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How do LBVs trigger supernova explosions? What secrets are hidden in their evolutionary paths?
Sharp and translucent blue variables (LBVs) are quite unique in the interstellar universe. They are rare, extremely massive stars with complex evolution processes. These stars exhibit unpredictable characteristics in their continuously changing spectra and brightness, and their dramatic changes often attract the attention of astronomers. For example, S Doradus is one of the bright LBVs located in the Large Magellanic Cloud. The evolution of LBVs and how they trigger supernova explosions remains a mysterious area of exploration for the astronomical community.
How do LBVs transition from stable stars to the stage of imminent supernova explosion during their evolution?
The discovery and history of LBVs
The mystery of LBVs had begun to unravel in the 17th century, when astronomers noticed unusual changes in P Cygni and eta Carinae. However, it was not until the late 20th century that scientists truly understood the nature of these variable stars. John Charles Duncan first discovered three variable stars in the Triangulum Galaxy (M33) in 1922, followed by further observations by Edwin Hubble in 1926. It was not until the 1970s that LBVs began to be formally named and associated with S Doradus type variable stars. Subsequent studies have further confirmed their connection to other variable stars.
Physical properties of LBVs
LBVs are regarded as extremely massive unstable supergiants, exhibiting a series of spectral and photometric changes. They usually exhibit the characteristics of B-type stars in their "quiescent" state, and often have unusual emission lines. In the Hertz-Sprang–Russell diagram, LBVs are located in the S Doradus instability zone, a region where stars can exceed hundreds of thousands of times the brightness of the Sun. The high brightness and temperature of these stars are astonishing. General outbursts can cause the temperature of LBVs to drop slightly, however in some cases some LBVs may show larger changes in brightness during the outburst.
The evolution of LBVs
The average life span of LBVs is relatively short, usually only a few million years, with the life cycle of the LBV stage even shorter. The high luminosity of these stars puts them in a state of rapid evolution, and many LBV precursors have been found to be possibly related to supernovae. Recent theoretical studies have shown that LBVs are usually the final evolutionary stage of more massive stars before they explode, and this process is crucial for their mass loss. In fact, the phenomenon of LBV explosion may directly affect its subsequent supernova performance.
Supernova-like explosion
The most fascinating thing about LBVs is that they undergo "giant outbursts," resulting in dramatic increases in mass and brightness. Taking eta Carinae as an example, it is a typical representative among LBVs, while P Cygni also showed similar explosive behavior 300 to 400 years ago. These events vary in duration, with some lasting years or even decades, and many events initially classified as supernovae are instead re-evaluated because of their special characteristics. Examples of these supernova imitators, such as SN 1954J and SN 2005gl, are possible precursors to LBV.
Summary
The existence of LBVs and the supernova explosions they trigger reveal many mysteries hidden in stellar evolution. While the current study provides valuable insights, there are still many unknowns about the behavior of these mysterious variable stars and their consequences. As technology advances, future observations and model building may unlock more secrets of these giant stars' brief but dazzling lives. When we think about supernovae caused by LBVs and their evolutionary paths, we can’t help but ask: How many unknown variables are there in the universe waiting for us to explore?
