We are golden --- caught in a Devil's Bargain
And we've got to get ourselves back to the Garden"
--- Joni Mitchell
Since we've reached the end of the list of 58 primary stars in the Nautical Almanac let's take a grog break and clarify a few points of interest about the navigational stars:
THE NAUTICAL ALMANAC
The Nautical Almanac was first published by His Majesty's Navy (that would be George III) in 1766, and periodically thereafter. The U.S. Navy used the British book until 1855, when the United States began publishing an independent edition. In 1958, the U.S. and U.K. decided to publish The Nautical Almanac jointly, since the only differences in the two works had to do with spelling and phrasing.
The earliest extant star almanac is a 4000 year old Chinese scroll, although the ancient Mesopotamians, the Indians, and the Aboriginal Americans of today's Latin America kept various types of star charts that are at least that old. Stonehenge and New Grange in Ireland are precise megalithic astronomical devices that are older yet.
The earliest surviving European star almanac was drawn up by Ptolemy around the end of the first century after the time of Christ. He identified 48 "Classical" constellations and their brightest stars. He also (erroneously as it turned out) posited that the Sun and the planets revolved around the Earth, an idea that went largely unchallenged for 1600 years.
There are other star almanacs that give information on navigational stars. Mostly, their differences have to do with the order in which the stars are listed, and whether or not there are additional stars listed in their "Of Interest" sections.
WHY THE STARS?
Mankind has been fascinated by the heavens since the day humans could peer above the horizon. The various bright points of light to be seen were considered gods or messengers from the gods, and their arrangements were seen as messages or beings as well. Each culture placed its heroes and villains in the sky, and storytellers and wise men and wise women imbued the sky with magical and portentous properties based on the star arrangements they recognized and the qualities they assigned to them.
Interestingly, the identities and qualities of the constellations and stars are remarkably consistent across the world. Until humans began being technologically adept, the positions of the stars were used to determine planting seasons, the best and worst times to coronate a king, and as indicators of personal well-being or ill-being. That art and science was called astrology, and the earliest astronomers were astrologers. Amazingly, the ancient wisdom concerning the stars still seems to have power, though no one knows quite why.
After the fall of Rome, circa 500, Europeans lost much of the ancient knowledge and wisdom that had been handed down about the stars for generations, but just as Europe fell into darkness, the Middle East emerged into light. Islamic astrologers and astronomers preserved and enlarged upon the ancient knowledge of the stars, and, unsurprisingly, they documented it in the language of Islam, Arabic. Most of the 6000 stars that can be seen with the naked eye have Arabic names as a result.
ALPHA BETA GAMMA DELTA
Besides the Arabic names of stars, some kept their ancient Greek and Roman names, and as Western science reemerged around 1600 CE, various scientists began cataloguing the stars, often by brightness. The brightest star in a constellation was called "Alpha", the second "Beta", and so on. This naming system was inconsistent, and as telescopes have grown more powerful we recognize that what may appear to to the naked eye to be a single star may be a multistar system, leading to such bizarre appellations as Alpha Beta Alpha {insert name of constellation here} and so on.
Spica, the brightest star in the constellation Virgo (Virginis) is also named Azimech, Spica Virginis, Alaraph, α Virginis, 67 Virginis, HR 5056, BD -10°3672, HD 116658, GCTP 18144, FK5 498, CCDM 13252-1109, SAO 157923, and HIP 65474 in different naming systems.
MAGNITUDE AND LUMINOSITY
Magnitude is a logarithmic numerical system based on powers of ten which assigns every star a numerical brightness. The system used to work on a 1-10 basis, but with improvement in astronomical measurements the scale has been adjusted to a fineness. Sirius is a magnitude zero star (rated -1.46). Regulus is a first magnitude star (rated +1.40), and so on. Scientists also now distinguish between "apparent" magnitude (magnitude as seen from the Earth) and "absolute " magnitude (as seen from a fixed distance in space). I've stuck with the simpler old 1-10 system, which is sufficient for our purposes. Most of the stars in the Nautical Almanac are magnitude 0-2 stars. It is hard to see anything dimmer than a magnitude 4 star with the naked eye, but space telescopes like the Hubble can see beyond the 12th magnitude.
