Space appears dark at night despite there being billions of stars and galaxies. This question has puzzled scientists and astronomers for centuries. While there are many stars out there, the universe is mostly empty space. In addition, light from distant stars has not reached Earth yet. However, with powerful telescopes, we can peer deeper into space and observe light from the earliest stars and galaxies. While parts of the night sky look dark to the naked eye, the universe is filled with light our eyes cannot see.
How much of the universe can we see?
The observable universe contains about 200 billion galaxies. Each galaxy contains billions of stars. So in total, there are about 1,000,000,000,000,000,000,000 (1 septillion) stars in the observable universe. That’s a huge number of stars! Yet, on a clear dark night, we can only see about 9,500 stars with the naked eye. Why can’t we see the rest?
The answer lies in the vast distances between us and the stars. The closest star to our solar system is Proxima Centauri, which is about 4.2 light years away. That means the light from Proxima Centauri takes 4.2 years to reach Earth. Most stars are much farther away. For example, the Andromeda galaxy is 2.5 million light years away. So the light we see left that galaxy 2.5 million years ago. The most distant galaxies we can see are over 13 billion light years away, meaning we see them as they were 13 billion years in the past.
Due to the finite speed of light, we can only see a small fraction of the universe. Scientists estimate we can observe about 93 billion light years in all directions. Beyond that distance, light has not had enough time to reach us since the Big Bang 13.8 billion years ago. The rest of the universe extends beyond what we can see.
Observable Universe Radius: 93 billion light years
So the observable universe forms a sphere with a radius of about 93 billion light years. As light from more distant stars and galaxies has not reached us yet, we cannot see them. This is why parts of the night sky appear dark – there are no stars or galaxies close enough in those patches to be visible to the naked eye.
Why is space mostly empty?
Another reason is that space is mostly empty. Although there are trillions of stars in our observable universe, they are scattered across unimaginably huge voids of space. To understand just how empty space is, let’s consider an example.
If the Sun was the size of a pea, the nearest star Proxima Centauri would be 350 miles away! On this scale, even our Milky Way galaxy would be the size of the continental United States. And galaxies are separated by millions of light years of empty space. This illustration shows that space is mostly void, with only occasional dots of stars and galaxies.
So a random line of sight is likely to pass through these huge voids and not encounter any stars or galaxies. Similar to looking out to sea and not seeing any ships for miles and miles. That is why much of the night sky appears dark in any given direction.
Density of Matter in the Universe
The overall density of ordinary matter in the universe is extremely low. On average, there is:
- 1 atom per cubic meter
- 4.5 protons per cubic meter
- 0.25 solar masses per cubic megaparsec
With matter so sparsely distributed, it is easy to see why space appears mostly dark and empty.
Brightness of stars
Not only are stars extremely far apart, most are also too dim to see with the naked eye. The intrinsic brightness of a star depends on its size, temperature and distance from us:
- Large, hot stars put out more light and can be seen from farther away.
- Small, cool stars emit less light and cannot be seen from as far.
- More distant stars appear fainter than closer ones, even if their luminosities are the same.
Our Sun is a medium-sized yellow star of moderate brightness. Let’s consider how far away other stars would need to be to appear as faint as the Sun:
|Distance for Same Brightness as Sun
|Small red dwarf
|16 light years
|Medium yellow star like Sun
|16 light years
|Large blue giant
|1600 light years
This table shows that except for the brightest blue giants, most stars would need to be within a few thousand light years to appear bright enough to see with the naked eye. Beyond that distance, they become too faint to detect without a telescope no matter how luminous they are intrinsically.
In summary, the vast majority of stars are simply too dim for their light to reach us visibly across intergalactic distances. So those faraway stars cannot contribute to the night sky brightness.
Light pollution from Earth
The proliferation of artificial lighting on Earth in modern times has made it harder for us to see faint stars. Light pollution washes out the night sky, especially for urban residents. Our ancestors had much darker skies free of this light pollution. Before the invention of electric lights in the 19th century, the only nighttime light came from dim candles and fires.
Today, our cities shine bright all through the night with millions of lights. Street lamps, neon signs, office buildings and car headlights all contribute to skyglow. This extra light scatters and reflects off dust and gas molecules in our atmosphere. The result is a pale orange haze that covers the night sky.
Light pollution has gotten so bad that nearly 80 percent of people in North America and Europe cannot see the Milky Way from where they live. Our excessive use of artificial lighting has obscured the stars for much of the world’s population.
The effects of light pollution include:
- Reduced visibility of stars, especially faint ones
- Milky Way cannot be seen from most cities and suburbs
- Night sky appears orange/gray instead of black
- Disorients nocturnal animals and affects ecosystems
- Prevents astronomical research and observation
Light pollution is yet another reason why the night sky appears dark, as stray light washes out the faint starlight.
Expansion of space
Not only is observable universe limited to how far light has travelled, but space itself is expanding at an accelerating rate. As space expands, it stretches out the wavelength of light travelling through it. This redshift effect causes the light from distant galaxies to shift toward the red end of the spectrum. The further away a galaxy is, the more its light is redshifted.
In fact, some galaxies are so far that their light has been stretched into infrared wavelengths invisible to human eyes. All we see is darkness in the parts of the sky those galaxies occupy. The ongoing expansion of space prevents a significant fraction of star and galaxy light from reaching Earth in visible wavelengths.
Blocking by dust and gas
Interstellar gas and dust absorb and scatter starlight travelling through space. These tiny particles include:
- Cosmic dust – Microscopic grains of silicates and ices
- Hydrogen gas – The most common element in space
- Dark nebulae – Dense molecular clouds
When light passes through these clouds, some is absorbed and converted to other forms of energy. Shorter wavelength blue light is more prone to scattering. The remaining light that gets through is reddened and diminished. This dust extinction limits our ability to see distant stars and galaxies located beyond dense clouds.
Opaque nebulae completely block light
In addition, there are extremely thick and dense regions known as opaque nebulae. These massive, dark clouds are composed mainly of hydrogen and helium gases. Stars and galaxies located behind them are completely obscured from our view. So opaque nebulae create black voids in space where no background light can penetrate.
In summary, there are several reasons why the night sky is predominantly dark:
- Observable universe is limited to 93 billion light years
- Space is mostly empty between stars and galaxies
- Stars rapidly decrease in brightness over large distances
- Light pollution washes out faint starlight
- Expansion of space redshifts and dims light
- Interstellar gas and dust absorb or block light
While there are trillions of stars in the universe, they are separated by vast tracts of empty space. Their light is too faint and diffuse for most distant stars to be visible. Nearby stars can only illuminate a tiny fraction of the night sky. These factors combined result in the cosmic dark void that surrounds our small island of starlight in this galaxy.
Yet, what we can see is illuminating. The starry sky inspired our ancestors for millennia. And astronomers continue using ever more powerful telescopes to peel back the darkness bit by bit, discovering new wonders hidden by the night.