Sky & Telescope - February 2019 - 31

CR A B NEBUL A: A DA M BLOCK / M OUN T LEM M ON SK YCEN TER / UNIV ERSIT Y OF A RIZON A;
ROSSE'S SK E TCH: PHILOSOPHICAL TR ANSACTIONS OF THE R OYAL SOCIE T Y OF LOND ON
(1844) / PUBLIC DO M AIN; MITCHELL'S SK E TCH: INSTIT U TE OF ASTRONO M Y LIBR A RY /
UNIV ERSIT Y OF CA MBRIDG E / CC BY 4.0

he dying star had about 10 times the
mass of our Sun and had just lost its
ability to fuse elements in its core.
Without the outward pressure of fusion
to hold back its own crushing gravity, the
inner core collapsed within seconds into a
20-kilometer-wide sphere of neutrons. The
outer core of the star immediately crashed
onto this abruptly smaller and superdense
core at 23% the speed of light. A titanic
shock wave explosively rebounded off the
star's neutron core, completely disrupting
its outer layers in a star-shattering Type
II supernova. The intensely sudden and
cataclysmic death of the star was brieﬂy
brighter than the entire Milky Way galaxy.
Around 6,500 years later Chinese
astronomers saw a brilliant "guest star" in
the pre-dawn sky near Zeta (ζ) Tauri on
July 4, 1054. For 23 days it was visible in
broad daylight and at night it had a reddishwhite color. It took more than a year and a
half for it to gradually fade from sight.
Some 700 years after that, the stillexpanding debris nebula of the supernova
inspired Charles Messier to start his
catalog of things that looked like comets but didn't move relative to the stars.
Discovered 27 years earlier by John Bevis, Messier independently found the nebula while searching for Comet De la Nux
(C/1758 K1). By 1781 Messier's list included 102 more objects,
which was extended to 110 by various modern authors based
on notes he and his collaborator, Pierre Méchain, left behind.
The nebula was then given a nickname that didn't ﬁt its
actual appearance, and was used as a damning example of all
that was wrong with 19th-century nebular astronomy. In the
20th century, scientists gradually pieced together its incredible nature, a process that continues today. Now I'm here to
convey why the remains of that shattered star are so important, and to show what's possible to see of it with a large
amateur telescope in the early 21st century.

The Nickname
In 1844 Lord Rosse made a sketch of Messier 1 that inspired
its nickname, the Crab Nebula. The sketch looks more like a

t EARLY SKETCHES (Top) Lord Rosse's 1836
disavowed sketch of the Crab Nebula as seen
through his 36-inch scope. This sketch inspired
the Crab Nebula nickname. As per Lord Rosse's
notes in the original publication, south is up.
(Bottom) R. J. Mitchell's 1855 sketch of the Crab
Nebula using Lord Rosse's 72-inch telescope.

pineapple - or a tadpole - than a crab,
and has the additional charm of looking
nothing like the actual object. Lord Rosse
even repudiated this sketch, but the Crab
Nebula name stuck anyway.
The deﬁnitive view of M1 through Lord
Rosse's 72-inch telescope was sketched
by R. J. Mitchell in 1855. Although it no
longer looks like a pineapple, it still looks
different than the object we see today.
But that may be more a matter of style;
except for the ﬁn, the overall shape is
pretty close to an unﬁ ltered view in a
large amateur telescope today. On the
other hand, a somewhat wispier ﬁn can
be seen in the title image on page 30.
It's also possible the ﬁn is just less
noticeable today. After all, the Crab has
had 163 years to expand since this sketch
- nearly 17% of its existence. Or perhaps
a 72-inch speculum metal mirror just produces a different
view than aluminized glass mirrors and CCD cameras.

Subjective Science
In 1876 the German astronomer Wilhelm Tempel used a
collection of Crab Nebula drawings made by some of the
most well-regarded astronomers of the day - including
himself - to show how useless these drawings were scientiﬁcally. It was a point well made, because each sketch looked
like it was of a different object.
Tempel was emphasizing the need for a standardized
method of sketching deep-sky objects, but also showing the
futility of using subjective visual observations; they weren't
accurate enough for scientiﬁc use.
There was no hope of being able to quantify objects like
the Crab Nebula with visual observations, because even
the best-trained astronomers of the 19th century were still

orn of Taurus
The Crab Nebula is amazing no matter how you look at it.

sk yandtele scope.com * FE B RUA RY 2 019

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Sky & Telescope - February 2019

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