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May 20th, 2017
Supernovae are naturally occurring thermonuclear explosions that happen when a massive star explodes. This phenomenon marks the death of stars. However, not all stars end their lives as supernovae and there are several types of this phenomenon. What makes the difference is the initial mass of a star and its chemical composition.
Facts about Supernovae
- Supernovae are the final explosive disruption of stars, and are more massive than our Sun.
- Supernovae are very bright and they emit more energy at the peak of the explosion than a whole galaxy, like the Milky Way, with 100 million stars typically emits. A supernova emits the same energy in a few months that our Sun will emit in its entire life!
- Because supernovae are bright they can be observed from vast distances, across the known Universe. Therefore, if astronomers can confirm the precise type of a supernova, they can use it to measure the distance to its host galaxy. This is why certain types of supernovae are referred to as standard candles.
How are Supernovae Formed?
It is a common misbelief that stars are simple hot glowing spheres of gas. In fact, stars have complex structures and at least we should split them to a core and an envelope that is surrounding the core. The properties of these two are very different and to understand supernovae we need to consider the core.
In lower mass stars following the end of a burning stage the core starts contracting, and consequently its density increases. When the core is dense enough electrons are forced to fill certain orbitals, or states. We know from chemistry that only a certain number of electrons can occupy certain states, this is called the Pauli exclusion principle. As the core contracts and the density increases electrons are forced to occupy higher states, which requires more energy. This adds another pressure in addition to gas and radiative pressure, which together are enough to stop the core from contracting even if there is no burning in the core. In this condition the material of the core enters a degenerate state and the star becomes a white dwarf. If the mass of a white dwarf increases above 1.4 solar mass (Chandrasekhar mass limit), for example due to mass transfer from the companion star in a binary, or when the temperature rises just enough to start nuclear fusion in the core material, the whole star explodes in a fraction of a second as a supernova, leaving behind an expanding nebula.
In more massive stars the density is always large enough to start burning heavier elements before core contraction can result in a degenerate core. The star evolves along the fuel sequence until its core is composed of iron (Fe), and the core is surrounded by several shells or layers burning lighter elements. But iron cannot be burned further, therefore the core simply runs out of fuel. At this stage, if the core mass is above 1.4 solar masses, not even degeneracy can stop contraction and the core collapses. As a result a neutron star or a black hole forms at the core and the envelope of the star is fused in a supernova and dispersed, leaving behind a nebula.
Types of Supernovae
There are numerous observable properties of supernovae, based on which, one can categorize them (e.g.: the light variation, rate of fading, the features in their spectra, how and where the explosion starts inside the star, or the location of supernovae in galaxies). However, most of these features are related to whether a degenerate object accumulates mass and collapses or a thermonuclear runaway happens in a degenerate core.
This type of supernovae usually occurs in low mass interacting binary stars where one member is a white dwarf. When the white dwarf gets material from the companion, enough to increase its mass to 1.4 solar masses and the central temperature and density are enough to start carbon burning, it happens violently. In a fraction of a second the entire white dwarf explodes in a thermonuclear runaway.
There are several subtypes of Type-I supernovae depending on where and how the fusion is started, center or off-center in the core, or even in the envelope of the star. The explosion may also start with a core collapse. The most distinctive feature of Type-I supernovae is the lack of hydrogen in their spectra.
These supernovae originate from massive stars and hydrogen is always present in their spectra. When a 4-8 solar mass star reaches the end of its available fuel sequence, its core cannot sustain the pull of gravity and collapses. Interestingly, when even heavier stars collapse to black holes there is little energy release, hence they are not observable as supernovae, but such massive stars are very rare.
Only a few supernovae occurred in recorded history were bright enough to be visible by the naked eye. The most recent ones were Tycho's supernova in constellation Cassiopeia in 1572 and Kepler's supernova in Ophiuchus in 1604. Both were bright enough to be visible for weeks, even during the daytime. They are named after the famous astronomers who observed them regularly.
One of the best observed and analyzed supernova was SN1987A in the Large Magellanic Cloud 30 years ago, which is one of the satellite galaxies of the Milky Way. Similarly bright supernovae are rare events in our Galaxy, but thanks to large telescopes hundreds are discovered in distant galaxies every year.
There were numerous supernovae in the past, many of them have left spectacular nebulae behind, such as Messier 1, which is also known as the Crab nebula in constellation Taurus.