This past Friday, recent post-baccalaureate of the Columbia Astronomy program, Nicholas Hunt-Walker, gave a talk entitled, “Supernovae: Going out with a Bang!” In his talk, Nick gave a clear overview of the main points of stellar evolution that lead up to two different types of supernovae: mass-accretion and core-collapse. He told us that mass-accretion type supernovas (also known as Type Ia) arise from binary systems where one star has already gone through all of the evolutionary phases, becoming a hot, dense stellar remnant, called a white dwarf. The other star, being less massive at birth, takes longer to evolve, eventually becoming a red giant star. When this occurs, the red giant star becomes so puffed up that its outer layers are close enough to the white dwarf companion that it becomes more gravitationally attracted to it. Thus, the white dwarf starts to siphon off the envelope of the red giant star and forms an accretion disk around itself. When enough matter has piled on the white dwarf it ignites thermonuclear burning in its core again. However, since it no longer has an envelope of mass around the core like a normal star, it can’t contain the increase in pressure and temperature and thus ignition becomes a runaway event that blows the white dwarf apart!
The second type of supernova (also called Type II) comes from the core collapsing in a massive star that is capable of fusing Hydrogen in its core up to Iron. Once this occurs, the star can no longer create enough energy in the core to support the many solar masses of matter above it. In seconds the star implodes on its self and then blows apart, usually leaving a neutron star or black hole behind. Nick received a myriad of good questions about the particulars of these processes and he patiently responded to everyone who asked.
He then went on to show us many beautiful examples of supernova remnants and how you can tell the difference between the type of supernova that created them from the characteristics of the remnants themselves. One good example of the observational difference is that a core-collapse supernova can have a compact stellar remnant in its center whereas a mass-accretion supernova can’t since the central object is totally destroyed! Nick also told us why we view the spectra of these remnants in different wavelengths like the X-Ray, Radio, and Optical. He explained that with this data we can better figure out what the progenitor star was made of and how and when it exploded.
Finally, Nick shared some historical accounts of famous supernova like the Crab supernova, seen by Chinese and Arab astronomers in 1054, and Tyco Brahe’s supernova, seen by, well, the man himself, in 1572. There continued to be many good questions from the audience about supernovae and their remnants. It was only in the interest of time for observations on the roof that the Q&A session ended.
We had a great turn out and our attendees made full use of our facilities by watching astronomy visualizations on our 3-D wall, ran by graduate student Yuan Li, and by going to the roof to stargaze. It’s cool to think that some of the stars we observed will be become supernovae in the future.
Thanks to the 9 volunteers and the 140+ people who attended our lecture and observing night!