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On the Light of the First Stars
The following points are made by Piero Madau (Nature 2006 440:1002):
1) New work reports the detection of copious high-energy gamma-ray =
emission from two "blazars" -- a class of active galaxy -- around 2 =
billion light years from Earth. This observation indicates that such =
radiation can travel largely unimpeded through the Cosmos, and implies =
that the infrared glow of the first stars in the Universe and their =
remnants is fainter than previous measurements had led us to believe. If =
true, that could influence our ideas of how and when the first =
structures in the Universe evolved.
2) The formation of structure in the Universe is believed to proceed =
hierarchically, with smaller galaxies merging, through the action of =
gravity, to build more massive ones. But the timing and sequence of the =
events through which the very first galaxies and stars formed remain =
largely unknown. According to current theories, the first dwarf galaxies =
hosted metal-free stars over a hundred times more massive than the Sun. =
These stars shone intensely for only a few million years and then either =
blew themselves apart in gigantic supernova explosions, or collapsed to =
form the first massive black holes.
3) Astronomers have long been rummaging through the Universe for =
tell-tale signs of these dramatic beginnings. When the first stars =
ignited, they emitted large numbers of photons at ultraviolet =
wavelengths. These photons "reionized" the surrounding atomic hydrogen =
gas that had formed as the Universe cooled. Recently, astronomers using =
NASA's Wilkinson Microwave Anisotropy Probe (WMAP) reported the latest =
detection of photons produced soon after the Big Bang. Their data show =
that these "cosmic microwave background" photons became polarized =
(tending to oscillate in only one direction perpendicular to their line =
of travel) by scattering on free electrons in the early Universe. The =
level of polarization allows the era of reionization to be pinpointed to =
some 400 million years after the Big Bang, when the Universe was just 3% =
of its present age.
4) So how much of the background light that we see comes from the first =
stars? As the Universe aged and expanded, part of the ultraviolet =
radiation emitted by these stars was absorbed again by re-formed atomic =
hydrogen. Lower-energy ultraviolet light escaped this fate, but was =
stretched to longer, redder wavelengths. Therefore, although the early =
stellar populations were twinkling so long ago that current telescopes =
cannot detect them, their combined energy output is recorded in diffuse =
light that reaches Earth in the near-infrared region of the =
electromagnetic spectrum, at wavelengths of a few micrometers. Resolving =
this infrared glow is, however, a daunting task, because many other =
celestial sources -- among them older stars in closer galaxies, active =
galactic nuclei known as quasars, and the bright foreground sources in =
the Milky Way and the Solar System -- emit radiation at similar =
wavelengths.
Full Text from ScienceWeek
http://scienceweek.com/2006/sw060512-3.htm
Posted by
Robert Karl Stonjek
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<DIV>
<DIV><FONT size=3D5><STRONG>On the Light of the First =
Stars</STRONG></FONT></DIV>
<DIV><BR>The following points are made by Piero Madau (Nature 2006=20
440:1002):<BR><BR>1) New work reports the detection of copious =
high-energy=20
gamma-ray emission from two "blazars" -- a class of active galaxy -- =
around 2=20
billion light years from Earth. This observation indicates that such =
radiation=20
can travel largely unimpeded through the Cosmos, and implies that the =
infrared=20
glow of the first stars in the Universe and their remnants is fainter =
than=20
previous measurements had led us to believe. If true, that could =
influence our=20
ideas of how and when the first structures in the Universe =
evolved.<BR><BR>2)=20
The formation of structure in the Universe is believed to proceed=20
hierarchically, with smaller galaxies merging, through the action of =
gravity, to=20
build more massive ones. But the timing and sequence of the events =
through which=20
the very first galaxies and stars formed remain largely unknown. =
According to=20
current theories, the first dwarf galaxies hosted metal-free stars over =
a=20
hundred times more massive than the Sun. These stars shone intensely for =
only a=20
few million years and then either blew themselves apart in gigantic =
supernova=20
explosions, or collapsed to form the first massive black =
holes.<BR><BR>3)=20
Astronomers have long been rummaging through the Universe for tell-tale =
signs of=20
these dramatic beginnings. When the first stars ignited, they emitted =
large=20
numbers of photons at ultraviolet wavelengths. These photons "reionized" =
the=20
surrounding atomic hydrogen gas that had formed as the Universe cooled.=20
Recently, astronomers using NASA's Wilkinson Microwave Anisotropy Probe =
(WMAP)=20
reported the latest detection of photons produced soon after the Big =
Bang. Their=20
data show that these "cosmic microwave background" photons became =
polarized=20
(tending to oscillate in only one direction perpendicular to their line =
of=20
travel) by scattering on free electrons in the early Universe. The level =
of=20
polarization allows the era of reionization to be pinpointed to some 400 =
million=20
years after the Big Bang, when the Universe was just 3% of its present=20
age.<BR><BR>4) So how much of the background light that we see comes =
from the=20
first stars? As the Universe aged and expanded, part of the ultraviolet=20
radiation emitted by these stars was absorbed again by re-formed atomic=20
hydrogen. Lower-energy ultraviolet light escaped this fate, but was =
stretched to=20
longer, redder wavelengths. Therefore, although the early stellar =
populations=20
were twinkling so long ago that current telescopes cannot detect them, =
their=20
combined energy output is recorded in diffuse light that reaches Earth =
in the=20
near-infrared region of the electromagnetic spectrum, at wavelengths of =
a few=20
micrometers. Resolving this infrared glow is, however, a daunting task, =
because=20
many other celestial sources -- among them older stars in closer =
galaxies,=20
active galactic nuclei known as quasars, and the bright foreground =
sources in=20
the Milky Way and the Solar System -- emit radiation at similar=20
wavelengths.</DIV>
<DIV><BR>Full Text from ScienceWeek<BR><A=20
href=3D"http://scienceweek.com/2006/sw060512-3.htm">http://scienceweek.co=
m/2006/sw060512-3.htm</A></DIV>
<DIV> </DIV>
<DIV>Posted by<BR>Robert Karl Stonjek</DIV></DIV></BODY></HTML>
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