ORION NEBULA: ORIGIN, LOCATION, CHARACTERISTICS AND DATA - ARTE - 2024

The Great Orion Nebula is an emission nebula, the center of which resembles a butterfly. It is south of the constellation Orion and is easily visible to the naked eye, as a faint whitish spot in the middle of Orion's belt.

Nebulae, named for their diffuse shape, are vast regions in space filled with interstellar material: dust and gas. The Orion Nebula was first described by the French astronomer Nicolas-Claude Fabri de Peiresc in 1610, although ancient civilizations like the Maya, for example, have records of similar objects. However, it is not possible to be sure that it was indeed the same Orion nebula.

Figure 1. The Orion Nebula in the constellation of the same name, taken with Hubble. Source: NASA

In fact, Galileo does not mention it, although it is known that he examined the region with his telescope and detected some stars within it (known as the Trapezoid). Neither did other notable astronomers of antiquity.

But since it can now be easily seen with the naked eye, the nebula may have brightened by the birth of new stars.

Charles Messier cataloged it in 1771 as object M42, a name by which it can also be searched on the web and in astronomy applications for telephones.

From an astronomical point of view, nebulae like Orion's are important, since stars continually form there.

By virtue of the force of gravity, it is there that the aggregates of matter arise that later condense and constitute the seed of the stellar systems. Stars are continually forming inside the nebula.

Location

The Great Orion Nebula is relatively close to the solar system, 500 parsecs (1 parsec = 3.2616 light years) or 1270 light years. As we said, it is located in Orion's belt, made up of the three bright stars diagonally in the center of the constellation's quadrilateral.

Those three stars are Mintaka, Alnilam and Alnitak, although colloquially they are called the Three Marys or the Three Wise Men.

Figure 2. The constellation Orion and the location of the Orion Nebula, seen from Earth. Source: Pixabay.

Size from Earth

From Earth, the angular diameter (the size of the angle at which the object is viewed from Earth) of the nebula in the sky is about 60 arc minutes.

By comparison, that of Venus, an easily visible celestial body, ranges from about 10 to 63 arc minutes, depending on the time, but the appearance of Venus is brighter for reasons of proximity.

You can get an idea of ​​the size of the nebula and its true brightness by comparing the distances: 1270 light years = 1.2 x 10 ¹⁶ km, against the Venus-Earth distance of just 40 x 10 ⁶ km.

How to observe the Orion nebula?

The Orion Nebula is an emission nebula, which means that it emits light in the visible range. It is visible in the east, towards dawn from July, but the best time to observe it is during the winter months in the northern hemisphere or in the summer in the southern hemisphere.

It is visible with the naked eye if the sky is dark and clear. And while it is true that it can be visible from a large city, it is best to get as far away as possible from light pollution.

Through binoculars or a small telescope, the nebula looks like a small pearly spot, although a slight pink tint is sometimes observed. This is not the most common, because the eye is not as sensitive to colors as photographic film.

So the observer will not see it as in the photograph shown in figure 1. This requires larger telescopes or taking long exposure photographs, which also usually receive post-processing to bring out details.

Despite this, even if it is observed only with binoculars, the nebula is an image of astonishing beauty, even more so knowing that stars are being born inside it at this precise moment.

Finding the nebula is easy, as mentioned before, since Orion is one of the best known constellations. Similarly, an application like Sky Map will show your location immediately. And with modern telescopes you can program the search so that the focus is automatic and locate the Trapezoid inside.

The Trapeze

At the center of the Orion Nebula are four stars known as The Trapezoid (Θ-Orionis). Galileo discovered three of them in 1610, but curiously he did not leave a record of the nebulosity that surrounded them, which Fabri de Peiresc did.

The stars that make up the Trapezoid are bluish-white and very bright. They are also massive, having between 15 and 30 times the mass of the Sun.

Figure 3. The Trapezium stars at the center of the Orion Nebula. Source: Wikimedia Commons. ESO / M.McCaughrean et al / CC BY (https://creativecommons.org/licenses/by/4.0).

They are relatively close: 1.5 light years apart approximately separates each. They are part of a much larger stellar conglomerate, about 10 light-years across and of about 2,000 stars, called the Trapezium cluster.

