Brilliant Supernova Blazes to life in the Pinwheel Galaxy, Messier-101

As featured in The Astronomy Picture of the Day for May 22, Type II supernova SN 2023ixf is seen here in the host galaxy, Messier-101 or the “Pinwheel Galaxy”. Imaged by Craig Stocks in this composite animation, the “before” and “After” images clearly show this super “New” Star or Supernova. Based on the supernova’s spectra, we know it was the spectacular end to a high-mass star or a Type-II, Core-collapse supernova.

Brilliant Supernova in a Nearby Galaxy

Click for Supernova video

Imagine inhabiting a planet within 100 light years of a Type II supernova. Take a good look at the image above. The pink emission region immediately to the east (left) of the supernova would be about 100 light years. At the time the second image (the “after” image in the 2-image composite) was obtained, the supernova was at magnitude 11 (for comparison, the faintest star visible to the unaided eye is between 6 and 6.5). The galaxy is 21 million light years distant with an 11th-magnitude source puts the absolute magnitude of the supernova at -18.4!! Absolute magnitude is a source’s apparent magnitude measured at 10 parsecs or about 33 light years.

Placed at 33 light years distant, this supernova would light up the sky 100x brighter than the full moon, cast a shadow and turn the black sky dark blue!

An absolute magnitude of -18.4 is more luminous than the typical Type-II supernova, suggesting its mass was above 20 solar masses. By comparison the famous supernova of 1987, SN-1987a (see below) had, at its peak brightness, an absolute magnitude of -15.5. The progenitor for that supernova was a 20 solar-mass class B blue giant star.  The full-moon has an apparent magnitude of -12.6. The supernova in M-101 would then present with an apparent brightness 100x brighter than the full moon at 33 light years! The smaller or more negative the number, the brighter the object.

Why 100 light years?

The minimum safe distance from any supernova is about 100 light years. Since all Electromagnetic Radiation (light) travels at the speed of light, the instant the supernova is visible would be the same instant the hypothetical planet would be bathed in a dangerous (or lethal) dose of gamma rays. At a distance of 100 light years, the supernova would be 9x fainter than it was at 33 light years (10 parsecs) or 2.5 magnitudes fainter with an apparent magnitude of -16.

Named according to the standard naming convention, SN 2023ixf was discovered by Japanese amateur astronomer Koichi Itagaki on May 19 and was subsequently located on automated images from the Zwicky Transient Facility two days earlier. It was featured on NASA’s Astronomy Picture of the Day for May 22nd.

What would it Look Like from 100 Light Years?

The red supergiant star Betelgeuse, the brightest star in Orion marking his right shoulder is seen here in the immediate aftermath of its inevitable demise as a Type-II supernova. Betelgeuse has unexpectedly brightened again and has piqued the interests of the world’s astronomers as it did 2 years ago when it underwent greater than normal fluctuations in brightness. At 427 light years from earth, Betelgeuse is at a distance 4x greater than our hypothetical planet at 100 light years. So, if it went supernova now, Betelgeuse, as illustrated above, would be 16x dimmer than SN 2023ixf at 100 light years or 3 magnitudes fainter.

The Sun’s Proton-proton Chain

All stars are dynamic, self-regulating, self-contained energy sources that produce their energy by transmuting 4 hydrogen nuclei (protons) into1 Helium nucleus (an alpha particle). This Proton-proton chain is the principal energy source for stars up through about 5 solar masses, beyond that mass an additional nuclear fusion cycle is principally used. This process continues in stars such as the sun until about 12% of the original core hydrogen remains.

The Sun as a Red Giant and Ultimate White Dwarf

After this core-compliment of hydrogen has been consumed, the fusing, nuclear core contracts, heats-up to over 100 million kelvin and begins to produce carbon and oxygen in a process known as the “Triple Alpha” process, a 2-stage process where 2 helium-4 nuclei fuse to form Beryllium-8. This is followed with a second step where another helium nucleus fuses with the Beryllium-8 to form a Carbon-12 nucleus. Occasionally, another helium-4 combines with the carbon-12 to form Oxygen-16. This Triple-Alpha process continues for about 10% of the sun’s original life or about 1 billion years after which there is insufficient mass to continue this progressive alpha process of building heavier nuclei.  A Carbon-oxygen stellar core remains for stars such as our sun.

This dying stellar ember is surrounded by an expanding shell of gas, gas that used to be the sun’s outer layers and becomes what’s known as a “planetary nebula”. This core provided the earth and the solar system with light, heat and energy for over 10 billion years is now a White Dwarf star. We skipped a number of important details in this process since this isn’t about the evolution of solar-mass stars, it’s about the terrifying end to a high-mass star as a Type-II, core-collapse supernova. Those details include multiple expansions to become first, a sub-giant star when the sun exhausts is hydrogen, then a second phase in its evolution when it expands fully to become a red-giant star, engulfing the inner planets.

Stellar Nucleosynthesis of Heavy Metals

For higher-mass stars such as the progenitor star of SN 2023ixf, the process continues. For stars greater than 8 solar-masses, this alpha-burning process continues up through a silicon burning cycle that lasts 24 hours, a cycle that produces the heavy metals Iron and Nickel as the final step. At this point, unable to continue the process of producing energy with the addition of a He-4 nucleus to Iron, the core collapses at 30% the speed of light. The stellar core is in free-fall and soon rebounds off an unimaginably dense nickel-iron core, releasing the all this pent-up gravitational potential energy in a final, spectacular release of energy, obliterating the star as a Type-II core-collapse supernova, leaving behind either a neutron star or black hole in its wake.

Click for video (evolution of 15 solar-mass star)

Some famous examples of Type-II supernovae

SN 1987a

Over three decades ago, astronomers spotted one of the brightest exploding stars in more than 400 years. Since that first sighting, the doomed star, called Supernova 1987A, has continued to fascinate astronomers with its spectacular light show. The NASA/ESA Hubble Space Telescope is one of many observatories that has been monitoring the blast’s aftermath. This image shows the entire region around the supernova. The most prominent feature in the image is a ring with dozens of bright spots. A shock wave of material unleashed by the stellar blast is slamming into regions along the ring’s inner regions, heating them up, and causing them to glow. The ring, about a light-year across, was probably shed by the star about 20,000 years before it exploded. Astronomers detected the first bright spot in 1997, but now they see dozens of spots around the ring. Only Hubble can see the individual bright spots. In the next few years, the entire ring will be ablaze as it absorbs the full force of the crash. The glowing ring is expected to become bright enough to illuminate the star’s surroundings, providing astronomers with new information on how the star expelled material before the explosion. The pink object in the centre of the ring is debris from the supernova blast. The glowing debris is being heated by radioactive elements, principally titanium 44, created in the explosion. The debris will continue to glow for many decades. The origin of a pair of faint outer red rings, located above and below the doomed star, is a mystery. The two bright objects that look like car headlights are a pair of stars in the Large Magellanic Cloud. The supernova is located 163,000 light-years away in the Large Magellanic Cloud. The image was taken in December 2006 with Hubble’s Advanced Camera for Surveys.

The famous Crab Nebula in Taurus

First witnessed by Chinese astronomers in 1054, the famous Crab Nebula is now a supernova remnant. The nebula is powered by a rapidly rotating neutron star at its center. Colloquially known as a “Pulsar”, this object was the progenitor star’s nuclear core and its rotational energy is what’s providing 100% of the remnant’s radiant energy.


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