PERSPECTIVE: ORIGINS 185Aging stars exhale
gas and dust into the
interstellar medium.The Gas-Stars-Gas CycleVisual imageVisual imageVisual imageX-ray imageGas expanding away
from a supernova
explosionGas and dust in the
interstellar medium.Stars forming
from clouds of
interstellar gas■ Figure P-13
Matter cycles from the interstellar medium to form stars and back into the interstellar medium. Our galaxy is slowly converting hydrogen into elements heavier
than helium and enriching the interstellar medium.
(^1012) Hydrogen
Helium
Carbon, nitrogen, oxygen
Carbon, nitrogen, oxygen
Iron
Iron
Elements
heavier than iron
1010
108
106
104
102
1
1012
0
1 50 100
a Atomic mass number
150 1 50 100
Atomic mass number
Relative abundance
150
Hydrogen
Helium
b
■ Figure P-14
The abundance of the chemical elements
is usually plotted as in the graph in part
(a). Hydrogen is most common, followed
by helium. Carbon, nitrogen, oxygen, and
iron also seem abundant, but heavier ele-
ments are quite rare because they are made
only in supernova explosions. Plotting
these abundances on a linear scale as in
the graph in part (b) provides a more real-
istic impression. Most of the universe is
hydrogen and helium. The atoms of which
you are made are hardly more than a tiny
impurity in the cosmos.