The night sky is full of beacons, but some shine in a way that makes them nearly invisible. Astronomers have discovered a new class of objects: ultrasoft X-ray sources. They are so "cool" by X-ray standards that their emission falls at the boundary between ultraviolet and soft X-rays, where ordinary telescopes are almost blind.

These sources are binary systems where a compact object (a black hole, neutron star, or white dwarf) steals gas from a companion star. The infalling matter heats up and glows, but ultrasoft sources have lower temperatures than typical X-ray binaries. So their light shifts into the ultraviolet, which is absorbed by interstellar dust and gas. Previously, such objects simply went unnoticed, like a dim flashlight next to a spotlight.

The discovery is important for understanding the life of galaxies. Ultrasoft sources may be progenitors of type Ia supernovae: those very "standard candles" used to measure distances in the universe. Moreover, their ultraviolet radiation can ionize gas in galaxies, affecting the birth of new stars.

Now that the blind spot has been found, astronomers can re-examine old images and look for such objects in neighboring galaxies. Perhaps these invisible beacons are not so rare after all.