Stars can be seen as fundamental components of the universe, just as atoms are the basic building blocks of matter. For most of human history, the stars that wandered through the universe seemed to be eternal. But like all life on Earth, stars undergo birth, evolution, and death. However, we didn't know this fact at first, until the advent of a map, which gradually revealed the evolution of stars for us.
In the early 20th century, astronomers used spectrographs and photographic techniques to collect a large amount of data on stars, including information related to the temperature and brightness of stars. Danish astronomer Ejnar Hertzsprung and American astronomer Henry Russell independently plotted a large number of stars on a two-dimensional chart to create the hertzsprung-Russell diagram, which is today famous.
In the Hertzsprung chart, the vertical axis shows the luminosity of the stars, and the horizontal axis shows their surface temperature. The position of the star in the graph shows information about its stages and mass. Stars capable of burning hydrogen to fuse into helium are located in a diagonal region, the so-called main sequence stars. Red dwarfs, like AB Swordfish, are located in cold, dim corners, with temperatures of about 3,000°C and about 0.2 percent of the Sun's luminosity. When a star runs out of all its hydrogen, it leaves the main sequence and becomes a red giant or supergiant according to its mass. A star with the mass of the Sun burns up all the fuel and eventually evolves into a white dwarf. | Image source: ESO
This deceptively simple diagram has revolutionized our understanding of stars. The longitudinal axis of the Hertzsprung chart maps the luminosity of the star, that is, the energy output; the horizontal axis maps the surface temperature of the star. What is shown in the image is actually like a group photo of stars taken at random points in time, scattered in different locations in the universe, with different brightness and color.
In the Hertzsprung chart, it can be found that most of the stars, including the Sun, are distinctly clustered on a diagonal line, extending from one corner of the plot to another. Astronomers refer to stars distributed diagonally as main sequence stars.
Today, the structures we observe in the universe are driven by gravity and stars. Gravity brings matter together, and the light emitted by stars illuminates the universe. When a star is in the longest and most mundane period of its life, it is called a main sequence star. During this period, the star is very stable, and the inward gravitational pull cancels out with the outward pressure, so the star does not change much.
Fusion creates an extroverted force that cancels out the gravitational pull inward. The balance between the two sustains the main order of life of the star. | Image reference: Standford University
During the main sequence star period, stars acquire energy by fusing hydrogen into helium. This fusion reaction can be carried out through two processes, one is the so-called proton-proton chain, and the other is the carbon-nitrogen oxygen cycle. In Sun-like stars, proton-proton chains dominate the production of energy, while the carbon-nitrogen-oxygen cycle accounts for only about 1%. For stars that are heavier and hotter than the Sun, the main energy supply reaction is dominated by the CNO cycle.
In addition, why does the main sequence band present a special diagonal pattern from high luminosity, temperature to low luminosity, temperature? The secret lies in the fact that the center of a massive star has a strong gravitational squeeze, raising the temperature at the core. The rate of nuclear fusion is very sensitive to temperature, which means that hydrogen in massive stars burns hot and fast, producing enormous amounts of energy. Therefore, the main sequence band also contains the mass information of the star, the massive star in the high luminosity, temperature region, and the low mass star in the low luminosity, temperature region.
From the Hertzsprung chart, we also see many stars of moderate brightness and very low temperatures, as well as many stars of extreme brightness and very low temperatures. Through calculations, astronomers can conclude that these bright, cold stars are much larger in size than our Sun. From this information, astronomers have discovered giant stars that are 10 times the size of the Sun, as well as supergiants that are 100 times the size of the Sun. The variety of giant and supergiants shown on the Hertzsprung chart is the most important evidence of stellar evolution.
When a star runs out of hydrogen, it begins to enter the end of its evolution. Helium in stars first turns into carbon and then fuses into heavier and heavier elements. In a star similar to the Sun, once the star has metabolized the heaviest element it can possibly fuse, its outer layer is pushed away, leaving only a dense core, wrapped in a white dwarf, surrounded by a cloud of gas known as a planetary nebula. For more massive stars, its ending is even more dramatic: in a supernova explosion, the core of the star leaves behind a neutron star or black hole (which cannot be plotted on a hertzspring due to the extremely complex properties of neutron stars and black holes).
The lifespan of stars is determined by mass: the more massive the shorter the lifetime, the more mysterious the end. | Image credit: ESO/M. Kronmesser
Our lives are too short compared to the lives of stars to truly observe their evolution. But by randomly measuring a large number of stars, we can witness older stars that are in the giant or supergiant stage, and then through statistical analysis, we can see whether their theoretical evolution trajectories are consistent with those shown in the Hertzsprung chart. The results show that the answer is yes. So this map tells us not only what the stars really are, but also how these glowing spheres have changed over the course of billions of years of cosmic history.
Today, although we have a lot of information about stars, there are still many mysteries lingering. Scientists don't fully know the details of how gas clouds and dust collapse to form stars, or why most stars form constellations. There are too many details to be further determined through a combination of observation and theory.
To reveal the birth and early evolution of stars, we need to be able to peek deep inside dense clouds of dust and gas, where star formation began. However, due to the obscuration of dust, these regions cannot be observed in the visible light band, but must be observed in the infrared band, which is why for astronomical research in this field, we urgently need high-resolution infrared space telescopes like the Webb Space Telescope (JWST).
Image of the "Pillar of Creation" of the Eagle Nebula in visible light (left) and in the infrared light band (right). In the infrared light band, the light emitted by the young star that is forming penetrates clouds of dust and gas. | Image credit: NASA, ESA/Hubble and the Hubble Heritage Team
The last unsolved mystery related to the life cycle of stars has transported astronomers back into the distant past. We know that every cycle has a beginning, so how did the first stars in the universe form? JWST will give astronomers the opportunity to observe the first rays of starlight in the universe, and it will help us fill in the gaps in the early chapter of the universe's history, giving us a better understanding of how the universe moved through the lifetimes of stars and how we got to where we are today.
#创作团队:
Text: No two Beidou
#参考来源:
https://bigthink.com/hard-science/understand-hertzsprung-russell-diagram-astrophysics/
https://bigthink.com/13-8/how-to-read-hr-diagram/
https://bigthink.com/13-8/hr-diagram-stars-evolve/
https://webbtelescope.org/webb-science/the-star-lifecycle
https://jwst.nasa.gov/content/science/birth.html
#图片来源:
Cover image: hubblesite.org