Science - USA (2020-06-05)

SCIENCE sciencemag.org 5 JUNE 2020 • VOL 368 ISSUE 6495 1047

should cool to black over billions of years.

But if the white dwarf orbits in a binary

pair with another star, a more spectacu-

lar fate may await it. The classic type Ia

scenario was proposed in 1973 by John

Whelan and Icko Iben. They mapped out

the fading light of supernova SN 1972e for

one full year, and realized its brightness

could be explained by the decay of about

one solar mass worth of radioactive met-

als. These, they proposed, were forged in

a white dwarf that had grown to a size at

which the pressure and temperature in its

core would be high enough to fuse carbon,

causing a thermonuclear blast. Whelan

and Iben suggested the white dwarf might

grow to that mass threshold if its grav-

ity stole hydrogen gas from a companion

star such as a puffed-up red giant, which

doesn’t clutch its outer layers too tightly.

Later modeling showed achieving this

growth was a tricky balancing act. If a

white dwarf gobbles up hydrogen too fast,

the hydrogen layer that forms on its sur-

face can get hot enough to blow up pre-

maturely, in a more modest thermonuclear

explosion called a classical nova or, if it

happens repeatedly, a recurrent nova. If

it accumulates too slowly, the white dwarf

can tiptoe up to a mass 1.44 times that of

the Sun. At that threshold, known as the

Chandrasekhar limit, theorists predict the

pressure inside will cause electrons and

protons to fuse into neutrons, and the

white dwarf will quietly collapse into a

neutron star.

But if the hydrogen is added at just the

right rate, a white dwarf, especially one

rich in carbon and oxygen, can respond

Red giant

Hydrogen layer (Not to scale)

builds up

Hydrogen

Red giant

survives

Core ignites

White dwarfs

merge

Heat of merger

sparks fusion

White dwarf binary

White dwarfs

on point

of merger

Helium is

pulled across

Helium layer

explodes

~14,000-kilometer

diameter

Shock waves

ignite core

Second dwarf

thrown from blast

Cor

Earth

White dwarf

Sun Red giant

Supernova

Mass = 1.4 Suns

Supernova

Mass ≥ 1.4 Suns

Supernova

Mass ≤ 1.4 Suns

Nuclear options

Type Ia supernovae, critical as

cosmic rulers and chemical forges,

are explosions of white dwarfs. But

why would a burned-out star go

nuclear? For decades, researchers

thought a giant companion star

donated gas to the white dwarf until

it was heavy enough to ignite. Two

alternative theories have emerged,

both involving white dwarf pairs in

spiraling dances to the death.

Stellar cinder

When a Sun-size star

runs out of hydrogen

and helium fuel, it

collapses to a white

dwarf, an Earth-size

ember made mostly

of carbon and

oxygen. Left alone,

it could smolder

for billions of years.

But a more explosive

fate may await if it is

paired with another star.

Classic scenario

The white dwarf steals gas, gaining enough

weight to start carbon fusion—leading to

a runaway explosion.

White dwarf merger

Merging white dwarfs generate heat to

spark carbon fusion. But the explosions may

be superficial and lopsided.

Double detonation

Just before a merger, one dwarf steals a thin

layer of helium that detonates, igniting

a bigger explosion in the core.

Orbits shrink as stars

emit gravitational

waves

GRAPHIC: C. BICKEL/SCIENCE

Published by AAAS