SUPERNOVAE
CONTENT
I would be surprised if you had not heard of a Supernova or Nova, but if not, here is all the information you will ever need to know about the destructive forces of these hidden beasts.
NOVAE
Every few years most astronomers are aware of "new stars" that seem to appear out of nowhere before fading into nothing again. These are called Novae. There have been a couple of good examples in the last few years or so, including: Nova Cygni, Nova Aquila & Nova Sagittarius. It's fair to say that most astronomers are aware that stars are not NEW stars at all, but simply faint stars that for some reason had an increase in luminosity and become visible to the naked eye. What I shall do here is briefly explain what happens during the outburst of a nova, and it's cause.
Firstly, Novae only occur in binary star systems.... why? Well the cause of Novae is when a normal main sequence star sheds matter onto it's white dwarf companion. This occurs due to the large gravitational attraction of the small hot white dwarf. The surface of the normal main sequence star is close to 95% Hydrogen, and so deposits almost pure Hydrogen gas onto the white dwarf. The gas slowly gets peeled of the larger star and then begins to spiral faster and faster toward the white dwarf before being deposited onto the surface. The speed at which the gas is deposited is a major factor in determining how the layer is structured. This is because the gas which is layered on the surface releases gravitational energy which heats the white dwarf.
The quicker the gas is deposited onto the surface the hotter the star becomes. If the matter is deposited slowly then the white dwarf surface remains cool enough for the surface of the star (including the deposited material) to remain degenerate*. Over a long period of time (hundreds or thousands of years) this hydrogen rich layer will increase dramatically in mass. If the temperature at the base of the surface layer gets hot enough, the hydrogen rich layer begins to fuse into helium. The energy of the fusion reactions do not depend on the temperature of the gas if it is degenerate*. The heated Hydrogen increases the rate at which it fuses, which raises the temperature even further and therefore increases the fusion rate again. This effect is known as a thermonuclear runaway effect, and in a matter of hours it can increase the luminosity of the white dwarf by a factor of one million. This is the cause of the Novae outburst. The previously (but continual) accreted matter is blasted into space at around 1,000 metres per sec. Over the following tens of thousands of years the white dwarf will and cool, and repeatedly accrete matter from its binary partner.
The number of novae in a galaxy far out-weight the number of supernovae. About 50 a year can be detected, although only about 2-3 of those are visible to the naked eye. For comparison, no supernovae has been observed in the Milkyway since 1604.
(*electron degeneracy : When a gas becomes degenerate, the lowest electron energy levels are filled, and the electrons are forced into higher and higher energy levels, filling the lowest unoccupied states. The density of a degenerate gas is not temperature dependant)
Supernovae
A Supernova is simply a star that explodes. Like the events that shape a planetary nebula, a supernova is the death of a star in a huge explosion. The energy released by supernovae is much higher than that of any novae or planetary nebulae. A star will only become a supernova if it is born with a mass greater than 5-8 times that of the sun, anything smaller will simply become a planetary nebula.
The main types of supernovae are, type I, type Ia, and type II
A type I supernova is caused by the collapse of a white dwarf star left over from a planetary nebula. A type I supernova can only occur in a binary star system. Although you may expect both stars in a binary system to be the same age, a man called John Crawford proposed an idea to the contrary. He proposed that the secondary star (the one about to become a white dwarf) was in fact once a much more massive star, even bigger than the now seen primary star in the system (which should be only a few hundred million years old - unlike the secondary which should be many billions of years old). The massive star completed it's hydrogen burning phase and attempted to become a red supergiant (such as Betelgeuse & Eta Carina). As it grew, it found itself being distorted by the secondary (today's primary) younger star. The once huge primary star starts to pour of material onto the secondary star increasing it's mass to slowly become the primary. Over the course of a few million years the two stars actually switch place. The now primary star will itself become a red giant in time and start to pass it's material back onto the secondary (now white dwarf). The higher gravity of the white dwarf pulls more and more mass off the primary, gradually becoming more dense. The stars mass will eventually increase above the Chandresekhar limit. This states that when a white dwarf becomes more than 1.4 times the mass of the Sun, the laws of Quantum mechanics prohibit the matter at the centre of the white dwarf from being any denser. The star collapses in a catastrophic explosion called a type I Supernova. The white dwarf star has now become a Neutron star (see below).
The type Ia supernova is very similar to the standard nova which is described above. The difference is the amount of energy released. The example of a nova above is described for a slow deposition of hydrogen. The cause of a type Ia supernova is the same, but for a fast deposition of hydrogen. In this circumstance, the rate at which the gas falls onto the surface of the white dwarf is high enough for the surface temperature to increase so it cannot become degenerate. Without degeneracy the thermonuclear runaway effect cannot occur. Without this runaway effect the rate of fusion goes on at an almost steady rate. Without a runaway effect and a steady rate of fusion, there is no surface explosion of gas, and so the white dwarf becomes hotter, smaller, and gains more mass. Sooner or later the temperature at the bottom of the newly deposited hydrogen layer reaches a temperature of 600 million oK, now carbon starts to fuse into oxygen. In a less massive star, this increase in energy would expand the size of the core, but, because the gas at the surface of the white dwarf is degenerate, it only increases the temperature. Just as with the previous case, this then leads to the runaway thermonuclear effect. Almost 100% of the gas as the centre of the star is instantly changed by several nuclear reactions to form metals like Fe, Ni, and Co. At this time the entire star becomes convective as huge amounts of energy are produced. As in a normal convection system the cooler material absorbs the heat of a hotter material until equilibrium is reached. In this case the extremely hot carbon rich gas is carried by convection currents into the core of the white dwarf where it is rapidly consumed by fusion reactions. Because this takes place in the core of the star, the energy produced tears the star apart leaving nothing left. The remnants of the star are carried outward at speeds approaching 6,000 metres per sec. The amount of energy released in a Type Ia supernova explosion in 1 second, is equivalent to the entire energy the sun will produce in it's 10,000,000,000 year main sequence.
