Milky Way

The Milky Way as viewed from the Atacama Desert in Chile.

The Milky Way is a delicious^{[citation NOT needed]} chocolate candy bar the galaxy in which we live. The term "galaxy" itself comes from the Greek word for milk, so "Galaxy" is just a fancy way of saying "Milky Way", which makes sense given that until into the 20th century most people thought the Milky Way and its largest companion galaxies, the Clouds of Magellan, were all there was in the universe.

Observing it[edit]

The Milky Way can be seen from those — sadly less and less common — fortunate places with little or no light pollution to speak of^{[note 1]} as a dim glowing band arching through the sky and broken by dark zones, that correspond with dark nebulae, that cannot be resolved into stars with the naked eye — things, of course change with a pair of binoculars and using a telescope at low power the spectacle of countless stars filling the field of view is quite daunting. It's best seen from the Northern Hemisphere on summer (winter on the Southern one), and those people down under are luckier since they see those brighter parts over their heads, and not low on the horizon as happens above the equator^{[note 2]}.

Some astronomical history[edit]

The origin of the name "Milky Way" comes from its "milky" appearance in the sky, and for example Greek mythology narrates how it's the milk of the goddess Hera after Heracles sucked so hard from one of her nipples that it was scattered over the sky (and Hera presumably quite pissed off). Later Aristotle would propose the Milky Way are just stellar exhalations being ignited in the upper atmosphere^[1], and Democritus would suggest it's composed of countless stars so close together that cannot be resolved with the eye^[1], a view shared by a number of Arab astronomers. However it was not until Galileo Galilei pointed a telescope towards it, resolving countless stars^[2], that Democritus as well as those who shared his ideas were proven right.

Since we're inside it, determining its shape and where our Solar System is located was not, and still isn't, an easy task (see next section), with the earliest attempts putting the Sun at its center^[3] After spotting many curious spiral-shaped objects in the sky, that with powerful telescopes turned out to be clusters of stars too far away to be easily resolved — galaxies — rather than nebulae within the Milky Way^[4], it was clear our galaxy was just one more of the bunch. Nowadays we have refined the location of the Solar System within the Milky Way considerably, for example the specific arm of the Milky War we are located in is called the Orion Arm.^[5] The Sun's position well away from the chaotic galactic center has also allowed for long periods of stability throughout Earth's evolutionary history for complex life to evolve and flourish, and given that the Sun orbits the galactic core at roughly the same speed as the galactic arms do, we are also (relatively) safe from harmful supernovae impacts. Most of the time the Solar System lies outside of any galactic arms, although right now we are in one, but that will change shortly, astronomically speaking of course.^[6]

Structure and properties[edit]

Illustration of the Milky Way with labelled structural features

There's an analogy by Isaac Asimov in one of his popular science books where he compares to decipher the structure of our galaxy, with us inside it, with attempting to map a city while living in a low house on its outskirts and having foggy weather. The actual situation is far worse, since we cannot leave that house,^{[note 3]} have no stuff as drones^{[note 4]} to map it (and, of course, no Internet to check), and we're not entirely sure how far away are those other houses in suburbs distinct to ours. The result of this is that, in order to map the structure of our galaxy, we must use many different tracers (young stars, variable stars, gas…), cope with the interstellar dust, that in visible light absorbs and reddens starlight to the point of hiding the stars behind it, and take assumptions on the distance to those objects too far away to measure their parallaxes, even if those assumptions are correct on more than one occasion as per the Cepheid Variable.

The basic picture that has emerged is that the Milky Way is just one of two hundred billion galaxies in the observable Universe, and a large barred spiral galaxy^{[note 5]} of moderately loosely wound arms^{[note 6]}, with a diameter of around 100,000 light-years^[7] (and perhaps at least a half times more)^[8] and several hundred billion stars^[9][10] (to settle in the middle, say 250 billion of them; its exact number is very poorly constrained, as it depends of the unknown proportion of low-mass stars that make the bulk of Milky Way's ones), plus hundreds of billions of planets^[11], a similar if not higher number of planets not bound to any star^[12], and there's a hell of a lot of moons, asteroids, and comets, free-floating or not. Just look at ʻOumuamua for an example.

