SN2021gmj: What Happened, Distance & Viewing Guide

Q: What type of supernova is SN2021gmj and what kind of star exploded?

SN2021gmj is a Type II Core-Collapse Supernova, marking the death of an evolved red supergiant star (with an initial mass between 8 and 25 times our Sun) that preserved its vast outer hydrogen envelope. Having exhausted all nuclear fuel through successive stages of fusion (hydrogen, helium, carbon, neon, oxygen, and silicon), its inert iron core could no longer withstand gravitational pressure. In less than a quarter of a second, the iron core collapsed into nuclear density, triggering a catastrophic outward shockwave that blasted the star's outer layers into interstellar space.

Q: How far away is SN2021gmj from Earth and how old is the light reaching us?

SN2021gmj is located approximately 47.9 Million Light-Years from Earth (cosmological redshift z = 0.003312, luminosity distance d_L = 14.695). Because electromagnetic radiation travels at 299,792 km/s, the photons detected by modern telescopes began their cosmic voyage 47.9 million years ago during the early Cenozoic era, following the extinction of the non-avian dinosaurs. While that light traveled across intergalactic space, Earth's continents shifted and biological evolution shaped the history of our planet.

Q: What does the cosmological redshift of SN2021gmj tell us about the expansion of space?

SN2021gmj exhibits a measured spectroscopic redshift of z = 0.0033. Under Hubble's Law, this redshift corresponds to an apparent recessional velocity of approximately 992.9 km/s away from our Milky Way galaxy. This redshift is not motion through space alone, but the stretching of light waves as the fabric of the universe itself expanded during the millions of years the photons traveled to our telescopes.

          🔭 Deep Field (~2.1′ FOV) · Highest-Definition Optical Field (0.25″/pix)
        

● SN2021gmj Explosion Site

High-magnification deep optical view into SN2021gmj's host environment. Pan and scroll to explore the cosmic neighborhood where this star exploded.

⚡ Quick Observer Facts & Telemetry

IAU Transients DB

🌐 Distance to Earth

47.9 Million Light-Years

Lookback Cosmic Time

✨ Peak Brightness

Mag 15.1

Astro Camera (CCD)

☀️ Peak Radiance

167.8 million Suns

Combined Stellar Energy

🚀 Shock Velocity

~991 km/s

~0.3% Speed of Light

🌌 Host Galaxy

NGC 3310

Center Coincident

🧭 Constellation

Ursa Major

The Great Bear

💥 Explosion Physics

Type II

Core-Collapse Supergiant

📅 Discovered On

2021/03/20

DLT40

CHAPTER I

The Iron Core Collapse of a Dying Supergiant

On 2021/03/20, astronomers scanning the heavens flagged a sudden, violent pinpoint of light that had flared into visibility. Designated SN2021gmj, it represents a catastrophic stellar explosion classified as a Type II supernova.

The progenitor of SN2021gmj was a mammoth supergiant star, shining with the furious vigor of an object at least 8 to 25 times more massive than our Sun. For millions of years, it synthesized heavier and heavier elements in concentric onion-like shells: hydrogen burning into helium, helium into carbon, carbon into oxygen, neon, and silicon. But when silicon fused into iron, the stellar engine ran out of fuel. Iron fusion absorbs energy rather than liberating it; within fractions of a second, the iron core collapsed under its own gravity, rebounding into an immense cosmic shockwave that blasted the star into pieces.

Astrophysical Mechanism Summary

Type: Type II • Progenitor: Massive Red/Stripped Supergiant Star • Velocity: ~991 km/s (~0.3% c)

CHAPTER II

A Message Across Deep Cosmic Time

The light from SN2021gmj is a dispatch from an ancient past. Located approximately 47.9 Million Light-Years away (redshift z = 0.003312), the photons detected by telescopes today began their journey 47.9 million years ago during the early Cenozoic era, following the extinction of the non-avian dinosaurs.

While this burst of electromagnetic radiation traversed the cold void of intergalactic space at 299,792 kilometers per second, continents on Earth drifted, mountain ranges rose, and entire ecosystems rose and fell. To look into a telescope at SN2021gmj is to gaze directly into prehistoric cosmic time.

CHAPTER III

Incandescence of 167.8 million Suns

At the height of the outburst around 2021/03/19, SN2021gmj surged to a peak apparent magnitude of 15.1 and an intrinsic absolute magnitude of -15.732. At that instant, this single dying star radiated with the collective power of approximately 167.8 million Suns combined, outshining whole dwarf galaxies and illuminating the surrounding interstellar medium.

