杏吧原创

IT鈥橲 BEEN dark for a long time, ever since the fireball of the big bang faded
away. But now, all over the Universe, the stars are coming out. These are no
ordinary stars. Burning 10 million times brighter than our own Sun, they race
through their lives at breakneck speed and die spectacularly. In the first and
greatest of all firework displays, space is rocked by a rash of titanic
explosions, each brighter than a hundred galaxies. Only there aren鈥檛 any
galaxies. Not yet. Welcome to the age of the megasuns.

Evidence is mounting that the first stars were monsters, hundreds of times
the mass of the Sun. Now, astronomers have begun to produce realistic
simulations of these megasuns, illuminating their strange lives and deaths. The
research offers solutions to two intergalactic puzzles. And it reveals that
these monsters may have left behind gigantic fossils.

The biggest stars in our sky are only about a hundred times as massive as the
Sun. Theorists believe that anything larger would be unstable. The greater a
star鈥檚 mass, the hotter it is, and the greater the pressure from internal
radiation. When that pressure nearly balances the force of gravity, adding just
a little heat causes a star to expand a lot.

In the 1970s, Immo Appenzeller of the Landessternwarte in Heidelberg,
Germany, showed that this makes stars of more than a hundred solar masses
unstable. Their inner regions, where nuclear energy is generated, pulsate,
making the nuclear burning occur in spurts. Each spurt is like a nuclear
explosion, ejecting matter from the star about once a day. 鈥淧eople thought these
pulsations would reduce a 200 to 300-solar-mass star to perhaps 10 solar masses
by the time it died,鈥� says Stan Woosley of the University of California at Santa
Cruz.

They were assuming a composition like that of present day stars. Today, in
our galaxy, stars form from gas that is a concoction of different elements.
Hydrogen and helium left over from the big bang are spiced with heavier atoms
such as carbon, oxygen and iron, that were forged in the cores of other stars
and spewed into space when those stars exploded.

But the first stars formed when the Universe contained few atoms heavier than
hydrogen and helium. 鈥淲hat nobody seems to have realised is that instabilities
are much weaker if there are no heavy elements,鈥� says Woosley. Heavy elements
provide a far larger target for radiation bouncing around inside the star,
making the effective pressure much larger.

The first to realise that this could make a crucial difference to ultraheavy
stars were Isabelle Baraffe, at the 脡cole Normale Sup茅rieure in Lyon, and
Alex Heger, also at the University of California, Santa Cruz. In a paper to be
submitted to The Astrophysical Journal, they describe simulations of
simple hydrogen and helium stars which aren鈥檛 prone to those destabilising
pulsations. So in the early Universe, monster stars of perhaps 300 solar masses
could have survived.

Better still, the lack of heavy elements could have encouraged the formation
of these megasuns. Today, molecules such as carbon monoxide (CO) and hydrogen
cyanide (HCN) can cool down interstellar gas clouds to 20 kelvin or so. They
have many energy levels that can be excited by molecular collisions, even at
quite low temperatures, so they can radiate away the thermal energy of the gas.
The more heat is removed, the lower the pressure of the gas and the easier it is
for gravity to begin crushing the cloud. So heavy elements allow even relatively
small gas clouds to collapse into stars.

In the early Universe, the only molecule around was molecular hydrogen. This
has no energy levels that are excited at low temperatures, and so is less
effective than other molecules at radiating away heat. Tom Abel at Harvard
University, Greg Bryan at MIT and Michael Norman at the University of Illinois
have found that the collapse process could have occurred only in more massive
clouds with stronger gravity.

Stars that time forgot

Heavy elements also reduce the loss of mass in the late stages of collapse.
This is not a well understood process, but we know that during star formation,
the central condensation of gas can become so hot that its radiation blows away
the rest of the in-falling material. In today鈥檚 Universe, the radiation can push
against heavy elements and dust grains made of heavy elements, but when there
was only hydrogen and helium to push against less of the in-falling material
would have been blown away. Stars of this ancient era could therefore have grown
fat more easily. Abel thinks they could have ended up 300 times the mass of the
Sun.

So what would these monsters have been like? In a separate paper submitted to
The Astrophysical Journal, Woosley, Heger and their Santa Cruz
colleague Chris Fryer, describe a new computer simulation that brings these
extinct megasuns to life.

