sne.space
📡 Radar ❓ FAQs
🔭 Pro Cockpit 📖 Story View
Type Ib/c Supernova

The Story of Supernova AT2021ssu

An extraordinary stellar explosion that occurred 491.6 Million Light-Years away in deep space.

⏱️ ~6 min deep read 🔬 24 Astrophysical Q&As 🔭 Viewing Guide Included 🪐 Constellation Pisces
🔭 Deep Field (~2.1′ FOV) · DSS2 Optical (China-VO mirror)
● AT2021ssu Explosion Site

High-magnification deep optical view into AT2021ssu'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
491.6 Million Light-Years
Lookback Cosmic Time
✨ Peak Brightness
Mag 17.7
Large Observatory
☀️ Peak Radiance
1.6 billion Suns
Combined Stellar Energy
🚀 Shock Velocity
~9,795 km/s
~3.3% Speed of Light
🌌 Host Galaxy
LEDA 101452
Center Coincident
🧭 Constellation
Pisces
The Fishes
💥 Explosion Physics
Type Ib/c
Core-Collapse Supergiant
📅 Discovered On
2021/07/08
ATLAS
CHAPTER I

A Core-Collapse Class Transient

On 2021/07/08, astronomers scanning the heavens flagged a sudden, violent pinpoint of light that had flared into visibility. Designated AT2021ssu, it represents a catastrophic stellar explosion classified as a Type Ib/c supernova.

AT2021ssu 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 AT2021ssu.

Astrophysical Mechanism Summary
Type: Type Ib/c • Progenitor: Massive Red/Stripped Supergiant Star • Velocity: ~9,795 km/s (~3.3% c)
CHAPTER II

A Message Across Deep Cosmic Time

AT2021ssu has a luminosity distance of approximately 491.6 Million Light-Years (redshift z = 0.033223). That figure is not the light-travel time: under flat ΛCDM the lookback time is ≈453 Myr, when the light set out 453.1 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

Incandescence of 1.6 billion Suns

At the height of the outburst around 2021/07/07, AT2021ssu surged to a peak apparent magnitude of 17.7 and an intrinsic absolute magnitude of -18.1557. At that instant, this single dying star radiated with the collective power of approximately 1.6 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 Ib/c light curves; an object-specific nickel mass for AT2021ssu 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 AT2021ssu'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 AT2021ssu.

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 Pisces

AT2021ssu is associated with cataloged host LEDA 101452 . In our terrestrial sky, it resides in the constellation Pisces (The Fishes) at Right Ascension 01:19:51.554 and Declination +10:33:20.31.

CHAPTER VII

The Scientific Surveillance Campaign

Following its discovery by ATLAS, observatories worldwide swung their lenses toward AT2021ssu. In the sne.space, AT2021ssu 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 5.4 years ago (2021/07/08). No reliable current brightness estimate — no photometry has been published.

Planetary Safety Note: Even though AT2021ssu was a titanic explosion, our planet sits safely outside the lethal 50–100 light-year kill zone. At a distance of 491.6 Million Light-Years, the blast poses zero physical hazard to Earth's biosphere.

🌌 Cosmic Neighbors & Contemporaries 105,868+ transients indexed

Cataloged transients closest to AT2021ssu in discovery time, spatial sky neighborhood, and cosmological redshift:

⏱️ Closest in Time
AT2021spi Exploded same day
Type Candidate 2021/07/08 · Mag 20
AT2021spl Exploded same day
Type Candidate 2021/07/08 · Mag 20.3
AT2021spm Exploded same day
Type Candidate 2021/07/08 · Mag 20.1
🔭 Closest on the Sky
MLS121009:011848+104311 18.5′ away
Type Candidate Discovered 2012/10/09
PSN J01190082+1018191 19.5′ away
Type Transient Discovered 2014/08/19
AT2021yze 23.2′ away
Type Candidate Discovered 2021/09/15
🌌 Same Cosmic Era (Redshift)
AT2015cv z = 0.033224
Type Candidate ~455.1 Mly (approx.)
SN2018hck z = 0.033226
Type Ia ~455.1 Mly (approx.)
SN2016fmt z = 0.03322
Type II ~455.1 Mly (approx.)

