Space

Astrophile: Cool echoes from galaxy's biggest star

Echoes of a 19th-century spectacle (Image: X-ray: NASA/CXC/GSFC/M. Corcoran et al.; Optical: NASA/STScI)

Astrophile is our weekly column covering curious cosmic objects, from within the solar system to the furthest reaches of the multiverse

Object: Massive star
Mass: 150 suns

Stars like Eta Carinae – the most massive in our galaxy – are so rare, it's sheer luck that we even know this behemoth exists. Kris Davidson of the University of Minnesota in Minneapolis likens the chances of seeing something like Eta Carinae from Earth to the likelihood of walking out of your house in the morning and seeing an 8-foot-tall man walk by: "There are people 8 feet tall, but how often do you meet them?"

Now light echoes are effectively taking us back in time to watch a past outburst of this heavyweight. This might offer clues as to Eta Carinae's explosive future and the nature of such massive, ageing stars in general.

At about 150 times the sun's mass, Eta Carinae is the heaviest known star among the trillions in our galaxy. When it destroys itself in the not too distant future, it could blast thousands of planets with beams of gamma radiation, though Earth itself is fortunately not in the firing line.

Eta Carinae has already been seen nearly blowing itself apart once in recorded history, in the 1840s, when it became the second brightest star in the sky. That outburst was spectacular, but it wasn't until the 20th century that we really learned to appreciate Eta Carinae's awesomeness.

In the latter part of the century, astronomers discovered that it is 8000 light years away, compared with 8 light years for the brightest star in the sky, Sirius. That means it is intrinsically much brighter than any other known star in the galaxy, even when it is not erupting. It owes that brightness to its colossal mass.

Light echoes

Such heavy stars are short-lived, burning through their nuclear fuel quickly only to explode as supernovae. Eta Carinae appears to be nearing this stage: the 1840s eruption may have been a symptom of its advancing age.

If modern instruments had been available back then, the explosion might have provided some clues as to how much mass these massive stars shed towards the end of their lives, which governs whether their remains form a black hole or neutron star. Now, nature has provided the next best thing: echoes of the eruption are allowing us to watch the event all over again.

A team of researchers led by Armin Rest of the Space Telescope Science Institute in Baltimore, Maryland, found the echoes by observing the area around Eta Carinae with the 4-metre Victor M. Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile.

Blue variables

The echoes are light that was emitted during the 1840s eruption, but not directly towards Earth. Instead, it bounced off interstellar dust clouds before heading our way, delaying its arrival here by more than 160 years.

The latecoming light has already revealed one surprise. Its spectra suggest the material involved in the eruption was relatively cool, at about 5000 K. That could change our understanding of what made Eta Carinae explode.

By contrast, material seen erupting from stars called luminous blue variables have reached 7000 K. These eruptions are thought to be caused by the stars' tremendous brightness: their outer layers cannot easily resist the pressure of so much light pushing out at them and sometimes get peeled off and thrown into space, producing the eruptions.

Previously, Eta Carinae was thought to have exploded like this. Now it looks as though some other trigger must have been at work.

Nuclear nudge

One alternative could have been a sudden acceleration of nuclear reactions in the star's core, with the extra energy released nudging the star to throw off its outer layers.

Davidson, who was not involved in the new study, says it would be premature to abandon the light-pressure explanation based on these new observations alone. However, he says further observation of the echoes could help us better understand what the future holds for Eta Carinae and other massive stars.

"We've learned more from Eta Carinae than we have from all the other massive stars combined," Davidson says. "Time and time again it's done something that no one predicted."

Reference: arxiv.org/abs/1112.2210

http://www.newscientist.com/