History UsefulNotes / TheMilkyWayGalaxy

The Milky Way[[note]]Some professional astronomers know it as the Galaxy (with an uppercase G)[[/note]] is the galaxy in which [[UsefulNotes/TheSun our Sun]] is located. Almost everything we can see in the night sky with the naked eye[[note]]The exceptions are the [[https://en.wikipedia.org/wiki/Andromeda_Galaxy Andromeda]] and [[https://en.wikipedia.org/wiki/Triangulum_Galaxy Triangulum]] galaxies (both large galaxies very close to ours), and some other galaxies farther away said to have been spotted with people with very keen eyes.[[/note]] is inside it, or at least is orbiting it (globular clusters[[note]]Usually large, high-density clusters of stars, as old as the Milky Way, with a more or less spherical (globular) shape[[/note]]) or is close to it (the Magellanic Clouds[[note]]a pair of small galaxies thought to be orbiting ours[[/note]]). This page hopes to help readers who want to learn more about our galaxy.

The Milky Way[[note]]Some professional astronomers know it as the Galaxy (with an uppercase G)[[/note]] is the galaxy in which our Sun is located. Almost everything we can see in the night sky with the naked eye[[note]]The exceptions are the [[https://en.wikipedia.org/wiki/Andromeda_Galaxy Andromeda]] and [[https://en.wikipedia.org/wiki/Triangulum_Galaxy Triangulum]] galaxies (both large galaxies very close to ours), and some other galaxies farther away said to have been spotted with people with very keen eyes.[[/note]] is inside it, or at least is orbiting it (globular clusters[[note]]Usually large, high-density clusters of stars, as old as the Milky Way, with a more or less spherical (globular) shape[[/note]]) or is close to it (the Magellanic Clouds[[note]]a pair of small galaxies thought to be orbiting ours[[/note]]). This page hopes to help readers who want to learn more about our galaxy.

Creator/IsaacAsimov, in one of his books about science, gave an analogy that explains very well the difficulties we have when studying the structure of our galaxy, comparing them to those of someone who wanted to make a map of his/her city living in a small building in the suburbs in foggy weather. Worse, we cannot leave our house, nor send drones for the reasons previously mentioned, and — of course — we cannot look up that information on the Internet. We're on our own.

Creator/IsaacAsimov, in one of his books about science, gave an analogy that explains very well the difficulties we have when studying the structure of our galaxy, comparing them to those of someone who wanted to make a map of his/her city living on a small building on its suburbs and having foggy weather. Worse, we cannot leave our house, nor sending drones for the reasons depicted before, and — of course — we cannot look up that information on the Internet. We're on our own.

Let's compress that model, assuming now the Sun is the size of a grain of sand (around 0.5 millimeters). At that scale, you'd need a microscope to see the Earth at 54 millimeters from the Sun, and you'd find the closest star (Proxima Centauri) as another grain of sand at a bit more than ''14 kilometers''. The Milky Way at that scale would still have a size of ''330,000'' kilometers, almost as big as the distance between the Earth and the Sun, and our Sun would be at around 94,000 kilometers from its center. And before you go to NASA to tell them to send a probe to it, remember that light in this model — the fastest thing in the Universe — would move at the snail pace of almost 390 millimeters ''per hour''[[note]]And the less we say of our space probes, the better[[/note]]. Just for the record, stars are not fixed; they move around the center of the Milky Way as Earth moves around the Sun. The latter moves at a speed of 220 kilometers ''per second'' (and there are stars even faster[[note]]Some of them boosted to speeds so high that have departed the Milky Way to never return[[/note]]). At this scale, our Sun would move at 0.3 millimeters per hour.

Most of the spiral arms of the Milky Way are named after constellations (marked in '''bold''') that they cross as seem from Earth. So we have the ''3-kpc near arm'' and ''3-kpc far arm[[note]]Like its partner, the 3-kpc near arm is so named because it's located at 3 kiloparsecs (10,000 light-years) from the center of our galaxy[[/note]]'', that together form the ring that surrounds the Milky Way's central bar, the '''Norma Arm''', that becomes the ''Outer Arm'' as it continues outwards, the '''Scutum'''-'''Centaurus''' Arm, the '''Perseus Arm''', and the '''Carina'''-'''Sagittarius Arm'''. Of these arms, the two most important are believed to be '''Scutum'''-'''Centaurus''' and '''Perseus'''[[note]]The study of the Milky Way's spiral structure is hard, not only because what has been explained above, but also because as we're quite close to its plane we see how the spiral arms overlap, so while seems clear that all arms branch from the ring formed by the 3-kpc ones it's not well known where they attach. To make things worse, the Galactic Center makes the study of the opposite region of our galaxy difficult. It's assumed the parts we can't see are symmetrical with those we can.[[/note]]

Meanwhile, dust is concentrated within a disk that coincides with the plane of the galactic disk, forming the dark band that crosses the equator of galaxies that are seen edge-on. There's much less dust than gas: just 1 percent of the gas mass of the disk is in the form of dust. However, as we've explained before that dust is very efficient at blocking the light coming from distant stars. Luckily, however, as most of it lies concentrated within a disk the farther we look from the Milky Way the less (much less) of it, as well as stars, we'll find and the farther we can look, to the point of being able to observe external galaxies (and millions of them.)

