A primordial star at the
outer edges of our Milky Way galaxy upsets current theories of star formation in the universe. The
star simply shouldn't exist since it lacks the materials astronomers have long
thought necessary for low-mass stars to form, scientists say. When Lorenzo Monaco of the European Southern Observatory in Chile and colleagues examined the elemental composition of the
oddball star, prosaically named SDSS J102915+172927 (image
below), they discovered that it has a mass smaller than that of the Sun, and is
probably more than 13 billion years old.
"This star has the
composition that is the nearest that has been found up to now to the big bang
composition," says Piercarlo Bonifacio of the Paris Observatory, France.
The low concentration of
chemical elements heavier than hydrogen and helium suggests it is the most
primitive star ever discovered, yet the exact ratio of these heavier elements
suggests it is younger. Much, much younger.
"In some sense it is a
perfectly normal star, but it is different because it's in a very low metal
range," Monaco says. The relationship between a star's age and its
elemental composition stems from the way the early universe evolved.
The first stars are thought
to have condensed out of the hot soup left over from the big bang and contained
only hydrogen, helium and a trace of lithium. Most were giants tens of times
more massive than the sun, that quickly exploded as supernovas spewing elements
from carbon to iron, which subsequent generations of stars incorporated. The
process occurred again and again, with younger generations of smaller stars
acquiring larger fractions of heavier elements. Which is how our Sun eventually
formed.
Until now, the universe
seemed to agree. Astronomers had found only three stars with very low amounts
of heavier elements. They were low-mass, and oxygen and carbon dominated the
traces of heavier elements, which meant they passed the carbon-oxygen threshold
needed to form a low-mass star – despite having a very low concentration of heavier elements
overall.
But SDSS J102915+172927 is
different and a mystery: it's composed almost entirely hydrogen and helium,
making it look like one of the very first in the universe. When Monaco and
colleagues used two spectrographs at the Very Large Telescope in Chile to examined its elemental composition, they found it had
the lowest content of heavier elements ever seen yet – 4.5 millionths that of
the sun.
But similar to modern
stars, its oxygen and carbon levels do not dominate over the other heavier
elements. This means there is not enough carbon and oxygen overall to meet the
critical threshold needed to form a low-mass star. According to the theory,
this star should not have been able to form. One theory is that that the star
is indeed near-primordial and that its nursery was cooled interstellar dust
rather than carbon and oxygen.
It's also possible that
low-mass, low-metal stars like this one could be detritus from giant stars'
birth, suggests Abraham Loeb of Harvard University
according to New Scientist.
Image credit at top of page: With thanks to Corbis
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