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 during which the light overcomes them. In one year, light travels 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, is just one light second from us. To the nearest star to the sun - Proxima Centauri - four point twenty four hundredths of light years. Our Milky Way galaxy is one hundred thousand light-years across. To the nearest galaxy to us - Andromeda - two and a half million light-years. Space 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 picture. 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 work for astrophysicists?

For closely spaced objects, we use a method called trigonometric parallax. The meaning is pretty simple. Let's do an experiment. Stretch out your hand with a protruding thumb and close your left eye. Now open the left and close the right. It seems like the finger has shifted, while the objects in the background remained in place. With the stars the same thing. But the distance to them is much greater than the length of your hand. And the Earth is not so great. Even telescopes located at the equator opposite each other would not be able to determine the displacement of the star’s position. Therefore, we have been observing for six months, since during this time the Earth passes half its orbit around the Sun. Measuring the positions of stars in winter and summer is the same as observing an object with either the left or the right eye. Nearby stars appear displaced against the background of more distant stars and galaxies. But this method is only suitable for distances of not more than a few thousand light years. Objects outside our galaxy are so far away and parallax is so small that even our most sensitive technique is not able to fix it.
In this case, we rely on a different 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.

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 blurry. But, fortunately, we have another type of standard candle - the famous supernova type 'One A' (Eng. Ia). Supernovae - giant stellar flares - is one of the options for star death. These flares 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. Namely, Type A supernovae 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 fading rate, we can use these supernovae to determine distances up to several billion light-years.

And why is it so important to look at remote objects? Remember how fast the light moves. For example, the light emitted 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 the way she was eighty years ago. And those intricate galaxies are millions of light years from us. It took their light millions of years to reach us. Thus, the whole universe, in a sense, is itself a time machine. The farther we look, the more young the universe we see.

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 that is required of us is to decrypt it.