Stellar Distances

Goals: Explore the nearby stars and investigate their parallax values and uncertainties.

Before starting, turn on: stars, mwVis

You will be using: thresh command

In this tutorial, you will learn about stellar distances and parallax. Let's begin at Earth looking toward Orion (press the Home Button if necessary).

Seeing Only the Nearest Stars

First tell Partiview that you want to see only those stars within 25 light-years of the Sun (make sure stars is the active data group). Do this by using the thresh command on the data variable distly:

thresh distly 0 25

This removes all stars outside this range (their labels remain, though). Partiview reports that there are 103 stars in this range.

Let's see how this looks from outside the solar neighborhood. Pull away from the Sun to see all of these stars. Turn off the constellations and the visible Milky Way (mwVis). You may need to brighten the stars with the Slum Slider. You can see that Sirius, Procyon, and Altair dominate the scene.

Let's expand our scope and increase the limit to 100 light-years using

thresh distly 0 100

Now you see a nice sphere of stars surrounding the Sun. Turn on the ecliptic coordinates, which are traced on a 100-light-year sphere. As you orbit, notice the midplane of the sphere (the sphere's “equator”). This is the ecliptic, the plane that contains Earth's orbit around the Sun and generally defines the plane of the Solar System.

Stellar Distances Are Determined by Parallax

Return all stars to view (see all) and let's now look at the parallax. A detailed definition of parallax and how astronomers calculate a star's distance can be found in “Parallax and Distance.” Briefly, parallax is an angle astronomers measure that is used to solve the lengths of a right triangle, thereby yielding a star's distance. This angle comes from Earth's path around the Sun. If a star is close, it will appear in one place with respect to the background stars (those stars that surround it in the sky but are farther away). Six months later, it will appear to have moved relative to the background stars. This motion is so small that astronomers need powerful telescopes to detect it.

In the Atlas, we can look at the distance of stars in terms of the parallax angle. The range of values shown in the report generated by the datavar command is 0.64 to 722.33 milliarcseconds (mas) with a mean of 8.35 mas. The closer the star, the larger this angle will be, so let's look at stars with large parallax angles:

thresh plx 100 800

There are 187 stars in this range resembling our view of the stars within 25 light-years.

Parallax Uncertainty and the Plane of the Solar System

Associated with each of these parallax measurements is an uncertainty. These uncertainties come from the methods and instruments used to measure the parallax, for no measurement is free from uncertainty. To see how this uncertainty propagates through the stellar catalog, let's look at those stars with small errors.

The parallax uncertainty ranges from 0.19-76.92 mas, with a mean of 24.7 mas. Look at the stars with very little uncertainty using the command

thresh plxerr 0 3

This shows stars with highly accurate distances. As you might expect, these are nearby stars, but you'll notice they are not distributed in a sphere. The reason for this is somewhat obscure.

If you still have the ecliptic coordinates on, you'll notice that the long axis of symmetry of the star distribution intersects the ecliptic poles. Think about what astronomers observe on Earth. As Earth orbits the Sun, the parallax of a star near the pole will make a circle in the sky. Conversely, if you're looking at a star near the ecliptic (in the same plane), it will appear to move back and forth along a line. Between these extremes, the parallax motion is in the shape of an ellipse.

It's easier to measure the width of an angle formed by a circle than a line, so the uncertainty for a star in the ecliptic will be more than that of the same star at the ecliptic poles. Therefore, our distances are more accurate at the ecliptic poles than near the ecliptic.

Let's see how this uncertainty in the measured parallax angle translates into distance uncertainty in the Atlas.

Digital Universe Atlas

Digital Universe Guide

Stellar Distances