Digital Universe Guide - Hayden Planetarium

44 3. THE MILKY WAY ATLAS
and reports its number, name, range, and mean value. For example, the first line, data variable 0 called
BVcolor, is the (B − V ) color of the star and ranges from −0.63 to +2.057 with the mean value of 0.535.
In the tours that follow, we will use these data variables to explore the stars around us and even the
Galaxy we live in.
Tutorial: Stellar Brightness
Goals: Examine a star’s brightness as we see it from Earth.
Before starting, turn on: stars, constel, mwVis
You will be using: thresh and see commands
In this tutorial, we examine the brightness of the stars in our sky. This is described by what
astronomers call the apparent magnitude and what we call the appmag data variable.
We can attribute the system of magnitudes to Hipparchus, the Greek observer from the second
century BC. He developed the first star atlas and set the brightest stars in the sky to first magnitude and
the dimmest stars to sixth magnitude. Now, with more precise scientific methods, we have quantized the
magnitudes, and some stars were found to be brighter than 0 and, therefore, have negative magnitudes.
For example, Sirius has an apparent magnitude of −1.4 and the Sun’s is −26.
Beginning from the home position (press the Home Button) and looking toward Orion, you can see
the bright stars that form the hunter as well as Sirius and Procyon, two nearby bright stars. Let’s see
how bright they are. Recall from our data variable report that the range in apparent magnitude, mV , is
−1.44 < mV < 14.85 (not including the Sun). If you wish to see only the bright stars in the sky, you can
select them for display by using the thresh command. To see only stars in the range −1.4 < mV < 1,
type
thresh appmag -2 1
We simply use −2 instead of −1.44 to take everything from the lower limit to mV = +1. Partiview reports
# thresh 3(appmag) min -2 max 1 (15 of 100659 selected)
echoing the threshold, then telling you that it found 15 stars in that range. You can confirm that the sky
is empty of stars except for the 15 brightest ones in the sky.