The total energy released by the cataclysm was on the order of 10⁵¹ to 10⁵³ ergs. Over 99% of this energy was emitted within the first 10 seconds as a dense burst of trillions of neutrinos, with only 1% driving the visible blast wave.

CHAPTER IV

The Radioactive Furnace: Why Supernovae Glow for Months

Unlike a conventional terrestrial explosion that cools and goes dark in seconds, SN2021gmj shone brightly for weeks and months. The secret behind this prolonged celestial glow is nuclear physics: the extreme heat and pressure of detonation synthesized vast quantities of radioactive Nickel-56 (⁵⁶Ni).

With a half-life of 6.075 days, Nickel-56 decays into Cobalt-56 (⁵⁶Co), emitting gamma rays and high-energy positrons that heat the expanding ejecta from within. Cobalt-56 in turn decays with a half-life of 77.2 days into stable Iron-56 (⁵⁶Fe), powering the steady exponential radioactive tail observed in the light curve.

CHAPTER V

Cosmic Kiln: Seeding the Elements of Life

Supernovae are the premier chemical foundries of our universe. SN2021gmj forged and liberated tons of newly synthesized elements: vast reservoirs of oxygen (the most abundant heavy element in living organisms), carbon, nitrogen, magnesium, and silicon.

As Carl Sagan famously observed, "We are made of star-stuff." The iron atoms that carry oxygen in human hemoglobin and the calcium in our bones were originally forged in explosions identical to SN2021gmj billions of years ago.

CHAPTER VI

Galactic Setting in Ursa Major

SN2021gmj detonated inside NGC 3310. In our terrestrial sky, it resides in the constellation Ursa Major (The Great Bear) at Right Ascension 10:38:47.268 and Declination +53:30:30.31.

CHAPTER VII

The Scientific Surveillance Campaign

Following its discovery by DLT40, observatories worldwide swung their lenses toward SN2021gmj. In the Open Supernova Catalog, SN2021gmj is documented across 7 photometric measurements in passbands such as G and 0 spectroscopic epochs. These multi-wavelength observations allow astrophysicists to model the expanding photosphere, measure shock velocities, and probe circumstellar interactions.

CHAPTER VIII

Stargazer's Field Guide: Can You See It Tonight?

Can I see it tonight? No — this supernova is physically extinguished.

Supernovae are brief, explosive cosmic catastrophes. They brighten over days to weeks and then permanently fade into darkness as their radioactive nickel-56 and cobalt-56 fuel decays. This explosion occurred 5.5 years ago (2021/03/19). Accounting for cosmological time dilation at redshift z = 0.0033, the rest-frame age is +2013.3 days. By standard radioactive decay templates, its optical brightness has decayed by Δm ≈ 20.8 magnitudes to an estimated magnitude 35.9, rendering the transient undetectable to all ground-based observatories.

Instrument Breakdown: At its maximum brightness in 2021/03/19, it reached magnitude 15.10 (Amateur CMOS Rig). Tonight, pointing a telescope at these coordinates will reveal only the background host galaxy (NGC 3310).

Instrument Class & Aperture	Sensitivity Limit	At Peak Maximum (m=15.10)	Tonight (Est. m≈35.9)
Naked Eye Dark sky site (Bortle 1–3) with no optical aid	m ≤ 6.0	❌ Below limit	❌ Below limit
Binoculars (50mm) Standard 7x50 or 10x50 handheld binoculars	m ≤ 9.5	❌ Below limit	❌ Below limit
Small Backyard Scope (4" / 100mm) Entry 4-inch (100mm) refractor / reflector	m ≤ 12.0	❌ Below limit	❌ Below limit
Medium Amateur Scope (8"–12") 8-inch to 12-inch Dobsonian or Schmidt-Cassegrain	m ≤ 14.5	❌ Below limit	❌ Below limit
Amateur CMOS Rig Cooled monochrome/color CMOS camera with multi-hour stack	m ≤ 19.5	✅ Detectable	❌ Below limit
2m–3m Research Telescope University or regional observatory (e.g. Palomar 60", Calar Alto)	m ≤ 22.0	✅ Detectable	❌ Below limit
Giant 8m–10m Observatories Keck (10m), VLT (8.2m), Gemini, Subaru optical imaging	m ≤ 25.0	✅ Detectable	❌ Below limit
Space Observatories Only Hubble Space Telescope (WFC3) / JWST (NIRCam deep stack)	m ≤ 30.0	✅ Detectable	❌ Below limit

Planetary Safety Note: Even though SN2021gmj was a titanic explosion, our planet sits safely outside the lethal 50–100 light-year kill zone. At a distance of 47.9 Million Light-Years, the blast poses zero physical hazard to Earth's biosphere.

