Basically, these types of stars had been camouflaged in the light of other galaxies in a sense:

Until now, no one had considered the effects of these stars on the light coming from more distant galaxies. In distant galaxies the light of each pulsating star is mixed with the light of many more stars that do not vary in brightness.

“We realized that these stars are so bright and their pulsations so strong, that they are difficult to hide. We decided to see if the pulsations of these stars could be detected even if we couldn’t separate their light from the sea of unchanging stars that are their neighbors,” said co-author Dr Charlie Conroy of the Harvard-Smithsonian Center for Astrophysics.

This is an extremely fascinating find and with more telescopes being built, including in China, we are likely to find more of these.

thanks to sci-news.com for the great info

thanks to NASA or the pic

And here is a video of another unusual pulsating star.