Astronomy

ASTRONEWS

Black hole

Black

hole Black hole

Intermediate-mass black holes

100 to 10,000 solar masses?

Example:

Possible black hole in 47 Tucanae:

2,300 MSun

Example:^

Milky Way’s supermassive

black hole (Sagittarius A*):

4 million MSun

Stellar-mass

black holes

2 to 100 solar masses

Supermassive black holes

1 million to several billion solar masses

Example:

Cygnus X-1: 14.8 M^

Sun

TIPPING THE SCALES

Black holes come in many sizes.

How are they categorized, and

how does each class measure up?

16 ASTRONOMY • JANUARY 2018

MERCURIAL ICE. A review of MESSENGER data indicates there’s enough water ice at Mercury’s poles to cover

the state of Rhode Island.

Like their smaller counterparts, supermassive

black holes weighing in at over 1 million times

the mass of the Sun can exist in binary pairs.

Scientists believe such pairs most commonly form

when galaxies merge — over time, each galaxy’s

supermassive black hole falls to the center of the

resulting larger galaxy, eventually orbiting and

merging with each other. But just how closely

can two gargantuan black holes orbit each other

before they collide?

In September 18’s Nature Astronomy, an

international team of astronomers reported the

discovery of the tightest-ever orbiting supermas-

sive black hole system. Located 400 million light-

years away near the center of the spiral galaxy

NGC 7674, the two black holes are separated by

less than 1 light-year. The previous record holder

has a separation of nearly 25 light-years.

To resolve such a tiny separation from such a

large distance, the team took advantage of a

technique called very-long baseline interferome-

try, which uses multiple radio telescopes net-

worked together to function as one massive

telescope. This provided an angular resolution

about 10 million times better than the human

eye, enough to resolve the two black holes.

The combined mass of the black holes is about

40 million times the mass of the Sun. Though they

are tightly bound, their orbital period is a sluggish

100,000 years, meaning it will be a while yet

before they merge to create a single object. — J.P.

Supermassive black holes

can form tight pairs

Novae occur when a white dwarf, the rem-

nant of a Sun-like star, collects material from

a nearby donor star. The resulting runaway

nuclear reaction causes a powerful explosion

visible from vast distances.

But some novae seem too bright. Now,

astronomers have shown that such explosions

don’t defy the laws of physics, but result from

amplification making them appear brighter.

The findings, published September 4 in Nature

Astronomy, prove a theory developed by Brian

Metzger, an astronomer at Columbia

University and co-author on the paper.

“Astronomers have long thought the

energy from novae was dominated by the

white dwarf, controlling how much light and

energy are emitted,” co-author Laura

Chomiuk of Michigan State University

explained in a press release. “What we discov-

ered, however, was a completely different

source of energy — shock waves that can

dominate the entire explosion.”

“This event shows that shocks are the main

event,” added Metzger.

White dwarfs in binary systems can pull

material from companion stars, and it slowly

builds on the stellar remnant’s surface. Once a

tipping point is reached, a nuclear reaction

begins. A wave of material is ejected from the

white dwarf, with another wave right after it.

The second wave, hotter and faster than the

first, collides with the cooler, slower-moving

material to produce heat and light that boost

the appearance of the nova. “The bigger the

shock, the brighter the nova,” said Chomiuk.

“We believe it’s the speed of the second wave

that influences the explosion.” — A.K.

Shocks boost nova brightness

These black holes form after the death

of a massive star (≥8 M

matter or by merging with other black black holes can grow by accreting nearby Sun). Stellar-mass

holes, causing gravitational waves that

can be seen by the Laser Interferometer

Gravitational-wave Observatory (LIGO).

Intermediate-mass black holes could

provide a link between stellar-mass and

supermassive black holes, with masses

between 100 and 10,000 M

Sun or more.

No intermediate-mass black holes

have been definitively discovered yet,

but several candidates have been

identified for further study.

Supermassive black holes reside in the centers of galaxies.

They are the engines that power quasars and jets spanning

hundreds of light-years. Astronomers still aren’t sure how

these huge black holes form, but they, too,

can merge and grow when their host

galaxies collide. — A.K.

ASTRONOMY

: ROEN KELLY

PARTNER DANCE.

Located at the center

of galaxy NGC 7674,

these two compact

radio sources are

less than a light-year

apart and correspond

to two accreting

supermassive

black holes orbiting

TIFR-NCRA AND RIT, USA each other.

SHOCKING DEVELOPMENT. Material ejected

during a nova, colored yellow in this artist’s concept,

gives off gamma rays as shock waves travel through it.

Observing optical light and gamma rays from a recent

nova helped astronomers to determine the role shocks

play in a nova’s brightness. BILL SAXTON, NRAO/AUI/NSF