The Nebulae (part 2)
Each star is born from a cloud and when it dies it leaves behind a cloud (apart from the smaller ones); nothing is created, nothing is destroyed, but everything is transforming.
Whether it is a slow and pulsating death, as we have seen for the planetary nebulae or a rapid and explosive death like for the supernovae, the progenitor star leaves a sort of cosmic signature that will enrich the surrounding interstellar medium giving life to new generations of stars and, why not, to planets that maybe one day will host life.
Let’s imagine the stars as cosmic “lombriconi” that with their tireless work prepare the ground “fertilizing” areas of their belonging galaxy but to do this their sacrifice is necessary: we are here not thanks to our Sun but thanks to the star that dying enriched the cloud from which our star was then born “filling it” with the oxygen we breathe, the calcium that makes up our bones or the iron present in our blood.
The dark nebulae are among the coldest objects that are known, their temperature being close to absolute zero (-273.15 ° C) stopping to a “miserable” -263 ° C (but using the Kelvin scale then 10 K) and they are mainly made up of hydrogen (remember that it is the most abundant element in the universe) and interstellar dust that are the real responsibles for observing these nebulae. As the name itself suggests, these do not shimmer neither in reflected light nor in emitted light, so how can we observe them in the cosmic darkness? That’s simple, the powders that compose them block the light emitted by stars placed behind the cloud and here they emerge as large black patches on a bright background. A striking example is the “coal sack” nebula (southern hemisphere):
As you can see the blackness of the powders emerges, obscuring the myriad of stars placed behind them; if instead we want to jump in our hemisphere here is the well known “horse’s head”:
Here the situation is a little more complex, it is not just a dark nebula but, as you can see from the colors, there is also an emission component (IC434 behind the equino). Shooting done by Hubble have shown that within the dark nebula there is a mechanism of star formation: in fact these dark regions like all nebulae, are real stellar nursery; as soon as “part” of the formation of the first star, the gravitational “sconquasso” triggers the process like a domino.
Another fine example of dark nebula is given by Barnard 68 in Ophiuchus; it is one of the closest to the Earth being “only” 500 a.l. making it one of the most studied ever.
There is a totally different story instead for the supernova remains that are formed after the death of a star with a mass of more than 8 solar masses. The first object of the Messier catalog belongs precisely to this type of emission nebulae: M1 is in fact what remains of a colossal explosion that took place in 1054 and was observed by various people of the Earth; it was so bright that it could be observed for 1 month even during the day and remained visible for more than two consecutive years. The supernova was recorded by Arab and Chinese astronomers while in the new world, the Anasazi portrayed it in cave paintings in Chaco Canyon in the company of the Moon.
When a star explodes, leaving behind a nice fuss, even though the types of stellar explosions are many, more or less all of them generate remains like the Nebula Veil or the Crab Nebula or the beautiful “pearl necklace” that has left behind SN 1987A. As already said there would be no life without the death of the stars so welcome these “barrels” both for the “fertilizer” action and for the visual or photographic show for us astrophiles.
Courses and appeals, death generates life that in turn generates death and since in the universe everything is recycled, the atoms that compose us in previous cycles have belonged to other planets, stars or nebulae, so when you raise your eyes to the sky again do it thinking that maybe you are observing a distant uncle or a cousin.