The Milky Way
Today I’m going to be talking about our very own galaxy: the milky way. It contains our solar system, over 100 billion stars, at least as many planets as stars, a bunch of huge clouds of interstellar gas, and a supermassive black hole at its center. It’s a very intriguing place, so let’s dive start talking about it!
But first, let’s talk about its name. “The Milky Way” isn’t the most majestic name we could have given our wonderous galaxy, so why was it picked out of all the other possible contenders? Well, this name actually comes from a Greek myth about the goddess Hera, who when trying to feed a baby demigod, spilt milk all across the sky, hence creating the Milky Way. In other parts of the world, our galaxy goes by other names. In China it’s called the “Silver River,” and in the Kalahari Desert in Southern Africa, it’s called the “Backbone of Night.”
And all of these names also fit with the Milky Way’s appearance. The Milky Way is visible from Earth as a hazy band of whiteish light, some 30° wide, arching the night sky. When observing the night sky, the term “Milky Way” is limited to this band of light, although all the individual stars in the entire sky are still part of the Milky Way Galaxy, The light originates from the accumulation of unresolved stars and other material located in the direction of the galactic plane. Brighter regions around the band appear as soft visual patches known as star clouds, which are groups of stellar clusters, which is what I talked about last episode! The most conspicuous of these is the Large Sagittarius Star Cloud, a portion of the central bulge of the galaxy. Dark regions within the band, such as the Great Rift and the Coalsack, are areas where interstellar dust blocks light from distant stars. The Great Rift especially serves to block off part of our view of our galaxy, dividing it vertically. It covers one third of the Milky Way, and is flanked by strips of numerous stars. The clouds of this rift are an obstruction to millions of the galaxy’s stars, blocking of some of the most spectacular views, not to say that the Milky Way isn’t already pretty spectacular.
The area of sky that the Milky Way obscures is called the Zone of Avoidance since, well, the darkness basically avoids that area of the sky. A good chunk of this darkness is additional interstellar dust, which is why astronomers mainly rely on radio or infrared telescopes to study our galaxy. It also has a relatively low surface brightness, mostly due to interference from our sun. This means the Milky Way’s visibility can be greatly reduced by background light, such as light pollution or moonlight. This makes the Milky Way difficult to see from brightly lit urban or suburban areas, but very prominent when viewed from rural areas when the Moon is below the horizon. Maps of artificial night sky brightness show that more than one-third of Earth’s population cannot see the Milky Way from their homes due to light pollution, which is rather sad since the Milky Way is truly a beautiful sight.
As viewed from Earth, the visible region of the Milky Way’s galactic plane occupies an area of the sky that includes 30 constellations. The Galactic Center lies in the direction of Sagittarius, which is also where the Milky Way is brightest. From Sagittarius, the hazy band of white light appears to pass around to the galactic anticenter in Auriga. The band then continues the rest of the way around the sky, back to Sagittarius, dividing the sky into two roughly equal hemispheres. The galactic plane is inclined by about 60° to the plane of Earth’s orbit, officially known as the ecliptic.
The Milky Way is the second-largest galaxy in our Local Group, after the Andromeda Galaxy, and its stellar disk is approximately 170,000–200,000 light-years in diameter and, on average, approximately 1,000 lightyears thick. The Milky Way is also approximately 890 billion to 1.54 trillion times the mass of the Sun. Those are some big numbers, so to get a better idea of the size of our galaxy, let’s compare the relative physical scale of the Milky Way and our solar system. If the main Solar System, which is the region stretching from the sun out to the orbit of Neptune, were the size of a US quarter, the Milky Way would be approximately the size of the United States, ignoring Hawaii and Alaska. So as you can see, our little planet is barely worth mentioning when you take a look at vastness of all that surrounds us. But that’s not all, there is also a ring-like filament of stars rippling above and below the relatively flat galactic plane, wrapping around the Milky Way at a diameter of 150,000–180,000 light-years. Some scientists believe this halo of stars may be part of the Milky Way itself, though work still needs to be done to see if they are truly joined to our galaxy.