🌌 Cosmic Neighbors & Contemporaries 110,222+ Transients Indexed

Cataloged supernovae closest to SN2021gmj in discovery time, spatial sky neighborhood, and cosmological lookback epoch:

⏱️ Closest in Time

AT2021gmm Exploded same day

Type Candidate 2021/03/20 · Mag 17.7

AT2021gmo Exploded same day

Type Candidate 2021/03/20 · Mag 18.9

AT2021gnx Exploded same day

Type Candidate 2021/03/20 · Mag 19.8

🔭 Closest on the Sky

SN1991N 0.4′ away

Type Ic Discovered 1991/03/29

AT2021apt 16.3′ away

Type Candidate Discovered 2021/01/13

SN2014S 31.0′ away

Type II Discovered 2014/02/21

🌌 Same Cosmic Era (Redshift)

AT2018arb z = 0.003312

Type Candidate ~45.4 Mly lookback

SN2022zut z = 0.003309

Type Ia ~45.3 Mly lookback

SN2018aoq z = 0.003319

Type II ~45.5 Mly lookback

❓ Frequently Asked Questions About SN2021gmj

Scientific & observational Q&As indexed from astronomical databases & the Open Supernova Catalog

What type of supernova is SN2021gmj and what kind of star exploded? Astrophysics & Progenitor

SN2021gmj is a Type II Core-Collapse Supernova, marking the death of an evolved red supergiant star (with an initial mass between 8 and 25 times our Sun) that preserved its vast outer hydrogen envelope. Having exhausted all nuclear fuel through successive stages of fusion (hydrogen, helium, carbon, neon, oxygen, and silicon), its inert iron core could no longer withstand gravitational pressure. In less than a quarter of a second, the iron core collapsed into nuclear density, triggering a catastrophic outward shockwave that blasted the star's outer layers into interstellar space.

What was the progenitor star doing in the millions of years leading up to SN2021gmj? Astrophysics & Progenitor

Before detonating as SN2021gmj, the progenitor lived a short, furious stellar life of roughly 10 to 30 million years. In its interior, temperatures and pressures reached astronomical extremes, burning through nuclear fuel in an 'onion-skin' arrangement of concentric shells: hydrogen burning into helium for millions of years, helium into carbon for hundreds of thousands of years, carbon into neon for centuries, oxygen into silicon for months, and silicon fusing into iron in mere days! Once iron filled the core, fusion could no longer extract energy, dooming the star to sudden gravitational collapse.

How far away is SN2021gmj from Earth and how old is the light reaching us? Cosmic Distance & Time

SN2021gmj is located approximately 47.9 Million Light-Years from Earth (cosmological redshift z = 0.003312, luminosity distance d_L = 14.695). Because electromagnetic radiation travels at 299,792 km/s, the photons detected by modern telescopes began their cosmic voyage 47.9 million years ago during the early Cenozoic era, following the extinction of the non-avian dinosaurs. While that light traveled across intergalactic space, Earth's continents shifted and biological evolution shaped the history of our planet.

What does the cosmological redshift of SN2021gmj tell us about the expansion of space? Cosmic Distance & Time

SN2021gmj exhibits a measured spectroscopic redshift of z = 0.0033. Under Hubble's Law, this redshift corresponds to an apparent recessional velocity of approximately 992.9 km/s away from our Milky Way galaxy. This redshift is not motion through space alone, but the stretching of light waves as the fabric of the universe itself expanded during the millions of years the photons traveled to our telescopes.

How bright did SN2021gmj become at its peak, and how many Suns does that equal? Explosion Energetics

At peak brightness, SN2021gmj achieved an apparent magnitude of 15.1 around 2021/03/19. Corrected for cosmic distance and foreground interstellar dust, its intrinsic absolute magnitude was -15.732. At this peak, the exploding star radiated with the incandescent brilliance of approximately 167.8 million Suns combined, briefly outshining the cumulative starlight of entire dwarf galaxies!

