Largest galaxy found + New type of star covered in helium burning ashes

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The largest galaxy ever found is absurdly enormous & strangely ordinary
https://www.iflscience.com/space/the-lar...-ordinary/

EXCERPTS: Astronomers have found a radio galaxy with a diameter 100 times as wide as our own Milky Way, yet the finders describe it as being “suspiciously ordinary” in every aspect other than its immense size. [...] They named the largest example they found Alcyoneus, after a giant enemy of Hercules whose name meant “mighty ass”. Alcyoneus is ... located around 3 billion light years away.

Yet when the authors tried to investigate what makes Alcyoneus so big they came up short. The galaxy that is producing these enormous ejecta is composed of stars with combined masses of 2.4 x 1011 times that of the Sun, about half that of the Milky Way. Its supermassive black hole has 400 million solar masses, or a hundred times Sagittarius A* at our own galaxy's heart, but is more than 100 times smaller than the largest known. If all it took to produce a giant radio galaxy was a black hole this big, there would be a lot more of them in the universe.

Besides its size, the one thing that stands out about Alcyoneus is that the pressure in its lobes is the lowest yet found, which, the authors state, makes it “the most promising radio galaxy yet to probe the warm–hot intergalactic medium.”

[...] The authors compare the features of Alcyoneus we can measure with those of other GRGs. If anything, its stellar and black hole mass are slightly on the low side, offering no clue as to why its radio jets are so enormous. Similarly, its luminosity density is fairly typical. ... Perhaps statistical analysis of larger GRGs will shed more light on the situation, but for the moment it remains a mystery as to how this galaxy could grow so big... (MORE - missing details)


Astronomers discover a new type of star covered in helium burning ashes
https://ras.ac.uk/news-and-press/news/as...ning-ashes

RELEASE: A team of German astronomers, led by Professor Klaus Werner of the University of Tübingen, have discovered a strange new type of star covered in the by-product of helium burning. It is possible that the stars might have been formed by a rare stellar merger event.The fascinating results are published in Monthly Notices of the Royal Astronomical Society.

While normal stars have surfaces composed of hydrogen and helium, the stars discovered by Werner and his colleagues have their surfaces covered with carbon and oxygen, the ashes of helium burning -- an exotic composition for a star. The situation becomes more puzzling as the new stars have temperatures and radii that indicate they are still burning helium in their cores -- a property typically seen in more evolved stars than those observed by Werner and his team in this study.

Published alongside the work of Professor Werner and his team, a second paper from a group of astronomers from the University of La Plata and the Max Planck Institute for Astrophysics offers a possible explanation for their formation. "We believe the stars discovered by our German colleagues might have formed in a very rare kind of stellar merger event between two white dwarf stars," says Dr Miller Bertolami of the Institute for Astrophysics of La Plata, lead author of the second paper. White dwarfs are the remnants of larger stars that have exhausted their nuclear fuel, and are typically very small and dense.

Stellar mergers are known to happen between white dwarfs in close binary systems due to the shrinking of the orbit caused by the emission of gravitational waves. "Usually, white dwarf mergers do not lead to the formation of stars enriched in carbon and oxygen," explains Miller Bertolami, "but we believe that, for binary systems formed with very specific masses, a carbon- and oxygen-rich white dwarf might be disrupted and end up on top of a helium-rich one, leading to the formation of these stars."

Yet no current stellar evolutionary models can fully explain the newly discovered stars. The team need refined models in order to assess whether these mergers can actually happen. These models could not only help the team to better understand these stars, but could also provide a deeper insight into the late evolution of binary systems and how their stars exchange mass as they evolve. Until astronomers develop more refined models for the evolution of binary stars, the origin of the helium covered stars will be up for debate.

"Normally we expect stars with these surface compositions to have already finished burning helium in their cores, and to be on their way to becoming white dwarfs. These new stars are a severe challenge to our understanding of stellar evolution." explains Professor Werner.
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