6. SUPERNOVA 1987A

On the evening of February 23, 1987, a young Canadian astronomer named Ian Shelton walked outside of the dome at the Cerro Tololo InterAmerican Observatory and took a photo of a nearby galaxy, the Large Magellanic Cloud (LMC) with his Nikon Camera. Shelton developed the photo and immediately noticed a bright star where none had been seen before. He told his colleagues, and within hours the word had sped around the world, by phone, e-mail and fax. This was a nearby supernova -- the brightest one to be seen since 1604 AD, when Johannes Kepler observed a supernova in our own galaxy.

Astronomers immediately began to observe the supernova (called SN1987A) with every telescope they could use. Unfortunately, the great telescopes in Hawaii and the Northern hemisphere could not observe the supernova because it was far to the south. But astronomers could observe it almost continually with telescopes in Chile, Australia, and South Africa. In this image of the LMC, you will see a big bright nebula of glowing hydrogen gas in the upper left, called 30 Doradus or the Tarantula Nebula. In this magnified image of the Tarantula Nebula with SN1987A you can see the supernova to the lower right. In the pair of images at the top of this page, the arrow on the right panel points to the star that blew up. Before it blew up, it was already a fairly luminous (about 10⁵ times the Sun) blue star; but after the explosion it suddenly became 1000 times brighter (about 10⁸ times the Sun), as seen on the left panel.

From its location in the LMC, the light from SN1987A must travel for about 160,000 years before it reaches the Earth. So, when Ian Shelton first saw the supernova explode in 1987, he saw an event that had actually occurred in about 158,013 BC (1987 - 160,000 years). Today, in 2002, we are seeing a delayed-action re-run of the actual event. When we say that SN1987A is 15 years old, we really mean that it is 15 years older than when we first saw it.

Neutrino observations: Actually, there were two "observatories" in the Northern hemisphere that did observe SN1987A. They were huge tanks of water deep underground, all sides of which were covered with instruments designed to detect flashes of light that might occur in the water due to the interactions of fundamental particles. One, called the Kamiokande experiment, was in Japan; and the other, called the IMB experiment, was in Ohio (deep under Lake Erie). These experiments were designed to detect rare proton decay events predicted by theories of elementary particles, and the builders had no idea that they might detect neutrinos from supernova explosions. The neutrinos from SN1987A came right through the Earth and entered these tanks of water from beneath. A very few of them (12 in the Kamiokande detector and 8 in the IMB detector) interacted with atomic nuclei (protons) in the water to make very fast positrons, which in turn made light flashes in the water that the experiments detected. You can see the results here. From these observations, scientists were able to infer three properties of the neutrino burst: (1) the total energy (about 0.1M_Sunc², or 1/10 the mass-energy equivalent of the Sun); (2) the temperature of the neutrino source (40 billion K); and (3) the duration of the neutrino signal (about 10 seconds). All these values were just what scientists expected from theoretical calculations of the collapse of the core of a star to form a neutron star. So these observations of neutrinos from SN1987A provided very strong evidence that the supernova explosion was accompanied by the formation of a neutron star, as illustrated here.

Despite this evidence, and twelve years of intensive observations, nobody has found any further evidence of a neutron star at the center of the debris of SN1987A. Why? We don't know.

Optical light and gamma rays: The graph below shows a record of the optical light curve of SN1987A for the first 1400 days after the explosion was first observed. The crosses are the actual data. The graph shows that the supernova

continued to brighten for the first 100 days after the initial flash, reaching a maximum brightness of about 3rd magnitude, bright enough to see with the naked eye. But then it faded rapidly, with the brightness dropping by a factor of 2 every 77 days for the first 500 days, as indicated by the dashed line.

The light from SN1987A faded at almost exactly the same rate observed in laboratories for the decay of the radioactive nucleus Cobalt-56 into the stable nucleus Iron-56 (the half-life of Cobalt-56 is 77 days). This was not a great surprise, because astronomers had long suspected that supernova explosions were responsible for the formation of the heavy elements in the universe. Theoretical calculations of the formation of heavy nuclei at billions of degrees (the temperature expected during the explosion) indicated that about this much Cobalt-56 would be formed. But the fact that the supernova light decayed just as expected was the strongest confirmation to date of the idea that supernova explosions really did make the heavy elements -- and, for the first time, we could measure exactly how much Cobalt-56 was made (0.07 Solar masses).