How much total energy was released by SN2021gmj, and where did that energy go? Explosion Energetics

The collapse of SN2021gmj's progenitor released a staggering 10⁵³ ergs of gravitational binding energy—more energy than our Sun will radiate across its entire 10-billion-year lifespan! Astonishingly, 99% of this titanic energy was emitted within 10 seconds in the form of trillions of nearly massless neutrinos. Only about 1% (10⁵¹ ergs) drove the physical kinetic blast wave, and a mere 0.01% (10⁴⁹ ergs) was radiated as the visible starlight observed by telescopes.

How fast are the supernova ejecta and shockwave of SN2021gmj expanding through space? Explosion Energetics

The debris and shockwave of SN2021gmj erupted into space at an astounding velocity of approximately 991 km/s (measured spectroscopically). This corresponds to roughly 0.3% of the speed of light (Mach 2,890 in air)! At this blistering speed, the expanding debris shell traverses the entire diameter of planet Earth in just 12.85 seconds, carving a giant bubble in the interstellar medium.

What powers the prolonged glow of SN2021gmj weeks and months after detonation? Radioactive Engine

While the initial flash of SN2021gmj was driven by shock breakout heating through the stellar envelope, its prolonged visibility over weeks and months was sustained by the radioactive decay of approximately 0.05 to 0.15 solar masses of Nickel-56 (⁵⁶Ni) forged in the core shock. As ⁵⁶Ni decays into ⁵⁶Co (half-life: 6.1 days) and then into stable ⁵⁶Fe (half-life: 77.2 days), gamma rays and positrons thermalize within the expanding ejecta, preventing the debris from instantly freezing in the vacuum of space.

What chemical elements did SN2021gmj create and disperse into the universe? Nucleosynthesis & Elements

Core-collapse supernovae like SN2021gmj are the primary creators of life-sustaining elements in the cosmos. The explosion manufactured and dispersed immense reservoirs of oxygen (the single most abundant heavy element in the universe), alongside carbon, nitrogen, neon, magnesium, silicon, sulfur, and calcium (which builds terrestrial bones and teeth). In the ultra-dense, neutron-rich shockwave, rapid neutron capture (r-process nucleosynthesis) forged heavy elements like gold, platinum, and uranium.

Did SN2021gmj leave behind a black hole, a neutron star, or nothing at all? Cosmic Remnant

The crushing core collapse of SN2021gmj's progenitor forged an ultra-dense compact stellar remnant at the center of the detonation. If the progenitor had an initial mass under ~20 solar masses, it left behind a neutron star (pulsar)—packing the mass of our entire Sun into a city-sized sphere barely 20 kilometers wide, spinning dozens or hundreds of times per second. If the progenitor exceeded ~25–30 solar masses, gravity overcame neutron degeneracy pressure, creating a permanent stellar-mass black hole.

What will SN2021gmj's explosion site look like in 1,000 to 10,000 years? Cosmic Remnant

Over the coming millennia, the explosion site of SN2021gmj will undergo three dramatic evolutionary epochs: During the next few centuries (Free Expansion phase), the ejecta shell will continue expanding at thousands of km/s. Between 500 and 10,000 years (the Sedov-Taylor adiabatic phase), the forward shock will sweep up hundreds of solar masses of interstellar gas, heating it to tens of millions of degrees and glowing in bright thermal X-rays (similar to the famous Cygnus Loop or Cassiopeia A). Eventually, the cooling shock will compress nearby giant molecular clouds, triggering the gravitational collapse of new stars and solar systems!

In which galaxy did SN2021gmj explode, and where is it located relative to the galactic center? Galactic Environment

SN2021gmj is associated with NGC 3310. High-precision astrometry from optical sky surveys pins the explosion coordinates directly to the galaxy's underlying stellar population.

Where is SN2021gmj located in the night sky and which constellation is it in? Sky Coordinates

In the celestial sphere, SN2021gmj is located at Right Ascension 10:38:47.268 and Declination +53:30:30.31, situated in the constellation Ursa Major (The Great Bear). Because its declination is +53:30:30.31, it is primarily placed in the Northern celestial hemisphere.

Across which photometric filter bands was SN2021gmj monitored? Astronomical Observations

SN2021gmj was tracked across 7 photometric observations across a baseline of 372.2 days utilizing filter bands including G. Data were captured by observatories and survey networks including GAIA. Multi-color photometry tracks the temperature evolution of the fireball, verifying the rise time to peak and the rate of radioactive decline.