Luminosity has to do with the actual amount of visible light a star is emitting. Most stars we can see from Earth are far more luminous than our Sun. Stars (including the Sun) also emit non-visible rays like infrared, ultraviolet, x-rays, gamma rays, and other forms of radiation.
Mass is the amount of substance in an object. Density is the amount of mass per given volume of the object. Radius is the distance from the core of the object to its perimeter. The diameter of a star would be equal to the distance across both of its radii. The circumference of a star is the distance around its perimeter.
Let's say there is a star named White Dwarf 1 (WD1) which glows dimly and, when studied, turns out to be the same size (in radius, diameter and circumference) as our Moon. But we discover that WD1 has a very large mass --- a lot of substance --- and that it is very dense --- that substance is packed tightly into a smallish sphere --- so that each teaspoonful of the stuff of WD1 weighs about as much as the Earth. Such stars do exist.
DISTANCE
A light year is not a measure of time but of distance, the distance a beam of light travels in a year --- 5.9 TRILLION miles (that's 6 million million miles). Considering that many stars lie hundreds of light years away from Earth the vastness of space can be dimly grasped.
AGE
The age of stars is often hard to determine. Age is measured by the type of radiation emitted, the chemical makeup of the star, and other factors. Also, stars have highly individualized lifespans. One star may move through its life in a scant few hundred thousand years while other stars may live for billions of years. Whether a million year old star that has passed its Main Sequence is "old" compared to a five billion year old star in the prime of its Main Sequence is a matter for debate.
TEMPERATURE
I've cheated here. Star temperatures are usually measured on the Kelvin scale, measured in degrees Kelvin or simply Kelvins. 0 on the Kelvin scale is Absolute Zero (the point when all motion ceases). Since most of my audience is unfamiliar with Kelvins (and with Celsius) I've converted all temperatures to the familiar Fahrenheit scale.
COLORS
Human beings get a little shortchanged when it comes to seeing the colors of stars. Although stars shine in all colors of the visible spectrum --- red, orange, yellow, green, blue, indigo, and violet --- they also shine in frequencies we can't see, the infrared and the ultraviolet and beyond. Since we can't see them they appear invisible to us (we can use instruments to find such "black" stars).
Stars may also appear brownish or very dark red on one end of the spectrum or dark blue or purple on the other end. Both extremes are virtually invisible to our eyes against the blackness of space.
We also get short shrift in the middle of the visible spectrum. Since our eyes evolved to take maximum advantage of the greenish-yellow light frequency of our own Sun we tend to see green and yellow stars as white. Occasionally, local conditions around an observed star may make it appear yellow or green, but it is a rare human who sees these star colors naturally.
As a rule, we can most easily see bright red, bright orange, bright yellow, bright blue, bright blue-white and bright white stars.
Color is an indicator of temperature. Red stars are the coolest, blue stars the hottest. Color may also be a guide to size: Red stars tend to be bigger, blue stars smaller, though this is not a fixed rule. Temperature and color have much to do with the chemical makeup of a star as well.
A star's Main Sequence is equivalent to its adulthood. After a dramatic birth and years (sometimes centuries sometimes billions of years) of a dynamic adolescence, a star will eventually settle down to its business of fusing hydrogen atoms into helium atoms and emitting energy in the process.
Depending on the star's size, density, mass, and its reserves of hydrogen, this "main sequence" can last a very long time or not. Eventually, a star will run out of hydrogen atoms to fuse. At that point it leaves its main sequence and begins fusing helium into carbon, and then carbon into heavier elements. The further down the fusion road it goes the faster this portion of the star's life progresses.
Eventually, the star is left burning heavy elements like iron. Bigger stars tend to become unstable at this point and explode into supernovae while smaller stars collapse into white dwarfs. The remnant matter of the star --- dust, gas and elements --- tends to coalesce over eons to give birth to new stars. And so it begins again.
No comments:
Post a Comment