Wednesday space exploration | Thursday observation guide
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Author: Wang Qiru
Proofreading: Alex Mu Fu Astronomy Proofreading Group.
Later period: Kutlija Li
Credit: NASA /SDO
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Last Tuesday, we discussed the birth of sun-like stars, low-mass stars, massive stars and brown dwarfs. Now let's look at a star like the sun and see how it evolved after its birth.
Wheel galaxy under x-ray
Credit: NASA /CXC
The beginning of extinction-the birth of sub-giants
Solar-like stars occasionally erupt flares and sunspots in the main sequence stage, but in most cases, their properties will not suddenly change on a large scale. When the solar-like star's fuel, hydrogen, begins to run out, the abundance of helium has increased to a certain extent. With the depletion of central hydrogen, the nuclear reaction stops and the main combustion zone moves to the outer layer of the core. Without the support of nuclear reaction, the outward pressure in helium nucleus will be weakened, which will lead to the contraction of the nucleus. The contraction of helium core will release gravitational potential energy, raise the central temperature and heat the combustion layer covering the core. At this time, the temperature exceeded100000 Kelvin, but it was not high enough to cause nuclear fusion of helium atoms. However, this temperature can make the fusion of hydrogen nuclei faster than before. Hydrogen burns at an alarming rate in the shell of an unburned core composed of helium "ash" around the center of a star. This stage is called hydrogen shell combustion. The energy generated by the hydrogen shell is increasing, while the helium core is shrinking inward, with a slight increase in luminosity and a decrease in surface temperature. After about 1 100 million years, the radius of the original sun-like star increased to more than three times that of the sun and became a subgiant.
Here, we take Capella (α Aur) as an example. This yellow giant is about 2.5 times the mass of the sun, with a radius of 12 times and an apparent magnitude of -0.08. , 42 light years away from the earth. Capella is currently in the middle stage of the transformation from the main sequence star to the red giant star, which is the sub-giant star stage we just mentioned.
Capella appeared in the night sky in the northeast of Shanghai tonight.
Credit: SkySafari pro
The first stage of extinction-red giant star
Now, our old star is far from the main sequence and is no longer in a stable equilibrium state. Hydrogen is transformed into helium at an accelerated speed, and the increase of gas pressure caused by hydrogen combustion increases the outer radius of unburned stars. When the core is shrinking and heating, the outer layer is expanding and cooling. In this change, the star becomes a red giant. The red giant is very big-about as big as the orbit of mercury; On the contrary, its helium core is very small-only several times the size of the earth. The density of the nuclear center is very high, and about 25% of the mass of the whole star is compressed in a core the size of a planet.
A common example of a sun-like star in the red giant stage is arcturus, whose mass is 1. 1 times that of the sun, its radius is 25 times that of the sun, its apparent magnitude is -0.04, and it is 36 light years away from the earth. Arcturus is the third brightest star in the whole night sky. At present, it is in the stage of hydrogen shell combustion and rising along the branch of red giant. Its luminosity is 170 times that of the sun, and most of its radiation is in the infrared range of the spectrum.
Arcturus (left) and the sun (right).
Credit: Cosmic Sandbox
The second stage of extinction-helium flash and carbon deflagration
For a star similar to the sun, after it left the main ordinal number for hundreds of millions of years, the burning of the hydrogen shell made the core of the helium star bigger, and the heat generated by it pushed the core temperature to 65.438+0 billion Kelvin, at which time the helium in the core began to burn. At this time, the gravity of the star began to prevail again, the star contracted under the action of gravity, and the electron gas density of the star began to increase. Here we can classify the results caused by electron degeneracy of sun-like stars with different masses:
The first is a star with a mass of 0.8~2.2 times that of the sun. For such stars, when the mass of degenerate helium nuclei exceeds the critical value (0.45~0.50) due to accumulation, helium will ignite in degenerate gas. Generally speaking, the pressure of non-degenerate gases is directly proportional to the temperature. The local temperature rise caused by ignition in non-degenerate gas will be accompanied by the increase of pressure, and the star nucleus will expand, thus restraining the temperature rise and being in equilibrium. However, the volume and pressure of degenerate gas have no response to temperature, and what happens after ignition in degenerate gas is positive feedback, which leads to intense fusion of helium and thermal runaway, and the corresponding phenomenon is called helium flash.
Followed by stars with a mass of 2.2~8 times that of the sun. For such a star, helium will ignite under non-degenerate conditions and burn normally. At the core of a red giant, two helium nuclei merge to form a Be nucleus, which is a very unstable isotope and usually decays into two helium nuclei in a short time. However, when the density of the red giant's core is high, the Be nucleus may meet other helium nuclei before decay and polymerize with them to form carbon. For such a star, carbon will ignite under degenerate conditions. When carbon starts to ignite, there is convection in the center of the star core, which transfers the energy released by carbon combustion, so thermal runaway will not happen immediately. But in the end, carbon flashes will form, and then shock waves will be generated. When the shock wave propagates to the unburned medium, it will gradually ignite the medium and then develop into explosive combustion. The corresponding phenomenon is called carbon deflagration.
