The hallmark of the summer sky was the band of light stretching from the northeastern horizon to the southern horizon—the Milky Way.
On either side of the Milky Way were two bright stars. One was Lyra Alpha in the west, also known as Vega; the other was Aquila Alpha in the southeast, also known as Altair. Together with Cygnus Alpha, they formed the Summer Triangle. As long as it was not cloudy or rainy, this distinct geometric shape could be seen even in cities with light pollution.
However, to observe the Cepheid variable Eta Aquilae, one had to go somewhere far from artificial lights. It was one of the few Cepheid variables visible to the naked eye, with an apparent magnitude of 3.7 to 4.4 and a light cycle of roughly seven days.
According to Henrietta's theory, the longer the cycle of luminosity variation, the brighter the light emitted. Yet Eta Aquilae and Delta Cephei actually had similar apparent magnitudes, with Delta Cephei even somewhat brighter.
They were discovered one after the other by John Goodricke in 1784. Like Henrietta, he had become deaf and mute because of a fever. In the winter of 1782, he discovered Beta Persei—the binary system of the "Demon Star" with Medusa's eye—and proposed the theory of eclipsing variables.
When an expanded yellow giant contracted under gravity, pressure accumulated in the helium ionization layer until radiation pressure exceeded gravity, causing it to expand once more.
It was like the surface tension of liquid. Before the "balance" was broken, no matter how much it looked as though the liquid was about to spill over, it would not because of that tension. Once the "balance" was broken, there would be no containing it.
As time accumulated, so did the pressure. The greater the energy produced, the brighter