Light seconds, years and centuries: how huge distances are measured

Light is the fastest object known to us. Its speed is so great that we measure large distances, expressing them through the time over which the light overcomes them. In one year, the light passes about nine and a half trillion kilometers, or, in other words, one light year. For comparison: the moon, to which the astronauts of the Apollo mission flew for four days, just one light second from us. The nearest star to the sun, Proxima Centauri, is four full twenty-four hundredths of light years. Our Milky Way galaxy is one hundred thousand light-years across. The nearest galaxy, Andromeda, is two and a half million light years away. Cosmos is unimaginably huge.

But wait, how do we even know the distances to stars and galaxies? Looking at the sky, we see a flat two-dimensional image. If you point your finger at one of the stars, you will not be able to determine how far it is. So how did this happen to astrophysicists?

For closely spaced objects, we use a method called trigonometric parallax. The meaning is pretty simple. Let's do an experiment. Extend your arm with your thumb sticking out and close your left eye. Now open the left and close the right. It seems as if the finger has shifted, while the objects in the background have remained in place. With the stars is the same. But the distance to them is much more than the length of your hand. And the Earth is not that big. Even telescopes located opposite each other at the equator could not determine the displacement of the star's position. Therefore, we are observing for six months, since during this time the Earth passes half of its orbit around the Sun. Measuring the positions of stars in the winter and summer is the same as observing an object with either the left or the right eye. Near stars look shifted against the background of more distant stars and galaxies. But this method is only suitable for distances of no more than several thousand light years. Objects outside our galaxy are so far away and the parallax is so small that even our most sensitive technology is not able to fix it.
In this case, we rely on another method, using landmarks called "standard candles". Standard candles are objects whose constant brightness, or luminosity, we know well. For example, if you know the luminosity of a light bulb and ask your friend to take it and move away from you, the amount of light you receive will decrease in proportion to the square of the distance. Thus, comparing the amount of light received with the constant brightness of the light bulb, you can determine the distance to your friend. In astronomy, the role of light bulbs is played by stars of a special type, called Cepheids. They are internally unstable, and look like a constantly inflating and deflating ball. And since the pulsations change their brightness, we can calculate their luminosity by measuring this cycle. Moreover, the brighter the star, the longer the cycle. Comparing the observed light of these stars with their luminosity, obtained by means of the mentioned measurements, we can determine the distance to them.

This, unfortunately, is not the end of the story. The maximum distance at which we can still distinguish individual stars is about forty million light years. After that they become too blurred. But, fortunately, we have another type of standard candle - the famous 'One A' supernovae (eng. Ia). Supernovae - giant star flashes - this is one of the options for the death of stars. These flashes are so bright that they overshadow the galaxy in which they occur. So even if we cannot distinguish individual stars in a certain galaxy, we are still able to see a supernova explosion. It is the One A supernovae that are suitable for the role of standard candles, because brighter supernovae decay more slowly than less bright ones.

Due to the relationship between brightness and extinction rate, we can use these supernovae to determine distances up to several billion light years.

Why is it so important to look at distant objects? Remember how fast the light moves. For example, the light radiated by the sun reaches the earth in eight minutes, which means that we see the sun as it was eight minutes ago. When you look at the Big Dipper, you see her as she was eighty years ago. And those difficult to distinguish galaxies are millions of light years away. It took their light millions of years to reach us. Thus, the whole universe is itself in a sense a time machine. The further we look, the younger we see the universe.

Astrophysicists are trying to read the history of the universe and understand how and where we came from. The universe constantly sends us information in the form of light. All we have to do is decipher it.