Why does the sky become dark at night?

Today, we’ll try to answer a deceivingly simple question asked in the title of this article. Let’s talk in detail about the “dark sky paradox”!

Contents

You might be thinking: “What’s there to discuss? The sky is dark at night because the Sun is down!”. But think about the stars.

Our Milky Way galaxy alone contains 100 to 400 billion stars, and the observable Universe contains 100 to 200 billion galaxies. That’s a huge number of stars, to say the least! Add to this the assumption that our Universe might actually be infinite. Any small patch of the sky you choose to look at should contain myriads of stars, and so the night sky should be dazzlingly bright.

However, in reality, this is not the case. This discrepancy is called Olbers’ paradox, and we’re going to resolve it in a couple of minutes.

What is Olbers’ paradox in simple terms?

Formulation of the “dark sky paradox” is usually attributed to German astronomer Heinrich Wilhelm Olbers, though many other famous people (including Johannes Kepler and Edgar Allan Poe) also contributed to solving this problem. Here’s the paradox in simple terms:

If the Universe is infinite, and there’s an infinite number of stars in it, then the entire sky should be covered with stars. We should be able to see a star in any direction we look, and so the night sky should be brightly lit. Why is it dark then?

To understand Olbers’ paradox better, imagine yourself in the midst of a thick forest. Anywhere you look, you’d see a wall of trees with no gaps between them. And it’s only a forest, which can be big but is by no means infinite. Now try to extend this analogy to an infinite Universe filled with stars!

Possible (but wrong) solutions to Olbers’ paradox

Over centuries, there have been several attempts to explain the puzzling phenomenon of the dark night sky. Let’s consider some of these explanations and rule them out.

The stars in the Universe are distributed fractally, not uniformly

Why it’s wrong: This hypothesis could have explained the paradox, but modern astronomical data don’t support it. The Universe seems to be almost perfectly isotropic (i.e., the same in all directions).

The Universe has a finite number of stars

Why it’s wrong: Even if it’s finite, the number of stars in the Universe is still enormous and would be enough to light up the entire night sky.

We don’t see some stars because they are too far away and thus too faint

Why it’s wrong: Let’s divide the infinite Universe into even spherical layers with the Earth in the center. If one layer contains a certain amount of stars, then the layer positioned two times further away from the Earth would contain four times more stars due to the Universe’s homogeneity. However, according to the inverse-square law, the stars in the further layer would also shine four times dimmer when observed from the Earth. This means that the total luminosity of each of these layers would be the same. So every layer of stars would produce the same amount of light, no matter how far away it is. As a result, the sky would be uniformly lit.

Space is filled with interstellar dust that blocks the light from distant stars

Why it’s wrong: The starlight would inevitably heat up the dust. According to the law of conservation of energy, the dust would soon start to reradiate the absorbed light and shine as brightly as the stars.

What is the best solution to Olbers’ paradox?

There are two main factors that can explain the “dark sky paradox”. Due to both of them, we see empty spaces between the stars in the night sky.

  • 1. The Universe is not infinitely old

Our Universe is about 13.8 billion years old. Light takes time to travel, so we can only observe objects that are located 13.8 billion light-years away — not further. As the Universe is infinite in size, many stars and galaxies are invisible to us simply because their light has not reached us yet.

  • 2. The Universe is expanding

The expansion of the Universe was discovered by Edwin Hubble in 1929. He saw that the light from distant stars and galaxies “stretched out” as they were rapidly moving away from us. This phenomenon is known as “redshift”: it happens when light increases its wavelength and shifts towards the red end of the electromagnetic spectrum. The light waves from very distant objects get stretched so much that they become infrared. Human eyes can’t see infrared light, so very distant stars and galaxies become invisible to us.

Bottom line: Everywhere you look in the sky, there is a star or galaxy. You simply can’t see many of them because either their light hasn’t reached us yet or it has shifted into the infrared spectrum. That’s why the night sky appears dark to a human eye.

Space

The simplest answer to this question requires that you use your imagination to picture our local star – the sun – and its family of planets. The sun’s light pours outward to illuminate every portion of our solar system so that the space around the sun is almost entirely flooded with light.

But there are dark places. These are in the shadows of planets, moons and other objects in orbit around the sun. And it’s these shadows that create night. Earth’s shadow extends over a million kilometers into space. Our planet’s shadow is so long it can brush the face of the moon during a lunar eclipse. Lunar eclipses are relatively rare. But, every day, as Earth turns on its axis, the part of the planet you’re standing on turns for a time so that you face into Earth’s shadow. When you face into the shadow, it’s night. When Earth turns so that you again face the direction of the sun, it’s day.

On any clear evening, you can see the curved line of Earth’s shadow ascending in the eastern sky. It gets higher in the east as the sun sinks below the western horizon.

Posted 

October 9, 2009

 in 

Space

The EarthSky team has a blast bringing you daily updates on your cosmos and world. We love your photos and welcome your news tips. Earth, Space, Human World, Tonight.

Table of Contents

  • The Quest
  • The Expanding Universe
  • Suggested Reading

The night sky is dark because the Universe is expanding. Distant stars and galaxies move further and further away from us. Light might be the fastest thing in the Universe, but its velocity is still finite. Light emanating from stars situated a million light-years away will take a million years to reach us.

Every day, as the Sun rolls to the west, it pulls the blue blanket of silk wrapped around it along with it. As this blanket slides to the west, the east is steadily exposed to the bare darkness of outer space. Space may be dark, but it’s not empty. The night sky is mottled with countless stars, perhaps shining even brighter than our Sun, around whom revolve worlds unexplored.

