The unexpected discovery of a river of stars among the galaxies of the Coma cluster


Giant Come Stream
The “Giant Coma Stream” of stars in the Coma Berenices galaxy cluster captured by the William Hershel Telescope. Credit: William Herschel Telescope/Román et al
Sergio Messina Sergio Messina 5 minutes

The Coma cluster is a cluster containing thousands of galaxies about 350 million light years from us iin the direction of the constellation “Coma di Berenice” (Latin Coma Berenices), from which it takes its name.

Berenice, the wife of King Ptolemy of Egypt, was known for her beautiful hair, which was transformed into the constellation Coma Berenice.

over the years, this cluster has been the subject of repeated studies. In fact, it is one of the first clusters of galaxies in which there is a presence dark matter.

Visible matter, that which we see and know, is thought to make up only 4% of cosmic matter. Of the remaining 96%, 27% is dark matter and 69% is dark energy. Both dark matter and dark energy are so called because they are not visible and their nature is not understood. However, its gravitational effects on visible matter can be measured.

The search for dark matter and energy represents one of the most stimulating challenges of modern physics and astrophysics. Many projects have been and are dedicated to this study, such as the ESA Plank and Euclid missions.

In a study aimed at characterizing the stars in halo galaxies in the Coma cluster, the William Herschel telescope, which has a 4.2 meter primary mirror and is located on the island of La Palma in Spain, was used for observations.

What was discovered

An international team of researchers, analyzing images collected by the Herschel telescope, happened to discover a very strange structure, namely a stream of stars that extends between galaxies about ten times the diameter of our Galaxy.

This kind of discovery is called “serendipity”that is, an accidental discovery that was not anticipated and obtained while searching for something completely different.

The Coma Galaxy Cluster
Part of the Coma Berenices galaxy cluster captured by the Hubble Space Telescope. Almost all bright points are galaxies, mostly ellipticals. Acknowledgments: NASA, ESA and the Hubble Heritage Team (STScI/AURA).

What makes the discovery unique is the fact these stars form a structure similar to a stream of stars (or if we want a star bridge) outside the galaxy, where only gas and dust exist at extremely low densities. simply said, the “right” star structure, but in the wrong place. In fact, other similar but much smaller structures have been observed in our Galaxy.

This structure was called “Giant Coma Stream” and it is remarkable how it can resist the gravitational pull of nearby galaxies, maintaining its shape instead of being torn apart by external gravitational forces.

The origin of this structure is not yet known and it is ruled out that it could be the remains of a galaxy disturbed by gravitational effects.

Future developments in research

The results of this discovery were published in the journal Astronomy & Astrophysics by a team of researchers with the first signature Javier Roman. The same team performed computer simulations that were able to show how dynamically such structures can exist.

Coma cluster
Original Herschel Telescope (WHT) image of the Coma Berenices galaxy cluster zoomed in on the “Gian Coma Stream”. Credit:

The next step, the team’s astronomers say, will be to look for other similar structures and revisit the “Giant Coma Streamwith more powerful telescopes such as the Space Telescope James Webb or A very large telescope to ESO or in the near future toExtremely Large Telescope (ELT).


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