Where are the stars, daddy? My astronomy book says the night sky is full of stars, but the sky looks dark. Why daddy?

 To the inhabitants of densely populated cities, urban, suburban areas, and even the outskirts, the night sky might not seem to be of much particular interest. The palette of heavenly colours that inspired Vincent van Gogh to paint The Starry Night has been washed away factitiously, leaving behind the moon, the four kindred planets - Mars, Venus, Jupiter and Saturn, and some of the brightest stars, viz., Sirius, Arcturus, Antares, Rigel, Betelgeuse, Vega, plus some others with comparable limiting magnitude. Any absence of these lonely sentinels makes the night sky appear as a dark, gloomy, if not vacant, and downright depressing canvas. 

On the very first day of our kindergarten school, we are taught to recite a very famous lullaby: 

                   Twinkle, Twinkle, Little Star

                   How I wonder, what you are...

In the subsequent stanzas, the poem does a great job of telling us all we need to know about the stars, how they shine like bright beacons even if they lie far beyond our mortal reach. Unknowingly, we are also made aware of the fact that the brightest of the stars (which we later come to know as the Pole Star/North Star) guides the lost traveler to the safety and comfort of his home. As we grow up to attend high school and college, we learn that our little, twinkling stars, are actually suns, albeit, giant nuclear reactors transmuting millions of tons of hydrogen into helium per second, and therefore, liberating the resultant energy in all bands of the electromagnetic spectrum, including visible light. This starlight can travel unabated for billions of light-years through space and time, piercing the abysmal darkness of the universe. At first, confounded with the knowledge of the stupendously large number of planets, stars and galaxies that makes our universe whatever it is, including the Milky Way Galaxy, our galactic address, it is natural to turn our gaze upon the night sky, with a sunken heart, for we feel hopelessly puny in comparison to the grand scheme of things. 

On the premise of his The Starry Night, Vincent van Gogh once said, ''I have... a terrible need... shall I say the word?.. of religion. Then I go out at night and paint the stars".
Image Credits: Public Domain, via Wikimedia Commons

Unfortunately, in our fast-paced society, the starry night sky has become an exceedingly rare sight. Right now, millions of children across the world are being deprived of the chance to see the Milky Way and its stars. But, a hundred years ago, our great grandfathers have had the opportunity to instil in their kids the natural wonder while they stood in awe, gazing upon the Milky Way Galaxy as a great arch across the firmament. For those who have not yet seen the galaxy, it appears as a thick white band with its bright central region obscured behind the clouds of interstellar gas and dirt. Every single white dot in the background is a star belonging to our home galaxy. As such, an estimated number of 7,000-10,000 stars are supposed to be visible from the remotest and the secluded locations on our planet, and 2,500-3,000 stars from virtually any corner of our planet, if not the least. However, since the 1950s, and particularly after the 1980s, the stars have been disappearing from the night sky. But why? What is happening to the stars that for time immemorial have stood like bright beacons, breathing life onto our ancestors and theirs? 

  The Milky Way arches over ESO's 3.6 metre observatory at La Silla mountain peak, in new Mexico.
Image Credits: ESO/S. Bruiner, CC BY 4.0, via Wikimedia Commons

The answer is light pollution. Among all of humanity's, monumental losses, loss of clean air, fresh water, fertile soil, and chirping birds, loss of the crystal clear, star-filled night sky stands as an epitome of our hedonistic ways of life. It so happened that during the second half of the past century, when we went on a frenzy of procuring the brightest of the brightest light, brighter than the Sun and the Moon, we deliberately turned a blind eye to the environmental cost we might have to pay at a later time. But as always, a lack of knowledge is a greater threat than sheer ignorance, for none of us could ever imagine in their wildest dreams that the electric light bulb, one of the most transformative and transcendental inventions other than the invention of the controlled use of fire, the wheel and language, could have a dark side. Like all familiar kinds of pollution and pollutants, light pollution too is a real deal. As per the standard textbook definition, a pollutant is a foreign substance of anthropogenic origins (i.e., made by humans) and, when introduced into the natural environment can cause adverse changes and modifications. In this way, electric lights too can affect the natural environment, because for every unit of electricity we put into a typical light bulb (say an incandescent or an LED), and for every hour it stays on, we are taking away something, best described as the natural darkness of the night. 