The model stars start out like the gas that emerged from the big
bang鈥�76 per cent hydrogen and 24 per cent helium. But despite this bland
makeup, they soon start to fuse their hydrogen into helium via the so-called CNO
cycle, in which carbon, nitrogen and oxygen act as catalysts. Where did these
heavy elements come from? The enormous weight bearing down on the stellar core
creates temperatures so enormous that helium nuclei fuse into carbon. 鈥淵ou only
need about one carbon in every 10 million atomic nuclei to run the CNO cycle,鈥�
says Heger. 鈥淭hese stars easily produced that amount.鈥�

With core temperatures of hundreds of millions of degrees, nuclear reactions
proceed at an extraordinary rate, generating huge quantities of energy. A
typical megasun has a surface temperature of 100 000 K and shines as brightly as
10 million Suns. If such a star were to replace Alpha Centauri, the nearest star
to the Sun, it would appear 50 times as bright as the full Moon. 鈥淲e live in the
dull twilight of the Universe,鈥� says Heger. 鈥淲e can only imagine what it would
have been like in the blazing dawn.鈥�

The brilliance of megasuns may finally explain one of the most puzzling
features of our Universe. Until about 300 000 years after the big bang, the
Universe was filled with radiation that bounced around off free electrons and
protons. But when these particles combined to form hydrogen, the radiation broke
free of matter and became today鈥檚 cosmic microwave background.

The mystery is that the gas between the galaxies is now ionised once more,
even at the farthest limits probed by our telescopes鈥攁 red shift of 5 or
6, corresponding to a time when the Universe was less than a sixth of its
present age.

So what re-ionised the Universe? Some scientists think it was light from
quasars, the luminous cores of some galaxies where gas spirals into huge black
holes via a white-hot accretion disc. But Abel among others believes it was
intense ultraviolet radiation from the prodigiously luminous first stars. They
would have formed as far back as a red shift of 20, when the Universe was less
than a twentieth of its present age. And a relatively small number of megastars
could have had a powerful effect. For their combined ultraviolet output to have
completely ionised the Universe, only a tiny fraction of the gas in the Universe
need have gone into forming them. One ten-thousandth could have been enough.

Stars that pump out 10 million times the light of the Sun might seem
impressive. However, their violent deaths, after about 3 million years, defy the
imagination. The model stars envisaged by Woosley, Fryer and Heger prove to be
so stable that they end their lives with almost all their huge original mass.
This makes their explosions rather interesting, to say the least.

The astronomers looked at the deaths of megasuns with two masses, 250 and 300
times that of the Sun. These masses were not picked at random. It turns out that
stars either side of 270 solar masses respond very differently to a crucial
event in their lives called the 鈥減air instability鈥�.

As heavy elements are formed by fusion reactions, they settle into the core
of the star. Their weight compresses the core, heating it up. After a few
million years it becomes so hot鈥攁bout a billion degrees鈥攖hat the
gamma-ray photons inside it reach a critical energy at which they begin to
create electron-positron pairs. Because massive stars are supported against
gravity mainly by the pressure of the photons they contain, when some of this
radiation turns into electrons and positrons the pressure goes down. With its
support suddenly removed, the core collapses, shrinking catastrophically at up
to 1000 kilometres per second. This compression heats it up so much that it
ignites carbon, oxygen and neon in an orgy of nuclear burning.

In a 250 solar mass star, the nuclear energy unleashed blows the star apart
in a 鈥減air-instability supernova鈥�, or 鈥渉ypernova鈥�. So violent is the explosion
that no remnant is left behind. 鈥淲e鈥檙e talking about a star that for a month or
so burns more brilliantly than a hundred present-day supernovae, or a hundred
normal galaxies,鈥� says Woosley.

These hypernovae could explain another puzzle鈥攖he traces of heavy
metals scattered among intergalactic gas, even at the greatest distances our
telescopes can reach. These elements are sparse compared with their
concentrations in our Galaxy, but still too abundant to have been made in the
big bang. According to Woosley and Heger, a hypernova makes and spews out huge
quantities of heavy elements鈥攗p to 50 solar masses of iron alone. If there
were enough of them to ionise the Universe, there were also enough to explain
that early contamination.

For a 300 solar mass star, the end is even more astonishing. Freyer and his
colleagues believe the end-point could be an object that briefly pumps out more
energy than 10 billion galaxies. For in a star of 300 solar masses, the runaway
burning is not powerful enough to overcome the gravity of the star. Instead, the
core shrinks catastrophically to form a 30-solar-mass black hole. But this isn鈥檛
the end.

