UY Scuti
UY Scuti — a red supergiant in Scutum long celebrated as a candidate for the largest known star. Its fame rests on a 2013 measurement that put its radius near 1,700 solar radii; but that assumed a distance of about 9,500 light-years, and better Gaia-era astrometry points to a nearer distance and a substantially smaller star, closer to 900 solar radii (Healy et al. 2023). Its Gaia parallax is itself unreliable for so large and unstable a star, so its true size stays uncertain by roughly a factor of two — and the 'largest star' title has since moved on. To the eye it is a faint, deeply reddened point, lost in the dense Scutum star cloud.
Key facts
Science
Physical
- Radius
- 4.23 AU · 909× wider than the Sun
- Spectral type
- M4 Ia (red supergiant)
Composition
- By mass
- A cool red supergiant whose true size is genuinely contested. The 2013 interferometric measurement that made it famous — about 1,700 solar radii, 'the largest known star' — assumed a distance near 9,500 light-years. Gaia-era astrometry favours a closer distance and a smaller star, near 900 solar radii (Healy et al. 2023), though Gaia's parallax is unreliable for so large and pulsating a star, leaving the radius uncertain by roughly a factor of two. Its mass is poorly constrained and is omitted here rather than guessed. The title of largest known star has since passed to other, equally uncertain candidates.
Temperature
- Surface
- 3,550 K (3277 °C)
Radiation & temperature
- Surface temperature
- 3,550 K (3277 °C)
- Peak wavelength
- 816 nm · near-infrared (mostly invisible to the eye)
Discovery & history
- Name
- UY Scuti — the variable-star designation 'UY' in Scutum, the Shield.
Recorded in the 19th-century Bonn Survey (BD) as a variable star. Its size made headlines after Arroyo-Torres and colleagues measured its angular diameter with the VLT in 2013; the later downward revision is a textbook lesson that a star's measured size is only ever as good as its distance.
Images
Images from open archives · credit shown per image