LET’S HAVE A LOOK AROUND OUR HOUSE!
Photo Credit: Caltech 2Mass All-Sky Survey of approximately a half-billion stars. CLICK photo for larger view and see below for information and links.
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 our various social media pages. That question is “If we reside within the plane of the Milky Way galaxy, how then, can we know what it looks like?”
That’s not by any stretch of the imagination a silly question and it’s perhaps the fault of television and media misspeaking or giving incomplete information when they briefly touch the concept then move on, never to elaborate further, leaving you with the question long after. So that’s what I want to do with this post and after I hope you have some closure on this question. Also, as many of you know, I’m not an astronomer, astronaut or an academic of any sort so I will be giving you the information that I know personally. Another person may explain it differently with more details etc……Let’s begin.
CURRENT MILKY WAY MODEL: It’s accepted today that the Milky Way is an average barred spiral galaxy that stretches approximately 100,000 light years in diameter and contains roughly (very roughly) 100-200 billion stars. The plane of the galaxy is about 1000 light years thick and the nucleus has a central bulge and at its core is a quiet, non-feeding (at this time) supermassive black hole.
There are an estimated four arms to the galaxy. They are the Scutum-Centaurus, Sagittarius, Perseus and Norma arms. There are also “spurs” like the Orion spur, which is where the Sun resides and the Sun itself is approximately two-thirds the distance from the center of the galaxy.
That’s roughly where we stand with the model of the galaxy we live in.
HOW DO WE KNOW THAT?: Good question and there are many pieces to the answer and even the answer itself isn’t concrete. For example even the knowledge of four main arms just came to light in late 2013! Decades ago it was believed to be the case but later observation determined only two; now, we are back at four. Also, you can look around on different pages for NASA, ESA, observatories, space telescopes etc. etc. and the differences in star estimates are incredibly varied. From anywhere to 100-400 billion stars which to me just means “We don’t know.” The recognized estimate however is between, 100-200 billion.
The main tool in any astronomer/cosmologists tool kit is observation. Aside from meteors and geology, literally everything we know about the universe is gleaned through the observation of light and the interpretation of that light. We look out into the universe and see billions upon billions of galaxies; spirals, elliptical, irregular, dwarfs, lenticular etc. So then, how do we know what type we are in? First, we can look at the countless photos and observations of the plane of our galaxy and by process of elimination remove most of the different types of galaxies out there by simple eyeball observation alone. We’re certainly not merging, elliptical, irregular etc. so that narrows our options considerably.
Next, as we look to the millions or even billions of spiral galaxies out there we see them at literally every possible angle and orientation. So we have a pretty darn good idea of what a spiral or barred spiral galaxy looks like on edge and for that matter, at various stages of evolution. Another thing we see in almost every galaxy is a supermassive black hole at its nucleus. Therefore, it’s been estimated and recently confirmed that indeed, the Milky Way also has a supermassive black hole at its nucleus.
Add in the different telescopes observing in the different wavelengths of the electromagnetic spectrum and the picture of our home becomes very clear. So no, we can’t leave the galaxy to get a photo of it but we can certainly still see the galaxy from the inside. We have amassed an amazing amount of tools to observe with and those observations we can compare with other similar objects. Not just hundreds, but millions of similar objects.
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. Heck, let’s assume 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, picture window so I can see approximately that I appear to be on the second floor. I can also icicles so it’s a fair assumption that they’re hanging from the roof thus, the house is a two story house. Looking out the window I can also roughly get a feel for my location. I’m definately not in a city and not in the country so that also feeds me information. I can look down the hallway and see 5 doors, so having seen many houses in the past I can assume at least 3 bedrooms, 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. Having seen many houses in the past I can estimate what’s in the kitchen as well and its approximate size.
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 out the window and roughly determine the climate then inspect the walls, floors etc. and even determine what the house is constructed out of. I could go on and on and honestly this is pretty cool now that I’m sitting here thinking about it all. 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 precisely 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.
2Mass All-Sky Survey: http://www.ipac.caltech.edu/2mass/
2Mass page for this image: http://www.ipac.caltech.edu/2mass/gallery/showcase/allsky_stars/index.html