My Trip Through the Milky Way

I've been on a science spree lately.

A little over a year ago the National Geographic channel was showing a multi-episode series dramatizing the life and achievements of Alfred Einstein.  Using clever stories and images, the show did an impressive job bringing you a few steps closer to understanding hard-to-get-your-head around things, like Einstein's Time Dilation.  So I could really grasp it, though I embarked on an exploration of a few dozen YouTube videos on Time Dilation.  I finally understood it and why it happens.  Coolest of all is the fascinating proof of Time Dilation occurring on a human scale (i.e. not interstellar).

But that's for another blog post.  Since my Einstein fetish, I've moved onto curiosity about the Milky Way Galaxy.  It started with the question that most people ask about the Milky Way: "How can I see the Milky Way in the sky if I am in the Milky Way?" led me to learning the answers to a multitude of other questions.  

So here's how I'd explain the Milky Way to a friend.  Hopefully my explanation is not too marred by my very thin amount of scientific knowledge, and hopefully I didn't rely on bad sources, but if anyone spots anything, please feel free to tell me about it.

 How can we see a galaxy that we are in?

Artist's conception of the Milky Way (source)

Okay, you can't.  There are no photos of our galaxy.  The pics you see that show a spiral galaxy, and are labelled "Milky Way", are digital artistry, projections based on what other galaxies look like, and based on the clever measurements and inferences scientists can make.  Or, if it really is a photo, then it is a real pic of another galaxy.  "NGC 6744" is a spiral galaxy popularly used to show us what our own galaxy looks like.  Or you may be seeing a real photo, not of the Milky Way's entire spiral galaxy, but parts of the Milky Way stitched together from views in the sky.  See examples of these below.

Photo of Galaxy NGC 6744, having a spiral structure similar to the Milky Way's.  (source)

Composite of many sky images of the Milky Way taken at different times and locations, by Axel Mellinger. (source)

The Spiral Arms

The way to talk about where things are located in the Milky Way is by referencing the spiral arms of the Milky Way.  The schematic below wonderfully depicts what we need to know:

(source)

There's the Norma and Cygnus arms, Sagittarius, Scutum-Crux, and Perseus.  We (our solar system) reside in this dip-shit little afterthought of an arm called the "Orion Spur"!  It's closer to the edge of the Milky Way than the center, about 2/3 of the way out from the supermassive black hole "Saggitarius-A" at the galactic center.

"Far" is actually close

You've see those fantastic Hubble images, often of spectacular dust clouds (such as the so-called "Pillars of Creation")?  You probably know that they are impossibly far away and impossibly huge.  But where in the universe are they, in our galaxy, or another?  To put it another way, how far away are such things, on the Milky Way scale?  I'm sorry to tell you that they are barely off of our front porch in our Orion Spur!  Yes, these things whose distance from us that are beyond comprehension, whose light we see comprise photons that were emitted thousands of years ago...are right here in our "neighborhood".  The Pillars of Creation are a "mere" 7,000 light years away from us, putting them not only in our Milky Way, but still comfortably in our little neighborhood of the Orion spur (10,000 light years in length).  

Those wonderful Exoplanets we're learning about, the ones possibly hosting life, but whose citizens we have no hope of ever meeting, or vice-versa?  You guessed it:  in the Orion Spur.

For comparison, the center of our galaxy, super massive black hole Saggitarius-A, is 25,640 light years away.  The total width of the Milky Way is around 100,000 light years.  So, the rest of the universe?  Forget about it.  The Milky Way alone is just one of trillions of galaxies.  Feeling small, punk?

Answering the Big Question

Finally there was my big question, the one that took several videos and articles for me to understand:  when you see one of those photos taken in deep country, away from city lights, and there's a big, beautiful arc of colorful galactic soup that is the Milky Way...how can something that we are IN be something we can point to?

The answer was very cool to me.  It starts with describing the shape of the galaxy; it's a disk (not a sphere).  Better yet, to borrow Dave Fuller's illustration, it's more like a deep dish pizza:  a disk, but one with significant height.  Consider the pizza for a minute: looking at it from the top, our solar system would be a tiny fleck of pepper located 2/3 from the center.  But the fleck is not on the surface, we are inside the pizza.  Let's say the pizza is 1" high, and we are right in the middle, 1/2" down.

Look at the Sun in the Spiral Arms diagram above.  Which side of it are we on?  Well, that depends on the calendar.  When we are facing out, we are looking out towards part of the Perseus arm only, so we see what is around us in our Orion spur, and what is in the Perseus arm.  That's all.  That is when there is very little to see in the sky, not the source of many spectacular photos.  During the times we are facing in, we are seeing the superimposition of our Orion spur, Saggitarius arm, the Scutum Centaurus arm, the Norma arm, and the galactic center...and then back through the arms in reverse order again.  That's a lot of layers of lasagna, and what produces the really pretty photos.

Okay, back to our location inside the pizza.  Let's look around.  Turning around 360 degrees, looking straight ahead, we see cheese everywhere.  And there's more or less of that cheese, depending on whether we're facing inward or outward.  But we can look in other directions too:  upwards or downwards, past either the top or crust of the pizza, to whatever is above or below the Milky Way.

Now the mindblower:  when you look at the vertical stripe of the MW in the sky, that is you looking straight ahead through the pizza, looking at cheese, cheese, everywhere.  To the one side of the stripe is your "out and above" view from the pizza, and the other side of the strip is your "out and below" view from the pizza!

That's it, that's the awesome discovery.

Where are the Constellations?

I've got another disappointment for you.  Almost all those constellations we know and love, and the stars that comprise them, that are so impossibly far away, are once again...in our shitty little Orion Spur!  With a few exceptions, we don't see any points of light (stars, other galaxies, nebulae, etc.) that aren't in the Orion Spur.

"But wait," you ask:  You told us that the stuff to the left and right of the visible Milky Way band is us "looking out" beyond our galaxy...yet our favorite constellations are spread all over the night sky.  So don't those constellations have to exist outside the Milky Way, inside some other galaxy?  No.

I made that mistake too at first.  What I forgot is that our Orion Spur surrounds us in every direction.  When you're "looking out" you are still looking through the surroundings of the Orion Spur first, before you see the things that are "out there".

Will Voyager Take a Picture of the Milky Way?

The farthest human-made objects from earth are the two Voyager spacecraft launched between 1979 and 1980, and they keep going and going at 35,000 mph away from our solar system.  Will they leave the Milky Way, and in theory, be able to photograph the Milky Way?

I wish I could have answered, "yeah, got a sec?"  Because in 40,000 years, we'll at least get the first fly-by of a star other than our Sun: the star Gliese 445, about 4 light years away from earth.  And that is, you guessed it, still in the Orion Spur.  In fact, at it's narrowest, the Orion Spur is 3,500 light years, so Voyager will remain in the 'hood for at least 35 million more years.   After that, then?

As an object, bombarded by cosmic rays and high energy charged particles, Voyager could last another 100 million years before dissipating into dust, so we've got that going for us.  We do have a problem with power supply though; eventually, in fact, as soon as 2025, its onboard Plutonium-238 powerplant will cease to provide the power it would need for photos and communications.  But in theory, could Voyager someday be positioned to take that picture?

Unfortunately no, it won't leave the Milky Way, ever.  Voyager was supplied with the necessary rocket fuel to leave the sun's orbit; no extra fuel to achieve an escape velocity from the Milky Way, which itself is a gravitational/orbital system.  It will stay in Milky Way orbit forever, and like our sun, complete its circle around Saggitarius-A once every 230 million years.