FU Orionis stars are pre-main sequence stars and are considered to be a stage of T-Tauri stars. These star exhibit extreme variability over a period of time. These stars can gain a magnitude difference of about 6 degrees brighter such as what happened in the case of Fu Orionis in 1936. These stars can be referred to as FUors.
A T-Tauri Star is a young star, about 10 million years old and one that has yet to be converting Hydrogen into Helium or Carbon. It is the stage in a star life when it has passed the protostars stage and is now pushing away the excess accretion disk that orbits in. They get their name from the star FU Orionis in the constellation of Orion (surprise!). There are not that many FU Orionis stars out there compared to T-Tauri stars.
FU Orionis was deemed to be unique until a star exhibiting the same style was discovered in the constellation of Cygnus. V1057 Cygni was spotted in 1970 where the star brightened by a magnitude of 5.5 It didn't fade back down as quickly as it brightened but over time. It had been considered to be a Nova but because it because of the speed of dimming, that idea was rejected. A.A.V.S.O.
FU Orionis will be in the current state it is in for the next hundred years before it runs to a normal state. There's plenty of time to see it. After the FU Orionis stage, it will become a T-Tauri star again until it has enough mass to begin converting Hydrogen into Helium or Carbon.
At the conversion stage, the star will have entered Main Sequence stage which our star is currently in. A star can be in the main sequence stage for millions, billions or trillions of years. The smaller a star is, the longer it'll live. Smaller stars are more energy efficient so a star like Proxima Centauri, a red dwarf star could possibly live for a trillion years.
On a side note, Proxima Centauri might not get to a trillion because it could be destroyed by Rigil Kentaurus going supernova in a several* billion years time. A giant star like Regulus, a large blue star could only expect millions of years of life.
Our Sun could have gone through a FU Orionis stage as it is below 3 solar masses, anything higher, the star wouldn't necessarily become a T-Tauri star or a FU Orionis for that matter. Our star probably had gone through a FU Orionis stage as that would then explain why some elements are more prevalent on Mars than they are here. Sci-Tech Daily
FU Orionis is a star that at a magnitude of about 10 is not something that can be seen with the naked eye. You would need something like a 10x50 pair of binoculars or better. The picture was generated using Stellarium, a totally free software to download and use. Magnitudes
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