Sky and Telescope - September 2018 - 27

FER MI: N ASA GSFC; SK E TCH: LE A H TISCIONE / S&T

u FERMI BUBBLES Spanning some 50,000 light-years from end to end,
the Fermi bubbles were formed by one or more energetic events in our
galaxy's core. The pink regions are based on gamma-ray data, the purple
edges on X-ray observations. The bubbles are to scale.

the bend, one end of the noodle would collide with the other.
Collision begets shocks and compression in gas, which lead to
little knots that collapse into stars.
Several simulations indicate that, with this method, star
formation is plausible at the disk's location. Furthermore,
this scenario might actually favor the birth of massive stars
like those in the disk. When large amounts of gas pour onto
a black hole, the stuff nearest the black hole forms a tutu
around it and heats up due to internal friction. Work presented by Chris Frazer (University of North Carolina, Chapel
Hill) at the American Astronomical Society meeting this
past January suggests that the blazing glow from this accretion disk heats gas at much greater distances from the black
hole. The radiation reaches all the way to where the OB disk
lies, hundreds of times farther out than the accretion disk.
Because gas generally needs to be cold in order to condense
into stars, the radiation could affect star formation, warming
things up enough that protostellar clumps would have to pack
on more gas before they collapsed.
"The most startling point about this entire story - which
we are all still dazzled about - is how robust this star-formation process actually is," Frazer says. "Even extreme radiation
feedback doesn't appear to disrupt star formation."
The key to the OB disk's origin might be its age. Of the
O stars, about half have already converted into Wolf-Rayet
(WR) stars. WR stars are extremely hot, massive stars that
spew out hefty winds of material. They're a short-lived stage
of evolution for the largest stars, providing an accurate clock
for when the disk's stars were born, Naoz says. Their presence
indicates that the stars are about 5 to 6 million years old.
Another major event is thought to have happened in the
galactic center around that time: the creation of the Fermi

bubbles (S&T: Apr. 2014, p. 26). Two gargantuan lobes that
extend roughly 25,000 light-years above and below the Milky
Way's disk, the Fermi bubbles were likely blown out by an
energetic event, either an outburst from the black hole or a
slew of star birth and death in its neighborhood.
"It's fascinating that the disk of stars is a comparable age,"
Ghez said during a recent talk at MIT. If a big gas cloud fell
onto Sgr A*, it would be natural for the cloud both to "wake
up" the black hole by feeding it and to fragment into new
stars. "I'm pretty convinced."

ONE WAY TO MAKE THE OB DISK Unlike the S cluster, the disk
of OB stars likely formed in place, perhaps from a giant cloud of
gas that accreted onto the black hole.

1 Massive gas cloud

falls onto black hole.

2 Cloud forms
good descriptor for G1 through G5 - and for many
other objects in the galactic center. If the eccentric
Kozai-Lidov (EKL) mechanism is forcing binary stars
to merge, some of them would look exactly like the
G objects, until they settled down to be S stars. Furthermore, binaries on bizarre, beeline-like routes like
the ones G1 and G2 follow around Sgr A* are the most
susceptible to the EKL mechanism.
However, preliminary data suggest G3's orbit is fairly
circular, which wouldn't favor a quick EKL-spurred
merger. It could be that some G objects formed different ways. Furthermore, the EKL effect wouldn't explain
how stars wind up on these strange orbits in the first
place, a puzzle that remains bothersome.

gigantic disk
around black hole.

3 Disk feeds black

hole, perhaps turning
on jets, and forms a
disk of stars.

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

27

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

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