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Type IIb Supernova

The Story of Supernova SN2021dex

An extraordinary stellar explosion that occurred 825.2 Million Light-Years away in deep space.

⏱️ ~6 min deep read 🔬 24 Astrophysical Q&As 🔭 Viewing Guide Included 🪐 Constellation Ursa Minor
🔭 Deep Field (~2.1′ FOV) · DSS2 Optical (China-VO mirror)
● SN2021dex Explosion Site

High-magnification deep optical view into SN2021dex'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
825.2 Million Light-Years
Lookback Cosmic Time
✨ Peak Brightness
Mag 18.97
Large Observatory
☀️ Peak Radiance
1.4 billion Suns
Combined Stellar Energy
🚀 Shock Velocity
~16,000 km/s
~5.3% Speed of Light
🌌 Host Galaxy
Field Transient Host
Center Coincident
🧭 Constellation
Ursa Minor
The Little Bear
💥 Explosion Physics
Type IIb
Core-Collapse Supergiant
📅 Discovered On
2021/02/12
ZTF
CHAPTER I

A Core-Collapse Class Transient

On 2021/02/12, astronomers scanning the heavens flagged a sudden, violent pinpoint of light that had flared into visibility. Designated SN2021dex, it represents a catastrophic stellar explosion classified as a Type IIb supernova.

SN2021dex is classified in a core-collapse spectroscopic family. Events of this class are generally linked to the death of a massive star after successive nuclear burning stages, but no unique progenitor mass, lifetime, or pre-explosion identification is established for this object. The narrative of onion-shell fusion ending in iron-core collapse describes the class model, not a measured biography of SN2021dex.

Astrophysical Mechanism Summary
Type: Type IIb • Progenitor: Massive Red/Stripped Supergiant Star • Velocity: ~16,000 km/s (~5.3% c)
CHAPTER II

A Message Across Deep Cosmic Time

SN2021dex has a luminosity distance of approximately 825.2 Million Light-Years (redshift z = 0.055). That figure is not the light-travel time: under flat ΛCDM the lookback time is ≈739 Myr, when the light set out 738.6 million years ago during the Neoproterozoic, hundreds of millions of years before complex animal life diversified.

Luminosity distance incorporates cosmic expansion and exceeds the light-travel distance. The two quantities must be reported separately.

CHAPTER III

Incandescence of 1.4 billion Suns

At the height of the outburst around 2021/02/26, SN2021dex surged to a peak apparent magnitude of 18.97 and an intrinsic absolute magnitude of -18. At that instant, this single dying star radiated with the collective power of approximately 1.4 billion Suns combined, outshining whole dwarf galaxies.

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

After peak, many supernovae are powered by the radioactive chain ⁵⁶Ni → ⁵⁶Co → ⁵⁶Fe. That is the standard model for Type IIb light curves; an object-specific nickel mass for SN2021dex is not published here.

Nickel-56 decays to Cobalt-56 (half-life ≈6.1 days), then to stable Iron-56 (half-life ≈77.2 days), heating the ejecta from within. Typical class yields must not be quoted as measured values for this event.

CHAPTER V

Cosmic Kiln: Seeding the Elements of Life

Supernovae enrich galaxies with heavy elements. Events of SN2021dex's class are associated with oxygen-group and intermediate-mass products (O, C, Ne, Mg, Si) in galactic chemical-evolution models. No measured elemental yields are published for SN2021dex.

As Carl Sagan observed, "We are made of star-stuff" — a statement about nucleosynthesis in general, not a claim that atoms from this specific remnant are identified in the Solar System.

CHAPTER VI

Galactic Setting in Ursa Minor

No host galaxy is identified for SN2021dex in this catalog record . In our terrestrial sky, it resides in the constellation Ursa Minor (The Little Bear) at Right Ascension 15:31:29.453 and Declination +79:49:13.76.

CHAPTER VII

The Scientific Surveillance Campaign

Following its discovery by ZTF, observatories worldwide swung their lenses toward SN2021dex. In the Open Supernova Catalog, SN2021dex is documented across 0 photometric measurements 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? Unknown — no measured brightness.

Discovered 5.7 years ago (2021/02/12). No reliable current brightness estimate — no photometry has been published.

Planetary Safety Note: Even though SN2021dex was a titanic explosion, our planet sits safely outside the lethal 50–100 light-year kill zone. At a distance of 825.2 Million Light-Years, the blast poses zero physical hazard to Earth's biosphere.

