Theory The Milky Way Galaxy

Our Milky Way Galaxy is our galaxy, that set of stars gathered into a gigantic, rotating spiral structure, like the ones popularized by astronomical imagery, like the Whirlpool, or Andromeda Galaxy. From our perch, at 26,000 light years from the galactic center, on a secondary spiral arm known as the Orion Arm, astronomers progressively built a image of our Milky Way Galaxy. By the 1950's, they had painted a broad-brush picture, determining that the Milky Way’s stars are distributed in a central bulge, wrapped by serpentine stellar arms and surrounded by a thin, spherical halo. In the 1970's and 1980's, researchers deduced how this structure had built up over billions of years, beginning with a vast cloud of dark matter, gas and dust. The visible components collapsed into a disk-like structure, which then bulked up by devouring smaller, satellite galaxies. Our Milky Way swallowed up a smaller galaxy somewhere between 11.6 billion and 13.2 billion years ago . A period of enhanced star formation in our Milky Way Galaxy likely occurred seven billion years ago. As 80 percent of the stars in the Milky Way central region formed between eight and 13.5 billion years ago, by the beginnings of the Galaxy, a lull of six billion years followed. And one billion years ago during less that 100 million years, stars with a combined mass as high as a few tens of million Suns formed in this central region. That resulted in over a hundred thousand supernova explosions. That chronology, on a other hand, matches the episodes of galactic cannibalism which are known at the Andromeda galaxie. The stars burst in the Milky Way the event was most likely caused by a gas inflow from the Sagittarius dwarf galaxy, which at the time was very close to our own. Astronomers later filled in the details and in the stellar halo, they found remnants of small galaxies that had been stretched out into star-studded debris streams. Our Milky Way has a shape of a flat disk, 100,000 light-years wide, with a central bulge and large spiraling arms attached. Our Milky Way weighs in at about 1.5 trillion solar masses as it's the central supermassive black holes which accounts the most, at 4-million-solar-mass or black matter. With that mass, our Milky Way Galaxy is a hefty one. Because our Sun is located inside the Galaxy however, we cannot see our Milky Way from the outside, but from the inside instead, or the Milky Way which ornates our night skies. The Milky Way is just the Milky Way Galaxy as seen edge-on! Our Sun, which is one among hundred of billion stars which constitute the Milky Way Galaxy, is located 28,000 light-years away from center and it is orbiting the Galaxy in 250 million years. It is also lying about 20 light-years above the center of the plane of the disk. A most prominent dark lane in the Milky Way stretches from the constellations Cygnus to Sagittarius and is often called the Great Rift, or sometimes the Dark Rift. The Galactic disc is filled with a diffuse mixture of gas and dust, or the interstellar medium that pervades space, filling the large gaps found between stars as that medium also may turn denser and give birth to new stars. Dust thus is a minor but crucial component of the interstellar medium in our Galaxy. Our Milky Way features two satellite-galaxies, with the Large and Small Magellanic Cloud (LMC and SMC respectively) which are orbiting around it. The SMC displays a central bar

click to to a map of our Milky Way Galaxy. map courtesy NASA

Our Milky Way Galaxy had been born about 10 billion years ago in a environment where galaxy mergers or black holes are playing a important role. Like most large-scale structures in the Universe, the Milky Way likely owed much to dark matter in terms of formation. As dark matter pervaded the infant Universe, it eventually clumped at every scale. Some clumps turned spherical, with the main one, which was to yield our Galaxy, some 1 million light-years in diameter with a mass 10¹² times the one of the Sun. As those areas of dark matter were holding a haze of hydrogen and helium, those cooled and turned into stars providing for early galactic sets, which then turned a galaxy through collisions, dissipations, cooling, heating and explosions. Smaller dark matter halos, as far as they are concerned, provided for dwarf galaxies, those small, irregular aggregations of stars and gas. It may be possible that some clumps of dark matter never produced any stars at all, or that their stars turning supernovae blasted them away. Such a history explains the current aspect of our Milky Way Galaxy. Also, the Milky Way likely began as faint, blue, low-mass object containing lots of gas, without a flat disk with a bulge in the middle, both of which grew simultaneously later into the majestic spiral seen today. Our Galaxy built up 90 percent of its stars between 11 billion and 7 billion years ago as the star formation rate passed from 15 stars, to 1 star a year. By comparison, massive elliptical galaxies build a central bulge first. Our galaxy black hole has driven large amounts of energy into the Galaxy in the past, through the so-called 'Fermi Bubbles'

Our Milky Way Galaxy must been considered both with it usual, spiral aspect as some constituents are located mostly outside that structure. Let's begin with such constituents

