An image of an ultra pure tellurium crystal. Image: MIT |
Nearly
13.7 billion years ago, the universe was made of only hydrogen, helium and
traces of lithium—byproducts of the Big Bang. Some 300 million years later, the
very first stars emerged, creating additional chemical elements throughout the
universe. Since then, giant stellar explosions, or supernovas, have given rise
to carbon, oxygen, iron, and the rest of the 94 naturally occurring elements of
the periodic table.
Today,
stars and planetary bodies bear traces of these elements, having formed from
the gas enriched by these supernovas over time. For the past 50 years,
scientists have been analyzing stars of various ages, looking to chart the
evolution of chemical elements in the universe and to identify the astrophysical
phenomena that created them.
Now
a team of researchers from institutions including the Massachusetts Institute
of Technology (MIT) has detected the element tellurium for the first time in
three ancient stars. The researchers found traces of this brittle,
semiconducting element—which is very rare on Earth—in stars that are nearly 12
billion years old. The finding supports the theory that tellurium, along with
even heavier elements in the periodic table, likely originated from a very rare
type of supernova during a rapid process of nuclear fusion. The researchers published
their findings online in Astrophysical Journal Letters.
“We
want to understand the evolution of tellurium—and by extension any other
element—from the Big Bang to today,” says Anna Frebel, an assistant
professor of astrophysics at MIT and a co-author on the paper. “Here on
Earth, everything’s made from carbon and various other elements, and we want to
understand how tellurium on Earth came about.”
‘In the halo of the Milky Way,’ a rare element found
The team analyzed the chemical composition of three bright stars located a few
thousand light-years away, “in the halo of the Milky Way,” Frebel
says. The researchers looked at data obtained from the Hubble Space Telescope’s
spectrograph, an instrument that splits light from a star into a spectrum of
wavelengths. If an element is present in a star, the atoms of that element
absorb starlight at specific wavelengths; scientists can observe this
absorption as dips in the spectrograph’s data.
Frebel
and her colleagues detected dips in the ultraviolet region of the spectrum—at a
wavelength that matched tellurium’s natural light absorption—providing evidence
that the rare Earth element does indeed exist in space, and was likely created
more than 12 billion years ago, at the time when all three stars formed.
The
researchers also compared the abundance of tellurium to that of other heavy
elements such as barium and strontium, finding that the ratio of elements was
the same in all three stars. Frebel says the matching ratios support a theory
of chemical-element synthesis: namely, that a rare type of supernova may have
created the heavier elements in the bottom half of the periodic table,
including tellurium.
No ordinary supernova
According to theoretical predictions, elements heavier than iron may have
formed as part of the collapsing core of a supernova, when atomic nuclei
collided with huge amounts of neutrons in a nuclear fusion process. For 50
years, astronomers and nuclear physicists have modeled this rapid process,
named the r-process, in order to unravel the cosmic history of the elements.
Frebel’s
team found that the ratios of heavy elements observed in the three stars
matched the ratios predicted by these theoretical models. The findings, she
says, confirm the theory that heavier elements likely formed from a rare,
extremely rapid supernova.
“You
can make iron and nickel in any ordinary supernova, anywhere in the
universe,” Frebel says. “But these heavy elements seem to only be
made in specialized supernovas. Adding more elements to the observed elemental
patterns will help us understand the astrophysical and environmental conditions
needed for this process to operate.”
Frebel
is continuing the search for heavy elements in space. For example, selenium—which
is similar to tellurium—has yet to be detected in the universe. Tin, Frebel
says, is also a difficult element to spot, as are many elements along the same
row as tellurium in the periodic table.
“There
are still quite a few holes,” Frebel says. “Every now and then, we
can add an element, and it adds another data point that makes our work easier.”
