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Intergalactic Supernovae by Joe Renzetti
It is very difficult for the human mind to comprehend the vastness of the Universe. We can look at distant galaxies in our telescope and read up on their sizes, distances and the mass of their stars. But a galaxy is usually still just seen as a faint fuzzy patch in our night sky. So, when we occasionally get a glimpse of a supernova and see a new bright star suddenly appear in a galaxy millions of light years away, we can better comprehend that vastness and the incredible power that the supernova unleashed.
A Star is in a constant battle between the force of gravity trying to collapse it and the internal energy released from nuclear fusion trying to expand it. For most of a star’s life these forces are in balance. But then, at the end of its life, the star expands and blows away its outer layers, leaving behind an incredibly
Most stars go out with a whimper. But a few high mass stars go out with a bang- a huge, violent bang known as a supernova.
A supernova occurs in a star that has more than 8 times the mass of our Sun. Stars of this size are extremely hot and luminous, as they “burn” through their fuel at a much faster rate. These stars will progress up the periodic table, progressively generating heavier elements and creating energy in the process. Once silicon fusion starts, however, the star becomes a ticking time bomb. Silicon fuses into several heavy elements, including iron- which builds up in the ever denser, ever shrinking, ever hotter core of the star.
At this stage the iron core removes energy from the star instead of creating it, accelerating the shrinking while compressing and heating the core even more. Then, in a fraction of a second, the core collapses and the temperature rises to over 100 billion degrees as the iron atoms are crushed together. With no outward energy to counterbalance it, the core’s gravitational force pulls on the star’s matter above,
which begins falling at fantastic speed. At the same time a shockwave created by the collapse of the core moves outward against the incoming material. Also at the same time, the core generates vast amounts of neutrinos that slam into the incoming material, causing it to reverse course and explode outward. This explosion is the supernova.
The energy released in a supernova is so huge it can be seen halfway across the Universe. It can outshine all the stars in its host galaxy combined. This is why intergalactic supernovae that occur in galaxies millions of light years away can be observed in an amateur telescope.
Supernovae are classified according to their light curves (a graph of the rise and fall of their brightness) and the absorption lines of different chemical elements that appear in their spectra. There are two main classifications of supernova: “Type I” and “Type II’’. Type I supernovae are without hydrogen absorption lines in their spectrum and Type II supernovae are, you guessed it, with hydrogen absorption lines in their spectrum.
The Type I supernovae are also subdivided into three subclasses:
Type Ia: no hydrogen lines, no helium lines, strong silicon lines
Type Ib: no hydrogen lines, strong helium lines
Type Ic: no hydrogen lines, no helium lines, no silicon line
Type Ia class supernovae occur in a white dwarf binary system. When a white dwarf’s binary companion is close enough, the white dwarf can steal matter from its partner. When the absorbed matter reaches 1.4 times the mass of our sun, the white dwarf reaches what is known as the Chandrasekhar limit and it explodes as a supernova and completely vaporizes.
Type Ib and type Ic supernovae are often referred to as stripped core-collapse supernovae. They are massive stars which lose their outer layers in a stellar wind before the core collapses. Type Ib supernovae lost their hydrogen-rich outer layer, revealing the helium-rich layer immediately below. Type Ic supernovae suffered more mass loss as supergiants, losing both the hydrogen-rich layer and the helium-rich layer, revealing the carbon-rich layer below.
Type II supernovae are more common. They occur in high mass stars during the events described above.
They are also divided into subcategories:
Type IIb: weak hydrogen spectrum that changes to become like a Type Ib
Type II-L: light curve has a linear decline
Type II-P: light curve has a distinct plateau during the decline
Type IIn: has narrow or intermediate width hydrogen emission lines in the spectra
The light curves of a Type II supernova rise to a peak brightness followed by a decline. They
have an average decay rate of 0.008 magnitudes per day. Because the Type II-P has a plateau period
the luminosity decays at a slower rate of 0.0075 magnitudes per day, compared to 0.012 magnitudes per day for Type II-L. As a result, a Type II-P can be visually observed for a longer period after its peak.
While Type II supernovae can vary significantly in brightness, all Type Ia supernovae have approximately
the same absolute magnitude after their light curves have been corrected for the timescale “stretch-factor” (an adjustment made which accounts for the rate at which the brightness of a supernova declines.)
Because Type Ia supernovae have a known brightness, they are used as a standard candle to determine the distance to a galaxy, once the stretch-factor is accounted for. Detecting Type 1a intergalactic supernovae are valuable not just for distance measurements but also for calculating the expansion of the Universe and to probe dark energy.
