People wonder why space is dark even though it is full of stars, as there are about 200 billion trillion stars swimming there. Some believe that the reason is that the stars are very far from Earth, but in fact this is not the main reason.
Bubble expansion and its effect on star illumination
Imagine if the universe was so old that light took a long time to reach us from distant stars, says Brian Jackson, associate professor of astronomy, Boise State University. In this imaginary scenario, all stars are completely stationary, so imagine a giant bubble containing Earth. At its center, if the bubble is about 10 light-years in diameter, it will contain approximately 12 stars, and the further away from the bubble we travel, the darker the stars will appear from the terrestrial viewpoint.
Jackson continued, “If the bubble continues to expand to 1,000 light-years, then to 1 million light-years, then to 1 billion light-years, the farthest stars in the bubble will appear dimmer. However, the amount of stars inside the larger and larger bubble will increase, and each will contribute.” In sky lighting, although distant stars will appear dim, there are many of them, making the night sky appear very bright.
The shining of stars in the universe
Jackson explained that through the imaginary bubble illustration, he imagined that the stars do not move and that the universe is very old – 13 billion years old, which seems like a long time but is actually a short time from the cosmic aspect. For example, it takes light from the farthest stars… It’s about 13 billion light-years away to reach Earth, meaning that the actual bubble containing all the stars we see only extends about 13 billion light-years from Earth.
He stressed that according to this, there are not enough stars in the bubble to fill all our line of sight, so when we look in some directions in the sky, we can see bright stars, but when we look in some other areas, we find ourselves in front of dark spots in which we cannot see any stars. This is because the stars in those dark spots are very far away, and their light has not yet reached Earth, and over time, the light from those distant stars will need time to reach us.
Doppler shift
You find some people asking, “Will the night sky eventually be completely illuminated?” Jackson replied, saying: The universe is expanding as galaxies far away from Earth move away at nearly the speed of light. As a result of this divergence, light from moving stars bleeds into wavelengths that the human eye cannot see. Seeing it, this phenomenon is known as “Doppler shift”.
He added that if the light had enough time to reach us, we could not see the light of distant stars with our eyes, so the night sky would not be completely illuminated, and if we waited longer, all the stars would eventually end up burning out, because like the sun, they have a limited lifespan of about 10 years. Billions of years.
Astronomers expect that in the distant future – after trillions of years – the universe will turn into complete darkness, and there will be only the remnants of stars such as white dwarfs and black holes. Although the night sky is not entirely bright with stars, we live in an exceptional period in the history of the universe. We have a rich and complex night sky, full of light and dark, and we are blessed with this wonderful view.
