LIGHT POLLUTION - NOT A NEW PROBLEM FOR ASTRONOMERS

By Doug Daniels

I got my first look at a really dark night sky when I was just four years old - it was in 1944 during the second World War. At that time, cities had a night time blackout imposed upon them. There was no street lighting, no advertising lighting and even the buses and the few cars moving on the roads had their headlights reduced to narrow slits. In consequence, the night sky was so dark that the Milky Way could be seen even from London! It must have created quite an impression on me because less than a decade was to pass before I built my first telescope. During that decade however, the rot had already began to set in and by the time I was ready to take a closer look at the night sky, the Milky Way was no longer visible and at least two stellar magnitudes were lost to view.

I began to think seriously about the effects of light pollution recently, because I was asked to submit evidence to a Parliamentary Select Committee which is currently considering the threat imposed to the study of astronomy by this insidious menace. It was pointed out that the night sky is in fact a Site of Special Scientific Interest (an S.S.S.I.), which has at present no legislation to protect it.

Recalling the wartime blackout, reminded me that it was only due to such a blackout imposed on the city of Los Angeles, that the American astronomer Walter Baade was able to make the observations that were to totally alter our conception of the Universe.

The first reasonable 'modern' estimate of the extent of the Universe was made in 1906 by Jacobus Kapteyn. At that time photography was the 'new technology' and astronomers were taking full advantage of it. By taking numerous photographs of the Milky Way and counting the numbers of stars of different magnitudes contained in different sections, and assuming that the stars were all of average size, Kapteyn was able to estimate the distances that the stars would have to be in order to register their magnitudes on the photographic plates. These observations were based on the relationship between a star's apparent and absolute magnitude. Just as when we look down a road at night illuminated by the dreaded street lamps - the lamps close to us look brighter than those further away, although they are all actually the same brightness. The amount by which light diminishes with distance obeys a law, which states "the apparent magnitude is proportional to the absolute magnitude divided by the square of the distance to the observer." Astronomers define the absolute magnitude of a star as the apparent magnitude that it would have if placed at a distance of 10 Parsecs from Earth.

Kapteyn conceived our galaxy as a lens shaped structure 23,000 light years thick with the Solar System close to its centre. He came to this conclusion because the Milky Way divides the sky roughly into two halves and it appears more or less evenly bright in all directions. This assumed symmetry however, had one significant flaw - the globular star clusters are not evenly distributed over the whole sky but are found in far greater concentrations only in certain areas.

Just six years later, in 1912, Henrietta Leavitt was studying the stars of the Lesser Magellanic Cloud (LMC). She discovered that the LMC contained a number of Cepheid Variables. These are stars, which vary in magnitude in a very precise way. Henrietta's study revealed a relationship between the brightness of the stars and their period of variability. She plotted the results on a graph and produced the Period/Luminosity Curve. The LMC was particularly convenient for this study, as all the stars are more or less at the same distance from us.

It has to be remembered that at this time, our 'galaxy' was considered by many astronomers to comprise the entire 'Universe'. They had no real conception of 'other galaxies'. However, it was also in 1912 that Vesto M. Slipher first described the 'red shifts' observed in the spectra of certain 'nebulae', indicating that they were moving away from us. This work was to be developed later in 1929 by Edwin Hubble.

In 1918 Harlow Shapley used the Period/Luminosity curves for RR Lyrae stars to show that the Globular star clusters were distributed as a kind of 'halo' which surrounded the central bulge or nucleus of our Galaxy. These observations indicated that our sun was nowhere near the centre of our Galaxy.

Two years later, in 1920, Edwin Hubble used the newly constructed 100 inch telescope on Mount Wilson to take long exposure photographs of the Great Andromeda Nebula (M31), resolving the outer region into stars. So M31 was really another distant galaxy. Hubble searched for Cepheid Variables amongst the stars of M31 and was successful. Using the Period/Luminosity relationship he calculated the distance of M31 to be in the order of 750,000 light years. He then went on to measure other galaxies of similar structure. The trouble was that all these galaxies seemed so much smaller than our own and furthermore, spectroscopic observations of these galaxies were now indicating that the Universe as a whole was expanding - despite what Einstein thought! To add to the confusion, the figures obtained from these observations were indicating an age for the Universe of just 2 billion years. Geologists knew that the Earth was at least twice as old as this! Clearly something was amiss!

Now we come to the part played by light pollution. The outbreak of World War two and the imposition of a blackout on the city of Los Angeles allowed the astronomers at Mount Wilson to re-photograph the Andromeda Galaxy. The darker skies freed from the curse of light pollution, allowed far longer exposures and they were able to resolve the central region of M31 into stars. Walter Baade found that the stars in the central region were redder and fainter than the stars in the outer spiral arms. There appeared to be two distinct types of star in the Andromeda Galaxy. Baade called the blue outer stars Population I and the redder stars of the central region Population II.

Because of our Sun's position in our Galaxy, most of the stars we see are Population I. Such stars occur in the outer spiral arms of galaxies where they are formed from the high concentrations of dust and gas to be found in these regions. Now the size of our Galaxy was determined from the Cepheids in the LMC, which actually belong to Population II. The LMC is not a spiral galaxy and contains little or no dust and gas. The size of the Andromeda Galaxy, on the other hand, was deduced from the study of Population I stars in the outer spiral arms. If the Cepheids of Population I stars behaved differently from the Cepheids of Population II stars, then this might explain the apparent smallness of other galaxies and resolve the discrepancy concerning the age of the Universe.

Baade made a careful study of the Cepheid Variables of both Pop.I and Pop.II stars and by 1952 was able to announce that Pop.I stars did not comply with the Period/Luminosity law of Henrietta Leavitt. By virtue of the fact that Pop.I Cepheids were over four times brighter than previously assumed, they were therefore twice as distant.

Overnight the Universe doubled in size. The distant galaxies became bigger and once again our bit of the Universe was relegated to the fourth division! The Andromeda Galaxy was now at least 50% bigger than our own and some 2.25 million light years away. Because the galaxies were further apart, the Universe was therefore much older too, so the conflict with the geological evidence on Earth was also resolved.

It is a sobering thought that all these discoveries were possible because the lights of Los Angeles were turned off!

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Last updated by Julie Atkinson   28-Jan-2018