Supernova!

A Quarter-Century Perspective on 1987a

Supernova 1987A, in the Large Magellanic Cloud, a nearby galaxy. Astronomers in the Southern hemisphere witnessed the brilliant explosion of this star on Feb. 23, 1987. Shown in this NASA/ESA Hubble Space Telescope image, the supernova remnant, surrounded by inner and outer rings of material, is set in a forest of ethereal, diffuse clouds of gas. This three-color image is composed of several pictures of the supernova and its neighboring region taken with the Wide Field and Planetary Camera 2 in Sept. 1994, Feb. 1996 and July 1997. Courtesy Hubble Heritage Team (AURA/STScI/NASA/ESA).

It had to have been quite an exciting thing for Ian Shelton and Oscar Duhalde when they first saw a brightening star on a photographic plate that hadn’t been there night before.  Or, for Albert Jones of New Zealand, and Rob McNaught in Australia, who saw the same brightening and must have wondered “What??!”.  In Chile, Ian stepped outside the Las Campanas Observatory in Chile to visually check that area of the sky. Sure enough, there was a hugely bright star in the Large Magellanic Cloud that wasn’t that bright the night before.  All three observers had discovered the supernova of the century, named Supernova 197a.  It was the last explosive gasp of the dying blue supergiant star Sanduleak -69° 202 (called the “progenitor star”), and an eye-opener for scientists studying supernovae, particularly a type called “core collapse” or Type II.

When massive stars like the one that died to form Supernova 1987a come to the ends of their lives, they have basically run out of fuel to consume in their cores.  Stars begin by fusing hydrogen to helium in their cores. The result is heat and light.  Eventually the star runs out of hydrogen as fuel, so it begins to fuse helium, then carbon, and so forth, until it gets to iron.  At that point, fusing iron takes more energy than the process can put out, and that’s when the fusion action stops. Dead. And, there’s no way that the core can support the mass of the layers above it. So, it collapses.  The outer layers collapse, too, and when they hit the core, they rebound out, forming a huge shock waves that blows everything but the core out into space. That’s what we detect as a supernova.

Hubble images show the sequence of ring expansion around Supernova 1987a. Courtesy Mark McDonald via Creative Commons Share-Alike License.

Supernova 1987a was immediately surrounded by an expanding ring of debris.  Astronomers immediately began looking for that ring, and eventually the Hubble Space Telescope took images and data of it a few years later.  Today, 25 years after the first detection, astronomers are still watching the debris expand. As it does, it collides with material (gas and dust clouds) that the star shed earlier in its death process. When the shock wave and expanding debris make contact with that material, everything lights up.

Supernova 1987a has given astronomers new insight into the types of stars that become Type II supernovae.  For one thing at the time of Supernova 1987a’s discovery,  blue supergiants were not considered likely supernova candidates for a variety of reasons. Yet, here was one exploding in a supernova. So, astronomers had to go back and re-examine their ideas and theories about these kinds of high-mass stars.

For one thing, the progenitor star, Sanduleak -69° 202, just wasn’t on people’s radar as a possible supernova candidate. It didn’t show any hints that it was about to blow itself up. That raises a lot of questions about what we know of high-mass stars and their death cycles.

A composite image of supernova 1987a taken 20 years after the explosion was first detected. Data came from NASA's Chandra X-ray Observatory and Hubble Space Telescope. The outburst was visible to the naked eye, and is the brightest known supernova in almost 400 years. This shows the effects of a powerful shock wave moving away from the explosion. Bright spots of X-ray and optical emission arise where the shock collides with structures in the surrounding gas. These structures were carved out by the wind from the destroyed star. Hot-spots in the Hubble image (pink-white) now encircle Supernova 1987A like a necklace of incandescent diamonds. The Chandra data (blue-purple) reveals multimillion-degree gas at the location of the optical hot-spots. These data give valuable insight into the behavior of the doomed star in the years before it exploded. Credit: X-ray: NASA/CXC/PSU/S.Park & D.Burrows.; Optical: NASA/STScI/CfA/P.Challis

The progenitor star was a very compact and blue; not the kind of star to explode like this. So, there had to be another influence. It turns out there was more than one star involved; this system was a binary. One idea is that both the progenitor star and its companion were engulfed in an envelope of material.  The companion may have dissolved in some way, and that affected the progenitor star, and helped send it down the road to supernova-hood.  There are other explanations, and current and ongoing studies of the supernova remmants and the immediate neighborhood may help solve the mystery of why a blue supergiant exploded as it did.

