Cosmic Vision: Unravelling Rainbows

Professor Katherine Blundell OBE

When light is dispersed into its constituent colours, it can become possible to discern rich dynamical information about an evolving system in space, for example cosmic explosions, collisions or accelerations.

This lecture explored how such dispersion can be designed to reveal the dynamics of distant worlds

Professor Katherine Blundell OBE


Katherine was appointed Gresham Professor of Astronomy in 2019. She is a Professor of Astrophysics at Oxford University and a Research Fellow at St John’s College. Before this she was one of the Royal Society’s University Research Fellows, a Research Fellow of the 1851 Royal Commission, and a Junior Research Fellow at Balliol College, Oxford.

Her research interests include the evolution of active galaxies and their life cycles, the accretion of material near black holes and the launch and propagation of relativistic jets (jets of plasma emitted by some black holes). In her research she uses electromagnetic imaging and spectroscopy, as well as computational techniques.

She is also a renowned science communicator and set up a worldwide network of five schools-based Global Jet Watch observatories, which collect data on evolving black hole systems and nova explosions in our Galaxy, helping to inspire the next generation of scientists in South Africa, Chile, Australia and India.

Her awards include a Philip Leverhulme Prize in Astrophysics, the Royal Society’s Rosalind Franklin Medal in 2010, the Institute of Physics Bragg Medal in 2012, the Royal Astronomical Society’s Darwin Lectureship in 2015 and an OBE for services to astronomy and the education of young people in 2017.

Professor Blundell’s first lecture series for Gresham College is called Cosmic Concepts, starting 2 October 2019, and she will be looking at how concepts developed in physics and cosmology have led to some of our most surprising discoveries about the Universe.

The following are notes from the on-line lecture. Even though I could stop the video and go back over things there are likely to be mistakes because I haven’t heard things correctly or not understood them. I hope the Professor Blundell and my readers will forgive any mistakes and let me know what I got wrong.

The lecture

Rainbows can only be seen when there is a certain distribution of raindrops that can disperse white light from the sunlight. Too much cloud associated with the raindrops means the rainbow isn’t visible.

A rainbow is a meteorological phenomenon that is caused by reflection, refraction and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky. It takes the form of a multicoloured circular arc. Rainbows caused by sunlight always appear in the section of sky directly opposite the sun.

In a primary rainbow, the arc shows red on the outer part and violet on the inner side. This rainbow is caused by light being refracted when entering a droplet of water, then reflected inside on the back of the droplet and refracted again when leaving it.

In physics, refraction is the change in direction of a wave passing from one medium to another or from a gradual change in the medium. Refraction of light is the most commonly observed phenomenon, but other waves such as sound waves and water waves also experience refraction. How much a wave is refracted is determined by the change in wave speed and the initial direction of wave propagation relative to the direction of change in speed.

A correct explanation of refraction involves two separate parts, both a result of the wave nature of light:

1) Light slows as it travels through a medium other than vacuum (such as air, glass or water). This is not because of scattering or absorption. Rather it is because, as an electromagnetic oscillation, light itself causes other electrically charged particles such as electrons, to oscillate. The oscillating electrons emit their own electromagnetic waves which interact with the original light. The resulting “combined” wave has wave packets that pass an observer at a slower rate. The light has effectively been slowed. When light returns to a vacuum and there are no electrons nearby, this slowing effect ends and its speed returns to c.

2) When light enters, exits or changes the medium it travels in, at an angle, one side or the other of the wavefront is slowed before the other. This asymmetrical slowing of the light causes it to change the angle of its travel. Once light is within the new medium with constant properties, it travels in a straight line again.


Above left: A ray of light being refracted in a plastic block. Above right: When a wave moves into a slower medium the wavefronts get compressed. For the wavefronts to stay connected at the boundary the wave must change direction.

Reflection is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves. The law of reflection says that for specular reflection the angle at which the wave is incident on the surface equals the angle at which it is reflected. Mirrors exhibit specular reflection.

All objects reflect light to some degree. If an image isn’t formed then the reflection is classed as diffuse. Light bounces off in all directions due to multiple reflections by the microscopic irregularities inside the material. The light sent to our eyes by most of the objects we see is due to diffuse reflection from their surface, so that this is our primary mechanism of physical observation.


Above left: specular reflection. Above right: diffuse reflection


Light rays enter a raindrop from one direction (typically a straight line from the sun), reflect off the back of the raindrop, and fan out as they leave the raindrop. The light leaving the rainbow is spread over a wide angle, with a maximum intensity at the angles 40.89–42°. (Note: Between 2 and 100% of the light is reflected at each of the three surfaces encountered, depending on the angle of incidence.

White light separates into different colours on entering the raindrop due to dispersion, causing red light to be refracted less than blue light.

Rainbows span a continuous spectrum of colours. Any distinct bands perceived are an artefact of human colour vision, and no banding of any type is seen in a black-and-white photo of a rainbow, only a smooth gradation of intensity to a maximum, then fading towards the other side. For colours seen by the human eye, the most commonly cited and remembered sequence is Isaac Newton’s sevenfold red, orange, yellow, green, blue, indigo and violet, remembered by the mnemonic Richard Of York Gave Battle In Vain (ROYGBIV). Although one wag thought it would be a good idea to change the V to L (replace violet with Lilac) because Richard Of York Got Buried In Leicester.

Rainbows can be caused by many forms of airborne water. These include not only rain, but also mist, spray, and airborne dew.


The above image shows a primary rainbow with a secondary rainbow. The secondary rainbow has its colours in the opposite order to the primary rainbow. This is caused by the light being reflected twice on the inside of the droplet before leaving it.

The image also shows that inside the primary rainbow it is lighter and whiter than outside the rainbow. This phenomenon arises because the raindrops that disperse the sunlight refract the sunlight into cones, whose size depends on the wavelength of the light, with red light being scattered over a larger angle than blue light. Where the cones add together you get white light, which brightens the sky, but because they are different sizes, not all the wavelengths meet at the edges and the wavelength dependence of the scattering gives rise to the rainbow. Colours are separated because each wavelength associated with a particular colour is dispersed differently. Red is refracted the least and violet the most.


