New structure discovered in the Milky Way: Cepheus spur

The massive blue stars (scientifically known as OB stars being between the spectral classes O and B) they have a peculiarity that makes them especially interesting for astrophysicists: they have a ephemeral life of a few million years.

In the same way that the dating of rocks reveals the level of geological activity of a planet, the presence of OB stars in the Milky Way is an indicator of activity in our galaxy, since they indicate star-forming regions. Wherever they are, it can be said that the galaxy is ‘alive’, they are areas where new stars are originating.

By making the most detailed maps of massive blue stars in our neighborhood and that of the spiral arms of our galaxy, a structure has been discovered that connects the arm of Orion, where we are, with that of Perseus

On the other hand, these short-lived stars do not have time to move away from the areas where they are born, the spiral arms, so they are also excellent references for mapping those galactic structures.

With these advantages and in this context, an international team of researchers led from the Astrobiology Center (CAB, CSIC-INTA) has carried out a comprehensive update of the largest existing catalog of massive OB stars in our galaxy: the so-called catalog Alma Luminous Stars (ALS), compiled two decades ago and with almost 20,000 objects.

The authors have for months crossed the old data of each star with that recently provided by the Gaia mission of the European Space Agency (ESA). Specifically, the information from Gaia DR2 (Data Release 2), thus obtaining an updated catalog, although soon they will do so even more with the even more precise data from Gaia EDR3.

Map of the spiral arms of the Milky Way

But for now, the results achieved so far, published in the Monthly Notices of the Royal Astronomical Society (MNRAS), have already made it possible to trace for the first time the most detailed map of the spiral arms of our galaxy.

Spiral arms of the Milky Way. Between that of Orion and Perseus would be the recently discovered Cepheus spur. / NASA / JPL-Caltech / ESO / R. Hurt

How it stands out Michelangelo Pantaleoni González, CAB researcher and lead author of the study, “having such an up-to-date sample of stars has led us to review which aspects of our galactic environment were manifesting more clearly, and that’s where the surprise arose.”

The spur of Cepheus

The map is so detailed that it has allowed to discover something no one had seen until now: a branch of the spiral arm where our solar system (Orion’s) is located. Researchers have dubbed it “The spur of Cepheus”: spur (spur in English) because it is how they are called this type of structures between arms and Cepheus because it is the constellation where it is more prominent.

The new structure is about 10,000 light-years long and extends outward toward the next arm (that of Perseus), also rising above the plane of the galaxy.

Regarding its origin, Jesus Corn Apellániz, a CAB researcher and study co-author, explains: “It had recently been proposed that there is something called radcliffe wave as an oscillation in the vertical distribution (with respect to the galactic plane) of the young stars in our environment. That study presented oscillation as a phenomenon in one dimension and now we have seen that it occurs in two dimensions ”.

Its origin is related to corrugation: the galactic plane has ‘wrinkles’ like an unstretched fabric, where this spur is the crest of the undulation and the valley is formed by other star-forming regions such as the Orion and Rosette nebulae

“The Cepheus spur – he adds – is the crest of the undulation and the valley is formed by other regions of star formation such as the Orion and Rosetta nebulae. This phenomenon is known as corrugation, that is, the galactic plane has wrinkles like a cloth laid on the ground without stretching and this is the best demonstration of its existence in the solar environment ”.

For his part, Pantaleoni concludes: “It is interesting to note that the huge amount of data obtained with the Gaia mission and the use of statistical tools has allowed us to draw interesting general conclusions about our environment, such as signs of the warping of our galaxy (warped) and corrugations of the disk, which are probably relics of the convulsive evolution of the Milky Way ”.


M Pantaleoni González, J Maíz Apellániz, RH Barbá, B Cameron Reed. “The Alma catalog of OB stars. II. A cross-match with Gaia DR2 and an updated map of the solar neighborhood ”. Monthly Notices of the Royal Astronomical Society, 2021.

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