What happens when a star behaves like it exploded, but it’s still there?
About 170 years ago, astronomers witnessed a major outburst by Eta
Carinae, one of the brightest known stars in the Milky Way galaxy. The
blast unleashed almost as much energy as a standard supernova explosion.
Yet Eta Carinae survived.
An explanation for the eruption has eluded astrophysicists. They
can’t take a time machine back to the mid-1800s to observe the outburst
with modern technology.
However, astronomers can use nature’s own “time machine,” courtesy of
the fact that light travels at a finite speed through space. Rather
than heading straight toward Earth, some of the light from the outburst
rebounded or “echoed” off of interstellar dust, and is just now arriving
at Earth. This effect is called a light echo. The light is behaving
like a postcard that got lost in the mail and is only arriving 170 years
later.
By performing modern astronomical forensics of the delayed light with
ground-based telescopes, astronomers uncovered a surprise. The new
measurements of the 1840s eruption reveal material expanding with
record-breaking speeds up to 20 times faster than astronomers expected.
The observed velocities are more like the fastest material ejected by
the blast wave in a supernova explosion, rather than the relatively slow
and gentle winds expected from massive stars before they die.
Based on this data, researchers suggest that the eruption may have
been triggered by a prolonged stellar brawl among three rowdy sibling
stars, which destroyed one star and left the other two in a binary
system. This tussle may have culminated with a violent explosion when
Eta Carinae devoured one of its two companions, rocketing more than 10
times the mass of our Sun into space. The ejected mass created gigantic
bipolar lobes resembling the dumbbell shape seen in present-day images.
The results are reported in a pair of papers by a team led by Nathan
Smith of the University of Arizona in Tucson, Arizona, and Armin Rest of
the Space Telescope Science Institute in Baltimore, Maryland.
The light echoes were detected in visible-light images obtained since
2003 with moderate-sized telescopes at the Cerro Tololo Inter-American
Observatory in Chile. Using larger Magellan telescopes at the Carnegie
Institution for Science's Las Campanas Observatory and the Gemini South
Observatory, both also located in Chile, the team then used spectroscopy
to dissect the light, allowing them to measure theejecta’s expansion
speeds. They clocked material zipping along at more than 20 million
miles per hour (fast enough to travel from Earth to Pluto in a few
days).
The observations offer new clues to the mystery surrounding the
titanic convulsion that, at the time, made Eta Carinae the
second-brightest nighttime star seen in the sky from Earth between 1837
and 1858. The data hint at how it may have come to be the most luminous
and massive star in the Milky Way galaxy.
“We see these really high velocities in a star that seems to have had
a powerful explosion, but somehow the star survived,” Smith explained.
“The easiest way to do this is with a shock wave that exits the star and
accelerates material to very high speeds.”
Massive stars normally meet their final demise in shock-driven events
when their cores collapse to make a neutron star or black hole.
Astronomers see this phenomenon in supernova explosions where the star
is obliterated. So how do you have a star explode with a shock-driven
event, but it isn’t enough to completely blow itself apart? Some violent
event must have dumped just the right amount of energy onto the star,
causing it to eject its outer layers. But the energy wasn’t enough to
completely annihilate the star.
One possibility for just such an event is a merger between two stars,
but it has been hard to find a scenario that could work and match all
the data on Eta Carinae.
The researchers suggest that the most straightforward way to explain a
wide range of observed facts surrounding the eruption is with an
interaction of three stars, where the objects exchange mass.
If that’s the case, then the present-day remnant binary system must
have started out as a triple system. “The reason why we suggest that
members of a crazy triple system interact with each other is because
this is the best explanation for how the present-day companion quickly
lost its outer layers before its more massive sibling,” Smith said.
In the team’s proposed scenario, two hefty stars are orbiting closely
and a third companion is orbiting farther away. When the most massive
of the close binary stars nears the end of its life, it begins to expand
and dumps most of its material onto its slightly smaller sibling.
The sibling has now bulked up to about 100 times the mass of our Sun
and is extremely bright. The donor star, now only about 30 solar masses,
has been stripped of its hydrogen layers, exposing its hot helium core.
Hot helium core stars are known to represent an advanced stage of
evolution in the lives of massive stars. “From stellar evolution,
there’s a pretty firm understanding that more massive stars live their
lives more quickly and less massive stars have longer lifetimes,” Rest
explained. “So the hot companion star seems to be further along in its
evolution, even though it is now a much less massive star than the one
it is orbiting. That doesn’t make sense without a transfer of mass.”
The mass transfer alters the gravitational balance of the system, and
the helium-core star moves farther away from its monster sibling. The
star travels so far away that it gravitationally interacts with the
outermost third star, kicking it inward. After making a few close
passes, the star merges with its heavyweight partner, producing an
outflow of material.
In the merger’s initial stages, the ejecta is dense and expanding
relatively slowly as the two stars spiral closer and closer. Later, an
explosive event occurs when the two inner stars finally join together,
blasting off material moving 100 times faster. This material eventually
catches up with the slow ejecta and rams into it like a snowplow,
heating the material and making it glow. This glowing material is the
light source of the main historical eruption seen by astronomers a
century and a half ago.
Meanwhile, the smaller helium-core star settles into an elliptical
orbit, passing through the giant star’s outer layers every 5.5 years.
This interaction generates X-ray emitting shock waves.
A better understanding of the physics of Eta Carinae’s eruption may
help to shed light on the complicated interactions of binary and
multiple stars, which are critical for understanding the evolution and
death of massive stars.
The Eta Carinae system resides 7,500 light-years away inside the
Carina nebula, a vast star-forming region seen in the southern sky.
The team published its findings in two papers, which appear online Aug. 2 in The Monthly Notices of the Royal Astronomical Society.
The Hubble Space Telescope is a project of international cooperation
between NASA and ESA (European Space Agency). NASA's Goddard Space
Flight Center in Greenbelt, Maryland, manages the telescope. The Space
Telescope Science Institute (STScI) in Baltimore, Maryland, conducts
Hubble science operations. STScI is operated for NASA by the Association
of Universities for Research in Astronomy in Washington, D.C.
Credits:
Illustration: NASA, ESA, and A. Feild (STScI)
Science: NSF and AURA
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Contacts
Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Nathan Smith
University of Arizona, Tucson
520-621-4513
nathans@as.arizona.edu
Armin Rest
Space Telescope Science Institute, Baltimore, Maryland
410-338-4358
arest@stsci.edu
Credits:
Illustration: NASA, ESA, and A. Feild (STScI)
Science: NSF and AURA
Related Links
This site is not responsible for content found on external links
- The science paper by N. Smith et al.
- The science paper by N. Smith et al.
- University of Arizona's Release
- Gemini Observatory's Release
Contacts
Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Nathan Smith
University of Arizona, Tucson
520-621-4513
nathans@as.arizona.edu
Armin Rest
Space Telescope Science Institute, Baltimore, Maryland
410-338-4358
arest@stsci.edu