🌌 Cosmic Neighbors & Contemporaries 105,768+ transients indexed

Cataloged transients closest to SN2021dex in discovery time, spatial sky neighborhood, and cosmological redshift:

⏱️ Closest in Time
AT2021csr Exploded same day
Type Candidate 2021/02/12 · Mag 17
AT2021ctc Exploded same day
Type Candidate 2021/02/12 · Mag 20.9
AT2021ctb Exploded same day
Type Candidate 2021/02/12 · Mag 19.9
🔭 Closest on the Sky
Gaia18ciy 14.0′ away
Type Candidate Discovered 2018/08/22
FRB20220119A 23.8′ away
Type FRB Discovered 2022/01/19
AT2020qzc 24.9′ away
Type Candidate Discovered 2020/08/09
🌌 Same Cosmic Era (Redshift)
SN1998ei z = 0.055
Type Transient ~753.4 Mly (approx.)
SN1999ak z = 0.055
Type Ia ~753.4 Mly (approx.)
SN1999ao z = 0.055
Type Ia ~753.4 Mly (approx.)

❓ Frequently Asked Questions About SN2021dex

Scientific & observational Q&As indexed from astronomical databases & the Open Supernova Catalog
🔍 24 of 24
What type of supernova is SN2021dex and what kind of star exploded? Astrophysics & Progenitor
SN2021dex is a stripped-envelope supernova (Type IIb), consistent with core collapse after loss of the outer hydrogen (and for Ic, helium) envelope. The specific progenitor channel for this object is not uniquely determined by the spectral type alone.
What was the progenitor star doing in the millions of years leading up to SN2021dex? Astrophysics & Progenitor
The specific progenitor of SN2021dex has not been observed. Events of this spectroscopic class are generally associated with massive-star core collapse after successive nuclear burning stages (hydrogen → helium → carbon → neon → oxygen → silicon → iron). Typical lifetimes of such progenitors are tens of millions of years, but that is a class expectation—not a measured age or evolutionary track for this star. No pre-explosion imaging identification is cited in the available record for SN2021dex.
How far away is SN2021dex from Earth and how old is the light reaching us? Cosmic Distance & Time
SN2021dex has a luminosity distance of approximately 825.2 Million Light-Years (redshift z = 0.055, d_L = 253). Under a flat ΛCDM cosmology (H₀≈70 km/s/Mpc, Ωₘ≈0.3), the corresponding lookback time is ≈739 Myr — when the light set out 738.6 million years ago during the Neoproterozoic, hundreds of millions of years before complex animal life diversified. Luminosity distance is larger than the light-travel distance because cosmic expansion stretches the path; the two must not be equated.
What does the cosmological redshift of SN2021dex tell us about the expansion of space? Cosmic Distance & Time
SN2021dex exhibits a measured spectroscopic redshift of z = 0.0550. The cockpit reports a user-facing recession velocity of 16,035.8 km/s, computed as the redshift Doppler-equivalent v = c·((1+z)²−1)/((1+z)²+1). Cosmological redshift is not generally equivalent to special-relativistic motion through space; this number is a convenient velocity label for z, not a measured peculiar velocity and not supernova ejecta speed. Bare c·z (≈16,489 km/s) is a different low-z approximation and is not mixed here.
How bright did SN2021dex become at its peak, and how many Suns does that equal? Explosion Energetics
At peak brightness, SN2021dex achieved an apparent magnitude of 18.97 around 2021/02/26. Corrected for cosmic distance and foreground interstellar dust, its intrinsic absolute magnitude was -18. At this peak, the exploding star radiated with the incandescent brilliance of approximately 1.4 billion Suns combined, briefly outshining the cumulative starlight of entire dwarf galaxies!
How much total energy was released by SN2021dex, and where did that energy go? Explosion Energetics
Core-collapse events of this broad class are often discussed with a ~10⁵³ erg gravitational binding energy of which ~99% may escape as neutrinos—again a class scale, not a measured budget for SN2021dex.
How fast are the supernova ejecta and shockwave of SN2021dex expanding through space? Explosion Energetics
Spectroscopic line blueshifts imply an ejecta velocity of approximately 16,000 km/s (~5.3% of c) for SN2021dex. This is distinct from cosmological recession velocity inferred from redshift.
What powers the prolonged glow of SN2021dex weeks and months after detonation? Radioactive Engine
Classified core-collapse light curves are often sustained after peak by radioactive ⁵⁶Ni → ⁵⁶Co → ⁵⁶Fe decay, with typical nickel yields of order 0.05–0.15 solar masses—again a class scale, not a measured yield for SN2021dex. Shock-breakout heating may dominate the earliest optical flash; without a published bolometric fit, neither nickel mass nor the dominant power source is established for this event.
What chemical elements did SN2021dex create and disperse into the universe? Nucleosynthesis & Elements
Events of SN2021dex's broad spectroscopic class are discussed in galactic chemical evolution as sources of oxygen-group and intermediate-mass elements (and, for some channels, iron-peak material). No measured elemental yields are published for SN2021dex.
