From: Amara Graps (amara@amara.com)
Date: Sun Jul 21 2002 - 06:06:39 MDT
Sun Is Made of Iron, Not Hydrogen
>"Data from NASA's Galileo probe of Jupiter's helium-rich atmosphere
>in 1996 reveals traces of strange xenon gases -- solid evidence
>against the conventional model of the solar system's creation,
>Manuel says."
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Hmm
I sincerely doubt these years that there a 'conventional model'.
>Based on these findings, they concluded that the solar system formed
>directly from the debris of a single supernova, and the sun formed on the
>supernova's collapsed core.
I'm not sure his 'one' supernova assumption is right. Evidence points
to a series of supernovae explosions forming our Local Bubble.
Consecutively? Simultaneously?
(btw, deja-vu this discussion.. last winter here?)
Amara
[Economist.com]
The American Astronomical Society meeting
Fire burn and cauldron bubble
Jan 10th 2002 | WASHINGTON, DC
From The Economist print edition
A cluster of nearby stars may hold the key to two scientific puzzles
NEXT time you find yourself looking at the night sky in spring, make
sure you turn to the constellation of the Scorpion and, if you are far
enough south, especially to the place where it meets the Centaur.
There, a bunch of bright stars is beating a hasty retreat from the
neighbourhood, having wrought havoc even as it left a precious legacy:
starry nights.
Astronomers call these stars the Scorpio-Centaurus OB association (O
and B stars being the most massive in the heavens), or "Sco-Cen" for
short. Because it contains such big, bright stars, and because Sco-Cen
is reasonably close by astronomical standards, it makes a fine sight in
the sky. These are also the reasons why it is the prime suspect in some
violent goings-on a few million years ago. Large stars lead short lives
that often end in an explosion known as a supernova. And a supernova in
the solar system's neighbourhood is just what astronomers have been
looking for, in order to explain why the sun is camped in the middle of
a galactic wasteland.
This wasteland is known as the Local Bubble, and it has been a puzzle
ever since its discovery in the 1970s. The bubble is an absence of
matter: the interstellar gas within a radius of about 300 light years
of the sun is far less dense than the galactic average. It is also very
hot-about 1m°C-though its tenuous nature means that it would not scorch
you.
Astronomers owe a lot to the Local Bubble. Without it, starlight would
have a much tougher job reaching the earth, so fewer stars would be
visible. In addition, even those that were would tend to appear a dull,
red colour, since the more exciting frequencies of light would be
absorbed in transit. On top of that, one end of the bubble pokes out of
the plane of the galaxy, providing human astronomers with a window on
to the rest of the universe that less favoured alien races might envy.
What is less clear is why the bubble is there in the first place. The
presumption is that it was created by a supernova. The shockwave from
this would have carried almost everything before it, and what was not
swept away would have been heated up enormously.
Supernovae, however, tend to leave traces. The shockwave itself is
usually visible, and the collapsed remnant of the star frequently forms
a rapidly spinning object known as a pulsar. But the Local Bubble is a
quiet place: just thin, hot gas and stars, almost all of them small
ones like the sun that are up to no particular mischief. No shockwave,
in other words. And no pulsar.
A possible solution to this puzzle has just been announced at the
winter meeting of the American Astronomical Society (AAS) in
Washington, DC, by Jesus Maiz Apellaniz, a researcher at the Space
Telescope Science Institute in Baltimore. All the evidence, according
to Dr Apellaniz, points to stars now deceased that were once members of
Sco-Cen.
Exploding the bubble
To persuade his peers that he is right, Dr Apellaniz needs to show
three things: that Sco-Cen is the kind of star cluster which produces
supernovae; that it was at the scene of the crime when the Local Bubble
was formed; and that a supernova exploded at the right time. Putting
together research by himself and several other scientists, he thinks he
can do all three.
To start with, there is almost no way that a star group like Sco-Cen
could have failed to produce supernovae in the past. The O and B stars
after which it is named are being formed continuously out of the dust
and gas it contains. Such stars burn so profligately that they can
survive for only a few million years before they run out of fuel. When
that happens, they collapse. If the collapse is big enough it results
in an explosion-in other words, a supernova.
Evidence that this has actually happened comes from a European Space
Agency satellite called Hipparcos, which is recording the positions of
stars with such precision that repeated measurements can track their
movements. Three Dutch astronomers, Ron Hoogerwerf, J.H.J. de Bruijne
and Tim de Zeeuw, investigated a number of "runaway" stars that
Hipparcos has located near Sco-Cen. These stars are racing away from
the cluster at speeds of up to 200km a second.
Such runaways are thought to have been members of binary systems that
were blown apart when one of the partners became a supernova. The
Hipparcos measurements make it possible to "rewind" the film, and show
that pairs and even triplets of these stars were extremely close
together in Sco-Cen several million years ago. That confirms that
supernovae were produced in the cluster.
The question is, could any of those explosions have caused the Local
Bubble? It appears that they could, for the whole Sco-Cen association
is on the move. It is sailing through space away from the sun, and by
"hindcasting" its journey, it looks as though parts of it may have been
as close as 100 light years from the sun a few million years ago.
Knowing the birth-rate of large stars in associations of the Sco-Cen
type, and knowing how long such stars live, Dr Apellaniz concludes that
Sco-Cen may have produced as many as six supernovae during the 10m
years it was in the sun's neighbourhood. That would have been enough to
blow away the interstellar gas and create a bubble.
New brooms sweep clean
Besides explaining the Local Bubble, this interpretation also sheds
light on the recent history of the earth. Three years ago, some German
scientists discovered two thin, but globally distributed, layers of
sediment in the ocean floor. One was about 5m years old, the other 2m
years old. Both were enriched in an unusual isotope of iron that
calculations suggest would be produced in large quantities by stars
exploding as supernovae.
That timing is suggestive: 2m years ago, life on earth underwent a
convulsion. Many species suddenly became extinct in a transition
between the epochs known as the Pliocene and the Pleistocene.
In the past, palaeontologists who study such mass extinctions have
tended to dismiss nearby supernovae as possible causes. Although such
stellar explosions produce a lot of radiation of a sort that would be
bad for life on earth, it has always been assumed that these
radioactive particles would be slowed down and dispersed by the
magnetic fields that are associated with interstellar gas.
However, as Dr Apellaniz and his colleagues Narciso Benitez and Matilde
Caņelles of Johns Hopkins University argued in a second paper at the
AAS meeting, two supernovae in quick succession might be far more
dangerous. The first would sweep a path clean for the radioactivity of
the second, and the result would be a blizzard of subatomic particles
that would, among other things, destroy the ozone layer that protects
terrestrial life from damage by ultraviolet light.
The first forms of life to suffer would be plankton, soon followed by
species that eat plankton, such as marine molluscs. And those are
exactly the species that fell first in the Pliocene/Pleistocene
transition. Other changes would follow, on land as well as at sea, with
species that survived the transition, such as humanity's ancestors,
being able to take advantage of the reduction in competition. The
stellar broom that swept the skies clean for human astronomers to
marvel at might thus have had a hand in the evolution of those
astronomers' ancestors in the first place.
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