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.