Because we're inside it and lack the privileged view that we've of other galactic systems from the outside, parameters such as its mass -be it in visible or dark matter- and total luminosity are poorly known and the literature offers often quite different values^[13][14]. However estimations tend to converge in the Milky Way having around 50 billion solar masses in stars^[15] plus between 5 and 7.5 billion solar masses of gas -almost all as hydrogen-^[16] and 50-75 million solar masses of interstellar dust^[16]. When invisible dark matter is included, this climbs to a mass of up to around 1 trillion solar masses^[17]. Its total luminosity in visible light varies also wildly depending of the estimation used, but tends to hover over 20-30 billion times the Sun's luminosity -corresponding to absolute magnitudes of -20.9^[14] and -21.3^[13] respectively^[18]-.

The Milky Way is divided in the parts that follow:

Galactic bulge[edit]

Sagittarius A* imaged by the Event Horizon Telescope, with lines overlaid to mark the orientation of polarization of the magnetic field.

Our Solar System is located at around 27,000 light years from the center of the Milky Way, located in the direction of the constellation of Sagittarius, and takes 240 million years to complete a turn around it. This center is hidden by extensive dust and cold gas clouds, that disallows us to study it in visible light and forces to use other wavelengths as the infrared or the radio waves able to penetrate said clouds.

The center of the Milky Way is occupied by a dense and spheroidal agrupation of old, metal-rich^{[note 7]} stars called the bulge, that has a radius of 10.000 years, around 15 per cent of the mass in stars of the galaxy^[15] and where few things of interest happen^[19]. This changes dramatically at its very center, where in addition to those old stars one finds not only much younger ones, including short-lived massive stars, but also a number of star-forming regions and young, massive clusters of stars like the Arches or the Quintuplet, that not only are among the most massive star clusters known in our galaxy but also contain some of the most luminous stars known. At the heart of this region finally lies a supermassive black hole, Sagittarius A^*, with a mass of 4 million times that of the Sun^[20] that is "dormant", in the sense of little matter falling on it compared with other galaxies, whose centers show far more activity.

The bulge is threaded with a bar (actually two, one within the other)^[21] primarily composed of old stars, with a not well determined radius (up to 16.000 light-years)^[22], and the ensemble may be surrounded by a gas-rich ring often identified with the innermost arms of the Milky Way that contains most of its star formation activity, and that is thought would be the most visible feature of our galaxy from external ones^[23].

Galactic disk[edit]

This is where our Solar System is located. As has been described above, this feature has a size of 100,000 light-years (and probably even quite more). However, it's pretty thin — most of its stars are within 1,000 light-years above or below its equatorial plane. Unlike the bulge — except its very center — the disk contains a mixture of stellar populations, from old to intermediate-aged ones as the Sun, and finally young ones, and is rich in both gas and dust. Its bluish color contrasts with the yellowish tint of the bulge showing how star formation is still going on there.

The most prominent feature of the disk are the spiral arms, that emerge from the central bar or the mentioned central ring. Despite that, the issues that we've to study the Milky Way's structure hit pretty hard there and while we know our galaxy has four spiral arms^[24] (two if we just use old stars as tracers)^[25], we don't know its exact layout — it does not help either that the arms are far from regular and, as per other spiral galaxies, have spurs, branches, and twists and there's no consensus on this. They are^{[note 8]}:

• 3-Kpc Arm — Perseus Arm (actually two tightly wound arms, the Near 3 kpc Arm and Far 3 kpc Arm, that probably are associated with the ring mentioned above that encircles the central bar. 3-Kpc refers to its distance from the Galactic center (10,000 light-years). Perseus is also one of the two most conspicuous arms of the galaxy.)
• Norma — Outer Arm (Outer refers to it being the outermost (known) arm. The longest one.)
• Scutum — Centaurus Arm (With Perseus the Milky Way's most important spiral arm.)
• Sagittarius — Carina Arm (Once considered a major arm, but now just a minor one.)
  • In addition to these arms there are also spurs branching of them (at least two). From our perspective, the most conspicuous is the Orion — Cygnus spur/arm, as our Solar System is within it.

The spiral arms are the places where the disc's star formation is concentrated, and in pictures of external galaxies^{[note 9]} are quite photogenic because of its even bluer tinge contrasting with the pink and red of emission nebulae, where hot young stars have ionized the hydrogen causing it to glow, and the cold gas clouds where stars are forming (or will form in the future). Young stars there, besides a few, massive, and dense star-forming regions, generally lie within associations and denser clusters.

