Sky & Telescope - USA (2020-01)

Big, Bold, Bright, Beautiful

64 JANUARY 2020 • SKY & TELESCOPE

While you’re in the neighborhood, check out Rigel,

Orion’s lower right kneecap. It’s big and bright — and fun to

watch twinkle. When it’s near the horizon, Rigel looks like

a police car fl ashing red and blue. Turbulent air acts like

dozens of prisms, splitting the starlight into its spec-

trum and fl ashing different wavelengths at you at

random. In a telescope you’d swear the mothership

was making a dive straight for you, lights ablaze.

When Rigel rises high enough to steady out,

crank up the power and look for its binary compan-

ion less than 10′′ away. The difference in magnitude

is pretty extreme (0.3 and 6.8) and their separation

is pretty tight, so the secondary will be just outside

the glare of the primary. But if the seeing is relatively

steady it’s usually there as a distinct separate star.

Up alongside Orion’s left side you’ll fi nd M78, one

of the brightest refl ection nebulae in the sky. M42 is an

emission nebula, its gas glowing like a neon light, but M78 is

simply refl ecting starlight. You can see the two stars that are

casting most of M78’s light; they’re off-center to the north

end of the nebula. That end is the brightest, and the rest of

the nebula trails away to the south in a cone shape that gives

it a cometary appearance. There’s a third, dimmer star near

the nebula’s southern edge. Stars are forming within this

nebula, too. Most are still hidden within the gas and dust,

but infrared studies have discovered nearly 200 young stars

in several still-developing clusters.

As long as you’re still in the neighbor-

hood, check out Barnard 33, the Horse-

head Nebula. Ha ha ha! Just checking

to see if you were paying attention. The

Horsehead Nebula is big and beauti-

ful, but it’s neither bold nor bright, and

unless you’ve got a 12-inch scope or

greater and dark, dark, really dark skies

— and a hydrogen-beta fi lter — it’s not

likely to show up.

Collinder 91, on the other hand,

stands out nicely about 9° to the east of

M78. Not far from the Rosette Nebula (which you should

also check out if you have dark skies), Collinder 91 is a fairly

bright open cluster that also has the neat added property of

having its stars arranged in a pattern that suggests something

astronomical: an analemma.

What’s an analemma? It’s the pattern the Sun makes in

the sky if you photograph it at the same time of day for an

entire year. The Sun climbs up and down with the seasons

due to Earth’s axial tilt, but it also lags left and right as the

Earth speeds up and slows down in its elliptical orbit. The

combined effect of these two factors makes a big bowling-

pin-shaped fi gure eight in the sky.

Collinder 91’s major stars are arranged in just such a pat-

tern. The narrow end of the analemma doesn’t quite close

the loop — it looks like the photographer still has a few weeks

left to go — but the rest of it makes a remarkably good fi gure

M78

Cr 91

Rosette

Nebula

Rigel

Betelgeuse

α

β

β β

ε δ

γ

γ

κ

λ

ζ

ORION

ERIDANUS

MONOCEROS

M42 Trapezium

6 h 5 h +10°

–10°

0 °

St

ar

m

ag

ni

tu

de

s

2

1

3

4

5

6

JE

RR

Y^ O

LTI

ON

(^4

)

pBY ORION’S SIDE M78 is

one of the brightest refl ection

nebulae in the sky. FOV=35′

tSMALL ANALEMMA Not

as complete as a proper solar

analemma, Collinder 91 nev-

ertheless forms a recognizable

fi gure 8 in the sky. FOV=75′

N

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