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ERROR IN VIDEO - There are up to 200 BILLION stars in our galaxy...not 200 million.

How many stars are there in the universe? This question leads Hank to a couple other questions - How many stars can we see from Earth? How many stars are there in our galaxy? - but the answer to the original question proves elusive.

T*mbl SciShow. http://scishow.tumblr.com

References:
http://www.universetoday.com/22380/how-many-stars-are-in-the-milky-way/
http://en.wikipedia.org/wiki/Messier_81
http://apod.nasa.gov/apod/image/0709/milkywayband_gleason.jpg
http://www.universetoday.com/24328/how-many-stars/

#### Introduction

How many stars are there? Buried in that one little question are a number of other bigger questions, and the majority of those answers to those questions are "we have no freaking idea", but that doesn't mean there's not a lot to learn, so let's start asking.

(Intro music plays)

#### How Many Stars Can Be Seen From Earth?

The first "how many stars are there" question that we can try to answer is "how many stars can I see?" Go to the middle of Nowhere, Siberia or Alaska for a super clear night with the moon not risen and no moisture in the sky, preferably on the top of a mountain. If you happen to have really fantastic eyesight, you'll be able to count two to three thousand points in the sky. But those are just the ones that are visible to you at the moment.

An exact number is impossible to come by because different eyes can see different numbers in different places. You can never see all of the sky at one time because, of course, the earth is getting in the way of the rest of the night sky, but there are about five thousand stars visible from the surface of the earth. Of course, there are some dots in the sky that aren't stars -- many man-made satellites are visible from earth, as well as Mercury, Venus, Mars, Saturn, Jupiter, Neptune and Uranus; all occasionally visible to the naked eye, as well as two galaxies: Andromeda and the much, much dimmer M81.

#### How Many Stars are in the Milky Way Galaxy?

Which brings us to the question of how many stars there are in our galaxy, because while we can see a fair number with the naked eye, hanging out in our galaxy are more than two hundred million [annotation: BILLION (oops)] stars. Two hundred million! But of course, it is impossible to count them all. There are so many that when we point telescopes at more dense areas of the Milky Way, there's just a solid mass of stars that we can't see beyond and unless we leave our galactic neighborhood, we'll never be able to see beyond.

But two hundred million is just the low end of the estimates. By measuring how fast the galaxy spins, and thus its gravitational effect upon itself with dark matter subtracted, scientists can figure out that the rough mass of the Milky Way, which has resulted in estimates of up to four hundred million stars. And that's a lot, but nothing compared with the number of stars that there actually are.

#### How Many Stars are in the Observable Universe?

Because though we can only see two galaxies with our unaided eyes, the observable universe has a lot of galaxies. Again, we don't know how many, but estimates range from one hundred billion to a trillion. And the Milky Way is not an exceptionally large galaxy. There's so many galaxies that when NASA pointed the Hubble Space Telescope at a tiny bit of dark sky and left the shutter open for a couple of days, the photograph that resulted contained thousands of pin pricks, spirals, and smudges: each one a galaxy.

Not that this number will mean anything to the human mind, but that gives us estimates of up to one septillion stars. As Carl Sagan said, "Greater than the number of all the grains of sand on all the beaches on Earth."

#### How Many Stars are there in the Entire Universe?

Oh, and yeah, when I said the observable universe, that's just the areas of the universe that we can possibly see. Where the light that has been produced has had time to travel to our eyes. Beyond that, there are no doubt, more galaxies, indeed, the number may be infinite. And so, when you ask how many stars there are, just know that we can't know. But it's definitely a number greater than I can conceive.

#### Outro

Thank you for watching this episode of Scishow, if you have any other fun questions you'd like us to answer, we're on Facebook and Twitter, and of course down in the comments below. And if you wanna keep getting smarter with us here at Scishow, you can go to YouTube.com/Scishow and subscribe.