M 51, also known as the Tourbillon galaxy (Whirlpool Galaxy in English) is a couple of galaxies located around 27.4 million light years in the constellation of Hunting dogs.
This famous couple is composed of a regular spiral galaxy massive whose diameter is estimated at 100 000 light years and a small irregular galaxy .
Spring star, it is one of the most famous galaxies of astrophotographers, with very pronounced colors and presenting beautiful nebulae, accentuated here by the joint use of a Halpha Hydrogen filter.
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First of all, remember that in astronomy deep sky objects are night sky objects other than those of the Solar System (planets, comets and asteroids), single stars and multiple stars, to know about those which are mainly photographed by amateurs:
- star clusters (group of stars physically linked by gravity)
- nebulae (celestial objects composed of gas or interstellar dust)
- galaxies (vast set of stars and interstellar matter whose cohesion is ensured by gravity)
There are of course a multitude of other types of celestial objects outside the solar system, but these three categories contain the types of objects that mostly amateurs photograph.
Also, in general, stellar clusters and nebulae photographed by amateurs (with a few exceptions) are located within our galaxy, the Milky Way.
Galaxies, on the other hand, are obviously by definition outside our own galaxy.
As a preamble, a few orders of magnitude:
- Diameter of the Earth: 12,700 kms
- Distance Earth-Moon: 380,000 kms or 1.3 light seconds
- Earth-Sun distance: 150 million kms or 8 light minutes
- Diameter of the solar system: 20 billion kms or 18.5 light hours
- Distance Sun-Proxima of the Centaur (nearest star): 41,000 billion kms or 4.3 light years
- Diameter of the Milky Way: 100,000 light years
- Distance Milky Way - Andromeda Galaxy: 2.5 million light years
- Estimated diameter of the observable universe: 93 billion light years
A star cluster is a local concentration of stars of common origin and linked together by gravitation.
These objects are classified into two families according to their appearance : the open clusters and the globular clusters .
Star clusters are maintained by the mutual gravitational attraction of their members.
Due to internal influences (collisions with other members of the cluster, stellar evolution ) and external (collisions with massive objects and influence of galaxy host), they slowly “evaporate”. Their lifespan varies from a few million years for sparse associations to several billion years for massive globular clusters.
The closest and brightest star clusters are visible to the naked eye.
Due to their rather homogeneous stellar population and their relatively well-known distance, clusters play an important role in astrophysics and in astrometry .
Star clusters
>>> Open clusters are groups of approximately 100 to 10,000 stars of the same age and present even inside a galaxy .
These are, in general, young stars born in the same molecular cloud and which gradually begin to move away from each other.
Several open clusters are visible to the naked eye , such as Pleiades (M45) and Hyades , both in the constellation of Taurus , as well as M44 (the cluster of the Beehive or the Crèche) in the constellation of Cancer, or even the most distant double cluster of Perseus .
They are thought to form within molecular clouds , the large clouds of gas and dust that constitute the diffuse nebulae .
As the time required for their formation is much shorter than their lifespan, all the stars in an open cluster are more or less the same age and since they come from the same nebula, they have the same chemical composition .
Thus, the only differences existing between the stars of an open cluster come from their mass . Most open clusters are dominated by their stars blue giants of type O or B , which are very bright but short-lived. We can estimate the age of an open cluster by looking at the ratio between the number of blue, yellow and red stars: the more blue stars there are, the younger the cluster.
>>> The globular clusters are very dense, typically containing a hundred thousand stars distributed in a sphere varying in size from twenty to a few hundred light years .
Their density is thus significantly higher than that of open clusters and the stars in these clusters are generally red giants .
Most globular clusters are very old and probably formed at the same time as their host galaxy. However, some globular clusters of blue color have recently been observed and their color is normally representative of hot and young stars.
It is not yet known whether globular clusters can form relatively late in the life of a galaxy.
There are 150 globular clusters in our galaxy, the Milky Way . But there are undoubtedly others, which remain undetectable because they are masked by the galactic disk .
Globular clusters are part of the galactic halo, they orbit around the galactic center.
It is through their study that Harlow Shapley , in 1918 , was able to determine the position of Sun within the Galaxy. Since globular clusters contain the oldest stars in a galaxy, they also make an important contribution to the study of star evolution. and galaxies.
The density of stars in globular clusters being very high, collisions or near-collisions between stars are sometimes possible there, unlike other regions of a galaxy.
Nebulae
We designate "nebula" any region of the interstellar medium particularly rich either in gas (most often hydrogen ), or in interstellar dust , or both.
Nebulae can originate from stars at the end of their life, but also play a key role in the birth of the stars that they can form by collapsing under the effect of gravity .
We distinguish :
- nebulae in emission which are clouds of gas ionized in the interstellar medium, which absorb the light of a nearby hot star and re-emit it in the form of various colors at lower energies and which group together the H2 regions, planetary nebulae, supernova remnants and Wolf-Rayet bubbles.
- reflection nebulae which, unlike emission nebulae, do not emit their own light but reflect that of nearby stars or that they contain
- dark nebulae, whose density in gas and dust is such as to mask the light of stars distant background and stands out against this bright background. We also talk about dark nebula .
