showing it as a brighter gray object is more realistic.

Although we can strive for accurate “true-color” representations of celestial objects, they will never be entirely like the way they would appear to us in real life. Even our spacecraft don’t take color images. Instead they take grayscale images at various wavelengths, which can be combined to create color images. Whether we generate more brilliant or more true images is a matter of preference.

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Recently I wrote about the average color of the universe, as determined by a survey of more than 230,000 galaxies. While knowing the overall color of these galaxies is a fun little factoid, it isn’t particularly useful from a scientific standpoint. However the color was determined by the average spectrum of the galaxies, which is quite scientifically useful. This “cosmic rainbow” tells us about the history of star formation in the universe.

You can see the average spectrum in the figure below. One of the things you’ll notice is that isn’t simply a continuous spectrum. There are wavelengths that are particularly bright or dark. These are emission (bright) or absorption (dark) lines that are particularly common for the galaxies. Several of the lines are labeled by the element that causes the line. By looking at the relative brightness of these lines, we can determine the relative abundances and temperatures of typical stars. This is because young stars have hotter atmospheres than older stars, so the emission and absorption lines of a star changes over time.

Credit: Karl Glazebrook & Ivan Baldry

Given this average, you can fit it to models of historical star formation. If most stars formed earlier in the universe, then the line spectra would resemble older stars, since most of the present stars would be older. If instead stars formed at a fairly constant rate, then you would see much less bias toward older stars.

What we find is that stars haven’t been produced at a continuous rate within the universe. Instead, there was a peak of star production between 6 and 10 billion years ago, and that the rate of production has been declining ever since. Most of the stars we observe are more than 5 billion years old. New stars are still being formed, but the level of star production is nothing like what it was. The universe has shifted into middle age.

Of course this just further supports that the universe began with the big bang. A peak of star production is exactly what you’d expect in a universe that begins with raw hydrogen and helium, with each generation of stars releasing some material back into the wild via supernovae and the like, but locking part of the material into red dwarfs, neutron stars and the like.

Hints of the big bang in a cosmic rainbow.

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At the turn of the 21st century, the Anglo-Australian Observatory made a large survey of galaxies in our universe, known as the 2-degree-field galaxy redshift survey (2dFGRS). It measured the spectra and redshifts of more than 230,000 galaxies. The main goal of the survey was to determine the distribution of galaxies within a radius of about 4 billion light years. A statistical analysis of this distribution could then be used to put constraints on things like dark matter and neutrino mass (which I’ll talk about another day).

But with the spectra of 200,000 galaxies you can also look at the average spectra of all of them, and in 2001 and 2002, Karl Glazebrook and Ivan Baldry did just that. The average spectrum is useful because it shows the wavelengths that are more common and less common, which in turn tells us the type of spectra stars typically have. This is important because the spectra of a star changes over time. So this gives us an idea of the evolution of stars on average.

Of course you can also take this one step further to determine the color of the universe. You just have to determine the color we would see if we looked at the average spectrum of all the galaxies at the same time. This is a bit more challenging than you might think.

For one, since different galaxies are redshifted by different amounts, you have to account for their redshift. This was done when determining the average spectra of the galaxies, and is pretty straightforward. More challenging is accounting for just how we would observe such a spectra, because sometimes the colors that reach our eyes are not the colors we see.

For example, consider the color yellow. If you look at a yellow square on your computer monitor, your eyes are not actually detecting yellow light. You are actually looking at red and green pixels. The red and green light stimulates the red and green cones in your retina similar to the way truly yellow light stimulates both of them, so the square appears yellow. In a sense, the yellow square on your monitor is a simulated yellow.

Another aspect to deal with is the fact that the sensitivity of our eyes depends on the color. Our eyes are most sensitive in the green region, and less sensitive in the red and blue. Then there is the fact that colors appear differently in different light. When we determine an average color, should it be for daylight adjusted light, dark adapted vision, or some other condition (known as the white point).

Glazebrook and Baldry decided that the best version would be a dark adapted (equal energy) white point, with a gamma correction of 2.2 to account for observed brightness. The result is a pale tan seen in the image below. It represents the color we would see if we could observe all the surveyed galaxies at once (and at rest relative to us). The color was given the name Cosmic Latte.

The color of the universe gradually changes over billions of years. In the past, when the universe was populated with younger stars, the color was lighter and more blue. In the future, as larger stars age and die the color will become darker and more red.

Perhaps in a few billion years we will have to change the name to Cosmic Mocha.

