Star Endured Unique Explosion That Didn't Destroy
Written by Nancy Atkinson
There's 'smoked but didn't inhale,' 'promised but didn't deliver,' and now there's 'exploded but didn't destroy.' Eta Carinae, the galaxy's biggest, brightest and perhaps most studied star after the sun, appears to be driven by an entirely new type of stellar explosion that is fainter than a typical supernova and does not destroy the star. Astronomer Nathan Smith proposes that Eta Carinae's historic 1843 explosion was, in fact, an outburst that produced a fast blast wave similar to, but less energetic than, a real supernova. This well-documented event in our own Milky Way Galaxy is probably related to a class of faint stellar explosions in other galaxies recognized in recent years by telescopes searching for extragalactic supernovae.
"There is a class of stellar explosions going off in other galaxies for which we still don't know the cause, but Eta Carinae is the prototype," said Smith, a UC Berkeley postdoctoral fellow.
Eta Carinae (η Car) is a massive, hot, variable star visible only from the Southern Hemisphere, and is located about 7,500 light years from Earth in a young region of star birth called the Carina Nebula. In 1843, observers saw Eta Car brighten immensely. Visible now is the resulting cloud of gas and dust, known as the Homunculus nebula, wafting away from the star. A faint shell of debris from an earlier explosion is also visible, probably dating from around 1,000 years ago.
But these shells of gas and dust are moving relatively slowly at 650 kilometers per second (1.5 million miles per hour) compared to the blast shell of a regular supernova.
Presumably blown off by the star's fierce wind, the shells of gas and dust are moving slowly - at speeds of 650 kilometers per second (1.5 million miles per hour) or less - compared to the blast shell of a supernova. But new observations by Smith show filaments of gas moving five times faster than the debris from the Homonuculus, which would equal speeds of materials accelerated fast blast wave of a supernova explosion.
The fast speeds in this blast wave could roughly double earlier estimates of the energy released in the 1843 eruption of Eta Carinae, an event that Smith argues was not just a gentle surface eruption driven by the stellar wind, but an actual explosion deep in the star that sent debris hurtling into interstellar space. In fact, the fast-moving blast wave is now colliding with the slow-moving cloud from the 1,000-year-old eruption and generating X-rays that have been observed by the orbiting Chandra Observatory.
"These observations force us to modify our interpretation of what happened in the 1843 eruption," he said. "Rather than a steady wind blowing off the outer layers, it seems to have been an explosion that started deep inside the star and blasted off its outer layers. It takes a new mechanism to cause explosions like this."
If Smith's interpretation is correct, supermassive stars like Eta Carinae may blow off large amounts of mass in periodic explosions as they approach the end of their lives before a final, cataclysmic supernova blows the star to smithereens and leaves behind a black hole.
"Looking at other galaxies, astronomers have seen stars like Eta Carinae that get brighter, but not quite as bright as a real supernova," he said. "We don't know what they are. It's an enduring mystery as to what can brighten a star that much without destroying it completely."
Source: EurekAlert
Filed under: Astronomy
September 10th, 2008 at 5:16 pm
Nathan, Nathan, Nathan. I am so envious of you!!
Not only do you already have umpteen first-author papers, but now your very own Nature paper.
Fascinating result!! Keep up the good work.
I will not that if the pulsed pair instability mechanism is the correct interpretation, it would need to work on very different timescales. It's been over 150 years since the Great Outburst of Eta Car, but SN 2006jc went bang just two years after its own LBV eruption.
September 10th, 2008 at 5:18 pm
The way I always understood it, a Supernova was the death of a star, either forming a black hole or just ejecting its mass in all directions.
A Nova (not super) was a stellar explosion which greatly increased the brightness of the star briefly, but did not kill the star/expend all fuel.
September 10th, 2008 at 5:24 pm
A nova is the thermonuclear ignition of accreted matter on a white dwarf's surface. Kind of like pouring gasoline on a hot stove burner…
September 11th, 2008 at 12:54 am
Further to Don's comment, as the matter is accreted on to the surface it has to come from beyond the white dwarf. As a result novae only occur in binary systems, and close binaries at that!
September 11th, 2008 at 7:57 am
Is an infalling hot Jupiter big enough to trigger one of these?
I doubt it, but . . .
September 11th, 2008 at 12:08 pm
@John: Not by long shot! Anyway, Eta is an ultramassive binary star, it's pretty much impossible that planets were able to form there.
September 12th, 2008 at 6:30 pm
Perhaps at such large stellar masses, the pressure pushing out on the star is not always enough to keep some of the mass from collapsing into a microscopic black hole. The black hole would be very short-lived and explode within the star creating the stellar explosion.
September 12th, 2008 at 7:21 pm
Correction - scale the size of the black hole up from microscopic to whatever size it would have to be to create an explosion that fits the observable data.
September 14th, 2008 at 4:40 pm
Maybe Eta Carinae is actually a binary massive star. 1 with 80+ the mass of the Sun and the other 30+ times the mass of the Sun and they orbit each other in extremely eccentric orbits with a period of 165 years or so and in every orbit, each star's outer layers hits the outer layers of the other star, which releases a lot of matter into space, while the gravity between the 2 stars tug on each other star's core, which releases extreme amounts of energy into space from their cores into space making it appear as an explosion smaller than a supernova every 165 years or so. Or less years every time. Possibly when they nearly collide, their gravity doesn't let the other companion get far as it did last time.
September 15th, 2008 at 6:57 am
@RetardedFishFrog: The conditions needed to create a black hole can only oocur upon total core collapse - and that destroys the star completely. Furthermore, a black hole which, upon explosion, would release enough energy to create this effect would be stable for much longer than the lifetime of the universe (and hence the star…)
@Richard Diaz: Eta IS a binary system! This was shown more than ten years ago. The secondary seems to be a B type star. Not sure what the mass is, maybe 10 solar masses or so. The orbit is 5.2 years. They form a colliding wind binary, and, yes, the orbit is eccentric, each time the stars approach each other, the X-ray signature and the spectrum change. So, you had some good ideas, but I fear the secondary is not responsible for the explosive behaviour. You probably need much larger tidal forces for that, for example when a massive star passes really close to a supermassive black hole, it may explode.