• The Milky Way Galaxy first is wrapped in a vast, spherical halo of dark matter which can represent up to 80 percent of a galaxy's mass. With the dark matter halo included, our Milky Way likely span over 1.9 million light years across. Small dark matter clumps called halos as linked with dwarf galaxies are predicted to exist by the thousands in relation to our Milky Way Galaxy but astronomers have found far fewer than expected. Dark matter also plays a role into bounding such satellite dwarf galaxies together. It might that is took billions of years since about 10 billion years ago for galactic dark matter to condense from the halos it formed then and that its dominating effect now is only seen on the rotation velocities of galaxy discs. That halo of dark matter clumped together into smaller structures due to gravity as thousands of large dark-matter clumps might orbit the Galaxy. Occasionally such clumps are getting eaten by a mass of dark matter at the centre of the Milky Way, in a process akin to the Galaxy's consumption of small visible satellites
• A diffuse halo of stars extends outwards from the Milky Way perhaps 326,000 light-years in every direction, forming a rough sphere with a total mass of about 10⁹ times that of the Sun. It likely is nothing more than the remnant of all the dwarf galaxies that got disrupted over billions of years, during the Milky Way's history. The stellar halo divides into outer and inner. Stars in the outer halo are very old, about the age of the first generation of stars in the Universe and the halo is rotating in a direction opposed to that of the Milky Way. Stars in the inner halo are barely younger, at 11.4 billion years old as that halo rotates in the same direction than the Milky Way. Our Milky Way Galaxy endured a merger about 10 billion years ago with a object slightly more massive than the Small Magellanic Cloud making that the inner halo is dominated by debris from that encounter as a dynamical heating of the precursor of the Galactic thick disk occurred, thus contributing to the formation of this component. Our galactic disk may be parted between a thin inner disk containing gas, dust and young stars, inside a thick outer disk consisting almost entirely of older stars. The outside disk likely originated from a deal of energy brought by a collision, which inflated a original thin disk. It is likely that the outer halo formed mostly from disrupted dwarf galaxies as the inner a remnant of the aggregative formation collapse of the Milky Way. Stars circle around the Milky Way at hundreds of miles per second. The fastest class of stars in our Galaxy are called hypervelocity stars, which are thought to start their life near the Galactic centre to be later flung towards the edge of the Milky Way via interactions with the supermassive galactic black hole. About twenty stars could be travelling fast enough to eventually escape from the Milky Way. Most of the high velocity stars spotted however are racing towards as their origin is still ill-explained. They could come from other galaxies, or the Large Magellanic Cloud whence they would have been -- in the first case -- flung out via interaction with a central black hole, or from a binary system. A alternative explanation is that the stars could be native to our Milky Way Galaxy's halo, accelerated and pushed inwards through interactions with one of the dwarf galaxies that fell towards the Milky Way. Hundreds or thousands of hypervelocity stars could exist in the vicinity of the Milky Way
• A enormous halo of hot gas, with a diameter about 600,000 light-years has been found lately; it looks like composed of warm gas at temperatures between 100,000 and one million degrees K and a hotter component with a temperature greater than a million degrees K. The mass of the hot gas halo is much greater than that of the warm gas. A immense halo of gas was observed enveloping the Andromeda galaxy stretching about a million light-years from its host galaxy, halfway to our own Milky Way galaxy. Halo is estimated to contain half the mass of the stars in the Andromeda galaxy itself, in the form of a hot, diffuse gas. Such halos are thought to form at the same time as the rest of a galaxy and enriched in heavy elements, which might be due to supernova explosions. In our Galaxy Milky Way, the galaxy's halo is not static and is spinning in the same direction as the disk of the Milky Way and at a similar speed—about 400,000 mph (640,000 km/h) for the halo versus 540,000 mph (870,000 km/h) for the disk. The halo is only visible in the X-rays as it also encompasses both the Magellanic Clouds. As far as both Magellanic Clouds are concerned, they both are orbiting each other as the Large pulled out a huge cloud of gas from the Small, called the 'Leading Arm,' at 1-2 billion yeaers old, which in turn is dismantled by the Milky Way Galaxy to which it is connected. A trailing, counterpart arm, the 'Magellanic Stream' is following both Magellanic clouds. A explosion near our Milky Way's supermassive black hole at the centre sent a beam of energy and radiation throughout and beyond that even impacted the Magellanic Stream, about 200,000 light years away. That was a Seyfert flare which created two gigantic ionisation cones, thought to have occurred 3 million years ago -- which is not that long at Earth's geological scale -- and last 300,000 years. The Magellanic Stream is a vast bridge of matter likely due to tidal forces between both the Magellanic Clouds and/or our Galaxy. Galaxies generally, are also surrounded by large, yet nearly invisible cloud of dust and gas, called the circumgalactic medium, or CGM. The CGM acts as a giant recycling plant, absorbing matter ejected by the galaxy and later pushing it right back in
• Remnants of primordial mergers are seen like streams of stars looping around the Milky Way, or some dwarf galaxies are still in the process of being desintegrated by gravity. Hundreds of enormous, high-velocity gas clouds generally also whiz around the outskirts of our Milky Way Galaxy. They were likely blown out out by some cluster of supernova explosions. One of those, the Smith's cloud, which was discovered by 1963 in the Netherlands will crash back unto the Perseus Arm in thirty million years from now, triggering star formation. It is moving in space at a speed of 700,000 mph. The study in 2018 of the motions of six million stars in the Milky Way disk show that our Galaxy must have been perturbed between 300 million and 900 million years ago bringing to a wave disturbance of those stars. That was due to a close flyby of the Sagittarius dwarf galaxy as it is currently in the process of being cannibalised by the Milky Way.
• The pace at which the Milky Way has been forming stars during about 10 billion years necessitates a reservoir of gas beyond the quantity disponible in the spiral arms. Astronomers think that such a reservoir is lying, under the form of a halo of ionized hydrogen, extending some 1.5 million light-years from the galactic center. That reservoir likely is permanently replenished by a processus of galactical 'chimney,' or 'fountains' linked to supernovae events. The stellar winds which are generated in such occasions are just blowing gas out from the galactic plane into that halo. Like dew falling by a cold morning, the gas returns to the disk's plane through some process still badly known. Parts of radiation emitted inside the Milky Way are captured in the Galaxy’s magnetic field
• Two bubbles at last on either side of the galactic central bulge, perpendicular to the disk, with small, faint gamma-ray jets inside them, are extant too, at each 24,200 light-years long. Both formed early when the supermassive galactic black hole of our Galaxy was much more active than today, a few million years ago only likely. Matter falling into the black hole sent jets of energy out, creating shockwaves in the surrounding gas. Explosions of the current supermassive black hole occurring several times over the last ten thousand years, likely also helped. The structure is spanning the sky from the constellation Virgo to the constellation Grus. Astronomers are still endeavouring to explain how the structure formed, as most likely explanation is that it is the result of a large and relatively rapid energy release, the source of which remains a mystery, like a ancient particle jet expelled by our Galaxy's supermassive black hole, or the result of gas outflows from a burst of star formation, perhaps the one that produced many massive star clusters in the Milky Way's center several million years ago. In the X-rays, subtle evidence are extant too for bubble edges close to the galactic center, or in the same orientation as the Milky Way. Such giant lobes, dubbed Fermi Bubbles, as discovered by 2010, suggest that a violent event in the galaxy's core launched energized gas into space at 2 millions mph (or 3 millions km/h). Those colossal bubbles of gas weighing the equivalent of millions of Suns, have been found by 2017 to be the result of the last meal of our galactic black hole, when it consumed a large bump of infalling gas about 6 to 9 million years ago. Galactic winds are common in star-forming galaxies and it looks like there's a link between the amount of star formation and whether or not these outflows happen as there is a high concentration of star formation close to the core of our Milky Way Galaxy albeit it producing a few stars generally. Such galacic bubbles generally, might be the source of cosmic rays due to that the bubbles' outer regions generate shock waves as they expand and collide with surrounding gas. Two channels of hot, X-ray emitting material further were discovered streaming outwards from Sagittarius A*, linking the immediate surroundings of the black hole and the bubbles together. Scientists think that these act as a set of exhaust pipes through which energy and mass are transported from our Galaxy’s heart out to the base of the bubbles, replenishing them with new material

