Astronomers estimate that the Milky Way contains at least 100 billion brown dwarfs - stellar objects that have failed to turn into full-fledged stars. The research of scientists shows how much this type of stars is actually common in our galaxy and how actively they take part in the formation of new stars. The numbers show that for 2-3 stars of other classes there is at least 1 brown dwarf.
This type of space objects clearly stands out from the rest. They are too large and hot (15-80 times more massive than our Jupiter) to be classified as planets, but too small to be full-fledged stars - they do not have enough mass to maintain stable hydrogen fusion in the core. However, brown dwarfs initially form in the same way as ordinary stars, which is why they are often called failed stars.
Back in 2013, astronomers began to suspect that brown dwarfs are quite common in our galaxy, counting their approximate number in the region of 70 billion. However, new data presented at the National Astronomy Meeting, held recently at the English University of Hull, suggest that there may be about 100 billion of such space objects in our galaxy. Considering that the entire Milky Way may contain an estimated 400 billion stars, the number of brown dwarfs is both impressive and disappointing.
To refine the results, astronomers have studied more than a thousand brown dwarfs located within a radius of no more than 1,500 light years. Since stars of this class are very faint, observing them at longer distances is extremely difficult, if not impossible. Most of the brown dwarfs we know have been found in new star-forming regions known as clusters. One such cluster is the object NGC 133, which contains almost as many brown dwarfs as ordinary stars.
This seemed very strange to Alex Scholz of St Andrews University and his colleague Koralka Muzhich of the University of Lisbon. For a more detailed understanding of the frequency of birth of brown dwarfs inside star clusters of different densities, the researchers decided to look for more distant dwarfs in the denser star cluster RCW38.
Astronomers used the NACO adaptive optics camera on the ESO's Very Large Telescope to view a distant cluster about 5,000 light years away. As with previous observations, this time, scientists also found that the number of brown dwarfs in this cluster is almost half of the total number of stars in it, which, in turn, suggests that the frequency of birth of brown dwarfs does not depend at all on composition of star clusters.
Color image of the core of a young but massive star cluster RCW 38, for which data were obtained with the NACO adaptive optical camera installed at the ESO Very Large Telescope
“We found a large number of brown dwarfs in these clusters. It turns out that regardless of the type of cluster, this class of stars is quite common. And since brown dwarfs form together with other stars in clusters, we can conclude that there are indeed a lot of them in our galaxy,”Scholz comments.
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We can talk about a figure of 100 billion. However, there may be even more of them. Recall that brown dwarfs are very dim stellar objects, so their even dimmer representatives could simply not get into the field of visibility of astronomers.
At the time of this writing, the results of Scholz's latest research were awaiting critical review by outside scientists, but the first comments on these observations to Gizmodo were given by astronomer John Omira from Saint Miguel College, who was not involved in the work, but believes that the figures reflected in it may be are correct.
“They come to the number 100 billion, making a lot of assumptions for that. But in fact, the conclusion about the number of brown dwarfs in a star cluster is based on the so-called initial mass function, which describes the distribution of masses of stars in the cluster. When you know such a function and you know with what frequency a galaxy forms stars, then you can calculate the number of stars of a certain type. Therefore, if we omit a couple of assumptions, then the figure of 100 billion really seems real, commented Omira.
And by comparing the number of brown dwarfs in two different clusters - with a dense and less dense distribution of stars - the researchers showed that the environment in which stars appear is not always the key factor in regulating the frequency of occurrence of these types of stellar objects.
“Brown dwarf formation is a universal and integral part of star formation in general,” says Omira.
Professor Abel Mendes of the Planetary Habitability Laboratory, another astronomer who was also not involved in the study in question, says the numbers in the new work may indeed make sense, especially given the fact that there is a significant more compact stellar objects than larger ones.
“Small red dwarfs, for example, are much more common than all other types of stars. Therefore, I would suggest that the new numbers are more likely even the lower limit,”says Mendes.
There is, of course, a downside to this fertility of brown dwarfs. A large number of failed stars also means a decrease in the potential for habitability. Mendes says brown dwarfs are not stable enough to support what is commonly called the habitable zone. In addition, not all astronomers like the term "failed stars" itself.
“Personally, I prefer not to call brown dwarfs 'failed stars' because, in my opinion, they simply don't deserve the title of stars,” comments Jacqueline Facherty, astrophysicist at the American Museum of Natural History.
“I would call them rather“overgrown planets”, or simply“superplanets”, since in terms of their mass indices, they are nevertheless closer to these astronomical objects than to the stars,” the scientist says.
NIKOLAY KHIZHNYAK
