The Milky Way is ＂getting fat＂ to house stars

　　It is well known that if a person consumes more energy than they expend, the body may gain weight and vice versa. It only takes a scale to measure a person’s weight gain and loss.

　　How the galaxies in the vast universe, especially the Milky Way where humans live, are changing is a major problem plaguing astronomers around the world. Recently, an international research team led by European Space Agency astronomer Dr. Andrew Fox wrote in the Astrophysical Journal that the gas inhaled by the Milky Way is more massive than the gas exhaled, and it is in the process of "getting fat".

　　So, what is the mystery behind the "breathing" and mass changes of the Milky Way? What effects will this "fattening" have on the Milky Way?

　　Gas material exchange, activate "a pool of spring water"

　　Gas is constantly being "blown" out of the Milky Way, but these gases will be "sucked" back into the Milky Way and fall on the Milky Way. What does this "breathing" mean?

　　"This is the cycle of gas and dust matter brought about by the birth and death of stars," Zuo Wenwen, an associate researcher at the Shanghai Observatory of the Chinese Academy of Sciences, told Science and Technology Daily in an interview. Stars collapse from clouds of gas molecules in the silver disk to form. Stellar winds during stellar evolution, as well as supernova explosions that occur when massive stars evolve to the end of their lives, throw most of the material outward and send shock waves to the surrounding interstellar material, forming a shell-like structure of expanding gas and dust, known as a supernova remnant.

　　"Stars can be seen as originating from dust and returning to dust when they die," Zuo Wenwen said.

　　The entire life cycle of a star from birth to death accomplishes a large-scale transfer – transferring the gas and dust material in the Milky Way disk to the more peripheral halo of the Milky Way. Moreover, a star accumulates a large amount of metal elements during its lifetime. In astronomy, elements larger than the number of helium atoms are usually called metal elements. These metal elements are like a star working hard – working hard to burn itself and strive for the wealth it has accumulated in its lifetime. It occasionally "consumes" in its daily life, that is, throws some matter through the stellar wind phenomenon; more at the moment when the massive star is dying, the "possessions" it has accumulated in its lifetime are scattered, enriching the elemental composition of the entire galaxy, and igniting the spark of the origin of the next generation of stellar life.

　　As time goes by, the gas and dust in the halo will gradually clump together, causing gravity to cause these clumps of gas to fall back into the disk and begin the next round of star formation.

　　The death of a star creates the birth of a new star, and the end point is the starting point. The cycle is repeated, "born to death". The Milky Way also completes the exchange of gas and matter with the surrounding environment in the "sacrifice" of countless stars, like a lake with a pool of living water in it.

　　High-speed molecular cloud, tagged "floating population"

　　So, is the great lake in the Milky Way "rising" or "leaking"? Many researchers want to find out.

　　The answer given by this study is the former, that is, the gas inflow is greater than the outflow.

　　The study used ultraviolet data from the Hubble Space Telescope to study 187 high-speed molecular clouds, and determined their velocities in the Milky Way’s standard stationary reference system based on the movement of absorption lines relative to the wavelength of the stationary reference system. They were classified into inflow-speed molecular clouds and outflow-speed molecular clouds. Through calculations, the researchers estimated the inflow rate to be 0.53 ± 0.17 times the mass of the sun per year and the outflow rate to be 0.16 ± 0.06 times the mass of the sun per year, indicating that the Milky Way is currently in a period of inflow dominance.

　　Zuo Wenwen pointed out that the attractive forces of the Milky Way may drag some of the intergalactic medium in, and may also drag some of the gas from its satellite galaxy.

　　The Science and Technology Daily reporter noticed that the main object of this research is the high-speed molecular cloud. There are countless gas and dust in the Milky Way, why did the researchers only aim at the high-speed molecular cloud?

　　Zuo Wenwen mentioned that there is an interstellar medium between stars, and an intergalactic medium between galaxies. A galaxy is not a system with closed boundaries.

　　As a result, no gas would voluntarily label itself an "outsider" or a "native." So how do researchers define which gases are outflow or inflow "floating people" and which are "permanent" residents of the galaxy? The starting point for solving these problems is the high-speed molecular cloud.

　　Usually, the "resident" gas in the Milky Way disk rotates at the same speed as the Milky Way disk. The gas in the high-speed molecular cloud is moving faster than the rotation speed of the Milky Way disk, which means they are likely a type of inflow or outflow gas. Then observe the speed direction of the molecular cloud and analyze whether it is moving towards or away from the Milky Way disk to determine whether the molecular cloud is inhaled or exhaled by the Milky Way.

　　Of course, some scholars have also pointed out that the study ignores the high-speed gas structures that already exist in the silver disk, such as Fermi bubbles, which will undoubtedly bring errors to the experiment.

　　Zuo Wenwen also said that the study is based only on gas clouds with lower temperatures (about 10,000 Kelvin), and the annual inflow and outflow of gas are all lower limits. More data is needed to obtain more accurate results.

　　The significance of respiration regulates the life cycle of stars

　　"The formation of stars is regulated by the relationship between gas inflows and outflows. Therefore, studying the gas cycle process is very important for studying star formation and galaxy evolution." Zuo Wenwen said that the Milky Way is the galaxy we live in, and has relatively rich observational data to study the gas cycle problem.

　　Perhaps many people will wonder what might have happened if the galaxy had been in a state where more gas was flowing in than out?

　　"More gas is accumulating in the galaxy than is flowing out. The Milky Way provides the raw materials for star production – gas, dust, which contributes to subsequent star formation." On the contrary, if more gas is flowing out of the galaxy than is flowing in, Zuo Wenwen said, one day, the raw materials for star formation will be lost, and no new stars will form in the galaxy. In fact, while inflows and outflows determine whether a galaxy will have continued star formation, it is also important to pay attention to how large the gap is and how long it lasts.

　　In 2018, astronomers at Tohoku University in Japan wrote in the journal Nature that the Milky Way experienced a period of dormancy that lasted for billions of years between two "baby boomers" of star formation, and actually "came back to life" after "death," a phenomenon that is inseparable from the galaxy’s gas cycle.

　　According to this study, the Milky Way began to form the first generation of stars by inhaling a large amount of cold gas early on. About 7 billion years ago, shock waves from the star collapse explosion heated the gas within the galaxy to high temperatures. This caused the cold gas to stop flowing into the galaxy, and with it, star formation stopped. Over time, the Milky Way’s hot gas gradually radiated and cooled, and began to inhale new cold gas 5 billion years ago. This led to the formation of the second generation of stars, including the Sun. More importantly, other studies suggest that the Milky Way’s neighbor, the Andromeda galaxy, may have gone through a similar process. This suggests that massive spiral galaxies tend to experience a "dormant period" of star formation, while smaller galaxies do not.

　　In fact, the concept that galaxies "breathe" also applies to smaller systems in the universe, such as stars and even planets. Compared to the "weight gain" of the Milky Way, the sun and the earth are losing weight. According to NASA and MIT, the sun loses 1.3245 quadrillion tons of mass each year, and the earth loses 1 to 50,000 tons each year.

　　As the Universe Today website puts it: "Whether we are talking about planets, stars or galaxies, they all experience birth, life and death. During this time, they may gain or lose a few pounds. The cycle of life unfolds on a cosmic scale." (Intern reporter, Yu Ziyue)