The Milky Waynote is the galaxy in which our Sun is located. Almost everything we can see in the night sky with the naked eyenote is inside it, or at least is orbiting it (globular clustersnote ) or is close to it (the Magellanic Cloudsnote ). This page hopes to help readers who want to learn more about our galaxy.
First Things First: observing it
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 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 time to see it is in the months of July-August, when it can be seen at midnight. People in the southern hemisphere are more lucky, even if it's winter by then there, than those in the northern one, since the brightest parts of the Milky Way corresponding 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
Mapping the Milky Way
Being inside our galaxy, and of course being unable to send a probe outside itnote , puts us at a disadvantage in studying the Milky Way. To make matters worse, the space between the stars is filled with dustnote , 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.
Isaac Asimov, 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.
The example, however, differs from the Real Life 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 them to others that are farther away. These ways are using other wavelengths that are not affected (or at least not so much) by the interstellar dust to observe those distant objects, such as radio waves, infrared, ultraviolet, or X-raysnote
We have ways to observe those buildings. Now another problem kicks in: how can we determine how far away they are, so we can put them on a map? This is a big one, and is related to the ways we use to determine distances to objects outside the Solar System. Trigonometric parallaxesnote , 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
Another method, less precise, is related to the spectrum of a star. Two stars of the same temperature but differing in their luminosity (ie: one being a Sun-like star while the other is a supergiant) 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
Finally, astronomers have even less precise ways but often the only methods usable 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 Centre have faster speeds than those at higher distances,note estimating its distance.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 they within our galaxy (and, thus, its structure) or outside it, is anything but an easy task. At least we've advanced considerably from the earliest attempts, when the telescope did not exist and we had to resort to conjectures impossible to probe, or later (up to the beginning of the twentieth centurynote ), 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 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 from the centre and between two spiral arms. Early on, it was thought we were at its center something that, as depicted below, would be bad for the health of Earth's life.
The size and structure of our galaxy
Before describing the parts that compose our galaxy, it's very important to have in mind something: it's big. Really fuckin' big and truly humongous by human standards, 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 their 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 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 40,000,000 kilometers from its center. 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 Moon, 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 hournote . 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 fasternote ). At this scale, our Sun would move at 0.3 millimeters per hour. So face it; there's no way to be able to grasp the size just of our galaxy. You can, to a point, excuse writers of their errors.
What are the parts of the Milky Way Galaxy?. Let's find out:
The bulge. 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 larger neighbor, the Andromeda Galaxy, and especially the one in the Sombrero Galaxy.
The center of the Milky Way is in the direction of the constellation Sagittarius and, as explained above, interstellar dust makes its study almost impossiblenote , and we must use wavelengths other than visible light to study it. This has led us to realize 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 luminousnote stars, among them some of the most luminous ones of the entire Milky Way and, also, some concentrated within two massive star clusters known as the "Arches" and "Quintuplet" clusters, 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 black hole known as Sagittarius A*, surrounded by a couple of stars and gas clouds orbiting it. This hole has a mass of around 4.1 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 regions.
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 galaxiesnote have central bars, more or less long (here◊ and 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:
The Disk. The disk is the largest part of the Milky Way in size, reaching to 100,000 light-years (and maybe even morenote ), but it's quite thin (no more than 2,000 light-years thick), and here's where we can find most of its stars and of all ages: from stars as old as the Milky Way itself to others still forming, as well as most of the matter that fills the space between the stars. It's the part where our Sun is located. Its most notable feature is the presence of spiral arms, features 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 regionsnote 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 A theory that explains quite well how spiral arms develop is the density wave theory. To explain how it works, we'll use a traffic jam as an analogy. Cars move inside and outside of it, increasing the density of cars within it, but the jam's density does not, or at least not as fast as the cars outside. Translating it to a galaxy, stars and gas are the cars and the jam is the spiral arms. The former leave the arm unscathed, but the latter is compressed until stars are born, the hottest and most luminous of them ionizing the hydrogen of the interstellar medium as it's explained above forming emission nebulae. Those stars, however, are short-lived and explode as supernovae before being able to leave the arm, much unlike less luminous stars as the Sun that can enter and leave an arm many times. This theory means that the spiral arms themselves do not move, or at most move very slowly, and that they simply are zones where the density of stars and interstellar matter, as well as the star formation activity, are high compared to the space between them.
