Solar system distances
Sun
Objects are not to scale
Inner Oort Cloud
Main
asteroid belt
1
Logarithmic scale
Oort Cloud
Kuiper Belt
10
100
Astronomical units (AU)
1,000
10,000
100,000
The Oort Cloud is an extended region of icy bodies left over from the formation of the solar system. These objects appear tightly packed in illustrations,
but they are actually spread out with great distances on the order of 31 million miles (50 million km) between neighboring bodies. ASTRONOMY: ROEN KELLY
gravitational waves differently
from Einstein. If the nature of
gravity is different from what
Einstein states, then gravita-
tional waves might be allowed
to disperse.
LIGO and Virgo have both
looked for dispersion in the
gravitational waves detected
from merging black holes and
neutron stars, but have so far
found that all the different fre-
quencies are arriving at the
detector at the exact same time,
just as Einstein predicted.
Robert Naeye
Contributor
Q: I’VE HEARD THAT THE
OORT CLOUD CONTAINS
TRILLIONS OF ICY BODIES.
WHAT WOULD BE THE AVER-
AGE DISTANCE BETWEEN
THESE BODIES?
Larry Guldenzopf
Milwaukie, Oregon
A: In movies you usually see
an asteroid belt full of rocks
close to one another. In reality,
this would be an unstable situ-
ation because objects that are
close to one another will col-
lide often as they orbit the Sun.
Any belt of objects will grind
down the population until col-
lisions are rare, and thus the
objects generally will not be
found near each other in a sta-
ble long-lived belt.
In our main asteroid belt
between Mars and Jupiter,
between about 2.1 and 3.3
astronomical units (AU; 1 AU
is the average Sun-Earth dis-
tance, 93 million miles [150
million kilometers]) from the
Sun, there are some 1 million to
1.5 million asteroids larger than
0.6 mile (1 km). Each of these
asteroids is on average 1.8 mil-
lion miles (3 million km) apart,
or about eight times the Earth-
Moon distance. These asteroids
are small compared with our
Moon and thus would generally
not be observable from each
other. Collisions between aster-
oids still do happen, with a few
per year that create dust clouds
we can detect with telescopes.
The Oort Cloud has many
more objects than the main
asteroid belt, some trillion
objects larger than a kilometer,
but it also occupies a much
larger volume of space from
5,000 AU to beyond 20,000 to
50,000 AU from the Sun. Oort
Cloud objects larger than 1 km
have some 31 million miles (50
million km) between each
other. This is about the dis-
tance between Earth and Mars
at their closest approach.
The large distances between
small objects in our solar system
make empty space the norm
and mean spacecraft can travel
safely among the planets.
Scott Sheppard
Staff Scientist, Department of
Terrestrial Magnetism, Carnegie
Institution of Washington,
Washington, D.C.
Q: MY UNDERSTANDING IS
THAT THE MATERIALS FOR
OUR SOLAR SYSTEM AND
OTHERS NEARBY WERE
CREATED BY A SUPERNOVA.
WHERE IS THE BLACK HOLE
FROM THAT SUPERNOVA?
John Goodemote
La Grange, Illinois
A: The heavy elements we see
throughout the galaxy today,
including our solar system,
were indeed created in and dis-
persed by supernovae, as well as
by colliding neutron stars such
as those observed in August.
However, not all supernovae
end in black holes. Current
models predict that a star must
have a mass at least 20 times
that of the Sun before the core
collapse at the end of its life
will result in a black hole. If
any nearby supernovae didn’t
meet this initial criterion — if
their parent stars were only
between eight and 20 solar
masses — they would have left
behind a neutron star, not a
black hole.
Neutron stars are difficult to
observe, particularly when they
are old and lack a binary com-
panion. They are typically on
the order of 12 miles (20 km)
in diameter and cool off over
time unless they are actively
accreting material from a
nearby companion star. Black
holes have the same observa-
tional problem — they only
“light up” and become
detectable when infalling mat-
ter forms a disk that emits
radiation. A lone black hole
with no companion accretes
slowly, so it has a minimal disk
and remains virtually invisible.
Finally, nothing in our
Milky Way is static. Every star
(and neutron star and black
hole) revolves around the cen-
ter of the galaxy, and also
moves relative to the objects
around it. This ultimately
means that astronomers aren’t
able to reliably wind back the
clock to determine what our
Sun’s surroundings looked like
before it formed. A black hole
or neutron star left by a nearby
supernova isn’t in the same
place it was billions of years
ago. We may never find the
exact supernova remnant or
remnants that left behind the
material from which our solar
system formed.
Alison Klesman
Associate Editor
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