The Trapezoid can be observed with the help of a small telescope and in very clear skies an additional fifth star is distinguished. Larger telescopes allow up to 8 stars to be distinguished.

The color of the Orion nebula

At first glance the nebula is whitish in color, although sometimes, under suitable conditions, the human eye is able to detect a slight pinkish hue.

True colors show up in images taken with long exposures and come from the energy emitted by excited gas molecules.

Indeed, the stars inside the nebula have temperatures of around 25,000 K. Therefore, they are capable of emitting enough ultraviolet radiation to ionize hydrogen, which is the majority component in the region.

The combination of wavelengths emitted by the gas's molecular excitation (in red, blue and violet) produces the distinctive pink color.

In some photographs it has also been possible to see green areas, corresponding to different energy transitions that only occur in places with the physical conditions of the nebula.

Features and data

Due to the great stellar activity in its interior, the Orion Nebula is of great astronomical interest. Inside it there are a large number of stars in formation, called protostars.

Since this is a very short stage in the life of a star, it is not easy to find protostars for study. And because the great Orion Nebula is far from the plane of the galaxy, what it contains is not easily confused with other objects.

For all these reasons astronomers and astrophysicists have studied it extensively.

Facts about the Orion Nebula

-The age of the nebula is estimated at less than 2 million years, which is the same age as the stars in the clusters that form it.

-Hydrogen is the most abundant element in the nebula and that is why we see it in a reddish or pink color, because the red light from the hydrogen emission lines is the most intense.

-The stars are surrounded by bright filaments that extend to distances of 8 parsecs. Some of these filaments are meeting fronts between particles that move slowly with others that move more slowly.

-In the interior of the nebula, stars with protoplanetary disks and brown dwarfs have been detected, among other objects.

Protoplanetary disks are made of material that rotates around newly formed stars, giving rise to planetary systems like ours.

About 85% of the stars in the nebula are surrounded by disks of gas and dust, although this does not necessarily mean that they develop a planetary system like ours.

For their part, brown dwarfs are bodies halfway between stars and planets, because they did not have enough mass to create the fusion reactor that gives rise to a star.

Given the high rate of stellar births, there are numerous brown dwarfs in the great Orion nebula.

-The Orion Nebula is a visible part of the large Orion Molecular Cloud or Orion Molecular Complex, which groups together different types of nebulae and other astronomical objects, such as the Barnard loop (horn-shaped in the following image) and the well-known dark Horsehead nebula.

Figure 4. Orion's Molecular Complex, the crescent shape corresponds to Barnard's Loop. Source: Wikimedia Commons. Rogelio Bernal Andreo / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

-The Orion Nebula is gradually dispersing and is expected to become extinct in a few hundred thousand years, a blink from the point of view of the universe. There is still time to enjoy such a spectacular celestial phenomenon.

Structure

The following diagram shows the structure of the Great Orion Nebula and the adjacent regions.

By means of infrared shots, the structure is much better distinguished, since gas and dust are transparent at these wavelengths, while visible light is totally scattered or absorbed.

The stars of the Trapezoid, already described, shifted to the left in the image.

Inside the nebula, the outer molecular cloud can also be distinguished and within it, the following objects, visible in infrared:

Figure 5. Schematic of the Orion Nebula. Source: Pasachoff, J. 2007. The Cosmos: Astronomy in the New Millenium. Third Edition. Thomson-Brooks / Cole.

-The Becklin-Neugebauer object, visible in infrared, is an intermediate mass protostar, that is, a star at a very early stage that is not yet incorporated into the main sequence.

- Masers or natural sources of microwave emission, are typical formations of molecular clouds.

-The Kleinmann-Low nebula, a very active star-forming region in the center of the Orion nebula. It contains a star cluster surrounded by dust and gas that also includes protoplanetary disks.

References

1. Kutner, M. 2003. Astronomy: a physical perspective. Cambridge University Press.
2. Pasachoff, J. 2007. The Cosmos: Astronomy in the New Millenium. Third Edition. Thomson-Brooks / Cole.
3. Seeds, M. 2011. Foundations of Astronomy. Seventh Edition. Cengage Learning.
4. Wikipedia. Orion Nebula. Recovered from: en.wikipedia.org.
5. Wikiwand. Trapezium Cluster. Recovered from: wikiwand.com