Almost everything in the universe has mass, from atoms and subatomic particles (such as those studied by the Large Hadron Collider) to huge clusters of galaxies. So far, the only things that scientists know without mass are photons and gluons.

It is unimaginable that the celestial bodies in space are so far away. So, how do astronomers determine the mass of objects in the universe?

Suppose a typical star is quite large, usually much larger than a typical planet. Why should we care about its quality? It is important to know this information because it reveals clues about the past, present and future of star evolution.

Astronomers can determine the mass of stars in several indirect ways. A method called gravitational lens measures the path of light bent by the gravity of nearby celestial bodies. Although the amount of bending is small, careful measurement can reveal the gravitational mass of the celestial body when it is dragged.

It was not until 2 1 century that astronomers used gravitational lenses to measure the mass of stars. Before that, they had to rely on binary stars to measure the mass of stars. The so-called binary star refers to two stars orbiting the same center of gravity. The mass of binary stars is easy for astronomers to measure. In fact, a multi-star system provides a textbook example of how to calculate its mass. This is a bit technical, but it is worth studying to understand what astronomers must do when calculating the mass of celestial bodies.

First, they need to measure the orbits of all the stars in the multi-star system. They also need to measure the orbital speed of a star, and then determine how long it takes a given star to orbit a complete orbit. This is called "orbital period".

Once all this information is known, astronomers will do some calculations to determine the mass of stars. They can use the formula V = SQRT(GM/R), where v is the circumferential linear velocity, SQRT is the "square root", g is gravity, m is mass, and r is the radius of the object. By rearranging the equation to solve the mass m, the physical problem is transformed into an algebraic problem.

Therefore, astronomers use mathematics and known physical laws to calculate the mass of distant celestial bodies without touching stars. However, not every star can calculate its mass according to this formula. In non-binary or multi-star systems, there are other physical quantities that help astronomers calculate the mass of stars. For example, astronomers can use luminosity (in astronomy, luminosity is the total energy radiated by an object in unit time, that is, radiation flux. ) and temperature to measure the mass of stars. The quality of stars with different luminosity and temperature varies greatly. When this information is plotted on a chart, it shows that stars can be arranged according to temperature and brightness.

A truly massive star is one of the hottest stars in the universe. Smaller stars, such as the sun, are colder than giant stars. The chart of the temperature, color and luminosity of a star is called Hertz Bragg-Russell chart. By definition, it also shows the mass of a star, depending on its position on the chart. If it lies on a long and tortuous curve called the main sequence, then astronomers know that its mass is neither too big nor too small. The stars with the largest and smallest mass fall outside the main sequence.

Astronomers have a good grasp of the birth, survival and extinction of stars. This life-and-death sequence is called "stellar evolution", which is a series of rapid changes experienced by stars in the course of their lives. The biggest factor to predict the evolution of a star is its birth mass, that is, its "initial mass". Low-mass stars are usually colder and darker than massive stars. Therefore, by observing the color and temperature of stars and their "residence" in Hertz Bragg-Russell diagram, astronomers can know the quality of stars well. By comparing similar stars with known mass (such as the above binary stars), astronomers can know the mass of a given star well, even if it is not a binary star.

Of course, stars don't keep the same mass all their lives. With the increase of age, the mass of stars will gradually decrease, and these lost masses will be gradually consumed as nuclear fuel, and eventually they will experience huge mass loss at the end of their lives. If they are as massive as the sun, they usually form planetary nebulae. If their mass is much greater than that of the sun, they will die out in a supernova event, in which the core will collapse and then expand outward in a catastrophic explosion, blowing most of their materials into space.

By observing the types of stars that die like the sun or die of supernovae, astronomers can infer what other stars will do. Astronomers know the mass of these celestial bodies, and they also know how other stars with similar mass evolve and die, so they can make some pretty good predictions based on the observation of the color and temperature of celestial bodies to help them understand their own mass.

Observing stars is more important than collecting data. The information obtained by astronomers is folded into a very accurate model to help them accurately predict what stars in the Milky Way and the whole universe will do when they are born, aged and dead, all based on their mass. Finally, this information helps people know more about stars, especially our sun.