SuperNova 1987A – A Tribute to Hubble Space Telescope - by Albert Lim (2000)

One of the most significant scientific result of the Hubble Space Telescope must be it’s many years of continuously observations of Supernova 1987A (SN1987A) which have given astronomers a detailed and in-depth view of a supernova remnant in the making. This has enabled astronomers to predict models and verify them along the way with Hubble observations. This is one of the most powerful of the scientific methods and it has deepen our understanding not just of supernova itself and the making it’s remnant, but also on the surrounding interstellar medium as a whole. The significance of Supernova 1987A is that it is the brightest since Johannes Kepler’s observation of the supernova of 1604. Almost immediately after SN1987A explosion, astronomers recognised that this is a rare opportunity since it’s proximity is so close that obtaining images of the explosion at various stages in the course of it’s evolution is entirely possible - after Hubble launched in 1990, it provided the necessary resolution to image details in the changes in shape and dynamics of this rare event.

Fig 1 - The 11.6 ton Hubble Space telescope was launched from the cargo bay of the Space Shuttle Discovery on the 24th of April 1990.

The story of the line of observations of SN1987A made by others and later by Hubble makes for a compelling and fascinating story. In February 1987, Canadian astronomer Ian Shelton at Las Campanas Observatory discovered a new star in the Large Magellanic Cloud (LMC). This was the result of an explosion of an ageing massive blue supergiant star of about 20 solar masses called Sanduleak -69 degrees 202. Astronomers name the supernova explosion Supernova 1987A.
Astronomers believed that the star swelled up to become a red supergiant, puffed away some of it’s mass, then reheated and contracted to become a blue supergiant.

At this point, the star’s core collapse in less than a second. A wave of neutrinos had heated the core to over 10 billion degrees triggering off the massive supernova explosion. The neutrinos were picked up arriving earlier then the bright light from the explosion by deep underground detectors such as the IMB detector in Ohio as well as the Kamiokande II in Japan. In May 1987, the International Ultraviolet Explorer (IUE) satellite confirmed that the progenitor star had already passed through the red giant phase through discoveries made of abundances of chemical elements in the supernova debris consistent with such a phase. By July 1987, X-rays were detected emanating from the supernova site by the Japanese Astro-C satellite named GINGA, as well as the German High Energy X-ray Experiment (HEXE). Later in 1987, high energy gamma-rays were also detected by several research missions including the Solar Maximum satellite. By December 1989, the European South Observatory’s (ESO) New Technology Telescope revealed for the first time a glimpse of a ring-like structure around the supernova.

It was only in April 1990 that the Hubble Space Telescope (HST) was launched. By August 1990, Hubble’s Faint Object Camera had showed clearly for the first time a narrow ring surrounding the supernova at a distance of about 0.75 light year. This ring was believed to have formed by the blue supergiant star about 20,000 years prior to it going supernova when it puffed off its outer layers. Meanwhile, about this time, radio astronomers at the Australian National Telescope Facility (ANTF) also determined brightening radio waves were coming from the regions of supernova debris located between the ring and it’s central position. By 1992, the German X-ray satellite ROSAT detects brightening X-rays coming from the same regions as the radio sources detected by ANTF radio astronomers.

Fig 2 : Hubble finds mysterious mirror image pair of rings around SN 1987 A in May 1994. Astronomers then predicted that the rings are probably surfaces of an hourglass shape bubble predicted to be blown into space by the supernova.

In May 1994, Hubble’s Wife Field and Planetary Camera 2 (WFPC2) revealed 2 surprisingly thin outer loops of glowing gas surrounding the supernova. At this point, astronomers all over the world scrambled and  raced to explain the processes which formed such unusual features.

Fig 3 : This series of 4 images shows the evolution of SN 1987A debris from February 1994 to February 1996. Material from the stellar interior was ejected into space during the explosion which took place in February 1987. For the first time, Hubble revealed the geometry of a stellar explosion enabling astronomers to relate it to the geometry of the ring systems surrounding the supernova. These Hubble images provided important clues to the dynamics of the supernova explosion and the structure of the progenitor star Sanduleak -69 degrees 202.

By January 1997, Hubble’s WFPC2 showed the predicted hour-glass dumbbell-shaped structure one-tenth of a light year across consisting of 2 blobs of debris in the centre of the supernova site. They are amazingly speeding away from each other at nearly 6 million miles per hour (nearly 1 billion km per hour).

Fig 4 : Hubble’s long slit STIS spectrograph imaged the entire ring system and dissect it’s light into it’s component colours. Each colour represents specific elements in the ring’s gases. Oxygen appears as single green ring while Nitrogen and Hydrogen appears as triple orange rings while Sulphur are double red rings. Through these dissections, astronomers hope to put together a clearer picture of how these rings formed some 30,000 years ago.

By May 1997, Hubble'’ Space Imaging Spectrograph (STIS) produced a detailed ultraviolet image of the inner ring and provided identification of specific gases such as Oxygen, Nitrogen and Hydrogen. By dismantling the ring into these components, astronomers were able to assemble a model for the ring formation to some level of success. By June 1997, Hubble’s STIS again measured the speed of the gas in

Ultraviolet (the gas is invisible at optical wavelengths) ejected by the explosion as it crashes into the ring ejected prior to the supernova explosion and established it at 33 million miles per hour (over 50 million km per hour) inside the inner ring.

Fig 5 : Left : Hubble’s Wide Field and Planetary Camera 2 captured this image of the ring around Supernova 1987A in 1994. Right : This 1997 Hubble image showed the brightening of the knot at the upper right (indicated by the arrow) caused by the powerful collision of the outward moving blast wave and the innermost parts of the circumstellar ring. The collision causes the gases in the ring to heat up and brighten.

Images made by Hubble in 1997 also revealed for the first time a brightening knot on the upper right side of the ring (see Fig 20).  Hubble for the first time showed the signature of the above collision and allowed astronomers to validate ideas that they have already built up from over the past 10 years of observations. At this point, it is interesting to note that astronomers are already predicting that the whole ring would eventually be glowing in a matter of years. This was predicted to happen in 2002.

In the latest to the continuing saga of Hubble with Supernova 1987A, Hubble is finally providing evidence to the earlier predictions. The real fireworks in Supernova 1987A is beginning to show and Hubble is invaluable because it is providing astronomers with unparalleled views. ( As a side-track, it is not unreasonable to expect the Subaru telescope to join in the race and provide even more significant findings with it’s far superior resolving power to Hubble’s. ) As the central ring lights up, astronomers hope to have a chance to observe the old materials surrounding the star. Astronomers hope to map out the distribution of these materials to see the true structure of the gas. This would help in the understanding how the material got there in the first place and shed light on the initial processes which could in turn explain the formation of the rings. The scientific results accumulated by Hubble’s continuously observing of this supernova remnant in the making must surely be one of it’s most significant.

Fig 6 : The latest breakthrough by Hubble in scientific results on SN 1987A concerns this image showing the inner ring beginning to glow all around as predicted earlier. These  collisions of the fastest moving debris moving at 40 million miles per hour are created by the explosion in February 1987.  These collisions cause the gas in various parts of the inner ring to glow as they are heated to millions of degrees. Subsequent observations by Hubble is expected to reveal the full extend of this celestial fireworks and provide evidence for the first time in history for humans to witness a supernova remnant in the making

All 1987A images above by the incredible Hubble Space Telescope.