The Night Sky August 2014 – Our Home in the Universe The Milky Way
Hi everyone, I’m Darrell Heath with the UALR College of Arts, Letters, and Sciences, welcome to The Night Sky.
For me, one of the most satisfying aspects of stargazing is gaining a deeper understanding of how I fit into a much larger picture. The cosmic perspective I’ve personally gained from this pursuit has been both humbling and awe inspiring.
In this episode I hope to share some of that cosmic perspective by leading you on a voyage across our Milky Way Galaxy. We are going to use a mental scale model developed by the Astronomical Society of the Pacific to help us gain a better understanding of the size of the Galaxy we call home and where we all fit into it.
Looking up at the night sky we are immediately struck by the sheer number of stars that we see. From a dark sky locale it is possible to see several thousand of them shimmering like diamonds in the dark.
Our Sun is just one of several hundred billion stars within the Milky Way Galaxy. The Galaxy itself is a flattened disc with a pair of sweeping spiral arms that give it the look of a celestial pinwheel. From edge to edge our galaxy is about 100,000 light years wide and averages about 1,000 light years thick.
Here is where we begin to encounter a major stumbling block in our ability to comprehend the true scale of our galaxy: numbers. How many is a hundred billion? How big across is 100,000 light years? The mind reels at comprehending such large figures and, in truth, our brains simply cannot accommodate these vast scales.
Remember, a light year is not a unit of time. It’s a unit of distance where one light year equals the distance that light will travel in a year’s time within the vacuum of space. Light travels at the finite speed of 186,000 miles per second,in one year’s time that comes out to around 6 trillion miles.
Obviously we need a better way to visualize these huge distances. Let’s shrink down our own solar system to something we are easily familiar with.
On average, the Sun is about 93 million miles away and it takes 8 minutes for light to travel the distance between the Sun and the Earth. We can therefore say that the Sun is 8 light minutes away. Think about it: if the Sun were to magically disappear we wouldn’t know about it for 8 minutes! It takes the Sun’s light 5.5 hours to travel all the way from the center of the solar system out to Pluto. So, from side to side, our Solar System is a total of 11 light hours across.
Imagine now that a quarter coin represents our Solar System in our mental scale model. On the scale we are going to use here the quarter will contain the Sun and all of the planets out to the orbit of Uranus; Pluto would be just off its edge. The Sun and planets are going to be so small we would need a microscope to see them!
With our solar system shrunk down to the size of a quarter our Milky Way Galaxy would now be some 2,500 miles across. It just so happens that the distance from Los Angeles to New York is 2,500 miles.
Remember, in reality our galaxy is some 1,000 light years thick but with a coin-sized solar system inside a North America-sized galaxy it now shrinks down to 25 miles thick. Astronomers think that our solar system is located about half way out from the center of the galaxy and on our North American-sized model that would place our Sun and its family of planets somewhere within the Rocky Mountains. If we were inside a spaceship orbiting above the Milky Way could we look outside a window and see home? Commercial airplanes typically cruise at about 7 miles above the Earth; imagine trying to see our quarter-sized solar system from 7 miles up. It would be impossible. Not only would we be an insignificant speck, we wouldn’t even be noticed. Now we are beginning to comprehend how unbelievably vast the size of our Milky Way Galaxy is.
Let’s consider all of those stars inside our galaxy. We don’t really know how many stars actually exist in our Milky Way Galaxy but estimates range from 200 to 400 billion. Glancing up at the thousands we can see from a dark sky locale they appear to be very close together. But are they?
Let’s assume there are 200 billion stars in our galaxy. What could represent them all?
Well, let’s imagine a football field with a wall 4 feet high enclosing it. With the scale model we are using something the size of birdseed would represent the size of the stars. Actually, we are being over generous here as the birdseed is far too big to represent the size of the average star that exists within the Milky Way. 200 billion birdseeds would just about come to the top of our 4-foot wall around the football field. Now we have to distribute those seeds across our North American-sized galaxy. A third of them would be concentrated within the center of North America, say around Kansas and Iowa. The remaining birdseed has to be spread out across the rest of North America and some 25 miles deep. We now realize that all those stars we see upon the night sky are not so crowded together after all.
From our perspective inside the galaxy when we look straight up or down we see few stars but when we look edge on towards the center of the Galaxy we see untold numbers fading away into a distant haze.
The summer is the best time of the year to put some of this knowledge to use in our observing. From June to August go to a dark sky locale with little light pollution and step outside after midnight on a moonless night. Arching across the sky from the northeast to the southwest you will see a faint band of shimmering light. This mysterious-looking pale band of light is the Milky Way. Remember, we are embedded within the disc of our home galaxy and that band of light is the disc seen edge on towards the center. The pale band of light is actually the combined glow from untold numbers of suns that our eyes and telescopes cannot resolve. If the night is especially dark, and free of too much haze and humidity, you can see dark, inky rifts that blot out the stars. These rifts are huge clouds of gas and dust that make up the spiral arm we reside in and from within them new stars will one day emerge.
During mid-August look to the southwest after midnight to find the star pattern known as “The Teapot” just above the horizon. Look to the right of the teapot’s spout and you are looking in the direction of galactic center. The dark rifts obscure our view of the center but radio telescopes can penetrate the clouds of gas and dust and they reveal to us a powerful radio energy source within a small region of galactic center. Recent studies have shown that huge stars are being hurled around this relatively small region at tremendous speeds. Astronomers believe that here is where a supermassive black hole lurks with a mass of some 4 million times that of our Sun.
Every star you see in the sky is a resident of the Milky Way Galaxy but our galaxy is not the only one out there. Over the course of 10 consecutive days around Christmas of 1995 the Hubble Space Telescope took 342 separate exposures of a seemingly blank patch of sky within the constellation of Ursa Major. This allowed the cameras of the space telescope to observe faint, deep objects in space. The result is what has arguably become one of the most important photographs ever taken: The Hubble Deep Field. Just about everything you see in this image is a galaxy, some of them so far away that we are seeing them as they were during the earliest times of the universe’s history. The image shows some 3,000 galaxies. All of them within a space upon the sky no bigger than a grain of sand held out at arms length.
We don’t know how many other galaxies there are in the universe but a conservative estimate would be anywhere from 100 to 200 billion.
If all of this makes you feel infinitesimally small and insignificant maybe you can take some comfort in knowing that you belong to the only known species capable of contemplating its place within the universe and looking up in both awe and wonder.