Our Star, the Sun

[Jamie]

We all know that the Sun is overwhelmingly important to life on Earth, but few of us have been given a good description of our star and its variations.

The Sun is an average star, similar to millions of others in the Universe. It is a prodigious energy machine, manufacturing about 3.8 x 1023 kiloWatts (or kiloJoules/sec). In other words, if the total output of the Sun was gathered for one second it would provide the U.S. with enough energy, at its current usage rate, for the next 9,000,000 years. The basic energy source for the Sun is nuclear fusion, which uses the high temperatures and densities within the core to fuse hydrogen, producing energy and creating helium as a byproduct. The core is so dense and the size of the Sun so great that energy released at the center of the Sun takes about 50,000,000 years to make its way to the surface, undergoing countless absorptions and re-emissions in the process. If the Sun were to stop producing energy today, it would take 50,000,000 years for significant effects to be felt at Earth!

The Sun has been producing its radiant and thermal energies for the past four or five billion years. It has enough hydrogen to continue producing for another hundred billion years. However, in about ten to twenty billion years the surface of the Sun will begin to expand, enveloping the inner planets (including Earth). At that time, our Sun will be known as a red giant star. If the Sun were more massive, it would collapse and re-ignite as a helium-burning star. Due to its average size, however, the Sun is expected to merely contract into a relatively small, cool star known as a white dwarf.

It has long been known that the Sun is neither featureless nor steady. (Theophrastus first identified sunspots in the year 325 B.C.) Some of the more important solar features are explained in the following sections.

Sunspots

Sunspots, dark areas on the solar surface, contain transient, concentrated magnetic fields. They are the most prominent visible features on the Sun; a moderate-sized sunspot is about as large as Earth. Sunspots form and dissipate over periods of days or weeks. They occur when strong magnetic fields emerge through the solar surface and allow the area to cool slightly, from a background value of 6000 degrees C down to about 4200 degrees C; this area appears as a dark spot in contrast with the Sun. The darkest area at the center of a sunspot is called the umbra; it is here that the magnetic field strengths are the highest. The less-dark, striated area around the umbra is called the penumbra. Sunspots rotate with the solar surface, taking about 27 days to make a complete rotation as seen from Earth. Sunspots near the Sun's equator rotate at a faster rate than those near the solar poles. Groups of sunspots, especially those with complex magnetic field configurations, are often the sites of flares.

Over the last 300 years, the average number of sunspots has regularly waxed and waned in an 11-year sunspot cycle. The Sun, like Earth, has its seasons but its year equals 11 of ours.

Coronal Holes

Coronal holes are variable solar features that can last for months to years. They are seen as large, dark holes when the Sun is viewed in x-ray wavelengths. These holes are rooted in large cells of unipolar magnetic fields on the Sun's surface; their field lines extend far out into the solar system. These open field lines allow a continuous outflow of high-velocity solar wind. Coronal holes have a long-term cycle, but it doesn't correspond exactly to the sunspot cycle; they holes tend to be most numerous in the years following sunspot maximum. At some stages of the solar cycle, these holes are continuously visible at the solar north and south poles.

Prominences

Solar prominences (seen as dark filaments on the disk) are usually quiescent clouds of solar material held above the solar surface by magnetic fields. Most prominences erupt at some point in their lifetime, releasing large amounts of solar material into space.

Flares

Solar flares are intense, temporary releases of energy. They are seen at ground-based observatories as bright areas on the Sun in optical wavelengths and as bursts of noise at radio wavelengths; they can last from minutes to hours. Flares are our solar system's largest explosive events which can be equivalent to approximately 40 billion Hiroshima-size atomic bombs. The primary energy source for flares appears to be the tearing and reconnection of strong magnetic fields. They radiate throughout the electromagnetic spectrum, from gamma rays to x-rays, through visible light out to kilometer-long radio waves.

Coronal Mass Ejections

The outer solar atmosphere, the corona, is structured by strong magnetic fields. Where these fields are closed, often above sunspot groups, the confined solar atmosphere can suddenly and violently release bubbles or tongues of gas and magnetic fields called coronal mass ejections. A large CME can contain 10.0E16 grams (a billion tons) of matter that can be accelerated to several million miles per hour in a spectacular explosion. Solar material streaks out through the interplanetary medium, impacting any planets or spacecraft in its path. CMEs are sometimes associated with flares but usually occur independently.

