A pillar of dust and gas in the Eagle Nebula.
A huge cloud of dust
Throughout the Milky Way, and other galaxies like it, are gigantic swirling clouds of dust and gas known as
nebula. It is within nebula that stars are born. Our star, the Sun, was created in one such nebula.
Something, perhaps the shock wave from an exploding
supernova
(dying star) triggered dust particles to be drawn together to form a
dense spherical cloud. The accumulation of dust set off a chain
reaction. As the core of the cloud attracted more dust, its
gravitational pull increased. More and more dust was sucked in and the
cloud collapsed in on itself. As this happened, the rotation of the
cloud increased in speed, as happens when spinning ice skaters pull in
their arms. The rotational forces at the equator of the cloud prevented
dust along this plane being drawn in, causing the cloud to flatten into a
disc spinning around a dense core.
A star is born
As more
and more mass accumulated at the centre of the disc, the temperature
increased dramatically. Eventually there was enough energy to set off
nuclear reactions. Hydrogen atoms fused to form helium, releasing
enormous amounts of energy in vigorous bursts. This marked the
birth of the Sun, although it would take between one and 10 million more years for it to settle into the main sequence star recognisable today.
A star forming region in the Orion Nebula.
The formation of the planets
The planets, and other
extraterrestrial objects such as asteroids, formed in the flat plane of
the spinning disc of dust. Electrostatic forces or sticky carbon
coatings made dust particles stick together to form clusters, which in
turn stuck together to form rocks. Mutual gravity caused these rocks to
come together, eventually to form planets. This 'coming together' of
material is a process known as accretion.
An
artist's drawing a black hole named Cygnus X-1. It formed when a large
star caved in. This black hole pulls matter from blue star beside it.
A black hole is a place in space where gravity pulls so much that
even light can not get out. The gravity is so strong because matter has
been squeezed into a tiny space. This can happen when a star is dying.
Because no light can get out, people can't see black holes. They are
invisible. Space telescopes with special tools can help find black
holes. The special tools can see how stars that are very close to black
holes act differently than other stars.
How Big Are Black Holes?
Black holes can be big or small. Scientists think the smallest black
holes are as small as just one atom. These black holes are very tiny but
have the mass of a large mountain. Mass is the amount of matter, or
"stuff," in an object.
Another kind of black hole is called "stellar." Its mass can be up to
20 times more than the mass of the sun. There may be many, many stellar
mass black holes in Earth's galaxy. Earth's galaxy is called the Milky
Way.
An
artist's drawing shows the current view of the Milky Way galaxy.
Scientific evidence shows that in the middle of the Milky Way is a
supermassive black hole.
The largest black holes are called "supermassive." These black holes
have masses that are more than 1 million suns together. Scientists have
found proof that every large galaxy contains a supermassive black hole
at its center. The supermassive black hole at the center of the Milky
Way galaxy is called Sagittarius A. It has a mass equal to about 4
million suns and would fit inside a very large ball that could hold a
few million Earths.
How Do Black Holes Form?
Scientists think the smallest black holes formed when the universe began.
Stellar black holes are made when the center of a very big star falls
in upon itself, or collapses. When this happens, it causes a supernova. A
supernova is an exploding star that blasts part of the star into space.
Scientists think supermassive black holes were made at the same time as the galaxy they are in.
This image of the center of the Milky Way galaxy was taken by the Chandra X-ray Observatory.
If Black Holes Are "Black," How Do Scientists Know They Are There?
A black hole can not be seen because strong gravity pulls all of the
light into the middle of the black hole. But scientists can see how the
strong gravity affects the stars and gas around the black hole.
Scientists can study stars to find out if they are flying around, or
orbiting, a black hole.
When a black hole and a star are close together, high-energy light is
made. This kind of light can not be seen with human eyes. Scientists use
satellites and telescopes in space to see the high-energy light.
Could a Black Hole Destroy Earth?
Black holes do not go around in space eating stars, moons and planets.
Earth will not fall into a black hole because no black hole is close
enough to the solar system for Earth to do that.
Even if a black hole the same mass as the sun were to take the place
of the sun, Earth still would not fall in. The black hole would have the
same gravity as the sun. Earth and the other planets would orbit the
black hole as they orbit the sun now.
Sagittarius A* is the black hole at the center of the Milky Way galaxy.
The sun will never turn into a black hole. The sun is not a big enough star to make a black hole.
WHAT IS SUNLIGHT
Sunlight is a portion of the electro magnetic radiation given off by the sun, in particular infrared, visible, and ultra voilet light. On earth, sunlight is filtered through the earth atmosphere and is obvious as day night when the Sun is above the horizon. When the direct solar radiation is not blocked by clouds, it is experienced as
sunshine, a combination of bright light and radiant heat. When it is blocked by the clouds or reflects off other objects, it is experienced as diffused light. The World Meteorological Organization uses the term "sunshine duration" to mean the cumulative time during which an area receives direct radiance from the Sun of at least 120 watts per square meter.
The ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a principal source of vitamin D
3 and a mutagen.