Supernovae visible to the naked eye are very rare. One occurs in the Milky Way every few hundred years, so there is no guarantee you will ever see one in the Milky Way in your lifetime. To ensure astronomers do not miss this rare event, The SuperNova Early Warning System (SNEWS) was created. SNEWS is a network of neutrino detectors. Because a burst of neutrinos is emitted well before the light from a supernova peaks, detecting it gives advance warning to astronomers that a supernova has occurred and may soon be visible. The neutrino pulse from SN1987A in the Large Magellanic Cloud arrived 3 hours before the associated light photons. This was before SNEWS was activated. Type Ia supernovae, however, would not be detected by SNEWS as they don’t produce a significant number of neutrinos.
The last Milky Way supernova to have been observed was Kepler’s Supernova, in 1604, in the constellation Ophiuchus.
Intergalactic supernovae occur at a much more frequent rate and give amateur astronomers an opportunity to see an exciting change in the normally static night sky outside our solar system. Before computer automated searches, such as OSU’s All Sky Automated Survey for SuperNovae (ASAS-SN), were established, amateur astronomers played a more active role in finding intergalactic supernovae. In order to find supernovae, we compare new galaxy images with older ones and look for a “suddenly bright” star. Each region targeted to be searched must initially be imaged to form a template against which all other images can be compared.
Fortunately, with the increased activity in amateur astrophotography online, finding a galaxy image for comparison is fairly easy using Astrobin and Google Image searches.
The chances of detecting a supernova increases in galaxies that have very active star forming regions. These would be merging and interacting galaxies and galaxies that have an active galactic nucleus. The ones that have an exceptionally high rate of star formation are known as “Starburst Galaxies”. These include: M82, M83, M51, M61, NGC 253, and NGC 6946. If you spot a supernova that isn’t on record, you can report it to the IAU Central Bureau for Astronomical Telegrams.
Despite the increasing success of automated searches by ASAS-SN and the Sloan Digital Sky Survey (SDSS), even very young amateurs can still get lucky and make a discovery. 14 year-old Caroline Moore discovered a supernova in UGC 12682 in 2008. 10 year-old Kathryn Aurora Gray discovered one in UGC 3378 in 2011. 10 year-old Nathan Gray discovered one PGC 61330 in 2013. Kathryn Aurora Gray and Nathan Gray are in fact siblings.
Amateurs can also contribute to supernova searches with confirmation imaging for ASAS-SN. Since their network of 14-cm survey telescopes capture images with only 7.8 arcsecond wide pixels (the resolution of an average DSLR with a moderate telephoto lens), they rely upon “unpaid professional collaborators” with larger telescopes to capture higher resolution images of suspected supernovae for confirmation.
Amateurs who simply wish to observe or image a recently discovered intergalactic supernova can follow the latest discoveries listed online at the “Latest Supernovae” site hosted by the Astronomy Section Rochester Academy of Science. They keep an updated list of all active supernovae over mag 17.0 with
their host galaxy and RA/Dec coordinates.
A supernova typically takes two to three weeks to reach peak luminosity. The time it takes to dim can range from several weeks to several months depending on the type of supernova.
The most intrinsically bright supernova ever recorded was ASASSN-15lh, but from Earth it only had a peak apparent magnitude of 16.9 as it was an extremely distant 3.8 billion light years away in the host galaxy APMUKS(BJ) B215839.70−615403.9 in the constellation Indus.
The supernova with the brightest apparent magnitude recorded in the telescopic era was SN1987a in the Large Magellanic Cloud, which peaked at magnitude +2.9. The vast majority of intergalactic supernovae are considerably dimmer and can be a challenge to see in the eyepiece of a telescope. That’s not to say it can’t be done, as famed amateur astronomer Bob Evans from Hazelbrook, Australia would attest. He holds the record of 42 visual supernova discoveries from 1981 to 2008.
Whether with an eyepiece or a camera, observing intergalactic supernovae connects you to the awesome power of creation from destruction. When we observe a supernova we are witnessing the final moment in the life story of massive star and a cataclysmic event beyond all human experience- one that makes its fury known across half the Universe. Yet, we are also seeing the unmistakable hints of our own origins. Life as we know it could not exist without the elements forged in the nuclear furnace of a high mass star long ago. That star ripped itself to shreds in a violent death so it could deliver the building blocks of life when our solar system was born.
Supernova images all captured and processed by Joe Renzetti
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