y
Paul Ricon
Photo:
The first stars left their mark on the cosmic infrared
background.
Astronomers have detected a faint glow
from the first stars to form in the Universe, Nature journal
reports. This earliest group of stars, called Population
III, probably formed from primordial gas less than 200 million
years after the Big Bang. These objects cannot be seen by any
present or planned telescopes. Nasa scientists detected the
stars from the imprint they have left on the general glow of
infrared radiation dispersed throughout the cosmos.
This glow, which is composed of radiation from
stars past and present, is known as the Cosmic Infrared
Background (CIB). The observations used in the latest study were
made by the Infrared Array Camera (Irac) on the US space
agency's Spitzer Space Telescope.
Photo:
Spitzer - Infrared telescope.
The US space agency’s Spitzer telescope is the fourth
in a series of large space telescopes. The $2bn facility will
fill gaps in astronomical knowledge left by the other three
observatories: Hubble (launched in 1990), Compton (1991) and
Chandra (1999).
The results present
the first evidence for cessation of the so-called cosmic Dark
Ages. The term, coined by the English Astronomer Royal, Sir
Martin Rees, refers to the period in cosmic history when
hydrogen and helium atoms had formed but had not yet had the
opportunity to condense and ignite as stars. Blazing into
existence: The first stars after the Dark Ages were
probably composed solely of hydrogen, helium and a little
lithium. After blazing into existence, their lives would have
been intense and short, burning up their hydrogen in only a
few million years. Energy radiated by the Population III stars
must have contributed to the CIB; the problem for researchers
is that many more much younger stars have also contributed. In
order to isolate a signal from the earliest stars, Alexander
Kashlinsky and his colleagues at Nasa's Goddard Space Flight
Center in Maryland carefully removed the contributions from
other stars and galaxies to the CIB. "It took us a year to
remove the signal sufficiently accurately in order to convince
ourselves there was something out there that could not be
explained by anything else we could think of," Dr Kashlinsky ,
he said.
The team discovered clustering in the
distribution of infrared light over and above that expected from
the combined effect of known galaxies. In fact, the total
contribution of foreground galaxies is small compared with the
residual signal ascribed by the authors to the primordial stars.
Massive stars: In order to contribute this large signal,
the primordial stars must have been extremely massive, in the
region of hundreds of solar masses, Dr Kashlinsky explained. "It
seems these first stars were quite unlike those we see today.
They were huge thermonuclear furnaces; few and far between, but
they burned ferociously because they were so massive," Dr
Kashlinsky explained.
The distribution of cosmic infrared light
suggests these stars were clustered together, which might be
partially explained if they were around only for a short time -
perhaps a few hundred million years. It is believed that these
earliest stars manufactured the metals that would become
important for later populations of stars. However, other
researchers wondered whether the analysis had missed, for
example, foreground galaxies with low luminosities. Richard
Ellis, of the California Institute of Technology (Caltech), in
Pasadena, said that "even a minor blunder in removing these
foreground signals might lead to a spurious result". He added:
"A number of untested assumptions involved in allowing for
unobserved galaxies could represent a weakness in the analysis."
Quick
facts:
Infrared telescope
Launch mass: 950 kg
Mirror size: 85 cm
Coolant: 360 litres helium
Mission length: 2.5-5 years
Instruments: Infrared Array Camera, Infrared
Spectrograph, Multiband Imaging Photometer. These will operate
at just a few degrees above absolute zero (-273 C).
Titan
– A Place Like Home?
Over
a billion kilometres away, Saturn's largest moon,
Titan, holds tantalising clues to how life began here
on Earth.
In the most
ambitious and expensive interplanetary space mission
of all time, the Cassini-Huygens spacecraft made a
seven-year trek across the Solar System to attempt
first contact with the Earth-like moon of Titan by
landing a probe on its unseen surface. The first close
up images of Saturn and its many moons were taken in
the early 1980s by the Voyager One Deep Space Probe.
One moon stood out from all the rest, the mysterious
moon of Titan. Unlike any moon that had ever been
seen, it had a thick almost Earth-like atmosphere. It
was also shrouded in a thick orange haze which
prevented Voyager from seeing down to the moon's
surface. Scientists knew they had to go back. Launched
in 1997, the Cassini-Huygens spacecraft was the result
of a unique transatlantic $3.2 billion collaboration
between NASA and the European space agencies. Steered
from NASA's JPL mission control in Pasadena
California, the craft took seven years to reach
Saturn. It took a long slingshot route via Venus
twice, the Earth and Jupiter to pick up enough speed
to reach its final destination. When it finally
arrived in July 2004, the spacecraft had to carry out
a very dangerous manoeuvre and pass between Saturn's
rings in order to get into orbit around the giant
planet. Even the tiniest grain of dust could have
ripped through the spacecraft and destroyed the
mission. On Christmas Day 2004, the European-built
Huygens probe was finally released from the Cassini
mothership, ready to descend to Titan.
The probe's trajectory had to be
absolutely spot on, as without any engines even a slight
misjudgement could not be corrected later and would mean
Huygens missing its target altogether. January 14 2005.
The Huygens probe finally reached Titan's upper
atmosphere. Mission control had now transferred to ESA
in Darmstardt, Germany, but all the scientists could do
was sit and wait, as the probe was running on automatic.
For any chance of success, the probe's heat shield had
to protect the craft from the fierce temperatures of
re-entry, and its three parachutes had to deploy
correctly in sequence to slow its descent. Amazingly,
long before they expected to hear from Huygens, the
probe's faint carrier signal was picked up on Earth by
the massive Robert C Byrd radio telescope at Greenbank
in West Virginia. Not much stronger than a mobile phone,
and travelling over a billion kilometres through space,
the signal was too weak to carry any real data, but at
least they knew the probe had survived entry and was now
under parachute. Some hours later, the scientific data
finally started coming through, relayed via the orbiting
Cassini. To their horror, one of the vital data-streams
had not been switched on. Fortunately most of the data
was coming through on the single channel, but crucially
half the images were lost.
After years of waiting, Titan was
finally revealed. With Huygens built to sniff and taste
the atmosphere on its way down, it discovered it was
similar in many ways to that of the Earth in its
infancy, four billion years ago. Titan's chemistry is
still a long way from what we see as 'living', yet it
was found to contain a rich cocktail of organic
carbon-based chemicals, thought to be important as the
precursors to life. Now visible beneath the impenetrable
orange haze, Titan appears to look a lot like Earth. The
images beamed back from over a billion kilometres away
show lake beds, river channels, gulleys and canyons. But
these river channels are gouged not by water, but by a
rain of liquid methane. The surface itself is not made
of rock, but of solid ice, and Huygens' landing site was
strewn with small round ice pebbles, lying in a bed of
icy sand grains. Although home to a somewhat cold alien
chemistry, in many respects Titan is driven by exactly
the same geological and meteorological processes that
shape and contour our own planet. Titan is certainly a
place like home.