And, finally, another object that may be what remains of a galaxy that suffered a similar fate to Sagittarius in the past is [[https://en.wikipedia.org/wiki/Omega Centauri Omega Centauri]], a globular cluster (the most luminous, brightest and largest of them).

Threading the bulge, we find a large bar that may be up to 30,000 light-years long, also mostly composed of old stars. This is not a feature exclusive to the Milky Way; many other spiral galaxies[[note]]known as barred spiral galaxies [[CaptainObvious because of the presence of that central bar]][[/note]] have central bars, more or less long ([[https://en.wikipedia.org/wiki/File:Hubble2005-01-barred-spiral-galaxy-NGC1300.jpg here]] and [[https://commons.wikimedia.org/wiki/File:The_Great_Barred_Spiral_Galaxy.jpg here]] you have two nice examples of barred spirals) and are assumed to appear because of their evolution. Surrounding the bar there's a ring of hydrogen packed so densely that star formation is taking place at a rapid rate, so much so that from other galaxies it would be the Milky Way's most noticeable feature. The high level of star formation is concentrated close to the spiral arms that emerge from that ring, and this brings us to:

Its most notable feature is the presence of spiral arms, features [[CaptainObvious so named]] because they curve looking like a spiral. In pictures of external spiral galaxies they stand out quite prominently because of their bluish tint — very often studded with the pinkish red of star-forming regions[[note]]Known as emission nebulae. Basically, this color is produced because the bright, hot stars contained within those regions break up apart the atoms of hydrogen, their chief component, ionizing them. When an electron is re-captured back by an hydrogen nuclei (a proton), it emits radiation of that color[[/note]] — contrasting with the yellowish color of the central bulge of the spiral galaxy.

Astronomers trace the spiral arms using objects that are contained within them such as young, luminous stars, star clusters[[note]]that usually include those luminous stars and/or are embedded within nebulae[[/note]], and nebulae.[[note]]Besides the mentioned emission nebulae, there are other types of nebulae. The two ones that interest us here are ''reflection nebulae'', in which the starlight is reflected by grains of dust within it having a bluish color, and ''dark nebulae'' — [[CaptainObvious obviously dark]], looking as a splotch of ink in the middle of a starfield and lacking stars that ionize or give light to reflect. Stars are born in those nebulae, formed by a mixture of molecules of hydrogen, helium, other molecules, and dust. When young, hot ones form within them hydrogen gets ionized giving birth to an emission nebulae. If no stars of that kind form, we'll have a reflection nebula.[[/note]] Because of the difficulties mentioned above in calculating the objects' exact distances, as well as that things look different depending on the method used to study those objects[[note]]Red supergiants, evolved massive stars that have low surface temperature, for example, are more prominent on the infrared than on the ultraviolet, the wavelength where their younger and hotter brethren are more conspicuous. See {{UsefulNotes/Stars}} to know more about stellar evolution.[[/note]], it's not an easy task. Nor does it help that spiral arms are not regular, but have branches, twists, and some irregularities.

By far, the most notable of them is the ''Sagittarius Dwarf Spheroidal Galaxy'', or ''Sgr dSph''.[[note]]Sagittarius because [[CaptainObvious it's in that constellation]]. Remember, too, that's were we can find the center of the Milky Way.[[/note]] Despite being very close to the Milky Way, as it's on the opposite side of the galaxy in respect to us it was not discovered until 1994. Sagittarius is so close, in fact (at 70,000 light-years from us and around 50,000 light-years from the center of the Milky Way) that the gravitational forces of the latter are destroying it[[note]]not [[TakingYouWithMe without revenge, as the spiral structure of the Milky Way could have been originated by interactions with that galaxy]] [[/note]], and giving the poor little galaxy the form of a loop surrounding ours. Sooner or later, it will be absorbed by our galaxy, and a similar fate awaits at least the closest of those small galaxies.

