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Type Candidate Supernova

The Story of Supernova AT2021xxd

An extraordinary stellar explosion that occurred 584.2 Million Light-Years away in deep space.

⏱️ ~6 min deep read 🔬 24 Astrophysical Q&As 🔭 Viewing Guide Included 🪐 Constellation Pegasus
🔭 Deep Field (~2.1′ FOV) · DSS2 Optical (China-VO mirror)
● AT2021xxd Explosion Site

High-magnification deep optical view into AT2021xxd'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
584.2 Million Light-Years
Lookback Cosmic Time
✨ Peak Brightness
Mag 20.7
Large Observatory
☀️ Peak Radiance
138.5 million Suns
Combined Stellar Energy
🚀 Shock Velocity
~11,552 km/s
~3.9% Speed of Light
🌌 Host Galaxy
Field Transient Host
Center Coincident
🧭 Constellation
Pegasus
The Winged Horse
💥 Explosion Physics
Type Candidate
Core-Collapse Supergiant
📅 Discovered On
2021/09/02
ZTF
CHAPTER I

An Unclassified Astronomical Transient

On 2021/09/02, astronomers scanning the heavens flagged a sudden, violent pinpoint of light that had flared into visibility. Designated AT2021xxd, it is an unclassified astronomical transient whose physical nature is not yet established.

AT2021xxd was reported as a changing source, but it has not been spectroscopically classified. Without a spectrum, the progenitor cannot be identified: a thermonuclear white-dwarf detonation and a core-collapse of a massive star are mutually exclusive histories that produce different spectra and light curves. No progenitor, explosion mechanism, or remnant outcome is established for this object.

Astrophysical Mechanism Summary
Type: Unclassified (no spectrum) • Progenitor: Unknown • Velocity: ~11,552 km/s (~3.9% c)
CHAPTER II

A Message Across Deep Cosmic Time

AT2021xxd has a luminosity distance of approximately 584.2 Million Light-Years (redshift z = 0.039305). That figure is not the light-travel time: under flat ΛCDM the lookback time is ≈534 Myr, when the light set out 533.7 million years ago during the Paleozoic era, long before the first dinosaurs appeared on Earth.

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

CHAPTER III

Brightness Record

At the height of the outburst around 2021/09/01, AT2021xxd surged to a peak apparent magnitude of 20.7 and an intrinsic absolute magnitude of -15.524. At that instant, this single dying star radiated with the collective power of approximately 138.5 million Suns combined, outshining whole dwarf galaxies.

The total energy release is unknown without a spectroscopic classification.

CHAPTER IV

The Radioactive Furnace: Why Supernovae Glow for Months

Without a classification, the power source of AT2021xxd cannot be identified; radioactive ⁵⁶Ni decay powers many supernovae but is not established for this transient.

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 AT2021xxd'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 AT2021xxd.

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 Pegasus

No host galaxy is identified for AT2021xxd in this catalog record . In our terrestrial sky, it resides in the constellation Pegasus (The Winged Horse) at Right Ascension 23:01:57.327 and Declination +17:21:28.04.

CHAPTER VII

The Scientific Surveillance Campaign

Following its discovery by ZTF, observatories worldwide swung their lenses toward AT2021xxd. In the Open Supernova Catalog, AT2021xxd 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 5.2 years ago (2021/09/02). No reliable current brightness estimate — no photometry has been published.

Planetary Safety Note: Even though AT2021xxd was a titanic explosion, our planet sits safely outside the lethal 50–100 light-year kill zone. At a distance of 584.2 Million Light-Years, the blast poses zero physical hazard to Earth's biosphere.

