Astronomy - USA (2020-03)

M48

27

28 b

_

`

_

c MONOCEROS

CANIS

MINOR

Procyon

Gomeisa

N

E

5°

64 ASTRONOMY • MARCH 2020

BINOCULAR UNIVERSE

March is a month

of transition, here

on Earth as well as

in the night sky. As the Sun

moves northward along the

ecliptic, the days are getting

longer and the chill of win-

ter begins to abate.

As Orion marches

toward the western horizon

in pursuit of Taurus, his

attending small dog, Canis

Minor, lags behind. Canis

Minor is one of those con-

stellations that makes me

wonder how anyone could

imagine seeing the name-

sake figure among its stars.

After all, Canis Minor is drawn from only two: Procyon

[A lpha (α) Canis Minoris] and Gomeisa [Beta (β)

Canis Minoris]. A small dog from two stars? Maybe a

hot dog, but that’s the best I can do.

Nicknamed the Little Dog Star, Procyon ranks

eighth among the sky’s brightest. Its prominence is

primarily due to its location just 11.4 light-

years away. Through our binoculars, it gleams

pure white, a spectral class F5IV–V. That’s a

shorthand way of saying that Procyon is about

40 percent more massive and 10 percent hotter

than our Sun, and also that it is a star in transi-

tion. Procyon’s core is rapidly running out of

fusible hydrogen, causing it to leave the main

sequence and transition to the subgiant por-

tion of the Hertzsprung-Russell diagram.

Procyon is also orbited by a white dwarf

known as Procyon B. There is no hope of see-

ing it through our binoculars. Even through

the largest amateur telescopes, Procyon B is

a difficult challenge.

At third magnitude, Gomeisa appears

much le s s i mpre s sive t ha n Proc yon. I f you a re

observing from a heavily light-polluted area, you might

need binoculars just to see it. But the Small Dog’s Beta

star would rank “alpha” if placed side by side with

Procyon; its diminutive appearance is due to being 15

times farther away. Gomeisa is classified as spectral type

B8V, a massive blue stellar inferno located to the far

upper left of the HR diagram’s main sequence. Like our

Sun, it is happily fusing hydrogen into helium in its core.

Maintaining its energy output, however, comes with a

cost: Gomeisa’s life span may be only 10 million years

versus our more judicious Sun’s projected 10 billion.

While the small dog leaves something to be desired

visually, its breed is clear. It’s a pointer. How do I know?

Because a line extending southeast from Gomeisa

through Procyon points to an underappreciated open

cluster, M 48. Follow the pointer for about 11°, or about

two binocular fields, and you’ll come to an equilateral

triangle formed by Zeta (ζ), 27, and 28 Monocerotis.

Look half a field south-southeast of Zeta to see a fuzzy

patch of light. That’s our target.

For nearly two centuries, M48 was considered “lost.”

History shows that Charles Messier discovered M48 on

February 19, 1771. After describing it in his log as a

“cluster of very [faint] stars, without nebulosity; this

cluster is at a short distance from the three stars that form

the beginning of the Unicorn’s tail [Zeta, 27, and 28

Monocerotis],” he noted its position and moved on. From

this, he later calculated its celestial coordinates — and

goofed. Messier mistakenly placed his discovery 5° north

of its actual location. Before it was officially found again,

it was independently discovered by others and awarded

the designation NGC 2548. The “Case of the Missing

Cluster” was finally cracked in 1959 by Theodore F.

Morris, a member of the Royal Astronomical Society of

Canada, and confirmed a year later by Harvard

University astronomer Owen Gingerich. Morris noted

that NGC 2548 had the same right ascension as the

missing Messier object, but that its declination was off.

After examining records at Paris Observatory,

Gingerich confirmed this assertion, and M48

was “found.”

More than 80 stars populate M48, with

some studies including more than 320 stars

after factoring in outliers. Based on the types

of stars found within, astronomers estimate

M48’s age at 300 million years. Collectively,

they lie about 2,500 light-years away.

M48 is a splendid target for binoculars. The

cluster’s densest portion covers about half a

degree, but by including stragglers, the full

span nearly doubles in size. Several 8th- and

9th-magnitude cluster stars immediately shine

through the mist created by fainter suns. My

10 x 50 s re ve a l a c ent r a l i z ed k not of st a rs , wh i le

my 16x70s add short threads of fainter points.

Look carefully, and you might also notice that a couple

of those stars display hints of pale yellow or orange.

Do you have a favorite binocular object that you

would like to share with the rest of us? Contact me

through my website, philharrington.net. Remember,

two eyes are better than one.

Canis Minor points to an array of unusual stars.

A month of change

Although it’s a small

constellation, Canis

Minor the Little Dog

contains interesting

stars, including its

brightest, Procyon.

ASTRONOMY: ROEN KELLY

Canis Minor is

one of those

constellations

that makes

me wonder

how anyone

could imagine

seeing the

namesake

figure among

its stars.

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