the last “local” in 1604. ongoing, making Supernova 1987A the This rarity of supernovae is what most intensively studied object outside makes the night of February 23, 1987, of the . It has been one of the most significant dates in detected at all wavelengths of light, modern . from high-energy gamma rays to low- frequency radio waves. In early March A shot seen ’round 1987, physicists in Ohio and Japan (and through) the world announced that they had even detected The nearest supernova in nearly four a burst of neutrinos from Supernova centuries was discovered that night by 1987A on the night of the explosion. two working in Chile and Neutrinos are ghostly subatomic an amateur astrophotographer from produced in nuclear reactions hen a massive dies, it really New Zealand. All three were that rarely interact with normal matter. Wgoes out with a bang. In fact, independently observing the Large The 12-second burst of neutrinos from such an explosion, known as a Magellanic Cloud (LMC), a small Supernova 1987A had arrived from a supernova, is one of the most energetic companion that orbits as a direction over the South Pacific Ocean events in the entire . During around the Milky Way. (The near Antarctica and had actually passed a supernova the dying star, referred LMC is visible only from the Southern through the itself to arrive at the to as the progenitor, can increase its Hemisphere and lies at a distance of underground neutrino detectors from brightness by 10 billion , 160,000 light years from Earth, below. Generated during the first few outshining the combined light of the meaning that it has taken light 160,000 instants of the supernova and traveling Dead Do Tell Tales: The Intriguing Case of Supernova 1987A billions of other stars in its home years to reach us.) They each noticed Stephen S. Lawrence galaxy. The nuclear fusion reactions that a bright new star had appeared in that take place during the star’s the LMC, where there had been Assistant Professor lifetime and during the supernova nothing remarkable just the night Department of Physics explosion itself are responsible for before. Realizing the significance of a creating all of the elements heavier supernova so close to home, the at the speed of light, the neutrinos tell than hydrogen and helium. The Chilean astronomers used a telegraph us the exact of the explosion. we breathe, the calcium in our service run by the International Supernova 1987A is the only bones, the iron in our blood, the gold Astronomical Union (IAU) to broadcast astronomical object besides the in our fillings – all of these elements the news of the discovery of Supernova that has been detected through its are products of supernova explosions. 1987A (so named as it was the first neutrino emissions. There is even evidence that the supernova discovered in 1987) to every formation of the sun and the Earth observatory and astronomy department A three-ring circus may have been triggered by the worldwide. As the Earth turned, After several years, Supernova 1987A shockwave from a nearby supernova astronomers in Australia and South had cooled and faded such that it could explosion. Africa scrambled to find the appropriate be observed only with the largest Unfortunately, these exciting and filters that would allow them to observe ground-based telescopes, or with momentous explosions are extremely such a bright, naked-eye star with large astronomers’ newest tool, the Hubble rare, happening on the average of one research telescopes. Telescope (HST). Figure 1 per century in any given galaxy. During the first few weeks there shows a composite image of the Modern searches now discover more was practically round-the-clock coverage aftermath of Supernova 1987A taken in than 100 supernovae each year, but of the supernova, with observations wavelengths of red light. Multiple they are all in distant . This taken from every southern continent images taken between 1994 and 1996 makes them very faint and difficult to (including Antarctica), using telescopes have been combined digitally to study in any great detail. Here in our on mountaintops, weather balloons, improve the resolution and sensitivity home galaxy, the Milky Way, we are high-altitude aircraft and . to the faintest emission. A number of long overdue: Johannes Kepler sighted Observations of the aftermath are still interesting features can be seen: 3 circles on it with glow-in-the-dark of the ring. Assuming that the inner paint: one around the narrow waist ring was truly circular and that the and one each around the widest ejecta were thrown off symmetrically portions of the upper and lower with similar velocity in all directions, balls. Now turn out the lights and it was predicted that we would see the tilt the top of