Astronomers trace the spiral arms using objects that are contained within them such as young, luminous stars, star clustersnote , and nebulae.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 objectsnote , it's not an easy task. Nor does it help 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 their 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 armnote , 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 Perseusnote In addition to them, there are a number of spurs such as the Orion-Cygnus arm. Note that the picture mentioned above is subject to change. For example, it's currently believed that Sagittarius-Carina is a minor arm while in the past it was thought to be one of the most important arms of our galaxy and Orion-Cygnus is thought to connect with Perseus. Our Sun is located between the Orion-Cygnus spur and the Sagittarius-Carina arm.
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 this one) and the more luminous the star the less common they arenote . Our galaxy is estimated to have between 200 and 400-billion stars, plus besides their planets, if they have them brown dwarfsnote , white dwarfs, neutron stars, and black holesnote , and finally a veritable sea of flotsam and jetsam that includes rogue planets, comets, and asteroids. While this seems like a lot of stuff, remember the sheer emptiness is space. If Han Solo had activated the hyperdrive to escape from those Star Destroyers in A New Hope without calculating a hyperjump, the most likely fate of the Millennium Falcon would have been to end up in the middle of nowhere and light-years from the closest star.note
Because 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 it 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 being 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 surrounding the galaxies in large haloes.
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 clustersnote and all of them very old, even nearly as old as the Universe itself. Rather than a halo, recent findings suggest it's better to talk of two halos: the stellar one, with an 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 lie, and a gaseous one that envelops it in a vast corona of hot gas extending 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
We mentioned above dark matter existing in a 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 of 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
Life in orbit: Satellite Galaxies of the Milky Way
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 Cloudsnote , two irregularnote 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 are much smaller (respectively 14,000 light-years and 7,000 light-years across) and less luminous than our galaxy, being located at, respectively, the 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 Tarantula Nebula, the largest known star-forming region in the Local Group. 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 Cloudsnote , 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, they're very hard to detect. Those galaxies are named dwarf spheroidals because of their small sizenote and shape, and there are around 30 known (and very likely 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 the Sagittarius Dwarf Spheroidal Galaxy, or Sgr dSph.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 itnote , 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.
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 hundred to one, if our measurements are correct. It's not very exaggerated to consider those galaxies as dark galaxies of dark matter, with stars as just "icing on the cake".
Another companions of the Milky Way are the known as "High-Velocity Clouds" (HVC), large clouds of hydrogen also galactic building blocks, that however did not form stars and are composed of almost pristine hydrogen. At least one of them, the Smith's Cloud, is doomed to collide with the Milky Way, being absorbed by it and producing a burst of star formation where it hits... 27 million years in the future, so take it easy.
And, finally, another object that may be what remains of a galaxy that suffered a similar fate to Sagittarius in the past is Omega Centauri, a globular cluster (the most luminous, brightest and largest of them).
A biography of the Milky Way
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, meaning more supernovae, and likely the black hole at the center of our galaxy blazing as a quasarnote . 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.
From those days to the present, the Milky Way has been growing absorbing gas from its halo and smaller galaxies, albeit less and less as the number of those objects is diminishing. As the interstellar gas is being consumed by new stars, there's less raw materials to form new stars and thus star formation is dwindling. Perhaps the most notable event for us, at least in the past of the Milky Way was the formation of the Sun, 4.55 billion years ago.
What does the future have 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 another, 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, Andromedanote 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 into each other again. 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 Milkdromeda 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 the fusion of the two black holes that lurk 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