AT2021actu

AT2021actu 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.

The specific progenitor of AT2021actu 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 AT2021actu.

AT2021actu has a luminosity distance of approximately 367.3 Million Light-Years (redshift z = 0.024974, d_L = 112.627 Mpc). Under a flat ΛCDM cosmology (H₀≈70 km/s/Mpc, Ωₘ≈0.3), the corresponding lookback time is ≈343 Myr — when the light set out 342.6 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.

AT2021actu exhibits a measured spectroscopic redshift of z = 0.0250. The cockpit reports a user-facing recession velocity of 7,393.6 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 (≈7,487 km/s) is a different low-z approximation and is not mixed here.

At peak brightness, AT2021actu achieved an apparent magnitude of 19.8 around 2021/10/24. Corrected for cosmic distance and foreground interstellar dust, its intrinsic absolute magnitude was -15.4314. At this peak, the exploding star radiated with the incandescent brilliance of approximately 127.2 million Suns combined, briefly outshining the cumulative starlight of entire dwarf galaxies!

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 AT2021actu.

Spectroscopic line blueshifts imply an ejecta velocity of approximately 7,394 km/s (~2.5% of c) for AT2021actu. This is distinct from cosmological recession velocity inferred from redshift.

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 AT2021actu. 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.

Events of AT2021actu'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 AT2021actu.

A core-collapse explosion associated with AT2021actu'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.

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 AT2021actu. 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.

No host galaxy is identified for AT2021actu 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.

In the celestial sphere, AT2021actu is located at Right Ascension 03:36:05.270 and Declination -19:13:34.05, situated in the constellation Eridanus (The River). Because its declination is -19:13:34.05, it is favorably placed for Southern Hemisphere observatories.

Along this line of sight, the catalog records a Milky Way foreground color excess of E(B−V) = 0.0425 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.

No calibrated photometry is published for AT2021actu. 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.

No spectrum has been published for AT2021actu. 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.

AT2021actu was officially reported on 2021/10/25 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.

Data for AT2021actu are compiled from international astronomical notices, the IAU Transient Name Server (TNS), and peer-reviewed astrophysical journals.

Throughout global alert streams and survey databases, AT2021actu has also been designated as: ZTF21acizlcg. 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).

As a core-collapse supernova, AT2021actu 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.

Core-collapse supernovae like AT2021actu 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.

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), AT2021actu occurred at a distance of 367.3 Million Light-Years. While historical naked-eye supernovae occurred within our Milky Way or its immediate satellites, modern discoveries like AT2021actu allow astrophysicists to probe diverse galactic environments, metallicities, and stellar populations across the broader universe.

Unknown — no measured brightness. AT2021actu was discovered 1846 days ago (2021/10/25), 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.

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 367.3 Million Light-Years, the inverse-square law dilutes the radiation by quintillions of times, making AT2021actu completely harmless to our biosphere and purely a fascinating spectacle for human exploration.