In the 1840s a star in the southern sky suddenly became the second brightest in the night sky, then faded from naked-eye view within a few decades. It had not died. This episode builds a biography of Eta Carinae, a system roughly 7,500 light years away that started life with an estimated 150 to 250 solar masses and has spent centuries swinging between obscurity and spectacle. We trace its quiet years as a fourth-magnitude star logged by Edmond Halley, its place in Chinese astronomy, and the Great Eruption recorded by John Herschel and by the Boorong people of Australia.

We then explain the physics behind the show: the temperature-sensitive CNO fusion cycle that made the core boil, the supernova impostor event that ejected 10 to 20 solar masses to form the Homunculus Nebula, and the 5.54-year cycle discovered in 1996 that revealed a massive binary companion. We cover the colliding stellar winds that heat gas to 100 million Kelvin, the only known natural ultraviolet laser in the universe, and what a future core-collapse or pair-instability supernova would mean for Earth.

• Why the backward magnitude scale made a 150-plus solar mass star look ordinary for centuries
• How the Great Eruption of 1837 to 1843 relieved internal pressure without destroying the core
• The eccentric orbit of Eta Carinae A and B, from Neptune-like distances down to closer than Mars is to the Sun
• How Lyman-alpha radiation pumps iron ions in the Weigelt blobs to create stimulated emission on an astronomical scale
• What a magnitude negative four supernova could do to Earth’s ozone layer, satellites, and astronauts, and why the star may already be gone