the hourglass away impact begin on the near side and then from you by 45 degrees. This is a take a year to spread around to the far reasonable model for the shape and side of the ring. relative positions of the three As if on cue, in April 1997 rings.) One can even see a faint astronomers using the Hubble Space glow from some very low-density Telescope detected a small region on gas in the “walls” of the hourglass, the slowly fading inner ring that was Figure 1. A Hubble image of as a faint fringe of emission just actually brightening and had the Supernova 1987A, taken in red light roughly outside the brighter inner ring. signature of newly accelerated, high- eight years after the explosion. The last Astronomers have a basic velocity gas. This “hot spot” was even glowing embers of the star (center) are sur- understanding of how the star may on the near side, as predicted. But rounded by three mysterious rings of gas. For have blown off this gas into a here the simple predictions broke scale, the brighter, inner ellipse has the same general hourglass-like configu- down. For nearly three years apparent diameter as that of a quarter viewed two miles away. ration, but it is still a mystery as to following the discovery this first hot exactly why the gas is concentrated spot grew brighter, but didn’t spread into three denser rings, with the out along the ring. And no other new 1) The grayish spot at the very center walls and end caps at much lower, hot spots were detected. of the image is the faintly glowing effectively invisible concentrations. Then in January 2000 my remains of the densest gaseous 3) Significant quantities of transparent colleagues at Columbia University and debris thrown off during the ejecta are also between the central I discovered evidence that a second explosion, called “ejecta.” These visible debris and the inner circum hot spot was forming, in ground-based ejecta represent the innermost stellar rings. The outermost layers data taken at the Cerro-Tololo Inter- layers of the original star, contam- of this material were launched from American Observatory in Chile. We inated by the products of the the explosion at approximately 10 used an infrared instrument designed runaway nuclear reactions that percent the speed of light, or to rapidly compensate for the powered the explosion. This gas is roughly 600 million miles per hour! blurriness of the Earth’s atmosphere, kept hot and glowing by the decay These layers have already cooled and special image processing of radioactive isotopes of nickel, and faded to near invisibility, techniques designed to detect subtle cobalt and titanium. producing the slight grayish glow changes in a source by subtracting 2) Three clumpy, elliptical rings of filling the inner ring. images taken at different times. circumstellar gas are centered on Surprisingly, the second spot was the former location of the Seeing spots with Hubble located not on the near side of the progenitor. These mysterious rings Based on measurements taken during ring, but most of the way around to the were shed by the progenitor roughly the first few nights of the explosion, back. Announced in another IAU 20,000 years before it exploded, and astronomers calculated that the telegram (now distributed via e-mail), are drifting outward at the outermost layers of ejecta would reach the new hot spot activity was quickly astronomically leisurely pace of the bright inner ring in approximately confirmed by other astronomers in 30,000 miles per hour. The rings 10 years. Their models showed that HST images taken almost simulta- are like huge fluorescent lamps, these fast-moving, low-density ejecta neously with our announcement. glowing as a result of a strong burst would drive a massive shockwave into Using precious HST orbits held in reserve of X-rays and ultraviolet light higher-density, slow-moving gas in the for such late-breaking discoveries, we produced during the first few hours ring. This shockwave would suddenly developed an innovative method of of the supernova; they have been compress the ring gas, briefly heating observing Supernova 1987A that allows slowly fading ever since. Observations it to millions of degrees and us, in combination with our difference show that the rings are three parallel accelerating it to high velocities. imaging techniques, to monitor the circles stacked one above the other Because the near side of the ring is entire inner ring for either the brightness and tipped toward us by about 45 about a light year closer to us than the changes or high-velocity signatures degrees. (Imagine taking an far side, we see events and changes that indicate the formation of a new hourglass and tracing out three there a year earlier than on the far side hot spot.

