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.
VLBA: Very
Long Baseline Array
The key observations that settled these questions came from radio
telescopes of the Very Long Baseline Array, “VLBA.” VLBA is a U.S.
research facility of ten telescopes located along a 5000-mile path from
Mauna Kea on the Big Island of Hawaii to St. Croix in the U.S. Virgin
Islands. (I can tell you there’s great scuba diving in the waters at
both ends.) Each dish is 82 feet wide and weighs 240 tons. By combining
observations from these telescopes, VLBA achieves a resolution
equivalent to one 5000-mile-wide dish (albeit one with huge holes).
VLBA has the highest imaging resolution on Earth, or in space, as good
as 0.2 milliarcseconds. That’s an angle ten million times smaller than
is spanned by the Moon. The dish at North Liberty, Iowa is shown below.
Humorous side to X-1
In 1975 Caltech Professor Kip Thorne and his good friend, the iconic
British astrophysicist Stephen Hawking, made a wager. Stephen bet
Cygnus X-1 wasn’t a black hole and Kip bet it was. Stephen said all his
theoretical work on black holes would be in vain if they didn’t
actually exist, but at least he would win the bet and receive a
four-year subscription to Private Eye magazine. If X-1 were shown to be
a black hole, Kip would win a one-year subscription to Penthouse. As
the evidence mounted, Stephen eventually conceded the bet and paid off
“to the outrage of Kip’s liberated wife.”
Best Regards,
Robert
December 1, 2011
Never too young or
too old
to wonder: Why?
Dr Robert
Piccioni,
Author of "Everyone's
Guide to Atoms, Einstein, and the Universe"
and " Can Life Be
Merely An Accident?"
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