We continue our journey through the Milky Way by taking a look at its shape and structure. The distinctive shape of a galaxy, the way the stuff inside of it is arranged, tells us a lot about its age and the age of its inhabitants' stars. Of course you may be familiar with the map of the earth and the outlines of its land, continents and vast world oceans, but you can also imagine the image of our galaxy with its glowing, galactic core and spiral arms as a map of the Milky Way. You have locations and places where stars are nearing the end of their lifespan and unexplored frontier on the outskirts of the galaxy and also regions of space alive with new star formation. Somewhere lost in that collection of billions and billions of stars is one star that holds in its gravitational grip, a planet called Earth that harbors life. Some of it intelligent, others not so much. Right. The shape of a galaxy also sheds light on its ongoing story of cosmic evolution. Most of the stars in our galaxy, the Milky Way, are arranged in a flat disk about 100,000 light years across. So if you were to travel at the speed of light, it would take you 100,000 years to travel from one end of the galaxy to the opposite end. The sun is located about two-thirds of the way from the center of the Milky Way, occupying a calm cosmic suburb of sorts. A place not too violent with new star formation, but also not too far off from downtown, the center of the galaxy. The collection of stars is thicker and brighter towards the center of the galaxy and thinner towards the rim, such that if we could see the galaxy edge on from the outside, it would look like two huge fried eggs stuck back to back. Structurally our galaxy, the Milky Way, is made up of three major parts. The central bulge or galactic core is much thicker than the main disk. It is for the most part due to its intense gravity what holds the galaxy together. And just like how everything in our solar system revolves around the sun, the stars of our galaxy revolve around the galactic core. Far from being static and immobile, all of the stars of our galaxy, including our sun, are actually involved in a giant circle dance of cosmic harmony. The radiation-rich center is home to a mysterious, sleeping giant, a supermassive black hole that for reasons yet to be understood by our science, was far more active a long time ago than it is today. Moving on to the main galactic disk as I mentioned earlier is where the majority of the stars of our galaxy are located, including our solar system. The disk is home to the spiral arms of our galaxy, which are a major source of new star formation. It is believed that the sun began its life somewhere within one of the spiral arms, and as it orbited the center of the galaxy, made its way outwards. We are currently at an age where our sun exists between spiral arms, the major arm perseus and the minor arm Orion. So keep that in mind next time any of your relatives from the Andromeda galaxy would like to post something to you through the mail. Anybody? The final component to all of this is an all-encompassing galactic halo. No, no, not that kind of halo. Galactic halo is a massive permeating spherical structure that is home to groups of really old stars that orbit the center of the galaxy like satellites, called globular clusters. They appear as glittering jewels of stars clumped together when viewed from a telescope. Really beautiful stuff. Go look it up, globular clusters. The halo also consists of something called dark matter that dominates the mass of the galaxy. This elusive substance is invisible to light, meaning we can't see it, hence the name dark, but its gravitational effect can be felt by the entire galaxy. This is one of the foremost mysteries of astronomy and the universe, and something that we'll be tackling together as this series progresses. But what we are left with, ladies and gentlemen, boys and girls, is an incomplete picture. There is still much to be learned about our island of stars in the cosmos, ancient and vast, from which we spring.