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One of the cool things we’ve been able to do is determine an exoplanet’s color. The planet is known as HD189733b, and as seen in an article published in Astrophysical Journal Letters, it has a blue color. This does not mean this planet is Earth-like in any way. Neptune is also blue, and it is hardly a paradise world. I’ve written about this exoplanet before, where it was found that the hot Jupiter-type planet has atmospheric winds of over 6,000 miles per hour. We now know that its atmosphere contains silicates which give it a blue color. It seems its atmosphere is filled with small droplets of silica that scatter blue light, giving it a blue color.

But just how did astronomers determine the color of a planet they can’t directly see? They adapted a method long used to determine the color of stars. Stars are categorized by what is known as a photometric system. Basically you observe the brightness of a star through different filters. These filters block out all light except for a narrow color range. By measuring the brightness at different colors (wavelengths) you can determine the overall color of a star. For example, if you measure the brightness of a star at a blue wavelength and a green wavelength (known as the visible, since green is in the middle of visible light), then you can compare them. If it is brighter in the blue than visible, then it will appear more blue. If more visible than blue, then it will appear white (not green, for reasons I’ve written about before).

Credit: NASA, ESA, and A. Feild (STScI)

In astronomy we take the difference between the blue and visible brightness measurements to give what is known as a BV color index, which is a common way to categorize stars. In this particular research, the team also made observations in the ultraviolet range, to create a UB color index as well. Combining these two (a photometric method known as UBV) they determined its color in the visible spectrum. The result can be seen in the image here, where the BV index is vertical axis, and the UB the horizontal. You can see the color of HD189733b is about the same blue as Earth, but lacking the greenish tinge of our home planet. It truly is a blue world.

Of course HD189733b is very close to its star, so how can we be sure that the color measured is really from the light of the planet? The team measured the polarization of the light to distinguish planetary light from starlight. If you looked at a vibrating string, for example, it could vibrate up and down, or side to side, or diagonally, etc. The orientation of the vibration is the polarization of the wave. Light waves have a similar orientation. For starlight, the orientation is all random, so any orientation averages out (we call such light unpolarized), but when light scatters off an object, the scattered light can be polarized. This happens when light is scattered in our atmosphere, something even the Vikings knew.

What the team did was measure the polarization of the light coming from the planet and star. Since the starlight is unpolarized, it wasn’t affected by the measurement, but the polarized light scattered by the planet’s atmosphere was affected. So the team could distinguish the planet’s light from the light of its sun. It’s a clever technique that we can use to determine the color of other planets as well.

For now, we know that at least one exoplanet is true blue.

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Suppose an advanced alien race discovered our little planet. From their great telescopes they could tell our atmosphere is rich in oxygen and water vapor, which would indicate this was a planet inhabited by living organisms. They therefore decide to send a probe to study our curious blue world. With their advanced technology, this alien species can send a probe across the vast distance of space using a device they call the Maguffin drive. The Maguffin drive can transport the probe to Earth almost instantly, but because of the tremendous energy it requires the probe can only stay on Earth for one second.

This might seem rather pointless, but these aliens also have great scanning devices that can take images of the Earth very quickly. So they send the probe, which collects large amounts of data during its one-second visit. Many of those pictures are images of us, going about our daily lives. In essence they have taken a snapshot of our planet.

Given such a snapshot, what could these aliens possibly learn about us? It turns out quite a bit. They would know, for example, that we are social creatures, and that we tend to live in large communities. They would see that we wear clothes. They would see how we walk and run. They would also learn how we are born, live and die, and about how long we live.

Obviously a second is far too short to observe a single person be born, grow to maturity and die of old age, but there are lots of us on the planet. In that one second they would see moments of birth, children suckling at their mother’s breast, children playing, teenagers arguing with their parents, adults working and people dying. All of these moments would allow them to piece together an understanding of our lives and cultures. There’s a lot that can be gained by a snapshot.

In astronomy we do much the same things with stars. Modern astronomy has been around for only a moment on cosmological scales. We can’t possibly observe the birth, life and death of a single star. We can, however observe lots of stars at different stages. We see stars being born, stars dying, forming planets, with solar systems, etc. From all of these observations we can piece together the life cycle of a typical star.