Sir Isaac Newton PRS (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, theologian, and author (described in his own day as a “natural philosopher”) who is widely recognised as one of the most influential scientists of all time and as a key figure in the scientific revolution.

In 1666, Newton observed that the spectrum of colours exiting a prism in the position of minimum deviation is oblong, even when the light ray entering the prism is circular, which is to say, the prism refracts different colours by different angles. This led him to conclude that colour is a property intrinsic to light – a point which had, until then, been a matter of debate.

From 1670 to 1672, Newton lectured on optics. During this period, he investigated the refraction of light, demonstrating that the multicoloured spectrum produced by a prism could be recomposed into white light by a lens and a second prism.

He showed that coloured light does not change its properties by separating out a coloured beam and shining it on various objects, and that regardless of whether reflected, scattered, or transmitted, the light remains the same colour. Thus, he observed that colour is the result of objects interacting with already-coloured light rather than objects generating the colour themselves. This is known as Newton’s theory of colour.

Opticks: or, A Treatise of the Reflexions, Refractions, Inflexions and Colours of Light is a book by English natural philosopher Isaac Newton that was published in English in 1704.


Newton showed that light is composed of different spectral hues (he describes seven — red, orange, yellow, green, blue, indigo and violet), and all colours, including white, are formed by various mixtures of these hues. He demonstrates that colour arises from a physical property of light — each hue is refracted at a characteristic angle by a prism or lens — but he clearly states that colour is a sensation within the mind and not an inherent property of material objects or of light itself. For example, he demonstrates that a red violet (magenta) colour can be mixed by overlapping the red and violet ends of two spectra, although this colour does not appear in the spectrum and therefore is not a “colour of light”. By connecting the red and violet ends of the spectrum, he organised all colours as a colour circle that both quantitatively predicts colour mixtures and qualitatively describes the perceived similarity among hues.

Newton’s contribution to prismatic dispersion was the first to outline multiple-prism arrays. Multiple-prism configurations, as beam expanders, became central to the design of the tuneable laser more than 275 years later and set the stage for the development of the multiple-prism dispersion theory.


The above images, taken by Professor Blundell on a flight to South Africa, aren’t technically showing a rainbow. The phenomenon is called a glory and arises due to diffraction and back scattering effects when a plane is flying over cloud and mist and when the Sun is low down in the sky (the image was taken just after sunrise from inside the plane). They show that the yellow/white light from the Sun is made up of a range of colours.

In physics, backscatter (or backscattering) is the reflection of waves, particles, or signals back to the direction from which they came. It is usually a diffuse reflection due to scattering, as opposed to specular reflection as from a mirror, although specular backscattering can occur at normal incidence with a surface.

A glory is an optical phenomenon, resembling an iconic saint’s halo around the shadow of the observer’s head, caused by sunlight or (more rarely) moonlight interacting with the tiny water droplets that comprise mist or clouds. The glory consists of one or more concentric, successively dimmer rings, each of which is red on the outside and bluish towards the centre. Due to its appearance, the phenomenon is sometimes mistaken for a circular rainbow, but the latter has a much larger diameter and is caused by different physical processes.

Glories arise due to wave interference of light internally refracted within small droplets.

Exploring the light that comes from distant astronomical objects is the theme of the lecture.

The different colours carry information. In everyday life their use can be symbolic. Popes wear white, cardinals wear red and emperors wear purple.


The inauguration of President Joseph R. Biden and Vice President Kamala Harris’s was awash in the colour purple. In this case it was simple as 1, 2, 3. Paint by numbers: red + blue = purple. It was a call for unity to heal the great schism between the Republican and Democratic parties’ reds and blues.

Colour is important in astronomy as you can’t prod or poke the objects.

Colour perception and colour representation are hard and it is mainly in the mind.

Digital images are represented by pixels. These are made up of small squares, each of a single colour. Collectively they give us a sense of colour distribution of whatever is being viewed. The human brain is very good at interpreting a mix of data, colours or a mix of pixels.

In digital imaging, a pixel, pel, or picture element is a physical point in a raster image, or the smallest addressable element in an all-points addressable display device; so, it is the smallest controllable element of a picture represented on the screen.

Each pixel is a sample of an original image; more samples typically provide more accurate representations of the original. The intensity of each pixel is variable. In colour imaging systems, a colour is typically represented by three or four component intensities such as red, green, and blue, or cyan, magenta, yellow, and black.

In some contexts (such as descriptions of camera sensors), pixel refers to a single scalar element of a multi-component representation (called a photosite in the camera sensor context, although sensel is sometimes used), while in yet other contexts it may refer to the set of component intensities for a spatial position.


The above is a series of a pictures of a lovely doggy. Zooming in you can just see the more obvious pixels in the doggy’s eye.

Sodium street lamps have a distinct orange/yellow light, which arises from two specific wavelengths. These wavelengths arise because sodium atoms in the sodium vapour have particular electron transitions between energy levels.



Initially the electrons have to be excited to a higher energy level and the light is emitted when the electrons fall back down.

The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to an atom or molecule making a transition from a high energy state to a lower energy state. The photon energy of the emitted photon is equal to the energy difference between the two states. There are many possible electron transitions for each atom, and each transition has a specific energy difference. This collection of different transitions, leading to different radiated wavelengths, make up an emission spectrum. Each element’s emission spectrum is unique. Therefore, spectroscopy can be used to identify elements in matter of unknown composition. Similarly, the emission spectra of molecules can be used in chemical analysis of substances.


The energy of the emitted light = hν or hf where h is Planck’s constant and ν or f is the frequency of the wave. The equation can also be written as E = hc/λ where c is the speed of light in a vacuum and λ is the wavelength of the wave.

So, seeing the particular orange/yellow colour infers the presence of sodium vapour in the street light.