Once the explosion DID occur, aside from the shock wave and light, there was also a huge burst of neutrinos — fast-moving particles that whiz across space.  One expert estimated that 10⁵⁷neutrinos were generated by the explosion, speeding away in all directions. A few of them hit Earth and were detected by the Kamioka experiment in Japan, and by detectors in Cleveland and the former Soviet Union.

All in all, only 19 neutrinos were detected from 1987a, but they told astronomers a story of core-collapse inside a massive star. They also suggest that a neutron star formed in the wake of the core collapse of the supernova 1987a progenitor star. As of today, that neutron star has yet to be observed. There are a number of reasons for that, including the formation of a black hole at the same site.  Astronomers are still looking.

So, 25 years after the appearance of Supernova 1987a, there’s still something to study.  The continued expansion of the shock waves and debris rings into the surrounding material in interstellar space will provide much data about the material and those interactions. The search for the neutron star (or whatever’s left of the progenitor star), continues. And, astronomers continue to use this event to bolster and tweak theories about massive stars and their ultimate ends. It’s been a fascinating quarter-century, and the data continues to flow.  No doubt Hubble Space Telescope and ESA’s Herschel Space Observatory will continue to watch this object, as will the other facilities (such as Gemini Observatory) around the world. It will likely be a target for James Webb Space Telescope. So, stay tuned for new images and data to mark the 25-year mark of this cosmic event. Supernova 1987a might have exploded, but it’s not dead yet.

It Was 82 Years Ago

The Rise of the Dwarf Planets

A Hubble Space Telescope image of Pluto (central object) and its four largest moons, Hydra (upper left), Charon (lower left), Nix (lower right), and P4 (upper right). Courtesy NASA/ESA/STScI.

February 18th is the 82nd anniversary of the discovery of Pluto, the dwarf planet.  The find was made in 1930 by an observer at Lowell Observatory in Arizona by the name of Clyde Tombaugh.  He had spent months searching through and comparing photographic plates of the sky, looking for a possible new planet. His discovery was confirmed, and the name Pluto was bestowed on March 24, 1930.  I had the pleasure to meet Clyde at a conference some years ago, when he spoke enthusiastically about his work to uncover this distant, frozen world.

Pluto is classified as a dwarf planet — which means it’s a special class of planet, much as white dwarfs are special classes of stars, and some galaxies are termed “dwarfs” based on the characteristics that differentiate them from spiral, elliptical, and irregular galaxies.

One of the fascinating things (among many) about Pluto is that its discovery really opened up a new phase of solar system exploration, resulting the discovery of more dwarf planets  in the outer solar system.

Granted, we’ve done quite a bit of solar system exploration since Clyde’s momentous discovery.  We’ve sent probes to most of the other planets, and studied them with ground- and space-based telescopes.  But, until recently, we didn’t have the technical wherewithal to do more than study Pluto from Earth (or Earth orbit, with Hubble Space Telescope, for example).  That changed when the New Horizons spacecraft was launched in 2006 on an voyage of exploration of the outer solar system.

New Horizons will arrive at Pluto in 2015.  It will study the planet’s atmosphere, surface characteristics, and its nearest moons.  After that, it will continue out to other outer solar system objects — in fact, its larger mission is to study the Kuiper Belt, a region of space that extends out from the orbit of Neptune and in which Pluto orbits .  It’s really the gateway to all the outer solar system worlds, including Pluto.

I mentioned that astronomers have found other icy worlds out in Pluto’s domain, and beyond. Eris is the most massive known dwarf planet (so far), and orbits the Sun out well beyond Pluto.  It’s an icy world roughly the size of Plut0. Then, there are Makemake, Haumea, Charon, Orcus, Quaoar, and Sedna.  They’re all smaller and more distant than Pluto, but there’s no doubt they’re worlds in their own right. Undoubtedly others are out there, making trans-Neptunian space a sort of new frontier.  This is why I see Pluto’s discovery as momentous. So, in celebration of Pluto Discovery Day, I raise a toast to Clyde Tombaugh — whose ashes are aboard the New Horizon spacecraft bound for Pluto space.  Not only did he discover a dwarf planet, but he also opened the gates to discoveries in a sector of the solar system once thought empty and barren.  It’s a bigger solar system than we thought, folks, and we have visionaries like Clyde to thank for helping us figure that out.

Attention, Luminous Blue Variable Fans!

