Image credit & copyright: European Southern Observatory (ESO)/Surge Brunier (links below).
With this post I’d like to address a question that’s been posed to me countless times by friends, family and those here on various social media pages. That question is, “If we reside within the plane of the Milky Way galaxy, how then can we know what the Milky Way actually looks like?” Maybe you’ve never pondered the thought until right now but by no stretch of the imagination is that a silly question and it’s perhaps the fault of television and media for misspeaking or giving an incomplete picture when they address our home star city. So that’s what I want to do with this post and after I hope you have some closure on this question.
Our Current Milky Way Model: Today it’s universally accepted that the Milky Way is an average barred spiral galaxy that stretches approximately 100,000 light years (30 kiloparsecs) in diameter and contains roughly 100-300 billion stars. Yes, that’s an incredible variance but that’s where we stand. The plane of the galaxy is about 1,000 light years (0.3 kiloparsecs) thick and at the nucleus is a central bulge containing a quiet, non-feeding (at this time) supermassive black hole known as Sagittarius A (Sgr A*) with a mass of 4.5 million suns.
As with other barred spiral galaxies, the Milky Way has arms; the current number is believed to be four. They are the two major Scutum-Centaurus and Perseus arms as well as the lesser Sagittarius and Outer arms. There are also “spurs” like the Orion spur, which is where the Sun resides, that reach across from one arm to another. The Sun itself is approximately 28,000 light years (8.7 kiloparsecs) from the center of the galaxy and it takes us roughly 250 million years to complete one orbit around the galaxy.
How Do We Know This?: Great question but it mostly comes down to observation, observation, observation. It’s also important to bear in mind that what we know isn’t concrete, it’s what we know so far and many current data points will change as newer and more accurate measurements are conducted. Information like the size, mass and structure (how many arms, etc.) of the galaxy isn’t precise yet as you can see how varied the estimates can be.
The most crucial tool in any astronomer/cosmologists tool kit is observation. I mean think about it; aside from meteors, the geology of our own planet and a few data points from other planets, literally everything we know about the universe has been gleaned through the observation of light (the electromagnetic spectrum) and the interpretation of that light. Every night all around the world we look out into the universe, back into time and see billions upon billions of galaxies; spirals, elliptical, irregular, dwarfs, lenticular etc. and that begs the question, how do we know what type we reside in?
The first step here is to critique the countless images of the plane of our galaxy and by process of elimination; we can pretty quickly remove most of the different types of galaxies out there with rudimentary eyeball observation alone. We’re certainly not in the process of merging with any massive partners. We’re not inside of an elliptical, irregular or dwarf galaxy so already we’ve eliminated most contenders.
Next we can narrow our focus to the billions of spiral galaxies out there. Not only are they extremely common, we see them at every conceivable angle and orientation. With that information in hand, we have a very good idea of what a spiral or barred spiral galaxy looks like on edge and at various stages of evolution. Almost all major galaxies have a supermassive black hole in their nucleus and once again through observational data we’ve (the royal “we” of course, I don’t do anything) determined that the Milky Way does have a supermassive black hole roughly 4.5 times the mass of our Sun. Much of this black hole’s (Sgr A*) data has been obtained by observing 6 stars cataloged as S1, S2, S8, S12, S13 and S14 orbit….nothing at extremely high speeds, especially S2.
I want to emphasize what I stated above about the observation and interpretation of light because although I made it sound simple, it’s very complex. Our eyes only pick up about 0.0035% of the electromagnetic spectrum, this is known as the visible portion of the ES and it ranges from about 390 nm to 70 nm. The ES’s visible spectrum resides roughly in the middle of its known energies. Let’s start at the long end of the ES and travel its length. It goes from radio waves, microwaves, infrared (that can be broken into far and near infrared) and then we find the visible spectrum. Remember your ROY G BIV; red, orange, yellow, green, blue, indigo and violet? So if red is the weakest (longest wavelength) of the visible colors, what’s inferior to red? That’s right, infrared. But we’re going the other way to let’s say we travel the colors down to violet, now what? As we leave the visible light spectrum once again to higher energies (shorter wavelengths), what’s the first wave stronger than violet? Yea, you got it, ultraviolet then X-rays and finally gamma rays. Each one of these requires their own instruments, preferably in space to observe and record everything we see in the universe.
As we can only see about 0.000003% of the galaxy with our eyes and even with the entire breadth of the electromagnetic spectrum at our disposal we still only see about 10% of the mass of our galaxy. The rest at this time remains invisible in the form of dark matter. In the future, new technology will hopefully open this field up to us just as gravitational waves are beginning to reveal themselves to us.
Here’s My Take on This: As I sit here typing (hunting & pecking) at the desk in my living room, let’s assume that I can’t go outside and look at my house. Hell, let’s assume that I can’t even leave my seat but I can still look around from my vantage point and having seen hundreds of thousands of houses in my life I can get a pretty good idea of what my house looks like from outside. I can see my general location very well, living room walls, stairs etc. I can see out the main picture window and smaller side window which tells me that I appear to be on the second floor. I can also see icicles hanging down in front of the window and if it’s a fair assumption that they’re hanging from the roof, I’m probably in a two story or split level house.
Looking out the window also allows me to get a feel for my general location and orientation. I’m certainly not in a city nor am I in the country. I know its 5:10 pm and I can see the Sun which sets in the West getting lower out my window so I now have my home’s orientation also. I can look down the hallway and see 5 doors and having seen many houses in the past I can assume at least 3 bedrooms, one maybe a closet and a bathroom. Perhaps there are 4 bedrooms and a bathroom and the rooms all have individual closets, either way I’m pretty close. I can also see where the hallway ends so I know that from the living room to the end of the hallway is somewhat in the neighborhood of how long the house is. I can also slightly see into another room which appears to be the kitchen. I can see a good size wooden table and quite a bit of natural light. Again having seen many houses in the past I can estimate what’s in the kitchen, its approximate size and how far the house goes in that direction.
Now if I had the ability to zoom in very close to anything within my vision I could inspect photos on the walls, calendar etc. and possibly even determine how many people live here, their nationality, what they like to do and the language they speak. I can look back out the window and roughly determine the climate; I inspect the walls, floors etc. and even determine what the house is constructed out of. I could go on and on and honestly it’s a fantastic thought experiment should you put the effort into it. Simply by my observations in visible light only and having seen many houses in the past I can reasonably determine what the house that I’m in looks like though I can’t go outside or even around the inside of the house to inspect further.
This is generally how we know what the Milky May looks like and is structured. So how do we know what the Milky Way looks like if we can’t see it? Well, the answer is that we CAN see it, just from the inside. We can’t move around it and or go outside and view it as a whole but from what we can see and what we have observed in millions of examples, it’s pretty clear.
Milky Way Panorama: https://www.eso.org/public/usa/images/eso0932a/
NASA Where are we?: https://www.nasa.gov/mission_pages/sunearth/news/gallery/galaxy-location.html
NASA Charting the Milky Way: https://www.nasa.gov/jpl/charting-the-milky-way-from-the-inside-out