Pictured above: Supernova 1987A ring 4 clumpy inner ring that have been 1987A. Consider taking an image of an struck by the blast wave first. The area of the centered on Supernova order of appearance and the rate of 1987A (as my colleagues at Columbia brightening of the hot spots may University and Las Campanas provide us with information about the Observatory in Chile have been doing degree of asymmetry in the ejecta for the last 13 years), 10 years after velocities and details about the shapes explosion. Any photons reaching the Figure 2. HST spectral images of the inner cir- of the ring protrusions too small to Earth on that night have obviously cumstellar ring of Supernova 1987A, showing resolve even with HST. And we expect traveled along a path that is 10 light emissions from hydrogen gas in April 1997 that the real fireworks are yet to come, years longer in distance than the (left), May 2000 (center) and April 2001 when the ejecta actually reach the full photons that arrived here directly on (right). inner surface of the ring. Observations the night of the explosion. By Note the increasing number and of Supernova 1987A over the next measuring the angle between the echo brightness of the hot spots (horizontal smears) several decades will provide us with an and the supernova in our images, and with time, sites where a shockwave from the unprecedented amount of detailed combining this with the light-travel explosion is beginning to destroy the ring. information on how supernova shock delay, we can use simple geometry to waves deposit energy into interstellar calculate the position of the dust cloud Figure 2 presents three epochs of gas, and how newly synthesized heavy relative to the supernova. It is very HST data on the inner ring (rotated elements get spread throughout unusual in astronomy to obtain such about 135 degrees with respect to the a galaxy. three-dimensional information on orientation of Figure 1). These images targets within our own galaxy, let alone show the inner ring in a specific Is there an echo out there? at even greater distances. wavelength of red light emitted by hot In addition to being an once-in-a- Figure 3 presents an example of the hydrogen gas. The slow-moving gas of the lifetime opportunity to study a light echo phenomenon. Containing an inner ring forms a simple undistorted supernova up close, Supernova 1987A area of the sky hundreds of times larger image, but emission from the turbulent has also provided us with a unique than Figure 1, it displays the results of high-velocity gas within a hot spot is probe of the LMC galaxy. For just a carefully aligning and subtracting two broadened by the Doppler effect from a few months the supernova was billions images taken from Chile in 1995 and single point in the image into a of times of brighter than the progenitor 1996. As noted earlier, most astronomical horizontal smear. The leftmost image, had been previously, and then it faded objects remain constant over the from April 1997, shows the discovery quickly to near invisibility. On timescale of a year, so the vast majority of “smear” of the first hot spot (at the 2 astronomical scales of time and the thousands of LMC stars that clutter o’clock position on the ring). The distance, this is like an immense the two original images are precisely center image shows the inner ring three flashbulb going off in the galaxy. Much canceled out by the subtraction. (A years later, in May 2000. The first hot as the burst from a fireworks shell relatively small number of rapidly spot is now much brighter, and five new during the grand finale will light up the varying and bright, saturated stars do faint spots have appeared (at 7 o’clock smoke from earlier shells, photons leave small artifacts). The large, faint, and between 10 and 12 o’clock). The from the brief maximum of Supernova clumpy arcs and rings that remain are the rightmost image shows the ring in April 1987A are sweeping outward through light echoes, whose faint signals would 2001. The first hot spot has slowed its interstellar space and illuminating normally be lost in the overwhelming rate of brightening, the other spots from clouds of gas and dust in the LMC. background of the LMC stars. Due to the May 2000 have increased dramatically, The grains of dust scatter and reflect order of subtraction, the locations of the and other new spots have appeared in those photons in random directions, echoes in 1995 are presented as white, the space of just one year (at the 3, 5 and with some fraction redirected towards positive values, and in 1996 as black, 9 o’clock positions). It is very unusual in the Earth. Astronomers refer to these negative values. Across the space of a astronomy to be able to see such rapid scattered photons as “light echoes.” year, the light echoes appear to expand changes in an object this large. Our Much as an air traffic