Astronomy

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have not yet been attempted.
However, the size of the exoat-
mosphere of other hot Jupiter
planets like HD 209458 b (the
first transiting planet ever dis-
covered) can be 10 times larger
than the already inf lated atmo-
sphere. HAT-P-67 b’s star is
evolving into a red giant, so it
is brighter than it was in the
past. Its increasing radiation
will cause more heating and
more expansion of the planet’s
atmosphere in the future.
Leslie Hebb
Assistant Professor, Department of
Physics, Hobart and William Smith
Colleges, Geneva, New York

Q: HOW FAR FROM EARTH
WOULD A SPACE CAMERA
HAVE TO BE TO OBTAIN A
FULL IMAGE OF OUR OWN
MILKY WAY GALAXY?
Alan B. Thomas
Warrington, England

A: The specific answer to this
question depends on the type
of camera you’re talking about.
Objects appear smaller with
greater distance, so a camera
with a smaller field of view
(that can image less of the
space in front of it in one shot)
must be farther from an object
than a camera with a large
field of view to take an image.
So, let’s assume a few things:
First, let’s consider a camera
like JunoCam on the Juno
orbiter currently circling
Jupiter. JunoCam has a pretty
large field of view for a space-
craft camera: 58° across. (For
comparison, a 35mm lens on
a DSLR has a field of view of
63°.) So, to capture the Milky
Way in one shot, JunoCam
would have to be at a distance
such that the entire disk of our
galaxy takes up 58° (or less).
Second, let’s assume the visible
disk of the Milky Way, which
is a spiral galaxy, is 100,000
light-years in diameter; this is
a pretty good estimate, though

it’s continually being refined

based on new, more accurate

measurements. Third, while

we know that the solar system

is nowhere near the center of

our galaxy, and is instead

about 26,000 light-years from

the center, let’s assume we’re

sending JunoCam straight up

from the center of the galaxy,

for simplicity.

Calculating an object’s

angular size — how large it

appears on the sky — is pretty

straightforward for small

angles, but gets a little more

complicated as you approach

larger ones (up to sizes of 180°).

Still, it’s possible, and regard-

less of the formula, the angular

size (here, we’re assuming 58°

to fit in JunoCam’s field of

view) depends on the object’s

physical size (in this case,

100,000 light-years) and the

distance to the object (what we

want to know). It turns out that

a spacecraft carrying JunoCam

would have to be launched to a

position about 90,000 light-

years above the plane of the

Milky Way to be able to look

back down and just barely fit

the entire disk of the galaxy in

its field of view.

Alison Klesman

Associate Editor

Q: DO CONSTELLATIONS

LOOK THE SAME FROM

THE OTHER PLANETS

IN THE SOLAR SYSTEM?

Ronald Hellman

New York, New York

A: When you look at the sky,

you’re seeing a two-dimensional

projection of three-dimensional

space — the stars are spread

out in all directions, including

distance, but we don’t get that

distance information when we

look up (or when we take a

photograph). The constella-

tions are simply specific pat-

terns picked out on the sky;

they don’t take distance into

account. Some stars in a con-

stellation may be close, while

others are not. For example,

Sirius (Alpha [α] Canis

Majoris) is 8.6 light-years away,

but Aludra (Eta [η] CMa) is

about 2,000 light-years distant.

Though these two stars are in

the same constellation and

appear near each other on the

sky, in reality they are thou-

sands of light-years apart.

If you were to travel far

enough away from Earth (we’re

talking light-years), the pat-

terns would certainly start to

change. But because the stars

are so distant compared with

the size of our solar system

(which is just light-hours

across), the projection effect

— the patterns of stars we see

— from Earth holds true on

the other planets circling the

Sun. From any planet in the

solar system, the same constel-

lations we see here on Earth

are visible and recognizable.

The biggest difference you’d

notice is their orientation

in the sky (whether they’re

high or low compared to the

horizon and zenith), which

depends on the orientation of

a planet’s poles, as well as your

location on the planet. Polaris

(Alpha [α] Ursae Minoris) is

the North Star when viewed

from Earth because of the tilt

of its poles, but this is not true

from the other planets.

Alison Klesman

Associate 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.

On March 11, 2004, NASA’s Mars Exploration Rover Spirit snapped an

eight-second exposure to test its abilities to study the night sky from

the martian surface. A portion of the stars in Orion the Hunter, including

the three belt stars and bright Betelgeuse, are visible in their familiar

configuration at the bottom and right of the image. NASA/JPL/CORNELL