Astronomers have used the world’s largest telescope to gain new
insights into the hidden cauldron that lies beneath the foggy surface
of stars. A team led by Prof. Ann M. Boesgaard of the University of
Hawaii Institute for Astronomy has found large deficits of lithium
and beryllium. These two light elements act as probes into the deeper
reaches of stars because they are so fragile.

The team found that the youngest stars still have the lithium and
beryllium that they were born with, but older stars have destroyed
up to 99 percent of their lithium and up to 85 percent of their
beryllium. Theoretical models of stars cannot account for this
wholesale destructive behavior.

Lithium and beryllium atoms are destroyed by nuclear fusion in the
hot interiors of stars. Lithium “burns” when the temperature is
about 2 million degrees Kelvin, and beryllium atoms “burn” deeper
in the stars, where the temperature is about 3 million K. The amount
of these two chemical elements remaining on the surface indicates
how deeply the surface layers penetrate into the interior. Strong
convective currents and other mixing mechanisms transport the atoms
at the surface of the star to its interior, where they can no longer

The stars in the young Hyades star cluster in the constellation of
Taurus, the Bull, have been studied extensively for the lithium
content. At 700 million years of age, the Hyades cluster has a
pronounced deficiency in lithium in stars that are 25 to 40 percent
more massive than the Sun. This is known as the “lithium dip.”

The new observations investigated the beryllium content of these
stars. Is there a “beryllium dip”? These are a far more challenging
observations to make, and so the team used the Keck I 10-meter
telescope atop Mauna Kea in Hawaii for its research. The twin Keck
telescopes are the world’s largest optical telescopes.

The team, which also includes Eric Armengaud, a French student
working in Hawaii, and Jeremy King, assistant professor at the
University of Nevada, Las Vegas, looked for — and found — a
beryllium dip in the Hyades and other young clusters. The beryllium
dip is not dramatic as the lithium dip because not as much of the
surface matter circulates down to the deeper level where beryllium
can be destroyed.

The temperature of the surface of our Sun is about 6,000 K, and its
core is about 15 million K. Measurements show that the Sun has lost
all but one percent of its original lithium but retains most of its
beryllium. This means the surface atoms have been mixed with the
material in the inside of the Sun down to the region where the
temperature is 2 million K, but not as deep as 3 million K. Studying
both elements lets us see the degree of mixing between the surface
and the interior of a star.

The destruction of lithium atoms takes time. Stars younger than the
Sun have not destroyed as much lithium as the Sun. Like the Sun,
they have not destroyed any beryllium.

The amount of destruction also depends on the mass of the stars and
here the pattern for lithium differs from the pattern for beryllium.
For the cooler, low-mass dwarf stars there are huge lithium
deficiencies, but beryllium is unscathed. For the warmer stars that
are between 25 and 40% more massive than the sun, both lithium and
beryllium are destroyed. For stars in the middle of that range there
is no lithium to be found at all, while beryllium is less affected,
but is deficient. Stars that are more than 60% more massive than
the sun have the full complement of both lithium and beryllium.

This strange pattern is not predicted by any theory about the
circulation of surface material to the interior.

The new research shows that the beryllium dip is present in the
intermediate-age clusters, Hyades, Coma, and Ursa Majoris. It mimics
the lithium dip, but it is not as deep. For the Pleiades and
Alpha Per clusters, which are one-tenth the age of the Hyades, there
is no beryllium dip and only a minor lithium dip. From this, the
team concludes that lithium and beryllium are “burned” while stars
are in the most stable phase of their lives, not in the tumultuous
period of formation. The effects of the “burning” are evident only
after stars attain the age of about 200 million years.

Theorists will need to reexamine their ideas in light of this new
beryllium data. The mix-master in the warmer dwarfs seems to be
different from the mix-master in the cooler dwarfs. Extra mixing,
induced by rotation, appears to be a possible explanation for the
warmer stars. Those stars with higher initial rotation destroy more
lithium and beryllium than those with slow rotation.

This study was funded by the National Science Foundation.

The Institute for Astronomy at the University of Hawaii conducts
research into galaxies, cosmology, stars, planets, and the Sun. Its
faculty and staff are also involved in astronomy education, deep
space missions, and in the development and management of the
observatories on Haleakala and Mauna Kea. Refer to for more information.


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Image Caption:

This diagram shows a cross-section of a star. On the right is the sun
and on the left is a star that is 25% bigger than the sun. The upper
layer shows the zone where large convective currents occur. For both
sample stars the level where lithium is destroyed (the dotted line)
is well below the bottom of the convection zone. The level where
beryllium is destroyed (solid line) is even deeper in the star. The
temperatures of the various levels are indicated in Kelvin degrees.
Both lithium and beryllium are destroyed in the larger star which
indicates that the surface layers have been mixed way down to the
deep layers of the star. However, convective currents cannot be the
cause of the mixing. Other mechanisms, including stellar rotation
and turbulence, must play the crucial mixing role. For stars like
the sun, only lithium is destroyed while beryllium is unaffected.
For those stars the mixing is not as deep, but, again, convection
must be augmented by additional mix-master mechanisms.