WASHINGTON, Oct. 19 (Xinhua) -- Discovery of the largest example of a "small" black hole -- one formed from the collapse of a single massive star at the end of its lifetime -- has led scientists to revaluate how black holes come into being, according to a report in the latest issue of Nature.

Since black holes can't be seen -- because they trap all matter and light that enters them -- they are detected by the gravitational effects they have on nearby stars or other matter that is near them.

This team made their unusually precise calculations by measuring the motion of a star as it orbited abound the black hole, known as M33 X-7. The black hole completes one orbit every 3.45 days around its massive companion star.

The "small" black hole, 15.65 times the mass of the sun, required the surprised astronomers to reevaluate a long-standing theory.

"The theory we operated with for the last decade was that single-star black holes are formed from the remnants of massive stars -- the more massive the star, the more massive the remnant. But, all of the stellar mass black holes were expected to be in the range of 10 times the mass of the sun or less, since only the core regions of the star would collapse," said Charles Bailyn, Professor of Astronomy and Physics at Yale, and a member of the research team.

In this particular case, "finding a black hole with such unusual characteristic points out that our understanding of the evolution of massive stars and the formation of black holes must be incomplete," said Bailyn.

This black hole is also the most distant stellar black hole ever observed and is located outside our galaxy -- in a dwarf galaxy, Messier 33, that orbits Andromeda.

Bailyn noted that "ginding black holes in different and distant locations gives us many more objects to study and opens up the opportunity to find extreme examples that test theoretical limits."