Supernovae
A supernova is the explosion of a star. Since the dawn of recorded history, we know that occasionally, a “new” star would appear in the sky. Nova is Latin for new. The most luminous and destructive of these are the supernovae. Supernovae are so powerful that the light emitted during the explosion can briefly outshine the light from an entire galaxy. The explosion occurs in an instant, so a supernova brightens very rapidly. However, the shock of the explosion heats up and illuminates the material all around the supernova, which itself becomes luminous, and fades more slowly, creating the light curve shapes characteristic of supernovae. For each type of supernova we describe below, we show only one spectrum, but be aware that the spectra of supernovae change with time as the supernova dims.
As more and more supernovae have been observed, a classification system has been defined based on the properties of different types of explosions. To better understand how astronomers can make these distinctions, you might want to take a look at the light curve and spectrum pages under the Key Concepts page linked above before reading further. From the observational differences between supernovae, astronomers have pieced together the physical differences as well.
Type 1 vs. Type 2
Supernovae can first be divided into two major classes: type 1 or type 2. Type 1 supernovae contain no signs of hydrogen, while type 2 supernovae do. Hydrogen can be detected at specific wavelengths in the spectrum where hydrogen signatures are either visible or not.
Type 1
First, let's talk about the different subclasses of type 1 supernovae. The differences between type 1a, type 1b, and type 1c supernovae can be observed between their spectra.
Type 1a
Type 1a supernovae are the most common and well-known type of supernova. They occur when a dense white dwarf star orbiting in a binary system pulls extra mass from its companion star until the white dwarf the pressure triggers a cataclysmic explosion caused by nuclear fusion reactions of carbon and oxygen. Because of the nature of white dwarfs, this collapse always occurs at about the same mass and so type 1a supernovae are thought to always blaze with uniform brightness. For this reason, astronomers use type 1a supernovae as "standard candles" whose apparent brightness can be used to accurately measure cosmic distances.
Below is a sonified video of observations of the type 1a supernova called 2011fe. The video scans over time (x-axis) and modulates pitch based on magnitude (y-axis). Lower pitch represents dimmer magnitudes. 343 observations were taken during a span of about 450 days in 2011-2012. There is a rapid increase in brightness to the peak of the explosion, followed by a gradual decline. The range of magnitudes in the video is extremely wide, with the brightest at around 10 magnitudes and the dimmest coming in around 17 magnitudes.
Below is the observed spectrum for the type 1a supernova. This video scans across a plot of brightness measured in flux or intensity of light (y axis) versus wavelength (x axis), moving from blue to red wavelengths from 3800 to 7200 Angstroms. Lower pitch represents weaker flux. The spectrum peaks near 4000 Angstroms and then dims moving toward longer wavelengths, punctuated by some small fluctuations. Strong dips in flux across the spectrum correspond to absorption by iron, calcium and silicon ions. Type 1a supernovae also show a characteristic strong absorption band of silicon near 6150 Angstroms.
Light curve from: Munari, U. et. al. 2013, New Astronomy, 20, 30-37
Spectrum from: Pereira, R. et. al. 2013, A&A, 554, 22
Type 1b
Type 1b supernovae are formed when the core of a massive star collapses under its own gravity. In the late stages of evolution of very massive stars, nuclear fusion reactions within the stellar core can no longer produce enough radiation to hold up the outer layers of the star. The result is a rapid stellar collapse followed by a rebound expansion and explosive release of energy. In a Type 1b supernova, the outer hydrogen layers of the collapsing star must have been expelled before the collapse, because there are no hydrogen features observed in the star's spectrum. However, we still observe helium signatures from type 1b supernovas.
Below is a sonified video of observations of a type 1b supernova. The video scans over time (x-axis) and modulates pitch based on magnitude (y-axis). Lower pitch represents dimmer magnitudes. During the observation period the supernova ranged roughly in magnitude from 18 to 15 magnitudes. Over a period of about 80 days in 2013, 343 observations were taken to capture the rise and fall in brightness that corresponded to the supernova explosion.
Below is the observed spectrum for the type 1b supernova. This video scans across a plot of brightness measured in flux or intensity of light (y axis) versus wavelength (x axis), moving from blue to red wavelengths from 3800 to 7200 Angstroms. Lower pitch represents weaker flux. Type 1b supernovae have strong absorption lines caused by helium - the lightest element after hydrogen, but also by heavier elements like iron and calcium, formed deep in the core of the massive progenitor star.