❓ Frequently Asked Questions About AT2021ssu

Scientific & observational Q&As indexed from astronomical databases & the sne.space
🔍 24 of 24
What type of supernova is AT2021ssu and what kind of star exploded? Astrophysics & Progenitor
AT2021ssu is a stripped-envelope supernova (Type Ib/c), 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 AT2021ssu? Astrophysics & Progenitor
The specific progenitor of AT2021ssu 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 AT2021ssu.
How far away is AT2021ssu from Earth and how old is the light reaching us? Cosmic Distance & Time
AT2021ssu has a luminosity distance of approximately 491.6 Million Light-Years (redshift z = 0.033223, d_L = 150.74). Under a flat ΛCDM cosmology (H₀≈70 km/s/Mpc, Ωₘ≈0.3), the corresponding lookback time is ≈453 Myr — when the light set out 453.1 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 AT2021ssu tell us about the expansion of space? Cosmic Distance & Time
AT2021ssu exhibits a measured spectroscopic redshift of z = 0.0332. The cockpit reports a user-facing recession velocity of 9,794.6 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 (≈9,960 km/s) is a different low-z approximation and is not mixed here.
How bright did AT2021ssu become at its peak, and how many Suns does that equal? Explosion Energetics
At peak brightness, AT2021ssu achieved an apparent magnitude of 17.7 around 2021/07/07. Corrected for cosmic distance and foreground interstellar dust, its intrinsic absolute magnitude was -18.1557. At this peak, the exploding star radiated with the incandescent brilliance of approximately 1.6 billion Suns combined, briefly outshining the cumulative starlight of entire dwarf galaxies!
How much total energy was released by AT2021ssu, 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 AT2021ssu.
How fast are the supernova ejecta and shockwave of AT2021ssu expanding through space? Explosion Energetics
Spectroscopic line blueshifts imply an ejecta velocity of approximately 9,795 km/s (~3.3% of c) for AT2021ssu. This is distinct from cosmological recession velocity inferred from redshift.
What powers the prolonged glow of AT2021ssu 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 AT2021ssu. 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 AT2021ssu create and disperse into the universe? Nucleosynthesis & Elements
Events of AT2021ssu'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 AT2021ssu.
Did AT2021ssu leave behind a black hole, a neutron star, or nothing at all? Cosmic Remnant
A core-collapse explosion associated with AT2021ssu'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 AT2021ssu'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 AT2021ssu. 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 AT2021ssu explode, and where is it located relative to the galactic center? Galactic Environment
AT2021ssu is cataloged with host galaxy LEDA 101452. No secure nuclear offset is published in this record.
Where is AT2021ssu located in the night sky and which constellation is it in? Sky Coordinates
In the celestial sphere, AT2021ssu is located at Right Ascension 01:19:51.554 and Declination +10:33:20.31, situated in the constellation Pisces (The Fishes). Because its declination is +10:33:20.31, it is primarily placed in the Northern celestial hemisphere.
How much Milky Way interstellar dust obscures our view of AT2021ssu? Interstellar Dust
Along this line of sight, the catalog records a Milky Way foreground color excess of E(B−V) = 0.0424 mag (source: Analytical Galactic Disk Model (IAU)), corresponding to roughly A_V ≈ 0.13 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 AT2021ssu monitored? Astronomical Observations
No calibrated photometry is published for AT2021ssu. 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 AT2021ssu's chemical makeup? Astronomical Observations
No spectrum has been published for AT2021ssu. 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 AT2021ssu and how was it first detected? Discovery & History
AT2021ssu was officially reported on 2021/07/08 by ATLAS. 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 AT2021ssu? Scientific Research
Data for AT2021ssu 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 AT2021ssu? Cross-Identifications
Throughout global alert streams and survey databases, AT2021ssu has also been designated as: ATLAS21barw. 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 AT2021ssu contribute to measuring the Hubble Constant and the scale of the cosmos? Cosmology & Distance Ladder
As a core-collapse supernova, AT2021ssu 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 AT2021ssu be detected on Earth? Multi-Messenger Astronomy
Core-collapse supernovae like AT2021ssu 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 AT2021ssu 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), AT2021ssu occurred at a distance of 491.6 Million Light-Years. While historical naked-eye supernovae occurred within our Milky Way or its immediate satellites, modern discoveries like AT2021ssu allow astrophysicists to probe diverse galactic environments, metallicities, and stellar populations across the broader universe.
Can I see AT2021ssu tonight with a backyard telescope or binoculars? Backyard Observation
Unknown — no measured brightness. AT2021ssu was discovered 1956 days ago (2021/07/08), 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 AT2021ssu 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 491.6 Million Light-Years, the inverse-square law dilutes the radiation by quintillions of times, making AT2021ssu completely harmless to our biosphere and purely a fascinating spectacle for human exploration.
Data sourced from IAU TNS, ALeRCE, WISeREP, and the sne.space. View All General Astrophysics FAQs →
View Full Scientific Data & Plots in Pro Cockpit →