Measuring the distances from the Earth to the stars is a completely different story from the measurements we are used to seeing. To measure distances up to hundreds, millions or even billions of light-years, astronomers must resort to more special, more “supercosmic” methods. Here’s why conventional instruments are useless and how astronomers calculate the distances to distant stars.
Conventional rangefinders are useless against extrasolar bodies
On Earth, radar can measure distances of hundreds of delicious kilometers. It can send a wave at the speed of light and then “timer”, wait until the wave hits the object and then bounce back to the receiver. It then only needs to measure the time it takes for the wave to travel back and forth to deduce the object’s distance. However, for more distant objects such as those located outside the solar system, radar is useless.
Take Proxima Centauri, for example, a red dwarf star located approximately 4.2 light-years from the Solar System. Radar waves will take up to 8.4 years from Earth to get there and back. And by the time the radar wave returns, it will have lost all its energy and there is nothing left to collect to measure the distance. Therefore, for objects outside the solar system like other stars, astronomers have to use the method of stellar parallax (Stellar Parallax).
Stellar Parallax is a method of measuring the direct distance from the earth to celestial bodies near the solar system.
If you hold up a finger and look at it with each eye, you will see it shift relative to the scene behind. This phenomenon of objects changing positions relative to each other through different points of view is called parallax. As the Earth revolves around the sun, the angle of view from the Earth to the stars changes. The stars that are near will move a lot against the background of the starry sky (which is very far away), and those that are far away will move less. By measuring the angle of displacement of a star relative to the background of the starry sky behind, and knowing the distance the Earth has moved between two “photographs” of that star, astronomers will infer the distance from the Earth to that star, by the trigonometry we were taught in high school.
For example, we have a triangle created between the star (temporarily called A), the position of the Earth in December (temporarily called B) and the position of the Earth in June (temporarily called C). We know the distance between B and C. Now just measure angle B and angle C to infer the length of sides AB and AC (ie the distance from the Earth to the star).
Stellar Parallax makes it possible for astronomers to directly measure the distances from Earth to celestial bodies near the solar system. This method can work effectively with objects a few hundred light-years from Earth. However, the more distant the star, the greater the error, so the measured distance will no longer be accurate for other stars or objects that are too far away.
Although parallax is no longer accurate at great distances, it is still the first step for astronomers to measure distances to objects even further away, using other methods of distance estimation.
Other methods of estimating stellar distances beyond the reach of stellar parallax are also based on stellar parallax
Based on the “beats” of the star Cepheid
A common example would be that astronomers would rely on Cepheid variable stars. They flashed in rhythm, as steady as a heart. The luminosity of this type of star is closely related to its “pulsation”. While the “beat” reveals its true luminosity, comparing that true luminosity with that measured from Earth (apparent luminosity) will reveal how far away it lies.
To make this technique accurate, however, astronomers must first use the parallax method to measure the distances of nearby Cepheid stars. Thereby correcting the table comparing the true brightness of Cepheid stars. Because Cepheids are exceptionally bright stars, they can be seen in galaxies tens of millions of light-years away.
Based on the speed of the supernova explosion
To estimate the distance between Earth and further galaxies, astronomers can rely on supernova explosions. These are extremely powerful explosions when a massive star explodes, usually lasting weeks to months. As with Cepheid stars, the lightening and fading of a supernova can reveal its true luminosity. When compared with the brightness measured from the earth, we can infer how far it is.
Type Ia supernova SN 1994D (lower left) is brighter than its host galaxy, NGC 4526. Image taken by the Hubble Space Telescope
Of course, this method also needs to be corrected by the asterisk and the Cepheid stellar distance estimation method. Because without knowing the exact distances to a few supernovae, there’s no way to determine their true brightness, and this method won’t work either.
Above are some basic methods for scientists to calculate the distance from Earth to the stars, a job that conventional telemetry devices can never do. Hope this article has brought you interesting information. Thank you for reading and I hope you learn more and more interesting things!
According to GVN360
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