When you look up at the night sky, you may have wondered many times how far away the stars and galaxies are! Some stars are close, and others are very, very far! But how do scientists figure out their distance? Well, there are quite a few methods that the scientists use. Let’s explore them one by one!
1. Measuring Distance of Stars with Parallax
Take your thumb out and hold it in front of your eyes at a distance of about 15 cm away. Now, when you close one eye and then switch to the other, the thumb seems to jump, right? That’s called parallax! It is the apparent displacement or difference in the position of an object when viewed from two different lines of sight or vantage points.
Astronomers use this same idea to measure the distance to nearby stars.
They take pictures of a star from Earth at two different times of the year – six months apart. That way, the Earth is on opposite sides of the Sun. Because of this, the star looks like it has moved a tiny bit compared to very far away stars. This tiny shift helps scientists use math and angles to calculate how far away the star is. This method works best for stars that are not too far away.
2. Using Brightness to Measure Distance to Farther Stars
For stars that are too far for parallax, scientists use their brightness. Some stars, like Cepheid variables, blink in a special pattern. The time it takes for them to blink tells us how bright they really are. Scientists have studied lots of Cepheids, so we know how to match their blinking pattern with their true brightness.
If we know how bright a star truly is, and we compare it to how bright it looks from Earth, we can tell how far away it is. This method is like using a flashlight: the farther it is from you, the dimmer it looks!
3. Standard Candles
Stars like Cepheids and exploding stars called supernovae are known as standard candles. That means we know exactly how bright they should be. Supernovae, especially a kind called Type Ia, are huge star explosions. When they happen, they shine with about the same amount of brightness each time—like a cosmic flashlight that always turns on at the same power.
If we see a Cepheid or a Type Ia supernova in another galaxy, we can measure how bright it looks from Earth. Then, we compare that with how bright we know it really is. The difference tells us how far away it must be. Just like if you see a flashlight far away, it looks dim, but if you know how bright it really is, you can figure out how far it is from you.
So, by using these standard candles, we can measure the distance to faraway stars and galaxies—even millions or billions of light-years away!
4. Redshift – Measuring Distance to Far Away Galaxies
Galaxies that are very, very far away are moving away from us. This happens because the universe is expanding—like a balloon getting bigger. As galaxies move away, the light they send us gets stretched.
When light stretches, it shifts toward the red end of the light spectrum. This is called redshift. It’s kind of like how a siren sounds lower as it moves away—that’s a change in sound waves. Redshift is the same idea, but with light waves.
The more redshift we see, the faster the galaxy is moving away. And the faster it’s moving away, the farther it must be from us.
Astronomers measure the redshift of galaxies to figure out how far away they are. This method is very useful for galaxies that are billions of light-years away—much too far for other measuring tools!
So, redshift is like a cosmic ruler. The redder the light, the farther the galaxy!
The Cosmic Distance Ladder
The cosmic distance ladder is a set of methods that astronomers use to measure distances in space, from nearby stars to faraway galaxies. Each method works best at a different range, like steps on a ladder. For nearby stars, scientists use parallax, which measures a star’s slight movement as Earth orbits the Sun. For more distant stars, they use Cepheid variable stars, whose blinking pattern tells us their true brightness. Even farther, they use bright exploding stars called Type Ia supernovae. For the most distant galaxies, they measure redshift, which shows how fast a galaxy is moving away as the universe expands. Together, these steps help build a complete picture of the size and structure of the Universe.
Knowing how far stars and galaxies are helps scientists learn more about the size and age of the universe. It also helps us understand how stars are born, how they live, and how they die.
So next time you see the stars twinkling in the sky, remember – some of that light may have started its journey millions of years ago!