astronomy@iowa

On April 13 a Type II supernova was discovered in the unusual galaxy NGC4088. This particular 90-second image was taken by Cedar Amateur Astronomers charter member Doug Slauson on April 21, using his 9.25-inch Celestron Schmidt-Cassegrain telescope with an attached SBIG STV at f/3.75.

At a distance of 55 million light-years, NGC4088 is a spiral galaxy in the Ursa Major cluster that has some properties of a barred spiral. Designated SN2009dd, the supernova is thought to be a type II supernova—the result of a cataclysmic explosion of a massive supergiant star at the end of its thermonuclear life. After fusing less massive nuclei into more and more massive nuclei, the supergiant star eventually ends up with iron in its core and a dead-end. Since iron has the greatest binding energy of all nuclei, all reactions involving iron are endothermic and rob the star’s core of energy resulting in an inability to support the star against the crush of gravity due to its extreme mass. In a fraction of a second, the massive star’s core implodes triggering a shock wave that eventually rips the star apart is a spectacular explosion.

The energy released in the supernova explosion is almost unimaginable—in a few seconds, the destroyed star gives off more energy than 10,000 sun-like stars will emit over their entire 10 billion year main sequence lifetimes!

Talk about a lucky strike! On the night of January 9, 2008, astronomers were using the Swift satellite to observe a supernova in the galaxy NGC 2770—90 million light-years distant in the constellation Lynx—when another one popped off in the same galaxy. A supernova such as SN 2007uy occurs when a massive star runs out of nuclear fuel in its core and the outward flow of energy is insufficient to balance the crushing inward force of gravity. In an instant, the star’s core catastrophically implodes triggering a shock wave that races outward through the star. Astronomers have long suspected that when the shock wave bursts through the star’s surface, a short x-ray burst would be produced. Because of the timing of such an event, astronomers have only seen the optical brightening of the exploding star, which lasts for many weeks, but have never seen the short x-ray brightening. Until now, that is.

On that day in January, astronomers Alicia Soderberg and Edo Berger were observing SN 2007uy in the distant spiral galaxy when they detected an x-ray brightening lasting five minutes that did not coincide with the observed location of the supernova. Because of the fortuitous observation and the design of Swift, the astronomers were able to make numerous measurements of the new supernova (designated SN 2008D) and test the theory of x-ray break-out as the shock wave rips the progenitor star apart. Happily the observations of Soderberg and 38 other astronomers confirm the theory after nearly 40 years.

Astronomers estimate that every 30 to 50 years a star explodes somewhere in a galaxy. That number is based on observations of supernovae—violent explosions of stars reaching the end of their lives—in other galaxies and if our Milky Way is typical, there should be two or three supernovae observed per century in our own cosmic backyard. The question is, where are they? Tycho Brahe observed a supernova in 1572 and his assistant and successor, Johannes Kepler, observed one in 1604. The last supernova observed in the Milky Way occurred in 1680.

Supernovae come in several distinct varieties: the “classic” Type II supernova and the more exotic Type I supernova. A Type II supernova results when a star at least nine times the mass of our sun reaches the end of its thermonuclear life and suffers a catastrophic collapse of its iron-rich core. Within a fraction of a second a shock wave is created as infalling matter rebounds off the now rigid degenerate neutron core. Eventually the shock wave rips the supergiant star apart leaving behind a tiny neutron star core, or in more extreme cases, a black hole.

A Type I supernova occurs when a sun-like star—which is part of a binary star system—has reached the end of its life and evolved into an Earth-sized white dwarf star bereft of nuclear fuel and destined to slowly cool into a stellar corpse of carbon and oxygen. Normally that white dwarf would live out its remaining time uneventfully, but when that star is part of a binary system, the white dwarf may accumulate material on its surface that is drawn from the surface of its companion star. If that happens at a sufficiently rapid rate, the material building up on the white dwarf’s surface may explosively detonate in an enormous fusion reaction as the deposited hydrogen is converted into helium. Normally this reaction occurs in the core of a star, but in this instance the reaction occurs on the stellar surface and rapidly consumes the white dwarf. Think of an Earth-sized hydrogen bomb that contains as much material as our sun. The ill-fated white dwarf is believed to blow itself out of existence in the detonation of the hydrogen.

So, back to our supernova count… While we seem to have a supernova deficit in our Milky Way, a new supernova remnant has recently been discovered by the Chandra X-ray observatory. With the assuming name “G1.9+0.3″ (the supernova remnant’s name derives from its location near the center of our galaxy), G1.9+0.3 is thought to be the result of a supernova that occurred about 140 years ago. It went undetected because the dense dust clouds surrounding it absorbed all but a tiny fraction of the visible light produced by the explosion. The orbiting Chandra observatory—named in honor of the Indian Nobel Prize-winning astrophysicist Subramanayan Chandrasekhar, who spent much of his career studying the dynamics of stellar evolution—was able to detect the x-ray emission of the young supernova remnant, however.

If estimates of supernova rates are correct, there should be about a dozen more supernova remnants waiting to be discovered in our galactic backyard. Given the arsenal of instrumentation available today, astronomical sleuths sifting through the dusty ribbons and star clouds of the Milky Way have a better chance than ever finding the missing supernovae.

Around 6:00 UT on March 19 a gamma ray burst shattered the existing record and became the most distant object visible to the unaided eye. Theoretically visible, that is, because it’s not known if anyone happened to be looking towards the constellation Boötes at the time the faint glow would have brightened to the naked eye threshold. With a redshift of z=0.96, the estimated distance to the burster is 7.5 billion light-years; halfway across the observable universe! Not exactly a recent event; the explosion—dubbed GRB 080319B—actually happened about 3 billion years before Earth formed.

Classified as “Top Secret” by the U.S. Department of Defense for many years, gamma ray bursts are thought to occur when a supermassive star reaches the end of its life and literally blows itself apart as a supernova (hypernova?) while its inert core collapses into a black hole.

So how would we see a gamma ray burst when our eyes can’t see gamma rays? Gamma rays are produced when the massive star’s core implodes and the outflow of energy is directed into superheated jets. When the jets of energy slam into surrounding matter, the matter is heated and glows in visible wavelengths.

Jai guru deva.

Photo credit: NASA/Swift/Stefan Immler, et al. NASA press release.