🌌 Cosmic Neighbors & Contemporaries 105,768+ transients indexed

Cataloged transients closest to AT2021xxd in discovery time, spatial sky neighborhood, and cosmological redshift:

⏱️ Closest in Time
AT2021xmr Exploded same day
Type Candidate 2021/09/02 · Mag 16
AT2021xor Exploded same day
Type Candidate 2021/09/02 · Mag 18.3
AT2021xos Exploded same day
Type Candidate 2021/09/02 · Mag 18.5
🔭 Closest on the Sky
AT2021sdh 5.6′ away
Type Candidate Discovered 2021/07/01
AT2021afcy 22.1′ away
Type Candidate Discovered 2021/11/01
PS15cdv 25.6′ away
Type Transient Discovered 2015/09/23
🌌 Same Cosmic Era (Redshift)
SN2004gl z = 0.039301
Type Ia ~538.4 Mly (approx.)
PTF11gjh z = 0.0393
Type Ia ~538.4 Mly (approx.)
SN2023acbs z = 0.0393
Type II ~538.4 Mly (approx.)

❓ Frequently Asked Questions About AT2021xxd

Scientific & observational Q&As indexed from astronomical databases & the Open Supernova Catalog
🔍 24 of 24
What type of supernova is AT2021xxd and what kind of star exploded? Astrophysics & Progenitor
AT2021xxd has not been spectroscopically classified. It was reported as an astronomical transient, which means an object was detected changing in brightness, but its nature is not yet established. Classification requires a spectrum: the presence or absence of hydrogen lines separates Type II from Type I, and Type Ia is identified by silicon absorption near 6150 Å combined with the absence of hydrogen. Until such a spectrum is published, claims about the progenitor star or explosion mechanism would be speculation. Check the sources below for follow-up spectroscopy.
What was the progenitor star doing in the millions of years leading up to AT2021xxd? Astrophysics & Progenitor
The progenitor of AT2021xxd cannot be identified without a classification. Stellar evolutionary history depends entirely on the explosion type: a Type Ia event would require a carbon-oxygen white dwarf in a binary system, whereas a core-collapse event would require a massive star exhausting its fuel within tens of millions of years. These are mutually exclusive histories that produce different spectra and light curves. No progenitor or precursor has been reported for this transient in the available literature.
How far away is AT2021xxd from Earth and how old is the light reaching us? Cosmic Distance & Time
AT2021xxd has a luminosity distance of approximately 584.2 Million Light-Years (redshift z = 0.039305, d_L = 179.127). Under a flat ΛCDM cosmology (H₀≈70 km/s/Mpc, Ωₘ≈0.3), the corresponding lookback time is ≈534 Myr — when the light set out 533.7 million years ago during the Paleozoic era, long before the first dinosaurs appeared on Earth. 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 AT2021xxd tell us about the expansion of space? Cosmic Distance & Time
AT2021xxd exhibits a measured spectroscopic redshift of z = 0.0393. The cockpit reports a user-facing recession velocity of 11,551.9 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 (≈11,783 km/s) is a different low-z approximation and is not mixed here.
How bright did AT2021xxd become at its peak, and how many Suns does that equal? Explosion Energetics
At peak brightness, AT2021xxd achieved an apparent magnitude of 20.7 around 2021/09/01. Corrected for cosmic distance and foreground interstellar dust, its intrinsic absolute magnitude was -15.524. At this peak, the exploding star radiated with the incandescent brilliance of approximately 138.5 million Suns combined, briefly outshining the cumulative starlight of entire dwarf galaxies!
How much total energy was released by AT2021xxd, and where did that energy go? Explosion Energetics
The total energy released by AT2021xxd is unknown. Energy budgets differ by orders of magnitude depending on mechanism: a thermonuclear Type Ia liberates ~10⁵¹ ergs mostly as kinetic energy, while a core-collapse event liberates ~10⁵³ ergs of which ~99% escapes as neutrinos. Without a spectroscopic classification, neither figure applies to this transient.
How fast are the supernova ejecta and shockwave of AT2021xxd expanding through space? Explosion Energetics
Spectroscopic line blueshifts imply an ejecta velocity of approximately 11,552 km/s (~3.9% of c) for AT2021xxd. This is distinct from cosmological recession velocity inferred from redshift.
What powers the prolonged glow of AT2021xxd weeks and months after detonation? Radioactive Engine
The power source sustaining AT2021xxd cannot be identified without a classification. Classified supernovae are typically powered after peak by the radioactive decay of ⁵⁶Ni → ⁵⁶Co → ⁵⁶Fe, but nickel yields and whether a shock-breakout flash occurred depend on the explosion mechanism. No spectrum is published for this transient, so no ⁵⁶Ni mass or decay-powered light-curve narrative applies.
What chemical elements did AT2021xxd create and disperse into the universe? Nucleosynthesis & Elements
