Star density

The “Nearest Stars” section in the Astronomical Companion labors the point that huge stars are rare and tiny stars (mostly red dwarfs) are numerous, and it follows from this that only our nearby sample of space is representative. Farther off, only the luminous giants are noticeable. Most of the naked-eye stars, most of the familiar stars for which we have names, are distant and untypical.

It occurred to me that I could illustrate this by showing the Astronomical Companion nearest-stars picture, which is of a sphere of space with radius 16 light years (this is part of the picture) –

– and comparing with it a picture of a somewhat more distant shell of space, such as between 30 and 60 light years, with all the stars in it, not just some as in the “brightest stars” picture.

The catalog of stars recorded by the European Space Agency’s Hipparcos satellite in 1989-1993 (I had to buy it on disks from the Netherlands) must come close to containing all the stars that inhabit that volume of space. So by plotting all of them, not just those above a certain visibility, we would show that the density of stars in our nearby space, and the ratio of rare giants to teeming midgets, holds for that more distant region.

The difficulty was that a picture of the same kind, with stalks connecting each star to a plane so that we can see how they are arranged in space, would be an unintelligible forest of stalks. I tried a view from the north, with stars limited to a wedge in declination – included only if within 10° of the celestial equator. Eventually the picture evolved to this:

It’s a view from galactic north: looking down on the plane of our Milky Way galaxy. Not actually the mid-plane, but the plane in which the Sun lies (which is an uncertain distance, maybe 30 light-years, north of the galaxy’s real median plane). We see stars out to 90 light-years from us, but only those that are within 5 light-years north or south of this Sun’s plane. So we are looking at a disk-shaped block of space 90 light-years in radius and 10 light-years thick.

The density of stars does look consistent: similar in our closer neighborhood and in the more distant parts. But there are differences. The dots for the stars are sized for their absolute magnitude – their true brightness, not their apparent magnitude as seen from Earth. Colored pink are those that actually look fainter to us: those of apparent magnitude 6 or dimmer. Most of the stars in the outer rings are too dim for the naked eye. In the inner region are the few stars that are among the brightest in our sky because they are near and are of about the Sun’s size or larger: Sirius, Procyon, Alpha Centauri, Altair.

In the outer regions it’s hard to find any named stars; I had to lower the magnitude threshold for them to 5 before finding any even with names of the unfamiliar kind that aren’t really used – Kaffaljidhm is a star in Cetus, Keid is in Eridanus, Alshain is in Aquila.

So does this picture really show the density of stars in our part of the galaxy? It’s an approach, but probably an understatement. My program told me it had plotted 187 stars. The Hipparcos satellite produced two sets of data, called Hipparcos and Tycho. Tycho goes down to about three magnitudes dimmer, and has entries for nearly 9 times as many stars, though with shorter lines of information about each. If I make my program read the Tycho set instead, it takes minutes longer, and packs the picture with 19,164 stars!

Are there really so many more faint stars relatively close around us? I’ll probably be returning to this.

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