The Milky Way[[note]]Some professional astronomers know it as the Galaxy (with an uppercase G)[[/note]] is the galaxy in which our Sun is located. Almost everything we can see in the night sky with the naked eye[[note]]The exceptions are the Andromeda and Triangulum galaxies (both large galaxies very close to ours), and some other galaxies farther away said to have been spotted with people with very keen eyes.[[/note]] is inside it, or at least is orbiting it (globular clusters[[note]]Usually large, high-density clusters of stars so old as the Milky Way, that have more or less spherical (globular) shape[[/note]]) or is close to it (the Magellanic Clouds[[note]]a pair of small galaxies thought to be orbiting ours[[/note]]). This page hopes to help readers who want to learn more about our galaxy.

As perhaps you may already know, the Milky Way can be seen without the help of any instrument in moonless nights at places far away of large cities as a hazy band of light crossing the sky[[note]]Why do we see it so and not as the magnificent spiral depicted in so many artistic depictions?. Simple: we're inside it, very close to its plane, and we see it edge-on, as a band, the same as per galaxies that are seen with that angle such as [[https://en.wikipedia.org/wiki/NGC_891 NGC 891]]. Being us inside it explains too why it circles the entire sky[[/note]]. If you look at it with a binoculars or, better, with a telescope, you'll see how that band explodes into countless stars as well as other objects such as star clusters or nebulae that you can locate with a star map.

The best epoch to see it is in the months of July-August, when it can be seen at midnight. People at the southern hemisphere are more lucky, even if it's winter by then there, than those at the northern one, since the brightest parts of the Milky Way -that correspond to the constellations of Sagittarius and Scorpius- can be seen high in the sky. They're also blessed with the view of the two Magellanic Clouds as well as one part of the Milky Way that cannot be seen from northern latitudes.[[note]]Conversely, from the northern hemipshere there are parts of the Milky Way that cannot be seen from the southern one. However, southerners are at advantage with those of us who live in the northern hemisphere since the parts they can see have are richer in objects to observe.[[/note]]

Being inside our galaxy, and of course being unable to send a probe outside it[[note]]Actually, we could... if we had the patience to wait ''hundreds of thousand of years'' until that probe reached a point outside the Milky Way, plus the time (another couple of thousands of years at best) needed for its transmissions to arrive here. This, of course, assuming we could receive them, which is debatable at best[[/note]], puts us on a bad situation to study the Milky Way. To make matters worse, the space between the stars is filled with dust[[note]]Among other things such as hydrogen[[/note]], that absorbs and reddens the ''visible'' light from distant stars to the point -to give an example- the Galactic Center is totally invisible in visible light, being located behind large clouds of dust.

[[Creator/IsaacAsimov Isaac Asimov]], in one of his books about science, gave once an analogy that explains very well the difficulties we have when studying the structure of our galaxy comparing them as the same that would have someone who wanted to make a map of his/her city living on a small building on its suburbs and having foggy weather. Worse, we cannot leave our house, nor sending drones for the reasons depicted before, or -of course- we cannot look for that information on Internet. We're on our own.

The example, however, differs from the RealLife situation in one important way: we may be totally stuck in our house, but at least since the second half of the twentieth century we have ways to make the buildings, that obstruct our view, transparent and are able to see through it to others that are farther away. These ways are using another wavelengths that are not affected (or at least not so much) by the interstellar dust to observe those distant objects, such as radio waves, the infrared, the ultraviolet, or the X-Rays[[note]]All but the radio waves and some parts of the infrared and ultraviolet cannot be used from Earth's surface, since Earth's atmosphere blocks them and space telescopes must be used.[[/note]]

We have ways to observe those buildings. Now another problem kicks in: how we can determine how far away they are, so we can put them on a map?. This is a ''big'' one and is related with the ways we use to determine distances to objects outside the Solar System. Trigonometric parallaxes[[note]]Trigonometry calculations that use the relative motion on an object against a backdrop of stars and galaxies very far away[[/note]], the most precise way to determine the distance of a body, become too tiny to be measured after a certain point and we have to resort to use indirect methods, that include the use of stars such as ''Cepheids''[[note]]giant stars undergoing regular variations on their brightness that depend of their luminosity, some of them with a parallax-known distance. They're named so, because the first example of that kind of star was found in the constellation of Cepheus. They're so bright that can be seen in other galaxies, giving us a very useful tool to determine their distance[[/note]].
Another method, less precise, is related to the spectrum of a star. Two stars of the same temperature but differing on their luminosity (ie: one being a Sun-like star while the other is a supergiant one) will have different spectra. Assuming a star with a given spectrum and whose distance is unknown has the same luminosity that other whose spectrum is similar and its distance is known, it's easy to calculate its ''estimated'' distance[[note]]Red giants, evolved low and intermediate stars, have at its brightest similar luminosities and that allows to use them as more precise distance indicators.[[/note]].
Finally, astronomers have others even less precise ways -but often is all we have- to determine the distance to a celestial object such as estimating the way the interstellar dust reddens and extinguishes its light compared with other one whose distance is more or less known, and calculating from it the distance, or determining the speed it's moving across the Milky Way from its spectrum, and since objects closer to the Galactic center have faster speeds than those at higher distances.[[note]]Things are more complicated than this; we'll return to that issue later[[/note]] estimating its distance[[note]]The latter method is used also with those objects that are not stars such as nebulae[[/note]].