Our galaxy alone houses around 200 billion stars and there are at least 100 billion galaxies in the observable Universe! If the sky is exposed every night to practically an infinite number of stars burning as bright as the Sun, why is the night sky so dark?

Recommended Video for you:

The Quest

The problem has bothered us since antiquity and was resolved less than a century ago.

Stephen Hawking reasoned why we make discoveries that make no difference to our daily lives whatsoever, by remarking that “Humanity’s deepest desire for knowledge is justification enough for continuing our quest. And our goal is nothing less than a complete description of the Universe we live in.”

Our “quest” began with Aristotle, who believed that all the stars, including the Sun, the moon and all the planets in our Solar System revolved around the Earth in concentric circles. However, the Greeks observed that many stars appeared to be fixed; they didn’t appear to move, regardless of the direction from which they were viewed. Ptolemy summarized this view in a model where, except for the thick, outer crust of fixed stars, every celestial body revolved around Earth.

What lay beyond the ‘fixed stars’ was never clear. This ambiguity, however, left room for hell and heaven.

However, although the model looked good on paper, it couldn’t reconcile the trajectories of planets it predicted with what was actually observed. The model that could reconcile prediction and observation was Copernicus’ model. He simply suggested that it was Earth and the planets that revolved around the Sun, not the other way around. Copernicus, a priest, to avoid being vilified as a heretic, published his ideas anonymously.

His model was confirmed when Galileo observed a troupe of moons revolving around Jupiter. However, to Kepler’s disappointment (OCD alert), the planets didn’t revolve around the Sun in perfect circles, but rather in slightly elongated circles or ellipses. Furthermore, his dismay worsened when he found that the force of attraction that binds the Sun and the planets isn’t, as he predicted, magnetic.

The force is, as we all now know, gravity. The theory of gravity was first laid down by Newton in his Principia Mathematica, possibly the greatest intellectual achievement in the history of physical sciences. Finally, Copernicus’ model replaced Ptolemy’s model completely, but Newton was still bothered by the inactivity of ‘fixed stars’. If gravity forced massive objects to attract, why weren’t they moving towards each other? In fact, if the stars were finite in number, shouldn’t they fall together at some point?

Newton argued that this wouldn’t be the case if there were truly an infinite number of stars distributed uniformly over infinite space. Now, because every star is surrounded by infinite stars, there is no center point to fall into. However, German philosopher Heinrich Olbers objected that if the Universe did consist of infinite stars, then every line-of-sight from the Earth’s surface should end up on the surface of one. This would cause the whole sky, during both night and day, to be as bright as the Sun. Olbers speculated that the matter between us and stars could absorb their light and prevent it from reaching us. However, so much heat and light would heat the intervening matter itself, causing it to turn incandescent. The brightness would rather be compounded. So, why isn’t this the case?

The answer to your and Olber’s question is as obvious as Copernicus’ model.

The Expanding Universe

Strangely, every astronomer who lived before the 20th century theorized a new model or modified older models essentially on the basis of a subtle yet tremendously crucial assumption – the Universe is static. Newton himself believed that the Universe had existed forever in an unchanging state; he believed in a static Universe whose inhabitants were just there since the beginning of time.

If there is one scenario where the night sky, exposed to an infinite number of stars, can’t be as bright as the Sun’s surface, it is the one in which the stars hadn’t been shining forever, but rather sprang into existence at some finite time in the past. In this scenario, the night sky is dominantly dark because either the light from newborn stars hasn’t reached us or the intervening matter isn’t sufficiently heated.

The Orion Nebula, one of our nearest star nurseries (Photo Credit: peresanz / Fotolia)

However, springing into existence compels us to ask what events caused this existence? And what events caused the very events that caused the stars to spring into existence? One can trace the ‘what caused’ questions to the very first cause. The Universe, in fact, does have a first cause. It’s called the Big Bang and it occurred some 13.7 billion years ago. Even more astonishing is the fact that, since its birth, the Universe hasn’t stopped growing.

Edwin Hubble, one of the most recognized astronomers in history, learned this in 1929 when he observed that regardless of where you look, distant galaxies are receding away from us at a tremendous velocity. He had made the revolutionary observation that the Universe is expanding! Subsequently, we calculated its age by tracing back the expansion to a single point, the point of infinite density that popped into existence abruptly and expanded into the voluminous Universe we observe today. Stephen Hawking believed this was “probably the most remarkable discovery of modern cosmology.”

The Big Bang model of the Universe. (Photo Credit: Wikipedia Commons)

As the Universe continues to expand, distant stars and galaxies move further and further away from us. Light might be the fastest thing in the Universe, but its velocity is still finite. Light emanating from stars situated a million light-years away will take a million years to reach us. What we observe therefore is not the star as it exists now, but how it existed a million years ago. Perhaps it might be dead, but we will only learn that after a million years.

Furthermore, the energy of light dwindles as the stars recede, just how the hoot of a receding train does. The wavelength of light blasted during the Big Bang has gradually stretched from gamma rays to microwaves. This finding led physicists to foster a presentiment that our quest might conclude grimly. The stars we observe are visible because they have neither receded so far as to emit wavelengths that precede the visible range nor do they shine bright enough to emit wavelengths that succeed it.

(Photo Credit: Van Gogh / Wikimedia Commons)

However, as the Universe continues to expand, the stars that this generation has studied will recede so far that the astronomy tools of the coming generations will be as useful as spectacles for the blind. They would exist in perpetual limbo. Currently, we live in the Goldilocks era of astronomy, if we wish to know why we’re here and where we came from, now seems to be the best time.

Suggested Reading

References

  1. NASA.gov
  2. Physics.org
  3. Library of Congress

Was this article helpful?