Sky & Telescope - April 2023 - 39

DENNIS DI CICCO & SEAN WALKER / MDW SKY SURVEY (2)
Rare Isotopes
Theoretical calculations indicate that novae also produce
rare isotopes of three of the most common elements in the
universe: carbon, nitrogen, and oxygen. To see how, we need
to look further at what happens in a nova.
When the accumulated gas on a white dwarf goes nuclear,
hydrogen fuses with itself to make helium. But this reaction
slowly raises the temperature so that the CNO cycle
eventually takes over, in which carbon, nitrogen, and oxygen
catalyze the hydrogen-to-helium reaction. Along the way this
sequence produces carbon-13, nitrogen-15, and oxygen-17.
Like beryllium-7, they survive the explosion, because it blasts
them into the deep freeze of space.
Scientists have long used some of these rare isotopes to
track the past. For example, living creatures prefer carbon-12
over carbon-13, so ancient rocks enriched in the lighter
isotope suggest that life once inhabited them even if it left
behind no fossils. Nitrogen-14 escapes from a planet more
easily than heavier nitrogen-15, so the ratio of those two isotopes
probes the atmospheric evolution of Earth and Mars.
The Nova Rate
Exactly how much nova nucleosynthesis contributes to the
galaxy depends on how many novae occur each year - a
number no one knows. " There's a lot of uncertainty in estimating
the nova rate for our galaxy, " says Allen Shafter (San
Diego State University), who has spent most of his career
trying to do just that.
Nowadays, observers usually find between 5 and 15 novae
in our galaxy each year. Most novae are distant and therefore
appear near the galactic plane. Since the Milky Way is so large
and dusty, many novae go unseen, which means the true rate
is greater.
Classical versus Recurrent Novae
Unlike supernovae, which normally mark the deaths of
stars, novae repeat. A classical nova is one in which the
interval between explosions is so long that astronomers
have seen only one eruption. In contrast, so-called recurrent
novae explode much more often. Examples include
RS Ophiuchi, T Pyxidis, and U Scorpii.
One way to estimate that rate is to use known novae and
then extrapolate to include those too far to see. In 2022
Adam Kawash (Michigan State University) and his colleagues
used the Gaia spacecraft and the All-Sky Automated Survey
for Supernovae to deduce an annual nova rate of about 26.
A second way to estimate the nova rate is to observe
another galaxy, where we can presumably see all novae, and
extrapolate its rate to the Milky Way. In 2022 Travis Rector
(University of Alaska, Anchorage), Shafter, and their colleagues
used more than two decades' worth of observations to
find that about 40 novae appear each year in the Andromeda
Galaxy. Andromeda resembles the Milky Way but probably
emits more light. If the Milky Way's luminosity is 70% that
of Andromeda - which is only an estimate, since we can't see
the Milky Way from the outside - then our galaxy's nova rate
should be around 28 a year.
In 2021 Kishalay De (now MIT) and his colleagues used
infrared observations of the Milky Way that penetrate
interstellar dust. From the number of novae these scientists
detected, they estimated a somewhat higher nova rate,
around 44 a year.
Shafter currently favors an annual nova rate around 30.
That's close to the number of 25 that Starrfield's team had
NOVA CASSIOPEIAE 2021 Left: Discovered on March 18, 2021, this nova (orange star at center) rose to fifth magnitude before it faded. It's around
magnitude 8 here, on March 21st of that year. Right: In an image taken in October 2022, the system looks unremarkable.
sk yand tele scope .o r g * APRIL 2023 39

https://skyandtelescope.org/

Sky & Telescope - April 2023

Table of Contents for the Digital Edition of Sky & Telescope - April 2023