Astronomy/Type Ia Supernovae

For 2012, the Astronomy event will focus on stellar evolution and type Ia supernovae.

A supernova is, in short, the explosion of a star. This term can apply to several different types of explosions, though, and so, like many other astronomical terms, there are classifications. Type Ia supernovae are explosions of white dwarves in binary systems that pull mass off of their partner and accumulate enough pressure for a supernova. Type Ib and Ic supernovae are formed when a large star is stripped of its outer hydrogen layers. The Type I supernovae are generally associated with binary systems. Type II supernovae are explosions of supergiant stars that occur when the star fuses iron in its core. Some Type II supernovae are hypernovae, occuring in hypergiants, even larger and brighter than regular supergiants.

Type Ia

Basically, these supernovae are the explosions of white dwarfs. All type Ia supernovae emit roughly the same amount of energy because they result from the same type of star (a carbon/oxygen white dwarf around 1.4 solar masses), making them a good tool to determine galaxy distances. These supernovae also have very distinctive light curves that fall off quickly and steadily, as compared to the gradual fall-off of Type II supernovae. The spectra is also distinctive, since exploding dwarfs don't have hydrogen absorption lines.

Causes

Type Ia supernovae occur because of the Pauli Exclusion Principle, which states that two particles of the same type can't be in the same quantum state (position, velocity, energy level, spin, etc.). Quantum mechanics says that as a white dwarf gains mass and its electrons are squeezed into a smaller and smaller space, they have to move faster to avoid being in the same quantum state as other electrons. As a white dwarf approaches the Chandrasekhar Limit, its electrons must move at nearly the speed of light! Since nothing in the universe can move faster than the speed of light, a white dwarf can't exist above 1.4 solar masses, and instead collapses into a neutron star or a black hole.

The most common model for a type 1a supernovae consists of a binary star system of two main sequence stars. The larger of the stars will expend the hydrogen in its core faster and evolve into a red giant before its partner. Eventually, the larger star becomes a white dwarf and the smaller evolves into a red giant. The orbital period of the binary star system then decreases and can be as low as a few hours. As the angular momentum of the system is lost, the stars spiral together with the white dwarf accreting gas off of the red giant. Ultimately, the white dwarf explodes for reasons listed above.

Another model is where two white dwarves orbit each other quickly and begin to fall toward each other. Eventually, they will collide in the center of the system, and the resulting body will have a mass of over 1.4 solar masses, thus exceeding the Chandrasekhar Limit and resulting in a supernova. RX J0806.3+1527, one of this year's DSO's, fits this model.

Use In Determining Distances

An important characteristic for Type Ia supernovae is that they can act as a standard candle. All Type Ia supernovae have an absolute magnitude of about -19.3 (sometimes cited as -19.6), so by measuring the apparent magnitude observed from the explosion on Earth, one can simply use the distance modulus formula to determine the distance to the object.

Other Supernova Types

Although the main topic this year is on Type Ia supernovae, you may find questions relating to other types of supernovae and how they differ from each other.

Type Ib and Ic

Type Ib and Ic supernova (also called stripped core-collapse supernovae) both result from the collapse of a massive star, much like Type II supernovae. However, the stars that produce Type Ib and Ic supernovae have lost their outer shell of hydrogen (and also helium, for a Ic) due to strong stellar winds or gravitational pull on the outer layers by a partner in a binary system. The spectra of Type Ib/c supernovae are very similar to that of a Type Ia, except for the fact that they lack an absorption line of ionized silicon at 635.5 nm. The spectra of Types Ib and Ic can be distinguished by the absence of helium lines at 587.6 nm for a Type Ic (since it has lost most of its helium).

Type II

These supernovae are the explosions of massive stars, resulting from the collapse of the star's iron core. When the iron core reaches the Chandrasekhar limit, the electron degeneracy pressure (in layman's terms, the unwillingness of electrons to be squeezed into a smaller and smaller space) which had kept it from collapsing before then, isn't enough to hold the core up. Protons and electrons in the core are forced together to form neutrons and neutrinos. The neutrinos produce a huge outward force simply because there are so many of them (they don't usually interact with regular matter).

Meanwhile, the outer layers of the star fall inward, due to gravity, as the core collapses. When the core stops collapsing because of neutron degeneracy pressure, the outer layers crash into the core and "bounce" outwards, creating a shock wave. Along with the outward pressure from the neutrinos, this shock wave is what causes the star (except for the core) to blow itself apart in a Type II supernova.

If the core is 2-3 solar masses or less, it evolves into a neutron star - a star made up almost entirely of neutrons. Neutron stars spin very fast due to how dense they are, and they have very strong magnetic fields, which can emit radio waves. As these stars spin, the radio jets are aimed out into space. When the jet is momentarily aimed towards Earth during its rotation, we see a regular, repeating pulse...which is why these stars are called pulsars.

If the core ends up with over 2-3 solar masses of matter (because some of the matter in the outer layers fell back onto the core when the star went supernova), even neutron degeneracy pressure can't support the core against its own gravity and it collapses into a black hole. Once the core contracts to a small enough size, the escape velocity becomes greater than the speed of light! Nothing - not even light - can escape the gravitational pull of the collapsed core. Because of this, we can't even see the black hole itself; all we can see are its effects on nearby matter.