Why do some stars become ‘supernova impostors’? Astronomers still don’t quite know
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Historical records show that about 170 years ago, the star Eta Carinae suffered an unusual explosion that made it one of the brightest stars in the southern sky. But it wasn’t a supernova, it was a “supernova imposter.” | Credits: NASA, ESA, Hubble; Processing and licensing: Judy Schmidt
Looking up at the night sky, you might imagine a star bursting into flames, burning thousands of times brighter than usual. It’s a cosmic explosion – a supernova! Except that’s not the case. The star endures.
These violent, non-lethal flares can cause a star to mimic a real supernova – leading to what we affectionately call “supernova impostors.”
These are huge starsprone to titanic tantrums, releasing huge amounts of their own material. Astronomers call this “eruptive mass loss,” and it’s a stellar drama that we’re still trying to fully understand.
Trying to understand these supernova impostors is like trying to weigh the output of a raging volcano without getting too close. We know it’s important, but measuring how much material these stars eject and what drives them to do so is surprisingly difficult.
Current methods of measuring mass loss, for example from infrared or radio observations, generally only show us what is happening at the moment. But these stars spit stuff out in spurts, not in a constant stream. And when we try to average all stellar populations, we lose the juicy details of each star’s behavior.
For decades, astronomers have concocted complex computer models to predict how stars live and die. These traces of stellar evolution are our cosmic crystal balls. But for truly gargantuan stars, the models often sputter, unable to complete their lives in the simulation. A big sticking point? This same loss of eruptive mass.
The models include one way of describing it, by imagining light pressure pushing matter out of the star, exceeding its stable luminosity limit – what scientists call super-Eddington conditions.
But the key to making it work is a floating efficiency setting – a dial that controls the strength of the explosion. And no one knew where to put it. This was a crucial and unconstrained value, which hampered our understanding of the evolution of these cosmic giants.
The difficulty of accurately modeling these phenomena means that despite growing observational evidence of violent eruptions, the underlying physical mechanisms remain poorly understood.
NGC 3184, also known as the Little Pinwheel Galaxy, with an arrow pointing to an impostor supernova observed in 2010 known as SN 2010dn. | Credit: Kevin Heider/Wikimedia Commons/CC BY-SA 3.0
But astronomers are very intelligent. A team led by Shelley J. Cheng at the Center for Astrophysics | Harvard & Smithsonian, along with Charlie Conroy and Jared A. Goldberg, decided to tackle this problem head on. new study published on arXiv.
Their idea? Instead of trying to measure every little burp of a single giant, they would take a census of red supergiants among our close galactic neighbors – what we call Local Group stellar populations. These are massive, late-stage stars, puffy and red, that shine across the cosmos. We know where they live. We know what they look like.
Wide-field surveys, like the PanSTARRS1 Medium-Deep Survey, have revolutionized our ability to spot these special transients and bright explosions, helping us map these red giants in distant galaxies. This observational power is crucial for collecting the data needed to calibrate eruptive mass loss.
The team used sophisticated models of MESA stellar evolution, adjusting this mysterious efficiency parameter to see what happened. Then they created fake stellar populations – basically, fake galaxies full of these modeled stars, sampling different masses and initial ages, just like real star-forming regions.
They then compared the predicted brightness distributions of these false stars to actual observations of red supergiants in the Small Magellanic Cloudthe Large Magellanic Cloud and the Andromeda Galaxy (M31). It was like trying to match a blurry photo of a crowd to a line of suspects, carefully adjusting until the image clicked.
What they discovered was that the efficiency parameter wasn’t just a random number. It showed a clear, positive trend in metallicity – the amount of heavy elements baked into a star.
Heavier elements, more violent eruptions. It’s a bit like adding baking soda to a volcano experiment: things become more alive.
With this calibrated eruptive mass loss, stars that start out truly massive – about 20 times the mass of the sun – can never even become massive. red supergiants in the models. Instead, these colossal stars lose so much material in their dramatic explosions that they skip this red supergiant phase entirely, evolving on a different path.
But the universeas always, contains more cards. This relationship between mass loss and metallicity appears robust, but we need to test it in more galaxies, not just our immediate neighbors, to confirm that this trend is truly widespread. Future simulations will also need to dig into the details: Does metallicity affect what triggers an eruption, or just the amount of substances that escape?
The saga of these spitting stars is far from over. Each new burst of observation, each refined model peels back another layer, showing us how dynamic and surprising the life of a star can be.
