In our galaxy, 100 billion brown dwarfs

Astronomers estimated that the Milky Way contains at least 100 billion brown dwarfs – star objects that failed to turn into full-fledged stars. The research of scientists shows how much this type of stars in our galaxy is really widespread and what active participation they take in the formation of new stars. Figures 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 others. They are too big and hot (15-80 times more massive than our Jupiter), so that they can be classified as planets, but too small to be full stars – they do not have enough mass to maintain a stable synthesis of hydrogen in the nucleus. Nevertheless, brown dwarfs are initially formed in the same way as ordinary stars, so they are often called failed stars.

Back in 2013, astronomers began to suspect that brown dwarfs are quite a frequent occurrence for our galaxy, after calculating their approximate number in the region of 70 billion. However, new data presented at the National Astronomy Meeting, held recently in the English University of Hull, suggests that about 100 billion may be present in our galaxy. If we consider that the entire Milky Way can contain up to 400 billion stars according to an approximate estimate, then the number of brown dwarfs is both impressive and disappointing.

To clarify the results, astronomers conducted a study of more than a thousand brown dwarfs located within a radius of not more than 1500 light years. Since stars of this class are very dull, watching them at longer distances seems extremely difficult, if not impossible, to do. Most of the brown dwarfs known to us were discovered in the regions of formation of new stars, known as clusters. One such cluster is NGC 133, which contains almost as many brown dwarfs as ordinary stars.

This seemed very strange to Alex Scholz from the University of St Andrews and his colleagues Coralki Muzhic from Lisbon University. For a more detailed understanding of the frequency of the appearance of brown dwarfs in the star cluster of different densities, the researchers decided to look for more distant dwarfs in the denser star cluster RCW38.

In order to view the distant cluster, located about 5000 light-years from us, astronomers used a NACO camera with adaptive optics installed on the Very Large Telescope of the European Southern Observatory. As well as the framework of previous observations, this time scientists also found that the number of brown dwarfs of this cluster is almost half of the total number of stars in it, which in turn indicates that the frequency of the birth of brown dwarfs is completely independent of the Composition of star clusters.


The color image of the nucleus of a young but massive RCW 38 star cluster, data for which was obtained using an adaptive NACO optical camera mounted on the Very Large Telescope of the European Southern Observatory

“We found a large number of brown dwarfs in these clusters. It turns out that regardless of the type of cluster, a similar class of stars occurs quite often. And since brown dwarfs form together with other stars in clusters, we can conclude that there are really a lot of them in our galaxy, “Scholz comments.

It could be a figure of 100 billion. However, there may be more of them. Recall that brown dwarfs are very dim star objects, so even dimmer representatives could simply not get into the field of view of astronomers.

At the time of this writing, Scholz’s latest research was expected to be critically examined by third-party scientists, but astronomer John Omira of St. Miguel College, who did not take part in the work, gave the first comments on these observations to Gizmodo, but believed that the figures reflected in it could be Are correct.

“They come to the number of 100 billion, making many assumptions for this. But in fact, the conclusion about the number of brown dwarfs in the star cluster is based on the so-called initial mass function, which describes the distribution of the masses of stars in the cluster. When you know this function and you know how often the galaxy shapes 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 does seem real, “Omir commented.

And comparing the number of brown dwarfs in two different clusters – with a dense and less dense distribution of stars – the researchers showed that the medium in which stars appear is not always the key factor regulating the frequency of occurrence of this type of stellar objects.

“The formation of brown dwarfs is a universal and integral part of star formation in general,” says Omira.

Professor Abel Mendes of the Planetary Habitability Laboratory, another astronomer who also did not participate in the study, said that the figures in the new work could indeed make sense, especially given the fact that in our galaxy it is essential More compact star objects, rather 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 figures – it’s probably even the lower limit, “says Mendes.

There is, of course, the downside of such fertility of brown dwarfs. A large number of failed stars also means a decrease in the habitat potential. Mendes says that brown dwarfs are not stable enough to maintain an environment that is commonly called an inhabited zone. In addition, not all astronomers like the very term “failed stars”.

“Personally, I prefer not to call brown dwarfs” failed stars “, because, in my opinion, they just do not deserve the title of stars,” comments Jacqueline Fachherty, astrophysicist of the American Museum of Natural History.

“I would call them more” overgrown planets “, or simply” superplanets “, since from the point of view of their mass indicators they are still closer to these astronomical objects, rather than to the stars,” the scientist says.