Closer to where the center of the primordial dark matter's halo lied, there where gas and dwarf galaxies swirled inwards to form an ever-increasing mass of gas and stars and gained a rotating momentum which flattened the proto-Galaxy into a disk, the Milky Way as we mostly know it, is found

• The very center of the Milky Way contains, like famely known now for most galaxies, a supermassive, galactic black hole. Such black holes like are due to the merger the black holes which lied at the center of some dwarfs, or more evolved primeval galaxies which gathered together to form the Galaxy, as some thousands of stellar black holes, themselves due to massive stars' supernovae events, also participated. The supermassive black hole, in the early phases of the formation of the Milky Way, also played a interactive role with the other structures and processes of the galactic formation. The supermassive black hole today, which is also known as 'Sagittarius A*' or 'Sgr A*' for short, is the size of the orbit of Mercury and, at a weight of a four-million-solar-mass, it likely plays, in terms of gravitational forces, a anchoring role for the structure of the Galaxy. It is a relatively calm one. Observations of a so-called dusty gas cloud named G2 in 2015 confirmed it made its closest approach to the supermassive black hole at the centre of the Milky Way in May 2014 and survived the experience. The black hole itself has not yet shown any increase in activity. Most of the highest-energy cosmic rays populating the innermost region of our Galaxy, and especially the most energetic ones, are produced in active regions beyond the galactic center and later slowed there through interactions with gas clouds. Highest-energy share of these particles, those reaching 1,000 TeV, move through the region less efficiently than they do everywhere else in our Galaxy, hinting to a kind of 'trap' at the center of it
• Those Milky Way's central regions have significantly more of the dense gas and dust to catter for new stars, compared to other parts of the Milky Way. The most massive stars in our entire Galaxy managed to form so close to each other, in a relatively small region, despite the low birthrate in the surrounding areas
• The so-called 'bulge' of our Milky Way Galaxy, is a collection of old stars arranged in a sphere, likely dating back to the origins of the Galaxy, 10 billion years ago and located at the center of the Milky Way. The Milky Way Galaxy’s central bulge is a dynamic environment of stars of various ages zipping around at different speeds. Astronomers in 2012 have catalogued about 84 millions stars in the bulge of our Milky Way Galaxy. That was realized like a 9 gigapixel image as combined from infrared vistas taken by the Visible and Infrared Survey Telescope for Astronomy (VISTA), a instrument at the ESO Paranal Observatory. If printed with the resolution of a typical book, the zoomable picture would be 30 by 23-feet (9 by 7-meter) long. A large number of faint red dwarf stars have been found in that survey. The discovery of RR Lyrae stars in the center of the Milky Way, which typically reside in dense globular clusters over 10 billion years old, is suggesting that the center of our Milky Way Galaxy likely grew through the merging of primordial star clusters. European astronomers in 2013 have made the best three-dimensional map yet of the central parts of the Milky Way and found that the inner regions take on the Greek letter a-shape or a X-shape. A population of ancient white dwarfs are the reminder of stars that once inhabited the core of the Milky Way Galaxy 12 billion years ago and support the idea that the Milky Way’s bulge formed first and that its stellar inhabitants were born very quickly -in less than roughly 2 billion years. The rest of the galaxy’s sprawling disk of second, and third-generation stars grew more slowly in the suburbs, encircling the central bulge. The Milky Way’s bulge includes almost a quarter of the Galaxy’s stellar mass. They generally are slightly more low-mass stars in the bulge, compared to those in the galaxy’s disk population, suggesting that the environment in the bulge may have been different than the one in the disk, resulting in a different star-formation mechanism. The bulge’s stellar inhabitants also move at a different rate than stars in the disk, allowing the astronomers to identify them. The central galactic bulge is a huge cloud of about 10 billion stars spanning thousands of light-years as its structure and origin are not well understood as deeply obscured by dense clouds of gas and dust as seen from the Earth. A region in Sagittarius is unusually dust-free however and offers a unique keyhole view into the Milky Way Galaxy's bulge. As seen from above, the Galaxy's center has the form of a highly elongated bar. That is characteristic of a barred galaxy that started out as a pure disc of stars as the flat bar formed billions of years ago and the inner part of it then buckled to form the X shape. Stars of the bulge look like they are streaming along the arms of a X-shaped bulge and they orbit up and down of the plane of the Milky Way! Similar X-shaped structures have been observed in the bulges of other galaxies and for the same reasons
