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Type II Supernovae

Page history last edited by Fatima Chaudhry 15 years, 1 month ago

Type II Supernovae

By Fatima Chaudhry and Michelle Chan

SN 1987A

http://hubblesite.org/newscenter/archive/releases/2007/10/

A supernova is an explosion that occurs with the collapse of a massive star. There are two main types of supernovae: type I supernovae and type II supernovae. Both types result from stars with different masses. They are mainly classified according to their spectra and the shape of their light curves. For example, the spectra of type II supernovae contain hydrogen lines, unlike that of type I supernovae.

This image shows the difference between the formation processes of type I and type II supernovae

http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/astpic/supnovform.jpg

This page will focus mainly on type II supernovae.

The Formation Process and Core Collapse

Because type II supernovae occur in stars much more massive than the sun, they evolve in complex ways. Unlike the sun which takes 10 billion years to finish its fuel, these massive stars take about 35 million years.

Since they are much more massive, nuclear fusion continues even past the helium fusion stage that occurs in stars of 1-8 solar masses. Now that there is less hydrogen, the star needs to counteract the gravity acting on it. So carbon and oxygen in the core fuse with helium atoms and create elements such as neon, magnesium, silicon and sulfur. These elements fuse together and produce more elements. Silicon and sulfur fuse to make nickel and iron. As fusion continues, heavier and heavier elements are produced. These heavy elements move to the center and the star becomes layered like an onion with the lighter elements as the outer layers. Iron, the heaviest, is at the centermost core.

Onion like formation of the star's layers

http://en.wikipedia.org/wiki/Type_II_supernova

However, the iron core is too compact and is unable to produce enough energy when fusing to maintain a nuclear reaction. With no more fusion taking place, there is nothing to counteract the force of gravity on the star. At that point, the mass of the iron core will go above the Chandrasekhar limit of 1.44 solar masses. When this happens, the core begins to collapse from an approximate diameter of 8000 km to 19 km at an unimaginably rapid rate. The energy produced during this collapse itself is more than 100 sun-like stars during their lifespan of 10 billion years. The core collapses so rapidly that it rebounds as soon as it collapses and the other inner layers of the stars hit the rebounding core and sends a shock wave through the rest of the star. The shock wave heats the outer layers and causes them to perform some more fusion before they are blown away. Neutrinos are produced which help move the outer layers further away. The shock wave also produces elements heavier than iron and exceeds the brightness of a billion suns.

What’s left over is called a supernova remnant. Inside this remnant, the result varies between a neutron star, a pulsar, a magnetar, or a black hole. Usually, the product is a neutron star. It is believed that if the star is below 20 solar masses, the end result will be a neutron star. Also, if the star is above 60 solar masses, it will skip the supernova process and collapse into a black hole.

Light Curves

When a massive explosion occurs, it’s the first sign of evidence that a supernova has occurred. This happens when the star has at least 9 times the mass of the sun, which these stars contain the mass needed to combine elements that has an atomic mass greater than hydrogen and helium. Thus shock waves, from the star, will release electromagnetic radiation that is seen as a UV flash. Supernova becomes visible in optical wavelengths as it enlarges, having an initial rise in the light curve; increasing in surface area of the star with a relatively slow decreasing temperature.

The highest point of the light curve would occur when the temperature of the outer layers of the star decreases. Being on the highest point in the light curve, type II supernova can be divided into two classes that are based on the shape of their light curves.

Type II-Linear Supernova (SNII-L)

This class of type II supernova has a fairly rapid linear decay or a steady decline after it reaches the maximum light on the light curve. The brightness of this class of type II supernova is uniform at ~2.5 magnitudes fainter than a type Ia Supernova.

Type II-Plateau Supernova (SNII-P)

This class of type II supernova stays bright for an extended period of time or shows a very slow decline (as if on a plateau) after it reaches the maximum light of the light curve. The brightness of this class of type II supernova shows a large dispersion, which is due to the differences in the radii of the progenitors.

Light Curve Shapes of Types II-P and II-L

http://astronomy.swin.edu.au/cms/imagedb/albums/userpics/typeiilightcurves1.gif

The light curves have an average decay rate of 0.008 magnitudes per day. This rate in type II supernova is much lower than the rate of type Ia supernova.

Recent News

Evidence of a supernova having occurred in our galaxy was found by Japanese scientists in an ice core sample. In these samples, three nitrogen oxide spikes were found and two of them were identified with SN 1006 and SN 1054. (02-23-09)
Astronomers have confirmed what they theorized to be the source of a type II supernova (star of more than 9 solar masses/ Red Supergiant) when they noticed that two stars had disappeared and in their place lie supernovae. The basic conclusion was that, as they had suspected, type II supernovae are a result of the collapse of Red Supergiants. (03-19-09)
However, just days after the above news, a star with a mass over a 100 times that of the Sun was collapsed into a supernova. It has been theorized that the star was not mature enough to fuse the iron core that it may have developed. According to previous theories, stars with a solar mass this huge, may not even become a supernova and collapse into a black hole. This new discovery will cause astronomers to rethink their previous theories on supernovae and the evolution of massive stars. (03-22-09)