The wonderful cover photo was taken by Alessandro Milani.
The North America Nebula, also known as NGC 7000 or C 20, is an HII region near the star Deneb, in the constellation Cygnus. It owes its name to the characteristic shape it assumes, which is incredibly reminiscent of the North American continent, especially the Gulf of Mexico and Florida.
It is one of the most photographed astronomical objects in the sky (even I have tried several times) because of its beauty and its brightness. Unfortunately it is not possible to see it with the naked eye, despite being a luminous subject, as it is very extensive: it has an apparent area almost ten times the full moon. However, it already appears partly visible with binoculars from very dark skies or with a telescope.
The nebula is mainly composed of hydrogen ionized by the UV radiation of violent and young stars that are inside and, together with the nearby Pellicano Nebula, is part of a large area HII including other stars and nebulae, as well as some open clusters such as NGC 6996.
The characteristic shape of these nebulae is actually a game of joints between the nebula in the background and a dark nebula, LDN 935, which absorbs the light emitted by the ionized gas at its back, before it reaches the Earth. As we can see in the picture below, in fact, in the infrared it is possible to pass the dust of LDN 935 and observe the nebula behind it in its entirety.
Why the visible light is blocked by dust while the infrared is not? Here an entire chapter opens up on the interaction between the interstellar medium and the radiation, so I will try to respond in a simple and direct way. The visible light has a wavelength between 400 and 700 nanometers, which is comparable with the size of the large dust. The dust acts in two ways on visible light: it diffuses it in all directions or absorbs it and re-emits it in the form of heat. That’s why in the infrared we can observe the nebula as if there were no dust! Part of the visible light is in fact absorbed and then re-emitted as infrared radiation.
Let’s now return to the North America Nebula. It is located in a region of the sky extremely rich in stars, clusters, nebula, or the Arm of Orion, which is also the arm of the galaxy where we are currently. This is why it is very difficult to give a precise dimension and distance to this nebula, even if, after the most recent studies, uncertainty has been reduced a lot and today scientists are more or less agreeing to say that the distance from Earth is equal to approximately 1956 light years with an uncertainty of 163 light years.
The North America Nebula is very interesting because it is very likely that there are still ongoing star formation phenomena, visible especially in areas bordering the dark nebula LDN 935. Of the 430 stars identified, in fact, almost 10% would be in the phase before the main sequence and 700 other stars would be very young and would be surrounded by dust and gas disks, which could one day join together to form the planets of new solar systems.