Make the Milky Way Galaxy the size of the U.S.A.

[JimD]

--------- #1233 - What Would it Look Like if the Milky Way was the size of the U.S.A.

- Scaling the Milky Way Galaxy on to the map of the U.S. helps us visualize the vastness of what we can see in astronomy. It adds to our imagination when we gaze into the night sky.

- Attachments - Spiral Galaxy


- It is hard to visualize our Milky Way Galaxy because we are inside it. Of course, we can look at other spiral galaxies and think the Milky Way must look similar. And, then there are the lightyears of distance involved that are also hard to comprehend. Our Solar System is 28,000 lightyears away from the center of our galaxy. On a good seeing night we can see the disk of the Milky Way as a band of stars stretching from the Constellation Sagittarius , the “Teapot”, in the southern sky to the Constellation Cassiopeia in the northern sky. The spout of the “ Teapot” is the center of our galaxy. That is where the central Blackhole resides. The band of stars is the disk we can see looking edgewise. The disk totally encircles us so we only see part of it each night throughout the year.

- We are lucky that the central Blackhole is 28,000 lightyears away because the central bulge is very crowded and dynamic with stars in furious motion. Our Solar System is about half way out from the center to the edge of the galaxy. The galaxy radius is 60,000 lightyears, the diameter 120,000 lightyears. That is the starlight observable part. The galaxy is 4 to 5 times larger if we count the invisible hydrogen gas and the Dark Matter that we can not see. Then there are dozens of Dwarf Galaxies orbiting our galaxy like moons orbiting large planets.

- To better visualize our galaxy we could superimpose the spiral arms of the stars on the map of the United States. The center of the galaxy would be in Topeka , Kansas. The arms would spiral out to each coastline and halfway into Canada and Mexico. If we use these dimensions then the 3,000 miles coast to coast corresponds to 120,000 lightyears distance. On light year would be 132 feet on this scale. One mile would be 40 lightyears. The Blackhole would be in Topeka, Kansas and our Solar System would be in Auburn, Indiana. That is my home town and the center of my universe my first couple decades. Auburn is in the northeast corner of Indiana very close to the Ohio and Michigan borders, some 700 miles from Topeka. However, using this same scale our entire Solar System would be only 2 inches across centered in Auburn.
- From my hometown all the stars we could see with the naked eye ( up to Magnitude 6.0) and out 1,000 lightyears would be within 25 miles of town. It would just barely get to the Michigan border. (See the footnotes for the math in these calculations. The scale is 40 lightyears per mile).

- The Solar System is 2 inches across and the Sun is so small you cannot see it. It would be 1/20^th the thickness of a piece of paper. One of the biggest stars would be the size of this period “.” Arcturus is a big star 37 lightyears away. To find it follow the curved handle of the Big Dipper. Continue the arc to reach the next brightest star half way across the sky. That is Arcturus. (Changing scales for a moment if the Sun were a marble Arcturus would be a beach ball 2,500 miles from the Sun.). However, on our scale Arcturus would be the size of a period less than a mile away and at the edge of the city limits in the small town of Auburn.

- The Ring Nebula, M57, is 2,300 lightyears away. The nebula’s ring is 119 feet across and 57 miles away in the direction south-southwest of Auburn.

- The Hercules Cluster, M13, is 25,100 lightyears away and 145 lightyears across. Its cluster of hundreds of thousands of stars would fill a sphere 3.6 miles wide. It would lie 628 miles south-southwest of Auburn.

- The edges of the Milky Way reaches the east and west coasts of the U.S. From there you have to travel ¼th the distance to the Moon to reach the Andromeda Galaxy, M31. It would be 62,500 miles away.

- The Virgo Cluster of galaxies, M87, would be 1,500,000 miles away.

- To get to the edges of the Observable Universe you would need to continue out 342,000,000 miles from Topeka in all directions. Remember, our Solar System is only 2 inches nestled in this vast distance. And, also remember that light took 13,700,000,000 years to reach us. Actually the Universe Horizon has moved away from us another 833,000,000 miles during that 13.7 billion years. That galaxy that we could see at the edge of the Universe is now another 1,175,000,000 miles away. Therefore, comparing our 2 inch Solar system on the U.S. map is a small speck looking across a 2,350,000,000 miles of Universe, plus beyond that we can never see because that light will not have enough time to reach us. ( The edges of the Universe’s “horizon” are 94,000,000,000 lightyears across, and still expanding.)

- I hope this journey across the galaxy map helps you visualize what you are seeing in the Milky Way disk stretching across the night sky. Most of astronomy is imagination. Best wishes and Clear Skies.

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(1) 120,000 lightyears is scaled down to 3,000 miles, or 40 lightyears = 1 mile.
(2) The distance from us to the center of the galaxy is 28,000 lightyears or 700 miles.
(3) Stars seen with the naked eye out 1,000 lightyears are 1,000 / 40 = 25 miles away.
(4) The Observable Universe is 1.3 *10^26 meters, there are 9.46 *10^15 meters per lightyear so that is 13.7*10^9 lightyears divided by 40 lightyears per mile = 342 *10^6 miles.
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[Dixon]

...remember that light took 13,700,000,000 years to reach us...The edges of the Universe’s “horizon” are 94,000,000,000 lightyears across, and still expanding.

A fun post! One question though: If the universe is about 13.7 billion years old, how can the diameter of it be 94 billion years? Shouldn't the diameter of the universe be 27 billion or so years? I say that because the energy and matter from the Big Bang has only had 13.7 billion years to expand outward, so the radius of the expansion should be 13.7 light years, thus the diameter would be 27 billion or so light years. It seems to me that, to be 94 billion light years in diameter, the universe would have to be 47 billion years old (for a radius of 47 billion light years, thus a diameter of 2 x 47 billion = 94 billion light years). Where did my reasoning go wrong?

[geomancer]

The fabric of space-time itself has expanded the ~28 billion light year diameter sphere visible to us to an actual diameter of 94 billion light years. All space is expanding, not just the visible limit. The visible universe is embedded in a larger volume of space-time that remains unseeable because there has not been time for its light to reach us.

[Dixon]

The fabric of space-time itself has expanded the ~28 billion light year diameter sphere visible to us to an actual diameter of 94 billion light years. All space is expanding, not just the visible limit. The visible universe is embedded in a larger volume of space-time that remains unseeable because there has not been time for its light to reach us.

Oh yeah--duh! Next dumb questions:

1. How do they get the 94 billion figure, and distinguish the portion of the diameter that's attributable to spacetime stretching from that portion that's attributable to simple explosive outward traveling of the light sources (rather than just assuming the universe is 47 billion light-years in diameter)?

2. If even the 94 billion light-year figure is only what we can see, does anybody have any notion of how much more there could be? Are they sure it doesn't go on infinitely? How? And are the figures I've heard for estimated size or mass or number of atoms in the universe bullshit, or were they just referring to the observable universe?