Intrinsic Brightnesses

Goals: Explore the intrinsic brightness of the stars and contrast the intrinsically bright and faint stars.

Before starting, turn on: stars, mwVis

You will be using: lum const, lum datavar, and the see commands

We've seen the brighter stars around the Sun, but are they the only stars nearby? If you've scanned the sky, you may have noticed the label Barnard's seemingly hanging out in space without a star beside it. Even if you fly right up to the label, it is exceedingly difficult to see the star itself. Barnard's Star is the second-closest star to the Sun. It is a cool star with a very low luminosity, making it difficult to see. How many more stars like this appear before us but are too dim to see? Let's find out.

Setting the Stars to the Same Luminosity

Let's leave reality for a moment and set the intrinsic brightness of each star to one constant value using this command:

lum const 0.003

Now all stars are on an equal footing; the brighter the star, the closer it is to us.

Sirius and Procyon look normal because they are among the closest stars, but Orion has disappeared. Most of the stars in Orion are far away. The Hyades, the star cluster in the face of Taurus the bull (turn on oc briefly to see where the cluster is), appears above Orion. Turn off the constellation lines and see if anything recognizable remains. In a universe where all stars had the same intrinsic brightness, we would have to invent new constellations.

Restore the stars to read the intrinsic luminosity from the stars.speck by using the lum datavar command . In stars.speck, the data variable that sets the luminosity of each star is called lum. Use it in place of the datavar argument by typing

lum lum

Now the familiar night sky is back. (If nothing appeared to change, be sure stars is the active data group.) With a constant luminosity set, you may now toggle between them by typing lum const and lum lum.

Intrinsically Bright Stars Lie in the Milky Way Band

We have set two selection expressions in the mw.cf file to cull stars by their intrinsic brightness, what astronomers call the absolute magnitude. If you would like to look at the intrinsically bright stars, type

see bright

The bright alias has been set to show only those stars with an absolute magnitude, M_V, in the range -10 < M_V < - 3. Remember, the smaller the number, the brighter the star. We mentioned previously that the brightness of the Sun is -26. Its absolute magnitude is around +5, rather dim.

Notice that these intrinsically bright stars lie mostly along the band of the Milky Way. Also, some of the brighter stars in the sky are missing, including Sirius, the brightest star in the sky.

Dim Stars Far Outnumber Bright Stars in the Galaxy

Now let's look at the intrinsically dim stars using the

see faint

command. Here we show those stars with 0 < M_V < 10, and you may notice that most of the stars in the sky fall into this category. There are many more intrinsically dim stars in the Galaxy than bright stars.

Next let's explore stellar distances and see how well astronomers have determined the positions of the stars.

Digital Universe Atlas

Digital Universe Guide

Intrinsic Brightnesses