X-treme stars you can see

In “X-treme stars of the cosmos,” we took you on a tour of some the galaxy’s biggest, hottest, and most distant stars. But you don’t need a space telescope to see stellar extremes; you just have to know where to look.

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
Record	Name	R.A.*	Dec.*	Distance	Mag.	Note
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)

* Right ascension is the celestial equivalent of longitude.
** Declination is the celestial equivalent of latitude.