- 3459 - SUPERNOVAE - why haven’t we seen more? There should be a few supernovae in the Milky Way every century, but we’ve only seen 5 in the last 1,000 years. It’s been hundreds of years since the last observable supernova. New research explains why: it’s a combination of dust, distance, and dumb luck.
------------- 3459 - SUPERNOVAE - why haven’t we seen more?
- The last supernova to be noted in any kind of reliable source occurred in 1604, as recorded by many astronomers around the globe, most notably Johannes Kepler. At the time, nobody had any idea why or how these “new stars” appeared in the sky and then disappeared.
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- Nowadays we know the answer: they are the result of either the death of a massive star, or a runaway nuclear event on a white dwarf. Astronomers have also been able to calculate the typical supernovae event rate for a galaxy like our own, and it comes out to a few of these explosions every hundred years.
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- But in the four centuries since Kepler’s event, there isn’t a single reliable eyewitness account of a new star appearing in our skies. And that’s despite the fact that in those centuries, our technological ability to monitor the sky has exploded.
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- It’s not that the Milky Way is somehow not producing supernovae. The Cassiopeia A nebula is a remnant of a supernova that went off about 325 years ago, and yet nobody saw it.
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- Why aren’t we seeing more supernova? It all comes down to location, location, location. Most supernovae occur in the thin, star-filled disk of the galaxy. And that’s where most of the dust is. Dust that is exceedingly good at blocking light signals. The core of our galaxy hosts many more supernovae than average and a lot more dust.
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- In order to be observable to the naked eye, the supernova has to occur in just the right location in the galaxy. The astronomers’ model predicts that most of the naked-eye supernova should occur near the direction of the galactic center.
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- But most of the recorded supernovae don’t happen near there at all. It could be that the impact of spiral arms, which can trigger their own round of star formation and associated supernovae, play a role.
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- The researchers estimate that we have about a 33% chance of observing the next death of a massive star, and a 50% chance of seeing the next destruction of a white dwarf. As to when that will occur, that’s purely up to chance.
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- For the first time, a team of astronomers have imaged in real-time as a red supergiant star reached the end of its life. They watched as the star exploding as a supernova. Their observations contradict previous thinking into how red supergiants behave before they blow up.
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A team of astronomers watched the supernova explosion, “SN 2020tlf“, during the final 130 days leading up to its detonation.
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- This is a breakthrough in our understanding of what massive stars do moments before they die. Direct detection of pre-supernova activity in a red supergiant star has never been observed before in an ordinary Type II supernova. For the first time, we watched a red supergiant star explode!
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- The discovery dates back to the Summer of 2020. At that time, the progenitor star experienced a dramatic rise in luminosity. “Pan-STARRS” detected that brightening, and when Fall came around the star exploded as SN 2020tlf. The supernova is a Type II supernova, where a massive star experiences a rapid collapse and then explodes.
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- The Keck Observatory’s “Low-Resolution Imaging Spectrometer” (LRIS) captured the supernova’s first spectrum. The LRIS data showed circumstellar material around the star when it exploded. That material is likely what Pan-STARRS saw the star ejecting in the summer before it exploded.
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- This providing direct evidence of a massive star transitioning into a supernova explosion. It’s like watching a ticking time bomb. Astronomers had never confirmed such violent activity in a dying red supergiant star where we see it produce such a luminous emission, then collapse and combust, until now.
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- Data from the “DEep Imaging and Multi-Object Spectrograph” (DEIMOS) and “Near Infrared Echellette Spectrograph” (NIRES) showed that the progenitor star was 10 times more massive than the Sun. The star is in the NGC 5731 galaxy about 120 million light-years away.
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- This explosion has given new insight into Type II supernovae and their progenitor stars. Prior to these observations, nobody had seen a red supergiant display such a spike in luminosity and undergo such powerful eruptions before exploding.
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- Red supergiant stars eject material prior to core collapse. But that material ejection takes place on much longer timescales than SN 2020tlf. This supernova emitted “circumstellar material” (CSM) for 130 days prior to collapse.
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- The bright flash prior to the star’s explosion is somehow related to the ejected CSM, but the team of researchers isn’t certain how they all interacted. The significant variability in the star leading up to collapse is puzzling. The powerful burst of light coming from the star prior to exploding suggests that something unknown happens in its internal structure. Whatever those changes are, they result in a mammoth ejection of gas before the star collapsed and exploded.
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- What may have caused the ejection of gas? One possibility is wave-driven mass loss, which occurs in the late stages of stellar evolution. It occurs when the excitation of gravitational waves by oxygen or neon burning in the final years before saupernova can allow for the injection of energy into the outer stellar layers, resulting in an inflated envelope and/or eruptive mass-loss episodes.
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- But current wave-driven models don’t match the progenitor star’s ejection of gas. They’re consistent with the progenitor star’s radius in its last 130 days, but not consistent with the burst of luminosity.
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- Given the progenitor mass range derived from nebular spectra, it is likely that the enhanced mass loss and precursor emission are the results of instabilities deeply rooted in the stellar interior, most likely associated with the final nuclear burning stages.
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- Energy deposition from either gravitational waves generated in neon/oxygen burning stages or a silicon flash in the progenitor’s final 130 days could have ejected stellar material that was then detected in both pre-explosion flux and the early-time spectrum.
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- If there’s one supernova that behaves like this, there must be more. The findings mean that surveys like the “Young Supernova Experiment” transient survey now have a way to find more of them in the future. If the survey finds more stars ejecting material like this one, then they know to keep an eye on it to see if it collapses and explodes.
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- There new ‘unknowns’ that have been unlocked by this discovery. Detecting more events like SN 2020tlf will dramatically impact how we define the final months of stellar evolution, uniting observers and theorists in the quest to solve the mystery of how massive stars spend the final moments of their lives.
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- We have about 5,000,000 more years before our star explodes. But, it is not big enough to go full supernova, it will become a fizzled planetary nebulae. Hardly worth waiting for.
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February 13, 2022 SUPERNOVAE - why haven’t we seen more? 3459
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