Much of the mass of the Milky Way seems to be made up of dark matter, an unknown and invisible form of matter that interacts gravitationally with ordinary matter. A dark matter halo is conjectured to spread out relatively uniformly to a distance beyond one hundred kiloparsecs from the Galactic Center, with one parsec being equal to 3.26 lightyears. Basically, this dark matter halo would be very, very big. Mathematical models of the Milky Way suggest that the mass of dark matter is 1–1.5 times 1012 times the mass of our sun, so it would be, very, very massive.
The total mass of all the stars in the Milky Way is estimated to be between 4.6 times 1010 and 6.43 times 1010 solar masses. In addition to the stars, there is also interstellar gas, comprising 90% hydrogen and 10% helium by mass, with two thirds of the hydrogen found in the atomic form and the remaining one-third as molecular hydrogen. The mass of the Milky Way’s interstellar gas is equal to between 10% and 15% of the total mass of its stars. Interstellar dust accounts for an additional 1% of the total mass of the gas.
To get an exact figure for the number of stars in our galaxy would be very difficult, as that would depend on counting the number of very-low-mass stars, which are difficult to detect, especially at distances of more than 300 lightyears from the Sun. As a comparison, the neighboring Andromeda Galaxy contains an estimated one trillion stars, and it is slightly bigger than our own galaxy. So the Milky Way would possibly contain ten billion white dwarfs, a billion neutron stars, and a hundred million stellar black holes, but it would be very difficult for astronomers to detect all these objects, at least with the technologies we have now. So the best we have so far is an estimate that our galaxy contains anywhere from 100 billion to 400 billion stars.
Filling the space between the stars is a disk of gas and dust called the interstellar medium. This disk has at least a comparable extent in radius to the stars, whereas the thickness of the gas layer ranges from hundreds of light-years for the areas of colder gas to thousands of light-years for areas of warmer gas. This medium includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It also blends smoothly into the surrounding intergalactic space. The energy that occupies the same volume as the interstellar medium but in the form of electromagnetic radiation is known as the interstellar radiation field.
In all phases, the interstellar medium is extremely tenuous by terrestrial standards. In cool, dense regions of the ISM, matter is primarily in molecular form, and reaches number densities of 1 million molecules per cm3. In hot, diffuse regions of the ISM, matter is primarily ionized, and the density may be as low as 1 ion per every 10,000 cm3. When compared to the 10 billion molecules per cm3 of a laboratory grade high-vacuum chamber, this is barely anything.
Of the gas in the ISM, by number 91% of atoms are hydrogen and 8.9% are helium, with 0.1% being atoms of elements heavier than hydrogen or helium, which are also known as “metals.” By mass this amounts to 70% hydrogen, 28% helium, and 1.5% heavier elements.
The ISM is usually far from thermodynamic equilibrium, which means it is often expanding and condensing as it is often heating and cooling. Scientists believe that many different features may be responsible for this heating, such as UV radiation from hot stars, collisions between gas molecules, and even cosmic rays, with just as many possibilities existing to explain the cooling of the interstellar medium.
The interstellar medium is responsible for the brightening and fading of stars, which occurs primarily because of interstellar dust, as it obscures and reddens starlight. On the average, stars near the Sun are dimmed by a factor of two for every 3,000 light-years. Thus, a star that is 6,000 light-years away in the plane of the Galaxy will appear four times fainter than it would otherwise were it not for the interstellar dust.
Despite only making up about 1% of the ISM, interstellar dust has powerful effects on the galaxy, such as by changing the polarization of background starlight. The dust is aligned in space to some extent, and this results in the creation of a preferred plane of vibration for the light waves, so instead of just passing through space haphazardly, they travel on this imaginary 2D surface. It is likely that the polarization effect arises because the dust grains are actually partially aligned by the galactic magnetic field. If the dust grains are paramagnetic, which means they act somewhat like a magnet, then the general magnetic field, in slowly line up the grains in the direction of the field. As a consequence, the polarization for stars in different parts of the sky lines up with the magnetic field of the milky way at said point in the sky, which also makes it possible to plot the direction of the magnetic field in the Milky Way.
