Many school texts state that black holes are "purely theoretical," but scientists are busy finding and studying them. For example, views from ground-based visual and radio telescopes combined with views from the Hubbel Space Telescope give evidence of a black hole in the giant elliptical galaxy NGC 4261.
 

Left: Ground Based Composite Visual/Radio View: The giant elliptical galaxy NGC 4261 is one of the twelve brightest galaxies in the Virgo cluster, located 45 million light-years away. Photographed in visible light (white) the galaxy appears as a fuzzy disk of hundreds of billions of stars. A radio image (orange) shows a pair of opposed jets emanating from the nucleus and spanning a distance of 88,000 light-years.

Right: HST Image of NGC 4261: A giant disk of cold gas and dust fuels a possible black hole at the core of this galaxy. Estimated to be 300 light-years across, the disk is tipped enough (about 60 degrees) to provide astronomers with a clear view of the bright hub, which presumably harbors the black hole. The dark, dusty disk represents a cold outer region which extends inwards to an ultra-hot accretion disk with a few hundred million miles from the suspected black hole. This disk feeds matter into the black hole, where gravity compresses and heats the material. Hot gas rushes from the vicinity of the black hole's creating the radio jets. The jets are aligned perpendicular to the disk, like an axel through a wheel. This provides strong circumstantial evidence for the existence of black hole "central engine" in NGC 4261.

Resources on Black Holes

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booksweb resources

Hubble Revisited: New Images From the Discovery Machine
-- Infant galaxies, distant quasars, exploding stars, black holes, supernovae, star deaths -- astonishing clarity.

An Earthling's Guide to Deep Space
Huble images and commentary for young people.

The Mystery of Black Holes -- written for ages 9-11.

Black Holes and Time Warps: Einstein's Outrageous Legacy

Black Holes: A Traveler's Guide

Responsibility for conducting and coordinating the science operations of the Hubble Space Telescope rests with the Space Telescope Science Institute (STScI) on the Johns Hopkins University Homewood Campus in Baltimore, Maryland. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc. (AURA).