All of this explains why in the news we often see articles about spiral arms of the Milky Way "disappearing" or "appearing"; estimating the ''actual'' distances to celestial bodies, be within our galaxy (and, thus, its structure) or outside it, is anything but an easy task but at least we've advanced very much from the earliest attempts, when the telescope did not existed and we had to resort to conjectures impossible to probe, or later (up to the beginning of the twentieth century[[note]]To be more precise, when telescopes were able to resolve what were thought to be nebulae in stars, showing they actually were galaxies like ours[[/note]]), when it was believed the Milky Way and its attendants -the Magellanic Clouds- were all that existed in the Universe. As time goes by and our instruments and technologies improve, most of the mysteries about the structure of the Milky Way will -hopefully- be unlocked -others will need to wait for the day (if it ''ever'' comes) we have [[FasterThanLight FTL]] travel or can hook to the Galactic Internet (if that thing exists ''and'' whoever operate it allow us to connect)-.
We've been able, too, to determine the location of our Sun in the Milky Way, between 26,000 and 28,000 light-years and -too- between two spiral arms. Early on, it was thought we were on its center -something that, as depicted below, would be ''bad'' for the health of Earth's life-.

Before describing the parts that compose our galaxy, it's very important to have in mind something: it's ''big''. [[YouCannotGraspTheTrueForm Really fuckin' big]] [[DepartmentOfRedundancyDepartment and truly humongous]] by human standars, and a good-sized galaxy but absolutely nothing compared with the inmensity of the Universe, Hypothetical aliens from the Andromeda Galaxy would see the Milky Way more or less as we see theirs galaxy from ours, but from galaxies farther and farther away it would be dimmer and dimmer to the point it would be just another galaxy among millions too faint to be observed except by Hubble-like telescopes, that nobody cared about.

In the [[UsefulNotes/TheSolarSystem page]] of Useful notes about the Solar System there's a nice scaled-down model of the Solar System, that shows the sizes and distances of its planets. What would be the size of the Milky Way there?. ''141,000,000'' kilometers, almost the distance between the Earth and the Sun, with our star being located at 40,000,000 kilometers of its center.
Let's compress that model, assuming now the Sun has the size of a grain of sand (around 0.5 millimeters). At that scale, you'd need a microscope to see the Earth at 54 millimeters of the Sun and you'd find the closest star (Proxima Centauri) as another grain of sand at a bit more than ''14 kilometers''. The Milky Way at that scale would still have a size of ''330,000'' kilometers, almost as big as the distance between the Earth and the Sun, and our Sun would be at around 94,000 kilometers of its center. And before you go to NASA to tell them to send a probe to it, remember that light in this model -the fastest thing in the Universe- would move at the snail pace of almost 390 millimeters ''per hour''[[note]]And the less we say of our space probes, the better[[/note]]. Just for the record, stars are not fixed; they move around the center of the Milky Way as Earth moves around the Sun. The latter moves at an speed of 220 kilometers ''per second'' (and there are stars even faster[[note]]Some of them boosted to speeds so high that have departed the Milky Way to never return[[/note]]). At this scale, our Sun would move at 0.3 millimeters per hour.

'''''The bulge'''''. The bulge is the central part of the Milky Way. It has a roughly spheroidal shape, with a diameter of 10.000 light years (one tenth the diameter of our galaxy) and its mostly composed of old, small, stars closely packed together, more than in the neighborhood of the Sun. There's also very little interstellar matter there.
This bulge is small compared to the one of our large neighbor, the Andromeda Galaxy, and especially the one in the [[https://en.wikipedia.org/wiki/Sombrero_Galaxy Sombrero Galaxy]]

