An event that occurred in 1937 caught the attention of astronomers around the world who little knew that the mystery would remain for many decades. A star located in the Orion constellation suddenly turned 1,000 times brighter. This was one of the first observations of stars suddenly brightening and seemingly unexplained.
For decades, astronomers have found stars with similar phenomena and the search for an explanation has become the interest of research groups. These stars became known as FU Orionis, which refers to the name of the 1937 star, Orionis North. What caught our attention is that this behavior seemed to be associated only with old stars, while FU Orionis are young.
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With new data from the ALMA observatory, a group of astronomers published an article in The Astrophysical Journal arguing that they found the reason for these phenomena. According to the article, this is the first time they have been captured observational data on the process that causes the brightness increase. And a physical explanation became possible.
Stars FU Orionis
In the constellation of Orion there are a pair of stars with masses of 0.6 solar masses and 1.2 solar masses. The stars are called North Orionis and South Orionis, respectively. They attracted attention in 1937, when the Star Orionis North suddenly became about 1,000 times brighter. Since then, astronomers began to search for an explanation for the phenomenon.
No observed FU Orionis stars have returned to their original brightness, although the event is expected to last only a few decades.
Stars that had similar events were named stars FU Orionis in honor of the couple from 1937. These stars attract attention because they are stars that are a few million years old and very young. V1647 Orionis was a FU Orionis phenomenon that occurred in 2004 and is one of the objects studied as well.
SOUL
The ALMA observatory located in Chile was built as a revolutionary tool for astronomy. It has 66 antennas and the main array has a diameter of 12 meters. The 66 antennas act together as a large interferometer to observe radio waves. ALMA observes the sky all day long, all year round.
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One of the great observations that highlighted the importance of ALMA was the photograph of the M87* black hole. Thanks to ALMA within the project that aimed to photograph a black hole, the photo was possible and published in 2019. In addition to its emphasis on science, ALMA is also a tourist site that attracts many visiting tourists.
Without explanation
The FU Orionis stars attract attention precisely because they are very young stars, only a few million years old. Brightness variation is not uncommon in stars, but this process usually occurs towards the end of a star’s life.. During the end of its life, the star presents variations in brightness that can increase or decrease depending on the stage.
This happens because the star expands when the hydrogen fuel runs out, at this stage it becomes a red giant. How bright a red giant will be depends on the mass the star had before the process began. In the red giant stage, layers of gases escape from the star’s surface, varying in brightness.
accretion disk
When we talk about accretion disks, it is natural to think of the famous accretion disks of black holes. These structures get their name because they are true disks of material, usually plasma, that spiral toward a central object. Any object that has a strong enough gravitational field can have an accretion disk.
Therefore, It is quite common to find accretion disks in stars and material that spirals towards them. This mainly happens in younger stars, since the rest of the material that formed the star can form a disk around it. There is a complex dynamic between the central object and the accretion disk.
End of a mystery
ALMA researchers used data from the observatory to map carbon monoxide (CO) around 1937 FU Orionis. They found a CO filament that describes an accretion pattern around the star. However, the filament is not massive enough to have caused a variation in brightness.
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The idea is that the filament is a remnant of a past event that destabilized the accretion disk. As it destabilized, more material fell toward the star, causing its brightness to increase considerably.. The brightness of the disk and the object depends on the accretion rate of the central object.
Sources and reference of the news
Hales et al. 2024 Discovery of an Accretion Streamer and a Slow Wide-angle Outflow around FU Orionis The Astrophysical Journal
