Causes
An asthma attack results from changes
in the small tubes in the lungs that carry
air. During an attack the muscles around
these airways tighten. This makes it
harder for air to pass through. The tubes
also swell up. This leaves less space
inside for the airflow. In addition, the
tubes fill up with extra amounts of a
thick fluid called mucus.
Scientists do not know exactly why
people get asthma. But once people
have the condition, their bodies tend to
overreact to certain things called
triggers. The triggers vary from person
to person. For some it may be
exercising. For others, breathing
polluted air or getting a cold may bring
on an asthma attack. Sudden weather
changes can cause attacks, too.
Asthma attacks are also often associated
with allergies. Allergies are a bodys
negative reactions to certain substances.
Things that cause allergic reactions may
trigger asthma attacks, too. These things
include plant pollen, mold, and tiny
insects that live in house dust.
Treatment
There is no cure for asthma. However,
several medications can help prevent
asthma attacks. Other medicines can
quickly relieve the symptoms of an
attack. People with asthma also try to
avoid the things that trigger their
attacks.
#More to explore
Allergy Lung Medicine
Astronomy
Astronomy is one of the oldest sciences
in the world. The study of astronomy
involves all of the objects outside Earths
atmosphere. These include the sun,
Moon, planets, stars, galaxies, and all
other matter in the universe. People
known as astronomers have studied
these objects for thousands of years.
Study of Astronomy
Astronomers study the universe in different
ways. Some are professionals,
meaning that they make their living at
the work. Others study objects in space
as a hobby. They are known as amateur
astronomers.
Some astronomers work in laboratories.
There they study actual samples of
objects that have been collected by
spacecraft. These include meteorites,
An astronomer points a telescope toward
the evening sky.
214 Astronomy BRITANNICA STUDENT ENCYCLOPEDIA
rock samples from the Moon, and dust
particles.
Many astronomers observe distant
objects with the help of telescopes. Telescopes
range in size. Some are small
enough to be carried by hand. Large
telescopes are often housed in buildings
known as observatories. The Hubble
Space Telescope is in orbit around Earth
as a floating observatory. The Hubble
magnifies and records images of distant
space objects. It then sends the information
to astronomers on Earth.
Astronomers also use cameras and other
instruments to record information about
distant objects. Photographs provide
astronomers with a visual record of their
observations. Other instruments can
help reveal faint, hard-to-detect objects
in outer space. These instruments measure
the light and other radiation coming
from the objects.
Some Early Astronomers
Thousands of years ago there were no
calendars or clocks. People kept track of
time by watching the sun and the stars.
It was important for them to know the
time in order to sow their crops and reap
their harvests. These people were the
first astronomers.
The most influential ancient astronomer
was Ptolemy of Alexandria in Egypt. He
lived in the AD 100s. He developed the
idea that Earth occupies the center of
the universe. He thought that the other
objects revolved around Earth. Most
people believed this idea of Ptolemys
for more than a thousand years.
In 1543Nicolaus Copernicus published a
revolutionary new theory.He believed
that Earth and the other planets revolve
around the sun. Copernicus was criticized
because he challenged Ptolemys theory.
Galileo Galilei, a great Italian scientist
and astronomer, supported Copernicus
theory with his observations.
Later astronomers made new discoveries
that also supported Copernicus idea. In
the early 1600s Johannes Kepler
explained how the planets travel around
the sun. In 1687 Isaac Newton learned
about gravitation and motion. His work
reinforced Keplers laws. Today astrono-
Astronomers study everything
they can see or detect in the sky.
Some astronomers study the
oddly shaped clouds of gas and
dust called nebulas.
Chinese
astronomers
noticed an
explosion in
the sky on July
4, 1054. This
was a supernova,
or the
explosion of a
star. Its
remains can
be seen today
in the Crab
Nebula.
BRITANNICA STUDENT ENCYCLOPEDIA Astronomy 215
mers know that Copernicus was correct.
But there are still many questions about
the universe. Astronomers continue to
try to answer those questions.
..More to explore
Solar System Telescope Universe
Asuncion
Population
(2006
estimate), city,
519,361;
(2007
estimate), urban
area,
1,870,000
Asuncion is the capital of Paraguay, a
country in South America. It is Paraguays
largest city. Asuncion is located
on the Paraguay River. From the citys
port, ships can travel to the Atlantic
Ocean.
Asuncion processes and ships much of
the food grown throughout Paraguay.
Factories in the city also process cotton
and make cloth, shoes, and tobacco
products. The government employs
many people in Asuncion. Others work
in service industries such as health care,
tourism, and trade.
The Guarani Indians moved into the
area hundreds of years ago. In the 1500s
people from Spain settled among the
Guarani and created a colony. The Spanish
named the city Asuncion in 1537.
Paraguay became independent from
Spain in 1811. Asuncion then became
Paraguays capital. During a war in the
late 1800s Brazil took control of Asuncion.
But the city was returned to Paraguay
about 10 years later.
..More to explore
Paraguay
Athapaskan
The Athapaskan (or Athabascan) were a
large group of Native American tribes
who spoke similar languages. They traditionally
lived in northwestern North
The presidential palace in Asuncion is
brightly lit at night.
An Athapaskan child shows his drawings of
reindeer.
216 Asuncion BRITANNICA STUDENT ENCYCLOPEDIA
America. Before 1400 some Athapaskans,
including the Navajo and the
Apache, moved to what is now the
southwestern United States. Others
migrated to the Great Plains or the
Pacific coast. Today the name Athapaskan
is usually used for tribes who stayed
in northern Canada and Alaska. They
include the Chipewyan, the Han, and
the Kutchin.
The Athapaskan lived in forests in areas
with long, snowy winters. They fished
and hunted for animals such as deer,
moose, rabbits, and squirrels. They
prized the reindeer for its skin and fur as
well as its meat. The Athapaskan also
gathered plants.
Living in such a cold environment, the
Athapaskan spent most of the year in a
constant search for food. They traveled
in small family groups. While hunting
they lived in portable, cone-shaped
dwellings that resembled the tepees
made by Plains Indians. They used
dogsleds to carry their belongings over
snow and ice.
For many years only a few French,
English, and Russian fur traders entered