The Death March of Betelgeuse

A Cloudy Precursor to a Violent Stellar End

You can’t see it in the evening skies right now, but the bright, old star Betelgeuse that makes up one of the shoulders of Orion, the Hunter (visible beginning late in the year), is giving up more of its secrets even as it continues down the road of old age and eventual disruption by a supernova explosion. Betelgeuse is a red supergiant star.  It’s so big that if you placed it in our solar system in place of the Sun, its “surface” would be out at the orbit of Jupiter.  But, Betelgeuse’s influences stretches far out beyond that.  Why?  You have to understand something about this big old star. It’s big. It’s old. And when big, old stars get older, they shed much of their material out to space in an intense stellar wind. In the final step of aging, such stars can lose as much as one solar mass (that is, the amount of mass it takes to make the Sun) in just about 10,000 years.

For Betelgeuse, scientists describe this mass loss as two processes:  the first occurred when huge plumes of gas began to snake their way out from the star into nearby space; the second one involves giant bubbles in the star’s atmosphere. Those bubbles move up and down through the atmosphere quite vigorously, similar to boiling water in a pan.

This picture of the dramatic nebula around the bright red supergiant star Betelgeuse was created from images taken with the VISIR infrared camera on ESO’s Very Large Telescope (VLT). This structure, resembling flames emanating from the star, forms because the behemoth is shedding its material into space. The earlier NACO observations of the plumes are reproduced in the central disc. The small red circle in the middle has a diameter about four and half times that of the Earth’s orbit and represents the location of Betelgeuse’s visible surface. The black disc corresponds to a very bright part of the image that was masked to allow the fainter nebula to be seen. Courtesy ESO/P. Kervella.

How do we know that this is what Betelgeuse is doing?  For one thing, astronomers have been able to image the plumes of material blowing away from the star. They used an instrument called VISIR (an infrared-sensitive camera) attached to the European Southern Observatory’s Very Large Telescope in Chile to measure the extent of the clouds of material coming off Betelgeuse. They found an interesting structure to the clouds (see picture at left). It almost looks like flames licking out from the star.  They’re not fire, but warm streams of  “star stuff” blowing away from Betelgeuse.

The astronomers’ observations show the plumes that are close to the star are probably connected to structures in the outer nebula now imaged in the infrared with VISIR. The nebula cannot be seen in visible light, as the very bright Betelgeuse completely outshines it.

Notice that the clouds of material are irregularly shaped, not symmetrical. This also tells astronomers that Betelgeuse hasn’t been losing its material at the same rate in all directions. In other words, the loss is not symmetrical.  This is indirect evidence that the bubbles in the atmosphere and their plumes are responsible for the nebula’s appearance.

So, what is this material that’s flowing away from Betelgeuse?

Based on the observations, it’s most likely that this stellar stuff is composed of silicate and alumina dust. This is the same material that forms most of the crust of the Earth and other rocky planets.

This is kind of interesting. Think about it.  It means that at some time in the distant past, the silicates that make up Earth were formed by a massive (and now extinct) star similar to Betelgeuse. It’s interesting to see evidence for that now, but in a star that is at least several hundred light-years away from us (possibly farther).

Now, you’re probably wondering when Betelgeuse will finally go supernova.  A good question. In cosmic timekeeping, it could be anytime, meaning anytime in the next million years. Stars die on lengthy timelines.  And, its distance will keep us from knowing that it happened until a few hundred years after the initial explosion. So, if Betelgeuse is, oh, say 500 light-years away (and we don’t know for sure how far away it is, so I’m using that number as an example), and it blows up tomorrow, we won’t see that flash in our skies until the year 2511.  We’ll probably see an influx of neutrinos before that, emanating from the direction of Betelgeuse. Eventually, sky observers will see it start to get very large and bright in the sky, and once the initial flash dies out, they’d start to see a colorful, glowing nebula where Betelgeuse used to be.  It would be a bright source in radio and x-rays, a new “thing” to study in the annals of violent star death.

For now, however, astronomers are marking the progression of Betelgeuse’s “change of life” events by observing it as much as they can, in as many regimes of light as they can. Those continue to tell the story of this star’s inevitable death march.