The center of the Milky Way is in the direction of the constellation Sagittarius and, as explained above, interstellar dust makes almost impossible[[note]]Not impossible, because there are a couple of patches of the sky where there's little dust and we can see stars of the bulge, the largest known as [[https://en.wikipedia.org/wiki/Baade%27s_Window Baade's Window]], for the astronomer who discovered it. However, none of them point to the ''exact'' center of the Milky Way[[/note]] its study and we must use stuff distinct to visible light to study it. This has led us to known that the apparent boredom of the bulge is just that, apparent and very interesting things are happening in the center of our galaxy.
While few in numbers compared with the old stars that compose the bulge, the Galactic center contains a lot of hot and luminous[[note]]and thus young, since those stars live no longer than a few million years, compared to a Sun-like star's ten-billion year lifespan[[/note]] stars, among them some of the most luminous ones of the entire Milky Way and, also, some concentrated within two massive star clusters know as ''Arches cluster'' and ''Quintuplet cluster'', as well as a ring of hydrogen, almost dense enough to form stars and increasing in mass and density, to the point that it is believed that within 200 million years star formation will break loose in that ring at a furious pace.
Just at the center of the Milky Way lies a massive [[UsefulNotes/BlackHoles black hole]] known as ''Sagittarius A*'', surrounded by a couple of stars and gas clouds orbiting it. This hole has a mass of around 4 million times that of the Sun (small compared to the ones thought to exist in other galaxies -Andromeda's may be up to 100 million times more massive than ours and there are some that are far bigger in other galaxies-) and feeds, albeit less than expected, on the gas that surrounds it. There are also large clouds of gas distorted by magnetic fields as well as some other star-forming gas clouds.[[note]]Nebulae?[[/note]]

Its most notable feature is the presence of spiral arms, features [[CaptainObvious named so]] because they curve looking like a spiral. In pictures of external spiral galaxies they stand out quite prominently because of their bluish tint -very often studded with the pinkish red of star-forming regions[[note]]Known as emission nebulae. Basically, this color is produced because the bright, hot stars contained within those regions break up apart the atoms of hydrogen, their chief component, ionizing them. When an electron is re-captured back by an hydrogen nuclei (a proton), it emits radiation of that color[[/note]]- contrasting with the yellowish color of the central bulge of the spiral galaxy.

Why do spiral arms form? One could think they're material structures. However, if they were, as the galaxy rotates and the stars closer to its center rotate faster than those farther away, in just a few rotations they'd become so tightly wound that they would become indistinguishable of the surrounding galaxy. [[note]]Of course, the Milky Way doesn't rotate very quickly.[[/note]]

Astronomers trace the spiral arms using objects that are contained within them such as young, luminous stars, star clusters[[note]]That usually include those luminous stars and/or are embedded within nebulae[[/note]], and nebulae[[note]]Besides the mentioned emission nebulae, there are other types of nebulae. The two ones that interest us here are ''reflection nebulae'', in which the starlight is reflected by grains of dust within it having a bluish color, and ''dark nebulae'' -[[CaptainObvious obviously dark]], looking as a splotch of ink in the middle of a starfield and lacking of stars that ionize or give light to reflect. Stars are born in those nebulae, formed by a mixture of molecules of hydrogen, helium, other molecules, and dust. When young, hot ones form within them hydrogen gets ionized giving birth to an emission nebulae. If no stars of that kind form, we'll have a reflection nebulae-[[/note]]. Because of the difficulties mentioned above of calculating the objects' exact distances, as well as that things look different depending on the method used to study those objects[[note]]Red supergiants, evolved massive stars that have low surface temperature, for example, are more prominent on the infrared than on the ultraviolet, the wavelength where their younger and hotter brethren are more conspicuous. See {{UsefulNotes/Stars}} to know more about stellar evolution[[/note]] it's not an easy task. Nor it helps that spiral arms are not regular, but have branches, twists, and some irregularities.
The most recent studies show four spiral arms traced by young stars and gas as well as two spiral arms marked by older stars, that emit most of its light in the infrared. It's not known why this happens, but this dichotomy can be found in other spiral galaxies when observing them in the infrared.
Most of the spiral arms of the Milky Way are named after constellations (marked in '''bold''') that they cross as seem from Earth. So we have the ''3-kpc near arm'' and ''3-kpc far arm[[note]]Like his partner, the 3-kpc near arm is named so because it's located at 3 kiloparsecs (10,000 light-years) from the center of our galaxy[[/note]]'', that together form the ring that surrounds the Milky Way's central bar, the '''Norma Arm''', that becomes the ''Outer Arm'' as it continues outwards, the '''Scutum'''-'''Centaurus''' Arm, the '''Perseus Arm''', and the '''Carina'''-'''Sagittarius Arm'''. Of these arms, the two most important are believed to be '''Scutum'''-'''Centaurus''' and '''Perseus'''[[note]]The study of the Milky Way's spiral structure is hard, not only because what has been explained above, but also because as we're quite close to its plane we see how the spiral arms overlap, so while seems clear that all arms branch from the ring formed by the 3-kpc ones it's not well known where they attach. To make things worse, the Galactic Center makes difficult the studies of the opposite region of our galaxy. It's assumed the parts we can't see are symmetrical with those we can.[[/note]]