A type II supernova is caused when a star with at least 8 times the mass of the sun becomes a supergiant. The process of becoming a supernova through the fusion of elements in the core, is the same for these massive stars as it is for smaller stars such as the sun. Having a larger mass, the larger stars are able to continue the fusion phase past Helium, which is normally the limit for sun type stars. This nuclear fusion is described in detail below.
As the star begins to run out of hydrogen, the core cools and contracts as the pressure decreases. As the core contracts further the temperature begins to increase as the density increases. Once the temperature reaches 100 million degrees K, the Helium core will begin to fuse into Carbon. Oxygen will also be produced via the reaction:
4He + 12C → 16O + γ → γ = gamma ray
At this stage Neon is also produced via the reaction...... 4He + 16O → 20Ne + γ
This stage lasts around 500,000 years.
Once the core has depleted its helium content, the core now contains carbon and oxygen. Yet again the core will begin to contract and heat up. Once the core reaches 500 million degrees K, the carbon and oxygen will begin fusing into Neon, Sodium, and Magnesium via the reactions:
12C + 12C → 20Ne + 4He
12C + 12C → 23Na + p → p = proton
12C + 12C → 23Mg + n → n = Neutron
This stage lasts around 600 years.
Contraction occurs once again when the supply of carbon and oxygen has gone, and the temperature of the core reaches 1 billion degrees K. The neon burning phase directly produces oxygen through the destruction of Neon by high energy photons via a process called photodisintegration:
γ + 20Ne → 16O + 4He
Magnesium is also produced through the interaction of helium nuclei with undisturbed Neon via the reaction:
4He + 20Ne → 24Mg + γ
This stage lasts around 1 year.
16O + 16O → 28Si + 4He
This stage lasts around 6 months.
The final stage occurs when the core runs out of Oxygen, and begins to destroy silicon via photodisintegration. This occurs when the temperature reaches 3 billion degrees K. The nuclei of Silicon are very tightly bound and require huge energies to break them apart. A photon energy of at least 1.3Mev is required. The break-up of silicon produces Magnesium via the reaction:
γ + 28Si → 24Mg + 4He
The released Helium helps to create Sulphur via the reaction:
4He + 28Si → 32S + γ
The sulphur then joins to Helium to create Argon via the reaction:
32S + 4He → 36Ar + γ
This continues like so.....
36Ar + 4He → 40Ca + γ
40Ca + 4He → 44Ti + γ
44Ti + 4He → 48Cr + γ
48Cr + 4He → 52Fe + γ
52Fe + 4He → 56Ni + γ
All these reactions proceed one after the other and are consumed in less than 1 day.
At this point energy is absorbed from the gas and no more elements can be produced. At this point the star resembles an onion, with several layers. The Iron core is surrounded by shells containing Silicon, Oxygen, Neon, Carbon, Helium, and Hydrogen at the surface. No energy can be released and so the internal pressure of the core is lost and begins to contract. The contraction will continue until it reaches the Chandrasekhar limit of 1.4Msun. At this point the core cannot be sustained by internal pressure and begins to collapse. The core collapse speed is purely dependant on the density of the core at the time of collapse. Given the enormous density at the centre of a 25 solar mass star is around 1012kg m-3 (10 billion tonnes per cubic meter), the entire core can collapse from several thousand miles in diameter to just a few miles across in 1millisecond (free fall speed).
The core stops collapsing when it reaches a uniform density comparable to that of the nuclear density.....the density of atomic nuclei. This is 2.3 x 1017 kg m-3 (230 trillion tonnes per cubic meter). Upon halting, the core will slightly rebound and send a violent shockwave through the star. The shockwave tears apart the outer material of the star revealing the intensely hot and bright core of the star, a neutron star. The outward explosion is the supernova type II.
Fact: The Iron in your blood and the calcium in your bones was produced over 5 billion years ago in the core of a supergiant star in the same process as above. The outward flowing material eventually collapsed to produced the sun and all the planets. If it were not for supernovae, you and I would not be here.
It has been estimated that there is a supernova event of some sort every second in the Universe. Unfortunately we have not witnessed one in our Galaxy since 1604.
This image has been reversed to give a before and after view of a supernova in action. Called SN1987a, a type II supernovae originally catalogued as Sanduleak -62o 202 was seen to explode in mid 1987 in the southern hemisphere.
The images below were taken 4 years after the supernovae brightness peaked.
3.9m AAO telescope (pink dot)
The stunning view from HST 2.4m
On a much larger scale you can see how supernovae can easily outshine many billion stars
HST image of SN94D (type Ia) in NGC 4526. The supernovae is the bright star to the lower left.
The most well known supernova remnant is probably M1, the crab nebula in the constellation Taurus. Seen below is a photo of the remnant which was first recorded on the 4th July 1054 AD by Chinese Astronomers. This star was so bright it was visible in the middle of the day, and at night could even cast shadows.
"...in the first year of the period Chih-Ho, the fifth moon, the day of Chi-ch'ou, a guest star appeared approximately several inches south east of Tein-Kuan.....after more than a year it gradually became invisible......".
The star actually exploded around 4,000 BC, and can still be seen in the images below taken by the VLT and HST respectively.
The above image (right) shows the star to left of centre (indicated). All the filaments, gas, and dust seen above all went through the above process thousands of years ago.
Maybe one day the remnant will form new stars, and possibly even planets with life. If we are alone in the universe, chances are that it will not be long before all that changes.
