Astronomy - USA (2020-03)

68 ASTRONOMY • MARCH 2020

ASK ASTRO Astronomy’s experts from around the globe answer your cosmic questions.

QI

HOW DO ASTRONOMERS

CALCULATE THE MASS OF A

NEUTRON STAR OR BLACK HOLE?

Shobha Kaicker

Mississauga, Ontario

AI

Astronomers can calculate mass in several

ways. The simplest is when the object is part

of a binary system. Regardless of the objects in the

binary — two stars, a star and a neutron star, a star and

a black hole, et cetera — their orbits follow Kepler’s laws

of motion, which allow a scientist to calculate mass

based on the speeds of the objects and the size of their

mutual orbit. Additionally, when two objects such as

Weighing

black holes

neutron stars or black holes merge,

the gravitational waves they pro-

duce tell astronomers the masses

of the original objects, as well as

the mass of the result their merger

leaves behind.

But what about a lone neutron

star or black hole? A single neutron

st a r may g ive it sel f away a s a pu l s a r.

Pulsars are spinning neutron stars

beaming out intense radiation

along their poles. If these narrow

beams happen to point at Earth,

astronomers can identify the

source as a neutron star. The sig-

nals from a pulsar are regularly

spaced as the star spins. And some

pulsars glitch, which makes them

temporarily speed up. Astronomers

believe glitches are caused by inter-

actions between material from the

neutron star’s core and its crust.

Tracking a glitch as it occurs can

reveal information about the star’s

internal temperature. Work pub-

lished in Science Advances in 2015

suggests that if researchers know

the age of the neutron star, they can

use glitches to measure the star’s

current internal temperature and

compare it with a model of what the

neutron star’s interior should look

like. The model, in turn, allows

researchers to calculate the star’s mass based on its tem-

perature and age.

While it’s not possible to accurately calculate the

mass of a lone stellar-mass black hole created by the

death of a single star, it is possible to calculate the mass

of a galaxy’s central supermassive black hole. That’s

because astronomers have observed a link between the

mass of a supermassive black hole and the mass of the

spheroidal component — the bulge — of the galaxy

around it. There is also a link between the motion of the

stars in a galaxy’s bulge and the mass of its supermassive

black hole. So, astronomers can measure one or both of

these quantities to derive the mass of a galaxy’s central

supermassive black hole.

In the case of the Milky Way’s supermassive black

hole, however, researchers observed the motion of stars

around the object over the course of years. This allowed

them to use Kepler’s laws to measure its mass: 4.3 mil-

lion solar masses.

Alison Klesman

Associate Editor

It is straightforward
to calculate the mass
of a neutron star in
a binary system,
depicted in this
artist’s concept (the
neutron star is on the
right, pulling material
off its companion star
at left). By watching
the objects orbit each
other, astronomers
can use Kepler’s laws
of motion to derive
their mass. NASA/
JPL-CALTECH/R. HURT (SSC)