Novae and Type I Supernovae

Sometimes, when a white dwarf forms, it is in a binary (or multiple) star system. If the companion star is close enough, the white dwarf call pull some of the material off the companion, and accrete it onto the surface of the white dwarf.


When the stellar material hits the hot surface of the white dwarf, the material can fuse quickly and create what is called a nova. Novae are not as energetic as supernovae, but will increase the luminosity greatly. However, the material accreted is rapidly used up and the nova dies down. Nova can occur many times and do have a predictable period that can be measured.


However, if the material on the surface of the white dwarf accretes too fast, the material can increase the mass of the white dwarf to over the Chandrasekhar limit. When this happens, instead of a nova explosion, the white dwarf undergoes a catastrophic collapse. What occurs is a Type I Supernova.


Since the white dwarf mass becomes larger than 1.4 solar masses, the electron degeneracy of the white dwarf cannot overcome the gravity the white dwarf experiences. The white dwarf catastrophically collapses, allowing all the material in the white dwarf to fuse rapidly. The outward explosion from this sudden release of energy completely destroys the white dwarf and companion star. The way astronomers differentiate between Type I Supernovae and Type II Supernovae is the lack of hydrogen lines in Type I. Because the white dwarf has no remaining hydrogen and the hydrogen from the companion star is completely used up in the fusion process, we know that Type I Supernovae can only be created by the sudden collapse of a white dwarf that accretes material from a companion star.

From Wikipedia, OOCalc chart

Looking at the above image, another difference between Type I and Type II supernova is that Type I are typically much brighter in the beginning but fade much quicker.

Black Holes

Black holes may be one of the most strange features in the universe. These objects are so dense, light cannot even escape from them!





Amazingly enough, almost all black holes started out as a hot, bright, large star. These stars go through the same evolution that leads to a neutron star, but unlike a neutron star, these objects are so massive, even neutron degeneracy is not enough to overcome gravity. Gravity compresses the star so much that instead of having a small core remnant after the supernova explosion, all that's left is what is called a singularity. Singularities are thought to be an object so small but so massive that it has an infinite density. In reality, this isn't true, but a naked black hole has never been observed, so the assumption is accurate for now.





There are a couple of things that define a black hole. One is its Schwarschild (pronounced Schwartz-Shield), which is the theoretical distance from the center of the black hole where the escape velocity from the black hole equals the speed of light. The speed of light is approximately 300,000 kilometers per second, or fast enough that a beam of light can circle the Earth 12 times at the equator. The escape velocity is given by the square root of 2 times G (the gravitational constant ~ 6.67*10^-11 m³/kg*s²) times the mass of the star or planet divided by the radius of the planet squared.





Or in equation form:

If you change V_esc to the speed of light, c, you can solve for R, and solve for the Schwarschild radius, R_s.

where M_Star is the mass of the Star, or in this case a black hole.

Next time, we will learn more about black holes, like how they are observed and what else we know about them.

Neutron Stars

Neutron stars are the final stage of a medium-high mass star. They come about after a medium-high mass star ends in a violent Supernova Type II explosion. What exactly are neutron stars?

Neutron stars are a second type of degenerate stars, much like white dwarfs. However, unlike white dwarfs, the degeneracy comes from neutrons, not electrons, hence the name. But neutron degeneracy only arises when the neutrons are much more closely packed than the electrons. Whereas a white dwarf is on the scale on the Earth, in terms of radius, neutron stars are only 10 km in diameter, about as big as the size of Manhattan Island. Masses for neutron stars are larger than the Chandrasekhar limit, but only goes up to two (2) solar masses. Anything larger than that, gravity breaks the neutron degeneracy, and the neutron star will collapse into a black hole, which is ever wackier.

Neutron stars can also spin rapidly, and if they are aimed correctly, they can beam jets of energy along magnetic field lines towards Earth. When we see a neutron star rotating in this manner and we detect the beam of energy, the neutron star is called a pulsar, even though the star does not pulsate. The beams of energy are similar to a lighthouse shining light out to the ocean at regular intervals. However, pulsars rotate so fast that they can have periods on the scale of milliseconds (rotating almost a thousand times a second). We will talk more about pulsars in the next post.

Supernova Type II

Supernova Type II Remnant

Image Credit:

NASA/CXC/Penn State/S.Park et al.; Optical: Pal.Obs. DSS

 

Let's give a quick explanation of how neutron stars and black holes form. Neutron stars start out as medium-high mass main sequence stars around Spectral Type B and A stars and black holes start out as heavy stars at Spectral Types O and B. Once the hydrogen in the core is exhausted, the outer layers fall onto the core, heating it up and allowing the core to fuse helium into carbon, nitrogen, and oxygen via the triple alpha process. Surrounding the core is an envelope of hot hydrogen, still fusing into helium via the CNO cycle. Once the helium in the core runs out, again the star collapses onto the core, heating it up, allowing the heavier elements in the core to, fuse into even heavier material, with a layer of burning helium surrounding the core, followed by a layer of burning hydrogen. These processes and cycles continue until the core is mostly iron. Iron is a unique element in such that in order to furs it, energy must be added and no temperature can cause iron to fuse into anything heavier. When the outer layers collapse onto the core in this case, the temperature rises dramatically, and the outer layers explode into space via a supernova explosion.

A cool thing about supernovae is that all natural elements heavier than iron are created in supernovae. All the heavy elements found naturally on Earth, formed in a supernova. For that matter, any element heavier than helium was also formed in a star. So we are all made up of star stuff.

This is called a Supernova Type II because it is created via a core collapse of a medium-high to high mass star. Supernova Type I are caused by the accretion of material onto a white dwarf and can come from white dwarfs in binary (or multiple-) star systems. Type II supernova have hydrogen present in their spectra, while Type I lack hydrogen.

Supernova Type II remnants can be identified by two properties: hydrogen in their spectra and a stellar object at the center of the supernova remnant. Depending on the mass of the star, the remaining stellar object will either be a neutron star or a black hole. The next two posts will talk about both these remnants.