Where Do Chemical Elements Come From?

hat these observers did not know is

that during the explosion, the star not

only emitted huge amounts of light—more

light than a billion suns—but also released

chemicals in space. Inside the star were most

of the first 26 elements in the periodic table,

from simple elements, such as helium and car-

bon, to more complex ones, such as manga-

nese and iron; and the giant explosion sprayed

them in space. During the explosion, other

elements were created as well, and after the

explosion, the chemicals in space combined

with each other to form ions and molecules.

These elements travel in space and ulti-

mately end up in planets like Earth, being part

of everything we see around us and ourselves.

The carbon in our cells, the oxygen in the

air, the silicon in rocks, and just about every

element, were all forged inside ancient stars

before being strewn across the universe when

the stars exploded.

During the past century, scientists have

been studying how chemical elements form in

stars and in outer space. Like genealogists—

experts who study the origins of people and

families—these scientists can track down

where most chemical elements came from

and how they descended from each other.

And, similar to forming a family tree, studying

the links between the chemical elements has

brought—and keeps bringing—many sur-

prises and interesting discoveries.

Stellar ovens

A young star is composed primarily of

hydrogen, the simplest chemical element.

This hydrogen ultimately leads to all known

elements. First, the two constituents of each

hydrogen atom—its proton and electron—

are separated. The high pressure inside the

star can literally squeeze together two pro-

tons, and sometimes, a proton will capture an

electron to become a neutron.

When two protons and two neutrons

band together, they form the nucleus of

helium, which is the second element in the

periodic table. Then, when two nuclei of

helium fuse with each other, they form the

nucleus of another element, beryllium. In turn,

the fusion of beryllium with helium produces

a carbon nucleus; the fusion of carbon and

helium nuclei leads to an oxygen nucleus, and

so on. This way, through successive fusion

reactions, the nuclei of most elements lighter

than iron can be formed (Fig. 1). Scientists

call this process nucleosynthesis (for “synthe-

sis of nuclei”).

In stars, these fusion reactions cannot

form elements heavier than iron. Up until

the formation of iron nuclei, these reactions

release energy, keeping the star alive. But

nuclear reactions that form elements heavier

than iron do not release energy; instead, they

consume energy. If such reactions happened,

they would basically use the star’s energy,

which would cause it to collapse.

Not all stars form iron, though. Some

stars explode before creating that many ele-

Figure 1. The chemical composition of a star

before it explodes into a supernova.

In 1054, Chinese astronomers recorded what they called a “guest star”

in the constellation of Taurus, the Bull. This star had never been seen

before, and it became brighter than any star in the sky. In the American

Southwest, a culture rich in astronomical tradition called the Anasazi

also witnessed this brilliant new star. Easily visible in broad daylight,

the observers could read by it at night. Today, we know the Chinese

and Anasazi were witnessing a huge star explosion, called a supernova.

6 ChemMatters, OCTOBER 2009

www.acs.org/chemmatters

NASA,ESA, HEIC, AND THE HUBBLE HERITAGE TEAM (STSci/AURA)

LAWRENCE LIVERMORE NATIONAL LABORATORY

Where Do Chemical

Elements

Come From?

By Carolyn Ruth

ments. In stars less massive than the sun, the

reaction converting hydrogen into helium is

the only one that takes place. In stars more

massive than the sun but less massive than

about eight solar masses, further reactions

that convert helium to carbon and oxygen take

place in successive stages before such stars

explode. Only in very massive stars (that are

more massive than eight solar masses), the

chain reaction continues to produce elements

up to iron.

A star is a balancing act between two

huge forces. On the one hand, there is the

crushing force of the star’s own gravity trying

to squeeze the stellar material into the small-

est and tightest ball possible. On the other

hand, there is tremendous heat and pressure

from the nuclear reactions at the star’s center

trying to push all of that material outward.

The iron nucleus is the most stable

nucleus in nature, and it resists fusing into

any heavier nuclei. When the central core of a

very massive star becomes pure iron nuclei,

the core can no longer support the crushing

force of gravity resulting from all of the matter

above the core, and the core collapses under

its own weight.

