Lithium, the third lightest element after hydrogen and helium, is one
of the few elements created in the big bang. But young stars have 10 times
more lithium than the big bang produced, so most lithium must have been
made in some other way. One likely possibility is that in space, high-speed
particles collide with heavy elements, breaking them into lighter ones such
as lithium. New stars then inherit this lithium as they are born.

Now Eduardo Martin and Rafael Rebolo of the Canaries Institute of Astrophysics
in Tenerife and Jorge Casares and Philip Charles of the University of Oxford
have obtained evidence that lithium can be made in another way. They say
that violent bursts of X-rays in ‘accretion discs’ swirling into black holes
and neutron stars may forge the element.

The first hint of this came in 1992, after Casares, Charles, and Tim
Naylor of Keele University discovered that a star system called V404 Cygni
contained a probable black hole. Orbiting the black hole is an orange star
whose spectrum reveals a large amount of lithium (New ÐÓ°ÉÔ­´´, Science,
8 February 1992). Then a team led by Thomas Marsh of the University of Oxford
discovered lithium in an orange star circling a black hole candidate in
A0620-00.

Now Martin and his colleagues have observed Centaurus X-4, a system
in which an orange star orbits a compact neutron star. The astronomers discovered
that this orange star also has a large quantity of lithium on its surface.

The rarity of lithium in most stars makes these findings surprising.
At even modest temperatures, nuclear reactions destroy lithium; and orange
stars have convection currents that drag the cooler material on their surfaces
into their hot interiors, thereby ensuring the removal of lithium. So only
young orange stars should have much of the element.

But because black holes and neutron stars are dead stars, the orange
stars in orbit around them are probably old. Also, the black hole or neutron
star will have stripped the orange star of its surface material, so material
now on the star’s surface was once deep inside the star. This factor, too,
suggests that the orange stars should not have any lithium.

Since the stars do contain lithium Martin and his colleagues believe
that something in the environment of the black holes and neutron stars actually
creates the element. Bursts of X-rays may do the trick, say the astronomers.
The accretion disc, or ring of hot material, that orbits each black hole
or neutron star emits X-rays, and occasionally an instability in the disc
causes a hot spot and an accompanying burst of this radiation. In fact,
both A0620-00 and V404 Cygni attracted black hole hunters after emitting
such X-ray bursts, in 1975 and 1989 respectively.

During an X-ray burst, nuclei of helium-4 bombard each other to produce
lithium-7 together with a proton. This nuclear reaction may occur both in
the accretion disc around the black hole and on the surface of the orange
star itself. The outburst also ejects lithium into the Galaxy, where the
element may enrich the clouds of gas that provide the new material for new
stars and planets.

To test their scenario for the production of lithium, Martin and his
colleagues propose that astronomers search black holes and neutron stars
for gamma rays, which are emitted by recently formed lithium nuclei. Gamma
rays have already been detected coming from another black hole candidate,
Nova Muscae 1991. These may signal the presence of freshly minted lithium,
say the astronomers.

Martin and his colleagues admit, however, that it is too early to know
exactly how much lithium black holes and neutron stars contribute to the
Galaxy. Instead, more work needs to be done on verifying their scenario
and calculating how much lithium is actually produced in the accretion disc
hot spots.

The astronomers will report their work in the 10 November issue of The
Astrophysical Journal.

Pluto, the smallest planet, is at present 4.5 billion kilometres from
the Sun, farther than any planet except Neptune. In 1978, when astronomers
discovered Charon, they used the moon’s orbital motion to find that the
total mass of Pluto and Charon is 1/400th that of the Earth. But they
could not tell how much of this mass was in Pluto and how much in its moon.

Now George Null, William Owen and Stephen Synnott of the Jet Propulsion
Laboratory in Pasadena, California, have used the Hubble Space Telescope
to follow Pluto and Charon while the two worlds circled each other. When
viewed through ground-based telescopes, Pluto and Charon usually look like
one object, but the Hubble Space Telescope can resolve the pair into two
separate images.

The scientists observed Pluto and Charon for 3.2 days, half the orbital
period. The two objects revolve around their centre of mass, which lies
closer to Pluto because it is more massive. The ratio of the distances of
Pluto and Charon from the centre of mass equals the ratio of the masses
of the two objects.

Null and his colleagues found that Charon lies 11 times farther from
the centre of mass than Pluto does, which means that Pluto is 11 times more
massive. For comparison, the Earth is 81.3 times more massive than the Moon.

Astronomers had already calculated the diameters of Pluto and Charon
by observing the two worlds as they repeatedly eclipsed each other from
1985 to 1990. The eclipses revealed that Pluto’s diameter is 2300 kilometres
and Charon’s diameter is 1190 kilometres. These figures were used to deduce
the volume of each object.

Null and his colleagues then determined each world’s density, a quantity
that indicates an object’s composition. They found that Pluto’s density
is 2.1 grams per cubic centimetre. Charon is less dense at 1.4 grams per
cubic centimetre.

Pluto and most moons of the outer planets consist of rock and water
ice in varying amounts. The greater a world’s density, the more rock and
the less ice the body has. For example, Jupiter’s moon Io, whose density
is 3.5 grams per cubic centimetre, is all rock, whereas some of Saturn’s
moons, whose densities are 1.2 grams per cubic centimetre, are nearly pure
water ice.

Pluto’s high density suggests that the planet is about 75 per cent rock
and 25 per cent water ice. But Charon’s lower density indicates that the
moon consists mostly of water ice, with little rock.

These results will constrain theories for the origin of Pluto and Charon.
According to one popular theory, first proposed by William McKinnon of Washington
University in St Louis, Pluto and Charon began their lives as separate worlds.
Charon then hit Pluto, blasting some of Pluto’s surface material into space,
and the two worlds became gravitationally bound. If Pluto’s core was rocky
and its mantle icy, most of the material blasted into space by the collision
would have been ice, accounting for Pluto’s high rock-to-ice ratio today.
Moreover, some of Pluto’s ice may have fallen onto Charon, explaining why
the moon has so much ice.

Null and his colleagues plan to submit their results to The Astronomical
Journal.