controller radially outward from a point centered latest difference imaging analysis have uses the timing and direction of the on the supernova, like ripples in the shown that there are now 13 hot spots reflection of a pulse of radar to surface of a pond. The big “bull’s-eye” spread around the entire circumference determine the location of an airplane, ring feature represents two large sheets of the inner ring. we can use a series of images of these of dust and gas, each hundreds of light- We now believe that the hot spots light echoes to map out the three- years wide. The brighter clumps actually represent the innermost dimensional distribution of dust clouds scattered along each arc represent small peninsulas and protrusions of the in the neighborhood of Supernova substructures within the sheets where 5 the dust density is much higher. Another could provide us with detailed interesting structure that the echoes have information on the internal structure of illuminated is the edge of a huge LMC dust clouds on extremely small “superbubble” of gas and dust that was spatial scales. swept up by the combined blue The research projects described here supergiant winds of hundreds of massive are being conducted in collaboration with stars located in a at its center. Professor Arlin Crotts and Ben Sugerman With improvements in telescope of Columbia University, Dr. Patrice technology and our image subtraction Bouchet and Dr. Steve Heathcote of Cerro techniques, we should be able to monitor Tololo Inter-American Observatories and Stephen Lawrence remembers the excite- the progress of the light echoes for Dr. William Kunkel of Las Campanas ment that swept the astronomy and physics decades to come, mapping out the dust Observatory. They are being supported communities when Supernova 1987A distribution over an immense volume of through grants from the National Science exploded during his junior year as an under- space within our neighbor galaxy. Foundation, NASA and the Space graduate. The intense activity triggered by This region of the LMC has been a Telescope Science Institute. this once-in-a-lifetime event was a factor in site of intense star-forming activity for his decision to study astronomy, rather than the last hundred million years. It is physics, in graduate school. He is particular- filled with a variety of energetic star ly pleased that his career has come full circle clusters, nebulae and the remnants of and allowed him "to study one of the most exciting astronomical events of our time with ancient supernovae and, as such, has the Hubble Space Telescope, astronomy’s attracted the attention of numerous HST most powerful research tool." programs, focused on a broad variety of Professor Lawrence earned a B.A. in objects. My colleagues and I have physics from the University of Chicago and recently been awarded a grant from the an M.S. and Ph.D. in astronomy from the Space Telescope Science Institute to University of Michigan. He is a member of search through the public archive of Phi Beta Kappa and recipient of the Ralph many hundreds of these HST images, Baldwin Thesis Prize in and searching for light echoes from Space Science for his dissertation, "Fabry- Perot Imaging of the Crab Supernova 1987A that just happened to , Cassiopeia A, and GK Persei." fall in the field of view of an unrelated Prior to joining the Hofstra faculty earlier target. At the superb resolution of HST, this year, Professor Lawrence held teaching any such serendipitous echo images positions as lecturer at the University of Michigan and visiting assistant professor at Bowling Green State University. He also held research positions as resident at the Observatorio Astronómico Nacional in Mexico and, most recently, postdoctoral research scientist at Columbia University. Professor Lawrence’s research interests include supernovae and supernova rem- nants, light echo phenomena and interstellar dust, and low surface brightness galaxies. He has authored a number of journal articles as well as circulars and conference proceedings relating to his research findings. He has used a variety of large telescopes in Arizona, Mexico and Chile, as well as the orbiting Figure 3. A subtraction of two ground-based Hubble Space Telescope, to conduct images, revealing the light echoes of his research. Supernova 1987A. Analogous to a sound echo, As pointed out in his article, Professor the large “bull’s-eye” pattern displays the Lawrence has received funding from the reflection of the supernova from two neighbor- National Science Foundation, National ing clouds of interstellar dust, delayed by sev- Aeronautics and Space Administration, and eral years as a result of the enormous extra the Space Telescope Science Institute in sup- distance the light had to travel. (Courtesy of port of his research. - SK Ben Sugerman)

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