When the star is 5,000 lightyears from Earth.
A lightyear is a measure of distance. It's how far light travels in a year at about 186,000 miles per second. That is, about 5.87 trillion miles.
The star nearest our Solar System, Proxima Centauri, is about four lightyears away. The light traveling from Proxima Centauri to Earth takes about four years to get here. Thus, the light is four years old when we see it.
Another nearby star is Barnard's Star, which is six lightyears from Earth. It's light takes six years to get here, so that light is six years old when we see it.
The star Polaris, which we refer to as the North Star or North Pole Star, is 680 lightyears away. The light takes 680 years to travel to Earth, so it is 680 years old when we see it.
Hubble Space Telescope recorded this image in 1995 of the star forming region of the Universe known as the Lagoon Nebula (M8). It is some 5,000 lightyears from Earth. The explosive energy from starbirth in and around the nebula bombards the dusty clouds of cool gas producing the intense starlight seen from Earth. The light that reaches Earth is 5,000 years old. That's one of the hot young stars at lower right.
Most of the stars we see with the naked eye at night are within a few hundred lightyears, although a handful are as far away as 2,000 lightyears.
We don't see stars 5,000 lightyears away with the naked eye.
Viewing the light from stars more than about 2,000 lightyears away requires the use of a telescope.
The most distant objects seen from Earth are quasars, Telescopes are required to view them. They are 10-15 billion lightyears away. Their light takes 10-15 billion years to reach Earth, so their light is 10-15 billion years old.
An example of an object 5,000 lightyears away from Earth is the Lagoon Nebula, also known as M8. A nebula is a cloud of dust and gas in space, usually illuminated by one or more stars. Nebulae represent the raw material of which stars are made.
So when we look at the night sky we are looking at the past. Let's say one of the stars we see in the sky went supernova and and is not there no more and let's say this supernova was 'small' and could've of been seen only if you were literally next to the star. Let's say this star is about 4ly away. So in 4 years we will not see the star anymore right?
All light we see is from the past. The light from a light bulb at 3 meter distance arrives 10 ns after it left the bulb in your eye. For short distances this delay is negligible (10 ns is 10 billionths of a second), but at astronomical scale it becomes significant. Light from the Sun takes 8 minutes and 20 seconds to reach the Earth, so when we see the Sun it's the Sun like it was 8 minutes ago. If the Sun would suddenly die we wouldn't notice for 8 minutes.
The same goes for other stars in our Galaxy. The light from a star at 4 light years takes 4 years to reach us; it's the definition of a light year.
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