Wednesday, February 6, 2008

Getting to know the Sun (I)

I became enthralled with astronomy at the age of 12, after sending in a cereal boxtop plus fifty cents for a toy telescope. After the telescope arrived, I recall eagerly using it for nightly excursions around the South Florida sky. It may have been a toy - but it had enough magnifying power to allow me to resolve star clusters, and see the larger craters on the Moon. Some years later, I graduated to a larger telescope I constructed myself, and then to an amateur-astronomer sized scope: a Tasco 2.4 inch refractor.

With each succeeding telescope I more or less looked at the exact same objects: the brighter planets (Jupiter, Mars, Saturn, and Venus), star clusters (like the Pleiades), and the Moon. The idea was always to see how the increased size of the telescope allowed me to see more detailed features. However, by the time I reached the Tasco I was beginning to get somewhat jaded. What I needed was to find some purpose in my astronomical pursuits - apart from simple star-gazing.

It was around that time that I discovered - in my telescope carrying case, a tiny, darkened piece of flint glass embedded in a thick metal frame. Curious, I took it out and examined it up close. Then I rummaged through the box to find a small pamphlet describing the use of "your solar filter". Evidently, the filter was screwed on to the front of the eyepiece assembly - just ahead of the eyepieces. Once in place, the Sun could be viewed safely and sunspots became visible. I was absolutely amazed that so many ominous looking dark spots could nearly fill up the surface and not make any difference in the brightness.

For the next several weeks I became completely captivated by my solar observations, specifically the transit of large sunspots across the Sun's disk. What particularly fuelled my interest was a book I had borrowed from the local library entitled Our Star The Sun, by Donald Menzel of Harvard Observatory[1]. In this fascinating book I learned that the Sun the "daytime star" - was the source of all life on the Earth, and actually "the only practical reason for the study of astronomy". Change the physical nature of the Sun by 1 percentage point, and the survival of the human race, and all life on planet earth, was threatened. In the words of Sir Fred Hoyle:[2]

...if the Sun were to vary a little, only a very little, we should soon be faced by a situation besides which the political crises that fill our lives would fall into entire insignificance

To many laymen, the Sun appears so hot and bright, that no conscious connection is made to the pinpoints of light seen at night. The Sun tends to be segregated from the other stars entirely. Why the extreme difference in appearance if the Sun is a star like the others? To illustrate, the nearest star to Earth other than the Sun is very similar in physical characteristics: size, brightness, mass and surface temperature. It is called Alpha Centauri, and is 4.3 light years away. This works out to 270,000 times further than the Sun's 93 million miles. It looks like a fairly bright pinpoint because of its distance. However, the Sun would look exactly like it if its distance could be magically increased 270,000-fold.

A physical principle used in astronomy states that the brightness of a light source - like a star, decreases as the square of its distance. In concrete terms, if I look at a 100 watt bulb and a 40 watt bulb from ten feet away, I will judge the 100 watt bulb to be significantly brighter (two-and-a-half times to be exact). However, if I were to move the 100 watt bulb to a distance of 100 feet - keeping the 40 watt bulb at ten feet, what will I see? The 100 watt bulb is now ten times further than it was for the original comparison, so its apparent brightness is now (1/102) = 1/100 of what it was, or equivalent to a 1 watt bulb. Thus, the 40 watt bulb will now appear 40 times brighter even though intrinsically it isn't.

The same principle applies to the more distant stars. There are stars thousands of times brighter than the Sun, but they appear as dim pinpoints because they are so much farther away. One would have to "move" them to the same distance as the Sun (93 million miles) to get an appreciation for their actual properties in relation to the Sun's.[3] (Though, if that feat could be achieved, all life on Earth would be incinerated in a microsecond!) What is the point of all this? Simply this: without an awareness of the inverse-square law for light, humans would be deluded into the false perception that their particular star (the Sun) was the biggest and brightest in the universe. This is assuming they included the Sun in the same category as the distant stars. It is certainly not intuitive.

