In addition to the stars, planets, moons, and macroscopic objects such as small meteors and comets that can be directly observed with the naked eye and telescopes, the space around these celestial bodies is actually filled with a lot of interstellar dust and thin gaseous matter. In the broader universe, there is another type of celestial body that plays a huge role in the evolution of the universe, that is, nebulae.
In many popular science articles and videos, we should have seen descriptions and images of cosmic nebulae, in the depths of the universe, there are many clusters of ethereal or colorful nebulae, the shape of these nebulae, according to each person's inherent memory and association, and present different "imaginary" states, which really makes people think about it and can't stop.
In astronomy, nebula refers to a "cloud-like" celestial body in the universe composed of interstellar dust, hydrogen, helium and other ionized gases and other materials. At the end of their lives, massive stars collapse inward due to the weakening of internal nuclear fusion, which then re-excites more intense and uncontrollable nuclear fusion in the outer layers of the star, triggering supernova explosions, and the huge energy throws out the material that makes up the stars, and finally evolves into nebulae.
Of course, less massive stars, such as the Sun, will form red giants at the end of their lives, and in the process, they will gradually throw out the outer layer of material to form nebulae, and the remaining inner cores will continue to evolve into white dwarfs.
After that, the nebula will gradually gather under the influence of gravity during the long evolutionary process, and when the matter gathers to a certain extent and reaches a certain scale, it is possible to form new stars again. Therefore, nebulae are known as the cradle of a new generation of stars in the universe. In a sense, nebulae and stars can be transformed into each other, and the "death" of stars means the "life" of a new generation of stars in the next billions of years.
Of all the nebulae, there is one type of nebula that is gaining increasing attention among astronomers, and this type of nebula is the planetary nebula. Why is it called that? The main reason is that when viewed from astronomical telescopes, they have a relatively clear outline, just as we can use astronomical telescopes to observe planets such as Uranus and Neptune, which have relatively bright surfaces and clear boundaries, and see the regular spherical shape of the target area.
Through astronomers' observations and research, planetary nebulae are relatively rare in the universe, they generally come from small and medium-sized stars with a mass less than 10 times that of the sun, when such stars enter the end of their life, the hydrogen in the inner core is depleted, and thus continuously eject constituent materials, including interstellar dust and gas shells, which can be about 1 light-year in diameter. Most of the final forms are ring-shaped or dumbbell-shaped nebulae, and it is rare for them to be able to truly form a disk.
Among the thousands of similar planetary nebulae that scientists have monitored, there are only a few dozen that can reflect the shape of a circular surface, which can be said to be very low. Therefore, planetary nebulae in the true sense of the word are more recognizable than other types of nebulae. Like the "cat's eye nebula" in the Milky Way that scientists have observed, even if we look at it from the image, it will feel very amazing and shocking.
For astronomers, observing and studying planetary nebulae is not only about their regular and perfect shapes, but also about "sniffing" important evidence and clues for the evolution of cosmic elements and celestial bodies. Whether it is a supernova explosion at the end of a massive star, or a red giant at the end of a small star, their interior has been consumed by hydrogen, the star can not bear its own mass weight, and the violent collapse makes its interior begin to consume helium, thus triggering more violent nuclear fusion, the temperature of the star rises very high at this stage, and the light emitted from the star is almost concentrated in the ultraviolet band, so as to efficiently heat the gaseous substances thrown out of the surrounding area, making it highly ionized.
The plasma gas envelope of the planetary nebula absorbs a large amount of ultraviolet energy and converts it into visible light, the most obvious of which is to emit a "dizzy" emission line, which is reflected in the green part of the star spectrum in the visible region.
Therefore, by observing this short-lasting, "fleeting" emission line, scientists can find key evidence of chemical enrichment in the cosmic space where the nebula is located, and in addition, some scientists can also use this to use the very bright planetary nebula as a yardstick to measure the distance between the Earth and the galaxy, because the brightest planetary nebula, no matter what kind of galaxy they are in, their absolute brightness is basically the same, just as La-type supernovae are the "standard candlelight" for cosmic observations Similarly, the high-brightness planetary nebula is highly efficient for measuring galaxies at distances of 70 million light-years and beyond.
The brilliant planetary nebula is not only a "brilliant firework" in the universe, but also a beacon for us humans to understand the evolution of the universe.