![]() ![]() Astronomical units and parsecsĪstronomers also use comparisons to measure how far apart things are. For example, the Andromeda galaxy is approximately three trillion times the mass of the Sun (or 3×10 41 – that’s a 3 followed by 41 zeros – Astros). ![]() On the other hand, Astro’s mass is how much stuff he’s made of – and it’s the same no matter which planet he’s on.Īstronomers use Earth and the Sun as handy units to measure mass. This would be easy to do on Earth, even easier somewhere with lower gravity like the Moon, and much harder somewhere with higher gravity like Jupiter. We can think of weight in terms of how hard it is to lift an 18kg Astro off the ground. On Earth the weight of an object, like Astro, depends on the mass of Astro and the gravitational force pulling him down to the ground. Mass is a measure of how much stuff something is made of. If we want to measure how heavy an asteroid is, we could do it with camels – but in space we’re more interested in mass than in weight. So if we want to talk about how big Betelgeuse is, it’s much more convenient to use the radius of the Sun as our unit, instead of the radius of Earth (or to describe it as 632 billion Astros). Laura DriessenĪt an even larger scale, consider the star Betelguese: its radius is 83,000 Earths, or 764 times the radius of the Sun. ![]() So, how far is a parsec? 1 pc = 3.261631 ly.Astro the whippet contemplating the wonders of the Universe (probably). ![]() An equivalent way to define the parsec is to say it is the distance at which 1 AU subtends an angle of 1 arcsecond, also shown in a diagram on the next page. Actually, the nearby star’s parallax across the entire baseline of the Earth’s orbit (2 AU) would be twice as much: 2 arcseconds at a distance of one parsec. What this means is at a distance of one parsec, a nearby star would undergo a one arcsecond (1/3600˚) trigonometric parallactic shift relative to very distant stars across a one AU baseline, as shown in the diagram on the next page. What’s a parsec, then? Parsec is an abbreviation of parallax arcsecond. An astronomical unit is very nearly equal to the average distance of the Earth from the Sun, i.e. It could disappear today (highly unlikely), and you wouldn’t see it disappear until 8 years and 7 months from now! In the solar system-which is much smaller than a light year-astronomers like to use the astronomical unit as the distance unit of choice. So, when you look at Sirius, you are seeing the star as it looked 8 years and 7 months ago. The brightest star in the night sky, Sirius, for example, is 8.6 light years away. When we look out into space, we also look back in time. A very useful property of using the light year as a measure of distance is that it also tells you how many years ago the light left the object you are observing. In a year, then, light travels 9.5 trillion kilometers, which is 5.9 trillion miles. Even so, the nearest known star beyond our solar system, Proxima Centauri, is 4.23 ± 0.01 ly away. The speed of light in terms you may be more familiar with is 186,282.4 miles per second, or a whopping 671 million miles per hour. In fact, this speed defines the exact length of a meter. The speed of light is very fast: 299,792,458 meters per second, exactly. A light year is the distance light travels in an average Gregorian year (365.2425 days) in the near-vacuum of space. But before we define the parsec, let’s look at some more familiar units of astronomical distance: the light year (ly), and the astronomical unit (AU). Download What's a Parsec? and more Astronomy Study notes in PDF only on Docsity!What’s a Parsec? A parsec (pc) is a unit of distance that is often used by astronomers. ![]()
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