Astronomy - USA (2020-08)

64 ASTRONOMY • AUGUST 2020

SECRET SKY

The deep sky is

filled with tele-

scopic trea-

sures. And like Russian

dolls, some hide addi-

tional bounty within; the

Trapezium star cluster in

the Orion Nebula (M42)

is a classic example. This

month we’ll plunge into

the heart of another —

the Ring Nebula (M57)

in Lyra the Harp, one of

the sky’s most famous

planetary nebulae — and

try to visually snatch a

stellar pearl from the surrounding mist.

Aside from sky conditions, one’s ability to

snag the central star requires seeing to at

least magnitude 15 through your telescope.

To find out how faint a star you can see, try

using the American Association of Variable

Star Observers’ star chart for HS Sagittae

(bottom right) before heading for the Ring.

This star lies about 17° southeast of M57 in

the tiny constellation Sagitta the Arrow.

Comparing the eyepiece view with this chart

will revea l your scope’s limiting magnitude.

Seeing faint is only part of the equation, however.

What’s paramount to success is your ability to over-

come yet another obstacle: the feeble “mist” filling the

Ring’s annulus. When seen at low magnification, the

mist appears bright, thus lowering the contrast between

the central star and its background. In 1988, several

observers at the 18-inch f/14 Clark refractor at Wilder

Observatory in Amherst, Massachusetts, couldn’t

detect the Ring’s central star when viewed at 250x in a

wide field of view because M57’s “hole” was filled with

light. After they changed the magnification to 1,000x,

however, the Ring filled the field of view and everyone

saw the central star.

In August 1995, at Lick Observatory on Mount

Hamilton in California, the Ring’s inner edge rimmed

the small field of view of the 36-inch f/19 Clark refractor

at 1,176x. The sight of its dark well revealed the central

star and another field star shining brightly against a

dark, high-contrast background. An immediate walk

down the hall to Lick’s 40-inch Nickel ref lecting tele-

scope and a glance at the Ring through it at a much

lower power showed a magnificently bright ring filled

with a luminous mist. The central star, however, was

invisible despite the increase in aperture.

The smallest telescope through which I’ve seen the

Ring’s central star is the 9-inch f/12 Clark refractor at

Harvard College Observatory in 1976, and the story was

no different. Seeing the star required magnifications

between 600x and 700x with the Ring nearly filling the

field of view. Much time and patience behind the eyepiece

were also required — two other important factors needed

to achieve success when using smaller apertures.

So, a search for the Ring’s central star has at least three

principal components: 1) the ability to see stars to roughly

magnitude 15; 2) a telescope that can handle magnifica-

tions of 600x or more; and 3) atmospheric conditions that

allow such magnifications to be employed. In addition, I

find that a limited field of view (of only a few arcminutes)

helps by removing peripheral distractions. Long-focal-

length instruments with orthoscopic or Ramsden-type

eyepieces and a Barlow lens to increase the magnification

will do the job nicely.

A final consideration is the eye’s sensitivity to color.

Observers with blue-sensitive eyes have a bet-

ter chance of seeing the Ring’s hot central star

than those with red-sensitive eyes. Young

stargazers — and people who have had cata-

racts removed — have a much better chance

at achieving success than more senior observ-

ers whose eye lenses have begun to yellow.

As always, send your thoughts and obser-

vations to [email protected]. I’m always

open to suggestions for other topics as well.

A faint treasure lies deep within the Ring.

Pearl in the mist

The Ring Nebula

(M57) in Lyra is one

of the great sights

through amateur

telescopes during the

Northern Hemisphere

summer. This Hubble

Space Telescope

image reveals the

central star nicely.

NASA/ESA/THE HUBBLE HERITAGE

(STScI/AURA)–ESA/HUBBLE

COLLABORATION

Seeing faint

is only part

of the

equation,

however.

BY STEPHEN

JAMES O’MEARA

Stephen is a globe-

trotting observer who

is always looking

for the next great

celestial event.

BROWSE THE “SECRET SKY” ARCHIVE AT

http://www.Astronomy.com/OMeara

This 30'-wide telescopic field of view centered on HS Sagittae

shows stars to magnitude 15.8. The bright stars at top right are

Beta (β, top) and Alpha (α) Sagittae, both of which glow at

magnitude 4.4. South is up and east is to the right. AAVSO