Way. Therefore, Earth and all the other
planets in the solar system are also in the
MilkyWay. Just as the planets revolve
around the sun, the stars in the Milky
Way revolve around its nucleus, or center.
It takes the sun about 200 million
years to travel around the nucleus of the
MilkyWay.
Viewed from above, the MilkyWay
looks like a giant pinwheel. The stars are
gathered in the flat shape of a disk, and
arms of stars spiral out from the nucleus.
The nucleus is a bulging cluster of stars.
It has a black hole at the center. A black
hole is an area with such a strong force
of gravity that nothing, not even light,
can escape from it.
The sun and its planets are located on
the inner edge of one of the MilkyWay’s
arms. The solar system is about 30,000
light-years away from the center of the
galaxy. (A light-year is the distance that
light travels in one year—about 5.8 trillion
miles, or 9.5 trillion kilometers.)
The whole galaxy is about 100,000
light-years across.
#More to explore
Galaxy • Solar System • Star • Universe
Millet
Millet is a grain that is an important
food in Asia, Russia, and western Africa.
Like corn, wheat, and other grains, millet
has seeds that can be eaten. Millet
belongs to the grass family.
The black hole
at the center of
the Milky Way
has as much
matter as
4 million suns.
The sun and Earth are parts of
the Milky Way galaxy.
BRITANNICA STUDENT ENCYCLOPEDIA Millet 129
Millet was probably first grown as a crop
in Asia or Africa more than 4,000 years
ago. Today India and Nigeria are leading
millet-producing countries.
There are several types of millet, including
pearl, proso, and foxtail. Most types
grow from 1 to 4 feet (0.3 to 1.2 meters)
tall and have thin stalks. Pearl millet is
an exception. It grows to 10 feet (3
meters) tall and has stalks about 1 inch
(2.5 centimeters) thick. Tiny groups of
flowers grow at the tops of millet stalks.
They produce the seeds, or grain.
People eat millet in porridge, a soft,
thick food made by boiling grains in
milk or water. Millet can also be prepared
and eaten much like rice. People
also grind millet into flour for making
flatbread. Farmers use millet as food for
livestock. Dried millet plants are used
for hay. Millet is used in birdseed mixtures,
too.
#More to explore
Grain • Grass
Millipede
#see Centipede and Millipede.
Mimicry
Mimicry takes place when one living
thing resembles another kind of living
thing. Mimicry helps animals and plants
in various ways. It can keep them from
being eaten or it can help them get food.
Forms of Mimicry
Mimicry helps protect some types of
butterfly in Brazil from birds. These
butterflies taste good to birds. However,
they have markings similar to those of
bad-tasting butterflies. Birds see these
markings and stay away.
Sometimes different types of dangerous
or bad-tasting living things look like
People eat the seeds, or grain, of millet
plants. Tiny flowers at the tops of the plants
produce the seeds.
The fly orchid is a type of plant
whose flower looks like a fly.
This helps the plant attract real
flies to pollinate it.
130 Millipede BRITANNICA STUDENT ENCYCLOPEDIA
each other. Sand wasps and yellow jackets
are different types of insect. But they
look similar, and they both have a painful
sting. After an animal attacks one of
these insects, it may learn to avoid all
similar-looking insects. For this reason,
many different stinging insects have
yellow and black stripes.
Some animals use mimicry to prey on
other creatures. The tongue of the alligator
snapping turtle looks like a worm.
The turtle lies in water with its mouth
open. When fish come over to get the
“worm,” the turtle eats them.
Animals may also use mimicry to take
advantage of other animals. The eggs of
the European cuckoo bird look like the
eggs of other birds. A female cuckoo lays
her eggs in the nests of these other birds.
When the eggs hatch, the other birds
care for the young cuckoos.
Plants can use mimicry, too. Passionflower
plants sometimes grow little
lumps that look like butterfly eggs.
These lumps keep butterflies from laying
actual eggs on the plants. In this
way, the passionflower avoids being
eaten by the caterpillars that would
develop from the eggs.
How Mimicry Happens
In any group of animals or plants, only
some survive and reproduce. Mimicry is
a trait that makes survival easier. Therefore,
many of those that have such traits
do survive, and the traits are passed on
to their offspring. After many generations,
the entire population has the useful
trait.
#More to explore
Protective Coloration
Mineral
Minerals make up Earth’s rocks, sands,
and soils. They are found on Earth’s
surface as well as deep underground.
Minerals are inorganic substances,
meaning that they do not come from an
animal or a plant.
Mineralogy is the science of minerals.
Mineralogists, or people who study minerals,
have identified about 1,500 minerals.
Some of the most common minerals
are metals—for example, gold, silver,
copper, and platinum. Diamond, quartz,
sulfur, mica, talc, and salt are other wellknown
minerals.
Properties of Minerals
Mineralogists use three main properties
to identify a mineral: color, hardness,
