Astronomy

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— means they can see only the
brightest deep-sky objects.
The Andromeda Galaxy
(M31) is the farthest object to
be (barely) imaged; the Orion
Nebula (M42) and the Magel-
lanic Clouds are among the few
visible deep-sky-objects. A
pan-sky image would require
about 150 images — over an
hour — and one or two Mars
days’ worth of the rover’s avail-
able bandwidth. So, targeted
imaging always wins. In the
future, perhaps a wide-angle or
zoom camera will be able to
accomplish a pan-sky image.
Mark Lemmon
Texas A&M University
College Station

Q: SINCE THE DISCOVERY
OF THE “PILLARS OF CRE-
ATI O N ,” WHAT CHAN G E S
HAVE OCCURRED WITHIN
THEM AND THE SURROUND-
ING AREA?
Bobbie Hibbert
Ashton, Idaho

A: The Hubble Space Tele-
scope’s iconic image of the
Eagle Nebula’s “Pillars of
Creation” heralded the instru-
ment’s rebirth in 1995 and
showed the public just how
incredible astrophotography
could be above Earth’s atmo-
sphere. “No one imagined a
reaction that would turn the
image into a cultural icon,”
Astronomy Contributing

Editor Jeff Hester — who actu-

ally took the photo — said in

April’s Hubble commemorative

issue. The image has graced

everything from U.S. postage

stamps to classroom posters.

And yet the Pillars of Cre-

ation might not even exist any-

more. NASA captured this

stellar nursery at a f leeting

moment. The region is packed

with gas and young stars. And

as those stars ignite, the gases

evaporate into space, as seen in

the green streaks that surround

the columns. But because the

nebula sits some 7,000 light-

years from Earth, the light we

see left the nebula 7,000 years

ago, as agriculture spread across

Europe. Intense ultraviolet

radiation from young stars has

stripped much of the gas since

that time.

We’re already starting to see

the signs of that destruction.

NASA revisited the Eagle Neb-

ula last year and this time took

images in the near-infrared

part of the light spectrum as

well as the visible.

The view pierced the dust and

revealed the pillars’ tenuous

true nature as well as infant

stars hidden among the gas.

One prominent jet, perhaps

from another forming star, has

already increased its reach some

60 billion miles (97 billion kilo-

meters) farther into space. And

scientists suspect that a nearby

supernova, like the one thought

to have brought our solar sys-

tem radioactive elements, might

have already ablated what was

left of the gas. Spitzer Space

Telescope images show the

supernova’s shock wave was

racing at the pillars 6,000 years

ago. We should know for sure in

a couple thousand years.

Eric Betz

Associate Editor

Q: CAN YOU EXPLAIN

MERCURY’S RETROGRADE

MOTION? DO OTHER PLAN-

ETS APPEAR TO DO THIS

ALSO?

Robert Schneider

Merida, Mexico

A: When astronomers talk

about retrograde motion, gen-

erally they refer to the apparent

motion outer planets make as

Earth passes them while orbit-

ing the Sun. Most of the time,

all the outer planets appear to

move eastward through our

sky. During retrograde motion,

however, each appears to

reverse direction and head

westward. Note that this is an

apparent motion. The same

thing happens when you, in a

car, pass another car going in

the same direction. While you

are passing that car (and

because you are traveling

faster), it appears to move in

the opposite direction.

The two inner planets, Mer-

cury and Venus, don’t exhibit

retrograde motion for the same

reason because they move

faster than Earth. So, our

planet never passes either of

them. Some astronomers, how-

ever, define retrograde motion

as any westward motion by a

planet. For those that agree

with that definition, even the

inner planets retrograde as

they move farther from the

Sun (or the horizon) in the

eastern morning sky or

approach the Sun (horizon)

in the western evening sky.

Michael E. Bakich

Senior Editor

Send us your

questions

Send your astronomy

questions via email to

[email protected],

or write to Ask Astro,

P. O. Box 1612, Waukesha,

WI 53187. Be sure to tell us

your full name and where

you live. Unfortunately, we

cannot answer all questions

submitted.

By comparing a new image

of the Eagle Nebula’s “Pillars of

Creation” to the original, astrono-

mers are beginning to watch the

famous stellar nursery evaporate

before their eyes. NASA/ESA/HUBBLE HERI-

TAG E T E A M (S T SCI/AURA)/J. HESTER, P. SCOWEN (ASU)

1995

2014