Photographing the street light and zooming in using a computer screen (see below) shows the pixels of the image are themselves composed of the pixels of the monitor. These are represented by varying amounts of red, blue and green light.


Fortunately, even though or eyes only have three colour receptors, we are very good at interpolating and interpreting the relative amounts of these three colours to give a richer range of colours than the 3 colour receptors might suggest.


Schematic diagram of the human eye with some key structures labelled

The retina includes several layers of neural cells, beginning with the photoreceptors, the rods and cones.


The more important distinction between rods and cones is in visual function. Rods serve vision at low luminance levels, but it is the cones that give us colour vision. There are three types of cones properly referred to as L, M, and S. These names refer to the long-wavelength, middle-wavelength, and short wavelength sensitive cones, respectively. Sometimes the cones are denoted with other symbols such as RGB suggestive of red, green, and blue sensitivities. This concept is wrong and the LMS names are more appropriately descriptive. Note that the spectral responsivities of the three cone types are broadly overlapping.


Above left: Spectral responsivities of the L, M, and S cones. Above right: A representation of the retinal photoreceptor mosaic artificially coloured to represent the relative proportions of L (coloured red), M (green), and S (blue) cones in the human retina.

What is incredible is that we humans can distinguish about 10 million shades of colour from just three and that no human will distinguish a colour in the same way as another.

For astronomy it isn’t really possible to take a coloured image directly. In astrophotography red, blue and green filters have to be used.

When light from an astronomical object is collected by a telescope it is passed through a red filter and photographed. A red filter will block all wavelengths except red. The process is repeated through a green filter (only lets green light through) and then through a blue filter (only lets blue light through).



Ironically none of the images look coloured but they do show that there are different distributions of the coloured light.

Each image is artificially coloured. So, the red filter image is artificially coloured red, the green filter image artificially coloured green and the blue artificially coloured blue.



The three images are then combined to give the image(s) below. An amazing multicolour image of the Trifid Nebula. Different coloured gases.

The Trifid Nebula (catalogued as Messier 20 or M20 and as NGC 6514) is an H II region in the north-west of Sagittarius in a star-forming region in a nearby spiral arm’s Scutum-cantered part. It was discovered by Charles Messier on June 5, 1764. Its name means ‘three-lobe’. The object is an unusual combination of an open cluster of stars, an emission nebula (a relatively dense, red-yellow portion), a reflection nebula (the mainly NNE blue portion), and a dark nebula (the apparent ‘gaps’ in the former that cause the trifurcated appearance also designated Barnard 85). Viewed through a small telescope, the Trifid Nebula is a bright and peculiar object, and is thus a perennial favourite of amateur astronomers.

The most massive star that has formed in this region is HD 164492A, an O7.5III star with a mass more than 20 times the mass of the Sun. This star is surrounded by a cluster of approximately 3100 young stars.

image (below left)

Charles Messier (26 June 1730 – 12 April 1817) was a French astronomer.

Messier and Lee Steven Lee (above right)

As mentioned earlier white light is a combination of different colours/wavelengths. The eye and brain can be “fooled” into perceiving a full spectrum of all wavelengths of visible light just by the eye being stimulated by three wavelengths. The different colours are produced by different ratios of the red, blue and green being perceived by the colour receptors (cones) in the retina. Our brain sensitively interprets the electrical signals received from these cells to fill in the whole range of colours.

The Trifid Nebula shows different coloured gases. The red gas is predominantly hydrogen whereas the blue gas is a combination of light from hot blue stars together with light from young blue stars reflected off the ambient gas further out.

At night, when we switch off the lights our eyes perceive it has gone dark. This is because our eyes are only sensitive to visible light.

However, all objects “glow” even when they are not irradiated. The wavelength of this “glow” or radiation depends on the temperature of the object.

Certain jobs rely on this, e.g., blacksmiths. When they are making things like horseshoes, they know that a particular red glow corresponds with a certain temperature


This is not the pattern in a rainbow (or is it?).

So, colour (the wavelength/frequency) indicates the temperature of an object. This is most simply understood in terms of a conceptual device in thermodynamics called a black body.

A black body or blackbody is an idealised physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The name “black body” is given because it absorbs radiation in all frequencies, not because it only absorbs: a black body can emit black-body radiation. On the contrary, a white body is one with a “rough surface that reflects all incident rays completely and uniformly in all directions.”

A black body in thermal equilibrium (that is, at a constant temperature) emits electromagnetic black-body radiation. The radiation is emitted according to Planck’s law, meaning that it has a spectrum that is determined by the temperature alone (see figure at right), not by the body’s shape or composition.

An ideal black body in thermal equilibrium has two notable properties:

It is an ideal emitter: at every frequency, it emits as much or more thermal radiative energy as any other body at the same temperature.

It is a diffuse emitter: measured per unit area perpendicular to the direction, the energy is radiated isotropically, independent of direction.

In astronomy, the radiation from stars and planets is sometimes characterized in terms of an effective temperature, the temperature of a black body that would emit the same total flux of electromagnetic energy.

Wien’s Displacement Law


When the temperature of a blackbody radiator increases, the overall radiated energy increases and the peak of the radiation curve moves to shorter wavelengths. When the maximum is evaluated from the Planck radiation formula, the product of the peak wavelength and the temperature is found to be a constant.

This relationship is called Wien’s displacement law and is useful for determining the temperatures of hot radiant objects such as stars, and indeed for a determination of the temperature of any radiant object whose temperature is far above that of its surroundings.

It should be noted that the peak of the radiation curve in the Wien relationship is the peak only because the intensity is plotted as a function of wavelength. If frequency or some other variable is used on the horizontal axis, the peak will be at a different wavelength.


Max Karl Ernst Ludwig Planck, ForMemRS (23 April 1858 – 4 October 1947) was a German theoretical physicist whose discovery of energy quanta won him the Nobel Prize in Physics in 1918.


