In the 1800s, scientists discovered the realm of light beyond what is visible. The 20th century saw dramatic improvements in observation technologies. Now we are probing distant planets, stars, galaxies and black holes where even light would take years to reach. So how we do that? Light is the fastest thing we know in the universe. It is so fast that we measure enormous distances by how long it takes for light to travel them. In one year, light travels about 6 trillion miles. It is the distance, we call one light year. The Apollo 11 had to travel four days to reach the moon but, it is one light second from earth. Meanwhile, the nearest star beyond our own sun is Proxima Centauri but, it is 4.24 light years away. Our Milky Way galaxy is on the order of 100,000 light years across. The nearest galaxy to our own, Andromeda is about 2.5 million light years away.
The question is how do we know the distance of these stars and galaxies? For objects that are very close by, we can use a concept called trigonometric parallax. When you place your thumb and close your left eye and then, open your left eye and close your right eye. It will look like your thumb has moved, while more distant objects have remained in place. This same concept applies in measuring distant stars. But they are much farther than the length of your arm, and earth is not large enough, even if you had different telescopes across the equator, you would not see much of a shift in position. So we look at the change in the star’s apparent location over six months, when we measure the relative positions of the stars in summer, and then again in winter, nearby stars seem to have moved against the background of the more distant stars and galaxies.
But this method only works for objects less than a few thousand light years away. So, for such distances, we use a different method using indicators called standard candles. Standard candles are objects whose intrinsic brightness, or luminosity that we know well. For example, if you know how bright your light bulb is, even when you move away from it, you can find the distance by comparing the amount of light you received to the intrinsic brightness. In astronomy, we consider this as a special type of star called a Cepheid variable. These stars will constantly contract and expand. Because of this, their brightness varies. We can calculate the luminosity by measuring the period of this cycle, with more luminous stars changing more slowly. By comparing the light that we received to the intrinsic brightness we can calculate the distance.
But we can only observe individual stars up to about 40 million light years away. So we have to use another type of standard candle called type 1a supernova. Supernovae are giant stellar explosions which is one of the ways that stars die. These explosions are so bright, that they outshine the galaxies where they occur. So we can use the type 1 a supernovae as standard candles. Because, intrinsically bright ones fade slower than fainter ones. With the understanding of brightness and decline rate, we can use the supernovae to probe distances up to several billions of light years away. But is the importance of seeing distant objects? Well, the light emitted by the sun will take eight minutes to reach us, which means that the light we see now is a picture of the sun eight minutes ago. And the galaxies are million light years away. It has taken millions of years for that light to reach us. So the universe is in some kind of an inbuilt time machine. The further we can look back, the younger we are probing. Astrophysicists try to read the history of the universe, and understand how and where we come from.
“Dream in light years, challenge miles, walk step by step” – William Shakespeare