This week, we take another look at the brightest star in the night sky, the Dog Star, Sirius. During the winter, it commands our attention with a kaleidoscopic display of dazzling color. Found nearly due south at twilight, Sirius is progressing slowly toward the western horizon. By mid-May, Sirius will set with the sun and be absent from the night sky until it returns in autumn.

As stars go, Sirius is relatively close at 8.6 light years distant. To get a better sense of just how far away that is, consider this: Many of the third-graders enrolled in schools around town were born the same year the starlight we see tonight left Sirius.

Like many stars we see in the night sky, Sirius is a multiple star system. Based upon his observations, German astronomer Friedrich Bessel noticed that the star had a slow wobble. In 1844, he offered an explanation by predicting that another star orbited Sirius. This secondary star had enough mass to noticeably pull on Sirius and alter its path.

While a secondary star was suspected, a visual confirmation had to wait until nearly two decades later, in 1862. Before preparing it for shipment, Alvan G. Clark tested a telescope with a lens he crafted for astronomer E. E. Barnard. As he pointed the telescope toward Sirius, he detected a faint point of light to one side of Sirius' immense glare. It was the secondary star.

Designated Sirius B and nicknamed "the Pup," the companion star is one of the first white dwarf stars ever discovered. White dwarf stars are incredibly dense, and Sirius B is no exception. It has a mass equal to the sun crammed into a space the size of Earth, giving it a huge surface gravity. On Sirius B, my weight would be over 61 million pounds.

Sirius B and Sirius orbit a common center of gravity with a period of about 50 years. The stars have an elongated orbit, causing their relative distance to vary from 744 million to 2.9 billion miles. This difference in distance is significant for a couple of reasons. First, it has enabled scientists to conclude that about 100 million years ago, Sirius B was a bloated red giant star. And second, even with the extreme glare of Sirius, it is just possible, though still very difficult, to visually observe Sirius B as it reaches its greatest distance from the primary.

Over the next several nights, skies over New Mexico will see a number of bright International Space Station passes. One of the brightest of the bunch will occur Thursday night starting at 6:23 p.m., as the ISS rises from the southwestern horizon. Keep watch and track it as it climbs into the sky heading northeast and the highest and brightest portion of the pass. The maximum predicted magnitude of ISS for this pass is -3.4, many times brighter than Sirius. More details are available at www.heavens-above.com. Create a login and set your location to the town nearest you for accurate pass information.

Be sure to get out under the night sky soon and take in a view of Sirius and the International Space Station before they are dog gone.

Here is a link to a Hubble Space telescope image of the subject Sirius and Sirius B:
www.spacetelescope.org/goodies/printlayouts/large/heic0516.jpg. Sirius B is the small dot at about 8 o'clock.

Peter Lipscomb operates Astronomy Adventures. Contact him at peter@astronomyadventures.com.