###### By Joshua N. Winn, Princeton University

## To study astrophysics, we need to get comfortable in switching our perspective back and forth from one scale to another, from the micro-world to the cosmos. This brings up the issue of units: units of measurement. Sometimes, we can use standard metric units for lengths: meters, kilometers, and so on. However, there are times we need to use units that are more appropriate for the scale we’re interested in.

### From Hundreds to Millions of Meters

Let’s begin by zooming out. Let’s say we start with a map of the Wondrium headquarters in Chantilly, Virginia, where the scale bar represents 100 meters. If we expand our field of view by a factor of 10, the scale bar is then a 1000 meters, a kilometer, and we can take in the whole city.

Let’s zoom again, another factor of 10. Now, we start to see regional features. Taking another step, our scale bar is 100 kilometers, and we can see the entire mid-Atlantic seaboard. Another factor of 10, and we can see the entire Earth, hanging in empty space. At this point we’ve zoomed out from hundreds to millions of meters.

### Units of Measurement

This brings up the issue of units: units of measurement. Sometimes, we can use standard metric units for lengths: millimeters, meters, kilometers, and so on. The scale bar we just discussed is a million meters long, or, one mega-meter. Another way to write that is with scientific notation, 10 to the 6 meters, since 10 to the 6th power is a million.

However, there are times we want to use units that are more appropriate for the scale we’re interested in. When we think about entire planets, meters are not so convenient. It’s better to measure things in units of, say, the radius of the Earth.

One Earth radius is defined as 6378 kilometers. That way we can say, the planet Neptune has a radius of about 4 Earth radii, and Jupiter’s is about 11. Four and 11 are much easier to comprehend than however many millions of meters. That’s why the Earth radius is a handy unit. We write it as R with a subscript, a little plus sign inside a circle—the astronomical symbol for Earth.

The next useful unit that we need is for the size of stars, rather than planets. So, let’s zoom out and zip inward to the center of the solar system, so we can take in the entire Sun. Its radius is about 700 million meters, or a little more than 100 times bigger than the Earth. The solar radius is our unit of choice when we discuss stars. We write it as R with another astronomical symbol—this time it’s a dot inside a circle.

This article comes directly from content in the video seriesIntroduction to Astrophysics.Watch it now, on Wondrium.

### AU and Light Year

Let’s keep zooming out. Once we get to the scale of 10 to the 13 meters, most of the other planets come into view. We’ve reached the scale of the solar system.

On this scale, the scale of planetary systems, the traditional unit is the radius of Earth’s orbit around the Sun. In fact, that’s such a useful unit, it’s called the Astronomical Unit, or AU. It’s about 215 solar radii, or 150 billion meters. And with the AU, we can easily describe the solar system. Mercury is about 2/5 of an AU from the Sun, and Jupiter is out at 5.2 AU.

We’re still not done zooming out, though. When we expand our scale again, beyond the solar system, we find ourselves in empty space for quite a while, until we get to 10 the 16th meters. Then, at last, some of the neighboring stars come into view. A good unit to use on this scale is the light year—the distance light travels in one year, which is just short of 10^{16} meters. For example, the nearest star, Proxima Centauri, is 4.2 light years away.

But in practice, astrophysicists don’t use light years, except when giving public lectures. The preferred unit is called the parsec, and it’s about 3.3 light years. The typical distance between stars is a parsec or 2.

### Using Parsecs with Metric Prefixes

From here, we need to zoom out 4 more factors of 10—4 more orders of magnitude—until the architecture of the Milky Way galaxy comes into view, at around 10^{20} meters. At this stage, we stop inventing units with different names and just keep using parsecs, but with metric prefixes, like kilo, for a thousand. The diameter of a typical spiral galaxy is 10 or 20 kiloparsecs.

It takes a couple more orders of magnitude to start seeing neighboring galaxies. The typical spacing between galaxies is a few megaparsecs, millions of parsecs. After another step, we see that the galaxies themselves group together to form clusters of galaxies, joined by what look like filaments, or webs of galaxies. And when we keep going, when we keep increasing our scale bar all the way to 10^{26} meters, the universe starts to look like random static, nowhere different from anywhere else.

The natural scale here is the gigaparsec, billions of parsecs. That’s the end of the line. The largest spatial scales about which we have any direct knowledge. By zooming out 26 orders of magnitude, we have a view of the entire observable universe.

### Common Questions about Astrophysics

**Q: What is Astronomical Unit?**

Astronomical Unit, or AU, is the radius of the Earth’s orbit around the Sun. With the AU, we can easily describe the solar system. Mercury is about 2/5 of an AU from the Sun, and Jupiter is out at 5.2 AU.

**Q: What is light year?**

Light year is the distance light travels in one year, which is just short of 10^{16} meters. For example, the nearest star, Proxima Centauri, is 4.2 light years away.

**Q: What is parsec?**

Parsec is the preferred unit of measurement by astrophysicists. It’s about 3.3 light years. The typical distance between stars is a parsec or 2.