Page 325 - ABCTE Study Guide_Neat

between fission and fusion; so fusing elements heavier than iron requires, rather than releases, energy.
When the iron core builds up in these massive stars, they explode in a humongous explosion called a
supernova.

A supernova releases as much energy in a year as the Sun
does in its 10 billion year lifetime. There is plenty of energy
to make all the elements that will normally not fuse. The
supernova also blasts these heavy elements out into space
to be recycled into the next generation of stars. The Crab
Nebula is the remnant of a supernova that occurred on July
4, 1054. The material, enriched in heavy elements, will be
recycled into nebulas such as the Orion Nebula that is in
the process of forming new stars. Our solar system formed
from a nebula that was also enriched in heavy elements by
a prior generation of stars that exploded as supernovas.

The core that remains after the supernova explosion will
collapse into a neutron star, which is a ball of neutrons with
the mass comparable to the Sun but compressed to the
size of a city. If the core is more than about two to three
times the mass of the Sun, it will collapse into a black hole.
To become a black hole, the Sun would have to be
compressed to a radius of less than three kilometers. A
The Crab Pulsar lies at the heart of the nebula and spins thirty
times each second. black hole is so highly compressed that its escape velocity
exceeds the speed of light. Hence nothing can escape!

Because they manufacture and recycle heavy elements, supernovas play a crucial role in how you got
here. The atoms in your body heavier than hydrogen were manufactured in the cores of stars and
recycled by supernova explosions.

Structure and Composition of the Earth

Lesson Objective

In this lesson, we are going to cover the important features of our Earth.

Previously we covered...

We earlier reviewed the basic concepts of astronomy, including our solar system and the age of the
universe.

Layers of the Earth

The Earth, like many celestial bodies, is not a uniform solid object; it has a series of layers of varying
densities and characteristics. Our planet is made up of many constituent elements and compounds that
are responsible for the components and physical properties of the planet we call home. The main layers
are:

Mantle – uppermost layer –2890 km deep

Lithosphere - includes the crust to 670 km deep