Astronomy

1 10 100 1,000 10,000 100,000

Astronomical units (AU)

JupiterSaturn

EarthMars

Main

asteroid belt Kuiper Belt

Inner Oort Cloud Oort Cloud

Logarithmic scale

Objects are not to scale

Sun

UranusNeptunePluto

WWW.ASTRONOMY.COM 45

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

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.

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

Solar system distances