Astronomy

Supernova

Radiation

breaks nitrogen

molecule apart

Supernova

emits radiation

Nitrogen atoms combine with oxygen

Nitrous oxide

combines with

free oxygen atom

Nitric oxide

combines with ozone (O 3 )

Converts into oxygen molecule (O 2 ),

nitrogen atom, and two oxygen atoms

=

=

=

1

2

3

Nitrous oxide is formed

Nitric oxide is formed

energetics needed to form the Local Bubble.
The Local Bubble is an irregularly
shaped region of hot (million-degree) but
tenuous gas (plasma) in which our solar
system and many other stars reside. A
chain of supernova explosions may have
formed it, perhaps the same ones that
deposited the iron-60.
It may sound unreasonable to have so
many supernovae all going off in the same
area at nearly the same time, but it isn’t.
The massive stars that make type II super-
novae are often born in associations, and
therefore clump together. The Orion
Nebula (M42), a favorite of amateur astron-
omers, is a large example of this.
The stars that make the powerful super-
novae have fairly short lives, so a group
that is born together with the same starting
mass will tend to explode more or less
together. Astronomers estimate that the
stars that dump the iron-60 each contained
about 10 times the mass of the Sun, and
should live only a relatively short 30 mil-
lion years. This line of thinking demon-
strates that the idea of a chain of explosions
is reasonable, but by no means proven.
For the first time, instead of general
expectations, we have a definite event to
discuss. It was not close enough (30 light-
years) to generate a mass extinction but
close enough to affect Earth. Compare
this with historical supernovae thousands
of light-years away — the ones with writ-
ten records, allowing us to find their rem-
nants in the sky from the descriptions of
their locations. For the events that
dumped iron-60, we don’t have such infor-
mation. Although there is a lot of uncer-
tainty about how many supernovae
occurred in this series, the last one clearly
happened about 2.5 million years ago, at a
distance of 150 to 300 light-years. It gives
us something to work with. My group has
been working out what kind of effects we
should expect.
Interestingly, Charles Sheffield wrote a
pair of science fiction novels, Aftermath
(19 98) a nd Starfire (2000), in which he por-
trayed a nearby supernova with surpris-
ingly accurate descriptions of many of the
effects that our group has calculated. Later,
when mineral evidence was found for the
dinosaur-killing asteroid or comet,
researchers also had been looking for evi-
dence of a nearby supernova. So all of this
is not new; the issue has been considered
at least since 1950. Still, what we have
found recently surprised us because the
important effects turned out to be different
from the ones usually discussed.

Hazard lights

First, we looked at the effects of blue light

generated by the supernova. It sounds silly,

but insomnia would be a hazard if the

event were visible on Earth’s night side. It

turns out that the blue wavelengths of light

are not at all healthy for sleeping creatures.

(Get rid of any blue LED alarm clocks!)

Both the intensity and color of such an

object in the night sky would be detrimen-

tal to sleeping animals, but only for a few

weeks at most.

A more commonly discussed hazard is

ozone depletion in Earth’s atmosphere,

resulting in a big increase in ultraviolet

light at ground level. This is a side effect of

radiation breaking up nitrogen gas (N 2 ) in

our atmosphere. The chemical bond is so

strong that life on Earth has generally lived

with a nitrogen shortage. (It can’t be used

unless atomic nitrogen [N] is freed from the

molecule.) Most radiation hazards break up

the nitrogen in the stratosphere, after which

the freed nitrogen makes compounds with

oxygen, thereby destroying ozone (O 3 ),

which is converted to ordinary oxygen (O 2 ).

Ozone in the stratosphere blocks the

part of the ultraviolet spectrum called

UVB, whose wavelengths are between 380

and 420 nanometers. UVB can cause severe

burning of the skin. It gets absorbed by

protein and, most importantly, DNA — the

localized burst of energy breaks chemical

bonds and can lead to cancer and muta-

tion. For many decades, the disaster sce-

narios of the effects of nearby supernovae

have hinged on this effect. It turned out not

to matter in this case. To explain why, we

have to talk about cosmic rays.

Cosmic rays

from supernovae

Cosmic rays are protons or atomic nuclei

that have been accelerated to high energies.

They are different from cosmic neutrinos

and gravitational waves, which have next to

no effect on us, and electromagnetic radia-

tion (photons). Electromagnetic radiation

includes radio waves, microwaves, visible

light, X-rays, and gamma radiation. We get

a lot of energy, more than a kilowatt per

square meter at the top of our atmosphere,

as sunlight. Electromagnetic energy forms

the basis of our understanding of the uni-

verse. Cosmic rays do, too. And a large

amount of research focuses on them.

About 90 percent of cosmic rays are

protons, the nucleus of the hydrogen atom.

That makes sense because hydrogen is the

most abundant element in the universe.

But the other 10 percent of cosmic rays are

Bye-bye, ozone

A supernova can deplete ozone in Earth’s

upper atmosphere. A series of chemical

reactions break up ozone into oxygen

molecules and oxygen atoms.

ASTRONOMY

: ROEN KELLY