The Milky Way has a total mass in stars that is estimated to be very roughly 50 billion times (5*10^10) the mass of the Sun. Its total number of stars, however, is considerably higher as most of the stars in our galaxy (and in the Universe) are small, low-luminosity stars named red dwarfs (no, not [[RedDwarf this one]]) and the more luminous the star the less common they are[[note]]Sun-like stars are assumed to be just 10% of the total stars of the Milky Way. The high-luminosity stars mentioned above are ''very'' rare, but as their luminosities are so high they can be seen from large distances, even in external galaxies[[/note]]. Our galaxy is estimated to have between 200 ''billion'' and 400 ''billion'' stars, plus -besides their planets, if they have them- brown dwarfs[[note]]Bodies too small to have initiated nuclear fusion of hydrogen. Failed stars, in other words[[/note]], white dwarfs, neutron stars, and black holes[[note]]The remnants of dead stars. White dwarfs are by far the most numerous, around 10% of the total number of stars in our galaxy[[/note]], and finally a veritable sea of flotsam and jetsam that includes rogue planets, comets, and asteroids. While this seems ''a lot'' of stuff, remember the sheer emptiness is space. If Han Solo had activated the hyperdrive to escape from those Star Destroyers in ''Film/ANewHope'' without calculating an hyperjump, the most likely fate of the Millennium Falcon would have been to end up in the middle of nowhere and at light-years of the closest star[[note]]And, yes, I know the Franchise/StarWars galaxy is ''not'' ours[[/note]].

Because of many of the stars of our galaxy are, as explained above, less luminous than our Sun the total luminosity of the Milky Way is not so high as one would expect. Different authors give different values because -again- being within it makes difficult to estimate that, but estimations seem to oscillate -in visible light- around 20-30 billion times that of our Sun, a typical luminosity for a large galaxy. For comparison, the Andromeda Galaxy may be twice as luminous and the Large Magellanic Cloud ten times less luminous.

There's more than just stars in the disk of the Milky Way. Immersed in the space between the stars, there's gas -mostly hydrogen and helium, but also more complex atoms, that often form molecules -even organic ones-, as well as dust-.
Hydrogen, besides ionized in emission nebulae, can be found in two flavors: neutral hydrogen (single atoms of hydrogen), and molecular hydrogen (hydrogen in molecules of itself formed by two atoms, quite often accompanied by other molecules). Both have very different distributions within the Milky Way: most of the molecular hydrogen is concentrated between the distance of the Sun and the ring mentioned above, while the neutral hydrogen also tapers off at the ring, but extends farther away than the stars, at a radius of up to 75,000 light-years. Its total mass (within the disk) is approximately one sixth of the total mass in stars of our galaxy, meaning that the Milky Way has already used most of its gas to form new stars.
Meanwhile dust is concentrated within a disk that coincides with the plane of the galactic disk, forming the dark band that crosses the equator of galaxies that are seen edge-on. There's much less dust than gas: just 1 percent of the gas mass of the disk is in the form of dust. However, as we've explained before that dust is very efficient at blocking the light coming from distant stars. Luckily, however, as most of it lies concentrated within a disk the farther we look from the Milky Way the less (much less) of it, as well as stars, we'll find and the farther we can look, to the point of being able to observe external galaxies (and millions of them.)

We can use the velocities at which stars and gas -that is more extended than the stars- move around the center of a galaxy to determine its mass, and we would expect those that are farthest away from the center, have a low velocity. However, when astronomers started to measure those velocities they found that speeds in the outermost regions of galaxies are actually ''much higher'' than expected. The most accepted explanation is to assume there are large amounts of unseen matter -the famous "dark matter"[[note]]A ''thing'' -for lack of a better name, it's not known what composes it- that does not emit or absorbs electromagnetic radiations and that just interacts with normal matter via gravitation[[/note]]- surrounding in large halos the galaxies.

'''''The Halo'''''. The ''halo'' is a large spheroidal zone that surrounds the Milky Way's disk. It's more boring than the disk or even the bulge, being almost gas and dust-free. There are few stars, most of them concentrated within globular clusters[[note]]As explained above, usually large and dense star clusters more or less spherical. Our galaxy may have around around 150, but other galaxies have up to thousands of them[[/note]] and all of them very old, even as old as the Universe itself.
Rather than a halo, recent findings suggest it's better to talk of ''two'' halos: the stellar one, that would have a extent double than the disk of the Milky Way (ie: a radius of 100,000 light-years) and where most of its stars and globular clusters would lie, and a gaseous one that envelops it as a vast corona of hot gas extending at hundreds of thousands of light-years, and that may be a remnant of the formation of the Milky Way with a mass equivalent to that of our galaxy[[note]]Other galaxies such as the Andromeda Galaxy have been found to have those gaseous haloes too. Most, if not all of them, have stellar ones too[[/note]].

