254
See also: Supernovae 180–81 ■ Cosmic radiation 214–17 ■ Hawking radiation 255B
lack holes are invisible.
They allow no matter
to escape, and, with the
exception of low-level Hawking
radiation at the event horizon,
even swallow up electromagnetic
light energy. Due to the difficulty
of detecting an invisible object,
black holes remained purely
theoretical concepts up to the
mid-20th century. However, such
a concentrated mass ought still
to create observable effects.
As it is dragged into a black hole,
matter will be heated to millions
of degrees as it is ripped apart by
gravitational forces, pouring out
X-rays into space in the process.
In the 1960s, astronomers
looked for cosmic X-ray sources
with a series of balloon- and rocket-
launched detectors. Many of the
hundreds of sources that they
found were assumed to be “X-ray
binaries”—star systems in which
a superdense stellar remnant, such
as a neutron star, tears material
away from its visible companion.
Among the first of these X-ray
binaries to be discovered, in 1964,was a strong source close to active
star-forming regions of the Milky
Way, in the constellation of Cygnus.
In 1973, Australian Louise Webster,
Briton Paul Murdin, and American
Tom Bolton independently took
measurements of the blue supergiant
star HDE 226868. They revealed that
it orbits an object far too massive
to be a neutron star. The only
candidate for the invisible partner,
Cygnus X-1, was a black hole. Black
holes were now more than mere
theoretical entities. ■A STAR THAT WE
COULDN’T SEE
DISCOVERING BLACK HOLES
IN CONTEXT
KEY ASTRONOMERS
Louise Webster (1941–1990)
Paul Murdin (1942–)
Tom Bolton (1943–)BEFORE
1783 English clergyman John
Michell suggests the existence
of a star whose gravity is so
strong that not even light can
escape it.1964 Cosmic X-rays are
detected by Geiger counters
in sounding rockets.1970 Uhuru, the first X-ray
observatory satellite, is launched.AFTER
1975 Stephen Hawking makes
a bet with theoretical physicist
Kip Thorne that Cygnus X-1 is
not a black hole.
1990 Hawking concedes the bet
and buys Thorne a subscription
to Penthouse magazine.2011 Further observations give
Cygnus X-1 an expected mass
of 14.8 suns (14.8 solar masses).An artist’s impression shows matter
flowing from the blue supergiant star
HDE 226868 into its black hole partner,
Cygnus X-1. The star is losing one solar
mass of material every 400,000 years.