Light curve from: Fremling, C. et. al. 2016, A&A, 593, 27,
Folatelli, G. et. al. 2016, ApJ, 825, 22,
Brown, P. et al. 2014, Astrophys. Space Sci., 354, 1
Spectrum from: Srivastav, S. et. al. 2014, MNRAS, 445, 2
Type 1c
Similar to type 1b supernovas, type 1c supernovae are also formed when the core of a massive star collapses under its own gravity. In type 1c supernova, both the hydrogen layer and the helium layer must have been driven away from the star before it goes supernova, leaving no signatures of either element in the star's spectrum.
Shown below is a sonified video of observations of a type 1c supernova. The video scans over time (x-axis) and modulates pitch based on magnitude (y-axis). Lower pitch represents dimmer magnitudes. When the video begins, the supernova is already near its peak brightness, or the most intense period of its explosion. When listening to the 328 observations, you can hear the gradual decrease in brightness that occurs over about 6 months. The brightness dims by over 4 magnitudes, from 13 to 17.
Next, a sonified video of a type 1c supernova spectrum is shown. This video scans across a plot of brightness measured in flux or intensity of light (y axis) versus wavelength (x axis), moving from blue to red wavelengths from 3800 to 7200 Angstroms. Absorption bands of calcium, silicon, oxygen and iron dominate the spectrum of this Type 1c supernova.
Lightcurve from: ITEP Supernova Research Group (link)
Spectrum from: Pereira, R. et. al. 2013, A&A, 554, 27
Type 2
And now, let's learn more about Type 2 supernovae, which occur in massive stars, between about 8 and 40 times as massive as our Sun. Like all stars, they generate light and heat from nuclear fusion in their cores, which successively fuses lighter elements into heavier ones. In a massive star, the later phases of fusion leave a core made of nickel, cobalt and then iron. Fusion of iron to even heavier nuclei no longer produces, but rather absorbs energy. Therefore, outward radiation from the core no longer supports the outer layers of the star, which comes crashing inward. The core itself collapses into pure neutrons, forming a dense, stiff ball, and in the process blasting the outer layers with particles called neutrinos. Although these neutrinos interact only weakly with normal matter like atoms and ions, the blast from the core is so powerful that it blows the star’s outer layers out into the galaxy at velocities of thousands of miles per second, forming a type 2 supernova explosion.
There are two subclasses of type 2 supernovae -- type 2-L and type 2-P. While the spectra of these two types are largely indistinguishable, and both contain hydrogen features, the distinction between type 2-L and type 2-P supernovas is in the shape of their respective light curves.
Type 2-L
After its early peak in brightness, the light curve for a type 2-L supernova shows a steady, linear decline. Below, there is a sonified video example of a type 2-L supernova observed in late 2009 and early 2010. The video scans over time (x-axis) and modulates pitch based on magnitude (y-axis). Lower pitch represents dimmer magnitudes. The video begins near the peak brightness of about magnitude 16, and decreases linearly towards almost magnitude 19, which represents dimming by a factor of about 15. 101 observations were taken almost daily over about 100 days.
Shown next is the sonified spectrum video for the type 2-L supernova example. This video scans across a plot of brightness measured in flux or intensity of light (y axis) versus wavelength (x axis), moving from blue to red wavelengths from 3800 to 7200 Angstroms.
Light curve from: de Jaeger, T. et. al. 2012, MNRAS, 490, 2,
Silverman, J. et. al. 2012, MNRAS, 425, 3,
Brown, P. et al. 2014, Astrophys. Space Sci., 354, 1
Spectrum from: Hicken, M. et. al. 2017, ApJ Supplement Series, 233, 1
Type 2-P
The light curve for a type 2-P supernova starts to dim, then exhibits a distinct level stretch, or plateau, where the explosion's brightness decays at a slower rate before continuing to dim more quickly. A sonified video of observations of a type 2-P supernova taken in 2013 and 2014 is shown below. The video scans over time (x-axis) and modulates pitch based on magnitude (y-axis). Lower pitch represents dimmer magnitudes. The video spans 483 days and each interval of time plotted corresponds to 1 day of real time. Some clear differences can be heard between this light curve and that of the type 2-L supernova. The video starts from peak brightness and remains bright for about 100 days -- this is the “plateau” period. Next, there is a period of rapid dimming: a difference of almost 2 magnitudes in about 10 days. Then, there is a long period of linear dimming over nearly 400 days, punctuated by a seasonal observation gap of about 100 days.
Shown next is the sonified spectrum video for the type 2-P supernova example. This video scans across a plot of brightness measured in flux or intensity of light (y axis) versus wavelength (x axis), moving from blue to red wavelengths from 3800 to 7200 Angstroms.
Light curve from: de Jaeger, T. et. al. 2012, MNRAS, 490, 2,
Spectrum from: Yaron, O.; Gal-Yam, A. 2012, Publ. Astron. Soc. Pac., 124, 917
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