Page 324 - ABCTE Study Guide_Neat

of the distance between them.

It is this gravitational force that causes the planets to orbit the Sun, the Moon to orbit the Earth, and so on.
According to Newton's first law, any object, including a planet, will travel in a straight line at a constant
velocity unless an outside force acts on it. The Sun's gravity acts on the planets causing them to deviate
from their straight-line motion. Hence they orbit in a circular or elliptical path around the Sun.

The Big Bang

Abbé Georges Lemaître, a Belgian who was both a cleric and a scientist, was the father of the big bang
model. Lemaître proposed a theory that is consistent both with Hubble's observation that the universe is
expanding and with the theological idea that the universe had a definite beginning point.

According to the big bang theory, the universe had a moment of formation when it appeared as an initial
fireball. What caused this primeval fireball and what existed before the fireball came into existence is
beyond the realm of questions science can answer. The fireball caused everything we can see in the
universe today to expand away from the site of the fireball; and we still observe that the universe is
expanding.

This expanding fireball is often presented as a giant explosion, but this needs some rethinking. In an
explosion, such as a bomb or firecracker, matter expands to fill up space that is already there. In the
expanding universe, space is expanding so that the galaxies in space are moving farther apart. A better
analogy than an explosion is a loaf of raisin bread. The raisins are spread throughout the bread and move
farther apart as the dough rises. Blowing up a balloon with little galaxies drawn on it also makes a handy
classroom analogy.

During the first few minutes after the primeval fireball began expanding, the matter in the universe was
made. Protons, electrons, and neutrons were made. The only elements made during the big bang were
hydrogen, helium, and trace amounts of lithium and beryllium. All other elements were manufactured later
in stars.

As the universe continued to expand and cool, matter began to clump. Large clumps formed galaxies,
such as our own Milky Way galaxy, which contains hundreds of billions of stars. We think all galaxies
were formed about the same time during the early history of the universe. When galaxies are young, they
often go through very energetic stages when they emit far more energy from their nuclei than can easily
be explained. Galaxies in this stage are quasars. The energy source for quasars, which is probably a
supermassive black hole, tends to reduce the amount of energy it emits as it ages. Hence quasars existed
only during the early history of the universe. Because of light travel time from the far reaches of the
universe, we only see quasars at great distances. Closer galaxies have finished with the quasar stage.

The first generation of stars that formed in our galaxy were mostly hydrogen and helium, because that
was what was made during the big bang. The nuclear fusion reactions that create stars fuse these lighter
elements into heavier elements. When stars run out of hydrogen fuel in their cores, they expand into red
giants, which might be about the size of Earth's orbit around the Sun (or larger). Stars of about the Sun's
mass can then fuse helium to carbon, and some oxygen, in their cores. These stars will then collapse into
slowly cooling, burned-out stars about the size of Earth that are called white dwarfs. If the white dwarf is
more than 1.4 times the mass of the Sun, it cannot be a stable white dwarf and collapses into a neutron
star. The protons and electrons are squeezed together into neutrons, and the neutron star collapses to
about the size of a city. During the transition from red giant to white dwarf, many stars gently blow off their
outer layers to form a planetary nebula, which is a shell of gas around a dying star.

Stars that are ten or more times the mass of the Sun can fuse elements heavier than carbon in their
cores. The extra mass provides enough gravitational force needed to squeeze these heavier elements to
undergo fusion, until elements about as heavy as iron on the periodic table are made. Iron is the boundary