A blackbody is something that doesn’t absorb or reflect a wavelength preferentially over others. If a blackbody is at a particular temperature then you can predict exactly what shape the spectra will take.


As the temperature of the blackbody increases, the peak wavelength decreases (Wien’s Law). The intensity (or flux) at all wavelengths increases as the temperature of the blackbody increases. The total energy being radiated (the area under the curve) increases rapidly as the temperature increases (Stefan–Boltzmann Law).

The total energy (E) emitted by a black body per unit area per second is proportional to the fourth power of the absolute temperature (T) of the body. This is known as the Stefan-Boltzmann Law.


where σ is a constant known as the Stefan- Boltzmann constant and has a value of 5.7 x 10-8 Wm2K-4.

The axes of the graphs are basically intensity (how bright the object radiates) (y axis) of the light and frequency or wavelength (x axis). The visible spectrum is the approximate range of frequencies/wavelengths that human eyes are sensitive to.

At 20oC there is no visible radiation. It is in the infra-red radiation region of the electromagnetic spectrum.

The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies.

The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from below one hertz to above 1025 hertz, corresponding to wavelengths from thousands of kilometres down to a fraction of the size of an atomic nucleus. This frequency range is divided into separate bands, and the electromagnetic waves within each frequency band are called by different names; beginning at the low frequency (long wavelength) end of the spectrum these are: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays at the high-frequency (short wavelength) end. The electromagnetic waves in each of these bands have different characteristics, such as how they are produced, how they interact with matter, and their practical applications. The limit for long wavelengths is the size of the universe itself, while it is thought that the short wavelength limit is in the vicinity of the Planck length. Gamma rays, X-rays, and high ultraviolet are classified as ionizing radiation as their photons have enough energy to ionize atoms, causing chemical reactions.


At 600 to 800oC the radiation just creeps into the visible range, as red or orange light. At 1300oC the light radiation spans the whole visible spectrum. Our eyes and brain will sum all that radiation into white light.

Objects which are glowing with heat are also glowing with light, but it is only at high temperatures that we have a sense of light being radiated in accordance with the temperature of the object.

Of course, if the temperature gets high enough the radiation ceases to be visible again.

It should be noted that we see everyday objects, not because of their temperature, but because they absorb some wavelengths of light and reflect the others into our eyes. For example, we see grass as green because it absorbs all wavelengths of light except green. The green light is reflected off the grass into our eyes.

As briefly mentioned earlier, temperature and wavelength are also associated with stars.

The image below left is an out of focus image of a cluster of stars. The image below right is an in-focus image of the same cluster of stars.


The out of focus image makes it easier to see that stars can come in different colours.

The yellow stars are cooler and the blue stars are hotter. The hotter stars are radiating more energy than the cooler stars. The stars’ different temperatures are the reason why they have different colours.

What colour is sunlight?

Below is the Sun’s spectrum. It is produced using a spectrograph.


An optical spectrometer (spectrophotometer, spectrograph or spectroscope) is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light’s intensity. The independent variable is usually the wavelength of the light or a unit directly proportional to the photon energy, such as reciprocal centimetres or electron volts, which has a reciprocal relationship to wavelength.

A spectrometer is used in spectroscopy for producing spectral lines and measuring their wavelengths and intensities. Spectrometers may also operate over a wide range of non-optical wavelengths, from gamma rays and X-rays into the far infrared. If the instrument is designed to measure the spectrum in absolute units rather than relative units, then it is typically called a spectrophotometer. The majority of spectrophotometers are used in spectral regions near the visible spectrum.

Spectrometers are used in many fields. For example, they are used in astronomy to analyse the radiation from astronomical objects and deduce chemical composition. The spectrometer uses a prism or a grating to spread the light from a distant object into a spectrum. This allows astronomers to detect many of the chemical elements by their characteristic spectral fingerprints. If the object is glowing by itself, it will show spectral lines caused by the glowing gas itself. These lines are named for the elements which cause them, such as the hydrogen alpha, beta, and gamma lines. Chemical compounds may also be identified by absorption. Typically, these are dark bands in specific locations in the spectrum caused by energy being absorbed as light from other objects passes through a gas cloud. Much of our knowledge of the chemical makeup of the universe comes from spectra.

Grating spectrometer is shown below.


A spectrograph is an instrument that separates light by its wavelengths and records this data. A spectrograph typically has a multi-channel detector system or camera that detects and records the spectrum of light.

In optics, a diffraction grating is an optical component with a periodic structure that splits and diffracts light into several beams travelling in different directions.

Diffraction refers to various phenomena that occur when a wave encounters an obstacle or opening. It is defined as the bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a secondary source of the propagating wave.

The Sun radiates light at all different wavelengths and our eyes “indirectly” see all these wavelengths (although you should not directly look at the Sun) as white light because they are mixed up.

The Sun doesn’t just radiate visible light. It radiates all the branches of the electromagnetic spectrum.

The sum of the wavelengths in the optical band leads us to conclude that sunlight is yellowy-white.

The Sun’s spectrum shows dark, sharp vertical lines. Von Fraunhofer was the first person to spot this. He made the first spectrograph using a few simple prisms. He investigated these dark lines


Joseph Ritter von Fraunhofer (6 March 1787 – 7 June 1826) was a Bavarian physicist and optical lens manufacturer. He made optical glass and achromatic telescope objective lenses, invented the spectroscope, and developed diffraction grating. In 1814, he discovered and studied the dark absorption lines in the spectrum of the sun now known as Fraunhofer lines.

In physics and optics, the Fraunhofer lines are a set of spectral absorption lines named after the German physicist Joseph von Fraunhofer (1787–1826). The lines were originally observed as dark features (absorption lines) in the optical spectrum of the Sun.

Absorption lines are dark lines, narrow regions of decreased intensity, that are the result of photons being absorbed as light passes from the source to the detector. In the Sun, Fraunhofer lines are a result of gas in the photosphere, the outer region of the sun. The photosphere gas has lower temperatures than gas in the inner regions, and absorbs a little of the light emitted from those regions.