By natural darkness of the night, we do not mean a pitch-black night, with no illumination whatsoever, and everything around us is blacker than the blackest paint. On the contrary, it is quite the opposite. We all know that the Sun is our greatest source of natural illumination, and that is why sunlight is often referred to as natural light. After sundown, as the long blue twilight blends into the black night, nature's luminaries such as the Moon at its different phase following the lunar cycle, the aforementioned planets, the white band of the Milky Way, and all of its stars, even the neighbouring Andromeda Galaxy, the zodiacal light, the gegenschein and the airglow resulting from the scattering of sunlight by interplanetary gas and dirt; and the occasional comets, meteors, once in a while supernovae, and finally, the aurorae light up the night sky in all shades of red, green, blue, yellow, orange and white. Apart from the celestial luminaries, terrestrial sources of natural illumination include the will-o-the-wisp, bioluminescent animals and fungi, forest fires and on a far broader perspective, atmospheric lighting during a storm also fall under the umbrella term. 

Just like Stephen Hawking said, 'Look up at the stars and not at your feet. Have holy curiosity'
Image Credits: Photo by pixnio.com

Nearly a hundred years ago, genius inventors like Edison and Tesla dreamt of free electricity and dedicated their lives to have it available to all. Thankfully, we have lived up to their dreams. We are continuing their legacy. Right now, electric lights are so cheap and easily available that every time we feel we need more lights, we can readily go out and buy another one, and then another one and ad infinitum. But the irony is in the fact that no matter how many lights we buy, we will always feel that it is not enough. It is our inherent human nature to be afraid of the dark. For millions of years, we have sought for means to vanquish our fear of darkness and be on the top of the food chain instead of living at the mercy of nature and Her elements. That is why we needed fire in the first place; our first means of artificial illumination. By the end of the twentieth century, we had at our disposal, all sorts of artificial luminaries, viz., discharge lamps, metal halide, led, laser, incandescent, fluorescent, and others. However, it is not the number that matters, but its efficient, optimum use. We must bear in mind that our eyes fail to detect contrast in presence of blinding lights and we are left with a feeling that the lights are not enough. As a result, we use far in excess than what is actually necessary. This excessive use of electric lights is the sole reason behind the loss of the stars. 

Half is normal, half is not. The Orion constellation as photographed under truly dark sky (left) and light polluted sky (right). Which is more preferable? 
Image Credits: Jeremy Stanley, CC BY 2.0. Image uploaded from flickr.com

As per the International Dark Sky Association, light pollution is the inefficient use of outdoor lighting or the excessive use/unwanted presence of illumination in places supposed to remain naturally dark after sundown. Although light pollution can happen both during the day and the night, it runs unnoticed unless and until the Sun goes down, and instead of the multitudinous stars, which we speak of, all we get to see is an empty sky. The dark side of the electric light is that it brightens up the night sky and destroys the natural contrast of the black backdrop of space needed to discern the faint dots of starlight. The different types of light pollution are classified as follows: 

1) Sky Glow: Sky glow is the most noticeable aspect of light pollution. It is the diffuse illumination of the night sky caused due to the scattering of light via the suspended aerosols in the air. Due to the intrinsic property of every surfaces to reflect light, let it be asphalt roads or a typical building's exterior wall, the light projected onto them gets reflected at varying angles (diffuse reflection) and goes towards the skies. Being reflected by the clouds in the lower atmosphere back towards the surface, this light causes an artificial glow and decreases the natural contrast of the night sky. Consequently, the stars become invisible as they fail to penetrate this veil of artificial illumination that surrounds every major city like a dome. 

