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In the southern constellation Carina lies a stellar monster. With a mass more than 100
times our Sun’s, Eta Carinae is one of the largest and most luminous star systems in
our Galaxy, shining more than 5 million times brighter than our Sun.
The star is surrounded by the Homunculus nebula, a pair of billowing clouds blasted off in
a 19th century event called "the Great Eruption". Inside is a pair of dueling giants. They blast
each other with winds that collide at millions of kilometers per hour, reaching temperatures
of millions of degrees.
The surrounding Homonculus makes it hard to get a clear look at the central binary. And
that makes it difficult to understand how these stars interact and why they weren’t
destroyed in the Great Eruption.
But thanks to supercomputer simulations, 3D modeling, light echoes from the surrounding
nebula, and new observations from the Very Large Telescope and the Hubble Space Telescope,
astronomers are beginning to understand how Eta Carinae survived its Great Eruption, and
the late stages of a star on the brink of destruction.
Welcome back to Launch Pad, I’m Christian Ready your friendly neighborhood astronomer.
Eta Carinae is a Luminous Blue Variable star, or LBV star. It’s a star that's evolved
off the main sequence and has reached the blue supergiant phase. At least one of the
stars in the system is likely to go supernova relatively soon. And we're going to talk about
that in great detail. But before we get too far into the system, I’d like to thank MagellanTV
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Throughout recorded history, Eta Carinae was known to be a variable star, gradually brighting
since it was first cataloged in the 1600’s. In the 1830’s, Eta Carinae suddenly brightened
to outshine the star Rigel. A few years later, it became the second brightest-star in the
entire night sky. This brightening event came to be known the Great Eruption and would last
for another 18 years before fading from view. We now know that this was caused by the larger
star of the pair expelling huge amounts of gas and dust. This is what created the Homunculus
Nebula that surrounds the system today. As the nebula grew, its thick lanes of dust obscured
Eta Carinae until the mid-1970’s when it could finally be glimpsed in the night sky
once again. By the 1990’s, Eta Carinae had settled into a 5.5 year cycle of brightness
variations that continue to this day.
Around that time, the Hubble Space Telescope made the most detailed image of Eta Carinae
yet taken. It revealed a star enshrouded inside two billowing lobes of gas studded with giant
knots, each about the size of our solar system.
Astronomers realized the Homonculus Nebula's shape and the star's 5 1/2 year variation
could be explained if Eta Carinae was a binary system with its companion star on a highly
eccentric orbit.
However, neither star in the system can be directly observed because of the surrounding
gas and dust. So their properties can only be estimated from careful observations of
the outflowing material.
Still, it’s clear these stars are both monsters. The secondary star is thought to be around
60 times the mass of the Sun, and shines more than 1 million times brighter. That's the
secondary! The primary star's mass is estimated to be 120 solar masses and shines at leas
5 million Suns brighter than the Sun.
However, the system very likely started out as much as 250 solar masses before the Great
Eruption and other outbursts. The two stars are so luminous that the radiation they produce
rips off their surfaces and blows them into space.
These stellar “winds” travel at millions of kilometers per hour, causing the system
to lose a Sun’s worth of mass every 1,000 years.
The winds from the two stars are fierce, but have very different properties. The primary's
wind blows at nearly 1 million mph and is very dense. The companion's wind is six times
faster, but much, much thinner, carrying away about 100 times less material than the primary’s
wind.
This sets up a Viper vs. the Mountain kind of duel. When the stars are far apart, the
winds collide head on, creating a huge cavity surrounded by 50 million degree gas.
This ultra-hot gas makes Eta Carinae a source of X-ray emission.
But as the secondary reaches periastron, it whips around the larger star, carving a tunnel
in the primary’s wind.
From our vantage point outside the system, the x-ray emission shuts off at this point.
Computer simulations reveal the smaller star's faster wind crashes into the tunnel it created.
As it does so, it carves finger-like protrusions into the primary’s slow, dense wind.
The central area where the winds actually collide is comparatively tiny — a thousand
times smaller than the Homunculus Nebula. That’s too small for even the Hubble Space
Telescope to resolve.
But by linking three of the Auxiliary Telescopes at the Paranal Observatory as an interferometer,
astronomers were able to directly observe this region for the first time.
They found the wind has a distinctive fan-shape, carved out when the secondary star was in
front of the primary.
These observations and simulations help us to understand Eta Carinae’s present-day
variability and wind outflow. But understanding how the Homunculus nebula was created in the
19th-century is a different matter.
The star is surrounded by a disk of material, likely created by the stars' throwing material
into the plane of their orbit. When the primary star erupted, the disk channeled the outflow
up and down, funneling about 10 solar masses worth of starstuff in each direction.
To model the Homunculus Nebula, another team measured its spectra in unprecedented detail.
A spectrum is obtained by passing starlight through a narrow slit into a spectrometer.
By measuring the doppler shift of spectral features, astronomers can determine how fast
material in the slit is moving towards or away from us.
The nebula is too large on the sky to be mapped in a single spectrum. Previous efforts with
Hubble used 5 slits, which gave a general idea of the expansion of the Nebula. Then
in 2014, an international collaboration used the Very Large Telescope to lay 100 slits
across the entire length of the nebula.
By measuring the velocities along different parts of the nebula, astronomers built up
a 3D model of the Homunculus. And I just happen to have one right over here!