The structure of the Sun, a G-type star: 1) Core; 2) Radiation zone; 3) Convection zone; 4) Photosphere; 5) Chromosphere; 6) Corona; 7) Sunspot; 8) Granules; 9) Prominence

The photosphere is a star’s outer shell from which light is radiated.


The image above is a zoomed in region of the Sun’s spectrum showing Fraunhofer “D” lines. These two lines occur at exactly the wavelength of sodium atoms of the vapour in street lights.

This is an exact fingerprint of sodium atoms and are the result of two electron transitions present in sodium atoms described earlier.


Electron transitions take place in all the elements and molecules and can give a precise fingerprint to identify that chemical. The colour can identify elements in hot gases.

Neon light to our eyes looks pinky purple. This arises because of how our eyes and brain interpolates and interprets the emission lines from neon gas.


On May 27, 2020, Andrea Alessandrini used a small, 2.6-inch refracting telescope equipped with a special filter to take this remarkable photo of the planet Mercury and its glowing tail of sodium. His exposure lasted 7½ minutes at an ISO of 1000. This is a composite of three images taken over three days. (Andrea Alessandrini)


The above left image shows Mercury through a sodium filter with a 24 million km long tail. Above right is Andrea Alessandrini, who took the photograph with a relatively simple telescope and digital camera.

The image of Mercury seems to show that it has a tail like those seen with comets. The fact that it is seen to glow through a sodium filter indicates the tail contains a lot of sodium. Mercury is the nearest planet to the Sun so it experiences the greatest amount of the Sun’s heat. Some of its atmosphere is blown away into the tail.

The comet tail is about 24 million km. The exposure only took about 7.5 minutes to create so the tail isn’t especially faint, but it is only really visible by filtering out all the light except the light emitted by sodium vapour.

Dispersing light from different gas clouds in nebulae can identify the chemical elements present.

Meteors in the night sky can display distinctive colours. Professor Blundell has seen quite a few green ones. Seeing green light coming from a meteor can indicate it contains copper or iron.


All the elements present in the periodic table will have distinct spectrums of emission lines when they are in a hot gaseous form.

The periodic table shown above is in order of atomic number/electronic structure of the elements.

The image below shows the relative abundance of elements on Earth. There are roughly equal amounts of carbon, hydrogen and oxygen.

Doing a version for different stars would give completely different arrangements of elemental abundance.


Most stars are predominantly hydrogen but some of them can contain different elements because they have evolved differently and undergone different nucleosynthetic processes.


Looking at different stars through a spectrograph shows they have different spectra, rising partly from their different temperatures but also because of their different chemical compositions.

Stars that are made up out of gas that was ejected from older stars that exploded will be more enriched with elements further down the periodic table than just hydrogen and helium. Stars that were formed in the early days of the Universe would contain pristine hydrogen gas only


Interior structure of a massive star just before it exhausts its nuclear fuel: High-mass stars can fuse elements heavier than carbon. As a massive star nears the end of its evolution, its interior resembles an onion. Hydrogen fusion is taking place in an outer shell, and progressively heavier elements are undergoing fusion in the higher-temperature layers closer to the centre. All of these fusion reactions generate energy and enable the star to continue shining. Iron is different. The fusion of iron requires energy, and when iron is finally created in the core, the star has only minutes to live.

The stars below can be found in the Orion constellation.

Orion is a prominent constellation located on the celestial equator and visible throughout the world. It is one of the most conspicuous and recognizable constellations in the night sky. It is named after Orion, a hunter in Greek mythology. Its brightest stars are blue-white Rigel (Beta Orionis) and red Betelgeuse (Alpha Orionis).


What is different about the image above is that the light from the stars have been resolved into spectra.


Betelgeuse (above) has quite a lot of red light. Even though it looks red in the night sky it doesn’t just emit red light.

Betelgeuse is usually the tenth-brightest star in the night sky and, after Rigel, the second-brightest in the constellation of Orion. It is a distinctly reddish semiregular variable star whose apparent magnitude, varying between +0.0 and +1.6, has the widest range displayed by any first-magnitude star. At near-infrared wavelengths, Betelgeuse is the brightest star in the night sky.


The image above shows that Rigel has a different spectral shape. It still emits red light but there is a lot more violet light

Rigel, designated β Orionis is a blue supergiant star in the constellation of Orion, approximately 860 light-years (260 pc) from Earth.

Rigel seems to be more bluey-white to our eyes than Betelgeuse. This is simply due to their having different temperatures. This can be understood in terms of black body radiation.

At 3600 K (0oC = 273K) Betelgeuse is a much cooler star than Rigel.

Rigel being much hotter emits light right across the visible spectrum, in approximately equal quantities.




Looking at the spectra of Messier 42 found in Orion’s belt shows a lot of red light, which is the result of excited hydrogen gas.

The greenish-whitish light radiating out is caused by illuminated oxygen.


You would expect M42 to give a different spectrum to the stars, which is the case.


Zooming in on M42 spectrum shows there are two bright emission lines. Bright peaks corresponding to hydrogen and oxygen. This is consistent with the gas known to be irradiating in the nebula. So being able to identify the spectra you can identify the elements present in a particular phenomenon in the night sky.


As mentioned earlier, spectrographs (an example is seen above) are used to study spectra in more detail. They disperse light in a similar way to raindrops dispersing sunlight.

The dispersing element in a spectrograph is a piece of glass with a grating etched into it. It disperses light according to the wavelengths (colours). All the mixed-up light enters the spectrograph and is separated out by the grating. The spectrum is then fed into a monochromatic camera. This is a camera that just responds to the intensity of the incident light and captures how faint certain regions are and how bright other regions are.

Astronomers can measurably quantify which wavelengths and intensities of light are obtained through the spectrograph. This is because the dispersing element disperses the light according to different wavelengths into different positions along the detector. The wavelength of the spectrum goes from shorter wavelengths on the bottom left of the image below to longer wavelengths on the top right.

