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 B.C. 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 scientific methods, we have quantized the magnitudes so that we have negative values. 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, m_V, is -1.44 < m_V < 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 < m_V < 1, type

thresh appmag -2 1

We simply use -2 instead of -1.44 to take everything from the lower limit to m_V = + 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.

In a dark sky free of light pollution, the human eye can see to about +6.5 magnitude. Let's simulate this by showing only stars in that range. We have defined a selection expression called eye in the mw.cf file that you can use to display the “naked eye” stars. To use it, type the command

see eye

This will display the stars according to the Partiview command thresh appmag -2 6.5. Partiview reports that there are 8,667 stars in this range. The next time you look toward the sky, remember that there are only about 6,000 stars visible to the human eye in the entire sky as seen throughout the course of the year.

Finally, we provide a selection expression that is calibrated to the completeness limit of the Hipparcos catalog. Most of the stars in the stellar data are from the Hipparcos satellite, and we know that the Hipparcos data are complete down to about m_v = 7.5. Beyond this, there are dimmer stars missing from the catalog at these distances. To see a complete stellar catalog with no stars missing, type

see m75

This displays about 25,000 stars and accurately represents the stellar population around the Sun down to apparent magnitude 7.5.

Return all the stars to view by typing

see all

We'll explore the intrinsic brightness of the stars in the sky in the next tutorial.

Digital Universe Atlas

Digital Universe Guide

Stellar Brightness

Stellar Brightness