A schematic diagram showing the different categories of light pollution
Image Credits: Author's Computer 

                 Apart from this diffuse reflection, sky glow also happens due to the improper installation of the luminaries. Most of the time, the lights are positioned in such a way that a major section of the resulting illumination, instead of falling onto its intended place, exists the lamp-head (as in the above pic) at shallow angles. This escaping light or waste light can be seen for miles away from a typical city (as shown below).

A typical city at night. Too much lights and too bright for the stars to be visible 
Image Credits: Photo by pixabay.com

2) Light Trespass/Intrusion: Apart from sky glow, another major contributor to light pollution is the effect of light trespass/intrusion, which is more of a social problem, but it contributes directly to the sky glow mentioned above. This happens when a typical light source illuminates regions beyond its intended place. It could also be understood as the spillage of light beyond its intended boundaries. For example, the job of a streetlight is to illuminate the region directly under the lamp head. But if for some reason, the same light creeps into the bedroom of the neighbouring buildings then we call it light trespass/intrusion. Similarly, when the light installed in parking lots goes beyond to illuminate the streets, or if the floodlights of a nearby football field is visible from our bedrooms, it is light trespass.

3) Glare: Sometimes it happens that the light we are using is too bright or installed at such an angle that it can distract/dazzle a person a mile up ahead. This is called glare and can be classified into three distinct categories. Glare is best described as the uncomfortable light that shines unto our eyes from an approaching car headlight causing temporary vision deficiency and loss of contrast. 

4) Over Illumination: As the name suggests, over illumination is the excessive use of lights, which serves no definite purpose. Some examples include keeping the lights on at monuments and statues during the dead of the night, illuminating bridges and etc. Over illumination results in light clutter, i.e., an excessive grouping of lights. 

A map of light pollution. Our cities are becoming so bright that they are easily recognisable from space 
Image Credits: David Lorenz, Attribution, via Wikimedia Commons

From and onwards the 1970s, a growing number of astronomers from around the world became aware of the detrimental effects of light pollution and the looming threat upon their careers. The oldest of the natural sciences, astronomy is all about observing the stars and constructing theories based on the observations using the most complicated optical instruments and telescopes. For this, astronomers require a crystal clear, crispy view of the heavens. But due to light pollution, the stars are slowly disappearing from the night sky. As regions across the world are racing towards a hundred percent illumination, it is feared that the ensuing pollution would be so great that land-based terrestrial astronomy would become a thing of the past. As of now, an estimated 83% of the global population lives under severely light polluted skies, while 25% of the total land area is affected by skyglow. 

It should be known that light pollution is not a single problem of the astronomers, for hundreds of convocations on the matter have revealed that light pollution is a serious threat to our well-being. Apart from the disappearing stars, other adverse effects of light pollution include depression resulting from sleep deprivation, insomnia due to trespassing lights, and increased risk of cancer (more research is being carried out on this subject). Excessive lights can also distract plants and animals, of which birds and insects are affected the most, for they fail to differentiate between night and day because of the blazing horizon from the city lights. All in all, light pollution is a serious menace. And it is high time we did something to curb its inefficient use.

Sources: 

1. Bogard, Paul. The End Of Night: Searching For Natural Darkness In An Age Of Artificial Light. New York, Boston, London. Little, Brown And Company, 2013. 
2. Mizon, Bob. Light Pollution: Responses and Remedies. New York, Springer, 2012
3. Blinded By Light. British Astronomical Association. Campaign For Dark Skies(CfDS). www.britastro.org/dark-skies. 2009.
4. Light Pollution: The Global View(2003). Astrophysics and Space Science Library, 284.
5.  https://theconversation.com/darkness-is-disappearing-and-thats-bad-news-for-astronomy-51989