Sky and Telescope - September 2018 - 25

u THE CENTRAL PARSEC These stills from a 3D visualization show the
stars within a parsec (3¼ light-years) of Sgr A*, based on the UCLA team's
observations from 1995 through 2012. Visible are the two populations of
young, massive stars: the inner tangle of S stars and the OB disk outside
it. Watch the movie at https://is.gd/uclagalacticcenter.

don't live long - maybe 100 million years or so. That's not
enough time for them to sink to the center.
Could astronomers be wrong and these stars did form
here? Not the S stars. In order for a cloud to collapse into a
star, the gravitational attraction of the front side of the cloud
to its back side must be stronger than either side's attraction
to the black hole. In the light-year around Sgr A*, it's essentially impossible to achieve those conditions. "For a gas cloud
to form a new star requires such an enormously high density,
it's almost stellar density," Genzel says. None of the gas seen
in the region is anywhere near that dense.
One alternative is that the S stars we see are the survivors
of binary systems torn apart by the black hole. Imagine a pair
of stars happily orbiting around each other, far away from
Sgr A*, Genzel explains. Take that pair and shoot it at the
black hole. As the binary approaches the black hole, it falls
deeper into the wide pit the black hole creates around itself in
spacetime. Much like a ball speeds up as it rolls down a hill,
the binary gains energy, its stars whipping faster and faster
around each other until finally they can't hold onto each other
anymore. The pair splits. One star gets stuck near the black
hole, orbiting it at a distance proportional to its original dis-

HILLS MECHANISM If a binary star system were to shoot by our galaxy's supermassive black hole, the black hole could tear the binary apart,
keeping one star in orbit around itself and sending the other shooting
away at an extreme velocity. This encounter, called the Hills mechanism,
might explain how the S stars came to huddle around Sgr A*.

ECCENTRIC KOZAI-LIDOV EFFECT In this three-body interaction, the
black hole plays gravitational havoc on a binary star system that's orbiting it - the two stars' orbits around each other elongate and flip every
which way. The elongation can bring the two stars dangerously close,
ultimately causing them to merge and create a new, young-looking star.

1 Binary stars start out in an
1 Start with a stable binary star

system, ignorant of the black hole.

2 An interaction (say, with a giant
molecular cloud) deflects the
binary toward the black hole.
The binary shoots toward the
black hole on a nearly
straight-in path.

3 The black hole kicks one star out
of the binary and takes its place.
The switcheroo boosts the
gravitational energy holding the
new "binary system" together.
The result is to widen the
separation approximately 1,000×
and
d also
l to
t eject
j t the
th deposed
d
d
star at 100s to 1,000s km/s.

elongated orbit around the
black hole.

2 The black hole's influence

elongates the stars' orbits
around each other, and the
orbits flip around a lot with
respect to the circuit
around the black hole.

3 The two stars merge!

4 New, young-looking star
orbits the black hole.

sk yandtelescope.com * S E P TE M B E R 2 018

25


https://www.is.gd/uclagalacticcenter http://www.skyandtelescope.com

Table of Contents for the Digital Edition of Sky and Telescope - September 2018

Contents
Sky and Telescope - September 2018 - Cover1
Sky and Telescope - September 2018 - Cover2
Sky and Telescope - September 2018 - 1
Sky and Telescope - September 2018 - Contents
Sky and Telescope - September 2018 - 3
Sky and Telescope - September 2018 - 4
Sky and Telescope - September 2018 - 5
Sky and Telescope - September 2018 - 6
Sky and Telescope - September 2018 - 7
Sky and Telescope - September 2018 - 8
Sky and Telescope - September 2018 - 9
Sky and Telescope - September 2018 - 10
Sky and Telescope - September 2018 - 11
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Sky and Telescope - September 2018 - 25
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Sky and Telescope - September 2018 - Cover3
Sky and Telescope - September 2018 - Cover4
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