Curiosities and celestial phenomena of spring
The astronomer Rafael Bachiller reveals to us in this series the most spectacular phenomena of the Cosmos. Topics of exciting research, astronomical adventures and scientific news about the Universe analyzed in depth.
Starting from a very detailed map of the positions of massive blue stars, Spanish astronomers have discovered a new structure in our Galaxy: a kind of spur that connects the arm of Orin with that of Perseus.
Blue, bright, massive and short-lived
The data obtained by the European Gaia space telescope are revealing surprising details of the structure of the Milky Way, our Galaxy. Astronomers want to know the detailed distribution of different types of stars and their relationship to the larger galactic structures, such as the disk, the spiral arms, or the halo. And it is that in these components the long history of the formation and evolution of the Milky Way is written. Thus, using statistical tools to reveal the main patterns of the distribution of different types of stars, astronomers gradually decipher the major episodes in the complex history of our galactic home.
Particularly interesting in galactic studies are the large blue stars (those of type O and B, in astronomer’s slang). These stars have very short lives, and their mere presence in a region of the Milky Way tells us about star-forming activity in that region. In general terms, these blue stars are distributed along the spiral arms, where interstellar clouds abound in which all kinds of stars are born. Because they have such a short life, the blue stars do not get very far from their place of formation. Therefore, the blue stars, which are large, very massive and bright, serve to delineate the most active regions of the Galaxy.
Bridge from Orin to Perseus
Astronomers Michelangelo Pantaleoni Gonzlez and Jess Maz Apellniz (Centro de Astrobiologa, INTA-CSIC) have spent several years studying the catalogs of these massive stars (which contain almost 20,000 of them) and refining the maps of their spatial distribution. Now, thanks to the arrival of the excellent data provided by the Gaia telescope of the European Space Agency (ESA), they are making a lot of progress in their work. Thanks to a study coordinated by Pantaleoni, the group has just published the most detailed maps made to date of the spiral arms of our galaxy.
By reviewing the distribution of stars in the Sun’s vicinity, researchers have discovered a structure that had not been identified until now. It is a branch that extends from our spiral arm (that of Orin) towards the outer direction of the Milky Way, where the next arm (that of Perseus) is located. At nearly 8,000 light-years long, this kind of filament is not exactly coplanar with the disk, but rather rises above the galactic plane. Such interarm structures are often referred to as ‘spurs’ (spur in English) and this one, in particular, has been baptized as ‘Cepheus spur’, since it is in this constellation where it can be better appreciated, projected on the celestial vault.
It is not easy to visualize the spatial structure of the Cepheus spur. To help us in this task, astronomers have made representations of their data in 3 dimensions showing the positions of massive blue stars up to a radius of about 12,000 light years from the position of the Sun. In a first animation, The Galaxy appears only on edge, while in this second animation the point of view flies over the Galaxy. In both videos, the stars on the spur are represented in yellow. A frame of these animated maps is shown in the figure attached here.
As we said at the beginning of this article, there are other groups of astronomers studying the structure of the Milky Way and, more specifically, the region of the solar environment. Thus, a team led by Joo Alves (University of Vienna), studying the distribution of gas in the Galaxy, had identified a kind of filament perpendicular to the galactic plane known as the Radcliffe ripple (see his publication in Nature at this link). Pantaleoni and Maz think that the Cepheus spur could be a manifestation of this undulation and go on to describe its structure in greater detail.
These works therefore show that the galactic plane is not as flat as previously thought. The Cepheus spur and the Radcliffe ripple are part of a protruding corrugation in the direction perpendicular to the plane. We could think that the galactic plane is similar to a wrinkled cloth placed on the ground, the corrugations will be the bulges that rise above the horizontal plane.
Collision between galaxies?
Astronomers do not yet know the precise origin of these interarm structures. Certainly the collision of the Milky Way with another galaxy of much smaller size, in the distant past, could have caused this type of filament. But this is something that will have to be investigated further. It would be interesting, for example, to perform computer simulations of such a hypothetical collision between galaxies. The distributions of the interstellar clouds and the different stellar populations resulting from the simulations could then be compared with the observations.
The results of Pantaleoni et al. Are based on the Gaia data set known as DR2 (Data Release 2). But Gaia will continue to make more and more data public in the future. In particular, the DR3 set, which is much more precise, should allow researchers to refine the maps they just published to achieve a much more detailed view of the solar environment.
The article by Pantaleoni Gonzlez et al., Entitled “The Alma catalog of OB stars. II. A cross-match with Gaia DR2 and an updated map of the solar neighbourhood” It has been published in a recent issue of the British journal Monthly Notices of the Royal Astronomical Society. The manuscript can be consulted at this link.
Rafael Bachiller is director of the National Astronomical Observatory (National Geographic Institute) and academic of the Royal Academy of Doctors of Spain.
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