Britannica Student Encyclopedia - 2010

gold was discovered in 1874. In 1876

the Indians fought U.S. troops in the

Black HillsWar. After being defeated,

the Indians were forced to leave the hills.

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Gold Rush • Sioux

Black Hole

A black hole is an area in space with an

incredibly strong force called gravity.

This gravity pulls in everything that gets

close. Nothing, not even light, can

Mount Rushmore National Memorial rises

above the forested slopes of the Black Hills.

A picture from the Hubble Space Telescope

shows a huge ring of gas and dust in

space. Scientists think the ring may surround

a black hole.

BRITANNICA STUDENT ENCYCLOPEDIA Black Hole 87

 

escape from a black hole once it has

been pulled in.

Black holes are not actually holes. They

have great amounts of material packed

in very tightly. They are remarkably

heavy for their size. This gives black

holes their strong inward pull. They are

called “black” because they are invisible.

That is because no light can bounce off

a black hole. All the light that gets near

one is trapped inside.

Finding a Black Hole

Because no one can see black holes, they

are hard to find. Scientists detect black

holes by noting their effects on objects

nearby. For example, just before material

enters a black hole, it gives off many

waves of energy called X-rays. Scientists

can detect those X-rays.

How Black Holes Form

A black hole can be formed when a huge

star uses up its fuel. A star is normally a

huge ball of a gas called hydrogen,

which serves as fuel for the star. The star

constantly changes the hydrogen into

other gases and in the process makes

energy. The energy pushes outward. At

the same time, the star’s gravity pulls

inward. This balance of pushing and

pulling keeps the star about the same

size. However, when a star runs out of

hydrogen, it cannot make the energy

that pushes outward. Gravity continues

to pull the star in on itself. If the star is

very large, its gravity is very strong. The

gravity crushes the star smaller and

smaller, and it becomes a black hole.

Scientists think that black holes can also

be formed in other ways, but they do

not know for certain how this happens.

They believe that a huge black hole lies

at the center of nearly every galaxy, or

group of stars, gases, and dust.

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Galaxy • Gravity • Star

Black Sea

The Black Sea lies in the southeastern

part of Europe. It is roughly oval in

shape. Though far inland, it connects

with the Atlantic Ocean through a series

of other waterways. The Bosporus Strait,

the Sea of Marmara, and the Dardanelles

Strait link the Black Sea to the

Aegean Sea, the Mediterranean Sea, and

finally, the Atlantic.

The Black Sea is located where Europe

and Asia meet. It is bordered by Ukraine

to the north, Russia to the northeast,

Georgia to the east, Turkey to the south,

and Bulgaria and Romania to the west.

Several great rivers empty into the Black

Sea. These include the Danube, Dniester,

Bug, Dnieper, Don, and Kuban. The

88 Black Sea BRITANNICA STUDENT ENCYCLOPEDIA

 

Crimean Peninsula juts into the Black

Sea from the north. The sea covers about

163,000 square miles (422,000 square

kilometers).

The Black Sea is important for transportation

year-round. It is the starting point

for the shipment of goods from eastern

Europe to the rest of the world. The

magnificent climate and mineral springs

around the Black Sea have made it a

major health and vacation center. The

Crimea is the most popular region for

visitors.

#More to explore

Aegean Sea • Crimea • Mediterranean

Sea

Blackwell,

Elizabeth

Elizabeth Blackwell was the first woman

to become a doctor in the United States.

During Blackwell’s life few people

would accept the idea of a woman doctor.

As a result Blackwell had to struggle

all her life to practice medicine.

Early Life

Elizabeth Blackwell was born on February

3, 1821, in Bristol, England. Her

family moved to the United States when

she was 11 years old. When she was

about 24 years old she decided she

wanted to become a doctor.

Blackwell applied to many medical colleges,

but none of them would allow a

woman to study there. Finally, in 1847,

Blackwell was accepted by Geneva

Medical College in Geneva, New York.

She graduated two years later at the top

of her class. Blackwell then traveled to

Paris, France, where she worked at a

hospital for women. She continued her

medical studies in England.

Career

In 1851 Blackwell traveled back to the

United States, but no hospital would

The ruins of an old fort lie near the Black

Sea in Turkey.

Elizabeth Blackwell

BRITANNICA STUDENT ENCYCLOPEDIA Blackwell, Elizabeth 89

 

hire her. She started a small clinic for the

poor in New York City. In 1857 the

clinic became the New York Infirmary

forWomen and Children. In 1868

Blackwell opened theWoman’s Medical

College at the New York Infirmary.

In 1869 Blackwell moved to England.

She helped to set up the National

Health Society. She also served as a professor

at the London School of Medicine

forWomen from 1875 to 1907. Blackwell

died in England on May 31, 1910.

#More to explore

Medicine

Blimp

#see Airship.

Blindness

People who are not able to see have a

condition called blindness. Blindness