When astronomers talk about blue stars, they're generally talking about hot, young stars known as blue supergiants. But the blue stars you can see in the image up top are stars just like their Sun... except they've mysteriously turned blue.
While blue supergiants are about 25 times the size of our Sun and 10 to 50 times as massive, these stars located in the hub of the nearby Andromeda Galaxy are middle-aged stars, more or less the same size and mass as our own. For reasons we still don't entirely understand, they have had most of their outer mass stripped off. That has left behind only the core, which is much hotter than their original outer layers and so burns bright blue. They also emit tons of ultraviolet radiation, making them one of the very few kinds of old stars capable of such a feat.
According to the latest survey by the Hubble telescope, there are about 8,000 of these stars all throughout the inner regions of the Andromeda Galaxy. These stars likely once looked like our Sun, but as they ran out of fuel and expanded outwards, the intense gravitational forces all around them stripped off everything but the blue-hot core.
The astronomers believe these stars, unlike our own, are rich in elements other than helium and hydrogen, and their heavier composition makes it easier for them to eject their outer core. That's the easiest way to explain how such bizarre stars could even exist — but we still don't know why these stars in particular are the ones that are burning blue. Whatever is going on here, it's a whole new chapter in astrophysics.