After 500 days the visible light faded even faster than the Cobalt-56 decay rate. That happened because after that time dust particles began to form in the supernova debris. The grains absorbed part of the optical radiation and converted it into infrared radiation. Moreover, the supernova debris had thinned out enough so that the gamma rays could escape directly without first becoming converted to optical light. In fact, gamma ray telescopes in space could observe these gamma ray photons, and they saw that the gamma ray photons had exactly the same energy as those produced by Cobalt-56 in laboratories on Earth. That clinched the idea that the supernova explosion made Cobalt-56. Think about it: the iron that makes your blood red was once radioactive Cobalt-56. It was produced in a supernova explosion several billions of years ago. The observations of gamma rays from SN1987A leave little doubt about that. The same with the oxygen you breathe, the calcium in your bones, and the earth you stand on. We are, literally, stardust. In fact, we might consider ourselves the consciences of supernovae: the creatures supernovae have created to worry about the consequences of their unruly outbursts. The optical light of SN1987A has continued to fade for the past 15 years, and now has visual magnitude about m_V = 21. (See Lightcurve.) The radioactive heating by Cobalt-56 has long since become negligible. We believe that the main radioactive energy source today is Titanium-44, which decays much more slowly (with a half-life of 54 years).

Spectrum evolution: The graph below shows the optical spectrum of SN1987A at three dates after outburst.

The top panel shows the spectrum on February 25, 1987, two days after outburst. You can see that the spectrum has a strong continuum that is rising at short wavelengths, punctuated by very broad emission and absorption lines of hydrogen and helium. This spectrum is very similar to that of a hot blue star (spectral type O). The broad absorption lines are caused by rapidly expanding gas above the photosphere; they are blue-shifted because we see the near side of the supernova, which is expanding toward us. We can measure the expansion velocity of the supernova debris from the blue shifts of these absorption lines. At early times, the measurements showed that the outer part of the debris was expanding with velocity 30,000 km/s, or 10% of the speed of light! The spectrum changed very rapidly. Seven weeks after outburst (middle panel), the photosphere had cooled to a temperature of about 3000 K (spectral type M). The spectral lines were considerably narrower. One might think that this result implied that the supernova expansion had slowed down, but that would be a mistake. Actually, the fastest moving gas had become so thin that it was invisible, so we were now seeing spectral lines produced by atoms in more slowly expanding gas deeper inside the supernova.

Six months after the explosion (bottom panel), the continuum was almost completely gone from the spectrum, which was now dominated by emission lines. This kind of spectrum is called a nebular spectrum, because it resembles that of a gaseous nebula, such as the Orion Nebula. As the name suggests, the expanding debris of the supernova has now become so thin that one can see right through it.

Today, 15 years after the explosion, we see that the spectrum of SN1987A is still dominated by emission lines. By analyzing these lines, we find that the supernova debris has cooled to less than 300 K. In fact, SN1987A is the coolest optically emitting object in the sky. The debris is far too cool to emit optical radiation, and certainly would be invisible but for the fact that it contains radioactive elements such as Titanium-44. In 15 years, we have seen the debris of SN1987A cool from one of the hottest places in the universe (T > 10¹⁰ K) to one of the cooler places (T < 300 K).

The mystery of the circumstellar rings. A few months after SN1987A went off, a small telescope on the International Ultraviolet Explorer (IUE) satellite observed that narrow emission lines began to appear in its ultraviolet spectrum. This was a surprise: the supernova debris was expanding with velocities of thousands of kilometers per second; yet, the ultraviolet emission lines were narrow, indicating expansion velocities of less than 10 km/s. The ultraviolet emission lines continued to brighten, reaching maximum brightness 400 days after outburst, and then began to fade.

Astronomers soon figured out that these emission lines couldn't be produced by the supernova itself. The emitting gas was moving far too slowly. Instead, they must be produced by gas nearby the supernova, which began to glow after being illuminated by the flash of ultraviolet and X-ray light from the supernova outburst. But what was this gas? The telescope on IUE did not have sufficient angular resolution to obtain an image.

Several years later, the Hubble Space Telescope obtained the image of SN1987A shown above. The supernova was surrounded by a remarkable system of three rings of glowing gas! Where did these rings come from? They must be related to the supernova because the supernova is at their center. But they could not have been ejected by the supernova explosion. In fact, if one divides the radius of the inner ring (6 x 10¹² km, or 0.6 light-years) by the expansion speed (10 km/s, or 1/30,000 times the speed of light), one finds that the inner ring must have been expanding for 20,000 years to grow to this radius. Thus, the inner ring must have been ejected 20,000 years before the supernova explosion.

This movie illustrates the actual three-dimensional shape of the triple-ring system. If this gas was ejected by the star, why does it have this remarkable structure? Astronomers really don't know -- this triple ring system is one of the outstanding mysteries of SN1987A. Some physical effect must determine the polar axis of the rings. We suspect rotation. But rotation of what? Many astronomers now believe that the parent star of SN1987A was actually a close binary system. Perhaps the inner ring was ejected while the merger took place, 20,000 years before the explosion, as illustrated below.

