Distance Uncertainty
Distance Uncertainty
Goals: Be aware of the underlying uncertainties in the stellar catalog.
Before starting, turn on: stars, mwVis
You will be using: altLbl, center and censize commands, 10ly, err
You've seen how the measured parallax angle determines the distance to a star and how the uncertainty in that measurement propagates into the stellar data. Now you'll see a visual representation of these uncertainties for select stars.
Let's go home in the Milky Way Atlas (press the Home Button). Instead of backing away from the Sun, let's fly forward toward the Hyades star cluster. If you don't know where the Hyades is, turn on the open clusters for a moment to find the cluster. Once you've centered the cluster, fly forward by moving the mouse to the right (in Orbit Flight Mode) while pressing the right mouse button. It may take a few seconds to get going since you're so close to the Point of Interest.
The Hyades forms a ‘V’ shape. As you fly out, you'll see what appeared to be the brightest star in the cluster, Aldebaran, pass by long before reaching the cluster. At 65 light-years, it's not actually in the cluster but sits between the cluster and Earth. Once you get to the cluster, stop. You're going to change the Point of Interest to examine the cluster more effectively. With the cluster in front of you, turn on the alternate star names and find the star θ2 Tauri (marked as “The2 Tau”). You may need to switch into Translate Flight Mode to get the proper perspective.
Changing the Point of Interest Location
Once you have located the star, bring it close and place the mouse pointer over it. Next press the Shift + ‘p’ keys (or use Shift + middle mouse button for a three-button mouse). This places the Point of Interest on this star. To confirm that the Point of Interest has moved, increase the size of the Point of Interest marker by using the Censize Slider. The censize command sets the size of the Point of Interest marker in the units of the data you are viewing, which in the case of the Milky Way Atlas is parsecs. You can move it with the slider or enter a value at the command line. Make it exactly 1 light-year by entering
censize 0.307(0.307 parsecs = 1 light-year). Now you have a measuring stick and an idea of the size and scale of the star cluster. With the Point of Interest centered in your view, switch back to Orbit Flight Mode and orbit the cluster with the left mouse button. Turn off altLbl to see the stars without their labels. Also turn on the 10-light-year grid to give yourself a reference to the location of the Sun and Solar System.
Seeing Uncertainty
While you orbit the Hyades star cluster, turn on the stellar distance uncertainty. You will see a series of red points, some with blue labels, come on around the star Ain. This is Ain's distance uncertainty. Notice how these points line up when you look toward the Sun. Astronomers know a star's position in the sky to great certainty. It's the distance that is difficult to obtain, so it therefore has a margin of error associated with it.
The star is placed at the parallax-derived distance of 154.94 light-years. However, the star could lie anywhere along the red line from 149 to 161 light-years. The red points are spaced in 1-light-year increments with the absence of a point corresponding to the published parallax distance.
Bring the Point of Interest back to the Sun. Rather than travel back to the Sun, place the mouse over it and use the Shift + ‘p’ to move the Point of Interest, use the Command Line. The center (or interest) command sets where the Point of Interest is. Type the command
center 0 0 0to move the Point of Interest back to the Sun so that you return to orbiting the Sun.
Pull away from the Hyades and pass near the Pleiades star cluster, with its bright stars and longer uncertainty marker (remember to turn on the open clusters if you have trouble finding the Pleiades). We highlight the distance uncertainty of Pleione, the first star in the handle of the dipper-like asterism. You will immediately notice the uncertainty has grown a lot from that in the Hyades. This increased uncertainty, about eight times greater, affects the overall shape of the star cluster. You may recall that the Hyades was a nice spherical cluster, what you might expect a star cluster to look like. The Pleiades, on the other hand, is stretched along the line of sight to the Sun, parallel to the distance uncertainty. Astronomers call this radial stretching a “finger of god.” Of course, this is an observational limitation. If the telescope were larger, the parallax (and resulting distance) would be more accurate and the cluster would look more spherical.
Above the Pleiades is the star Betelgeuse, the bright red giant in Orion's shoulder (you may have to pull back from the cluster to see it). If you look at its ray of uncertainty, you'll notice the star is not in the space left for the published trigonometric parallax distance, at 427 light-years. We actually placed the star a bit closer because of the parallax error. For this star, the parallax error was so high that we factored in the photometric distance (the distance derived from knowledge of the star's brightness and luminosity). With a weighted mean distance between each of these, the star ends up a bit closer.
We have chosen a handful of stars for this data group. A list appears in “The Stellar Distance Uncertainty.” Deneb, the bright star in Cygnus, is the farthest star we sample. It is a distant star that is intrinsically very bright. But because it's far away, the uncertainty ranges from about 2,000 to 7,000 light-years.
All Data Have Uncertainties
From outside the stellar data group, all these lines of uncertainty point to the Sun and should remind you that all data in the Atlas have uncertainties. From the nebulae and clusters to galaxies outside the Milky Way, each point has an uncertainty that depends on the technique used to determine its distance.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott