Why do we See what we See?

The western highlands of Scotland, bathed in the visible light of our home star.

When we think about the vast array of electromagnetic radiation all around us – from Gamma rays, X-rays , UV, Microwaves and Radio waves – a natural question to ask is why do human eyes see in a very narrow band we call ‘visible light’?

The answer is undoubtably tied to the energy output of our nearest star – the Sun. Its peak radiation just happens to be at this ‘visible’ band of radiation. I’ve illustrated this below with a black body radiation profile of our Sun.

Our eyes have therefore evolved to ‘see’ this particular narrow range of otherwise insignificant wavelengths. There’s nothing inherently important about visible light – in fact it makes up a tiny 0.0035 percent of the entire electromagnetic spectrum!

Understanding this makes me wonder about the potential sensory apparatus of life that might have evolved elsewhere in the universe. Other stars with different stellar classifications to our Sun have markedly different peak radiation profiles.

If we had evolved next to a source of intense gamma rays for instance, we would very likely be completely blind to visible light but adept at observing small granular differences in the intensity of gamma radiation.

The Northern Lights


The northern lights looking over the Beauly firth towards the Black Isle, Inverness-shire

After reports of a KP6 geomagnetic storm predicted to strike Scotland over the weekend, and clear skies on Sunday evening, I headed out after sunset to try and catch the northern lights.  This was a very early aurora excursion as nights have only just got dark enough for decent views of the night sky, let alone tracking down the faint and elusive northern lights.

My initial outing took my into the hills above Bunchrew where I bagged some lovely views of the summer Milky Way overhead.  Turning my attention north I noticed a faint arc of light on the horizon,  and sure enough some test shots picked up a vibrant band of purple and green auroral light.  However little structure was evident until I moved to lower elevations, reaching the Bunchrew shoreline just after 10.30pm.


The Milky Way near Cygnus, framed between trees above Bunchrew.

From this new vantage, in the dark looking over the Beauly Firth,  the northern lights stood out much more clearly as distant columns of white light, slowly morphing and scintillating above the horizon.  Some of the images (attached) show nice structure and the suggestion of wave like movement.

As our nights get darker many more opportunities to view the aurora will present themselves.  The best strategy is to simply get out there as often as you can when it’s clear, and try and escape the boundaries of light polluted towns and cities.  Aurora forecasts should only be used as a guide as they’re seldom reliable.  Remember to look north and where possible find some nice low horizons in this direction.

Good luck and clear skies.


The aurora is caused by the solar wind slamming into the earth’s atmosphere near the poles, ionising chemical elements which produce light at very specific quantised frequencies.



Space Camp in Thurso


Some photo highlights from the Summer Space Camp up in Thurso’s band new Newton Room.  I had a great time delivering Mars and astronomy based workshops on day 2.  We covered the observational history of Mars, its surface geology, the night sky, the life and death of stars and spectroscopy.  Interactive sections included Mars cratering, galaxy frisbees, star cluster balloons and DIY spectrascopes.

Picture rights Skills Development Scotland.

Urban Astronomy Inverness


The formation of a young protostar following the collapse of a previously inert dust cloud

We had a great turnout for March’s Urban Astronomy session last week at the Sea Cadet’s Hall in Inverness.  The indoor presentation massively benefited from our new giant screen, expertly erected by Robbie (pictured below).  Here’s a selection of slides from my presentation on naked eye observing and the life of giant stars.


Robbie putting the final touches to our new giant screen for indoor astronomy presentations and virtual sky guiding

Topics covered:

– Naked eye and binocular observing
– Satellites: Iridium Flares and ISS
– Colour, temperature and mass of stars
– The Hertzsprung-Russell diagram
– Protostar formation from dark nebulae
– Main sequence burning and final fate of stars
– White dwarfs, supernovae, neutron stars and black holes.

As ever there were some superb questions during and after the talk.  Stay tuned for upcoming events as myself and Caroline roll out the program.


In the simplest terms stars behave like black body radiators with colour linked to their surface temperatures.


The brightest stars in the night sky can be close – like Sirius – or giant stars very far away (eg. Betelgeuse, Rigel, Deneb).


The HR diagram.  An elegant and reliable tool for describing the evolution of stars from main sequence burning into their final stages of life

Solar Day at Abriachan

We were blessed with a lovely sunny day on Saturday for our day of Solar learning up at Abriachan.  We were fully prepared for indoor activities as forecasts were looking pretty grey.  But as the weekend swung around skies cleared and we ended up seeing plenty of Sun all day.


A day of fun solar learning

Since conditions were so good we moved everything outside, including the talk I’d prepared which was originally put together on powerpoint.  I demonstrated basic shadow time keeping and direction finding, and how solar eclipses take place using a scale model of the Moon and Earth (with the moon’s orbit inclined at 5 degrees).

Based on our model the Earth and Moon were around 3 meters apart with the former about the size of a large orange.  At this scale the Sun would be 10 meters in diameter and over a mile away!  With this model the relative rarity of total solar eclipses becomes clear (on average one every 18 months).

During the talk we also touched upon:

  • Sun gods and how our ancestors perceived the Sun as a perfect orb with no imperfections
  • The human fear of eclipses
  • The discovery of Sun spots and how they revealed that the Sun is spinning
  • How spectroscopy revealed that our Sun is in fact a star (at very close proximity)
  • Why the Sun is loosing mass – over 4 billion tons of hydrogen per second
  • The ultimate fate of our Sun – how it will eventually flare up as a red giant star before cooling and shrinking down to a white dwarf

After the talk Clelland took over for some fun outdoor activities including a scale walk of the solar system, DIY spectroscopes and solar lasers using big magnifiers.  We also did a fun experiment simulating the colour of the sky and sunsets using milk in water bottles.

In terms of solar viewing, I setup the 200mm with a full objective white light filter, and we also had a Sunspotter, kindly on load from Glasgow Science centre.  Both setups produced clear views of the Sun’s photosphere, but unfortunately there were no sunspots to see.  This isn’t entirely surprising given we’re bang in the middle of the 11 year solar cycle minimum, although large spots can appear suddenly at any time.  We hope to one day invest in a good quality hydrogen alpha filter for these events, as these reveal many more interesting features, like edge prominences and coronal loops.

Overall a fun day of learning with great interaction and questions from the adults and little ones alike.

Next Abriachan Astronomy Dates

I’m excited to be hosting two more astronomy events alongside the Abriachan forest team in March and April 2018.  Details and ticket links below.

Star Cluster Special – March 10th (moved from Feb 10th) 7pm-9pm


The Hyades and Pleiades Star clusters

Explore the great winter open clusters under moonless dark skies with campfire stories to follow. Outdoor binocular guiding under clear skies. Indoor talk, astronomy activities and virtual guiding in the classroom in the event of poor weather. Refreshments provided.

Ticket link: https://www.eventbrite.co.uk/e/dark-sky-observingwith-a-sta…

Solar Special and the Life of Stars – April 14th 2pm – 4pm


A typical G-type main sequence star – locals have dubbed this one ‘The Sun’

A Sun special exploring our nearest star and the life of giant stars. Outdoor sun projections and activities, with illustrated talk and refreshments. Suzann even has plans for a solar pizza oven!

Ticket link: https://www.eventbrite.co.uk/e/some-sunny-science-and-the-l…

All stargazing events organised in collaboration with the Abriachan team, astronomer Stephen Mackintosh and learning coordinator Suzann Barr. Campfire tales delivered by forest ranger Clelland.

For group bookings please email: abriachanforest@gmail.com

Exoplanet Hunting and other Habitable Worlds


Artists impression of a giant exoplanet occulting part of its parent star.

Recent advances in telescope and sensor technology have finally allowed us to start answering one of the great unknowns of the 21st century – how many habitable worlds are there in our Milky Way galaxy?

Until recently it was simply assumed that a fairly high fraction of stars ‘probably’ contained planets, and that some further fraction of those would be in the so-called habitable zone.  These assumptions were based from simple extrapolation from our own solar system (along with some inconclusive inferential data gathered in the 1990s).  But as has been painfuly demonstrated with the hunt for life in our solar system, assumptions like this need careful confirmation, and extrapolating from a sample size of one is seldom convincing.  Often, what we want to believe is, regrettably, not compatible with reality.  The video link from the late Carl Sagan at the end of the piece will give you a flavour for what people ‘guessed’ in the 1980s.

The good news is we now have concrete evidence that an abundance of other planets are out there orbiting other suns – in fact roughly 70% of all stars are accompanied by other worlds.  How do we know this?  There are two simple detection methods which I’d like to explain in more detail, which both ‘indirectly’ detect the presence of other planets.  These are:

  1. Transit Photometry
  2. Radial velocity

Transit Photometry

The first method involves closely observing a star over a period of time with a photometer, looking for any subtle changes in its brightness.  Any dip in brightness which is periodic and cannot be explained by general stellar dynamics is then assumed to be a large body passing between us and the star.

Incredibly, even amateurs with 12 inch backyard telescopes have been able to detect these illumination changes for very large planets, producing crude light curve plots that have later been verified by professional astronomers.


As the exoplanet passes in front of the host star the number of photons reaching the objective lens of the telescope drops in a characteristic way.

This method of detection works out to distances of several thousand light years, and allows an approximate calculation of the planet’s size, or radius.  Additionally, for certain nearby stars (if clear spectra can be obtained), drops in light from specific elements can be detected, telling us about the possible composition of the planet’s atmosphere.  For example, if the planet contained a Nitrogen rich atmosphere, we might detect a dip in the intensity of the spectrum corresponding to the wavelength of Nitrogen – this process is called ‘absorption spectrometry’.

The main disadvantage of the transit method is that it relies on a nearly perfect edge-on view of the planet-sun ecliptic from our earth bound position.   Otherwise we simply would not detect any occultation.  Thankfully, we can calculate roughly how often this orientation occurs and account for it statistically.  There’s no shortage of candidate stars out there with systems well aligned for detection.

Radial Velocity

The second method is similar, but instead focuses on the relative motion of the star.  Despite the huge difference in mass between a star and its satellites, as a planet orbits it will impart enough of a gravitational tug to make the star  rotate about a small local axis – tiny but detectable.  The larger the planet the bigger this wobble will be.


An exaggerated illustration of how a large star will rotate about a local axis due to the gravitational influence of a planet.

To detect this regular movement we can look at the light emanating from the star over time and try and detect if its spectra is being shifted by tiny amounts.  By the doppler effect, if the wavelengths of the star’s spectra appear shifted towards the red it must be moving away from us, and the opposite if the spectra is shifted towards the blue.  In this way the sun’s speed and local orbital radius can be calculated, and from that a determination made of the mass of the orbiting planet.

As a quick example, the Sun moves by about 13 m/s due to the influence of Jupiter, but only about 12 cm/s due to Earth.  Incredibly, velocity variations down to 1 m/s or even less can be detected with modern spectrometers, such as the HARPS.  The major limitation of this method is distance.  At the moment it’s generally only useful for star systems up to 200 light years away.

Bringing both methods together, however, lets us form a picture of an exoplanet’s size and mass, and therefore it’s overall density.  From that,  inferences can even be made about the internal structure of the planet.  All this information without even observing the planet!

The Complete Picture

So what do these methods tell us, so far, about the likely number of habitable or earth like planets in our Milky Way galaxy?  The answer is absolutely staggering.

Based on Kepler mission data, as many as 40 billion earth like planets in the habitable zone could be orbiting around red dwarf and sun-like stars.   Taking away the red dwarfs leaves an upper calculation of 11 billion around Sun like stars.  Just think about those numbers for a moment.

That’s as many as one earth-like planet in the habitable zone for every 10 stars in our local galaxy!  A stupendously high number of candidate worlds from which life may have originated.

But here again, we must be cautious.  11 billion is 11×10^9.  But what if the probability of life forming on rocky planets within the habitable zone was actually as low as low as 11 x 10^-9,  or even 11 x 10^-99?  Then there might only be one or no candidate planets containing life.  A depressing possibility, but one we should never allow our natural bias to discount.  The lesson here is that any large number can quickly be diminished in stature by an equally small probability.

At the moment we simply don’t know what this probability of emergent life is.  Some biologists are more optimistic and consider it relatively high for simple single cell life, but other figures, for more complex multi cellular organisms, are much more pessimistic.  But when we do know this figure, calculating the number of planets on which life has arisen will be comparatively simple, and Frank Drake’s famous equation for estimating the number of ‘technical civilisations’ will be one step closer to a final solution.


How did we view the question of ‘other planets’ in the context of life outside Earth in the 1980s.  Watch the late Carl Sagan to find out.