Astronomers have observed for the first time ancient stars whose existence may explain some of the "dark matter" in the universe
News agencies
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Astronomers in Washington have for the first time observed dozens of dark stars - the researchers called them "dead stars" - at the edge of the Milky Way galaxy. This discovery may help solve one of the biggest mysteries in cosmology: the riddle of "dark matter". This is the name researchers call a substance that they identify by the influence of force The gravity it exerts on other bodies, but cannot be seen because it does not emit light.
The first direct detection of dozens of "dead" stars could explain at least some of the dark matter. Independent scientists called the discovery "exciting and momentous," adding that it may shake up current theories of how stars and galaxies formed in the early universe.
38 new dwarves
The international team of researchers based in Washington announced Thursday that they have found 38 previously unseen objects known as "white dwarfs" in the Milky Way galaxy. Each of them is the compressed ash of a dying sun-like star that has collapsed to the size of Earth. The researchers estimated that such objects make up at least 3%, and at most 35%, of the dark matter.
The "dead" stars were found within a range of about 450 light years from Earth, in the spherical "halo" of stars and dark matter that surrounds the Milky Way galaxy (most of the bright and visible stars are concentrated in a flat disk). The "dead" stars appear to be very ancient remnants from the galaxy's infancy, 10 billion to 13 billion years ago.
"We found a previously unknown population of stars in the galactic halo that represents a fraction of the normal dark matter in the galaxy," said Ben Oppenheimer of the University of California, Berkeley, lead author of the paper published in the journal Science last Friday. The finding, Oppenheimer said, "raises many questions concerning our understanding of the history of star formation in our galaxy and the processes underlying star formation." Chris Impey of the University of Arizona, a theorist and observer who is not a member of Oppenheimer's research team, said: "The stars they found can easily compete with all the stars seen in the disk of the galaxy."
"Snails" and MACHO The hypothesis regarding the existence of dark matter was first raised in the 30s. Newton's universal theory of gravity states that the rotation speed of stars around the center of the galaxy depends on their distance from the center of the galaxy and the mass of the galaxy. When the researchers examined the rotation speeds of stars and gas clouds far from the centers of galaxies known as "spirals" (due to their shape), it was found that the measured rotation speed was much greater than expected. From this, the researchers concluded that the mass of the galaxy is much higher than they estimated based on the light coming from the stars - in other words, that it contains additional material that is not visible to the eye. Astronomers initially tried to dismiss the idea, but the evidence eventually became overwhelming, and according to them about 95% of the mass in the universe is hidden from view in one way or another.
What is that dark matter? There are researchers who claim that dark matter consists of different particles than those that make up the matter we know from everyday life (protons, neutrons and electrons - "the matter that is us", as Oppenheimer put it). The claim is that these particles have no electric charge and that the electromagnetic force does not apply to them. From this it follows that they cannot emit any electromagnetic radiation: no light, no radio waves, no X-ray radiation. Since the observation devices that exist today absorb electromagnetic radiation, this material will be "dark" for us.
A second answer to the question of what dark matter is is that it is "normal" matter, most of which is hydrogen gas, and scientists have predicted that this "normal" component will eventually turn out to be something similar to "dead" stars - brown and white dwarfs. Since dwarfs cannot be observed directly, the researchers proposed to locate them through their influence on the light coming from distant stars. The theory of general relativity teaches that heavy bodies warp space and therefore deflect the path of light rays propagating through space. This effect is similar to the effect of a magnifying glass optical lens, for example, which changes the path of light rays by refracting them. Because of this, the effect of heavy bodies on light rays is called "gravitational attenuation".
The intensive efforts to obtain direct evidence of "dead" stars did not bear fruit, but the international project named MACHO discovered over six years hundreds of cases of "gravitational dusting" in the Milky Way galaxy, indicating the presence of dwarfs. These objects appeared to have roughly the mass of white dwarfs. However, because no one has seen them, the issue remains controversial.
It is possible that the new findings of Oppenheimer and his team will resolve the controversy. White dwarfs had been discovered before, but not in the location or numbers that would have made them candidates to explain the missing mass. To find the white dwarf population, Oppenheimer's group looked for extremely cold stars (that is, those with a temperature of 4,000 degrees Celsius; the temperature at the center of the Sun, by comparison, is 15 million degrees). Without thermonuclear combustion, the dwarfs' only radiation comes from the slow leakage of what little heat is still left in them. The scientists estimate that it would have taken 10 to 13 billion years for the discovered dwarfs to cool and dim to such an extent.
To determine how many such objects might exist in the Milky Way's halo, Nigel Hambley of the University of Edinburgh analyzed digital photographs of Earth's southern sky taken over the past 30 years. From an area equal in size to 10% of the sky, he selected dozens of objects that looked like white dwarfs. In October, the team of researchers used a 4-meter telescope in Coro Tulolo, Chile, to determine the nature of the objects by analyzing the light emitted from them.
These findings "are the best observational evidence for the correctness of the idea that there are more ancient white dwarfs in the galactic halo than expected," said Bohdan Peczynski of Princeton University, himself one of the skeptical researchers. In fact, there are about 20 times more dwarfs than expected.
