The Milky Way: Our City of Stars

 Posted by DrJeff on August 10th, 2009

 Copyright 2009  |  About this blog


HST Sag Final2

This post is a Dr. Jeff’s Jeffism.


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There’s no sight like the sky on a cloudless night far from city lights. The heavens filled with seemingly countless stars is overwhelming. At those moments, I cannot help but wonder if on a planet orbiting that star over there might be someone also looking heavenward, and in their sky is our Sun as one star among many. It touches the depths of one’s soul to look up into the night sky and wonder who might be staring back.


All those stars you see are stars of the Milky Way galaxy, our city of stars. But the Milky Way contains far more than those dotting your sky. Many are too dim to be seen with the unaided eye. Yet a vast number of stars can be easily seen—you just need to recognize them. You see that double band of white light that arches from horizon to horizon above you? Those are Milky Way stars so numerous that they combine into a continuum of light stretching across the heavens. It’s as if milk was sprayed in a broad arch across the sky—which is the heart of the Greek creation story for the Milky Way. Why do I single out the Greeks? There’s is just one of many elegant creation stories embraced by human cultures over history and passed down from generation to generation. It’s because the english word ‘galaxy’ derives from the Greek ‘galaktos’ which means ‘milk’.


Take a look at the impressive photograph above. Those dots are all stars—stars of the Milky Way. (I hope you just said “WOW.”) It was taken by the Hubble Space Telescope in 1998. It’s the Sagittarius Star Cloud in the direction of our galaxy’s center. Yet the photo is of a tiny patch of sky. If you want to know how tiny, give a dime to a friend, have them walk 75 feet (23 meters) away from you and lift the dime over their head. Then see how much of the sky the dime covers up from where you’re standing. That’s the amount of sky in the photograph. Go ahead and try it with friends, with your child, or with your class.


You’ll notice I’ve drawn a white rectangle on the photo. The photo is big enough to contain about 175 of these rectangles. That’s the number of students in about 7 classrooms (if there are 25 students per class.)


The rectangle is special. It contains about 50 visible stars. How do I know? I zoomed in on the photograph, drew the rectangle, and adjusted its size so it contained 50 stars as best I could count them.


So in the photograph there are enough visible stars to give 50 to every one of the 175 students in the seven classes. That’s a lot of stars—about 8,750.


Now here’s the BIG question. How many stars are in the Milky Way Galaxy? And here’s the answer as a Dr. Jeff’s Jeffism:


There are likely enough stars in the Milky Way to give 50

to every human being on Earth.


Just think about that for a moment, and let it soak in.  Now think about this—I’ve only handed out the stars of a single galaxy.



How did I calculate my Jeffism you ask?

I needed two pieces of information—the human population of Earth, and the number of stars in the Milky Way. I can get the first by going to the always handy World Population Clock that we’ve used here and here on BotU before. From the clock I see Earth now supports 6.78 BILLION humans!


There are a number of references for the number of stars in the Milky Way, for example: 12, 3.  The number ranges from 200-400 BILLION!


So if there are 50 stars for every person on Earth, I’d have 50 x 6.78 billion stars total, or 339 BILLION stars—well within the predicted range for the number of Milky Way stars. Pretty cool huh.



Teachers and parents:

Have fun with your child or students by calculating how many Milky Way stars there are for everyone in their: school, city or town. state (if in the US), and nation. Assume there are 335 billion stars in the Milky Way.


Have you child or students research how many galaxies are estimated to be in the observable universe, and see if they can come up with a way to easily picture the total number of STARS in the observable universe. I have. It will be the subject of a future blog post. Promise.



Photo credit: NASA., ESA. and the Hubble Heritage Team (STScI/AURA). Note: the photo was taken using Hubble’s Wide Field Planetary Camera (WFPC) 2, with a field-of-view of 2.7 arc-minutes or 0.045 degrees.

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One Response to “The Milky Way: Our City of Stars”

  1. janellewilson Says:
    November 22nd, 2009 at 10:10 am

    Very cool! I had never heard the number of stars in our galaxy represented that way. I will share with my students when we hit our astronomy unit.