Flying Through an Icy Saltwater Spray

Cassini Probes Subsurface Oceans of Enceladus

As it swooped past the south pole of Saturn's moon Enceladus on 14 July 2005, Cassini acquired high resolution views of this puzzling ice world. Courtesy NASA/ESA/Space Science Institute

Take a look at that moon. It’s called Enceladus and it orbits the planet Saturn. It looks all serene and quiet, in this view. And, probably looks cold and lifeless.

But, in reality, this little world has a hidden ocean beneath that icy surface. And, as on Earth, when we we think “ocean”, we often think “life”.

How can such a cold-looking place support an ocean? Doesn’t it have to be… um… warm?  Well, as it turns out, a confluence of gravitational forces (between Saturn, Enceladus and neighboring moons)  keep the subsurface water just warm enough to exist as in a liquid-slushy state. Couple that with tectonic actions that crack the surface and allow water under pressure to escape out to space, and suddenly Enceladus is way more than a quiet, icy world. It’s a place that spouts plumes of ice crystals out to space.

And, since the Cassini Solstice mission spacecraft is there studying those worlds, and occasionally sails through these plumes, we get an instant sample of the interior saltwater reservoirs of Enceladus after it has sprayed out to space. The plumes originate from the ‘tiger stripe’ surface fractures at the moon’s south pole, and create the faint E-ring, which traces the orbit of Enceladus around Saturn.

Jets of icy particles bursting from Saturn's moon Enceladus are shown in this NASA/ESA/ASI Cassini image taken on 27 November 2005. This and other recent images of Enceladus backlit by the Sun show the fountain-like sprays of the fine material that tower over the south polar region. This image was taken looking more or less broadside at the 'tiger stripe' fractures observed in earlier Enceladus images. It shows discrete plumes of a variety of apparent sizes above the limb of the moon. Credits: NASA/JPL/Space Science Institute

Today, NASA and ESA released information about the sampling runs that the Cassini spacecraft has been making through the icy plumes that the spacecraft first discovered in 2005. During three passes, done in 2008 and 2009, the mission’s Cosmic Dust Analyzer measured the composition of icy grains that had just been ejected from Enceladus. Close to the moon, the plume particles are large and rich in salt. In fact, about 99 percent of the total mass of solid material ejected through these plumes is very salty — but  most of it falls back to the surface of this icy world.

So, how salty are we talking about?  It appears that the particles that are the saltiest have what scientists call an “ocean-like” composition. That tells us most of the ice that gets shot out in the plumes comes from liquid salt water inside Enceladus.

Scientists studying these plumes have come up with this scenario to explain the plumes: deep underneath Enceladus’ surface, perhaps 80 km down, there is a layer of water between the rocky core and the icy mantle (structural  layers). It is kept liquid by tidal forces generated by Saturn and some neighboring moons, as well as by the heat generated by radioactive decay (from elements that exist inside the moon and have been there since it formed).

Salt in the rock dissolves into the water, which accumulates in liquid reservoirs beneath the icy crust. When the outermost layer cracks open, the reservoir is exposed to space. The drop in pressure causes the liquid to evaporate, with some of it flash-freezing into salty ice grains: together these create the plumes.

Roughly 200 kg of water vapor gets lost every second as these plumes stream out to space.  According to the team’s calculations, the water reservoirs must have large evaporating surfaces, otherwise they would easily freeze over, and that would stop the plume action dead.

This is one of those findings that simply amazes me about what we are able to learn from our science missions in space. If I walked up to you on the street and asked you, “How can we figure out whether or not a frozen world out near Saturn has subsurface oceans, and how much salt would they have, if they exist?”  you would look at me blankly. It’s not on your daily radar, but that doesn’t make such a discovery unimportant.  If you stop to think about why we’d even want to know such a thing, it wouldn’t take long to make a connection between water, salt water, warm salt water and… life.  And, finding out places in our solar system where life could survive (and this is NOT a story about whether or not life exists there — that’s a whole other issue), tells us a lot about places where it DOES survive and exist. Mainly, here on Earth.  We know of extreme places on Earth that have similar environments to Enceladus — and life exists in those Earthly habitats.  So, now we know of at least one other place in the outer solar system where the conditions to harbor life mimic those on Earth.  Kinda makes Enceladus seem all warm, fuzzy, and neighborly, all of a sudden.  Not that life is there… but, the cradle of life is possible there.