The Milky Way Galaxy

 

The Milky Way Galaxy

(Excerpt from: How do we know? Available on Amazon.com: 

http://www.amazon.com/know-things-learned-science-ebook/dp/B00DTIEZYW)

by

Kenny A. Chaffin

All Rights Reserved © 2013 Kenny A. Chaffin

            You’ve seen pictures of our Milky Way Galaxy, right? That beautiful jeweled pinwheel spinning in space that is our home. Anyone would recognize their home wouldn’t they? But wait, where did that picture come from? The Milky Way Galaxy is 100,000 light years across and the furthest man-made camera capable of taking such a picture is only now (after traveling for 35 years aboard the Voyager spacecraft) at the edge of our tiny solar system. A solar system that is located deep inside a spiral arm of that galaxy in the picture. How could we possibly have a picture of it? The truth is we don’t. It’s all made up, an artists simulation. We’ve never actually seen it and probably never will. The vantage point shown in most simulated images would take hundreds of thousands if not millions of years to reach and an equivalent amount of time to send the picture back even at the speed of light. We humans are however quite resourceful and have used our intellect and our instruments to work out how our galaxy would look from a different vantage point.

            Via lactea is the Latin name from which The Milky Way is derived. Looking up into the night sky far from the light pollution of our cities it is a glorious sight to see, a glowing whitish milk-like swath of light across the night sky. It was unknown and unexplained for most of our history until Galileo in 1610 trained his telescope upon it and was astounded to find that it is actually composed of millions of tiny stars. We see this swath of glowing light across our sky because we are looking at the other stars in our galaxy edge-on from our vantage point in the Orion Spur about two-thirds of the way out from the center of the galaxy.

            So if we are inside how can we know what our galaxy looks like from outside? It’s a bit like trying to see the inside of your own eye. For that there is a special instrument with mirrors and lenses that will let you see the inside of your eye. To know what our galaxy looks like we use the instrument of science to assemble the pieces, the knowledge we have from observations both by eyesight, optical and radio telescopes, and our rational reasoning in such a way that we are reasonable confident we know what our galaxy looks like. But how? We started with the knowledge provided by Galileo that the milky swath across the sky is actually composed of millions of tiny stars. Knowing nothing else we could assume all stars were the same size and brightness and that the apparent brightness and size of a star would depend on its distance from us. Just as when you approach a car or streetlight the light becomes bigger and brighter the closer you get. The same applies to any other light, including stars. Of course some lights are inherently brighter or dimmer so this must be taken into account. Some stars might naturally be brighter or dimmer than others. If we could assign an average brightness to most stars we could then make a calculated guess as to their distance. But how do we know what distance corresponds to what brightness?

            Here’s where a bit of math and trigonometry magic comes in. Not to delve too far into other areas, but once we know the length and size of the Earth’s orbit around the sun we can use that distance to measure the stars. By locating a near-by (i.e. bright) star such as Andromeda and measuring its position against the far distant stellar background from opposite sides of the Earth’s orbit (e.g. measure the star’s position in June and December or any other 6-month interval) we can use trigonometry with the Earth’s orbital diameter and the angle to the star being measured (relative to the background stars) to determine its physical distance from the Earth.

            Given that, we can assume the same relative brightness for the same type of star and calculate the distance to that star based on its brightness and dimming of the light. This technique is called the ‘standard candle’ technique and is used in various guises throughout astronomy. It can be used to measure within our solar system, galaxy or between galaxies depending on what light source is being used as a standard candle.  Simple right? Of course this method is fraught with potential error and assumptions. Assumptions such as all similar stars having the same brightness, interstellar space being free of gas and dust which could affect the brightness of distant stars (this of course we knew, or guessed simply from looking at the Milky Way and its dark areas and wondering why there were no stars there).  More recently we've been able to observe the universe with infrared, x-ray and radio waves which allow us to see through interstellar dust and gas and validate optical measurements and obtain measurements we could never get through an optical telescope. 

The Milky Galaxy

            So to get back to the Milky Way and how we know what it looks like, we can measure the distances to the visible stars and plot them on a three dimensional graph. When we do we get a good approximation of a spiral type galaxy.

We know from Hubble’s discovery of distant galaxies that there are only a few types of galaxies – the spiral galaxy being one of the most prominent. So we make an assumption.

            From infrared surveys (infrared light passes through interstellar dust and gas) we get a reasonably clear picture of our galaxy from our position within it without the dust blocking other stars. 

            Additional detailed measurements and plotting of stars reveals further that our galaxy is not a true spiral galaxy, but a ‘barred’ spiral meaning that there is flat rectangle bar of stars extending from the central bulge. Even given our outstanding astronomical equipment there is still controversy over the number and size of the spiral arms and spurs such as the Orion Spur to which our sun and solar system belongs. 

This is a current artist’s conception of our galaxy based on data from NASA. The Sun is located at the lower center with the coordinate graph radiating from it.

            It is thought that there is a supermassive black hole at the center of our Milky Way galaxy just as there are in the centers of other galaxies. There is no danger in this as we are some 25-30,000 light years from the galactic center (depending on the calibration of our standard candles). We still have much to learn about galactic formation and that will be a topic for another of these essays. It could be that the black hole is the reason the galaxy forms around it or alternatively it could be that the gravity at the center of the spinning galactic disc prompts the formation of a supermassive black hole.

Using our radio telescopes, the Chandra x-ray observatory, the Hubble and James Webb space telescopes as well as others in all the various bands of the electromagnetic spectrum we will continue to refine our measurements and knowledge of what the Milky Way Galaxy looks like from outside as well as the interior details. We’ll never actually know for certain what it looks like from the perspective shown above (unless of course we suddenly invent some kind of warp drive that can take us there) but with science we will continue to refine our concepts of our galactic home and what it looks like to others from their homes in galaxies far far away.

  

References/Resources/Links

Milky Way Galaxy:

http://en.wikipedia.org/wiki/Milky_Way#Size_and_composition

Mapping the Milky Way:

http://www.pbs.org/wgbh/nova/insidenova/2012/06/mapping-the-milky-way-from-the-inside-out.html

Milky Way Galaxy:

http://commons.wikimedia.org/wiki/File:Milky_Way_Galaxy_and_a_meteor.jpg

2Mass Sky Survey:

http://www.ipac.caltech.edu/2mass/gallery/showcase/allsky_stars/index.html

Nasa Milky Way Artist Simulation:

http://commons.wikimedia.org/wiki/File:236084main_MilkyWay-full-annotated.jpg

Cosmic Distance Ladder (measurement):

http://en.wikipedia.org/wiki/Cosmic_distance_ladder

About the Author

Kenny A. Chaffin writes poetry, fiction and nonfiction and has published poems and fiction in Vision Magazine, The Bay Review, Caney River Reader, WritersHood, Star*Line, MiPo, Melange and Ad Astra and has published nonfiction in The Writer, The Electron, Writers Journal and Today’s Family. He grew up in southern Oklahoma and now lives in Denver, CO where he works hard to make enough of a living to support two cats, numerous wild birds and a bevy of squirrels. His poetry collections No Longer Dressed in Black, The Poet of Utah Park, The Joy of Science, A Fleeting Existence, a collection of science essays How do we Know, and a memoir of growing up on an Oklahoma farm - Growing Up Stories are all available at Amazon.com: http://www.amazon.com/-/e/B007S3SMY8. He may be contacted through his website at http://www.kacweb.com.