Sky & Telescope - February 2025 - 14

GAIA SKY / ESA / MPL 2.0
UNSEEN COMPANIONS by Ken Croswell
The vast majority of black holes elude detection.
Gaia has recently found three of these
invisible objects and will soon discover more.
T
he first black hole ever found, Cygnus X-1, epitomizes
the standard view of a stellar-mass black hole: Gas
from an orbiting star spirals around the dark object
and heats up dramatically, emitting a plethora of X-rays.
This powerful radiation is the black hole's " siren song " in the
words of the Canadian rock band Rush, which in 1977 wrote
a 10-minute song about a journey to Cygnus X-1.
But for every black hole that boldly broadcasts its presence
via X-rays, countless others keep things on the down-low.
In fact, the entire Milky Way Galaxy has only an estimated
1,000 black holes like Cygnus X-1 - versus roughly 100 million
X-ray-quiet black holes.
Now the European Space Agency's Gaia spacecraft is starting
to find the quiet ones. " Gaia is a total game-changer, "
says Kareem El-Badry (Caltech), whose team recently used
the spacecraft to locate two previously unknown black holes
in our galaxy. Launched in 2013 to measure the distances
and motions of luminous stars, Gaia isn't the first to find a
quiet black hole, but El-Badry estimates the spacecraft will
eventually uncover dozens more.
" It's wonderful that we have a new population [of black
holes] detected by a completely new method, " says Avi Loeb
(Harvard University), who was not involved in the recent
discoveries. " The precision is quite remarkable. "
Indeed: Observations published in the January 2024 Publications
of the Astronomical Society of the Pacific nailed down
the mass of Gaia's first black hole to about 1% - far better
than the mass determination of any other stellar-mass black
hole in the universe.
Such precision results from the spacecraft's exquisite
measurements coupled with the work of Johannes Kepler four
centuries ago. Although his three laws originally pertained to
the planets orbiting the Sun, they also describe how one star
orbits another - even if one of them is a black hole.
In particular, Kepler's third law allows astronomers to
measure stellar masses. It states that the greater the combined
mass of the two stars, the faster they whip around each
other for a given separation. Measuring their orbital period
and mean separation therefore reveals their total mass.
If a luminous star orbits a black hole, the star appears
to be going around nothing at all. By measuring the star's
changing position on the plane of the sky, Gaia sees this orbit
projected onto that plane. Meanwhile, ground-based observations
of the Doppler shifts in the luminous star's spectrum
reveal the star's motion along our line of sight (its radial
GAIA'S FIRST BLACK HOLE This visualization shows Gaia BH1
with its companion, a yellow main-sequence star. The black hole warps
nearby space, causing light rays that pass through the space to bend
around the black hole's location.
14 FEBRUARY 2025 * SKY & TELESCOPE
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Sky & Telescope - February 2025

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