If you look up at the sky, how far are you really seeing? People have wondered this since ancient times. It turns out the farthest object within our solar system that we can see with the naked eye is the planet Saturn which is 1.2 billion kilometers away (746 million miles). The movement of this planet was well charted by astronomers and astrologers long before the advent of telescopes.
But Saturn is only visible because it reflects the Sun’s light. The stars we see at night are much farther away but are still visible because they emit far more light. But as we step beyond the solar system, the distances become so vast that its no longer practical to use kilometers or miles. We have to look at light years. Nothing in the universe can travel faster than light which moves at about 300,000km/s. That means in 1 year, a photon (light particle) travels about 9,461,000,000,000 kms.
To give you an idea about extra-solar distances, the closest star outside our solar system is Proxima Centauri which is 4.243 light years away. For comparison, the Sun is only 499 light seconds (0.00001581 light years) away. This means the light we get from the Sun was actually emitted 499 seconds ago. We only know how the Sun was 499 seconds ago. If for some reason, a new sun spot emerged or something happened inside the Sun, we would only find out 499 seconds later. For Proxima Centauri, we see it as it was 4.243 years ago because that is how old it’s light is when it reaches us.
Deneb is the farthest bright star in the sky. It is 1500 light years away. Think about that, we are seeing the star as it was one and a half millennia ago. Or think of it the other way around. Say if an alien astronomer at Deneb could focus on Earth, it would see the Byzantine Empire in its glory days.
But that’s not all, at the end of their lives, stars can explode and release much more energy and light than they did during their lives. These are supernovae. In 1006, the brightest supernova in recorded history happened and could be seen even during the day. This was from a star 7,200 light years away.
As we move outside our galaxy, we come to the most distant object we can see with the naked eye, Andromeda which is another galaxy 2.6 million light years away.
But what if you invest in a telescope? Well, an 8 inch telescope can show you things that are as far away as 2 billion light years. Imagine that, you are not only peering into space, but also into a distant time and seeing the universe as it was 2 billion years ago.
It should be clear that as we look at objects farther away, we are also looking further back in time.
Hubble Space Telescope
The universe is believed to be about 13.8 billion years old. When the Hubble Space Telescope was launched, it allowed us to see galaxies which were made within a billion years of the universe coming into being. By taking prolonged exposures, Hubble can gather light for long periods of time enabling it to see objects otherwise too dim to be visible. In September 2012, NASA released an image that took 23 days of exposure (spread over 10 years) to make. This contained about 5,500 galaxies, the oldest of which was 13.2 billion years old. Here is the Hubble eXtreme Deep Field (HXDF) image.
But this is from an extremely small patch of the sky. Here is an image showing you how big a patch of the sky the HXDF represents by comparing it with the moon.
HXDF vs. Moon in night sky
There is one final point to be made here. The universe is expanding at an ever increasing rate. So while we see light that was emitted 13.8 billion years ago, the object that emitted the light has moved much further away during that time. In fact, it is estimated that the objects whose light we are getting from 13.8 billion years ago are now 45 billion light years away!