You can also label the image in terms of frequency but the higher frequencies are bottom left and the lower frequencies are top right.


If we know and understand the positions of all the bright points on the spectrum (they are identified using emission lines from calibration lamps) we can absolutely calibrate the positions of different spectra and understand exactly what they are.


Calibration lamps spectra. Calibration lamps can contain thorium, krypton and other elements which give rise to emission lines at very specific wavelengths. The points of light on the spectrograph are matched to the wavelengths of the calibration lamps, which are measured in the laboratory.

Emission lines produced by a telescope and spectrograph can be identified by comparing with the reference spectrum produced by the calibration lamps.

This is important because another piece of information that can be obtained from colour information is speed.

Colour information gives the speed when it is combined with the Doppler Effect. Wavelengths are elongated when an object is moving away from us.

The Doppler effect (or the Doppler shift) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who described the phenomenon in 1842.

A common example of Doppler shift is the change of pitch heard when a vehicle sounding a horn, approaches and recedes from an observer. Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by, and lower during the recession.

The reason for the Doppler effect is that when the source of the waves is moving towards the observer, each successive wave crest is emitted from a position closer to the observer than the crest of the previous wave. Therefore, each wave takes slightly less time to reach the observer than the previous wave. Hence, the time between the arrivals of successive wave crests at the observer is reduced, causing an increase in the frequency. While they are traveling, the distance between successive wave fronts is reduced, so the waves “bunch together”. Conversely, if the source of waves is moving away from the observer, each wave is emitted from a position farther from the observer than the previous wave, so the arrival time between successive waves is increased, reducing the frequency. The distance between successive wave fronts is then increased, so the waves “spread out”.

For waves that propagate in a medium, such as sound waves, the velocity of the observer and of the source are relative to the medium in which the waves are transmitted. The total Doppler effect may therefore result from motion of the source, motion of the observer, or motion of the medium. Each of these effects is analysed separately. For waves which do not require a medium, such as electromagnetic waves or gravitational waves, only the relative difference in velocity between the observer and the source needs to be considered, giving rise to the relativistic Doppler effect.


Change of wavelength caused by motion of the source.


An animation illustrating how the Doppler effect causes a car engine or siren to sound higher in pitch when it is approaching than when it is receding. The red circles represent sound waves.

Passing car horn


The Doppler effect for electromagnetic waves such as light is of great use in astronomy and results in either a so-called redshift or blueshift. It has been used to measure the speed at which stars and galaxies are approaching or receding from us; that is, their radial velocities. This may be used to detect if an apparently single star is, in reality, a close binary, to measure the rotational speed of stars and galaxies, or to detect exoplanets. This redshift and blueshift happens on a very small scale. If an object was moving toward earth, there would not be a noticeable difference in visible light, to the unaided eye.

Note that redshift is also used to measure the expansion of space, but that this is not truly a Doppler effect. Rather, redshifting due to the expansion of space is known as cosmological redshift, which can be derived purely from the Robertson-Walker metric under the formalism of General Relativity. Having said this, it also happens that there are detectable Doppler effects on cosmological scales, which, if incorrectly interpreted as cosmological in origin, lead to the observation of redshift-space distortions.

The use of the Doppler effect for light in astronomy depends on our knowledge that the spectra of stars are not homogeneous. They exhibit absorption lines at well-defined frequencies that are correlated with the energies required to excite electrons in various elements from one level to another. The Doppler effect is recognizable in the fact that the absorption lines are not always at the frequencies that are obtained from the spectrum of a stationary light source. Since blue light has a higher frequency than red light, the spectral lines of an approaching astronomical light source exhibit a blueshift and those of a receding astronomical light source exhibit a redshift.


Christian Andreas Doppler (29 November 1803 – 17 March 1853) was an Austrian mathematician and physicist. He is celebrated for his principle – known as the Doppler effect – that the observed frequency of a wave depends on the relative speed of the source and the observer. He used this concept to explain the colour of binary stars.

The Doppler Effect means it is pretty easy to measure the speed a galaxy is moving towards us or away from us.

Vesto Slither and Edwin Hubble found that the further away a galaxy is, the faster it is receding from us. (below left)


Vesto Melvin Slipher (November 11, 1875 – November 8, 1969) was an American astronomer who performed the first measurements of radial velocities for galaxies. He was the first to discover that distant galaxies are redshifted, thus providing the first empirical basis for the expansion of the universe. He was also the first to relate these redshifts to velocity (above centre)

Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an American astronomer. He played a crucial role in establishing the fields of extragalactic astronomy and observational cosmology.

He provided evidence that the recessional velocity of a galaxy increases with its distance from the Earth, a property now known as “Hubble’s law”, despite the fact that it had been both proposed and demonstrated observationally two years earlier by Georges Lemaître. The Hubble–Lemaître law implies that the universe is expanding. A decade before, the American astronomer Vesto Slipher had provided the first evidence that the light from many of these nebulae was strongly red-shifted, indicative of high recession velocities. (above right)

Georges Henri Joseph Édouard Lemaître (17 July 1894 – 20 June 1966) was a Belgian Catholic priest, mathematician, astronomer, and professor of physics at the Catholic University of Louvain. He was the first to theorize that the recession of nearby galaxies can be explained by an expanding universe, which was observationally confirmed soon afterwards by Edwin Hubble.

Colour can also give information about distance.

The spectrum from the stars is recorded. The emission lines are used to identify the types of stars present in a galaxy. The emission spectra are also examined to see is they are displaced towards the red end of the spectrum. By measuring the redshift it is straightforward to calculate the speed at which the galaxy is receding from us.


Here, λo, λe are the observed and emitted wavelengths (measured in a laboratory) respectively. c is the normal speed of light in a vacuum and v is the recession velocity.