Galactic halo[edit]

Surrounding the disk in all directions there's a far more diffuse halo occupied by old, metal-poor stars, many of them in more than 150 massive and dense star clusters named globular clusters because of their aspect (note how some of those clusters may be, however, more rich in metals)^[26], as well as a whole lot of dark matter as explained above. Further away there is a corona of hot gas with a mass comparable to the one of stars in the Milky Way^[27]. This halo is estimated to extend to a distance of roughly 320,000 light-years, but if one includes satellite galaxies (see next section) could extend much further away, perhaps even touching the one of the neighboring Andromeda Galaxy. There are also streams of stars connecting other galaxies to our own as the Milky Way cannibalizes its neighboring galaxies.^[28]

Environment[edit]

Map of the Local Group, showing the Milky Way and Andromeda in relation to nearby galaxies

The Milky Way is accompanied by a large number of satellite galaxies, most of them being small, very low surface brightness and hard to detect (so stay tuned for new examples of them being discovered), systems with very old stars and no gas or star formation that are thought to be the basic blocks to build big galaxies as ours. Most notably, those galaxies seem to be composed almost entirely of dark matter, with very little mass within stars in proportion^{[note 10]}. In some cases such as the Sagittarius Dwarf Spheroidal Galaxy, that loops around our galaxy, they are being ripped apart by the Milky Way's gravitational forces.

The Milky Way is actively pilfering gas from the nearest dwarf galaxies, rendering these galaxies barren of the neutral hydrogen needed to form stars.^[29]

In addition to these small systems one can find a pair of more conspicuous (but still small compared with the Milky Way) galaxies, the already mentioned Clouds of Magellan, that unlike the former are rich in gas and star formation. Both are connected by a big cloud of gas and in addition to that linked with the Milky Way by a stream of gas, the Magellanic Stream, ripped of them by the latter.

Much further away one can find a big galaxy, the famous Andromeda Galaxy that is twice as large and much more massive and luminous than the Milky Way. Together with several dozens of other generally (much) smaller galaxies, they form the Local Group, a small clustering of galaxies held together by their gravitational attraction against the Universe's expansion. The Local Group in turn is part of a much larger galactic grouping (or, rather, an ensemble of galaxy groups and clusters), the Virgo Supercluster, and pressing on an even larger structure nicknamed Laniakea.

Formation and evolution[edit]

A series of illustrations showing the expected progression of the Andromeda-Milky Way collision

The origin of the Milky Way is believed to be in the many matter overdensities originated in the cosmic inflation, just after the Big Bang^[30]. Some of them formed what would become today's globular clusters, while others formed small protogalaxies, that would collide among themselves to form a larger, spheroidal-shaped system where star formation took place at a fast pace, exhausting the gas and expelling the remaining one in the form of supernovae and stellar winds. As this primeval Milky Way had a very fast rotation the gas formed a disk around the spheroid component, where star formation would take place at a considerably lower rate continuing until now^[30].

The Milky Way has continued growing until now by merging with smaller galaxies as well as accreting the surrounding gas to form new stars. Unlike many other spirals as the neighboring Andromeda Galaxy, this process has been relatively calm with no mergers with galaxies of similar size to ours^[31] and has caused it to be atypical compared to other spirals as both its central supermassive black hole and halo are of relatively low mass and it has a large satellite galaxy (the Large Magellanic Cloud)^[32].

The fate of the Milky Way is to continue its assemblage by absorbing smaller galaxies (see the Sagittarius Dwarf Spheroidal above), up to the Large Magellanic Cloud around two billion years from now^[33], which will also transform it into a more typical spiral galaxy engorging a lot both its central black hole and halo^[32], as well as accreting gas from said corona of hot gas. However, as raw hydrogen is being consumed to form new stars, star formation is slowing down and models suggest it will stop in five billion years from now coinciding with the expected collision with the Andromeda Galaxy, that is in a collision course with ours, to form a new, much larger, galaxy ten billion years from now after a first close passage roughly 4 billion years in the future^[34][35] even if some research suggests such merger will take place much later^[36].

External links[edit]

• See the Wikipedia article on Milky Way., as well as references therein.
• The Galaxy Song — Stephen Hawking sings it with Eric Idle of Monty Python

References[edit]

Notes[edit]