There are also a few large clouds of dust in our galaxy. These clouds are also narrowly limited to the plane of the Milky Way, though very low-density dust can be detected even near the galactic poles. Dust clouds beyond 2,000 to 3,000 light-years from the Sun cannot be detected optically, because closer dust clouds and the general dust layer obscure these more distant views. Based on the distribution of dust clouds in other galaxies, scientists can conclude that they are often most conspicuous within the spiral arms of our galaxy as opposed to its center, especially along the inner edge of well-defined arms. The best-observed dust clouds near the Sun have masses of several hundred solar masses and sizes ranging from a maximum of about 200 light-years to just a fraction of a light-year. The smallest tend to be the densest, likely because of the characteristics of dust: when a cloud of dust contracts, it also becomes denser and more opaque.
The disk of stars in the Milky Way also lies on the galactic plane, but it does not have a sharp edge beyond which there are no stars. Instead, the concentration of stars slowly decreases with as the distance from the center of the Milky Way increase. For reasons that are not understood, beyond a radius of roughly 40,000 lightyears from the center, the number of stars per cubic parsec drops much faster the further out one goes. Surrounding the galactic disk is thar spherical Galactic Halo of stars I mentioned before, as well as many globular clusters that extend farther outward. But both of these are limited in size by the orbits of two Milky Way satellites, the Large and Small Magellanic Clouds, whose closest approach to the Galactic Center is about 180,000 lightyears. At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds, so those objects would probably be ejected from the vicinity of the Milky Way.
Now that you know more about the composition of the galaxy, you may be curious as to how it is actually structured. First of all, the milky way is a barred spiral galaxy. If you could look down on it from the top, you would see a central bulge surrounded by four large spiral arms that wrap around it. Spiral galaxies make up about two-third of the galaxies in the universe. Unlike a regular spiral, a barred spiral contains a bar across its center region and has two major arms. The Milky Way also contains two significant minor arms, as well as two smaller spurs. One of the spurs, known as the Orion Arm, contains the sun and the solar system. The Orion arm is located between two major arms, Perseus and Sagittarius.
Our solar system is about 25,000 to 28,000 lightyears away from the center of the Milky Way. The Galactic Center is marked by an intense radio source named Sagittarius A*, which is believed to be the supermassive black hole our galaxy was built around. Supermassive black holes are a regular part of large galaxies, so it makes sense that our own milky way would have one. Surrounding this black hole is an extended bulge of stars that is nearly spherical in shape and that consists primarily of Population II stars, though they are actually quite rich in heavy elements. There are also quite a few globular clusters peppered in around this region.
The nature of the Milky Way’s bar is under active debate. Some scientists estimate that it has a half-length of 3,000 lightyears, while others say its 16,000 lightyears. The orientation is also under hot discussion, with guesses ranging anywhere from 10–50 degrees relative to the line of sight from Earth to the Galactic Center. Certain authors advocate that the Milky Way features two distinct bars, one nestled within the other, however certain types of stars tell us that the Milky Way may actually have a very weak galactic bar. Either way, it is widely believed that the bar structure acts as a type of stellar nursery, fueling star birth at their centers. The bar is thought to act as a mechanism that channels gas inwards from the spiral arms through orbital resonance, in effect funneling the flow to create new stars.
Gamma-ray telescopes have detected that the Milky Way’s center is surrounded by a huge cloud of antimatter. In case you don’t know what antimatter is, I’ll give you a quick definition. Like the name suggests, antimatter is the opposite of matter, meaning it has properties opposite to regular matter, which the type of matter we are used to interacting with. In a single atom of antimatter, the structure is the same, but the charges are switched. So the antimatter’s “protons” actually have a negative charge and the “electrons” have a negative charge. What does this means for the Milky Way? Well, it is believed that these antimatter clouds disrupt the Milky Way’s magnetic field lines, especially when the move or rotate. This causes flares of deadly gamma rays, less deadly radio waves, and other types of waves as well. If such a flare of gamma and x-rays were to occur near to earth, well, we’d pretty much all die.
And on that cheery note, let’s talk about the galaxy’s spiral arms! Outside the gravitational influence of the Galactic bar, the structure of the interstellar medium and stars in the disk of the Milky Way is organized into four spiral arms. Spiral arms typically contain a higher density of interstellar gas and dust than the galactic center, as well as a greater concentration of star formation.
