The distance to the star is, according to various estimates, from 495 to 640 light years. In the event of an explosion, Betelgeuse can increase its brightness at least 10 thousand times, shining with the brightness of a crescent.
Some sources predict a maximum stellar magnitude of about the same as the full moon (-12.5m). The explosion will last for several months: the star will look like a shining point, bright as the full moon, and the color of hot glass at night, and it will also be easily visible during the day. After the explosion, the star's luminosity will gradually decrease and within several months or years it will cease to be visible to the naked eye. Orion's right shoulder will disappear for a while, and after a few centuries a nebula will appear in its place.
However, if one of the poles of Betelgeuse points to Earth, there will be more perceptible impacts. A stream of gamma rays and other cosmic particles will be directed to the Earth. Strong auroras will be observed and possibly a tangible decrease in the amount of ozone in the ozone layer with subsequent adverse effects on life on the planet. In the case of this orientation with respect to the solar system, the flare will also be many times brighter than if the star's axis is directed away from us.
There is another danger, in the event of an explosion as a Supernova Betelgeuse located at a distance of 320-500 sv. years from the Earth, the UV radiation flux in the range 200-300 nm will be 35-55 thousand erg / cm2 and will become the most powerful mutagenic factor in the last two million years. Its transformation into a supernova will be accompanied by the ejection of a shell of about 10-4 of its mass, consisting mainly of hydrogen and helium with the energy of each nucleus up to 1.8 GeV / 4 /.
On Earth, a Kamiokande II-type detector will register a neutrino burst with an intensity of 25-75 thousand events (recalculated based on the results of measurements at Kamiokande II of a burst from SN1987 / 5 /). A few hours (up to a day) after the registration of neutrinos / 5 /, even the brightest star, comparable in brightness to the full Moon, will become visible even during the day. For several tens - hundreds of years after the maximum, 40-50 years after the arrival of quanta and neutrinos on the Earth, relativistic nuclei (mainly protons and helium) of the Supernova shell will act on the Earth. - their total fluence at the boundary of the Earth's magnetosphere will be 5 * 105 - 2 * 106 part / cm2.
They will be partly captured by the magnetosphere, partly will cause the production of secondary particles in the atmosphere, and partly they will reach the Earth's surface, becoming the second strongest mutagenic factor after UV radiation.
Now the main question is when will this happen? Probably this has already happened, after all, before it is more than 600 light years, the consequences of this event we have yet to see.
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Something similar has already happened in the memory of mankind, in 1054, when a supernova exploded, which later turned into the Crab Nebula. This incident is described in detail in the Chinese chronicles, the brightness of the flash was so significant that it could even be observed during the day during the month!
Betelgeuse will soon become a mirror-like point, bright as the full moon. It will be possible to read a newspaper in the open air at night, and within six months it will be easily visible in daylight.
A source of light comparable to the Moon will appear in the sky; not only light will come to the Earth. During a supernova explosion, a huge amount of energy was released at a time, and everything that was in the immediate vicinity of the supernova was burned out by the "solar wind".
When this "wind" reaches us, it will certainly weaken a little, but even the most powerful magnetic storm that has ever raged on planet Earth will darken before this cosmic storm, which will destroy our magnetosphere for several months!