Welcome back everyone. So, what we want to do now is think about galaxies, as astrophysical objects. And because galaxies are much larger than stars we have to think a little bit about how astronomers measure distances. So, we're going to be thinking about distances in general, but particular, thinking about how we measure large distances. Okay, so, let's think about the different kinds of distances that we're interested in as we move from one astrophysical scale to the other. So, if we're thinking about the solar system, we're typically interested in what are called astronomical units. That's the size scale that measures the distance between the sun and the Earth's orbit and that is about ten to the five kilometers. And an astronomical unit is a pretty decent measure to use for anything that's associated with the solar system, even going out to the oort cloud which really defines the edge of the solar system. We're really talking about a hundred thousand AU or so. But the next scale up that we want to use are we often use are what are called Light Years, and that is the distance that light travels in a year. And if you want to get some measure of this, the distance between the Earth and the moon, is about 1.3 light seconds. The distance between the Earth and the sun, which is an astronomical unit, is about 8 light minutes. And the distance to the nearest star Proxima Centauri, is 4.2 light years. So that means of course when we're looking at the nearest star, we're seeing as it was 4.2 light years excuse me, 4.2 years ago. And this is an important thing to understand is that the relationship between distance and time in astronomy, so as we look ever deeper into the sky, we're looking ever farther back in time. So, another way that astronomers another unit that astronomers have for measuring distance is something called the parsec, and the parsec comes from measurements of parallax, which we've talked about before. And it turns out that one parsec is about 3.2 light years, which means that the distance to the nearest star is about 1.3 parsecs. So, in general, we'll be using either light years or parsecs, but you'll often in astronomy see people talking about or using parsecs as their basic measurement. So, okay, so that gives us our, our sort of sense, our yard stick, and now we're going to go out and measure things and we'll be talking about either millions of lights years, thousands of lights years, or, or millions of parsecs and it's important just to remember that number 3.2, that 3.2 parsecs or excuse me, 3.2 light years per parsec. Okay, now that we have that, we're ready to go on to talk a little bit more about galaxies. [BLANK_AUDIO]