The model reveals new details that hadn’t been noticed before, particularly in the lobe
expanding away from our view. Holes pierce the lobes at their ends, and the lobes are
carved with dimples and trench-like structures.
But the most intriguing features are these two wings tilted at an angle with respect
to the long axis of the nebula, and also with respect to the orbit of the two stars.
Prior to this model, it wasn’t clear if Eta Carinae’s binary nature played any role
in the formation of the nebula. The primary could have created an hourglass-shaped nebula
on its own. All it needed was a surrounding disk to funnel the outburst along the poles.
But that doesn't explain the deformations, the trenches, and especially the wings.
Chances are that Eta Carinae’s binary nature did play a role in the Great Eruption and
the resulting Homunculus. The question then is, what role exactly?
One idea is that the explosion occurred when the stars were near periastron, that is when
the secondary is closest to the primary. The secondary star could then have deflected some
of the outflowing material into the wings.
Another idea is that the eruption occurred when the two stars were father apart. The
secondary spends most of its time away from the primary, and their winds collide in a
way that forms a fan-shaped shock front. The angle between the two shocks is about 130
degrees, which is about the same angle between the wings.
However, the wings aren’t perpendicular to the hourglass lobes. Today the companion
star orbits the primary around the primary’s equator. But maybe the secondary’s orbit
was tilted when these wings were formed. Perhaps the eruption they were formed in also caused
the stars to exchange angular momentum, causing the secondary’s orbit to align itself with
the primary star’s rotation.
Granted, these are speculations but these are some of the ideas that researchers are
investigating.
And then there’s the question of what actually caused the Great Eruption? Obviously, we can’t
go back in time, but astronomers are doing the next best thing.
Because Eta Carinae lies within the surrounding Carnia nebula, some of its light bounces off
background clouds and is reflected back to us. Because of its finite speed, the light
from eruptions on Eta Carinae are delayed, arriving more than a century after the fact.
It's like a distant voice echoing off a canyon wall.
Astronomers analyzed these light echoes reflecting off different clouds in the Carina nebula.
The spectra showed that the Great Eruption occurred in at least two different stages,
maybe more. At first, the outflow speed was between 150 and 200 km/s (that's 340,000—450,000
mph). Believe it or not, that was the slow outflow. Then it increased to the current
expansion rate of 600 km/s.
But the big surprise came during the second stage of the eruption, which showed the winds
in the lobes reaching ultra fast speeds, but with asymmetric velocities. The winds moving
toward the Earth reached 10,000 km/s (22.3 million mph), but the winds moving away from
Earth were even faster, blasting out at 20,000 km/s!
The energy required to blast that much material off at such a high velocity would have been
more than enough to destroy an ordinary star. That means the Great Eruption was way more
powerful than previously thought. It wasn’t an eruption so much as it was an almost-supernova!
That would account for the sheer energy involved in the eruption, but the lobe on the far side
is moving twice as fast as the one on the near side. And this suggests the explosion
started on the far side of the star before migrating to the near side. As it migrated
around, it lost energy, resulting in the “slower” forward lobe.
So how does something so powerful happen in a binary system without actually destroying
one of the stars in the process? Well the answer may be that Eta Carinae wasn’t always
a binary, but may have started as a triple star system.
In this new model, the two large stars were orbiting each other close in, while a third
star orbited farther out. As the larger star evolved it expanded, losing mass to its companion
which bulked up in response.
Matter from the donor star formed a giant accretion disk around the gaining star's equator,
like water circling a drain.
In response the donor star moved away from the primary. Eventually, it interacted with
the outer third star, sending the third star into a death spiral around the primary.
The result is a supernova-like explosion that gets funneled into a bi-polar outflow by the
surrounding disk. The outflow expands outward in both directions, but the energy is greater
on the far side of the star where the impact occurred. Some of that energy gets lost as
the explosion propagates around to the near side of the star and we get the asymmetric
outflow we see today.
It’s a really interesting model, but even the astronomers who propose it will be the
first to tell you it needs fleshing out. And the way to do that is find and study more
light echoes.
Meanwhile, astronomers used the Hubble Space Telescope to examine Eta Carinae in literally
a new light. By imaging the star in ultraviolet, astronomers mapped out the magnesium blasting
from the star. To their great surprise, the magnesium isn’t where it was expected to
be.
The Great Eruption unleashed powerful shock waves that slammed into slower-moving material
that was thrown off in earlier outbursts. The shock waves compressed the nitrogen gas,
causing it to glow red in this image.
In this new image, magnesium was expected to be found in the same region as the red
nitrogen. Instead, it’s in the region between the outer lobes of the Homunculus and the
nitrogen gas.
The location of this gas is extremely important for understanding how the eruption began.
It suggests that it wasn’t expelled previously with the nitrogen, but rather was blasted
later from the star shortly before the creation of the bipolar lobes.
The magnesium may represent a new event just before the Great Eruption, but more observations
are going to be needed to measure exactly how fast the material is moving and determine
when it was ejected.
Now if you’d like to learn how massive stars like Eta Carinae evolve and die, I have a
video on that here for you to check out.
As always I’d like to thank my patrons for helping to make Launch Pad Astronomy possible,
with a special shoutout to Michael Dowling and Steven J Morgan for their Cosmological
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month, please check out my Patreon page.
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videos. Until next time, stay curious, my friends.
[beep]
The model reveals new details that hadn't been noticed before, particularly in the lobe
expanding away from our view....this is the wrong lobe, okay. This is the right lobe...right?
[beep]