'Twas the season when the vault of heaven

Bends its most scorching heat upon the earth,

And Sirius the Dog-star smitten by Hyperion's full might

Pitilessly burns the panting fields.'
- Silvae, Latin Epic 1st Century AD

Every dog has his day, and these days he's lying somewhere in the shade and wishing he weren't wearing a fur coat. Yup, it's the dog days of summer. My dictionary defines the dog days thusly:

1 : the period between early July and early September when the hot sultry weather of summer usually occurs in the northern hemisphere

2 : a period of stagnation or inactivity

The term 'dog days' comes not from all the dogs lolling about, but from Sirius¹, also known as the Dog Star, which is the brightest star in our skies. It's located in the constellation Canis Major - the Great Dog - hence the star's name.
You can spot Sirius in the southern sky (viewed from northern latitudes) during January. In the summer, however, it rises and sets with the sun, and people used to believe that its heat increases that of the sun, creating a stretch of hot and sultry weather. And so we pale and pallid northerners lie doggo for several weeks while folks from other parts of the planet snicker and tell us what REAL hot weather is like.

The Egyptians considered Sirius to be the embodiment of Isis, sister and consort of the god Osiris. Sirius' heliacal rising, when it again rose into visibility after being hidden by the Sun's light for 70 days, marked the beginning of the Egyptian year and roughly coincided
with the flooding of the Nile, a major event marked by feasting and celebration.

The star's colour is something of a mystery. Today Sirius appears white to slightly blue, but many ancient observers described it as red. In The Iliad, Homer compared the light from Sirius to Achilles' copper shield. The Roman poet Cicero said that the star shone with a ruddy light, and another Roman poet, Seneca, referred to it as 'deep red'. And this wasn't so much poetic license: the astronomer Ptolemy described the star as 'fiery red' in his star catalogue The Almagest. Nobody knows why the star has apparently changed colour. Various explanations have been put forth, such as changing atmospheric conditions on earth or the evolving of stars near Sirius, but all the explanations have dubious features and even the explainers remain unconvinced.

Which brings us to another mystery. In the early 1800's astronomers noticed some irregularities in the movement of Sirius. They supposed that Sirius must be affected by a second star, and in 1862 a faint companion star was finally spotted by telescope and named Sirius B. Sirius B is classified as a 'white dwarf star'. It is extremely small and dense but invisible to the naked eye. Sirius B's surface is 300 times harder than diamonds, while its interior has a density 3,000 times that of diamonds. It spins about 23 times a minute on its
axis, generating huge magnetic fields that tug on Sirius and affect its motion.

In the late 1940's, Dogon priests² surprised a pair of French anthropologists by telling them what they knew about Sirius. The priests claimed that it had a companion star that was invisible to the human eye. They also stated that the star moved in a 50-year elliptical orbit around Sirius, that it was small and incredibly heavy, and that it rotated on its axis. The Dogon named this companion star Po Tolo, 'tolo' meaning 'star' and 'po' the smallest seed known to them.

Which was spot on. But Dogon beliefs were supposedly thousands of years old. How could they know anything of a companion star whose existence wasn't even suspected before 1800 AD?

When asked about their astronomical knowledge, the Dogon say that they learned of Po Tolo from a group of amphibious extraterrestrials called the Nommos. Their myths describe the Nommos, who arrived in a vessel along with fire and thunder, as humanoid beings with fish
skin running down their bodies. The Nommos were more fishlike than human and had to live in water. They were also known as Masters of the Water, the Monitors, or the Teachers, and Dogon mythology says that the Nommos will return to the earth one day, this time in human form³.

The Dogon also tell of a third star in the Sirius system, one they named Emme Ya. Supposedly larger and less dense than Sirius B, this star also revolves around Sirius and boasts an orbiting planet from which the mythic Nommos came. To date astronomers have not identified Emme Ya, although they have not ruled out its existence. I've seen claims by some of the wiftier (ie, less scientific) groups insisting that this third star has indeed been found, but their belief seems to rest on the complete absence of hard evidence of the star's
existence.

So, have we been visited by aliens? A more likely explanation for the Dogon's astronomical prescience lies in the difficulties faced by anthropologists when questioning people from more 'primitive' cultures. It is easy to allow one's technological assumptions to guide the questions one asks and to colour the interpretation of the answers. In short, one finds what one expects to find. In addition, the Dogon may have been indulging in that age-old game of 'primitives' everywhere, namely Pull the Stranger's Leg.

Just how likely is it that intelligent life, or life of any sort, exists elsewhere in the universe? Here's what we know today.

Until recently astronomers believed that around five percent of stars may have planets orbiting them, but a recent survey by researchers at the University of California (Berkeley) has revised this estimate. They say that if a star is rich in metals, particularly iron and other heavy metals, then it stands a one-in-five chance of hosting planets⁴.

Of the stars they surveyed, those like our sun fall in the middle of the spectrum, with a 5 to 10 percent chance of hosting planets. Stars with three times more metal than the sun have a 20 percent chance of harbouring planets, while for those with 1/3 the metal content of the
sun, the chances drop to about 3 percent. The most metal-poor stars in the survey, with less than 1/3 the sun's metal abundance, had no planets.

Just how many stars are there in the universe? According to researchers at the recent General Assembly of the International Astronomical Union in Sydney, Australia, it's 70 sextillion. That's 70,000,000,000,000,000,000,000 (seven followed by twenty-two zeros).
'This is not the total number of stars in the universe, but it's the number within the range of our telescopes,' said one researcher.

If we're conservative and assume that only a small number of these stars have planets orbiting them, we're still looking at a boatload of planets. And NASA (National Aeronautics and Space Administration in the USA) estimates that 1 to 5 planets per star may be capable of sustaining life of some sort. So the odds are very much in favour of life existing elsewhere in the universe.

But it's a huge leap from life existing elsewhere to aliens dropping by to chat up the natives. I suspect that if we had been visited by extraterrestrials, we'd know about it and we wouldn't be having a very good time. However, such worries haven't stopped us from looking for them.

SETI is the acronym for the search for extraterrestrial intelligence. More precisely, it is a search for electromagnetic signals from other civilizations in the universe. SETI projects have sprung up around the world, run by both professionals and amateurs. The most well-known of the amateur projects are known as SETI@Home; computer users can download software that allows them to contribute their excess computing power to the hunt for aliens. The majority of SETI efforts search for radio transmissions, but a growing number of
programs look for light signals.

Here are links to some of the SETI projects around the world:

• SETI Institute
• SETI League
• SETI Australia Centre
• SETI UK Network
• SETI@Home USA
• SETI@Netherlands
• SETI.Germany
• SETI@Home France
• SETI@Home Spain

Editorial Note: There is also a thriving seti community here on h2g2 although, sadly, the originator is no longer here to oversee its pages: The SETI@home project and Meet the H2G2 Researchers for SETI@home.

Also, for those in the Northern hemisphere, this coming week is the best time to see the Perseid meteorite shower. The H2G2 Astronomy Society are always happy to receive and collate reports of this event - you can read about previous years on these pages:

• A Once-a-year Opportunity!
• The Perseids Shower 2000
• Perseids Project Results - 2000

This is a lot to ponder while lying around and looking up at the stars on a hot August night. D'ya suppose anybody's looking back?

Running With Scissors
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