Astronomy - USA (2022-02)

51

ORION

TAURUS

AURIGA Pleiades

10°

N

E

ε δ

ζ σ

ζ

119

UU

β

NGC

2281

60 ASTRONOMY • FEBRUARY 2022

BINOCULAR UNIVERSE

Usually at

this time

of ye a r,

many of us are

greeted with leaf-

less trees, barren

gardens, and snow-

laden landscapes.

But while much of

the daytime views

may be starved for

color, the night sky

is resplendent with

vibrant stars for

observers. I offered

some favorites back

in my January 2020

column, and this

month, we are back

in search of more

winter color.

But how is it that

astronomers mea-

sure a star’s color?

Citing it purely in words can be subjective. So, to quan-

tify a star’s color, researchers measure its

magnitude at three wavelengths: ultraviolet

(abbreviated UV), blue (B), and yellow

(V, for visual). The difference between the

measured B and V magnitudes, called the

B-V color index, is the best way to gauge a

star’s true color. The smaller the value, the

bluer the star. The bluest stars can range

down to −0.4, while the reddest exceed 2.0.

Our Sun, a yellow main sequence star, has a

B-V color index of 0.66.

To better appreciate star colors through

binoculars, try slightly defocusing the field.

Doing so diffuses starlight just enough for

our eyes to detect subtle colors that pass unnoticed

when in focus. I need no more than an eighth of a turn

to make a difference.

The three stars that make up Orion’s Belt are some

of the bluest stars in the winter sky. From west to east,

those are Mintaka (Delta [δ] Orionis), Alnilam

(Epsilon [ε] Orionis), and Alnitak (Zeta [ζ] Orionis).

With B-V color indices of –0.39, –0.18 and –0.21,

respectively, all three look like glittering sapphires

against a backdrop of celestial diamond dust. Mintaka,

at the northwest (upper right) end of the Belt, is

obviously the bluest. Compare all three to blue-white

Rigel (Beta [β] Orionis), which has a B-V value of –0.03.

As you marvel at Orion’s Belt, glance just 50'

southwest of Alnitak to Sigma (σ) Orionis. With a B-V

index of −0.21, Sigma is another one of winter’s bluest

stars visible with binoculars. As a bonus, if you have 12x

or higher power binoculars, look carefully and you may

notice that Sigma has a 7th-magnitude blue-white

companion star just 40" northeast. Sigma is a quintuple

star system through telescopes — and in fact the bright-

est object, Sigma Orionis AB, is actually three stars, but

it is impossible to split them with a scope. The compo-

nent visible through most binoculars is known as Sigma

Orionis E.

Let’s jump to the opposite end of the visible spectrum

and hunt for some orange and red stars. Aim at blue

Zeta (ζ) Tauri, the Bull’s southern horn, and glance

about half a field southwest. There, 4th-magnitude

119 Tauri smolders like an orange ember. You would

never know it from its modest brightness, but 119 Tauri

is a red supergiant like Betelgeuse. In fact, its B-V color

index of 2.08 makes it even redder than that well-known

star. But at a distance three times greater, 119 Tauri

receives little attention. Show it some love on the next

clear night.

With a B-V color index of 2.64, UU Aurigae is just

about winter’s reddest star visible through binoculars.

It varies between 5th and 7th magnitudes but is typi-

cally around 6th. You’ll find it about 7.5° east of

Theta (θ) Aurigae, along the pentagonal constellation’s

eastern side. Once there, compare its color

to that of 51 Aurigae, found one degree

north-northeast. Although they appear

similarly bright, 51 will appear as more yel-

low, compared to its ruddy neighbor.

Finally, glance about 5° north of UU Aur

to find a right triangle made up of three pale

yellow 5th-magnitude stars. Psi^2 (ψ^2 ) lies at

the right angle, Psi^4 (ψ^4 ) marks the triangle’s

northern vertex, and Psi^7 (ψ^7 ) t he

southeastern vertex. While surveying the

stars for this column early one morning last

fall, I unexpectedly noticed a small, fuzzy

patch of light adjacent to Psi^7. Glancing at my

chart, I discovered it was NGC 2281, an open cluster

containing about 30 stars. The sky was clear enough that

I was able to resolve a few of the cluster’s brightest mem-

bers against the faint glow through my 10x50s.

I’d enjoy hearing if you uncover other colorful stars

or any unexpected sights. Contact me through my

website, philharrington.net. Until next month, remem-

ber that two eyes are better than one.

Return to the winter skies for more colorful stars.

Celestial light show

Take a tour of winter’s

most colorful stars

this season. ASTRONOMY:

ROEN KELLY

All three

[stars] look

like glittering

sapphires

against a

backdrop

of celestial

diamond

dust.

BY PHIL

HARRINGTON

Phil is a longtime

contributor to

Astronomy and the

author of many books.

BROWSE THE “BINOCULAR UNIVERSE” ARCHIVE AT

http://www.Astronomy.com/Harrington