Les Dalrymple is a guide at Sydney Observatory and a passionate deep sky observer. Below he discusses one of his favourite objects to observe – a supernova.
Supernovae are usually associated with a gigantic star that has exhausted its nuclear fuel, undergone core collapse followed by a brilliant explosion leaving behind a pulsar or possibly a black hole. While best known by the public, this type of supernova (known as a Type II event) is only the second most common type — there are other ways to make a supernova explosion!
The most common is a Type Ia supernova. This type usually arises from a tiny, burned-out white dwarf star in binary system, closely orbiting another star, often a red giant. Matter (Hydrogen and Helium) from the red giant falls onto the white dwarf slowly, making it heavier. As the growing white dwarf’s mass increases so too does its gravity, temperature and density. Finally, as it approaches 1.4 times the mass of our Sun a critical point (known as the Chandrasekhar Limit) is reached where central temperature and pressure leads to a catastrophic thermonuclear detonation that consumes the white dwarf — a type Ia supernova!
Because most Type Ia explosions arise from white dwarf stars of exactly the same mass, they are (within a few percent) uniformly bright and release 10^44 joules of energy. A small percentage of Type Ia events are a little brighter or dimmer, than usual or display curious characteristics in the way they slowly dim. Some Type Ia events might be caused by binary white dwarfs that have spiralled together and merged, and this has led to some controversy over the mechanism behind Type 1a supernovae. It is this characteristic of uniformity that makes Type Ia supernovae occurring in other galaxies very useful tools for astronomers in measuring the size, mass, composition and expansion rate of the Universe.
Of the dozens detected each year by astronomers, the vast majority of supernovae are too faint (and therefore too far away) to be seen in backyard telescopes. But on the 30th December 2015, Astronomer’s Telegram 8474 announced a supernova in NGC 7213 that should be observable.
The discovery image above, taken on 29th December 2015, shows the “new star”. This supernova was classified a few days later as a Type Ia event.
NGC 7213 is one of the easiest galaxies in the night sky to find. It is nestled a mere ¼ degree southeast (and barely outside the glare) of the brightest star in Grus, 1st Magnitude Al Nair. It is giant spiral galaxy comparable to our own Milky Way with an exceptionally bright core and an active nucleus. NGC 7213’s small size in the eyepiece is due to remoteness — about 80 million light-years distant. Though small in apparent size, it is one of the few galaxies in the night sky that can be visually observed from the urban location of Sydney Observatory — thanks to its very bright core and nucleus. It is one I sometimes show our visitors who ask to see another galaxy.
The 7th of January 2016 provided the first clear moonless night since discovery of the supernova. Just after 10pm, after aligning my 46cm Newtonian telescope and slewing it to NGC 7213 under dark country skies, first the galaxy and then the tiny, faint point of light in its outer halo came into view. I estimated the brightness of the supernova as magnitude +14.2 and in perfect conditions a 30cm telescope might snare it. Though the supernova was not visually impressive (it is no “eye-candy”), it is always a wonderful experience to witness a cataclysmic explosion — one of the most energetic events that is visible to the human eye, across a tract of space so enormous, its light took about 80 million years to reach Earth. In other words, this event we see now, actually took place millions of years before the first Tyrannosaurus walked the Earth.
The supernova in NGC 7213 was not my first; I’ve previously observed about two dozen in other galaxies. However, we all wait with baited-breath for the next Milky Way supernova. The last one visible to the naked-eye occurred before the invention of the telescope, back in 1604. Like most astronomers, I hope to see one in my lifetime.
Update, Feb 09, 2016: Right on cue a possible SN has just been detected in the Centaurus A galaxy. Les writes, “Attention Southern Observers: Probable supernova in Centaurus A*. A 14th magnitude transient (likely supernova) was discovered in NGC 5128 (Centaurus A*) on 8th February 2016. The discovery was announced in Astronomer’s Telegram 8651. The new transient is currently 14th magnitude and located 0″.0 east and 0″.0 north of the center of NGC 5128 It will probably brighten. If confirmed, it is another discovery of the prolific Australia / New Zealand BOSS team.”