ETA CARINAE is a superstar. A moody diva prone to spectacular outbursts, Eta
lies about 7000 light years away in the southern hemisphere constellation of
Carina. The behaviour of this bright blue star has baffled astronomers for
centuries. At times it has been too faint to see with the naked eye, yet for a
while it was one of the brightest stars in the sky (see 鈥淎 history of
violence鈥�). It even exploded spectacularly in the middle of the 19th century,
but somehow managed to carry on shining.
One of the biggest and brightest stars in the Galaxy, Eta is poised uneasily
on the brink of instability. It blazes with violent outbursts of X-rays and is
surrounded by clouds that seem to contain ultraviolet lasers. But it is
beautiful as well as violent鈥攖he Great Eruption of the 1840s left behind
an expanding, strikingly symmetrical nebula in the shape of an hourglass five
hundred times the diameter of our Solar System.
To the frustration of astronomers, Eta鈥檚 outbursts are unpredictable. Unlike
most variable stars it has always seemed truly capricious. Until now, that is.
In the past two years, astronomers thought they had finally divined some method
in Eta鈥檚 madness, changes in its X-ray emissions that might lead to an
explanation of what triggers its extraordinary outbursts. But the strangest star
in the Galaxy has once again taken them by surprise.
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Eta Carinae鈥檚 instability is mainly due to its awesome size. It weighs about
a hundred times as much as our Sun and shines about 4 million times as brightly.
To get some idea of something so colossally brilliant, imagine standing on the
surface of a planet that orbits as far from Eta as the Earth does from the Sun.
Look up, and the star鈥檚 searing blue mass will fill half the sky. But look
quickly, because you will be vaporised in less than a second.
The radiation is even more intense inside the star. And this is the crux of
the problem: Eta is so bright that it is in danger of blowing itself to pieces.
It works like this. Very bright stars are caught in a perpetual tug-of-war. The
outward pressure from the radiation they emit (see 鈥淗eavy light pressure鈥�)
tries to blow them apart, while their gravity strives to make them collapse. For
small, light stars, the inward pull of gravity more than balances the radiation
blowing outwards. But heavier stars are much brighter because they have much
higher pressures and temperatures in their cores, accelerating the nuclear
fusion reactions that make them shine. So for really heavy stars, the radiation
blowing outwards begins to outweigh the effects of gravity. The limit is about
120 times the mass of our Sun. Stars heavier than this simply can鈥檛 exist.
Eta Carinae is a little smaller than this limit. But there are extra factors
pushing it towards catastrophe. Large stars are relatively short-lived, so at
around 3 million years old, Eta is nearing the end. During its lifetime Eta has
lost much of its outermost layers of gas into space as a tremendous stellar
wind. As a result, there is less gravity holding the star together. It hasn鈥檛
grown dimmer though, because its core has become richer in heavy elements that
speed the rate of nuclear fusion.
Worse still, the radiation pressure could be exacerbated by a peculiarity of
ionised iron, which is spread throughout Eta. Within a narrow temperature range
just below 500 000 掳C鈥攚hich occurs some way into Eta鈥檚
interior鈥攊ons of iron are exceptionally opaque to radiation. In 1993,
Richard Stothers and Chao-Wen Chin of NASA鈥檚 Goddard Institute for Space Studies
in New York worked out that this effect could produce a spherical shell of
opaque gas inside Eta and prevent the radiation created in the star鈥檚 core from
reaching its surface. The radiation pressure would build up until it inflated
the gas in the shell, making it tenuous enough to allow the radiation to break
through. As a result, Eta may contain a wraparound 鈥渧oid鈥� in its interior,
separating the super-hot core from outer layers of floating gas. This would
obviously be very unstable.
Radiation pressure may bring Eta to the brink of cataclysm, but does
something else push it over? One clue comes from the shape of the cloud of gas
left over from the Great Eruption. It is an hourglass figure鈥攁 pair of
鈥渙sculating鈥� (kissing) spheres, with a thin, ragged disc slotted in between. The
simple explanation for this shape would be that Eta is not one star, but two. If
one of the stars had a violent outburst, the gas released could escape freely
from above and below the plane in which the two stars orbit. But in the plane
itself, the gas would collide with the orbiting companion star and its wind, and
gradually slow down. The resulting cloud of gas, or 鈥渘ebula鈥�, would have an
hourglass shape.