Credit: NASA
The most brilliant stage of extinction-planetary nebula
According to the above, the core temperature of a star whose mass is 2.2~8 times that of the sun is enough to cause carbon nuclear fusion reaction. However, before the carbon core can reach the incredible high temperature needed to ignite carbon fusion, its density will reach the point where it cannot be further compressed. In fact, once carbon begins to form, it marks that the star has entered the countdown to death. As the core gets closer to its final high-density state, the intensity of nuclear combustion is also increasing, and the star cladding is expanding and cooling, and the maximum radius is about 300 times that of the sun-large enough to devour Mars. It is at this time that the combustion of the star becomes very unstable, which makes the star very complicated-near the peak of each pulse, the surface temperature drops to the point where electrons can recombine with other nuclei to form atoms. Every time atoms combine, extra photons will be generated, which will give the gas some extra "external thrust" and cause some gases to escape. In less than one million years, almost all the cladding of a star was ejected into space at the speed of tens of kilometers per second. With the depletion of the remaining fuel in the core, it began to shrink and heat up, forming a small and well-defined core mainly composed of carbon ash-it is hot, dense and still very bright, and only the outermost layer of the core is still gathering helium into carbon and oxygen. Far away from the inner core is an expanding cloud of dust and cooling gas-the coating erupts from this giant star and fills a space the size of the solar system. Such a spectacular and gorgeous phenomenon is elegantly called "planetary nebula". Theoretically, all stars with a mass of 0.8-8 times that of the sun will form planetary nebulae. M57 (Ring Nebula) is a planetary nebula ejected from a dying central star in Lyra. M57 is 2300 light years away from the Earth, with an apparent magnitude of 8.8. Its expansion rate is 65,438+0 arc seconds per century. It is one of the spectacular planetary nebulae.
M57, Ring Nebula
Acknowledgement: Wikipedia
The Last Stage of the Evolution of Sun-like Stars-White Dwarfs
The remnant carbon core of the star in the center of the planetary nebula will evolve with time, its cladding will gradually dissipate, and the core originally hidden under the veil of red giant atmosphere will become visible (it will take tens of thousands of years for the core to emerge from the veil of diffused gas). The inner core is very small, and when the cladding ejects to form a planetary nebula, the inner core has shrunk to about the size of the earth, even smaller than the earth. Its mass is about half that of the sun. This small "star" with hot surface is called "white dwarf".
B(Sirius B is a white dwarf with the largest known mass, especially close to the Earth, and also a weak companion of the famous bright star Sirius A.. It is located in Canis Canis, 8.6 light years away from the earth, with an apparent magnitude of 8.3. Although its mass is equivalent to that of the sun, its density is100000 times that of any celestial body we are familiar with in the solar system.
Credit: Cosmic Sandbox
Go through the final journey alone-black dwarf
Once an isolated star becomes a white dwarf, its evolution is over (white dwarfs in binary stars may have further activities). This lonely white dwarf has been cooling and darkening with time, and has finally completed a lifetime journey, becoming the ashes in a cold and dense space-black dwarf. Cool dwarfs don't shrink too much when they disappear. In the case of extremely high star density, even if the temperature of the star is close to absolute zero, the resistance of electrons to the backlog will support the star-the same as the electron degeneracy of the red giant core near helium flash. When the dwarf cools, it is still about the size of the earth.
Composition changes of sun-like stars
Credit: NASA
At this point, the sun-like star reached the peak of the evolutionary stage in a gorgeous and elegant way and ended in a lonely and cold way.
Reference materials:
[1] Wang Youfen; Shao zhengyi. Observation characteristics and search of brown dwarfs. Progress in astronomy. 20 13 (0 1): 19-38.
Wang Hongyan. Massive neutron stars may contain hyperons. Journal of Jilin University (Science Edition). 2020 (03):236-240.
Xu Lanping. Post-main sequence evolution of stars. Progress in astronomy. 1989 (04):50-58
[4] hold high; Xiao Ting. Research progress of molecular gas and star formation in galaxies. Progress in astronomy. 2020 (02):4-2 1.
About the author:
Wang Qiru, a native of Yantai, Shandong, majored in materials physics.
Undergraduate study, astronomy enthusiast, popular science creator.
Mao_mingyuan@astronomy.com.cn Mu Fu Astronomy welcomes everyone to contribute enthusiastically.
"Astronomical Wet Engraving" produced in Mu Fu.
Acknowledgements: NASA, ESA and J. kastner.