Many cosmologists believe that the universe
was created about 15 billion years ago with a cosmic explosion
they nicknamed the Big Bang. This explosion produced an expanding
cloud of the simplest known chemical elements: hydrogen (H)
and helium (He) (see Figure 1).
Atomic model of the two simplest elements, Hydrogen
From U.S. Nuclear regulatory commission.
Electron [Online]. Available: http://www.nrc.gov/
In certain places there happened to be higher concentrations
of this gas mixture than in others. Where gas particles were
close together, their gravity pulled them even closer. These
local concentrations of hydrogen and helium led to the growth
of the first generation of stars. But they looked more like
clouds of gas than adult stars, and they did not release any
light (see Figure 2).
Figure 2. The birth
of stars in the M16 Eagle Nebula
Note. From Hester, J. & Scowen, P. (1995, November).
Star-Birth Clouds M16 [Online]. Available: http://hubble.stsci.edu/gallery/
As the young stars grew, their gravity increased, allowing
them to continue pulling in hydrogen and helium. With a gain
in material, the pressure and temperature in the center of
the gas clouds increased dramatically. This pressure started
the process of nuclear fusion. This is how other chemical
elements were formed inside the first stars (see Figure 3).
Figure 3. Nuclear Fusion
of Hydrogen into Helium.
Note. From Cornell University Instructional Web Server
Information and Services
Hydrogen Burning [Online].
Because nuclear fusion is a process that releases a lot of
energy, the growing stars soon released light and heat. At
this point in development, the balls of gas began to shine.
They became recognizable as stars.
As the stars aged, the hydrogen, helium and other chemical
elements used to create energy in their centers were used
up. The stars began to cool, as though their nuclear furnaces
had been turned off. As the outer layers of gas continued
to cool down, they collapsed toward the center. What happened
next depended on the size of the star.
In some cases, the collapsing stars exploded violently, becoming
supernovas. The energized material in the expanding supernovas
rapidly created heavier chemical elements. Much of the material
that made up these stars was spewed into space (see Figure
Figure 4. Expanding
explosion debris from Supernova 1987A
Note. From Pun, C.S.J., Kirshner, R.P., & NASA
Photo No.: STScI-PRC97-03 [Online].
In other cases, the dying stars slowly released their contents
into space. Like the supernovas, the end result was the same.
The space between surviving stars was enriched with heavier
chemical elements as well as the simpler hydrogen and helium
that had made up most of the dying stars. From the heavier
elements, many of the gaseous atoms solidified to form small
grains or crystals that appeared like dust floating in the
cold of outer space.
These processes of the death of stars and the formation of
new stars and small dust grains happened again and again.
Each successive generation of stars had higher amounts of
the heavier chemical elements formed in the previous generation.
The composition of newer stars was different from that of
Our star, the sun, belongs to the generation of stars created
4.6 billion years ago. A cloud of gas, dust, and frozen ice
from between existing stars collapsed to form a nebula. This
nebula spun slowly. Most fell to disk and then moved inward
to form the sun. A small fraction of the material in the disk
formed solids which bumped into each other, stuck and grew
larger. As they grew larger, their gravity increased. These
larger particles were able to attract even more dust and ice,
increasing in size to become the cores of planets, moons or
asteroids. Some drifted away from the sun to become comets
(see Figure 5).
Figure 5. Protoplanetary
disk around a developing star
Note. From Encarta 98 Encyclopedia
Protoplanetary Disk [Online]
Since all nine planets formed from the same material, are
they the same? They are not. Mercury and Venus, being closest
to the sun, are both much hotter than the Earth, but Mercury
has no atmosphere while Venus is covered with thick, poisonous
clouds. Earth seems to be the only one of the planets hospitable
to life. What makes the planets so different from each other?
(See Figure 6.)
Figure 6. The three
inner planets: Mercury, Venus, Earth
Note. From California Institute of Technology (1995).
Mercury, Venus, Earth [Online]. Available: http://pds.jpl.nasa.gov/planets/index.htm
Cosmologists have evidence that the planets, moons, and even
asteroids differ in the amounts of the various chemical elements
from which they were made. These differences may provide scientists
with clues into how the solar nebula of swirling gas, dust
and ice formed into planets. They have made models of the
process of planet formation. To best test these models, they
need to know the starting conditions. What chemical elements
were present in the original solar nebula?
The sun still contains most of the material of the original
solar nebula. Its internal nuclear reactions have modified
the material at the suns core. However, the surface
layers, which have not mixed with the core in its present
state, have quite accurately preserved the original nebular
For practical reasons, we cannot send a spacecraft to the
sun to pick up samples for analysis. However, the sun shoots
out streams of its outer material, which we call solar wind.
The Genesis spacecraft will collect samples of these chemicals
and return them to Earth for scientists to analyze. These
studies will enrich our understanding of the formation of
the planets, their moons, and the asteroids (see Figure 7).
Figure 7. The
Genesis module with collectors deployed.