If Eta is indeed two stars, that could help to explain what triggers the
outbursts. One star鈥攖he primary鈥攚ould probably contain the bulk of
the mass, bringing it close to the instability limit. And its delicately
balanced state could be upset when the two stars approach one another most
closely. The stars in binary systems tend to have highly elongated orbits, and
at the point of closest approach they exert strong tidal forces on each
other鈥攍ike the tides raised in the Earth鈥檚 oceans by the Moon. This would
create a bulge on both stars and help to send the primary over the edge.
Unfortunately, the shape of the nebula isn鈥檛 conclusive, because a rapidly
rotating star could also produce an hourglass shape. During a time of relative
calm, a slow, dense wind blows off into space from the region around the stellar
equator, where the rotation is just great enough to spin material off. Now
imagine an explosion sending gas outwards at high speeds in all directions. Near
the equator, this gas will plough into the sluggish, dense wind and slow down,
but near the poles it will expand faster, blowing bubbles on either side.
This is where the new X-ray findings come in. In 1996, Augusto Damineli, an
astronomer from the University of Colorado at Boulder, was trawling through data
on Eta Carinae鈥檚 past emissions when he noticed something odd. There was a
pattern in the way the visible and infrared emissions had changed over the past
50 years: once every 5.5 years certain wavelengths disappeared and then
reappeared. Damineli realised that the most obvious explanation was that Eta
Carinae is really made up of two stars moving round one another in an elliptical
orbit, one of them periodically eclipsing the other.
If so, could they be interacting with one another tidally to trigger Eta鈥檚
giant eruptions? Sadly, when Damineli calculated the orbits of the two stars, he
discovered that they would be too far apart for this. Still, if Eta were a
binary star, astronomers could use the measured orbits to pin down Eta鈥檚 vital
statistics鈥攕uch as its combined mass, age and brightness鈥攎uch more
accurately. That in turn could help explain why Eta is so close to the edge and
what pushes it over.
Damineli prepared to test his two-star theory. If he was right, every 5.5
years, as the two stars neared the point of closest approach, their winds would
interact more strongly, creating huge shockwaves that heat the gas in the winds
to temperatures of about 60 million degrees. This hot gas would emit X-rays.
Such emissions should increase at first as the stars approach one another. Then
when one star passes behind the other, Damineli reasoned, the X-ray emitting
region where their winds collided would be hidden behind the nearer star鈥檚 dense
wind. So the X-ray emission should drop. When the other star re-emerges, the
X-rays should rise again sharply
(see Diagram). Sure enough, Damineli found
evidence for these dips using old X-ray and radio data. But there wasn鈥檛 enough
detail to confirm his theory.
In particular, the data could not rule out an alternative, single-star
explanation. According to this idea, Eta behaves like a geyser, storing up heat
in an opaque layer, which 鈥渂ubbles鈥� more and more fiercely until the heat can鈥檛
be contained and the star boils over, losing its heat and starting the cycle
again. This sort of event would also produce a sharp fall in X-ray emissions
after the outburst, but a much more gentle rise afterwards.
Happy New Year
With insufficient past data to go on, astronomers looked to the future.
Plotting his cycle ahead, Damineli predicted that there would be another event
near 1 January this year. Astronomers prepared to pay very close attention.
So what happened? First, there was a burst of X-rays, right on time. Damineli
was delighted. 鈥淎fter four years sharing this belief with only a handful of
astronomers, now I see the event progressing on schedule,鈥� he wrote on his
website. But during the first months of this year, as the X-ray emission began
to climb from the predicted trough, some doubts began to emerge.
For one thing, the simple two-star colliding-wind model suggests that the
X-rays should immediately leap back to their previous level as the hidden star
re-emerges. But the X-rays from Eta Carinae have risen only gradually, which
would tend to support the single-star idea. And that wasn鈥檛 all. During the
build-up to this predicted cataclysm, another layer of complexity emerged. A
team led by Michael Corcoran of the Goddard Space Flight Center in Greenbelt,
Maryland, using a satellite called the Rossi X-ray Timing Explorer, found
another period lurking in Eta Carinae鈥檚 X-ray emission. This one was just 85
days long.