The first step in this process is found in a graph known as the Hertzsprung-Russell (or HR) diagram. First created around 1910, this graph plotted the color of a star versus its brightness. The color was determined by measuring the apparent brightness when observed through a blue filter, and then again through a green (visible) filter. The difference between these two brightnesses is known as the B-V color index. We now know that a star’s color is determined by its surface temperature, so the color measurement was a measure of a star’s temperature.

The brightness measure was determined by measuring the apparent brightness (apparent magnitude) and the star’s distance from us. The distance is important because a bright star very far away will appear dimmer than a dim star fairly close. From a star’s distance and apparent magnitude one can determine its absolute magnitude. This is the apparent magnitude a star would have if it were 10 parsecs away. Apparent magnitude is a measure of a star’s actual brightness (or luminosity).

An HR diagram of local stars. Credit: Richard Powell

You can see a modern HR diagram here. One of the things you will notice is that the stars aren’t randomly distributed, but rather are clumped within certain groups. Most of the stars lay along the central diagonal line. The reddest (or coolest) stars along the line are also the dimmest, and the bluest (or hottest) stars are the brightest. This is what you would expect with a star, that cool stars are dim and hot stars are bright. Because the majority of stars lie within this region they are called main sequence stars.

But you’ll also notice a large clump of stars above the main sequence ones. These are much brighter than other stars of the same color, and they get brighter the cooler they are. At the far right of the region you have very cool stars that are also very bright. The only way for a star to be both cool and bright is for it to be very large. In other words even though it is cool compared to other stars it has much more surface area giving off light. They are therefore known as giant (or supergiant) stars.

Below the main sequence you see a scattering of dim but hot stars. To be hot but dim these stars must be quite small. Very hot, but little surface area. For this reason they are known as white dwarfs.

From such an HR diagram we can put together the life cycle of a typical star. It forms within a stellar nursery to become a main sequence star. There it spends most of its life, until it runs out of hydrogen to fuse in its core. As it begins to fuse helium it swells into a giant star, where it lives for a short time before collapsing down to a white dwarf.

We now have a very good understanding of stellar life cycles and the nuclear reactions that occur within a star. In 1910 when this diagram was first introduced we didn’t even have a complete understanding of atoms, much less nuclear physics. And yet from the beginning the diagram indicated that stars were dynamic, and that they could change over time.

All from a snapshot of the stars in our cosmic neighborhood.

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Despite all the wonderful color images we have from the Hubble space telescope, there is no color camera on the Hubble. The main reason for this is scientific. When observing astronomical objects, you’d like to get as much light as you can from the object. You also want to get as wide a range of wavelengths as you can.

The light detectors used in space telescopes are typically charged coupled devices or CCDs. When light hits a pixel in a CCD it induces a charge. The more photons that strike a pixel, the more charge you get. In this way you can measure the brightness by the amount of charge you get. A similar digital camera is known as CMOS sensor. These are typically used in cell phones and the like since they are inexpensive, but they tend to have less sensitivity.

To make a color digital camera, you have to put a layer of filters over the sensor so that a third of the pixels will only see red light, a third only green and a third only blue. This is similar to the way the cones in our eyes work. They are sensitive at red, green or blue, and our brain puts this information together to produce our color vision. In your cell phone, the red, green and blue pixels are put together to make a color image.

This is fine if all you want to do is put photos of your cat on Instagram, but all those filters block a portion of the light, which produces a darker image. Since the filter is built into the camera, this also means you can only take a color picture. This means you have no flexibility to image things at any other wavelengths.

So for most telescopes the CCD just measures brightness within the range of their sensitivity. For the Hubble’s Wide Field and Planetary Camera (WFPC) this ranges from infrared through the visible to ultraviolet. The Hubble then has filters that can be moved in front of the camera. So if you just want to look at infrared, there is a filter that lets you do that.

So if telescope cameras typically only see in shades of gray, how do we get all these wonderful color images? They are composite images made from grayscale images taken under a red, green and blue filter. These three images are then given the appropriate color, and layered to produce a color image. You can see this in the image above, where I’ve colorized the original filtered grayscales from the Hubble to produce a color image of the planetary nebula M57.

My result is pretty basic. To produce the truly magnificent color images you see takes a tremendous amount of skill and talent. Of course since these images are composites you can also create false-color images of objects taken in infrared, ultraviolet, radio and x-rays, none of which have color to our eyes.

Color images are not only beautiful, they serve to inspire us to understand and appreciate the universe around us. But astronomers find shades of gray much more useful.

