Australian Sky & Telescope - 04.2019

40 AUSTRALIAN SKY & TELESCOPE April 2019

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MESSIER’S MARVEL by S. N. Johnson-Roehr

POSS-II / STSCI / PALOMAR OBSERVATORY / CALTECH

WCharles Messier detected the open cluster

M93 on the night of March 20, 1876. The field

of view of this image is 30 arcminutes. Messier

estimated the cluster’s width at 8 arcminutes

while William Herschel pegged it as almost

twice as wide.

T HE THIRD WEEK OF MARCH 1781

was particularly busy for Charles

Messier, the famed French comet

hunter. Prompted by the discovery of a

faint object in Virgo by his interlocutor

Pierre Méchain, Messier spent the night

of March 18 observing eight new deep

sky objects in what we now know as

the Virgo Cluster. (The number bumps

to nine if you include the lenticular

galaxy detected by Méchain on March

4 but drops to six if you discount both

Méchain’s discovery and a galaxy

previously logged by Johann Elert

Bode.) The temperatures in Paris were

slightly warmer than typical that week,

so we can imagine Messier was feeling

comfortable and possibly excited by the

potential for further discoveries as he

settled in with his 100-mm refractor at

the Naval Observatory on Paris’s Left

Bank on March 20.

Messier discovered only a single

object that night, a modest open star

cluster. He duly entered the cluster into

the fourth edition of his ‘Catalogue of

Nebulae and Star Clusters’ as No. 93,

laconically describing it as “a cluster of

small stars, without nebulosity, between

the Greater Dog [Canis Major] and

the prow of the ship [Puppis, once part

of Argo Navis].” He estimated it to be

about 8′ (arcminutes) across.

Was Messier disappointed that he

logged just one deep sky object on

March 20? It must have been a bit

anticlimactic after the long list he’d

written two nights earlier. As it turned

out, M93 was one of his last original

discoveries (excluding comets), and the

fourth edition of his catalogue, which

was published in the Connoissance des

Temps for 1784, would be the final

version. Since he didn’t include his

feelings along with the positional data

for the modest open cluster, we can only

guess as to his mindset, but I suspect

he was relieved to have the majority of

what he considered nuisance objects

catalogued so he could get back to the

real task at hand... hunting comets.

Nothing in Messier’s commentary

suggests that M93 offered anything

particularly special to the observer,

but words can be deceptive. In fact,

the open cluster boasts a lengthy

observational history, partly because

it’s visually interesting and partly

because it’s an easy target, hovering

at the edge of naked-eye visibility.

Even the most modest scope can pick

up the 6th-magnitude cluster, and if

you’re well away from light pollution,

you should be able to spot it without

optical aid 1½° northwest of the yellow

supergiant Xi (ξ) Puppis.

As astronomers following Messier

soon noted, M93 looks good through the

eyepiece. Caroline Herschel made the

next recorded observation of the cluster

after Messier’s, sweeping it up with her

small scope on February 26, 1783. She

invited her brother, William, to examine

it under more magnification, and they

found it consisted of perhaps 100 or 150

stars that were “very beautiful, nothing

nebulous among them”. William

revisited M93 in November 1784 and

Celebrate autumn by recreating Messier’s historic discovery

of a striking star cluster.

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Starfish stragglers