Small Stars Turned Out To Be Space Factories Of Lithium
Astronomers have discovered that small stars the size of the Sun in the last stages of their existence do not destroy lithium, but rather produce large amounts of this metal. This discovery will help solve the so-called "lithium problem" of cosmology, the researchers write in the scientific journal Nature Astronomy.
"We found that all the small stars that turned into red giants in the last stages of their life were unusually rich in lithium. This contradicts the predictions of theories that describe the workings of the bowels of such luminaries, and also suggests that we are missing out on some physical process that causes this metal to appear in the cores of stars," the scientists write.
After the Big Bang, there were only three elements in the Universe – hydrogen, helium, and a very small amount of lithium. Only after 300 million years, when the first stars appeared, due to thermonuclear reactions in their depths began to appear heavier elements.
I can't say yet how this process went. On the one hand, they believe that the Universe may contain many more heavy elements than indicated by the theories that explain their formation due to supernova explosions. Therefore, astrophysicists assume that other processes, such as collisions of neutron stars, are involved in their appearance.
On the other hand, the opposite situation is observed with lithium. In the observable Universe, it is about three times less than predicted by cosmological theories. Because of this, scientists have been arguing for decades about why this is so and where the error that caused the erroneous prediction lies.
Space lithium factories
Astronomers led by Zhao gang, a Professor at the National Astronomical Observatory of the Chinese Academy of Sciences, have found a possible explanation for this problem. They found that the conventional idea that small stars similar in size to the Sun do not produce lithium, but rather destroy it, is wrong.
In their new study, Zhao and his colleagues measured the concentration of lithium in the matter of about 225,000 small stars. To do this, they used data from ground-based telescopes that participated in the GALAH space project, as well as the Gaia orbital Observatory.
Astronomers were interested in elderly small-sized stars from the so-called "red spot;" this is what scientists call a special category of stars that have entered one of the last stages of their existence. At this stage, reactions involving helium, rather than hydrogen, begin to dominate in their nuclei. As a result, they turn into red giants and dramatically increase in size.
Many such stars, as Zhao and his colleagues note, contain an unusual amount of lithium. Therefore, scientists have tested how this is typical for all-stars of this class that exist inside the milky Way. In total, astronomers have found about nine thousand similar stars that have already entered the "red spot" stage or are preparing for it.
By comparing them with each other, the astronomers tracked how the concentration of lithium in them changed. It turned out that as they transition to reactions involving helium, the proportion of lithium in the interior of all such stars did not fall, but rose very quickly. On average, by the time they reached this phase of existence, the concentration of lithium in the matter of these stars increased by about 40 times.
Why this happens and how exactly lithium occurs in the bowels of such elderly stars, astronomers do not yet know. But they suggest that this process is related to changes that occur in the bowels of stars before or immediately after the helium begins to burn in their bowels. Studying this process, scientists hope, will help to understand where the error lies in both cosmological and astrophysical theories.