Sky and Telescope - January 2017 - 26

9 10

Neutron star becomes black hole

Neutron star becomes black hole

Neutron star

Neutron star

Iron core
collapse

White dwarfs

O/Ne/Mg core collapse

About solar value
Metal-free

Metallicity (roughly logarithmic scale)

Black Holes, Part I

25

en No H
vel
op
e

Immediate
black hole

Immediate
black hole
No
remnant
40

100

140

Immediate
black hole

t REMNANTS OF MASSIVE STARS
When stars die, what they become
next depends on their masses and how
tainted their gas is with heavy elements,
called metals. The most massive stars
create black holes (gray regions) -
except for a subset that have just the
right characteristics to die in a so-called
pair-instability supernova, which leaves
no remnant. The cores of less massive
and/or more metal-tainted stars become
neutron stars, some of which then convert into black holes when too much material from the star's layers falls back on
them and triggers runaway gravitational
collapse (yellow regions). The green line
separates stars that keep their outer
hydrogen layer (left and lower right) from
those that lose it in winds. Many of the
universe's first stars should have fallen in
the black hole regions of this diagram.

260

Surveys have uncovered roughly 50 quasars, brilliant hot-gas
beacons powered by supermassive black holes, blazing only a
billion years after the Big Bang. The black holes have masses of
billions of Suns, comparable to the heftiest black holes we see
today. To be so big so early, these titans must have grown to
full size within 900 million years of the Big Bang.
But that's nonsensical. Black holes usually eat like obstinate toddlers - a little food goes down the throat, but most
is left on the plate, dropped on the floor, or flung away. If the
earliest objects adhered to this grazing strategy, they'd never
have beefed up in time.
Still, these black holes came from somewhere; there's no
cosmic stork. Astrophysicists have been grappling for years
with the questions of how these objects were conceived and
how they grew so fast. And thanks both to some high-powered
simulations and to observations of dwarf galaxies, it's finally
looking like the problem is not as intractable as it once seemed.

Creating Seeds
Astronomers have three main ideas for black hole genesis. The
basic question boils down to whether stars stick their noses
into the affair or not. For two of the theories, they do; the
third circumvents them.
The first scenario starts with star clusters. Stars in clusters
can sometimes bonk into each other and merge. If a cluster
is compact enough, its stars could suffer runaway collisions,
growing like The Blob into a single star a few hundred to a
few thousand times as massive as the Sun. This superstar
would then collapse into a black hole.
Calculations suggest that such cluster dynamics could
produce black hole "seeds" of several hundred to a couple
thousand solar masses. But this process is grossly inefficient,
and it's hard to explain why a dense enough cluster would
26

J A N U A R Y 2 0 1 7 * SK Y & TELESCOPE

exist to enable this chain of events. Of the three theories, this
one is the least talked about among astronomers.
The second (far more popular) star scenario is a soupedup version of "normal" black hole creation. When a single,
massive star dies catastrophically, its core can collapse and
become a black hole. The first generation of stars to form in the
universe, called Population-III stars, would have often died this
way. These stars probably weighed in at a couple hundred Suns.
The universe can't easily create stars that big anymore,
because of a little thing astronomers call metals. In astronomers' parlance, these are all the elements heavier than
helium. Stars fuse hydrogen and helium into metals, and
when the stars die they spread these heavier elements into the
galaxy around them.
Metals make things difficult for star growth, for two
reasons. First, metals cause gas to cool faster. Cooler gas fragments into smaller star-forming clumps than warmer gas,
meaning metal-rich stars will generally start smaller than
metal-poor ones. Second, metals make gas more opaque. A
star's light actually pushes on its outer gas layers, balancing the
inward pull of gravity. If a star has a lot of heavy elements in
its makeup, its gas will be less transparent. Instead of passing
through the outer layers, light will shove on them, the pressure
driving the layers away as massive winds. This process curtails
the star's growth and exacerbates stellar weight loss.
Population-III stars didn't have this problem, because they
contained no metals: there were no stars before them to pollute the gas that made them. Those that later collapsed into
black holes could have retained at least half of their original
mass, or about 100 solar masses (see graph above). High-end
exceptions might weigh in at 10 times that. Planted at the
center of a growing galaxy and fed by gas that sank there, these
seeds would have then grown into supermassive black holes.

S&T: LE A H TISCIONE, SOURCE: HEG ER E T A L. / ASTR OPHYSICAL JOUR NAL 20 03

Initial mass (solar masses)



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