Siriusly
Published on Mar 1, 2006 at 12:18 pm.
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Filed under stars.
Go out this time of year and look to the South. Look up (but not too high!), and you’ll see a very bright star down and to the left of Orion. That is the star Sirius. It is the brightest star in the night sky. The name itself means “searing one.â€Â Sirius has a magnitude of -1.44 (the smaller the number, the brighter the star, and negative is even brighter). Sirius is spectral type A1, which means that it is white hot, having a temperature of just over 10,000 K (that is 18,000 degrees Fahrenheit). Located only 8.6 lightyears away, it is the 6th closest star to the Sun. Well, that in itself tells you something — the sixth closest star is so far away that it takes light 8.6 years to get from there to here. Sirius is a binary star, with a companion called a white dwarf. The white dwarf companion, called Sirius B, is about 1/9000 as bright as Sirius A. A white dwarf is essentially a dead star, or a star that has shed its outermost layers into space with its core collapsing to an object about the size of the Earth.
Not surprisingly, with Sirius so bright, there is a lot of lore surrounding the star. I don’t have time to go into all of that, but I figured some astronomical trivia might be interesting. First of all, Sirius is often called the Dog Star. A lot of people think that this is because it is located in the constellation Canis Major, the big dog. Well, really it is the other way around. The constellation was more likely named for the star, and I am about to tell you why.
Sirius, like all stars, appears to rise about four minutes earlier each day. This is due to the motion of the Earth around the Sun. Eventually, Sirius is rising with the Sun. Then it is rising just before the Sun. The first day that Sirius can be seen rising just before sunrise or with the Sun is called the heliacal rising of the star. Coincidentally, this happens in late summer. The Egyptians noticed that this also was just before the Nile flooded each year. So, they would watch the skies, and when they saw the heliacal rising of Sirius, then they knew that they had precious little time to harvest their crops before the floods. Sirius began to take on more and more significance to the Egyptians. After the Greeks conquered Egypt, they became aware of the significance of Sirius to the Egyptians. Well, it turns out that this time of year is also about the hottest time of the year. So, the Greeks mistakenly assumed that the combined light of the Sun and Sirius (the brightest star other than the Sun) resulted in the summer heat.  Eventually, the Romans conquered the Greeks, and Sirius lore came to the Romans. They then went one step further. They noticed that dogs had rabies more late in the summer than at other times. They had no idea about the rabies virus, which dogs contracted more in the summer due to more activity and interaction with wild animals. Rather, the Romans believed that the heat from Sirius rising with the Sun was driving the dogs mad. So, their solution was to simply sacrifice dogs to Sirius for a couple of weeks after they noticed the heliacal rising of the star. These became known as the dog days of the summer, a term still used even though we no longer sacrifice dogs then. Sirius became known as the dog star. So, it was only natural that the constellation that Sirius belongs to should be a dog!
Another interesting bit of lore about Sirius is found in the Almagest, a massive compendium of astronomical knowledge compiled by Ptolemy in the about the 2nd Century. Ptolemy’s work was very carefully done. Nearly one and a half millennia later, astronomers noticed that Sirius, and a couple other bright stars, were not exactly where Ptolemy said that they were. His records were so accurate that this suggested that the stars must have moved. This was the discovery of what we call the proper motion of stars. Another interesting, and as yet not fully explained, thing about Sirius and the Almagest is that Ptolemy records Sirius as being reddish.  You can see reddish stars out tonight. The upper left star in Orion, Betelgeuse, is red. Up and to the right of Orion is a red star called Aldebarran. There is no way to mistake Sirius for being red. Much has been made of this. Some suggestions have been that perhaps a particularly thick clump of interstellar medium passed between the Sun and Sirius, but we have yet to find this clump or where it went. Another suggestion is that perhaps Sirius B was just finishing losing its outer layers into space and was settling down to be a white dwarf. This, too, seems not to work in that we think that it takes a longer time for a red giant to settle into being a white dwarf than the time since Ptolemy. Also, we see no indication of the gasses shed by the star, so it must have done this much farther into the past. We can also probably rule out Ptolemy making a mistake. He was very careful, and though mistakes are possible, it would be highly unlikely that he would make such a huge mistake on a star so bright and so important as Sirius. So, why the color? One possibility comes from what I have already said about Sirius. It was most important to the Egyptians when rising with the Sun. Any object, whether it is the Sun, Moon, or star, will appear redder near the horizon than it does higher in the sky because of the atmospheric effects on its light. So, perhaps the color that Ptolemy records for Sirius comes from the time that it was observed most intently. Really, we don’t know why he recorded the color as red.
At any rate, there is a bit of astronomical trivia for y’all.
Astroprof