How does this period fit in? Some have suggested that there could be a third
star in the system, orbiting the primary in 85 days. Or perhaps a natural period
of the main star鈥攅ither rotation or pulsation鈥攎odulates the stellar
wind. If there is a hotspot on the surface of Eta, rotation would imprint a
spiral pattern on the wind, like a Catherine wheel. Pulsation, on the other
hand, would create concentric spheres of denser wind.
Each of these models makes a different prediction for what should happen to
the 85-day periodicity after the point of closest approach. If the period were
produced by rotation or pulsation, the wavefronts in the wind that these produce
would hit an approaching star and its wind at briefer intervals as they get
nearer (leading to a shorter period), and relatively long ones as it recedes. In
the three-star model, the period should be largely unaffected. The problem is
that none of these predictions matches what actually happened: preliminary
results suggest that the 85-day period is still there, unchanged in length but
delayed by about 10 days. As leading Eta-watcher Kris Davidson of the University
of Minnesota in Minneapolis puts it, 鈥淓ta has repeatedly invented unexpected
ways to vary its X-ray emission. I鈥檓 not even completely sure that it is a
binary system after all.鈥�
Once again, it seems that Eta Carinae has defied astronomers鈥� attempts to pin
it down. Because of the regularity of its 5.5-year period, most researchers are
still rooting for a binary model, but they have lots of work to do. Still, with
the Rossi spacecraft following its X-rays, and the Hubble Space Telescope
probing the nearby gas, they have plenty of tools to throw at the problem.
Whatever the cause of Eta鈥檚 outbursts, the star will one day go too far. It
may lose almost all its mass into space, and end up as an undistinguished,
washed-up white dwarf star. Or, some time in the next 100 000 years or so, its
heart will collapse to create an incredibly dense neutron star, its body will be
blown to pieces and the skies of Earth will be graced by a tremendous supernova.
But maybe Eta, the perennial show-off, won鈥檛 be satisfied even with that. Some
astronomers believe that a few stars end it all in a yet more flamboyant
fashion. In a so-called hypernova, the stellar core collapses directly into a
black hole, and the explosion is a hundred times more energetic even than a
supernova. For such a superstar, perhaps, it would be a fitting exit.
THERE is no recorded observation of Eta Carinae before 1600, so it was
probably scarcely visible. But between about 1600 and 1837, under a variety of
鈥渟urnames鈥� (Argus, Navis and Roburis), Eta makes recorded appearances at many
different brightnesses鈥攏ot because of the crude state of astronomy at the
time, but because of the star鈥檚 changeable nature.
Eta鈥檚 real star turn, however, was on the Victorian stage. In the Great
Eruption, between 1837 and 1856, it shot to fame, going from a run-of-the-mill
鈥渘aked-eye鈥� star to the second brightest in the sky. The only one that seemed
brighter was Sirius鈥攁nd that is a thousand times closer to Earth than
Eta.
There was a lesser eruption in about 1890, and then Eta disappeared from
public view for some time, dimmed by gas and dust thrown out by the Great
Eruption. It has begun to brighten since the 1930s, as the nebula thins, and is
now just visible to the naked eye.
OF ALL the puzzles posed by the cloud of gas surrounding Eta Carinae, one of
the most baffling is the ultraviolet radiation coming from ionised iron: instead
of the two strong emission lines that you would expect, there is only one. In
1996, Sveneric Johansson of the University of Lund in Sweden suggested that
there could be natural ultraviolet lasers in dense clouds near to Eta. Iron ions
would be excited by light from the star, and then stimulated to emit a single
frequency of light by photons emitted from neighbouring ions.
But gas clouds usually churn, with different regions of gas moving relative
to one another. This movement would change the wavelength of light coming from
different parts of the laser (the Doppler effect), shifting it away from the
precise value needed to stimulate more ions to emit. This should scupper the
laser idea, and yet, with no other explanations on offer, it鈥檚 the only show in
town. Perhaps some small regions of gas are unnaturally still? So far, the
nebula is throwing up more questions than it answers.
WHEN you step outside in the daytime, you don鈥檛 expect to get knocked over by
the sunlight. But light does exert some pressure鈥攊t鈥檚 just usually very
small. Each photon carries a little momentum, and when it hits an object it
passes that momentum on. In full sunlight on the Earth, for example, there is a
force equivalent to less than a milligram of weight spread over the exposed side
of your body. Even at the same distance from Eta Carinae, the force would only
be a few million times greater. If you could avoid being vaporised, it would
feel like a gentle breeze.