We mentioned above dark matter existing in an halo surrounding our galaxy, and said that was believed to be the cause of the stars moving at higher velocities that one would expect. How much dark matter is believed to exist there?. ''Lots'', much more than the mass in stars in our galaxy. Assuming things work in those places so far away -which, according to most astronomers is what happens- as in our neighborhood, the mass of dark matter may be in the ''hundreds of billions'' of solar masses. It's quite daunting to think that most of the matter in our galaxy is... something, that just interacts with normal matter with gravity and maybe with itself with a kind of "dark force".[[note]]It's actually ''worse'' than that: it's currently believed normal matter makes just 5% of the Universe, and dark matter is what composes another 25% (more or less). The remaining 70% is made of something named ''dark energy'', even more unknown and that seems to be accelerating the expansion of the Universe, It turns out that 95% of it is unknown and invisible.[[/note]]

The Milky Way is not alone in space, being accompanied by a veritable number of small galaxies that orbit it. We already mentioned the Magellanic Clouds[[note]]Sadly invisible for people living at the Northern Hemisphere,[[/note]], two irregular[[note]]Irregular galaxies lack a well-defined form[[/note]] galaxies that for a long time were believed to orbit the Milky Way, but some astronomers now think are first-time visitors. Anyway, both -the Large Magellanic Cloud and the Small Magellanic Cloud as are known- are much smaller (respective sizes of 14,000 light-years and 7,000 light-years across) and less luminous than our galaxy being located at respective distances of 160.000 light-years and 200.000 light-years. However, they're more rich in gas than the Milky Way and are plentiful in young, luminous stars. In fact, the Large Magellanic Cloud contains the [[https://en.wikipedia.org/wiki/Tarantula_Nebula Tarantula Nebula]], one of the largest star-forming regions known. Both share a common envelope of hydrogen and are connected by streams of it to the Milky Way, suggesting the latter is interacting with them.

The other satellite galaxies of the Milky Way are far smaller and less luminous than the Magellanic Clouds[[note]]So little luminous are some of them that there are (high-luminosity) stars ''brighter'', even if they're far more massive than those stars[[/note]], almost all of them being composed of little more than (comparatively) few and very old stars with little no dust or gas and thus star formation. In addition to this, they're very extended, and because of that, as well as having few stars, very hard to detect. Those galaxies are named because of their small size[[note]]compared to other galaxies, of course[[/note]] and shape ''dwarf spheroidals'' and there are around 30 known (and very likely there are more waiting to be discovered). Those dwarf galaxies are believed to be the evolved building blocks that built the Milky Way, their gas lost long ago because of their feeble gravity being unable to hold it when supernovae expelled if from them and/or having been stripped by the halo of hot gas that surround the Milky Way.

By far, the most notable of them is one known as ''Sagittarius Dwarf Spheroidal Galaxy'', or ''Sgr dSph''[[note]]Sagittarius because [[CaptainObvious is in that constellation]]. Remember, too, that's were we can find the center of the Milky Way[[/note]]. Despite being very close to the Milky Way, as it's on the opposite side of the galaxy respect to use it was not discovered until 1994. Sagittarius is so close, in fact (at 70.000 light-years from us and around 50.000 light-years from the center of the Milky Way) than gravitational forces of the latter are destroying it[[note]]not [[TakingYouWithMe without a revenge, as the spiral structure of the Milky Way could have been originated by interactions with that galaxy]] [[/note]], and giving the poor little galaxy the form of a loop surrounding ours. Sooner or later, it will be absorbed by our galaxy and a similar fate awaits to at least the closest of those small galaxies.

If in the outskirts of the Milky Way dark matter is plentiful, in those dwarf galaxies it's even more in proportion. In the most extreme cases, dark matter may outnumber normal matter ''several hundreds to one'', if our measurements are correct. It's not very exaggerated to consider those galaxies are dark galaxies of dark matter, with stars as just "icing on the cake".

And, finally, another object that may be what remains of a galaxy that suffered a similar fate to Sagittarius in the past is [[https://en.wikipedia.org/wiki/Omega Centauri Omega Centauri]], a globular cluster (the most luminous as well as the brightest of them).

