Supernova Appears "Near" Earth, Reason for World's Attention

On May 19, 2023 (Coordinated Universal Time), the visit of Ukrainian President Volodymyr Zelensky to Hiroshima was widely reported on the ground, but on that day, a great event was also taking place in the sky.

It was the appearance of a supernova.

A supernova is a large explosion that occurs at the end of a star's life. When this light reaches the earth, it is observed as if a new star was born overnight.

In fact, supernovae explode frequently in some galaxies, and thousands of them are found each year in total. Most of them are distant and faint, and are found by automatic telescopes specially designed to search for supernovae.
However, Supernova 2023ixf, discovered on May 19, is a special supernova that stands out from the crowd and has excited astronomers, physicists, and observers in the northern hemisphere. Many telescopes, instruments, and detectors are still keeping a close eye on this supernova, both on the surface and in orbit.

What is so special about Supernova 2023ixf? Let us explain.
A star is a floating mass of gas in space that emits heat and light due to nuclear reactions in its interior. Some stars are heavy and some are light, but the heaviest stars, which have more than eight times the mass of the sun, collapse under their own gravity after exhausting their nuclei, which serve as fuel. This is called "gravitational collapse.

A gravitationally collapsed star becomes a neutron star, an extremely dense object, or a black hole, an object with gravity so strong that even light cannot escape.

Gravitational collapse progresses rapidly in about 10 seconds. A huge object called a star turns into a tiny neutron star or black hole with a radius of less than 10 km in about 10 seconds.
A pot or a cup dropped from a height of only one meter will make or break a lively noise. Gravitational collapse is a phenomenon in which a huge amount of matter forming a star falls from a height as high as the stellar radius, so this is already a big deal.

Stellar material rushes toward the center at supersonic speeds, colliding with each other and creating shockwaves that generate incredibly high temperatures and pressures, breaking atoms and nuclei apart. Some of the material that did not make it into the neutron star or black hole is bounced off and scattered through space at thousands of kilometers per second. Murderous amounts of gamma rays, neutrinos and gravitational waves are emitted.

This is the principle behind supernova explosions.

Observations of supernovae give us insight into nuclear reactions inside celestial bodies, from which we can gain knowledge of nuclear physics. In addition, the formation of objects governed by relativity, such as neutron stars and black holes, provides a test of relativity, which is difficult to experiment with. Investigating where and how often supernovae occur in the universe can tell us about the evolution of galaxies and the accelerated expansion of the universe.
For this reason, various telescopes, neutrino instruments, gravitational wave instruments, and other instruments are waiting for supernovae to occur.
This supernova 2023ixf was discovered by Koichi Itagaki on May 19, 2023 at 17:27:15 (UTC)*1, which is 2:27:15 on May 20 in Japan. The discovery was reported to the International Astronomical Union four hours later and given the name "2023ixf".
Upon receiving the report, automatic telescopes around the world immediately began observations. The first report was quickly followed by the next one.

Thus, in the current field of sudden-onset astronomical observations, automatic telescopes are competing with each other as if they were the eyes of a living horse or a robot ripping off a CCD.
This supernova occurred in a galaxy called M101 in the constellation Ursa Major. The distance to this supernova, like the distance to M101, is between 21 and 22 million light years.

Approximately 20 million light-years is a "close" distance in the wide universe. The first supernova within about 20 million light-years was SN 2014J, which exploded in M82 in 2014, so it has been nine years since the last supernova within about 20 million light-years. (Incidentally, SN 2011fe exploded in M101 in 2011 as well. For some reason, supernovae seem to occur frequently in M101.)

M101 is popular among astronomy enthusiasts because of its beautiful and gaudy shape, and has been called "the Kazaguruma Galaxy" or "the rotating fireworks galaxy.

Supernovae rarely occur in such nearby famous galaxies. This is because nearby famous galaxies are rare.
Incidentally, objects with "M numbers" such as "M101" and "M78" are called "Messier objects" and are on a list created by the French astronomer Charles Messier (1730-1817). Messier, a comet hunter, compiled a list of about 100 objects that were confusingly vague (perhaps after repeatedly being fooled by these objects and taking a blind fancy to them). The list was lined with nearby galaxies, gas clouds, supernova remnants, and other important objects. The confusing list of objects is now more famous than the comets discovered by Messier.

So, when a supernova appears in a galaxy represented by the "M number," researchers are excited because they know it must have originated in a nearby galaxy. This is because nearby supernovae are more likely to be detectable with observational instruments.

Messier objects also have the advantage of being famous.

Because famous objects are the subject of so much interest, there is a wealth of observational data on them. The Hubble Space Telescope has already found a star in M101 that appears to be in its pre-explosion state at 2023ixf.