Did SN2021dex leave behind a black hole, a neutron star, or nothing at all? Cosmic Remnant
A core-collapse explosion associated with SN2021dex's class forges 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 SN2021dex's explosion site look like in 1,000 to 10,000 years? Cosmic Remnant
Supernova remnants in general evolve through free-expansion, Sedov–Taylor, and radiative phases over centuries to millennia, sometimes becoming bright X-ray shells and occasionally compressing nearby molecular clouds. These are standard remnant-evolution models, not forecasts measured for SN2021dex. Whether this site will trigger star formation, reach particular X-ray temperatures, or match Cassiopeia A depends on local density and magnetic field—none of which are constrained by the data shown here.
In which galaxy did SN2021dex explode, and where is it located relative to the galactic center? Galactic Environment
No host galaxy is identified for SN2021dex in this catalog record. A blank host field means the association is unresolved here—it does not establish that the host is uncataloged, resolved in imaging, or securely linked by astrometry. Possible states (unidentified, hostless candidate, imaging detection without a catalog name, or a securely matched cataloged galaxy) require separate evidence that is not present in this field.
Where is SN2021dex located in the night sky and which constellation is it in? Sky Coordinates
In the celestial sphere, SN2021dex is located at Right Ascension 15:31:29.453 and Declination +79:49:13.76, situated in the constellation Ursa Minor (The Little Bear). Because its declination is +79:49:13.76, it is primarily placed in the Northern celestial hemisphere.
How much Milky Way interstellar dust obscures our view of SN2021dex? Interstellar Dust
Along this line of sight, the catalog records a Milky Way foreground color excess of E(B−V) = 0.0574 mag (source: Analytical Galactic Disk Model (IAU)), corresponding to roughly A_V ≈ 0.18 mag for R_V = 3.1. Band-specific corrections (e.g. A_r) differ from A_V and should be applied when available. Map-based values (IRSA / Schlafly & Finkbeiner 2011) must not be claimed unless that map was actually queried.
Across which photometric filter bands was SN2021dex monitored? Astronomical Observations
No calibrated photometry is published for SN2021dex. Without multi-epoch flux measurements, neither a light curve nor a peak magnitude can be established. Follow-up imaging from survey alert streams would be required to map its brightness evolution.
What did astronomical spectroscopy reveal about SN2021dex's chemical makeup? Astronomical Observations
No spectrum has been published for SN2021dex. Without spectroscopy, its chemical composition, expansion velocity, and physical classification cannot be established. Classification spectra from follow-up observatories would be required to identify diagnostic absorption and emission lines.
Who discovered SN2021dex and how was it first detected? Discovery & History
SN2021dex was officially reported on 2021/02/12 by ZTF. 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 SN2021dex? Scientific Research
Data for SN2021dex are compiled from international astronomical notices, the IAU Transient Name Server (TNS), and peer-reviewed astrophysical journals.
What other names and survey identifiers exist for SN2021dex? Cross-Identifications
Throughout global alert streams and survey databases, SN2021dex has also been designated as: AT2021dex, ZTF21aalimya, Gaia21bdb. 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 SN2021dex contribute to measuring the Hubble Constant and the scale of the cosmos? Cosmology & Distance Ladder
As a core-collapse supernova, SN2021dex 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 SN2021dex be detected on Earth? Multi-Messenger Astronomy
Core-collapse supernovae like SN2021dex 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 SN2021dex 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), SN2021dex occurred at a distance of 825.2 Million Light-Years. While historical naked-eye supernovae occurred within our Milky Way or its immediate satellites, modern discoveries like SN2021dex allow astrophysicists to probe diverse galactic environments, metallicities, and stellar populations across the broader universe.
Can I see SN2021dex tonight with a backyard telescope or binoculars? Backyard Observation
Unknown — no measured brightness. SN2021dex was discovered 2093 days ago (2021/02/12), but no calibrated photometry is published for it, so its current apparent magnitude cannot be stated. Without a magnitude there is no basis for a telescope-aperture recommendation. Use the coordinates and airmass chart above to plan a follow-up observation and report what you measure.
Does the radiation or shockwave from SN2021dex 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 825.2 Million Light-Years, the inverse-square law dilutes the radiation by quintillions of times, making SN2021dex 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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