Pretty interesting stuff.

(http://www.autoadmit.com/thread.php?thread_id=5865682&forum_id=2...#49879040)

Yes, a red supergiant star exploding as a Type II supernova at 25 light years could devastate or effectively destroy most life on Earth, primarily through catastrophic ozone depletion and radiation effects.

en.wikipedia.org

Red supergiants (massive stars 8–20+ times the Sun’s mass in their late stages) end their lives in core-collapse Type II supernovae. These events release immense bursts of gamma rays, X-rays, neutrinos, and later high-energy cosmic rays. At 25 light years (~7.7 parsecs), Earth sits right at or just inside the estimated “kill zone” for significant biosphere damage.

en.wikipedia.org

Key Effects at This DistanceOzone layer destruction: Gamma rays and X-rays from the supernova trigger chemical reactions in the upper atmosphere that break down ozone (O₃). Models based on observed supernovae (like SN 1987A) show that a Type II supernova closer than ~8 parsecs (26 light years) would destroy more than half of Earth’s ozone layer—potentially ~47% or more depletion. This doubles (or more) the biologically harmful ultraviolet (UV) radiation reaching the surface.

worldbuilding.stackexchange.com

Immediate radiation spike: The gamma-ray and X-ray flash arrives with the visible light (after ~25 years of travel time). While our atmosphere shields the surface from the worst direct radiation, the ozone loss exposes everything to intense solar UV. This damages DNA, causes mutations, burns skin/eyes, and kills or impairs plants, phytoplankton, and reef ecosystems (the base of ocean food chains).

earthsky.org

Prolonged cosmic-ray bombardment: Over years to centuries (or longer), accelerated cosmic rays from the supernova remnant increase ionization in the atmosphere. This leads to more nitrogen oxides, acid rain, further ozone loss, and direct radiation (muons) that penetrates to the surface. Some models suggest this could trigger mass extinctions even out to 30–50 light years, depending on the exact supernova energy and interstellar medium.

sciencealert.com

The result would likely be a mass extinction event: widespread die-offs of surface and shallow-ocean life, collapse of food webs, agricultural failure, and severe impacts on complex multicellular organisms (including humans). Phytoplankton and coral reefs are especially vulnerable, which could crash marine ecosystems.

earthsky.org

Microbial life in deep oceans, subsurface environments, or shielded habitats might survive, but surface biosphere habitability for most species would be destroyed or drastically altered for decades to centuries while the ozone recovers.Important CaveatsNot physical destruction of the planet: The supernova’s expanding shell of material travels at thousands of km/s but takes thousands of years to reach 25 light years and would be too diluted to strip the atmosphere or shatter Earth. The threat is purely radiative and atmospheric.

skyatnightmagazine.com

Not total instant sterilization: Earlier dramatic claims (e.g., complete atmospheric loss) are overstated. Modern models point to severe but not necessarily 100% extinction-level effects—though still catastrophic enough to end civilization and most complex life as we know it.

earthsky.org

Distance estimates vary slightly: Older studies pegged the critical distance for major havoc at ~25 light years; some updated models extend dangerous effects (via cosmic rays or X-rays) to 40–160 light years for noticeable atmospheric or biological damage, though 25 light years remains firmly in the high-risk zone.

earthsky.org

Fortunately, no red supergiant (or any supernova progenitor) exists within 25–50 light years of Earth. The nearest candidates, like Betelgeuse (640 light years) or Antares (550–600 light years), are far outside the danger zone. Supernovae this close are extremely rare—on average, one occurs within ~33 light years every few hundred million years.

en.wikipedia.org

In short, at exactly 25 light years, the explosion would deliver a biosphere-shattering blow via radiation and UV overload. Life on Earth would not survive intact.

(http://www.autoadmit.com/thread.php?thread_id=5865682&forum_id=2...#49879073)