Astrophoto: NGC 3324 by Brad Moore

If the universe extends forever and if it’s full of stars, why is the night sky dark? This is a question that has been asked by philosophers and scientists since Antiquity. Johannes Kepler sought an answer, as did Edmond Haley, many years after him. Just as an observer sees trees in all directions when standing in a forest, every line of sight in an infinite universe should end with the twinkling of a star. The net result should be a sky ablaze with heavenly light. Not only should the night sky be as bright, if not brighter, than during the day but the heat from all those suns should be sufficient to boil the Earth’s oceans away! Therefore, the starry scene depicted in the striking picture that accompanies this article, should appear to be missing stars when compared to gazing into the Cosmos above.

Edgar Allen Poe mused about this puzzle in his 1850 work entitled “The Power of Words”. He referred to combined illumination radiated by celestial light as the “golden walls of the Universe”. For example, an observer in a forest sees a screen of trees because the forest continues farther than it’s background limit- the average distance at which the line of sight is interrupted by a tree. Similarly, from any point in an endless Universe filled with stars, stars that are close should overlap stars that are farther away until every square inch of the view is filled with the light from a distant Sun.

Current estimates place the number of stars in the Universe at 70 sextillion (70,000 million million million), based on a 2003 survey completed by Australian astronomers. That’s ten times the number of sand grains on all the Earth’s beaches and deserts combined and certainly more than enough to fill the entire sky with starlight!

But, the night sky is not awash in the light of the Universe, so early theorists speculated that either stars were limited in number or their light somehow failed to reach the Earth. When interstellar dust was discovered, some thought the reason had been found. But, calculations quickly indicated that if the dust particles absorbed all of the missing starlight then the dust particles would, themselves, start to glow.

The answer was finally explained by implications from Albert Einstein’s Theory of Relativity.

Somewhere between ten and twenty billion years ago, the Universe was formed by an event called the Big Bang. Why it occurred and what preceded it remain the deepest mysteries but that it occurred now seems fairly irrefutable to most in the scientific community. All the matter and energy- essentially everything that ever was, is or can be- was confined to a concentrated, unimaginably dense state. Interestingly, it was not as if everything in the Universe was squeezed into some location surrounded by a space filled with nothing. In fact, it was the Universe- all the matter, energy and all the space that they fill. It’s external size was unimportant since it had no exterior surface; nothing existed outside of it- this is still true today.

Then, for reasons that are still being debated, this kernel of the Universe started to expand at an extremely rapid rate as if it had experienced an explosion.This expansion has never ceased, in fact, it’s rate has increased over time! More to the point of our discussion is the fact that the Universe began at a finite moment in time.

One other implication of relativity theory helps to explain our dark night skies, too. Light travels at a finite speed. However, it moves so fast that it’s speed is expressed in the distance it travels during one year. This is known as a light year and during that time, light will traverse 9.46 trillion (9.46 × 10¹²) kilometers or 5.88 trillion (5.88 × 10¹²) miles.

Space and time are intertwined. We cannot look out into space without also looking backwards in time. Space is vast and the separation between stars is enormous. For example, the average distance between stars is a few light years. But, this is close compared to other lengths measured by astronomy. The distance from our Sun to the center of our Galaxy is about 26,000 light years or 260 trillion kilometers! The distance from our Galaxy, the Milky Way, to the next closest galaxy, located in the constellation of Andromeda, is over 2 million light years. That means the light we see tonight from the Great Andromeda Galaxy (M31) left for Earth when there were no modern human beings, or Homo Sapiens, on this planet- although our evolutionary lineage was well established. The distance from Earth to the most distant object, a galaxy spotted by the Hubble space telescope, is about thirteen billion light years. We see this galaxy as it looked before our galaxy was formed!

So, the reason our night skies are black, the reason space is not filled with blinding light is because much of the light from stars that fill the sky has not had time to reach the Earth- many are so far away that they are simply undetectable at this time. Thus, even though the number of stars is essentially infinite the number of stars we can see is finite and this creates dark gaps in the sky that we see as the vastness of space.

There are also a few other factors that cause space to appear un-illuminated. For example, many stars die out or explode over time and this removes their contribution to the amount of light within the Universe. Additionally, starlight is reduced by red shifting– a phenomenon that is directly related to the Universe’s expansion. Red shifting is similar to the Doppler effect because both involve the stretching of light waves.

The Doppler effect describes the motion of a light source relative to an observer. Light from an object moving toward an observer becomes compressed toward higher frequencies, or the blue end of the light spectrum. Light from an object that is moving away becomes stretched toward the lower frequencies or red end.

Red shifting has nothing to do with the motion of a light source but, rather, with the distance a light source is located from the observer. Since space is expanding in all directions, light from a very distant source travels an ever-increasing distance and the widening distance, itself, stretches its light wavelengths toward the red. The more distant a galaxy, the longer the path its light must travel to reach Earth. Because the distance between the galaxy and Earth is also constantly increasing, its light is stretched toward the red end of the spectrum. Light from very far away galaxies can thus be red-shifted out of the visible spectrum into the infrared or, beyond that, into the realm of radio waves. Therefore, red shifting also reduces the extent of visible starlight that reaches Earth and makes the night sky appear darker.

The picture featured with this discussion was produced by astronomer Brad Moore, from his private observatory near Melbourne, Australia earlier this year. This scene is situated near the Great Carinae Nebula and is known as NGC 3324. It also has a common name of the Keyhole Nebula and both it and the Eta Carinae Nebula are located about 9,000 light years from Earth in the southern constellation of Carina. It is comprised of a young, bright cluster of stars, some of which are illuminating the surrounding, hydrogen rich nebula and causing it to glow.

Interestingly, this is also called the Gabriela Mistral Nebula because of it’s uncanny likeness to the Nobel Prize winning Chilean poet. Look closely and you can see her silhouette in the nebula.

The hues in this stupendous image are not real, however. They have been assigned to also represent the composition of the material that comprises this view. Oxygen is represented by red, green indicates the presence of hydrogen and sulfur is depicted by a blue hue. This picture required a 36 hour exposure through a 12.5 inch Ritchey-Chretien Cassegrain telescope and a 3.5 mega-pixel astronomical camera.

Do you have photos you’d like to share? Post them to the Universe Today astrophotography forum or email them, and we might feature one in Universe Today.

Written by R. Jay GaBany