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We generally think of asteroids as looking like gray rocks. While that’s true to our limited eyes, more sensitive instruments find they have a variety of colors. You can see an example of this in the image above of the asteroid Vesta. This false color image was made by observing Vesta at various wavelengths in the visible and infrared spectrum. It shows that Vesta has variations in color too subtle for us to see with our eyes.

The color variation is caused by the varying composition of Vesta. The green regions, for example, indicate the presence of iron. While Vesta is the only asteroid with such detailed color observations, other asteroids have been observed at different wavelengths to determine their overall color. It turns out the color of an asteroid correlates well with the family it is a part of.

Asteroids aren’t just scattered randomly through the asteroid belt. Because of the gravitational interactions of Jupiter and the other planets, asteroids tend to be clumped into groups or families. In 2002, the Sloan Digital Sky Survey (SDSS) observed more than 10,000 asteroids at different wavelengths to determine their overall color. What was found was that there was less color variation within a particular family than their was between families.

Asteroid families are defined by the similarity of their orbits. What this study showed is that asteroid families also share similar coloring. Since the coloring of an asteroid is determined by its composition, this means asteroid families have similar compositions. Asteroid families are chemically similar.

This has important consequences for the history of our solar system. It means that asteroid families formed within their own family. They didn’t form first and then get pushed into groups by the gravitational tugs of the planets. This means the orbital dynamics of the solar system likely stabilized before the asteroids formed.

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Our eyes see color through cones in our retina, where the three different types (S, M, and L) each have a slightly different range of wavelengths to which they are light sensitive. Through the response of these different cones our brains are able to distinguish different wavelengths of light, which we interpret as color. Telescope detectors typically have a much wider range of light sensitivity, which is good if you want to detect a great deal of light, but not so good if you want to observe a particular color range. So many telescopes have filters that block light outside a particular range.  

Credit: NASA, ESA

As a case in point, consider the Hubble space telescope. Hubble has a camera that mainly looks at visible light (the Advanced Camera for Surveys, or ACS) and another that mainly gathers infrared light (the Wide Field Camera 3, or WFC3).  Since the sensitivity range of each of these cameras is so wide, each has several filters that block all but a narrower region, known as a band.  The colors in the figure don’t correspond to the colors we see.  Our visible spectrum is roughly the V band, while B is violet to ultraviolet, i is red to near infrared, and all the others are infrared.  You can see how the different bands overlap, and how the amount of light each filter lets through can vary. In order to look at the broad spectrum of light from an object (which is not the same as the spectral lines), Hubble images an object through different filters.  These images can be combined to create a “color” image.  Though often it is a false-color image with different visible colors assigned to each band.

The figure shows another interesting aspect of the spectra, which deals with redshift. The wavelengths of light we observe from an object are not always the wavelengths that object emits.  The light from an object can be redshifted or blueshifted due to the motion of the object relative to us, and things like cosmic expansion.  For distant objects, the redshift due to cosmic expansion can be pretty severe, pushing visible and even ultraviolet light well into the infrared.  The white line in the figure shows the light typically emitted by a distant galaxy at different redshifts (known as z).  You can see that by the time you get to about z = 12, most of a galaxy’s light is so far to the infrared that Hubble can only detect a small portion of it.  This means for the Hubble telescope, z = 12 is about the largest redshift it can detect.

Still, there’s quite a bit that Hubble can see simply by reading the rainbow.

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In 1671, Isaac Newton submitted a letter to the the Royal Society outlining a new theory of light and color. While Newton is probably most famous for his theory of gravity—and the mythical apple—he was also deeply interested in the nature of light, and made one of the first detailed studies of the properties of light. The work he describes in this 1671 paper is so brilliantly simple you can do it at home. All you need is some sunlight and a couple of prisms.

In Newton’s time it was already well known that light passing through a prism would produce a spectrum of colors. It was generally thought that the color must somehow be contained within the prism glass, and when light passed through a prism it would be tinted various colors. Newton was able to clearly show this was not the case. To show this, Newton sealed off a room from light except for a small hole in a window shutter, so that a single beam of light could enter. He then placed the prism over the hole, so that the light would pass through the prism and project onto a far wall. He saw the expected spectrum of colors, but he noticed something unusual. The original hole was circular, but the spectrum on the wall was oblong. This was unexpected. If the colors appeared because the light was tinted by the prism, then the light beam shouldn’t change shape from circular to oblong. What’s more, Newton noticed that the spectrum had sharp edges along its long sides, but fuzzy edges at its short sides.

