Great
Attractor
218 million
light-yearsPerseus Cluster
248 million
light-yearsComa Cluster
319 million light-years
million light-years
The cosmic^100200300400
distance scaleA
cosmic perspective is always a
little unnerving. For example, we
occupy the third large rock from
a middle-aged dwarf star we
call the Sun, which resides in a
quiet backwater of a barred spiral galaxy
known as the Milky Way, itself one of bil-
lions of galaxies. Yet at the same time, we
can take heart in knowing that our little
tract of the universe remains exceptional
as the only place where we know life
exists. Our solar system hosts one abode
for life, Earth, but in decades to come we
may learn of others — perhaps biologi-
cal enclaves on Mars or Jupiter’s moon
Europa. Although our solar system likely
is not unique in this respect, definitive
evidence for life on worlds orbiting other
stars will be harder to find.
All told, the solar system contains
about 4 million trillion trillion pounds of
material, or about 1.0013 solar masses.
The number on the left side of the
decimal is the mass of the Sun itself, and
about 73 percent of what’s on the right
side is held in the giant planet Jupiter. The
remainder includes everything else: Earth
and the other planets, moons, dwarf
planets, asteroids and comets of all sizes,
as well as dust and icy grains. One couldcorrectly describe our planetary system
as consisting of Jupiter plus debris.
The star that brightens our days, the
Sun, is the solar system’s source of heat
and light as well as its central mass, a
gravitational anchor holding everything
together as we travel around the galaxy.
Its warmth naturally divides the planetary
system into two zones of disparate size:
one hot, bright, and compact, and the
other cold, dark, and sprawling. Here’s
how it all fits together.The hot zone
Light takes eight minutes, give or take a
few seconds, to reach Earth from the Sun’s
surface. The average distance is 92.96 mil-
lion miles (149.6 million kilometers). We’ve
barely begun our tour of the solar system,
and numbers using familiar units already
are cumbersome. So astronomers devised
the astronomical unit (AU; see “Solar sys-
tem yardstick,” p. 26) as a simpler way to
express distances at the scale of planetary
orbits. The average distance from Earth to
the Sun represents 1 AU.
Mercury lies closest to the Sun, at an
average distance of 39 percent of Earth’s,
but its eccentric orbit — the most elongat-
ed of all the eight major planets — means
this number varies. At its greatest distance,
called aphelion, Mercury is
0.467 AU from the Sun,
and at its closest point,We start your tour of the cosmos with gas and ice giants,
a lot of rocks, and the only known abode for life. by Francis Reddy
OUR SOLAR SYSTEM
Realms of fire and ice
OBJECT DISTANCE
Sun 8.3 light-minutes
Jupiter 43.3 light-minutes
Saturn 79.3 light-minutes
Pluto 329 light-minutes
Oort Cloud ~1 l i g h t- y e a r
Orion Nebula 1,350 light-years
Eta Carinae 7,500 light-years
Omega Centauri 17,000 light-years
Milky Way’s center 27,200
light-years
Large Magellanic Cloud 160,000
light-years
Small Magellanic Cloud 200,000
light-years
Andromeda Galaxy (M31) 2.5 million
light-years
Centaurus A 12 million light-years
Whirlpool Galaxy (M51) 26 million
light-years
Virgo Cluster 55 million light-yearsThe cosmic distance scale
It’s hard to imagine just how big
our universe is. To give a sense of its
vast scale, we’ve devoted the bot-
tom of this and the next four stories
to a linear scale of the cosmos. The
distance to each object represents
the amount of space its light has
traversed to reach Earth. Because
the universe is expanding, a distant
body will have moved farther away
during the time its light has trav-
eled. To show the entire cosmos in
30 pages, we had to use a scale of
1 inch = 55.3 million light-years, or
1 millimeter = 2.18 million light-years.Francis Reddy is the senior science writer for the
Astrophysics Science Division at NASA’s Goddard
Space Flight Center in Greenbelt, Maryland.