The collapse of the core happens so fast

that it makes enormous shock waves that

blow the outer part of the star into space—a

supernova. It is during the few seconds of

the collapse that the very special conditions of

pressure and temperature exist in the super-

nova that allow for the formation of elements

heavier than iron. The newly created elements

are ejected into the interstellar dust and gas

surrounding the star.

“The amount of elements released

through a supernova is truly phenomenal,”

says Stan Woosley, professor of astronomy

and astrophysics at the University of Califor-

nia at Santa Cruz. “For example, SN1987A, a

supernova seen in 1987, ejected 25,000 Earth

masses of iron alone.”

How stars make

elements heavier

than iron

Elements that are heavier than iron can

be assembled within stars through the capture

of neutrons—a mechanism called the “s” pro-

cess. The process starts when an iron nucleus

captures neutrons, thus creating new nuclei.

These nuclei can be either stable, that is, they

do not change, or radioactive, meaning that

they transform, or decay, into another element

after a certain amount of time, which can be

as short as a fraction of a second and as long

as a few million years.

Also, the newly formed nuclei can be

different versions of a given element. These

different versions of an element are called

isotopes. They all contain the same number

of protons in their nucleus but have differ-

ent numbers of neutrons. Some isotopes are

radioactive, while others are stable and never

change.

For example, nickel can appear in

the form of 23 different isotopes. They all

have 28 protons, but each isotope contains

between 20 and 50 neutrons. Of these 23

isotopes, only five are stable, while the others

are radioactive.

If a nucleus produced through the “s”

process is stable, it may capture another

neutron. If it is radioactive, it transforms into

another nucleus. This other nucleus can, in

turn, absorb another neutron, leading to a

heavier nucleus.

For example, nickel-64, which contains

28 protons and 36 neutrons, can absorb a

neutron, leading to nickel-65, which contains

28 protons and 37 neutrons:

Nickel-65 is radioactive. It exists for only 2

and a-half hours, and then transforms into

copper-65—the next element in the periodic

table, which contains 29 protons and 36 neu-

trons. This is a process called beta decay, in

which a neutron transforms into a proton and

an electron:

Copper-65 is stable, so nothing happens after

that.

This neutron capture mechanism, called

the “s” process, is extremely slow. Hundreds

or thousands of years might elapse between

neutron strikes. But another process, called

the “r” process, which stands for “rapid,”

allows for the rapid capture of neutrons.

Unlike the “s” process, which occurs inside a

star before it explodes, the “r” process hap-

pens only during the explosion of a star.

Exploding and

cooking elements

at the same time

When a star explodes into a supernova, it

produces a huge amount of light and releases

an extremely high number of neutrons (on the

order of 10 thousand billion billion neutrons

per square inch per second). These neutrons

are then rapidly captured by the various nuclei

that are also released by the exploding star,

producing new nuclei through the “r” process.

In this process, even though

many neutrons are available, only

a limited number can be added

to a given nucleus; otherwise,

a nucleus becomes radioactive

and breaks up. Neutrons in a nucleus are

thought to occupy shells—similar to succes-

sive shells on a hard candy. When a nucleus

gets “saturated” with neutrons, that is, when

its shells are filled up, it undergoes a beta

decay process to become the nucleus of the

next element on the periodic table. This new

nucleus, in turn, absorbs as many neutrons

Ni-65 (28 protons, 37 neutrons) ➞

electron + Cu-65 (29 protons, 36 neutrons)

Ni-64 (28 protons, 36 neutrons) + neutron ➞

Ni-65 (28 protons, 37 neutrons)

Combined X-ray and optical images of the Crab

Nebula.

The Crab Nebula is a six-light-year-wide expanding

remnant of a star’s supernova explosion.

ChemMatters, OCTOBER 2009 7

NASA, ESA, J. HESTER AND A. LOLL (ARIZONA STATE UNIVERSITY)

NASA/CXC/ASU/J. HESTER ET AL.

