Sky & Telescope - December 2022 - 63

kilometers per second. This interpretation fails to consider that
when measuring the apparent motion of distant objects across
the sky, they aren't often moving perpendicular to our line of
sight. The light bouncing off dust clouds at various distances
has travelled farther than the light coming directly from the
supernova itself, but the dust clouds aren't located at the same
distance as the explosion - a simple trick of perspective that
produces the faster-than-light illusion. A similar phenomenon
is seen in the expanding light echo surrounding V838 Monoceros,
which underwent a nova outburst in 2002.
Other Applications
Astronomers have used this same imaging technique to
discover several light echoes in the LMC from previously
unknown ancient supernovae. It's even possible to acquire
spectra of these light echoes to study the type of supernova,
centuries after the direct light would have first reached Earth.
These discoveries were a byproduct of the SuperMACHO
Microlensing Survey, which looks for evidence of dark matter
using the same image-subtraction technique to detect transient
distortions in the light of individual stars.
Such light echoes are also helpful for measuring the structure
of the interstellar medium, because the dust and gas
in space are cold and dark. When a supernova explodes, its
expanding flash illuminates the surrounding clouds, providing
astronomers with a three-dimensional " scan " of otherwise
invisible structures between us and the dying star.
This subtraction technique can also be used in search of
light echoes from novae and supernovae in Northern Hemisphere
skies. While most are extremely faint and require
large instruments to record, photographing them may be an
interesting project for imagers with fast astrographs, sensitive
cameras, and datasets spread over multiple years. Who knows
what might turn up?
¢ ROLF WAHL OLSEN images the night sky from his backyard
observatory in the outskirts of Auckland, New Zealand.
A
SN 1987A
3 light-years
C
Apparent motion
B
First light
arrives
Reflected light
arriving
1 year later
SUPERLUMINAL ILLUSION The flash from SN 1987A is an expanding
sphere that illuminates interstellar dust clouds as it travels
outwards. Though it appears to move faster than the speed
of light, the light paths are illustrated here using a Pythagorean
triangle of 3, 4, and 5 light-years, respectively. The initial flash
(A) is observed at Earth coming along the direct line of sight (C).
When the flash is scattered by interstellar dust (B), some of that
light is reflected towards us along a slightly different line of sight.
The light path from point A to B is only one light-year longer
than the direct line of sight, but its proximity to us gives the false
appearance of having traveled 3 light-years in one year, or three
times the speed of light.
3
sk yand tele scope .o r g * DECEMBER 2022 63
4 light-years
5 light-years
https://skyandtelescope.org/
Sky & Telescope - December 2022

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