An artist's concept of NASA's Nuclear Spectroscopic Telescope Array, or NuStar, X-ray telescope. The spacecraft has discovered the brightest pulsar ever detected. NASA

A NASA space telescope studying X-ray emissions from a nearby galaxy has discovered the brightest pulsar ever detected, the fast-spinning remnant of a collapsed star that shines so intensely it was initially mistaken for a massive black hole, a possible "missing link" between compact stellar-mass black holes and the unseen monsters lurking at the cores of many galaxies.

While the brilliant pulsar may, in fact, become a black hole some day, it is not yet a member of the family tree. But theorists cannot explain how an object just one-and-a-half times as massive as the sun can suck in enough raw material to generate the high-energy X-ray emissions detected by NASA's Nuclear Spectroscopic Telescope Array, or NuStar.

"Today we're announcing the discovery of a pulsating dead star that's beaming X-rays with the energy of about 10 million suns," said Fiona Harrison, NuSTAR principal investigator at the California Institute of Technology in Pasadena, Calif.

"This dead star, called a neutron star, packs about the mass of the whole sun into a region the size of San Francisco. Yet this little mighty mouse pulsar packs the power of a much bigger black hole. The discovery is astonishing, because no object like this has ever been observed to be even remotely this bright. Theorists didn't think that it was possible."

The pulsar was found at the heart of a galaxy known as M-82, some 12 million light years from Earth. Also known as the "cigar galaxy" because of its oblong shape, M-82 is a favorite target for amateur astronomers, bright enough to be seen in relatively small telescopes. Larger instruments reveal huge, distinctive jets of material streaming away from the galaxy's core at right angles to its disk.

Shining in the core of galaxy M-82 some 12 million light years from Eath are two ultra-luminous X-ray sources. One, the brilliant blue "star" just outside the reddish glow, is known as X-1. It harbors a black hole 400 times more massive than the sun. The bright star in the center of the red glow is known as X-2. While almost as bright as X-1, it is, in fact, a pulsar, the collapsed remnant of a star just 1.5 times as massive as the sun. Scientists do not yet know how X-2 generates such enormous energy.

NASA

When stars use up all of their nuclear fuel, the energy generated by fusion in the core stops and gravity takes over, causing the core to collapse. Stars like the sun can only collapse so far, becoming white dwarfs that slowly cool over billions of years.

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NASA X-ray telescope finds theory-defying pulsar