pace is called ‘space’ for a
reason. The gaps between the
stars are incredibly empty –
far more sparsely populated than any
vacuum we can create here on Earth.
Astronomers call this area the
interstellar medium (ISM). And though it
may be diffuse, it’s thought to make up
at least 10 per cent of the visible mass of
our Galaxy. Stars sit embedded in the
ISM, but are not isolated from it.
“Stars are born inside the interstellar
medium,” says Mikako Matsuura from
the University of Cardiff. “It’s like an
ecosystem.”
The Voyagers have been able to study
the birth of stars by recording the distinctive patterns
of ultraviolet light that are associated with star
formation, which are known as Lyman alpha
emissions. We haven’t been able to see this radiation
in our own Galaxy up till now, because the Sun’s
emissions cloud them out. But now that the Voyagers
are on the edge of heliosphere – the area
where the influence of Solar wind is
stopped by the ISM – they have
been able to detect it, helping us
map out previously
unobserved regions of star
formation in the Milky Way.
Throughout their lives,
stars turn lighter elements
into heavier ones through
the process of nuclear
fusion. Then when stars
die, they send those heavy
elements back out into the
interstellar medium. This
exchange of material ultimately
governs how quickly a galaxy
uses up its supply of gas.
Exploring this eternal feedback loop
allows us to learn more about stellar evolution.
The death of massive stars enriches galaxies with
elements such as carbon, oxygen and iron, which
changes the abundance of those elements over time.
“As the enrichment proceeds, the fraction of stars
ejecting carbon-rich molecules and dust particles
decreases,” says Patrick Roche, from the University of
Oxford. Given that we’re a carbon-based life form, this
means that the material suitable for living things like us
is gradually decreasing.
Voyager 2 has been instrumental in the study of
supernovae – huge stars that explode upon reaching
the end of their lives. When the supernova SN1987Aexploded in the Tarantula Nebula around 163,000
light-years from Earth in 1987, Voyager 2 was
quickly swivelled round for a closer look. Since then,
further observations have shown that the area
continues to evolve.
“Not only has the ejected material expanded and
cooled, the mass of dust and molecules has
grown quite markedly,” says Roche. In
other words, we’re observing the
process of enrichment in action.
Studying how stars form out
of the ISM also tells us
something about how
common our Sun and Solar
System are, compared to
other solar systems in the
Universe. And it turns out
ours is actually quite
unusual. “Our view of the
balance between high mass
stars and low mass stars has
shifted,” says Roche.
The latest observations
suggest that the ISM is filled with a
lot more low mass objects than we’d
realised. “Not only is our Galaxy teeming with
several hundred billion stars,” says Roche, “There
are almost as many brown dwarfs [an object larger
than a planet but smaller than a star] and planetary
mass objects out there, too.”
With the two Voyager probes rapidly approaching
the end of their lives, Matsuura and Roche have
both turned to the new Atacama Large Millimeter
Array (ALMA) – a bank of 66 radio dishes spread out
in the bone-dry Chilean desert.
“ALMA really makes a difference,” says Roche.
“For the first time, we have the sensitivity to look
at the ISM in detail.”FORMATION
OF STARS AND
GALAXIES
By studying what matter there is in the
interstellar medium, scientists hope to
learn more about how stars are born
Supernova
SN 1987AS
SCIENCE