There is one other reason to suspect that the parent star of SN1987A was a merged binary system. Astronomers actually had taken a picture and a spectrum of the parent star before it exploded, and they could see that it was a blue giant star, not a red giant as expected for a star that was about to become a supernova. Perhaps the same merger event that caused the ejection of the rings would expel the distended outer atmosphere of the red giant star, leaving a hot blue core behind. We now think that the time variation of the ultraviolet emission lines seen by the IUE satellite is the result of illumination of the inner ring by the initial X-ray flash of the supernova. More than that, we can infer the physical diameter of the ring by analyzing the variation of ultraviolet narrow emission lines observed by IUE. By comparing the physical diameter to the angular diameter of the ring seen in the Hubble Space Telescope Image, we can infer the distance to the supernova. Finally, by dividing the radius of the ring by the expansion speed of the supernova debris, we can estimate when the debris will hit the inner ring.

The answer is: today! This is exciting, because we have never before seen a crash like this. I and my students and colleagues have been working for several years to predict what we will see when the crash occurs and what we will learn from such observations. We predict that the ring will become hundreds of times brighter than it is today at optical wavelengths. It will also become very bright in the radio, infrared, ultraviolet, and X-ray bands of the electromagnetic spectrum. That's great, because astronomers are now building telescopes that can observe SN1987A in these bands with unprecedented sensitivity, angular resolution, and spectral resolution. The Hubble Space Telescope is the telescope of choice at optical and ultraviolet wavelengths. In 1999, NASA launched the Chandra X-ray telescope, and SN1987A was one of the first targets it observed. Before long, we'll be able to observe SN1987A at infrared wavelengths with NASA's SIRTF telescope. In about a decade from now, we'll get a much better picture at radio wavelengths with the Atacama Large Millimeter Array.

We simulated the impact of the supernova debris with the inner ring by solving the equations of gas dynamics on a supercomputer. We assumed that the ring was a torus (i.e., shaped like a donut). Click here to see the results (courtesy of John Blondin of North Carolina State University). In this simulation the yellow circle represents a cross-section cut through part of the ring and the supernova is off to the left. The blast wave from the supernova enters the ring 12.7 years after the explosion, or AD2000. It compresses the gas to high density and high temperature (shown as red). It continues to crush and shred the ring, until about 30 years after the explosion, when the ring is totally destroyed. Once we know the temperature and density of the shocked gas, we can calculate the intensity and spectrum of the radiation it emits in the optical, infrared, ultraviolet, and X-ray bands. With such calculations, we predicted that the ring will brighten by a factor of several hundreds after the impact occurs.

In 1997, I wrote on this page:

Today, all these predictions are coming true. With the Hubble Space Telescope (HST) we are seeing ultraviolet and optical emission from the fast-moving debris of the supernova. Below is a spectrum from the Space Telescope Imaging Spectrograph showing hydrogen gas from the supernova debris moving at a velocity of 1/20 the speed of light. The hydrogen emits ultraviolet line radiation when it is suddenly slowed down in a shock wave that is approaching the ring.

In January 2000, we first used the Chandra X-ray telescope to obtain the first image of the X-ray emission from SN1987A, showing that the X-rays are coming from roughly the same region inside the ring that is emitting the radio and ultraviolet line radiation. The results are shown below.

We also see a rapidly brightening "hot spot" on the ring, evidently the spot where the supernova blast is making first contact with the ring, as illustrated below.

All this action is summarized in this movie, which I made with the help of people at the Space Telescope Science Institute. The first part shows the flash of the supernova, and then the ring lighting up due to illumination by X-rays from the supernova flash. We see the near side of the ring first because it is closer. About 400 days after the initial flash, the whole ring was bright -- hundreds of times brighter than it is today. This would have been a great show; but nobody could see it. The HST hadn't been launched yet and no ground-based telescope could see the ring at that time. The next segment of the movie shows the ring fading over 10 years, while the invisible debris of the explosion races out toward the ring. Then the first bright spot appears on the ring, as we see today. Soon, several other spots will appear, growing and merging. Finally, 10 years from now, the entire ring will become hundreds of times brighter than it is today.

Check here for the details of a press conference that I and my colleagues gave at NASA headquarters in 1998. But that's already old news. Keep watching this page. This story will change fast!

Update, February 20, 2002: Just as we predicted, several other spots have appeared! The montage below shows how the image of the ring has changed from October 1994 to April 2001. A "hot spot" first appeared in the upper left quadrant of the ring in July 1997 and has continued to brighten ever since. We also see that several more rapidly brightening spots have now appeared all around the ring. To read more about these observations, check Onset of Titanic Collision Lights up Supernova Ring from the Space Telescope Science Institute.

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Last modified February 25, 2002
Copyright by Richard McCray