To get an idea of how stars live and die, we can’t just pick one and watch its life unfold in real time. Most stars live for billions of years! So instead, we do a population census of sorts. Much like you can study how humans age by taking a “snapshot” of individuals ranging from newborn to elderly, so too can we study the lives of stars.
But like all good things in life (and stars), there are exceptions. Sometimes, stellar evolution happens on more human timescales—tens to hundreds of years rather than millions or billions. One such exception is the topic of today’s paper: planetary nebulae, and the rapidly dying stellar corpses responsible for all that glowing gas.
All stars similar to our Sun, or up to about eight times as massive, will end their lives embedded in planetary nebulae like these. The name is a holdover from their discovery and general appearance—we have long known that planetary nebulae have nothing to do with planets. Instead, they are the former outer layers of a star: an envelope of material hastily ejected when gravity can no longer hold a star together. In its final death throes, what’s left of the star rapidly heats up and begins to ionize gas in the nebula surrounding it.
An international team of researchers analyzing decades of observations from many facilities, including NASA's Swift satellite, has discovered an unusual source of light in a galaxy some 90 million light-years away.
The object's curious properties make it a good match for a supermassive black hole ejected from its home galaxy after merging with another giant black hole. But astronomers can't yet rule out an alternative possibility. The source, called SDSS1133, may be the remnant of a massive star that erupted for a record period of time before destroying itself in a supernova explosion.
"With the data we have in hand, we can't yet distinguish between these two scenarios," said lead researcher Michael Koss, an astronomer at ETH Zurich, the Swiss Federal Institute of Technology. "One exciting discovery made with NASA's Swift is that the brightness of SDSS1133 has changed little in optical or ultraviolet light for a decade, which is not something typically seen in a young supernova remnant."
Abstract: We study the newly discovered variable star GSC 4560--02157. CCD photometry was performed in 2013--2014, and a spectrum was obtained with the 6-m telescope in June, 2014. GSC 4560--02157 is demonstrated to be a short-period (P=0.265359d) eclipsing variable star. All its flat-bottom primary minima are approximately at the same brightness level, while the star's out-of-eclipse brightness and brightness at secondary minimum varies considerably (by up to 0.6m) from cycle to cycle. Besides, there are short-term (time scale of 0.03-0.04 days) small-amplitude brightness variations out of eclipse. This behavior suggests cataclysmic nature of the star, confirmed with a spectrum taken on June 5, 2014. The spectrum shows numerous emissions of the hydrogen Balmer series, HeI, HeII.
There are arguably a lot of things defy categorization, but it’s not everyday that we find something that suggests we do away with our categories altogether. The authors of today’s paper believe that the recently-discovered Type II supernova ASASSN-13co — read that as “assassin”, please — might just be one of the latter. Its unusual characteristics call into question the validity of the two classes (II-P and II-L, more on that later) into which we usually group Type II supernovae. As a result, they suggest that we treat Type II supernovae properties as a continuum, rather than the discrete designations we’ve become accustomed to assigning.
Death Throes of Massive Stars
Type II supernovae are identified by the hydrogen in their spectra (meaning that they still have a hydrogen envelope when they die). They are formed when a star with mass of 8-50 times that of the sun dies through core-collapse.
All stars produce energy through nuclear fusion, but massive stars can fuse much heavier nuclei than stars the size of our sun – all the way to nickel and iron, which have the highest binding energy of all elements. While the fusion of the lighter elements is an exothermic process, fusing iron uses up energy instead, so fusing elements heavier than iron isn’t energetically favorable. As a result, a core of iron and nickel (which then decays into iron) builds up in the center of a massive star. The core is supported by electron degeneracy pressure. When the mass of the iron-nickel core exceeds the Chandrasekhar limit (about 1.4 solar masses), however, electron degeneracy pressure is not enough to stop the core from collapsing. As the core collapses, the protons and electrons in the core of the star merge to form neutrons and neutrinos. The neutrinos can escape and carry away energy. At the same time, the outer layers of the star fall inward until neutrondegeneracy pressure kicks in, stopping the collapse and causing the outer layers to rebound. The combination of the pressure from the neutrinos and the rebound of the outer layers off of the core causes the star to be torn apart in a huge explosion – a core-collapse supernovae.