The Milky Way was formed shortly after the Big Bang by the fusion ("bottom-up") of countless objects similar to globular clusters or irregular galaxies Magellanic Clouds-style, that formed of matter overdensities that existed by then. It's generally believed it formed from the outside to the inside, with the first being the halo followed by the bulge and finally the disk as the gas coalesced because of its fast rotation and conservation of angular momentum. In those early days things were much more entertaining than in the present days, with much more interstellar gas and thus more star formation -that meant more supernovae-, and likely the black hole at the center of our galaxy blazing as a quasar[[note]]A galactic nucleus with an unusually high luminosity (of the order of a galaxy and even more), caused by the accretion of large amounts of matter by a supermassive black hole located on it[[/note]]. However unlike many other galaxies, that have collided and merged with others of comparable size in the past, the Milky Way has had a more calm history in that sense with no collisions with large galaxies.

Perhaps the most notable event -for us, at least- in the past of the Milky Way has been the formation of the Sun 4.55 billion of years ago.

What has the future in store for the Milky Way?. We've commented before that our galaxy is not alone in space. Its nearest large companion is the Andromeda Galaxy, another spiral galaxy similar in some respects to ours but twice as large and bright, at a distance of 2.5 million light-years.
Both the Milky Way and Andromeda are approaching one to each other attracted by their gravities, and it's expected the two galaxies will collide within 3-4 billion years. From a planet in the Milky Way, Andromeda[[note]]Or the Milky Way from a planet at Andromeda[[/note]] will grow bigger and bigger with the disks of both galaxies deforming just before the collision. After a first glancing blow that will disrupt them, both will move away until their gravitational attraction stops their motion and cause them to fall again one to each other. After a few more close passes, the collision will end with Andromeda and Milky Way's final embrace as one giant elliptical galaxy nicknamed ''Milkomeda'' or also ''Milkdromeda'' [[note]]A galaxy with a more or less ellipsoidal shape (think on a football), made of old stars and with very little, if any, gas and star formation[[/note]].

Notice that, as space between stars is so huge as explained above, collisions between the stars, even in the crowded centers of both galaxies, will be very rare. Whatever gas that remains in both galaxies, however, will collide and be compressed, creating a burst of star formation. It may, too, be funneled to the center of the new galaxy to feed the supermassive black hole formed there by fusion of the two black holes that lurked before in the centers of both Andromeda and the Milky Way, forming a quasar that will shine with the light of an entire galaxy. So both galaxies go out with a bang![[note]]As for our Sun, simulations of the encounter suggest it's likely it will end in the outskirts of Milkomeda and the dangers of the collision affecting planetary orbits are pretty low. However, as the Sun's luminosity is growing over time and will render Earth unhabitable by then, it's highly doubtful someone will still be around to see the show.[[/note]]

The Milky Way[[note]]Some professional astronomers know it as the Galaxy (with an uppercase G)[[/note]] is the galaxy in which our Sun is located. Almost everything we can see in the night sky with the naked eye[[note]]The exceptions are the Andromeda and Triangulum galaxies (both large galaxies very close to ours), and some other galaxies farther away said to have been spotted with people with very keen eyes.[[/note]] is inside it, or at least is orbiting it (globular clusters[[note]]Usually large, high-density clusters of stars so old as the Milky Way, that have more or less spherical (globular) shape[[/note]] or is close to it (the Magellanic Clouds[[note]]a pair of small galaxies thought to be orbiting ours[[/note]]). This page hopes to help readers who want to learn more about our galaxy.

Astronomers trace the spiral arms using objects that are contained within them such as young, luminous stars, star clusters[[note]]That usually include those luminous stars and/or are embedded within nebulae[[/note]], and nebulae[[note]]Besides the mentioned emission nebulae, there are other types of nebulae. The two ones that interest us here are ''reflection nebulae'', in which the starlight is reflected by grains of dust within it having a bluish color, and ''dark nebulae'' -[[CaptainObvious obviously dark]], looking as a splotch of ink in the middle of a starfield and lacking of stars that ionize or give light to reflect. Stars are born in those nebulae, formed by a mixture of molecules of hydrogen, helium, other molecules, and dust. When young, hot ones form within them hydrogen gets ionized giving birth to an emission nebulae. If no stars of that kind form, we'll have a reflection nebulae-[[/note]]. Because of the difficulties mentioned above of calculating the objects' exact distances, as well as that things look different depending on the method used to study those objects[[note]]Red supergiants, evolved massive stars that have low surface temperature, for example, are more prominent on the infrared than on the ultraviolet, the wavelength where their younger and hotter brethren are more conspicuous. See (UsefulNotes/Stars) to know more about stellar evolution[[/note]] it's not an easy task. Nor it helps that spiral arms are not regular, but have branches, twists, and some irregularities.