WWW.ASTRONOMY.COM 47ones. The process of fusion itself creates
many more elements on the periodic
table, up to iron and nickel. Exploding
stars — supernovae — are the dying car-
casses of massive stars far heavier than the
Sun. Their bombastic blasts create heavier
elements still, and the blasts send them far
out into the surrounding galaxy. These
processes, over vast amounts of time, have
created and spread the variety of elements,
most far heavier than hydrogen and
helium, that we know of today, including
the stuff that makes up our bodies.
The cosmic origins of the elements,
then, are varied. About two dozen ele-
ments originate from dying low-mass
stars. These include carbon, nitrogen,
strontium, and tin. Another two dozen or
so elements come mostly from superno-
vae. These include oxygen, potassium,
sodium, arsenic, and aluminum. Two
elements arise from cosmic ray fission
— when energetic particles from space
impact Earth’s atmosphere and surface.
This process creates boron and beryllium.
About another two dozen elements
are created largely from merging neutron
stars — the clashes of super-dense, dying
stellar remnants made mostly of packed
neutrons. These include iodine, xenon,
cesium, platinum, and gold. And a small
number of elements are created, or at
least can be created, by exploding white
dwarf stars, the final, decayed ultra-
dense remnants of stars like the Sun.
These include titanium, vanadium, chro-
mium, manganese, iron, and nickel.
These are incredible facts to ponder
as you walk out under a starry sky on a
clear, moonless night. Look deep toward
the shimmering glow of the Milky Way,
and you’ll see many twinkling stars and
the unresolved light from millions more
that make up the hazy band running
across our sky. That oldest of all human
questions — “Why am I here?” —
actually has an answer. You’re here
because atoms created in the Big Bang
and in the bellies of stars have recom-
bined in a way to make you, billions of
years after their creation — with a big
thank you to your parents as well.The story of the stars
The story of elements in nature, of why
we are here, of our cosmic roots, is
strongly tied to the story of stars in our
galaxy and universe. And that means
exploring the lives of stars, how stars
come to be, what happens during their
lifetimes, and how they, too — like
humans — eventually die. We may notall know it, but we are part of the biggest
recycling program that exists: the birth,
life, and death of stars.
Stars are born in great clouds of gas
called nebulae. The word nebula comes
from Latin and means “cloud” or “fog.”
These clouds exist between the stars and
consist of mostly hydrogen and helium,
along with some other gases. They are
typically ionized, meaning they are
excited — energized — by hot stars
inside and nearby them, which causes
these clouds to glow. Thankfully, because
of this process, we can see nebulae from
very large distances, across our galaxy
and even in other nearby galaxies.
Interstellar clouds also contain various
amounts of dust. Astronomers believe
these veils of dust in the universe were
formed in supernova explosions.
Observations of the universe have
shown that not only is the cosmos
expanding, as we have known for more
than a century, but that the universal
expansion also is accelerating over time.
On large scales, everything is moving
away from everything else, and the uni-
verse is getting bigger. But various forces
are at work in the universe. One of the
most important, the very force that keeps
us on Earth’s surface, is gravity. TheA stereo view of the Crescent Nebula (NGC 6888) shows an oblate bubble of gas encapsulating its hot central star, with blebs of multicolored nebulosity
aligning here and there to form the three-dimensional cloud.