The recession velocity relates directly to the distance the galaxy is away from us. Slipher and Hubble worked on this to give Hubble’s law.’s_law

Hubble’s law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from the Earth at speeds proportional to their distance. In other words, the farther they are the faster they are moving away from Earth. The velocity of the galaxies has been determined by their redshift, a shift of the light they emit toward the red end of the spectrum.

The discovery of the linear relationship between redshift and distance, coupled with a supposed linear relation between recessional velocity and redshift, yields a straightforward mathematical expression for Hubble’s law as follows:


Where v is the recessional velocity, typically expressed in km/s. H0 is Hubble’s constant and corresponds to the value of H (often termed the Hubble parameter which is a value that is time dependent and which can be expressed in terms of the scale factor) in the Friedmann equations taken at the time of observation denoted by the subscript 0. This value is the same throughout the universe for a given comoving time.

D is the proper distance (which can change over time, unlike the comoving distance, which is constant) from the galaxy to the observer, measured in mega parsecs (Mpc), in the 3-space defined by given cosmological time. (Recession velocity is just v = dD/dt).

Using the techniques of Doppler and spectroscopic observations of galaxies it has been possible for the most distant know galaxy (see below) to be observed and to have its recession velocity determined.


The most distant know galaxy is GN-z11

GN-z11 is a high-redshift galaxy found in the constellation Ursa Major. The discovery was published in a paper headed by P.A. Oesch and Gabriel Brammer (Cosmic Dawn Center). GN-z11 is currently the oldest and most distant known galaxy in the observable universe. GN-z11 has a spectroscopic redshift of z = 11.09, which corresponds to a proper distance of approximately 32 billion light-years (9.8 billion parsecs). The object’s name is derived from its location in the GOODS-North field of galaxies and its high cosmological redshift number (GN + z11). GN-z11 is observed as it existed 13.4 billion years ago, just 400 million years after the Big Bang; as a result, GN-z11’s distance is sometimes inappropriately reported as 13.4 billion light-years, its light-travel distance measurement. (below left)

image (above right)

In December 2020 observers using the W. M. Keck Observatory reported a sudden brightening, which they interpreted as a gamma-ray burst.


The W. M. Keck Observatory is a two-telescope astronomical observatory at an elevation of 4,145 metres near the summit of Mauna Kea in the U.S. state of Hawaii. Both telescopes have 10 m aperture primary mirrors, and when completed in 1993 (Keck 1) and 1996 (Keck 2) were the largest astronomical telescopes in the world. They are currently the 3rd and 4th largest.

This animation shows the location of galaxy GN-z11, which is the farthest galaxy ever seen. The video begins by locating the Big Dipper, then showing the constellation Ursa Major. It then zooms into the GOODS North field of galaxies, and ends with a Hubble image of the young galaxy. GN-z11 is shown as it existed 13.4 billion years in the past, just 400 million years after the big bang, when the universe was only three percent of its present age.


GN-z11 is found in the constellation Ursa Major. The z11 refers to the fact that its red shift is 11.09.


The graph shown above shows that a red shift of 11 means that the lookback time for the galaxy is well over 13 billion years.

It is believed that light from this galaxy left 400 million years after the Big Bang and that is a record holder, at the moment. It is very surprising that galaxy formation could take place such a short time after the Big Bang.

The Big Bang theory is a cosmological model of the observable universe from the earliest known periods through its subsequent large-scale evolution. The model describes how the universe expanded from an initial state of extremely high density and high temperature, and offers a comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements, the cosmic microwave background (CMB) radiation, and large-scale structure.


Artist’s conception of GN-z11 galaxy. It is very irregular in its form.

The Doppler Effect is useful cosmologically for determining distances but also the dynamics of rapidly evolving objects in the galaxy.

SS43 is a famous microquasar

A microquasar, the smaller version of a quasar, is a compact region surrounding a stellar black hole with a mass several times that of its companion star. The matter being pulled from the companion star forms an accretion disk around the black hole. This accretion disk may become so hot, due to friction, that it begins to emit X-rays. The disk also projects narrow streams or “jets” of subatomic particles at near-light speed, generating a strong radio wave emission.

SS 433 is one of the most exotic star systems observed. It is an eclipsing X-ray binary system, with the primary most likely a black hole, or possibly a neutron star. The spectrum of the secondary companion star suggests that it is a late A-type star. SS 433 is the first discovered microquasar. It is at the centre of the supernova remnant W50.


Artist’s impression of SS 433


Spectra of the famous microquasor SS43. It has three prominent emission lines.

The green Hα arises from hydrogen gas. It is referred to as the stationary alpha line but it isn’t exactly stationary as it wiggles around because of material orbiting the black hole system and material being sucked into the black hole.

A black hole is a region of spacetime where gravity is so strong that nothing—no particles or even electromagnetic radiation such as light—can escape from it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole.

The blue shifted Hα line arises from hydrogen gas which is radiating light in its own rest frame at the normal sort of frequency to that represented by the green line at 6500Å but that the gas is moving towards the Earth at rapid speeds.

There is also a corresponding red shift Illuminated hydrogen gas moving away from the Earth


Every time you look at the spectra, it has changed.





The reason why the spectra changes is because the blue and red shifting gas arises from packets of plasma being squirted out from the vicinity of the black hole, about an axis, whose direction changes systematically and steadily with time. This is why the red and blue shifting changes gradually over time.

Spectroscopy (colour) gives information about dynamical motion


Part of an animation

Time resolved spectroscopy can also help with elucidating different dynamical situations.

The images below show two different dynamical situations. One is a series of red spectra and the other is a series of blue spectra.


At first, they look the same (as above).

However, if you wind up the spectral resolution to show them in more detail you can see they give very different shapes because they arise from very different dynamics.





On the left you’ve got a situation whereby two emission lines are rock steady in wavelength, but they go up and down in intensity with time. This arises because a ring of gas is constantly rotating around a central mass, but the intensity of the lines changes as a beacon of light is illuminating the gas (like a lighthouse beam) in turn.