Finding Chemicals Inside Stars

o determine which chemical elements are formed inside stars,

scientists use a technique known as visible spectroscopy. It

is based on a device, called a spectroscope, which spreads visible

light into its component colors by passing it through a prism or

grating.

These colors are called an emission spectrum, and their

position and intensity differ according to the chemical element

that emits the light. For example, the hydrogen’s emission spectrum

consists of four lines: purple, blue, green, and red, located at posi-

tions that correspond to their wavelengths. The emission spectrum

of helium consists of six lines that are purple, cyan, green, yellow,

orange, and red. In other words, atoms and molecules produce their

own “ngerprint” or “signature” when the light they emit is spread in

a spectroscope.

Astronomers also measure how much light is present at each

spectral line. The overall strength or weakness of all the lines of

an element depends on the number of atoms of

that element. The percentage composition of the

atoms in a stellar body can also be determined.

For example, by looking at the light emitted by the

sun, scientists have been able to determine the

relative number of atoms from specic elements

and infer their percentage by mass.

as it can hold, and then decays when it is

“saturated” with neutrons, and the cycle starts

again. When an element formed through the

“r” process becomes really heavy (total num-

ber of protons and neutrons close to 270), it

spontaneously breaks apart through a process

called nuclear fission.

“The neutrons add very rapidly at a

temperature of a few billion degrees, going

from iron to uranium in less than 1 second,”

Woosley says.

Elements created this way include tran-

suranium elements—elements whose number

of protons is higher than that of uranium—

such as curium-250, californium-252, califor-

nium-254, and fermium-257.

Our stellar origins

When a supernova spews its newly made

elements into space, the elements become

part of an enormous cloud of gas and dust,

called an interstellar cloud. The gas is made

of 90% hydrogen, 9% helium, and 1% heavier

atoms. The dust contains silicates (com-

pounds made of silicon), carbon, iron, water

ice, methane (CH

), ammonia (NH

), and

some organic molecules, such as formalde-

hyde (H

CO).

Such clouds are found so often between

stars in our galaxy that astronomers think that

all stars and planets have formed from them.

Except for hydrogen, which appeared when

SELECTED REFERENCES

Cowen, R. Bang. The Cataclysmic Death of Stars.

National Geographic, March 2007, pp 78–95.

Pendick, D. Archival Search Spots Supernova.

Astronomy, Jan 2009, p 18.

Pendick, D. Watching Echoes of a Supernova,

Astronomy, Jan 2009, p 26.

Soderberg, A. X-rays Mark the Spot. Sky &

Telescope, Nov 2008, pp 26–31.

Carolyn Ruth is an adjunct professor of chemistry

at Mercyhurst College, Erie, Pa. Her most recent

ChemMatters article, “Letting Off Steam,” appeared

in the April 2009 issue.

Figure 2. Sketch of a spectro-

scope and how it forms a

spectrum. The light emitted by a

source from space goes through

a narrow slit to form a beam of

light, which is then spread into

its components by a grating (a)

or a prism (b), resulting in the

light’s spectrum.

Figure 1. When light emitted

by hydrogen is spread

through a spectroscope,

it reveals a characteristic

emission spectrum specific

only to hydrogen.

the universe formed through the Big Bang

explosion, all of the elements on Earth have

been cooked for billions of years in stars and

then released in the universe through super-

nova explosions. The nitrogen in our DNA, the

calcium in our teeth, the iron in our blood, and

the carbon in our apple pies were all made in

the interiors of stars. The gold in jewels, tung-

sten in light bulbs, and silver in cookware

were all produced during stellar explosions.

We ourselves are made of “star stuff.”

Finding Chemicals Inside Stars

Supernova remnant ejected from the explosion of a

massive star that occurred some 3,000 years ago.

(a)

(b)

8 ChemMatters, OCTOBER 2009

www.acs.org/chemmatters

ANTHONY FERNANDEZ

NASA