Supernova Remnants · Constellation Cassiopeia

Cassiopeia A (Cas A / 3C 461)

Coordinates, kinematic distance (3.4 kpc), JWST/Chandra ejecta knots, and high-aperture observing guide for Cassiopeia A (3C 461).

Right Ascension (J2000.0)23h 23m 26.00s
Declination (J2000.0)+58° 48′ 54.0″
Primary ClassificationYoung core-collapse (Type IIb) supernova remnant & brightest extrasolar radio source
Best Observing SeasonAutumn (September–December)

Provenance-Backed Scientific Measurements

Every measurement below preserves its unit, uncertainty interval, observational method, reference epoch, quality status, and primary catalog source. See Uncertainty & Provenance for how competing measurements are handled.

Astrometric, Photometric, and Physical Parameters for Cassiopeia A (Cas A / 3C 461)
ParameterValueUncertaintyMethod & EpochQuality StatusSource Archive
Kinematic expansion distance3400 pc± 100 pc3D proper motion vs. Doppler velocity of optical ejecta knotsdefinitiveSIMBAD Astronomical Database (CDS)
Apparent shell diameter5 arcmin± 0.2 arcminRadio, X-ray, and infrared blast-wave shelldefinitiveJWST / MAST Science Archive & Mission Documentation
Fast optical knot expansion velocity5500 km/s± 500 km/sOptical/NIR spectroscopy of oxygen- and sulfur-rich ejecta knotshigh-confidenceJWST / MAST Science Archive & Mission Documentation
Measurement Uncertainties & Competing Solutions: Because thick interstellar dust in the Perseus arm extinguishes Cas A by A_V ≈ 5 to 8 magnitudes in visible light, optical integrated magnitude is poorly defined (~16 mag restricted to northern knots), whereas 3D kinematic expansion fixes its distance at 3.4 ± 0.1 kpc and its explosion arrival date around 1671–1680 CE.

J2000 Equatorial Sky Chart & Culmination Altitude

The chart below plots Cassiopeia A (3C 461 Young Supernova Remnant) at RA 23h 23m 26.00s, Dec +58° 48′ 54.0″ within Cassiopeia, alongside peak meridian transit altitudes for 40°N and 30°S observers. Use the RA/Dec Coordinate Converter to compute exact culmination altitude for your latitude.

J2000 Equatorial Sky Position (Cassiopeia) +90° 0° −90° 24h 12h RA 0h Peak Meridian Altitude 71° 40°N Lat 1° 30°S Lat RA: 23h 23m 26.00s Dec: +58° 48′ 54.0″ Epoch: J2000.0 First-party vector diagram derived from IAU / CDS SIMBAD / Gaia DR3 astrometry.

Astrophysical Context & Structure

Cassiopeia A (Cas A, 3C 461) is the youngest firmly dated core-collapse supernova remnant in the Milky Way and the brightest extrasolar radio source in the sky at frequencies above 1 GHz. Located 3.4 ± 0.1 kiloparsecs (11,090 light-years) away in Cassiopeia, its expanding ejecta knots trace back to an explosion whose light reached Earth around 1671–1680 CE (possibly glimpsed as a 6th-magnitude star '3 Cassiopeiae' by John Flamsteed in August 1680, though heavily dimmed by 5 to 8 magnitudes of interstellar dust).

Spitzer infrared light echoes scattered off nearby dust clouds enabled astronomers to obtain an actual optical spectrum of the 340-year-old flash, proving that Cas A was a Type IIb supernova—the core collapse of a red supergiant stripped of almost all its hydrogen envelope by a binary companion.

Chandra X-ray Observatory and James Webb Space Telescope NIRCam/MIRI images reveal an inside-out explosion where iron-rich core ejecta have overtaken silicon and sulfur shells, a mysterious inner infrared structure nicknamed the 'Green Monster,' and a central compact neutron star with a carbon atmosphere that exhibits neither radio pulses nor a pulsar wind nebula.

Theoretical Astrophysics Follow-Up: Supernova Nucleosynthesis & Neutron Star Remnants on FastCosmos.

Practical Visual Observing Guide

Minimum Instrument350–450 mm (14–18 inch) telescope under pristine dark skies (primarily an X-ray/IR/radio & imaging target)
Recommended Magnification150×–250×
Best Evening SeasonAutumn (September–December)
  • Located 2.1° west-northwest of 5th-magnitude Tau Cassiopeiae (midway between Beta Cassiopeiae/Caph and Messier 52).
  • Because of 6+ magnitudes of foreground visual dust extinction, Cas A is an extreme challenge visually, completely invisible in small and medium amateur telescopes.
  • Experienced deep-sky observers using 400 mm (16-inch) or larger apertures under pristine Bortle 1–2 skies at 150×–250× can detect the brightest northern arc of quasi-stationary flocculi and fast-moving knots (~16th magnitude).

Catalog Cross-Identifications & Aliases

Common Aliases: Cas A · 3C 461 · SNR G111.7-02.1

Cross-Catalog Identifiers for Cassiopeia A (Cas A / 3C 461)
Catalog AuthorityIdentifier
SIMBADNAME Cas A
WIKIDATAQ217163
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Reference Guide

Proper Motion, Radial Velocity & Redshift

How astronomers combine tangential proper motion (mas/yr), Doppler radial velocity (km/s), and cosmological redshift (z) to reconstruct 3D space motion.

Scientific Sources & Dataset Provenance

  • SIMBAD Astronomical Database (CDS) (2026-10 release): Wenger, M., et al. 2000, A&AS, 143, 9 (The SIMBAD astronomical database) — Centre de Données astronomiques de Strasbourg
  • JWST / MAST Science Archive & Mission Documentation (Cycle 1–4 Public Releases (2022–2026)): Rigby, J., et al. 2023, PASP, 135, 048001 (The Science Performance of JWST as Characterized in Commissioning) — NASA / ESA / CSA / STScI