WWW.ASTRONOMY.COM 11M
atter arranges
itself in wildly
different ways.
Museums dis-
play dense can-
nonball chunks of iron pyrite
too heavy to lift, while wispy,
barely-there hydrogen domi-
nates the cosmos. It’s every-
where. You wouldn’t float in
water if your body didn’t have
more hydrogen atoms than any-
thing else.
Why explore this topic? Sim-
ple: Some of the most famous
weird-density objects now
parade overhead.
We express a material’s den-
sity by revealing how much a
cubic centimeter (cm^3 ) of it
would weigh. One cm^3 is the
size of a sugar cube, so picture a
sugar cube composed of water,
iron, or gold. If water, it weighs
1 gram, which is ½8 of an ounce.
If iron, the sugar cube weighs
7.87 grams. If it is made of gold,
the little cube tips the scale at a
whopping 19.28 grams.
We know the average density
of planets by how quickly they
make orbiting spacecraft or
natural moons whip around
them. Turns out, the three near-
est worlds to the Sun — Mer-
cury, Venus, and Earth — all
share similar densities, between
5.2 and 5.5 grams per sugar
cube. That’s more than five
times the weight of water.
They’re the densest objects in
the solar system.
To achieve such a high aver-
age density, Earth’s fluffy surface
items like oceans, oaks, and
olives must be balanced by a
very dense interior: its nickel
and iron core. We know Earth
can’t be solid gold beneath thesurface because our overall den-
sity would then be 19 g/cm^3
instead of the actual 5.5.
By contrast, the Sun and the
planets from Jupiter outward
have densities between 0.7 and
1.6 g/cm^3. This isn’t surprising
considering they’re mostly com-
pressed hydrogen. The Sun is
actually quite normal: Most star
densities more or less resemble
that of water.
Now we’re ready for the
weird stuff.
We’ve known about Sirius the
Dog Star’s little companion,
Sirius B (affectionately called
the Pup), since the Civil War.
Small and compact, this “white
dwarf ” has collapsed to the size
of Earth. Conveniently, it’s now
a nice 10 arcseconds from Sir-
ius, a separation that approachesthe widest in the pair’s elliptical
50-year orbit. Steady nights and
a good telescope let you glimpse
the Pup firsthand.
Even easier is 40 Eridini B to
the right of Orion’s foot star
Rigel. The first white dwarf
identified, its 9th-magnitude
glow stands out easily through
even the crummiest telescopes.
Both 40 Eridini B and the
Pup are more than merely tiny
stars that, oddly enough, spin
slowly. They’re crushed down to
a density of 1 million g/cm^3 — a
million times water’s density. A
sugar cube of their material
weighs a ton. Imagine needing a
forklift to pick up a sugar cube.
They would make excellent gagitems at a novelty store: “Hey
Mike, could you pass me those
little dice?”
White dwarfs aren’t com-
posed of exotic elements.
They’re ordinary carbon and
oxygen, crushed super-solid.
Despite off-the-scale surface
gravities, they stop imploding
when Earth-sized.
Everything changes if a white
dwarf possesses more than 1.
times the Sun’s mass. Then
when its nuclear furnace grows
too weak to support its heavyouter layers, it keeps collapsing
until it’s a ball just 12 miles (
kilometers) wide — smaller
than Arches National Park. The
most famous such “neutron
star” floats overhead on winter
nights. You can find it next to
Taurus the Bull’s left horn —
the Crab Pulsar.
You need at least a 14-inch
telescope to glimpse this faint
16th-magnitude pinpoint in the
heart of the Crab Nebula.
(Some say it’s a challenge even
through a 20-incher.) If you
spot it, you’ve beheld the small-
est deep-space object anyone
has ever seen. And among the
fastest spinning. Neutron stars
are strangely different fromtheir white-dwarf cousins,
which leisurely spin in about a
day. Instead, these whirl dozens
of times per second. One per-
forms 716 spins in the same
time you’d say “Mississippi.”
The Crab Pulsar’s density is
100 trillion g/cm^3. A sugar cube
of its material weighs a hundred
million tons. Imagine taking a
giant cruise ship and crushing it
down until its size matches the
ball in a ballpoint pen. We’re
talking about a ballpoint con-
taining thousands of tons of
steel. Then alone you’d attain
neutron star density.
Interestingly, this about
matches the density of each
proton and neutron in your
body. So you yourself are
already made of such stuff —
some 30 octillion teensy specks
of it. Who can doubt we live in
a funny old universe?
Only a black hole could top a
neutron star’s density. The near-
est, V616 Monocerotis, is also
out these nights — to the left of
Orion’s Belt. But we can’t quan-
tify its density. Theoretically,
the density could be infinite,
which has no physical meaning
at all.
And when density reaches
this level of strangeness, it’s
time for a new topic.STRANGEUNIVERSE
BY BOB BERMANBalls of
crushed fire
FROM OUR INBOX
BROWSE THE “STRANGE UNIVERSE” ARCHIVE AT http://www.Astronomy.com/Berman.Contact me about
my strange universe by visiting
http://skymanbob.com.A SUGAR CUBE OF WHITE DWARF MATERIAL
WEIGHS A TON. IMAGINE NEEDING A
FORKLIFT TO PICK UP A SUGAR CUBE.Planetary prediction
The article “Top 10 exoplanets” in the October 2013 issue makes
much of the confusing array of solar systems and how surprised
many astronomers were to find that exosystems don’t reflect our
assumptions about planet formation.
Isn’t it likely that planet formation is a chaotic process (in the
mathematical sense)? Perhaps “sensitive dependence on initial
conditions” gives rise to many different branches of the planet
formation process. While perhaps we can speak of hierarchical
development once a system has begun to organize itself, we cannot
predict exactly the path it will follow. — Jack Butler, Eureka, CaliforniaWe welcome your comments at Astronomy Letters, P. O. Box 1612,
Waukesha, WI 53187; or email to [email protected]. Please
include your name, city, state, and country. Letters may be edited for
space and clarity.Seek out some strangely
dense objects this winter.