The table below guides you to a handful of stars visible without a telescope. Zeta Puppis tops the list as the hottest star naked-eye observers can see. R Cassiopeiae is truly an extreme star, taking top ranking as both the coolest (1/3 the surface temperature of our Sun) and the largest. Exchanged with our Sun, R Cassiopeiae would engulf all the planets except Uranus, Neptune, and Pluto. And S Carinae, the most distant naked-eye star at 12,000 light-years away, is a Mira-type variable star that brightens to magnitude 5.6.
There’s one other object on the list, but it’s no longer visible. Still, its incredible distance when it was visible — 2.5 million light-years — establishes its extreme-star credentials. In 1885, the light from a star that exploded in our neighboring galaxy, M31, finally reached Earth.
Astronomers gave it a standard variable star designation, S Andromedae, but had no idea how far away it was. The scientific concepts of an event as violent as a supernova, of the vast gulf between our Milky Way and M31, and even of galaxies themselves wouldn’t exist until the 20th century’s opening decades.
|Naked-eye extreme stars|
|Hottest||Zeta (ζ) Puppis||8h03m35s||–40° 00′ 11″||1,400 light-years||2.2||Surface temperature: 42,000 K — 7 times hotter than the Sun; 20 times the Sun’s size; 60 times the Sun’s mass; 800,000 times the Sun’s brightness.|
|Coolest and largest||R Cassiopeiae||23h58m25s||51° 23′ 19″||350 light-years||4.8||Surface temperature: 2,000 K — 1/3 the Sun’s; 1,800 times the Sun’s size; would extend nearly to orbit of Uranus; Variable star; mag. 4.8 is its brightest.|
|Farthest||S Carinae||10h09m22s||–61° 32′ 56″||12,000 light-years||5.6||Variable star; 5.6 is the brightest it gets.|
|Farthest||S Andromedae||0h42m43s||41° 16′ 04″||2.5 million light-years||6.0||Supernova in M31 (SN 1885, no longer visible — although you can see its galaxy)|
** Declination is the celestial equivalent of latitude.