What did astronomical spectroscopy reveal about SN2021gmj's chemical makeup? Astronomical Observations

Spectroscopic observations of SN2021gmj confirmed its astrophysical classification by dissecting its light into individual wavelengths. Absorption and emission line features reveal the chemical composition, expansion velocity, and temperature of the expanding fireball.

Who discovered SN2021gmj and how was it first detected? Discovery & History

SN2021gmj was officially reported on 2021/03/20 by DLT40. Discoveries are typically flagged by high-cadence robotic survey telescopes (such as ATLAS, ZTF, Pan-STARRS, ASAS-SN, or Gaia) and worldwide amateur astronomers scanning the night sky, followed by rapid spectroscopic classification by international observatories.

How many scientific publications and observatories have contributed data to SN2021gmj? Scientific Research

SN2021gmj is documented across 6 scientific references and archival data sources in the Open Supernova Catalog. These include discovery circulars and research datasets from 2016A&A...594A..13P, Transient Name Server, Gaia Photometric Science Alerts, Asiago Supernova Catalogue. All raw photometry and spectroscopy points are cross-indexed to their original bibliographic records for peer-reviewed verification.

What other names and survey identifiers exist for SN2021gmj? Cross-Identifications

Throughout global alert streams and survey databases, SN2021gmj has also been designated as: AT2021gmj, ZTF21aaqgmjt, ATLAS21izq, Gaia21fms. These cross-matched identifiers allow astronomers to cross-reference observations across the Zwicky Transient Facility (ZTF), the Asteroid Terrestrial-impact Last Alert System (ATLAS), Pan-STARRS, Gaia Photometric Science Alerts, and the IAU Transient Name Server (TNS).

How does SN2021gmj contribute to measuring the Hubble Constant and the scale of the cosmos? Cosmology & Distance Ladder

As a core-collapse supernova, SN2021gmj provides independent cosmological distance calibrations via the Expanding Photosphere Method (EPM) and the Standard Candle Method for Type II supernovae (SCM-II). By correlating the physical expansion speed of the photosphere (measured via spectroscopic Doppler shifts) with its photometric color temperature, astronomers determine direct geometric distances independent of secondary distance ladders.

Could gravitational waves or neutrinos from SN2021gmj be detected on Earth? Multi-Messenger Astronomy

Core-collapse supernovae like SN2021gmj are premier targets for multi-messenger astrophysics! During the collapse of the iron core, an intense burst of 10⁵⁸ neutrinos escaped into space hours before the shock broke out through the stellar surface (as famously seen in SN 1987A). Furthermore, violent core asymmetries and non-axisymmetric core bounce can emit high-frequency gravitational waves detectable by advanced interferometers (LIGO, Virgo, KAGRA) for events within the Milky Way and Local Group.

How does SN2021gmj compare to famous historical supernovae like SN 1987A or the Crab Supernova? Historical Comparison

Compared to historical landmarks like SN 1987A in the Large Magellanic Cloud (168,000 light-years away, naked-eye peak m = 2.9) or the Crab Supernova of 1054 (6,500 light-years away), SN2021gmj occurred at a distance of 47.9 Million Light-Years. While historical naked-eye supernovae occurred within our Milky Way or its immediate satellites, modern discoveries like SN2021gmj allow astrophysicists to probe diverse galactic environments, metallicities, and stellar populations across the broader universe.

Can I see SN2021gmj tonight with a backyard telescope or binoculars? Backyard Observation

SN2021gmj exploded 5.5 years ago (2021/03/20). Optical transient emission has completely faded along its radioactive decay curve. Today, pointing a telescope at these coordinates reveals the expanding remnant nebula or NGC 3310; the original optical transient is no longer detectable with amateur backyard equipment.

Does the radiation or shockwave from SN2021gmj pose any threat to Earth? Planetary Safety

No, Earth is in zero danger. Supernovae are violent events emitting powerful gamma rays, X-rays, and cosmic rays; however, the astrophysical 'lethal kill zone' for our planet's protective ozone layer is estimated at 50 to 100 light-years. At a distance of 47.9 Million Light-Years, the inverse-square law dilutes the radiation by quintillions of times, making SN2021gmj completely harmless to our biosphere and purely a fascinating spectacle for human exploration.

Data sourced from IAU TNS, ALeRCE, WISeREP, and the Open Supernova Catalog. View All General Astrophysics FAQs →

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The Story of Supernova SN2021gmj