 Greetings and welcome to the Introduction to Astronomy. In this week's special topic in astronomy, we are going to discuss the parsec and what that means in terms of measurements in astronomy. So what we first want to look at is what is a parsec? Well, it is a way of measuring large distances, and distances in astronomy are very large. If we try to measure the distance to the nearest star in kilometers, it would be tens of trillions of kilometers, very large numbers that our mind does not like to try to wrap itself around. So we try to simplify these by getting other units that measure very large distances. The parsec is defined as the distance at which an object would have a parallax of one arc second. I'm not going to go into all the details here, but essentially that is looking at the sun here, and there is the earth as the planet orbiting around, and you have a star out in the distance. At one parsec, this angle would be one arc second. Now, what is that? Well, one degree is equal to 60 arc minutes, which are written like this, or 3,600 arc seconds. So one arc second would be one 3,600th of a degree, and our moon would be about one half of a degree, so it would be almost one 2,000th the diameter of the full moon. It's an extremely small angle, but it is what we have to be able to measure to determine distances out into space. So we can look at some equivalents here. It is equivalent to 206,265 astronomical units, so something at one parsec distance would be more than 200,000 times the distance of earth from the sun. It is 3.26 light years, so one light year is then a smaller than a parsec. Light would have to travel for over three years to travel the distance of one parsec. Now, if we want to put that in kilometers, it would be 31 trillion kilometers or 19 trillion miles. So you see why we don't want to use these. These are very large numbers. How do we have a concept of what 31 trillion is? That's difficult, but we can wrap ourselves around numbers such as ones or tens or fifties because we have a direct concept of what those are. So here we can see those. We convert those very large numbers and we use things like light years and parsecs for determining distances for the nearest stars. Now let's look at the neighborhood around the sun, and what we see is some of the nearest stars to us, and in fact we find that there is no star other than of course our own sun within one parsec of earth. So here are some of the nearest stars listed and we have our sun and as we go out the distance we pass the Oort cloud within our solar system. We get to two light years, still no stars. We get to four light years, still no stars. Then we finally get to the Alpha Centauri system a little over four light years away and then we have things like Barnard star which is about six light years away and some of these other stars that have been discovered since that are a little over six light years away. Those are the closest stars to earth. So nothing within one parsec. But it is still a convenient distance to use to measure to the stars to avoid using the very large numbers of kilometers or that would be needed to express these distances. So let's finish up with our summary. First of all we talked about how a parsec is a measure of distance, how far things away something is. Its definition is the distance at which a star would have a parallax of one arc second and we know that no stars except for our sun are currently within one parsec of earth. Note currently, yes, sometimes stars do move closer and millions of years from now it is quite possible that we will have stars within one parsec of earth. It is simply that right now we do not have anything at that distance but all the stars move relative to each other so it is not that there will never be stars within one parsec of earth and not that there never have been but only that there are not any currently. So that concludes this special topic in astronomy on the parsec. We will be back again next week for another special topic in astronomy. So until then, have a great day everyone and I will see you in class.