• Our Galaxy is also a so-called 'barred spiral,' which means that a linear bar of stars, some 6,500–13,000 light-years long, is bisecting the bulge as the spiral arms anchor to. Spirals are frequently featuring a barred form, generally, as they currently represent about two thirds of all spiral galaxies. Bars may be a common stage in the formation of spiral galaxies, and may indicate that a galaxy has reached full maturity. Galactic bars also are thought to act, since the early phases of formation of a spiral galaxy, as a mechanism that channels gas from the spiral arms to the center, enhancing star formation. When the disk of gas and stars in a spiral galaxy is sufficiently massive, a stellar bar may form, yielding a barred spiral. A stellar bar consists of stars moving in a box-shaped orbit around the center. Over time, the bar may become unstable and buckle in the center. The resulting bulge would contain stars that move around the galactic center, perpendicular to the plane of the galaxy. In our Milky Way Galaxy, within that structure, there is a giant X-shaped structure of stars crossing at the center of the galaxy. That also is evidencing that our Milky Way did not endured any major merging events since that bulge formed as it would have been disrupted in that case.
• As far as the spiral arms of the Milky Way are concerned, gravitational interactions within the disk caused the orbits of the stars and gas clouds to pile up. Density waves resulted that formed the spiral arms. In some galaxies spiral arms might be the result fo shock waves inside the interstellar gas regions. Spiral arms are the place where stars keep forming as they are also the place where complex interactions keeps gas available for that. Stars in our Milky Way Galaxy are a mix and ancient, new or renewed stars as once born, they may spread in streams around. Filaments observed in the Galaxy, some longer than 100 light-years and 10 light-years wide, are similar to structures seen in star-forming regions. Matter cycles back and forth between stars and interstellar, ionized gas. Ultraviolet light from newborn stars, or spectacular events of when stars die, like supernovae or planetary nebula, is replenishing the gas reserves of the Galaxy, as a part of interstellar gas also turns into, denser, molecular clouds, where the actual star formation occurs. Recentest studies with NASA's Spitzer Space Telescope in the infrared allow to think that the Milky Way is a galaxy with a bar and two major arms -the Scutum-Centaurus and Perseus arms- connecting there, with both young, bright stars, and older, so-called red-giant stars. Three minor arms are found too -Sagittarius, Norma, Outer- filled with gas and pockets of young stars. Perseus and Scutum-Centaurus seem to be more prominent and jam-packed with stars, while the Sagittarius and Outer arms have as much gas as the other two arms but not as many stars. The arms are those long curved streams of gas and stars stretching out from the Galactic center and actually waves of piled up gas and stars sweeping through the galactic disc, triggering sparkling bursts of star formation and leaving clusters after that. Carbon monoxide gas is concentrated along the plane of our Milky Way in the densest clouds of gas and dust that are churning out new stars. Massive stars, although relatively rare, can have profound impacts on the galaxies they inhabit. These giant stars are so bright that their radiation blows powerful winds of stellar material away, affecting the chemical and physical properties of the gas in their host galaxies. The bar and the spiral arms of the Milky Way induce radial migration of stars and can trap or scatter stars close to orbital resonances. Dark matter-dominated dwarf galaxy Antlia 2 crashed into the Milky Way several hundred million years ago as the collision could have produced the large ripples that we see in the outer gas disc of the Milky Way today. Torques generated by the rotation of the vast Milky Way inner disk might generate a warp shape to its outer areas. That might also be due to dwarf galaxies orbiting, or falling into. The Milky Way is warping at a speed much faster than anticipated, suggesting it might be colliding with a dwarf galaxy. The warp is precessing and would complete one rotation around the center of the Milky Way in 600 to 700 million years. The nature of spiral arms is still debated as they might being stable or dynamic structures. They could move or dissipate and re-form over the course of a few hundred million years. Stars born in a same cloud in the Milky Way Galaxy stick together a long as a few billion years, in long-lasting, string-like groups into it. Youngest strings comprising stars younger than 100 million years, tend to be oriented at right angles to our Galaxy's spiral arms. Older stars generally, are believed to orbit the Milky Way faster than younger ones

