Millions of massive stars are fleeing the Milky Way

The Milky Way does not retain all its stars. Some are sometimes even flung into intergalactic space and embark on an unpredictable journey. A team of astronomers recently conducted an in-depth study of these fleeing objects to understand the mechanisms of these releases.

When astronomers study a collection of stars in the Milky Way, one of the parameters they analyze is their velocity distribution. Indeed, it provides clues about how the stars within the galaxy move relative to a reference point. The overall distribution of the speed of these objects in a given stellar population is also strongly affected by the rotation of the galaxy. Thus, a star that does not follow this harmony will necessarily attract the attention of astronomers.

Two types of stars

As part of this work, researchers conducted a survey based on two catalogs of massive stars, namely the Galactic O-Star Catalog (GOSC) and the Be a star spectra (BeSS). These list different types of massive stars, such as O-type stars and Be-type stars, and their respective subtypes.

In detail, O-type stars are among the most massive and hottest stars in the universe. These objects are relatively rare, but play a crucial role in star formation and galaxy dynamics. Due to their large mass, they have a relatively short lifespan and can end up in a supernova.

Be-type stars, also massive and luminous, are characterized by the presence of specific spectral emissions related to the presence of certain absorption lines in their spectrum. The term “Be” comes from the German word “B-emission”, which indicates these emission lines. These objects tend to have high rotational speeds, causing their equators to become thinner. They are also often surrounded by a disk of rotating material.

trees stars sky forest milky way night
©Hristo Fidanov/Pexels

Many objects on the run

The researchers also cross-referenced this data with data from Gaia, a European Space Agency (ESA) satellite that measures the positions, distances and movements of a large number of stars.

By analyzing the data, the researchers identified 417 O-type and 1,335 Be-type objects that were present in both Gaia’s catalogs and observations. Among these stars, they discovered 106 O-type runaway stars, representing 25.4% of the stars listed in the GOSC catalog. Of this sample, 42 were previously unknown.

As for the Be-type stars, they identified 69 runaways, accounting for 5.2% of the objects in the BeSS catalogue, with 47 new identifications. In general, O-type stars showed faster motions than others. All of these stars are apparently on a trajectory that will take them outside the Milky Way.

Note that this study focuses on only two catalogs. No one really knows how many runaway objects are about to leave our galaxy. However, some estimates indicate that this could be the case currently over ten million.

Two proposed mechanisms

To explain these results, the researchers put forward two competing scenarios: that of dynamic ejection and that of the binary supernova.

In the first scenario, stars, often formed in binary pairs, undergo gravitational interactions in dense environments. These interactions may result from encounters between binaries and single stars, creating runaway objects. OB associations, where O-type and B-type stars tend to form, represent dense environments conducive to this process.

The alternative scenario involves one star exploding as a supernova and affecting its binary partner. If conditions are favorable, the surviving object gains enough momentum to escape its bond with its partner. Sometimes it can then escape the gravity of the Milky Way and embark on an intergalactic journey.

This illustration shows a star exploding in a supernova, subjecting its binary star to the brute force of the explosion. Credits: NASA, ESA, Leah Hustak (STScI)

The fact that O-type stars have a higher velocity suggests that the first scenario is more likely. However, further research will be needed to confirm this.

Details of the research have been published in the journal Astronomy & Astrophysics.

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