Between Sun and Earth

The region between the Sun and the planets has been termed the interplanetary medium. Although once considered a perfect vacuum, this is actually a turbulent region dominated by the solar wind, which flows at velocities of approximately 250-1000 km/s (about 600,000 to 2,000,000 miles per hour). Other characteristics of the solar wind (density, composition, and magnetic field strength, among others) vary with changing conditions on the Sun. The effect of the solar wind can be seen in the tails of comets which always point away from the Sun.

The solar wind flows around obstacles such as planets, but those planets with their own magnetic fields respond in specific ways. Earth's magnetic field is very similar to the pattern formed when iron filings align around a bar magnet. Under the influence of the solar wind, these magnetic field lines are compressed in the Sunward direction and stretched out in the downwind direction. This creates the magnetosphere, a complex, teardrop-shaped cavity around Earth. The Van Allen radiation belts are within this cavity, as is the ionosphere, a layer of Earth's upper atmosphere where photo ionization by solar x-rays and extreme ultraviolet rays creates free electrons. Earth's magnetic field senses the solar wind its speed, density, and magnetic field. Because the solar wind varies over time scales as short as seconds, the interface that separates interplanetary space from the magnetosphere is very dynamic. Normally this interface called the magnetopause lies at a distance equivalent to about 10 Earth radii in the direction of the Sun. However, during episodes of elevated solar wind density or velocity, the magnetopause can be pushed inward to within 6.6 Earth radii (the altitude of geosynchronous satellites). As the magnetosphere extracts energy from the solar wind, internal processes produce geomagnetic storms.

http://www.sec.noaa.gov/primer/primer.html

[lunator]

Jamie

Good description.

I have 1 point to raise though.

The Sun is beleived to be 4500 Million years old (4.5 Billion using US figures)

It is expected to 'burn' Hydrogen for another 4500 million years before under going a 'helium flash' to initiate helium fusion and enter the Red Giant phase which is expected to last about 100 million years.

Cheers

Ian

[Ant]

It has long been known that the Sun is neither featureless nor steady. (Theophrastus first identified sunspots in the year 325 B.C.) Some of the more important solar features are explained in the following sections.

I had no idea that Baader solar filters had been available for that long... How the hell did they find that out? Very few sunspots are big enough to be seen with the naked eye! You can get them (i've seen two myself), but how on Earth would you find out without optical aid or some form of filter?

Over the last 300 years, the average number of sunspots has regularly waxed and waned in an 11-year sunspot cycle. The Sun, like Earth, has its seasons but its year equals 11 of ours.

I read recently (may even have been on SGL) that scientists are learning a fair bit about the sun's 11 year cycle - they think that the next maximum is going to be one or two years late but it is going to be an absolute corker...

So I for one can;t wait to see the Aurora again!

Ant

[OXO]

I had no idea that Baader solar filters had been available for that long... How the hell did they find that out? Very few sunspots are big enough to be seen with the naked eye! You can get them (i've seen two myself), but how on Earth would you find out without optical aid or some form of filter?

Reflected light from a pool of water or something like it would enable you to see the larger sunspots in those times Ant from what i have read also the use of Fabrics too. This was something from school i read and as such i have no clue as to what the book was called hehe.. and also if the book was any good so take it with a pinch of salt.

James

[Ant]

I'm sure that the light from the sun reflected in water would be too bright!

But you may have something with the fabric idea though (Fabric has lotsd of little holes) - when I was stuck in the car during the eclipse, I made a very small hole between my thumb and finger and the image that appeared on the palm of my other hand (my god, driving while projecting the eclipse from one hand to the other, I could be banned ) would have been bright enough and big enough to see sunspots - if I hadn't been noving at the time!

Ant

Sorry Jamie, meant to say thanks for posting that - I enjoyed reading it!

[OXO]

Never work for a Taxi firm Ant :shock:

As to the water i guess if it were subdued with some source of pollution or additive it could work? muddy eclipse anyone