More Outburst Data from Eta Carinae

There’s an unstable star out there that’s on the brink of destruction. It lies some 7,500 light-years away, embedded in a star-forming region called the Carina Nebula. The star itself is called Eta Carinae (Eta Car, for short), and it’s actually a double star system. Some 170 years ago, observers noticed that it was growing very bright. Over time, it became the second-brightest star in the sky.

This unexpected brightening came to be known as the “Great Eruption”, and astronomers of the time watched it with great interest — but didn’t have much in the way of sophisticated instrumentation to really dig into the object to tell what was going on there.  Today, modern telescopes are showing us what really happened during the Great Eruption.

The story goes like this:  the more massive member of the duo — a type of star called a luminous blue variable — began blasting out huge amounts of its own mass. Over the period of 20 years that it was seen to be erupting (from 1837 to 1858), this heaving star lost more than 20 solar masses of material.  Much of that “star stuff” can still be seen in a double-lobed cloud surrounding the system.

These images reveal light from a massive stellar outburst in the Carina Nebula reflecting off dust clouds surrounding a behemoth double-star system. The color image at left shows the Carina Nebula, a star-forming region located 7,500 light-years from Earth. The massive double-star system Eta Carinae resides near the top of the image. The star system, about 120 times more massive than the Sun, produced a spectacular outburst that was seen on Earth from 1837 to 1858. But some of the light from the eruption took an indirect path and is just now reaching our planet. The light bounced off dust clouds (the boxed region at the bottom of the image, indicating an area of space that lies about 100 light-years away ) and was rerouted to Earth, a phenomenon called a light echo. The image was taken in February 2000 by the U.S. National Optical Astronomy Observatory's Curtis Schmidt Telescope at the Cerro Tololo Inter-American Observatory (CTIO) in Chile. The three black-and-white images at right show light from the eruption illuminating dust clouds near the doomed star system as it moves through them. The effect is like shining a flashlight on different regions of a vast cavern. The images were taken over an eight-year span by the U.S. National Optical Astronomy Observatory's Blanco 4-meter telescope at the CTIO. Credit: NASA, NOAO, and A. Rest (Space Telescope Science Institute, Baltimore, Md.)

Eta Car (the LBV) is a massive dying star.  Such stars do not make up the majority of stellar systems in our galaxy, so of course, astronomers are quite interested in what sort of death process Eta Car will go through. Unlike our Sun, which will sort of gently swell to become a red giant (and lose much of its mass in a less-explosive manner), Eta Car will likely go out in a huge cataclysm called a supernova.  Some astronomers suggest it could be such a catastrophic event it would be termed a “hypernova.”

However it blows, Eta Car’s passing will afford astronomers with a ringside seat to stellar mass destruction. The first thing they’ll detect when it blows is a gamma-ray burst that could affect our communications satellites.  After that, they’ll be busy cataloguing the process of the explosion and the ring of debris that will be rushing out to space.  Some astronomers have suggested that this explosion could happen anytime from the next few years to a few million years from now.  Regardless of when it blows, it’s not likely to hurt us much on Earth, since the rotational axis of the system is pointed away from us.

Hubble Space Telescope has been observing Eta Car for a couple of decades now, watching subtle changes in the cloud surrounding the stellar pair, and analyzing the mixes of gas and dust in that cloud. The most recent observations actually pinpoint an echo of the light from the Great Eruption bouncing off more distant parts of the clouds.   The observations of the light echoes mark the first time astronomers have used spectroscopy to analyze a light echo from a star undergoing powerful recurring eruptions, though they have measured this unique phenomenon around exploding stars called supernovae. In spectroscopy, light from an object is captured by the telescope and sent to an instrument (called a spectrograph) that breaks that light into all its wavelengths. Each wavelength of light tells you something about the chemical makeup of the object, its speed through space, whether or not it is spinning, and gives a measure of its temperature.  For the Hubble observations, the spectrograph  captured Eta Car’s characteristic “fingerprints,” providing details about its behavior, including the temperature and speed of the ejected material.

The light echoes from Eta Car are telling astronomers that this restless, heaving old star system does not behave like other stars of its class.  The temperature of the outflow from Eta Carinae’s central region, for example, is about 8,500 degrees Fahrenheit (5,000 Kelvin), which is much cooler than that of other erupting stars. So,this gives some important clues about what’s happening inside the star.  There are other clues in the stream of light HST is studying, and light from the outburst is still on its way to Earth. Astronomers are expecting another brightening in about six months and that will give them more data to chew on as they seek to understand how this star is going through its death process. So, stay tuned.  There’s more news on the way from Eta Car!