Of importance, at last, our Milky Way Galaxy, like any other, is featuring a global magnetic field, the equivalent to our Galaxy to what the magnetosphere or the heliosphere are to Earth and Sun, respectively. There are interactions between interstellar dust in the Milky Way and the structure of our Galaxy’s magnetic field as interstellar clouds of gas and dust are also threaded by it and might play a role in the build-up of structure in the Milky Way. The arrangement of the magnetic field is more ordered along the galactic plane, where it follows the spiral structure of the Galaxy. Small clouds are above and below the plane, where the magnetic field structure becomes less regular. Galactic winds flowing from the center of a galaxy are aligned along a magnetic field and transports a very large mass of gas and dust -- the equivalent mass of 50 to 60 -- into intergalactic space million Suns. Such winds might also pull the magnetic field into near-galactic space. Numerical simulations following the current leading cosmological theory of galaxy formation, known as the Lambda Cold Dark Matter model, predict that there should be far more satellite dwarf galaxies orbiting big galaxies like the Milky Way. Astronomers refer to this discrepancy as the Dwarf Galaxy Problem

In terms of the future of the Milky Way Galaxy, most recent studies are showing that our Milky Way Galaxy is bound to encounter the nearest, large spiral M31, or the Andromeda Galaxy in a dramatic collision about 5 billion years from now as both galaxies eventually will turn into a large elliptical galaxy