Sky & Telescope - February 2023 - 57

Going Deep by Bob King
The Lure of Extragalactic Supernovae
By knowing where and how to look, you can spot an exploding star in your scope.
W
hy sit around waiting for a
galactic supernova when you can
observe an extragalactic one almost any
time? And not just one. A half dozen or
more become bright enough to view in a
10-inch scope most years.
French astronomer Ludovic Gully
discovered the first supernova ever seen
outside the Milky Way by accident during
a public observing event on August
17, 1885. He dismissed it as scattered
moonlight and didn't follow up. Three
nights later, German astronomer Ernst
Hartwig was showing some friends the
Andromeda Nebula (as the galaxy was
then called) through the 9-inch refractor
at Dorpat Observatory in Estonia,
when he noticed a " new " star near the
galaxy's nucleus. Hartwig wanted to
alert other astronomers of his discovery,
but observatory director Ludwig
Schwarz insisted he wait to re-observe it
without interference from moonlight to
verify the object wasn't an artifact.
No doubt chomping at the bit,
Hartwig nevertheless sent a letter
reporting the discovery, but it went
missing. He finally broke the news by
telegram on August 31st and was credited
with the supernova's discovery.
Supernova SN 1885A was just 16″
from the galaxy's nucleus and peaked
at magnitude 5.9 on August 21st. Many
described the star as yellow or gold
in color. It faded to magnitude 16 by
February 1886 and to this day remains
the only supernova observed in the
Andromeda Galaxy.
Extragalactic supernova discoveries
slowly trickled in following the 1885
blast, with new finds ranging between
one and 15 per year through the 1940s.
In the 1950s the pace quickened, and
these days astronomers can't haul them
in fast enough. In 2021 alone, the catch
came to 2,294!
What's in a Name?
To keep track of all these discoveries,
each new object is given the prefix " SN "
followed by the year of discovery and
a one- to four-letter suffix. The first
26 supernovae in a year are designated
with a capital letter from A to Z. Pairs
of lowercase letters are used for additional
discoveries: aa, bb, and so on
up to zz. In recent years, the number
of supernova discoveries has (ahem)
exploded. Now it's common to see lowercase
trios: aaa, bbb, and even quads.
As of mid-October 2022, we've already
reached SN 2022xuq!
David Bishop's Latest Supernovae
pLIGHT THE SPARK Supernovae are among the most powerful events in the universe. Type
Ia supernovae, such as SN 2018gv in NGC 2525 (marked with tickmarks above), tend to be the
brightest and can appear in all galaxy classes from ellipticals to spirals to irregulars. Type II are
primarily found in spiral galaxies and associated with their dusty spiral arms. Although generally
fainter, they remain visible longer.
website (https://is.gd/bishop_sn),
a clearing-house of supernova and
extragalactic nova discoveries, reported
21,086 supernovae in 2021. Of them,
2,294 were confirmed and received
official designations, with 18,790 stuck
in limbo as possible supernovae. The torrent
of new discoveries in recent years
has created a data overflow. The main
reason so many remain unconfirmed is
sk yand tele scope .o r g * FEBRUARY 2023 57
ESA / HUBBLE / NASA / A. RIESS / SHOES TEAM /
ACKNOWLEDGEMENT: MAHDI ZAMANI

https://www.is.gd/bishop_sn http://skyandtelescope.org

Sky & Telescope - February 2023

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