>>> H2 regions are emission nebulae, consisting of clouds mainly composed of hydrogen most of which atoms are ionized, sometimes extending over several light years .
Ionization is produced by the proximity of one or more stars very hot, spectral type O or B, which radiate strongly in the ultraviolet extreme, themselves formed from the cloud.
Later, the explosions in supernovae and the forts stellar winds caused by most massive stars of the star cluster will eventually disperse the remaining gas particles, leaving behind a star cluster such as that of Pleiades .
These clouds of ionized gas are visible at great distances, and the study of HII regions extragalactic is fundamental for determining distances and chemical composition others galaxies .
HII regions get their name from the presence of large amounts of hydrogen. ionized, denoted "HII", distinct from molecular hydrogen (H2) and atomic neutral hydrogen ( HI ). It is therefore the ion H +, that is to say a simple proton .
These clouds of ionized gas are visible at great distances, and the study of HII regions extragalactic is fundamental for determining distances and chemical composition others galaxies .
>>> The planetary nebulae are emission nebulae which are the mark of a star of low mass at the end of life and foreshadow the destiny of our Sun.
Often small, round and relatively bright, astronomers have long confused them with planets, hence their name, but we now know that planetary nebulae have nothing to do with planets. planets .
When a small star (less than eight solar masses ) ages and ends up consuming all of its hydrogen , then its helium , his heart collapses to form a white dwarf , while the outer layers are expelled by the radiation pressure . These gases form a cloud of matter that extends around the star at an expanding speed of 20 to 30 kilometers per second. This cloud is then ionized by photons ultraviolet emitted by the star which has become very hot.
They play a crucial role in enriching our universe, transforming primordial hydrogen into heavier elements and expelling these new elements into the interstellar medium.
>>> The supernova remnants are emission nebulae very extensive and are the result of the violent explosion of a star of high mass. They often have a characteristic filamentous structure that evokes lace.
A supernova can be formed in two ways:
a supergiant star which stops generating the energy of his heart and ends up collapsing under the effect of his own gravity . We are talking about a collapsed heart supernova. While the massive star is over eight solar masses , the compact residue is of the order of 1.5 solar masses
a white dwarf having accumulated enough material from or colliding with a nearby star to reach the critical mass , which generates a thermonuclear explosion. We are talking about thermonuclear supernova . In the event that there is accretion of matter and not collision, the binary star may eventually survive the explosion. The star that is reduced does not leave a compact residue behind.
>>> The bubbles of Wolf-Rayet have an aspect similar to that of the remains of a supernova. These gas bubbles result from the gradual expulsion of the outer layers of an extremely hot Wolf-Rayet star, which is one of the most massive stars known.
The two Wolf-Rayet bubbles the most famous are certainly the crescent nebula (NGC 6888) and the Nebula Thor's Helmet (NGC 2359).
>>> The nebulae by reflection are dust clouds who reflect the light one or more stars neighbors.
These stars are not hot enough to cause ionization from gas , as in the case of emitting nebulae , but are bright enough to allow sufficient dispersion to make dust visible.
Reflection nebulae are often locations of star formation
They are usually blue because scattering is more efficient for blue light than red (it is the same scattering process that gives us the blue sky and red sunsets).
About 500 reflection nebulae are known.
>>> Dark nebulae, or dark nebulae or absorption nebulae , are regions where dust from interstellar medium seem to be concentrated in large clouds, partially absorbing the light passing through them and therefore obscuring what is behind.
In the visible spectrum , they can only be detected by contrast on a star field or a bright nebula.
Like reflection nebulae, they are the place of star formation.
Galaxies
A galaxy is an assembly of stars , gas dust , vacuum and perhaps mainly from dark matter , sometimes containing a supermassive black hole at its center.
The Milky Way , the galaxy in which the Solar system , has a few hundred billion stars and has a diameter of the order of 100,000 light years .
Most typical galaxies have a similar number of stars, but there are also dwarf galaxies with roughly ten billion stars and giant galaxies with several trillion stars.
Galaxies are classified into three main morphological types: ellipticals , spirals , and irregular but a more extensive description of the types of galaxies has been given by Hubble, called Hubble sequence . .
>>> Elliptical galaxies have a profile ellipsoidal , giving them an elliptical appearance from any angle of view.
Their appearance shows few structures and they do not have many interstellar matter . Therefore, these galaxies contain few open clusters and have a rate of star formation not high.
Older and more evolved stars revolving around their center of gravity common in a random manner, therefore dominate these galaxies. In this sense, they present a certain similarity with the globular clusters , but on a larger scale
The largest galaxies are giant ellipticals.
It is believed that many elliptical galaxies formed due to a galaxy interaction which ended up merging. They can reach enormous sizes (compared to spiral galaxies, for example).
'On the other hand, these giant elliptical galaxies are often found in the heart of large clusters of galaxies .
>>> Spiral galaxies form the most emblematic class of galaxies. They are made of a spinning disc and composed of stars and interstellar medium , with a bulb central of generally older stars. From this bulb emerge arm relatively bright.