The Milky Way’s spiral structure is uncertain, since we can’t take pictures of our own galaxy while still inside of it. Hence, there is currently no consensus on the exact shape of the Milky Way’s arms. A perfect logarithmic spiral pattern would only crudely describe features near the Sun, because galaxies don’t form in mathematical ways. Instead, they commonly have arms that branch, merge, twist unexpectedly, and feature a degree of irregularity, so we can’t use any of our equations to predict them. The positioning of the Sun within a spur emphasizes that point and indicates that such features are probably not unique, and exist elsewhere in the Milky Way, perhaps on smaller scales that make it difficult for our technology to detect them.
Though the exact structure of the arms may not be known, they’re positions are! The outermost arm is named the outer arm, since sometimes astronomers get tired of coming up with names for difference space objects. The next outermost arm is the Perseus arm, which contains a surprisingly high amount of messier objects such as the famous Crab Nebula. After that is the Scutum-Centaurus arm, which is particularly rich in star formation. The arm that barely reaches out to the edges of our galaxy, which is also the 4th and final arm, is the Carina-Sagittarius arm.
It has been suggested that the inner portion of these arms rotate faster than the outer portions, which would cause the arms to be broken up slightly. Hence, the outer rings wouldn’t actually be connected to the center of the Milky Way and instead form something called a pseudoring.
The Galactic disk is surrounded by a spheroidal halo of old stars and globular clusters, 90% of which lie within 100,000 light-years of the Galactic Center. About 40% of the clusters in this halo are on retrograde orbits, which means they move in the opposite direction from the Milky Way rotation. The globular clusters can follow rosette orbits about the Milky Way, in contrast to the elliptical orbit of a planet around a star. A rosette orbit is a complicated orbit that is similar to the the shape of a flower or spiral.
Although the galactic disk contains dust that obscures the view in some wavelengths, the halo component does not. Active star formation takes place in the disk, but does not take place in the halo, as there is little cool gas to collapse into stars.
Some scientists believe the Milky Way may actually be bigger than we think it is. This comes from studying our neighboring galaxies, specifically the andromeda galaxy, all of which are much bigger than our own. Since there is no reason that would cause this size difference, scientists believe that it is likely that our Milky Way extends farther than we realize. For example, scientists have just recently discovered a ring of galactic debris, likely from a failed dwarf galaxy, that encircles the galactic disk and also interacts with it.
In addition to the stellar halo, there is evidence that our Milky Way also includes a gaseous halo with a large amount of hot gas. The halo extends for hundreds of thousands of light-years, much farther than the stellar halo and close to the distance of the Large and Small Magellanic Clouds. The mass of this hot halo is estimated to nearly be equivalent to the mass of the Milky Way itself. The temperature of this halo gas is predicted to lie anywhere between 1.8 and 4.5 million ° Fahrenheit, making it very, very hot.
As previously mentioned, the sun lies on the inner edge of the Orion arm, on something known as the Gould belt. The Gould Belt is a local, partial ring of stars in the Milky Way, about 3000 light years long. There are many similar rings peppered throughout our galaxy’s arms. Our solar system actually orbits the milky way, taking about 240 million years to complete one orbit around the center. This is known as a galactic year. The Sun is thought to have completed 18–20 entire orbits during its lifetime, but only 1/1250 of a revolution since the origin of humans. The orbital speed of the Solar System about the center of the Milky Way is approximately 490,000 miles per hour, which may sound super fast but it is only 0.073% of the speed of light. Currently, our solar system is headed in the direction of the constellation Scorpius, which is following its very own orbit around the galactic center. The stars and objects in the milky way rotate differentially, meaning that their orbital period depends on their location. While one would think that the closer you are to the center of the galaxy, the faster you would orbit, this isn’t actually true. Some of the stars closest to the Milky Way’s center have orbital speeds too low to be detected, while stars in the outer edges of the galaxy have orbital speeds greater than 22,300 light years per second.
Our galaxy is only one of many in a galaxy supercluster known as the virgo supercluster. But that’s a discussion for another day as now its time to finish off this post. I hope you enjoyed hearing more about our very own galaxy, although I’m not quite done talking about it yet. You may have noticed that in this post I didn’t even mention how our galaxy was formed, and that’s because its quite an interesting story which I believe deserves a dedicated article, so make sure to tune on in for that!