Application of the physical principle removes the Sun's specialness - placing it in the category of a very ordinary, garden variety star known as a "yellow dwarf". There are a multitude of stars that are thousands of times hotter and brighter, and hundreds of times bigger in size. Be that as it may, these same physically imposing attributes place those stars in an improbable position to support life-bearing planets. This will apply to the Sun in another 4 billion or so years: it will be approximately three hundred times its present diameter as a "red giant". All the inner planets, including Earth, will be reduced to burnt out cinders as the Sun expands to devour them one by one. If giant, Earth-smashing asteroids don't succeed in impressing upon humans the need to spread themselves around the cosmos, this certainly should.

A catastrophe like the one above can be well-predicted from nuclear physics. Astronomers know the Sun will one day consume its hydrogen, forcing it to burn less-efficient helium (into which the original hydrogen would have been converted).[4] When the helium is used up, an even less efficient fuel in the form of carbon remains. To compensate for the considerable loss in fuel efficiency (lower temperatures) the solar core must contract under the force of gravity. This generates a good deal of heat, causing the surrounding layers of the Sun's atmosphere to inflate. (Since a heated gas expands). The only major uncertainty is whether this inflation will amount to 100 times the present size, or 500. In terms of human life surviving, it won't make any difference: planet Earth will be just as well roasted.

Of course, solar changes need not be as dramatic as these for life's grip to become very tentative. A change in solar energy output by as little as two percent could threaten most species on Earth with extinction - since the planet would either be transformed into a vast, arid wasteland with daytime temperatures approaching 125 degrees or, alternatively, a frigid glacier with daytime temperatures averaging 50 below zero. Possibly, some hardy bacteria and viruses might survive - but not much else. It is difficult to see how humans could sustain themselves in a hostile environment nearly devoid of water.

In the early and mid-1980's, measurements made by an instrument called ACRIM (Active Cavity Radiometer Irradiance Monitor), aboard the SolarMax satellite, detected an increase of one-half percent in the Sun's brightness on several occasions - due to the presence of many large sunspots. The instrument was capable of detecting changes in energy as small as one-thousandth of one percent. These small order differences would amount to an increase in the Sun's surface temperature on the order of 100 degrees Fahrenheit.

Given that an increase in energy output correlates with the appearance of many spots, it is reasonable to suppose the opposite is true as well: a dearth of sunspots correlates with cooler temperatures. While the ACRIM did find a few such cases, the most notable study is that of John Eddy on the so-called "Maunder Minimum" - over the 17th - 18th centuries, when relatively few sunspots were visible from historical records.[5] Coincidentally, much lower than average temperatures were the norm, earning the period the nickname "little ice age".

In Menzel's book I discovered that the sunspots I observed could "grow" to be as large as 100 thousand miles in diameter - more than twelve times the diameter of the Earth! The Sun's surface was also the site of titanic explosions called solar flares which could engulf as many as one thousand Earths, and release an energy equivalent to two-thousand million megaton H-bombs exploded simultaneously! (The Sun itself has a diameter equal to nearly 4 Earth-Moon distances, and if it could be placed on an immense balance - 330,000 Earths would be needed to equalize the scales.)

For the remainder of my high school senior year, until I left for college, I used my Tasco with the solar filter to observe the passage of sunspots. Following Menzel's guidelines I was actually able to track the same group of spots across the solar surface and deduce the Sun's rotation period, of about 27 days. Thus began what was to be a lifelong fascination with the nearest star, and the basis for future research that has continued to this day. (Though the emphasis has changed from simple sunspot transits to their relationship to solar flares).

Is the Sun really as important as Menzel (and others) have portrayed it? Consider this: in 1973, the Skylab orbiting platform, with solar observing equipment aboard, detected a solar flare that wiped out twenty percent of the (then) ozone layer over North America. In March, 1989, a mammoth solar flare erupted in a region of very large spots, knocking out Ottawa's power grid. Nearly a half-million people were deprived of electricity, for nearly ten hours.[6] A massive magnetized cloud, from a solar eruption on January 6, 1997, is believed to have knocked out a Telstar 401 communications satellite on January 11.

Granted, "monster" flares such as these are somewhat rare, but they disclose how precariously human existence is in relation to the Sun's behavior. Fortunately for humans, the Sun is so steady in behavior that we scarcely notice it - unless we go out and get sunburned. If, by contrast, the Sun were a variable star that changed only a few percent each year in temperature and brightness, we would all be in jeopardy. A minor increase in temperature and brightness of a few percent would blind most inhabitants of Earth, and make the temperatures extremely uncomfortable - reaching the hundred-plus degree mark even at the poles, in winter.