In the Hubble classification scheme, spiral galaxies correspond to type S, followed by a letter (a, b, or c), which indicates the degree of coiling of the spiral arms as well as the size of the central bulb. An Sa galaxy has relatively ill-defined arms and has a relatively large central region. In contrast, a Sc galaxy has very open and well-drawn arms as well as a small bulb.
The spiral arms rotate around the center and are areas where the density of matter is higher visible because of their content of young and bright stars, due to the high density of matter which facilitates the formation of stars.
The majority of spiral galaxies are "barred": they have a linear band of stars at their center, from which the spiral arms emerge. In the Hubble classification, they are designated with an SB, followed by a lowercase letter (a, b, or c), again indicating the shape and arrangement of the spiral arms (in the same way as spiral galaxies not crossed out).
Bars are believed to be temporary structures that can arise as a result of density radiation from the core outward, or as a result of interaction with another galaxy involving the tidal force.
>>> Irregular galaxies show no regular or discernible structure and for this reason are classified separately in the Hubble classification of galaxies .
Most irregular galaxies are dwarf galaxies (they contain 10 million to one billion stars).
Their brightness is not affected by their irregular shape because they are often rich in bright young stars, but their small size makes them difficult to detect, which is why we only detect those rather close to us.
They often orbit around larger galaxies (spiral galaxies ) which create compression waves (due to gravity) which pass through them. nebulae and trigger very rapid star formations there.
They would have been very numerous in the past in the universe, but their number would have decreased following collisions (they would constitute 10% of galaxies).
They therefore do not have any characteristics of spiral galaxies or some elliptical galaxies . Irregular galaxies are generally quite rich in interstellar gas and dust (nebulae) and in young stars.
>>> Note that apart from these three main categories there are also:
- lenticular galaxies (S0 or SB0) disc without spiral arms well defined. They represent the transition between a elliptical galaxy and an spiral galaxy in the Hubble sequence . Formally, they resemble spiral galaxies devoid of spiral arms although they have a galactic bulb much larger in size than that of a standard spiral galaxy.
Lenticular galaxies have lost or transformed most of their interstellar matter , so that very little star formation , even though they can contain a large amount of dust .
- ring galaxies having a ring shape. The ring is made up of stars massive and relatively young blues, which are extremely bright.
The central region contains matter relatively dim. The astronomers believe that ring galaxies form when a galaxy passes through the center of a larger galaxy. Since a galaxy is essentially made up of empty space, this "collision" very rarely leads to real collisions between stars.
However the disturbances gravitational caused by such an event may cause wave star formation that spreads across the larger galaxy.
- galaxies with a polar ring in which a ring of gas and stars revolves around the poles of a host galaxy. These rings are believed to form when two galaxies gravitationally interact. One possibility is that matter is moved by tidal effect, due to the passage of another galaxy, to form a ring. The other possibility is that a small galaxy collides with a larger galaxy, orthogonally to the plane of rotation of the latter.
- interacting galaxies which are the result of the disturbance of a galaxy by the gravitational field a second and vice versa.
An example of a minor interaction is the case of a satellite galaxy disturbing the arm of a spiral galaxy primary.
A major example of an interaction would be the collision and reunification of two galaxies.
On our scale, the galaxies are so far away that we can hardly imagine the distance, but on the galactic scale, some are closer than others, resulting in massive interactions.
- starburst galaxies are characterized by high concentrations of gas and dust as well as a high number of young stars. The most massive of them ionize the surrounding clouds and create HII regions. These massive stars end up as supernovas, producing an afterglow that interacts with the surrounding gas. This sets off a chain reaction of star formation that propagates through the entire gas region. Such a starburst only ends when the available gas is consumed or dispersed.
Starbursts are often associated with interacting or merging galaxies. The prime example of a starburst galaxy is M82, which recently interacted with the larger M81. irregular galaxies often have nodes or the rate of formation is particularly high
-The Seyfert galaxies are spiral or irregular galaxies containing an extremely bright core which can sometimes exceed the luminosity of the entire surrounding galaxy. It is a type of active galaxy, named after Carl Seyfert who studied these objects during the 1940s.
The variations in the luminosity of the central core take place in less than a year; which implies that the region emitting this light must be smaller than a light year, since an object cannot change faster than the time taken for the light to travel through it.
- galaxy clusters or galactic clusters, are the association of more than a hundred galaxies linked together by the gravitation . Below 100, we are talking more about group of galaxies , even if the boundary between group and cluster is not clearly defined.
These clusters are characterized by their specific shape (spherical, symmetrical or any), as well as by the distribution and their number of galaxies (up to several thousand). They trained there 10 billion years and has . These clusters may themselves combine into larger groups to form superclusters .
Clusters of galaxies are stable structures, that is to say that their constituents cannot escape from the well of gravitational potential of the cluster. These structures are among the largest and most massive in the known visible visible universe today.
Our galaxy, the Milky Way , belongs to a group of galaxies , comprising about thirty galaxies, called Local group , itself included in the Virgo superclusters .
Some galaxies can also be "hybrid" between these categories, such as the famous Wagon Wheel galaxy (also called ESO 350-40) which is a lenticular galaxy ring.
