Astronomy

Local “Orion” Arm

Milky Way

Sun

Iron nucleus

26 protons

34 neutrons

Cobalt nucleus

27 protons

33 neutrons

Co-60

Neutron changes

to proton

Neutron changes

to proton

Nickel nucleus

28 protons

32 neutrons

2.6 million years 5 years Ni-60

STABLE ELEMENT

Sun

Auriga Perseus

Cloud

“Pleiades”

Bubble

LUPUS

OPHIUCHUS

Galactic center

South

“Coalsack”

TAURUS

CHAMAELEON

46 ASTRONOMY • APRIL 2018

We expect serious, life-threatening events
every couple of hundred million years or
so, on average.
We know there have been mass extinc-
tions and sudden changes in Earth’s cli-
mate. But because these events left few
clues behind, for a time we could only
speculate about any connection between
events on Earth and those in space.
Supernovae, the explosions of stars,
have been the main focus. A really nearby
event — 30 light-years away or closer —
would induce a mass extinction from radi-
ation destroying the ozone layer, allowing
lots of ultraviolet radiation through to
damage life on the surface. It has probably
happened a few times, based only on the
rate of supernovae, but we don’t have any
direct evidence. Somewhat more distant
supernovae go off more often, but scientists
assume the effects on Earth would be simi-
lar, though weaker.
In the past year, everything has
changed. Researchers like Brian D. Fields
at the University of Illinois at Urbana-
Champaign and his colleagues have
already predicted that a supernova near
enough to affect Earth would dump some
radioactive residue here that we might
detect in ocean sediments. There had pre-
viously been some claimed detections, but
many doubted them because the work is
literally at the level of counting atoms! I
wrote about more detections in Nature in

2016. These were data from the ocean bot-
      toms in a variety of locations around the
      world. That work was followed by others
      who reported data from the remnants of
      fossilized bacteria, from the Moon, even
      from cosmic rays, all finally creating a con-
      sistent picture.
      That picture is based on detection of

iron-60, an isotope. The dominant, stable

form of iron is iron-56, whose nucleus con-

tains 26 protons and 30 neutrons. Iron-60

has four extra neutrons. It is radioactive

and decays with a half-life of 2.6 million

years. Because our planet is 4.5 billion

years old, no original iron-60 should be

left on Earth, unless it came from space.

Iron-60 is created in supernovae, so

identifying times of great increase in the

isotope is a good sign that supernovae have

gone off not too far from Earth. On that,

everyone agrees. Researchers can date the

age of the event from the age of the sedi-

ments in which iron-60 is found. All recent

publications agree that something hap-

pened 2.5 million to 2.6 million years ago

at a distance between 150 and 300 light-

years from Earth.

Beyond that, there is no consensus.

There are indications of another event

7 million or 8 million years ago, also near

enough to deposit iron-60 on Earth. In

space, the iron would pass Earth as part of

the blast wave and be deposited for only a

short time. But such events are spread out

in time. One interpretation suggests that

the dust grains containing iron-60 were

caught up in interstellar clouds, which con-

fined them or modified their trajectory,

keeping them in our neighborhood so they

could fall to Earth more than once.

Another idea is that a lot of supernovae

occurred at various distances, as many as a

dozen or more. This concept would explain

the extended deposits on Earth and also

sounds reasonable, considering the

The years of decay

The Local Bubble

Iron-60 is a radioactive element that should no longer exist on Earth. It has a half-life of

2.6 million years, after which the part that has become cobalt-60 quickly turns into nickel-60,

a stable element. Because researchers find iron-60, they speculate that it arrived recently,

blasted into space (and onto Earth) by supernovae. ASTRONOMY: ROEN KELLY

The Local Bubble is a void in the interstellar medium where the density of hydrogen atoms

is only one-tenth that of the rest of the Milky Way. What gas remains is hot and emits X-rays.

Astronomers believe nearby supernovae in the past 20 million years created the bubble and

excited its remaining gas. ASTRONOMY: ROEN KELLY