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Radio Telescopes Illuminate Oldest-Known Black Hole
New observations have yielded the most
precise measurements ever made of a black hole, achieving a first-ever,
complete description of one of these elusive objects. It’s fitting that
the subject of this investigation is Cygnus X-1, a brilliant X-ray
emitter that was discovered in 1964 and provided the first observational
evidence that black holes really do exist.
We now know: that Cygnus X-1 is 6070
light-years from Earth (36,000 trillion miles); that its mass is 15
times that of our Sun; and that it spins 800 times per second
at its event horizon. This makes X-1 one of the most massive “stellar”
black holes yet found, but only a piker compared to the “supermassive”
black holes at the centers of all major galaxies, which contain up to
ten billion times our Sun’s mass.
All of a black hole’s mass is compressed
into its central “singularity”, an infinitesimal ball that is probably a
trillion, trillion times smaller than the smallest atom. The
singularity is surrounded by an “event horizon” that marks the point of
no return. The event horizon, as horizons on Earth, isn’t made of
anything material but is instead a collection of locations in space.
From the outside, the event horizon is the limit of what can be seen.
Anything that enters the event horizon will never exit. Once inside the
event horizon, nothing can travel fast enough to escape, not even light.
Our theories of black holes, based on
Einstein’s General Theory of Relativity, show that they are
astonishingly simple. A normal star contains a variety of elements that
are spread differently throughout the star depending on their mass.
Temperature and pressure vary enormously and immense columns of gas flow
from its core carrying heat to its surface. All that complexity
vanishes when the star collapses to become a black hole. Each black
hole, theory says, is completely described by three numbers: its mass,
its spin rate, and it total electric charge. Since all massive objects
rapidly become electrically neutral, we really need only the other two
numbers for a complete description. Astrophysicists characterize this
simplicity by saying that “black holes have no hair.” Since we now know
X-1’s mass and spin, we know all that can be known (without going
inside).
A black hole in empty space would be
extremely difficult to detect. After all, no light can exit a black
hole—that’s why they’re called “black.” Astronomers can detect black
holes indirectly when they influence their surroundings. X-1 has a
companion star, a brilliant giant star named HDE 226868 that is about 20
times more massive and 350,000 times brighter than our Sun. The two
partners orbit one another due to their mutual gravity, and because we
observe the telltale motion of the bright partner, we know that the two
objects are only 20 million miles apart, five times closer than Earth
and the Sun, and that they complete a full orbit in 5.6 of our days.
X-1’s companion star emits an intense
stellar wind—charged particles that shoot out in all directions—that
carries away enough matter each year to form an Earth-sized planet. X-1
captures some of that matter, which accumulates in a thin disk that
rotates around the black hole—an “accretion disk.” In effect, X-1 is
slowly cannibalizing HDE 226868. See the artist’s sketch below. It is
this in-falling matter from the companion star that is heated to
millions of degrees and emits the X-rays that astronomers observe.
Shooting out in both directions along the rotational axis of the
accretion disk are two long, slender jets of charged particles. Each of
Cygnus X-1’s jets carries 1000 times more power than our Sun emits.
The presence of such a massive companion
star that has not yet reached the end of its life shows that the black
hole cannot be more than six million years old. Additionally, VLBA data
show the pair moves through the galaxy at “only” 45,000 miles per hour
(Earth’s speed around the Sun is 70,000 mph). This “slow” speed seems to
rule out the creation of X-1 in a supernova, as such an immense
explosion would likely have given the black hole a much stronger kick.
More evidence that Cygnus X-1 is a black
hole comes from observations of “dying pulse trains.” Occasionally, a
substantial chunk of matter falls from the accretion disk and spirals
inward to oblivion. We may then observe radiation from that matter
coming to us in bursts. As the matter orbits the black hole in an
ever-tighter spiral, we see a pulse of radiation each time the matter
turns in our direction. As the orbit shrinks and the matter moves
faster, the interval between pulses shortens and the radiation’s
wavelength increases due to an ever-stronger gravitational redshift. If
the in-falling matter ultimately hit a solid surface, such as that of a
neutron star, we would see a final, powerful energy burst from the
impact. But since a black hole’s event horizon is simply a location in
space, there is no impact as the matter goes through it and hence no
final, powerful burst. Observations of dying pulse trains without final
bursts confirm X-1 is a black hole.
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