Final answer:
Observing stars through a telescope is like looking into the past because we're seeing the light that was emitted long ago, due to the finite speed of light and the universe's vast size. Modern telescopes allow us to see back almost to the Big Bang, providing insights into the universe's evolution.
Step-by-step explanation:
When people say they are observing the past while looking at stars through a telescope, they refer to the fact that the light we see now was emitted by the stars a long time ago. Due to the finite speed of light and the vast size of the universe, light from distant stars and galaxies takes a significant amount of time to travel across the cosmos and reach Earth. As a result, when we observe these celestial objects, we are actually seeing them as they were in the past, not as they are in the 'present' moment. For example, if a star is one million light-years away, the light we see from it tonight is actually one million years old, and that star could've drastically changed or even ceased to exist in the time it took for its light to travel to us. Modern telescopes and sensitive detectors make it possible to collect this ancient light from faraway galaxies, giving us insights into the formation and evolution of the universe. By looking at objects billions of light-years away, astronomers are essentially looking back in time, reconstructing the history of the cosmos and learning about its development from nearly the time of the Big Bang. Therefore, when we use a telescope, we're not only exploring space but also time, making these instruments akin to a cosmic time machine.