The nucleosynthetic products of AT2021xxd are unknown because its type is unresolved. Element yields depend directly on the explosion mechanism: a thermonuclear Type Ia produces mostly iron-peak elements such as 56Ni and 56Fe, whereas a core-collapse event produces large quantities of oxygen, neon, magnesium and silicon from successive burning shells. No spectrum has been published for this transient, so which of these occurred is not established.
Did AT2021xxd leave behind a black hole, a neutron star, or nothing at all? Cosmic Remnant
Whether AT2021xxd left a remnant is unknown. A thermonuclear Type Ia leaves no compact remnant (the white dwarf is fully disrupted), whereas a core-collapse event leaves a neutron star or black hole depending on progenitor mass. Without a spectroscopic classification, none of these outcomes can be asserted for this transient.
What will AT2021xxd'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 AT2021xxd. 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 AT2021xxd explode, and where is it located relative to the galactic center? Galactic Environment
No host galaxy is identified for AT2021xxd 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 AT2021xxd located in the night sky and which constellation is it in? Sky Coordinates
In the celestial sphere, AT2021xxd is located at Right Ascension 23:01:57.327 and Declination +17:21:28.04, situated in the constellation Pegasus (The Winged Horse). Because its declination is +17:21:28.04, it is primarily placed in the Northern celestial hemisphere.
How much Milky Way interstellar dust obscures our view of AT2021xxd? Interstellar Dust
Along this line of sight, the catalog records a Milky Way foreground color excess of E(B−V) = 0.0532 mag (source: Analytical Galactic Disk Model (IAU)), corresponding to roughly A_V ≈ 0.16 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 AT2021xxd monitored? Astronomical Observations
No calibrated photometry is published for AT2021xxd. 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 AT2021xxd's chemical makeup? Astronomical Observations
No spectrum has been published for AT2021xxd. 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 AT2021xxd and how was it first detected? Discovery & History
AT2021xxd was officially reported on 2021/09/02 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 AT2021xxd? Scientific Research
Data for AT2021xxd 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 AT2021xxd? Cross-Identifications
Throughout global alert streams and survey databases, AT2021xxd has also been designated as: ZTF21abxqdhy. 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 AT2021xxd contribute to measuring the Hubble Constant and the scale of the cosmos? Cosmology & Distance Ladder
AT2021xxd cannot currently be used for cosmological distance work because its classification is unresolved. The distance ladder relies on type-specific techniques: Type Ia supernovae use the Phillips relation between peak absolute magnitude and decline rate, while Type II events use the Expanding Photosphere Method or SCM-II. An unclassified transient fits neither, so no Hubble-constant constraint can be derived from it until a spectrum establishes its type.
Could gravitational waves or neutrinos from AT2021xxd be detected on Earth? Multi-Messenger Astronomy
No neutrino or gravitational-wave counterpart has been reported for AT2021xxd, and none is expected to be identifiable without a classification. The multi-messenger signature differs sharply by type: core-collapse events emit a neutrino burst at shock breakout (detected once, for SN 1987A), while thermonuclear Type Ia events produce negligible neutrinos and gravitational waves. Searching for a counterpart before knowing which mechanism applies would be unfocused.
How does AT2021xxd 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), the distance to AT2021xxd is not established for a historical comparison (unclassified; redshift z = 0.039305 is published but no luminosity distance is derived). While historical naked-eye supernovae occurred within our Milky Way or its immediate satellites, modern discoveries like AT2021xxd allow astrophysicists to probe diverse galactic environments, metallicities, and stellar populations across the broader universe.
Can I see AT2021xxd tonight with a backyard telescope or binoculars? Backyard Observation
Unknown — no measured brightness. AT2021xxd was discovered 1890 days ago (2021/09/02), 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 AT2021xxd 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 584.2 Million Light-Years, the inverse-square law dilutes the radiation by quintillions of times, making AT2021xxd 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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