The ability to see stars from billions of years ago is a fascinating concept that involves the vastness of the universe, the finite speed of light, and the expansion of space. Here's an explanation:
When we look at distant stars, we are essentially looking back in time. Light travels at a finite speed, approximately 299,792 kilometers per second (or about 186,282 miles per second) in a vacuum. This means that when we observe an object in space, we see it as it was when the light left that object and started its journey towards us.
Now, considering the vast distances involved in the universe, the light from very distant objects takes a long time to reach us. For instance, if a star is located, say, 10 billion light-years away, it means the light we see from that star today actually left the star 10 billion years ago. Therefore, we are effectively observing the star as it appeared 10 billion years ago.
The expansion of the universe does not prevent us from seeing these ancient stars. While the universe is indeed expanding, it is important to note that this expansion primarily affects the large-scale structure of space itself. The expansion stretches the space between galaxies and clusters of galaxies, causing them to move apart over time. However, on smaller scales, such as within our Milky Way galaxy, gravity is stronger than the force of expansion. Thus, the expansion of the universe doesn't significantly affect the local environment of stars within galaxies.
It is worth mentioning that the expansion of the universe does cause a redshift in the light from distant objects. This means that the wavelength of the light is stretched as the universe expands, shifting it towards the red end of the electromagnetic spectrum. By analyzing the redshift of light, scientists can determine how much the universe has expanded since the light was emitted.
In summary, when we observe stars that are billions of light-years away, we are seeing them as they appeared billions of years ago because light takes time to travel across vast distances. The expansion of the universe does not hinder our ability to see these ancient stars, as it primarily affects the large-scale structure of space.