Astronomers use different methods to measure the distances to different celestial bodies. For the nearest stars, astronomers use the simplest triangulation method. This is the method used by humans to judge the distance of objects. If we stretch out a finger and alternately close our left and right eyes to look at it, we will find that the finger has a positional (directional) change relative to the distant background. If we know the magnitude of this change and the distance between our two eyes, we can calculate the distance from the finger to our eyes through simple geometric relationships. Astronomers also use this method to measure the distances to stars near us. They take the long axis of the Earth's orbit around the Sun as the baseline. As long as they observe the same star at both ends of the long axis of the Earth's orbit with a time interval of half a year, they can obtain the angular separation of the star to the Earth's orbit diameter. Astronomers call half of this angle the star's annual parallax. Knowing that the semi-major axis of the Earth's orbit is about 150 million kilometers, the distance to the star can be inferred from the annual parallax, which is called geometric distance.
The triangulation method can only measure the distances to stars near the Sun. The nearest star is 4.22 light-years away from us, and its corresponding annual parallax is less than 1". The farther the star is, the smaller the annual parallax will be, and the more difficult it will be to measure. At present, the maximum range of star distance measurement by triangulation is about 500 light-years. The scale of the Milky Way is about 82,000 light-years, and the distances to most stars in it far exceed the range of application of triangulation; as for celestial bodies outside the Milky Way, triangulation is simply out of the question, and other methods must be used.
Astronomers have come up with another measurement method called "luminous distance measurement". The principle of this method is very simple: if we observe the same type of light bulb, we will find that the nearby light bulb is bright and the distant one is dim. If we can measure how bright the light bulb is, we can know how far it is from us. Because the brightness of the light can be expressed in "candles", this distance measurement method is also called the "standard candle" distance measurement method. The key to this method is, which celestial bodies in the sky are of the "same type", or what is their actual brightness?
Through long-term monitoring of stars, scientists have found some stars with very regular brightness changes, called "Cepheid variables". In 1912, American female astronomer Leavitt discovered that there is such a relationship between the light variation period of Cepheid variables and their actual brightness: the longer the light variation period, the more energy is released; the shorter the light variation period, the less energy is released. This means that we can use the light variation period to determine their actual brightness. Thus, as long as the light variation period of the Cepheid variable is extrapolated to obtain its actual brightness, its apparent brightness can be used to calculate its distance. Therefore, scientists call Cepheid variables "cosmic rulers". American astronomer Hubble first confirmed that the Andromeda Galaxy was not a celestial body within the Milky Way, but a huge galaxy composed of hundreds of billions of stars like the Milky Way, by using Cepheid variables. The application range of "cosmic ruling" by Cepheid variables can reach more than 50 million light-years, far exceeding the ability of triangulation. In addition to Cepheid variables, novae, supernovae, and RR Lyrae variables also have certain relationships between their light variation properties and actual brightness, but they are not as precise as Cepheid variables. Therefore, measuring the distances to nearby galaxies mainly relies on finding their Cepheid variables.
In more distant galaxies, many stars have become indistinguishable, and only the supernovae that explode in the moment of destruction can release light that eclipses the entire galaxy and be seen by us. Supernovae are products of the late stage of star evolution. When they explode, their brightness will increase by tens of thousands or even hundreds of millions of times in a short period of time, with a maximum brightness of 10^7 to 10^10 times that of the Sun's luminosity, and release 10^40 to 10^45 joules of energy. Among supernovae, Type Ia supernovae are the most conducive to distance measurement.Supernovae. These supernovae are extremely bright when they erupt, and their maximum brightness is very stable, making them an excellent standard candle. The measured brightness can reach more than 10 billion light years away. With their magnificent deaths, supernovae have become new yardsticks for measuring the universe, providing valuable clues for studying the history of the universe. American astronomers Saul Perlmutter and Brian P. Schmidt, and Australian astronomer Robert J. Kirshner won the 2011 Nobel Prize in Physics for their observations of supernovae, which revealed that the universe is expanding at an accelerating rate.
But the story doesn't end there. Scientists have discovered Hubble's law, which can be used to calculate distances based on the redshift (recessional velocity) of galaxies. However, in the earlier and more distant universe, even galaxies are hard to find. Scientists are still trying to find new ways to unravel the secrets of the early universe.