Sky & Telescope - April 2023 - 37

must admit, to start with, that the a priori
chances of our Sun's becoming an ordinary nova
once during its total life period are fairly high. "
Fortunately, we no longer have to fear
this disaster. You can thank Merle Walker,
who in 1954 used Mount Wilson's 100-inch
reflector to observe a star in Hercules that
had gone nova 20 years earlier. He found it
wasn't a single star like the Sun but instead
an eclipsing binary. The pair's orbital period
of just 4 hours and 39 minutes was the
shortest such period then known and meant
the two stars must be close together in order
to revolve this quickly.
So . . . maybe all novae come from close
binaries? Bingo! In the 1960s Robert Kraft
studied 10 old novae, including the one in
Hercules, and came to this very conclusion.
Today, we know that a classical nova
occurs when gas from a companion star
falls onto a white dwarf, a superdense star
that was once the core of a red giant (S&T:
Dec. 2022, p. 28). A white dwarf's surface
gravity is roughly 100,000 times greater
than Earth's. So as more and more gas piles
up, the extreme gravity compresses the gas,
which makes it hotter and hotter.
Eventually the bottom layer of the
accreted gas gets so hot it triggers nuclear
reactions, which fry the gas further. The
heat stimulates more nuclear activity, raising
the temperature still more, causing yet more
reactions, and a nuclear runaway results.
" Temperatures get very high very fast, "
says Sumner Starrfield (Arizona State
University), one of the scientists who in
the 1970s calculated which elements novae
should make. The temperature exceeds 100
million kelvin - far hotter than the Sun's
center. The ensuing explosion is so bright the
star reaches an absolute magnitude between
−5 and −10, corresponding to a visible luminosity
between 10,000 and 1 million times
that of the Sun. Both the white dwarf and its
partner survive, only to repeat the fireworks
10,000 to 100,000 years later.
From this theory, several deductions follow.
First, our solitary Sun won't explode -
even after becoming a white dwarf. Nor will
the nearest white dwarfs, such as Sirius B,
Procyon B, and 40 Eridani B, because they're
far from their partners. Still, if Sirius B did
explode tonight, it would outshine Venus
and might even rival the Moon, peaking
between apparent magnitudes −8 and −13.
Furthermore, novae don't come from
Naming Novae
A nova is named for the
genitive of its constellation
plus the year its
light reached Earth. A
great nova in Perseus in
1901 is therefore named
Nova Persei 1901. Novae
receive variable star
names as well; that one is
also called GK Persei. If
more than one nova had
erupted there that year,
the first to be recognized
would have been Nova
Persei 1901 No. 1, then
Nova Persei 1901 No. 2,
and so on.
Only two constellations
have sported as many as
five novae in our galaxy
during a single year. If
you know the sky and
the structure of the Milky
Way, you can probably
guess which constellations
they are. The first is
the home of the galactic
center and hosts hordes
of distant stars, Sagittarius.
The second is one
of its zodiacal neighbors,
Scorpius. In 2012 five
and possibly six novae
erupted in Sagittarius,
and in 2021 five appeared
in Scorpius.
But some constellations
have never had even one
recorded nova. Surprisingly,
one is right next to
Sagittarius in the zodiac:
Capricornus. And the
largest constellation of
all, Hydra, has yet to have
a recorded nova. Amusingly,
Ursa Major, the large
constellation best known
for the Big Dipper, has
never had a known nova,
either, whereas its much
smaller sidekick, Ursa
Minor, has eked out one.
spring chickens. It takes time for a star
to turn into a white dwarf and time for
the pair of stars to cozy up to each other.
Matteucci estimates that well over a billion
years must elapse before novae start going
off in a galaxy.
A Niche for Novae
But what good are novae? A supernova from
a massive star showers a plethora of life-giving
elements into space. In fact, with every
breath you take, you inhale oxygen forged
in 160 million different massive stars that
went supernova, according to Matteucci
and Donatella Romano (Italian National
Institute for Astrophysics, Bologna). Just
one massive star can launch a full solar
mass of new oxygen into the galaxy.
Pretty hard for a mere nova to compete
with that! The typical nova ejects only
0.01% to 0.001% solar mass of material
into space. It's like a 10-year-old setting up
a lemonade stand to challenge a giant corporation
that sells the stuff by the ton.
So how could a nova make its mark on
the cosmos? Why, create an element that no
other star makes.
The most promising element turns out
to be lithium (atomic number 3), a rare and
increasingly important metal here on Earth.
But lithium's story in novae actually starts
with another element, beryllium (atomic
number 4), which is even rarer in space
than platinum. Stars don't make beryllium;
that's why the element is so rare on Earth.
Its one stable isotope, beryllium-9, arises
between the stars, when cosmic rays smack
into interstellar atoms, splitting either
them or themselves.
But a nova explosion drives helium-3
into helium-4, forging unstable beryllium-7.
Normally the nova's high temperature
would destroy the element, but here, there's
a way to rescue it instead: " Get it out of
there - get it to low temperatures as fast as
possible, " says Starrfield. The nova's blast
spirits the beryllium-7 away to safety. The
isotope is radioactive, with a half-life of 53
days, and decays into lithium-7, the isotope
that accounts for 92.4% of Earth's supply.
Lithium is rare, but it's 90 times more
common in the cosmos than beryllium.
Perhaps 10% of terrestrial lithium came
from the aftermath of the Big Bang (S&T:
sk yand tele scope .o r g * APRIL 2023 37
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