Take a look up into the east some moonless night this coming month. Maybe you will notice a swath of dim light stretching from north to south across the night sky. It will help if you get out of town away from distracting lights. Just after it gets completely dark, which is kind of late this time of year, it will be high up in the sky. Later, look for it coursing right overhead. What is it?
Humanity has asked this question for thousands of years. The ancient Greeks thought it was the breast milk of a goddess. Aristotle thought it was the smoke of stars burning up in the upper atmosphere of the earth. He was totally off on the scale of it all. A hundred years ago astronomers thought it was the entire universe.
Now we know that it is the Milky Way Galaxy, our home in the vast cosmos. Our sun was born into this home about 5 billion years ago. It is a home that is shaped like a giant pinwheel of stars with a central bulge and spiral arms radiating outwards. Our sun is just one of 300,000,000,000 stars, more or less, cruising along through it at the hold-onto-your-hat speed of 500,000 miles per hour. The earth and the other planets are getting pulled along with it. Even at that staggering rate of speed, it still takes us 250,000,000 years to take one spin around the vast Milky Way.
Exactly how big is it? Big numbers make my head spin so let’s shrink it down in size a bit. Here is a project if you find yourself bored one hot summer afternoon. Make a scale model of our home galaxy using grains of salt to represent each of the stars. Now this is a huge reduction in scale. Stars are big. Our sun is 864,000 miles in diameter, and it is a small star. Grains of salt are tiny.
Not enough room
Head down to the grocery store with your shopping list. Salt. How much would you need? Remember those round cardboard cylinders of Morton Salt with the picture of a girl holding an umbrella? You will need about 10,000 of those. Maybe you know someone with a pickup truck you could borrow.
I think you will have trouble fitting the whole thing in your living room, but you could start there. Open up one of the 10,000 boxes and carefully remove one grain of salt with tweezers, placing it in the middle of your coffee table. Congratulations! That’s the sun! You only have 299,999,999,999 grains of salt to go. The next grain, representing Proxima Centauri, the closest star to us, needs to be 5 1/2 miles away!
Where is the earth? You will need a microscope to see it because it is about the size of a bacterium, but look for it 1 inch from that grain of salt on your table. Actually, our Milky Way model would not fit on the earth at all. It would take up a space almost as big as the distance from earth to the moon. Maybe that gives you an idea of how big our galaxy is.
Coming back closer to home, just a few inches from that grain of salt, Venus is climbing higher into the twilight sky and much easier to see. Look for it as a bright “star“ above where the sun went down. Jupiter and Saturn are rising in the southeast later in the evening. Dimmer Saturn will come up first followed by the much-brighter Jupiter. You will know you are looking at a planet if it is shining steady, not twinkling like the stars.
Actually, the Milky Way Galaxy, even though it is very big, is not the biggest thing we can see out there in the night sky. This fall I will show you where to see something even bigger.