The right-hand side shows a completely different dynamical situation. This occurs when two stars are orbiting around each other and their emission lines go backwards and forwards according to the Doppler Effect.

Time lapse spectroscopy or time resolved spectroscopy can make all the difference between being able to understand which dynamical situation you’ve got.

Time lapse spectroscopy such as that carried out by the Global Jet Watch observatories is an extremely powerful tool to understand the dynamic evolving Universe close to home.

The Global Jet Watch project consists of five observatories strategically distributed in longitude around Planet Earth, such that there is always one of these in darkness. Optical astronomy has to be done at night! Our observatories are equipped with research-grade instrumentation and telescopes. They are located in Australia, India, South Africa and Chile.

On December 2020 there was a great conjunction of Jupiter and Saturn. Luckily the skies were clear in India


20th December

The conjunction was first observed on the 20th December 2020

The first of these images, with Saturn starting in the top left, has three of Jupiter’s Galilean moons showing together with a star in the same line (Europa is transiting Jupiter during this observation).


21st December

The middle image, with Saturn and Jupiter almost vertically aligned, shows three of the Galilean moons again but this time with Io occulted by Jupiter (and the whole system has moved relative to the background stars so that the first star which was to the left of Jupiter in the first frame is now seen to the right of Jupiter).

The 21st of December 2020 was when Jupiter and Saturn had their closest approach.

Being able to observe before, during and after gave a sense of how rapidly they were moving through the sky with one respect to one another

The final image, with Saturn in the top right, shows all four of the Galilean moons with two shown on each side of Jupiter. Each image shows Saturn’s brightest moon, Titan, in orbit around it (in the first image it is to the right of Saturn, and by the final image it is almost above).


22nd December


22nd December.

The 22nd December 2020 image shows five moons (although I can’t see 5) in total. Four Galilean moons in orbit around Jupiter and one moon, Titan, around Saturn. These planets do have many more moons but these five were clearly observable in the short exposures

These are not easy observations to do, even if the skies are free of clouds (not the case at all the other observatories!).  This is partly because Jupiter and Saturn were so close to the horizon, so the telescope is looking through lots of atmosphere which can sometimes be turbulent.  This image shows how the finder scopes could see the conjunction, with the tops of trees threatening to get in the way!

These images were conducted by remote control over the internet of the GJW-IN telescope by Professor Blundell in the UK and Steve Lee in Australia, made possible thanks to TeamViewer, Sophos, Zoom and Prism with a camera made by FLI.

Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass one-thousandth that of the Sun, but two-and-a-half times that of all the other planets in the Solar System combined. Jupiter is one of the brightest objects visible to the naked eye in the night sky and has been observed since pre-historic times. When viewed from Earth, Jupiter is on average the third-brightest natural object in the night sky after the Moon and Venus. It is named after the Roman god Jupiter.

Jupiter has 79 known natural satellites. Of these, 60 are less than 10 kilometres in diameter. The four largest moons are Io, Europa, Ganymede, and Callisto, collectively known as the “Galilean moons”, and are visible from Earth with binoculars on a clear night.

Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine times that of Earth. It only has one-eighth the average density of Earth; however, with its larger volume, Saturn is over 95 times more massive. It is named after the Roman god of wealth and agriculture.

Saturn has 82 known moons, 53 of which have formal names. In addition, there is evidence of dozens to hundreds of moonlets with diameters of 40–500 meters in Saturn’s rings, which are not considered to be true moons. Titan, the largest moon, comprises more than 90% of the mass in orbit around Saturn, including the rings. Saturn’s second-largest moon, Rhea, may have a tenuous ring system of its own, along with a tenuous atmosphere.

Jupiter’s Galilean moons and their orbits are described in Katherine Blundell’s Inaugural Gresham Lecture “Faster than Light?” <YOUTUBE LINK> and the Great Conjunction is described in the “Witnessing Fireworks” Gresham lecture <YOUTUBE LINK>.  Alternative (non-YouTube) links are HERE.

What would a rainbow of Jupiter look like?

The spectrum is substantially elongated up/down as it samples data from across the equator of Jupiter shown on the right of the image below. Jupiter is shown with its stripes in the vertical direction for this particular illustration.


The wavelength increased from left to right and is characterised by dark lines.


Zooming in shows that some the lines are vertical (indicated by the white arrows in the image below). Those actually arise because of absorption by fairly cold gases in our own atmosphere. But there are also slanted lines which arise because of absorption by gases in Jupiter’s own atmosphere.


The part of the spectrum indicated by the blue arrows above arises from gas which is absorbing light coming from the Sun but is also moving towards us. It is blue shifted. The part of the spectrum indicated by the red arrows is caused by gases absorbing light coming from the Sun but moving away from us. It is red shifted. This moving towards and moving away is caused by Jupiter spinning on its axis. The equator is constantly rotating

From this is possible to derive a spectrum at all these different heights across the broad 2D spectrum and to give us information about the speed at which different bits of gas all across Jupiter’s equator are moving with respect to Earth.

The image below is a composite of a lot of different spectra.


All from different sampling points around Jupiter’s equator. There are two extremes all the way from the blue shift of gases moving towards Earth to the red shift of gases moving away.

From these observations it is possible to extract the speed at which Jupiter is rotating and then to derive the period of rotation, which turns out to be 10 hours. So, Jupiter spins on its axis in less than half the time that it takes the Earth to spin on its axis.

You can measure the period of rotation yourself if you have a telescope that is able to resolve the giant red spot on Jupiter. You can make successive observations to see how long it takes for the red spot to track across Jupiter and work out how long it would take to go all the way round. It’s about 10 hours.

Audience comments

Molecules have a very different spectrum to atoms – they are normally in the infra-red. The lines are due to the molecules bending and twisting rather than electron level transitions in atoms

Oxygen emission causes the green colour in Aurora.

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Bottom of Form


Astronomy Physics Science

Professor Blundell’s lecture series are as follows:

2020/21 Cosmic Vision

2019/20 Cosmic Concepts

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