The name Cepheid variable came from John Goodricke's discovery of Delta Cephei in 1784. At its brightest, its apparent magnitude was 3.7; at its dimmest, 4.4. Its period was 5.37 days.
Stars whose brightness changed in such a regular pattern could be seen as having a "heartbeat." Delta Cephei was a binary star. Its primary was a yellow supergiant or bright giant, while its companion was blue-white. Since the difference between them was not great, and neither had enough mass to undergo a supernova explosion, it would ultimately contract into a white dwarf.
Within Cepheus was a bright reflection nebula known as the Iris Nebula. If the weather was good, it could be observed through a large-aperture telescope. It was dust produced after a supernova explosion. It could be seen because a nearby star illuminated it, and the dust reflected that light, creating a blue color like an iris. It was sometimes nicknamed the "Iris Nebula."
Not every nebula was a galaxy. Even if there was red light at the center of the Iris Nebula, it was not a star, merely matter in the dust's center altered by ultraviolet light.
It was the universe's most dazzling "firework," though it signified the death of a massive star.
Like many yellow giants, Delta Cephei began burning helium after exhausting its hydrogen. At that point, however, it could not yet produce elements heavier than helium as red giants could. A helium ionization layer formed inside the star, causing ionized helium to continue ionizing. At first, the yellow giant expanded. Then, due to gravitational contraction, doubly ionized helium became singly ionized helium and released heat outward. As radiation pressure built up, it finally exceeded gravity, causing the star to expand again as before, only to contract once more under gravity. This cycle continued until