What delays things is rotation. Fryer, Woosley and Heger assume that these
stars would have been spinning as fast as massive stars do today. If so, much of
a collapsing megastar鈥檚 substance has too much spin to fall into the hole
immediately. Instead, it forms a super-hot, swirling accretion disc around the
hole, like the discs inside quasars.

Quasars consume up to about 10 solar masses of matter a year in this way,
making them brilliant sources of light鈥攐ften more powerful than their
surrounding galaxy. But the young black hole in one of Woosley鈥檚 simulations
sucks in about 100 solar masses of material in just 10 seconds, some 100 million
times faster than a quasar. Fleetingly, one of these objects would shine
brighter than all the galaxies in today鈥檚 Universe combined.

You might think these gargantuan explosions would be easy to see today, but
they鈥檙e not. Almost all the energy would have gone into jets of matter stabbing
out wards at nearly the speed of light. These jets would have slammed into the
thin gas around the star, generating gamma radiation. Because the Universe has
expanded so much since the age of the megasuns, the wavelength of this radiation
would have been stretched out somewhat, red-shifting it into X-rays.

X-rays are absorbed by the Earth鈥檚 atmosphere, and today鈥檚 orbiting
observatories are probably too insensitive or have the wrong spectral range to
see these fantastically distant flashes. But a mission called EXIST, to be flown
on the International Space Station in about 5 years鈥� time, might be able to spot
them.

Or the first sighting could come from NASA鈥檚 Next Generation Space Telescope,
the planned successor of the Hubble Space Telescope. The NGST might be able to
see clusters of these stars, although that would be at the limit of its
sensitivity. The explosions of the smaller kind of megasun are a better bet.
鈥淗ypernovae should be easy to see if we happen to point in the right direction,鈥�
says John Mather, project scientist on the NGST.

Gravitational waves could also afford us a glimpse of a megasun. According to
Fryer, when a black hole forms from a megasun it is initially elongated, and
rotates around its short axis鈥攍ike a cigar spinning on a table. This
should create copious gravitational waves: disturbances in space-time predicted
by Einstein鈥檚 theory of relativity.

Physicists hope to snare gravitational waves in an experiment called the
Laser Interferometer Gravitational-Wave Observatory. In LIGO, laser beams will
precisely measure the lengths of two pairs of tunnels, one pair under Louisiana
and the other in Washington state. Fluctuations in these lengths should reveal a
passing gravitational wave.

Unfortunately, the waves from a dying megasun would be weak by the time they
reach Earth. 鈥淭he problem is the distance,鈥� says Fryer. 鈥淟IGO won鈥檛 be sensitive
enough to detect such a weak source, but gravitational wave detectors further
into the future might.鈥�

Detecting megasuns directly is not the only way to prove their existence,
however. They should have left fossils. Megasuns formed before the galaxies we
see around us today, probably in smaller gas clouds between a thousandth and a
hundredth the mass of the Milky Way. These 鈥減roto-galaxies鈥� later collided to
make today鈥檚 big galaxies.

If the megasuns were numerous enough to ionise the Universe鈥攗sing a ten
thousandth of all the available gas鈥攊t is possible to estimate how many
would have formed from a mass of gas equivalent to the Milky Way. The answer is
about a hundred thousand. Not all of them would have left relics though: Woosley
points out that the hypernovae blew themselves to pieces. But some of those
hundred thousand should have left behind black holes weighing as much as a
hundred Suns鈥攖oo big to have been created by modern stars. They should be
distinctive.

Even so, they would be difficult to unearth. If they aren鈥檛 sucking in any
matter, they will be dark. But perhaps we will be lucky enough to catch a
hundred-solar-mass black hole as it gobbles a nearby gas cloud, or witness the
effects of an isolated black hole as it passes in front of another star, bending
the starlight with its gravity.

And if megasuns once ruled the Universe, they must have left us another
legacy, hidden under our noses. Their by-products would account for a thousandth
of the heavy elements in today鈥檚 Universe, including, for example, the oxygen
and iron in your own body. So if you want to see the first generation of stars,
hold your hand up to the light鈥攁s much as 100 grams of your own flesh and
blood may have been forged in megasuns at the dawn of time.