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Lone Black Holes Discovered Adrift in the Galaxy

Astronomers using the Hubble telescope and ground-based observatories have discovered the first examples of isolated, stellar-mass black holes adrift among the stars in our Milky Way Galaxy. They detected two of these lonely, invisible objects indirectly by measuring how their extreme gravity bends the light of a more distant star behind them. All previously known "stellar" black holes have been found orbiting normal stars. Astronomers determined the presence of those compact powerhouses by examining their effect on their companion star. These new results suggest that black holes are common and that many massive but normal stars may end their lives as black holes instead of neutron stars, the crushed cores of massive stars that end their lives in supernova explosions. The findings also suggest that stellar-mass black holes do not require some sort of interaction in a double-star system to form but may be produced in the collapse of isolated, massive stars, as has long been proposed by stellar theorists.

1. What are stellar-mass black holes, and how are they different from supermassive black holes?

Stellar-mass black holes are the compressed remains of giant, exploding stars called supernovas. These compact, gravitational powerhouses keep everything, including light, from escaping their stranglehold. Supermassive black holes, as their name implies, are monsters. They are millions to billions of times more massive than the Sun and are believed to reside at the hearts of most galaxies. Scientists aren't sure how they first formed, but they believe that these massive "eating machines" were created during the early universe. Stellar-mass black holes, on the other hand, can form at any time. Supermassive black holes are easier to detect because scientists know where to look: the centers of galaxies. All black holes, by their very nature, are invisible. But scientists hunting for supermassive black holes probe the centers of galaxies, looking for how the suspected monsters gravitationally influence the stars and dust near the cores.

2. Explain the technique astronomers use to find the "drifting" stellar black holes.

The black hole's gravity acts like a powerful lens, bending the light of a background star, so that it appears as two separate images when the black hole slowly drifts in front of it. However, the black hole's gravity also magnifies these stellar images, causing them to brighten as the black hole passes in front. Astronomers used ground-based telescopes to search for these passages, called gravitational "microlensing" events. The two pictures at left identify the brightening star [center of boxed region] for one of the black holes. They then tapped Hubble and its sharp vision to pinpoint the "lensed" star. The Hubble frame [picture at right] indicates that the lensed star was blended with two neighboring stars of similar brightness that could not be separated in the poorer-resolution, ground-based images. Hubble's identification of the lensed star allowed for an accurate estimate of the mass of the black hole.

3. If astronomers can't see the objects passing in front of the stars, how do they know they're black holes?

Careful analysis reveals that each black hole is approximately six times the mass of the Sun. If they were ordinary stars with this bulk they would be bright enough to outshine the more distant background star. The masses are also too large to be white dwarfs or neutron stars. This leaves a black hole as the most likely explanation.