This gave him the idea that white light might contain within it all the colors of the spectrum, and that each color was bent by the prism slightly differently, thus producing the smeared spectrum. To test his idea, Newton measured the angles by which each color was bent (refracted), and he found that red was consistently bent by a certain angle, while violet was consistently bent by a certain larger angle. This was true for all the colors in between as well. The prism bent each color of light by a particular angle, with red bent least and violet bent most. What Newton seemed to show was that light was not naturally white, being tinted when striking the prism. Rather, light was made of colors revealed by the prism and its variable refraction.

But how could Newton prove that this was the case? After all, it was possible that the prism did color light passing through it as well as bending colors differently. How could he show that color was a property of light, and not a prism. To prove his theory, Newton made a board with small hole and positioned it so that only the red part of the light passed through the hole. He then placed a second prism over the hole so that the red light passed through the prism and struck the far wall. If color was caused by the prism, then one would expect to see a full spectrum once again. What Newton saw was a single circle of red light. When he measured the refraction of the light through the second prism, it was the same as through the first prism. Newton did this with several colors, and always found the same result. The prism never colored the light, it simply refracted it according to its color. Light was therefore not a simple featureless thing. It possessed some intrinsic structure we perceive as color.

Through his work, Newton took the first step toward our understanding of the complexity and subtlety of light.

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Yesterday I talked about why you never see green stars.  Despite that sad fact, the color of a star is an important aspect of categorizing stars into types.

Early categorization of stars was by apparent visual magnitude.  The brightest stars in the sky were magnitude 1, the next brightest magnitude 2 and so on.  Typically this divided stars into six groups, so that a magnitude 6 star was just bright enough to be seen with the naked eye on a dark clear night.  The color of some bright stars could be seen with the naked eye, for example in the constellation Orion Betelgeuse is reddish, and Rigel is bluish.  But it’s hard to make out the colors of most stars, as they generally just look white to our limited eyes, so stars weren’t initially categorized by color.

By the late 1800s, astronomers were able to look at the spectrum of light from the stars.  This meant they could look at the absorption lines of of atoms in the photosphere of a star.  Since each type of atom or molecule has a particular pattern (kind of an atomic fingerprint), we could identify what types of atoms were in a particular star.  So astronomers started categorizing stars into spectral types A, B, C, etc.

By the 1900s, physicists gave us an understanding of the relationship between the color of light a star gives off and its temperature.  Because stars are relatively hot, they are essentially blackbody radiators.  Just as iron glows red as it heats up, then gets more yellow as it gets hotter, the coolest stars (with surface temperatures of about 2 000 K) glow dim red, while higher temperature stars glow orange, yellow, white and blue.

Using photographic methods, astronomers were able to measure the brightest color, and therefore the temperature of stars.  They found that their original spectral classification depended largely on the temperature of a star, so they changed to a categorization based on surface temperature (or color), resulting in the categories we have now of M, K, G, F, A, B, and O.  The coolest stars being M, and the hottest stars being O.  As measurements became more precise, each temperature category was divided into tenths and assigned by a number.  So an F3 star has a temperature 3/10 of the way between an F0 and A0 star.

While this temperature based color scheme was useful it didn’t take into account how stars can be radically different types of stars but still have the same surface temperature.  For example, Barnard’s Star is an M3 star with a mass of only 15% of the Sun, and a radius about 1/5 of our Sun’s.  Betelgeuse on the other hand is an M2 star with ten times the mass of our Sun and about 1 000 times bigger radius.  Barnard’s Star is a red dwarf, while Betelgeuse is a Red supergiant.  So stars were then further characterized by their luminosity (actual brightness).  Since Betelgeuse is a very bright star for its color, it is category I, while Barnard’s star is fairly dim, so it is category V.  Most stars are category V, since this is the category of stars in their stable period, known as the main sequence.  You can see a comparison of main sequence stars in the figure above.

Our Sun is a G2V star, meaning it is a main-sequence star on the cooler end of the G class.  If you look at the image below, you might think that our Sun is a rather small star.  While there are certainly much larger stars than the Sun, and even much larger than O type main sequence stars, that’s a little bit misleading.  More than 3/4 of main sequence stars are M class stars.  Only about 8% of main sequence stars are G class, so our Sun is larger than about 90% of all main sequence stars in our galaxy.    Expressed in that way, our star is relatively large.

The fact that M class stars are most common is part of the reason the majority of the extrasolar planets we find are around these red dwarf stars.  But, that’s a story for another time.

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