62 ASTRONOMY • DECEMBER 2015A: This is an excellent question
and one that is at the forefront
of cosmology research. The
short answer is that we do not
yet know.
Dark matter particles, being
responsive to Newton’s laws of
gravity, are certainly allowed to
clump just like the ordinary
matter that is made of protons,
neutrons, and other familiar
subatomic particles. Dark mat-
ter could, therefore, collapse
into objects of any size. In fact,
there is strong evidence today
for the existence of dark matter
“halos” — spherical clumps of
dark matter centered around
every galaxy that extend 10
times larger than the radius
spanned by the galaxy’s stars.
However, the existence of
“compact” dark matter objects,
such as stars or planets, is more
difficult to ascertain. Ordinary
matter has an advantage over
dark matter in forming stars or
planets because of its electro-
magnetic interactions, which
facilitate collapse into small,
compact structures.Astronomy’s experts from around the globe answer your cosmic questions.GATHERING
THE DARK
Q: WOULD THE THIN ATMO-
SPHERE AND ABSENCE OF
LIGHT POLLUTION ON MARS
MAKE FOR A FANTASTIC
NIGHT SKY, OR WOULD
MARTIAN DUST/TWILIGHT
SPOIL THE VIEW? WHY
DON’T WE HAVE BROAD
NIGHT-SKY PICTURES?
Dustin Cable
Menlo Park, CaliforniaA: Martian astronomy has
some advantages. The surface
pressure is near 8 millibars
— less than 1 percent of
Earth’s surface pressure —
meaning that atmospheric
refraction is insignificant. One
effect of that is the stars would
not twinkle. Another is truly
dark night skies. Twilight is
sometimes very long, such that
astronomical imaging needs to
be at least two hours away fromsunset or sunrise. But the
brightest lights on Mars are
Curiosity’s Mars Hand Lens
Imager’s LED lights and Chem-
Cam laser. And in the skies,
the moons are much smaller
and fainter than ours.
But the dust is a problem,
even in the darkest skies. Typi-
cally, stars near the zenith lose
nearly 1 magnitude due to
dust; that increases to 3 to 4
magnitudes at 15° altitude.
The rover cameras have sen-
sitivities similar to the human
eye (Opportunity’s Pancam
can just pick out magnitude 6
or 7 red stars on a clear night,
and Curiosity’s Mastcam can
see magnitude 7 blue stars).
That means only the brightest
stars are visible at lower alti-
tudes. The cameras’ low sensi-
tivity — and the fact that they
cannot move while exposingASKASTR0
Q: DOES DARK MATTER COLLECT INTO
DENSE CONCENTRATIONS LIKE STARS?
AND, IF NOT, WHY? David White, Arlington, MassachusettsWhether dark matter can
collapse into these MAssive
Compact Halo ObjectS (or
MACHOS, as they have been
popularly known since the
1980s) is uncertain.
From the theory side, the
clumping depends on the exact
particle physics properties of
dark matter particles, for exam-
ple, the presence of any non-
gravitational interactions
between them. In any case,
such clumping is certainly
allowed in principle.
On the observational side,
there exist upper limits on the
abundance of MACHOS — made
up of either ordinary or dark
matter — in our galaxy and the
neighboring Andromeda Galaxy
(M31). Convincing evidence for
the existence of MACHOS made
up of dark matter would be
incredibly exciting and would
provide significant new informa-
tion in our understanding of
dark matter properties.
Dragan Huterer
University of Michigan
Ann ArborAstronomers have strong evidence that halos of dark matter surround
most galaxies, like the one shown in this simulation. KAVLI INSTITUTE/SIMULATION:
HEIDI WU, OLIVER HAHN, RISA WECHSLER; VISUALIZATION: RALF KAEHLERNASA’s Spirit rover turned its panoramic camera skyward above Gusev Crater in 2005 to capture these views of Orion. The famous constellation appears
upside down to Earth’s Northern Hemisphere viewers because of the rover’s southerly path on the Red Planet. NASA/JPL/TEXAS A&M/CORNELL/SSI10-second exposure 30-second exposure 60-second exposureOrion
(Simulation using
Starry Night software)SaiphOrion
NebulaOrion’s
BeltRigelBellatrix