As it is, the issue of whether the Sun is variable or not has still not been settled. There is some circumstantial evidence, including variable tree-ring growth, that the Sun is a "long-term" variable star, and more recent evidence it is short term as well.[7] The former case implies changing its temperature and brightness a few percent every 10,000 years or so. There has been some speculation that just such a change occurred around 10,000 years ago and brought the last ice age to a close. More recently, in the 1800's, an extended period of cold weather gripped much of the planet prompting the term "little ice age" to be used. Interestingly, this corresponded to a period of below-average sunspot frequency now referred to as "the Maunder Minimum", after the astronomer who made the original association.

The possible variability of the Sun, as well its potential for violent eruptions (in solar flares) makes it a pre-eminent subject of astronomical, and human importance. Indeed, attention to the Sun's behavior extends beyond the domain of esoteric research. Defense agencies and the military, for example, as well as power companies and telecommunications systems, are regular consumers of solar data - specifically flares, but also the particle bursts that result from flares. The data is provided through the 24 hours monitoring of the Space Environment Services Center of the National Oceanic and Atmospheric Administration. This is critical because, if conditions are right, energetic particles can saturate the delicate electronic detectors on board a spy or communications satellite. (As occurred with the Telstar 401 in January, 1997).

None of this is mysterious, of course. For years short wave fadeouts known as Dellingers originated with the passage of large sunspot groups with their powerful magnetic fields, near the center of the Sun. Solar flares magnify the effects, especially with electronic detectors on aircraft and in satellites. In some cases, large flares (with high x-ray output) have been known to cause malfunctions in navigation systems aboard commercial aircraft.

All of these provide compelling reasons to study the Sun - so I 've never had any problems explaining why I am "into" solar research - specifically the prediction of flares from sunspots.
Actually, prediction is probably a fairly strong word. The term that is generally favored is "forecasting", and that is just about what many solar observers and researchers do: use their data to make as reliable a forecast (say on flare occurrence) as we can. More often than not, we do not fare any better than weather forecasters.

[1] Menzel, D.: 1958, Our Star the Sun, Harvard University Press, Cambridge, Mass.
[2] Hoyle, F: The Frontiers of Astronomy, Signet Science Books, New York, p. 19.
[3] In fact, there is an easier way. Astronomers use the level playing field called 'absolute magnitude' to compare stars at the same distance. In this scheme, the inverse square law is used to adjust the distance/brightness of all stars to a standard distance of 10 parsecs or 32.2 light years (1 parsec = 3.26 light years). The magnitude scale is really a logarithmic brightness scale, with every even 5 magnitudes corresponding to 100 times brightness difference, and every one magnitude difference corresponding to 2.512 times brightness difference (2.512 being the fifth root of 100).

In this scheme, the Sun's 'absolute magnitude' is rated at +4.8, and that of the dog star Sirius at (-1.6). Since the more positive scale refers to a dimmer star, this implies that Sirius is really some 363 times brighter than the Sun (e.g. 2.512 raised to the power (4.8 - (-1.6)) = 6.4. Note that 'absolute magnitude' is only meaningful for light sources, e.g. stars - not planets - which are only visible by virtue of reflecting sunlight.
[4] The nuclear burning within stars is nicely discussed in numerous texts I will cite at the conclusion of the series.
[5] See, e.g. Eddy, J.A. 1976, in Science, Vol. 192, p. 1189.
[6] This was discussed in a presentation by solar physicist Hal Zirin at the joint American Geophysical Union - Solar Physics Division of the American Astronomical Society Conference held in Baltimore on Thursday, May 26, 1994. (The Conference, marking the 75th Anniversary of the AGU, lasted from the 24th through the 27th of May). Zirin included a slide showing a transformer power cable - such as used in the Ottawa system -with its copper wires melted. (Each copper wire had the thickness of a man's thumb). What happened is that electrically charged particles from the huge flare caused large currents (> 10^6 A) to be induced inside the power lines and transformer wires. The resulting electrical load was simply too much for the circuit conductors to accommodate - something like a fuse blowing in a household circuit.
[7] See: 'A Fickle Sun Could Be Altering Earth's Climate After All', in Science, Vol. 269, (Aug. 4, 1995), p. 633.

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