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What’s on Orion’s arm?

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Yes, that’s right, it’s our solar system. Our sun is just one of 200 billion stars all locked on a gravity merry-go-round. We all revolve in a circle around a black hole known as Sagittarius A.

The name for this massive structure is the Milky Way Galaxy. Astronomers classify galaxies by shape, and the Milky Way is a spiral galaxy. If you could look down on the Milk Way from outside, the view would include a center bulge with arms spiraling out from it. Our solar system is on the Orion arm of the Milky Way. As we move around it, we go in the direction of Cygnus the Swan, a summertime constellation. 

The speed of that journey is 155 miles per second. That is a fast speed; however, traveling at this rate it would take 220 million years to make just one lap around it. As the sun travels with its 200 billion champions, the motion is not just on a level plane around but more like a merry-go-round horse bobbing up and down as they travel. 

Think about all the changes that have happened on Earth in the time it takes the solar system to travel once around the Milky Way. Sizing up our galaxy, it is about 120,000 light years across. Multiply this by six trillion to convert light years to miles. The spiral arms, which astronomers refer to as the stellar disk, are about 1,000 light years thick and the bulge in the middle is about 26,000 light years thick. The bugle’s shape is more like a football than a basketball; therefore, the Milky Way is a special spiral called a bar spiral, or simply SB for short. 

The Milky Way loses stars because of their aging process. It gains stars through star birth, and stealing stars from dwarf galaxies that lose stars due to the massive size of the Milky Way, which gives it a strong gravitation pull. If you could look down on this gigantic mass of stars to see were our sun is located in the stellar disk, you would see the sun is on the Orion arm of the galaxy, about 26,000 light years away from the center. The spiral arms receive their names from the constellation, which appears to be within the arm from our vantage point here on Earth. However, the stars we use to make the constellations are only in a 500-light years diameter of space around us, a very small portion of the Milky Way. 

Ever wonder why the winter sky has so many bright stars in it? Many are close by and some are large. The fact that Orion is the dominant winter constellation and we live on the Orion arm adds up to a beautiful winter sky. Where is the Milky Way in the sky? Every one of the 1,800 stars visible in the sky as you look up is part of the galaxy.

 If you know where to look, you can identify the parts, such as the main plane. At this time of winter during early evening, roughly 8 p.m., the main plane is overhead. Take a walk outside and look straight up; if your neighborhood is dark enough, you will see a faint glow of billions of stars looking like a cloud. Light pollution, city lights and other ground lights rob us of the view. However, a pair of binoculars helps to gain it back. Looking straight up you will see pin dots of starlight and as you turn, gazing in a more east or west direction, there will be fewer stars. 

Come to the Ingram Planetarium and watch IBEX: Interboundary Explorer to learn how NASA is mapping out the edges of the solar system, our place in the heavens.