The Nebulae (part 1)
Mankind has always tried to give an order to the disorder that he has outside of himself in various ways, this in order to have some sort of “control” on what surrounds him in the everyday life in which he is immersed. Examples are the constellations: the myths have been moved to heaven forcing the resemblance of heroes / monsters handed down in oral legends or written for generations, with that of certain asterisms or groups of stars. When as an association we do fieldwork the question that often comes is the following: ” where can you see a dog / hunter / king?” In fact it takes a rich imagination for this. We are led to do the same with nebulae: the witch’s head, North America, the homunculus, but why? It is called pareidolia and it is the instinctive need that our mind has to find ordered and familiar forms in new or disordered images. The first hominids would have developed this ability to perceive the danger even with very few clues (for example to escape a camouflaged predator) and here is a defense system developed and introduced by the brain millions of years ago, so that we can see today faces on Mars, clouds shaped like an elephant, a rabbit on the full moon or cosmic clouds with funny shapes.
Nebulae are among the most fascinating objects in the cosmos and are essentially divided into 4 main groups:
- emission nebulae (properly called planetary nebulae)
- reflection nebulae
- dark nebulae
- Supernova remains
In this article we will deal with the first two types; at an initial analysis, even to an absolute neophyte, the fundamental caratticeica that differentiates a one-to-reflection emission nebula catches the eye, just try it by observing the photos without continuing to read:
Image credit: Evangelos Souglakos
Image credit: https://www.nasa.gov/multimedia/imagegallery/image_feature_1209.html
What do you notice differently? Both are nebulae, but one is red (emission) while the other is blue (reflection) and this is the way to recognize them: look at their colour. What is the difference between the two? They are clouds of interstellar gas and dust but they have 2 different ways to “shine”:
- if inside a cloud there is a very powerful and hot star (generally class O or B), here the radiations of the star ionize the gas and in this case we will have an emission nebula (which owes its color to the hydrogen HII which emits in red);
- if the stars inside the cloud are not so powerful to ionize the gas, then they will only make the powders visible thanks to the scattering phenomenon. This is the real reason why they have this color, the scattering is more efficient in the blue (sorry but in what color do we see the sky? Exactly, it’s the same phenomenon!).
But can “red” and “blue” coexist? Of course yes, and an example is the so called Trifid because in it the types of nebulae are even three:
Image credit: R Jay Gabany
As you can see, there is the emission part (red), the reflection part (blue) and you can see filaments and darker areas that block the light behind it: it is a dark nebula. If we measured the spectrum of an emission nebula, we would be able to find the elements that compose it, and if we were to do the same thing with a reflection nebula we would obtain a spectrum very similar to that of the star that illuminates it. But in the list of types of nebulae in the first group I wrote in brackets a strange word: planetary nebula. What’s this?
Image credit: https://www.nasa.gov/multimedia/imagegallery/image_feature_211.html
Although they are to all effects emission nebulae and they differ from the first for the type of star that illuminates them, the planetary nebulae are in fact illuminated by a white dwarf, a degenerate stellar cadaver which is very hot and so powerful that is it able to ionize other gases beyond hydrogen (see only the red in the image?). This is the last phase of life of a star of small mass, stars up to 8 solar masses which after leaving the main sequence and become red giants are extremely unstable, they tend to pulsate for the continuous contraction and expansion given by the variation of ” velocity “and the temperature of fusion, pulsations that become stronger as the helium is exhausted thus dispersing the atmosphere in the surrounding space and leaving the core uncovered that is so hot as to” ignite “everything: this is how a nebula is born planetary (video)
Needless to say, since the Universe does not waste anything, the material expelled from the star will contribute to the formation of other stars. A name was never more misleading however, as the planetary nebulae have nothing to do with the Earth, Mars or Jupiter. The name was given to them by William Herschel, discoverer of Uranus, because he thought that these were records that would have created planets, today instead we know that it is the exact opposite. They have very short life (astronomically speaking), it is thought that in 10 / 15.000 years maximum they cease to exist. When we look at any planetary nebula it is as if we were looking at 5 billion years in the future: this is the “death” that will create our Sun and I do not know about you, but often when observing these wonders with the telescope or seeing the photos that are online, I feel a little ‘less sad.