This is the inaugural entry in what promises to be a very long, fun, and educational series (at least for me), where I examine individual objects, systems of objects, or classes of objects in the solar system. It is the environment in which our planet evolved, the backdrop for humanity's future development, and exquisitely beautiful, diverse, and exotic besides. We begin the series with our star, known typically as The Sun but occasionally personalized as Sol (hence, "solar").
However, it was not always part of the Orion Spur, and its current association with it is not permanent. Stars in a spiral galaxy like the Milky Way orbit the galactic core more or less independently of each other, so arm structure and its stellar membership are transient. But barring a radically close encounter with another star under conditions that would slingshot it into another galactic orbit, the Sun's path will stay roughly where it is relative to the center of the galaxy. On the other hand, the probability of such an event increases substantially in 3-5 billion years if the theorized collision of Andromeda and the Milky Way occurs.
An interesting consequence of our Sun's position in the galactic disk is that we can't see what's on the other side - the density of gas and stars in the core blocks our view, so typical representations of that side are merely an extrapolation. It will likely be a very long time before we have specific data about the other side - with current remote sensing technology, we would have to send a probe thousands of light-years out of the galactic plane to see over the core, which is unlikely to ever happen. But there are optical tricks (e.g., gravitational microlensing) whose long-term derivatives may some day permit a view of the other side without having a direct line of sight.
This is what the rest of the galaxy looks like from the location of the Sun, i.e. Earth (click for attribution):
It would look identical from anywhere else in the solar system, given how small our solar system's diameter is compared to the size of the galaxy.
Our solar system was born in a gas cloud stellar nursery similar to the Orion Nebula, and thought to have been just one of several thousand other star systems born there. Based on observed abundances of certain metallic elements associated with supernovas, it is thought that a nearby supernova several billion years ago sent a shockwave through our cloud and caused a cascade of local star formation to occur. Below is the Eagle Nebula - another stellar nursery - followed by a close-up of star formation within the Orion Nebula.
Originally, what was created was not exactly a star (although it's called a T Tauri star) - the local diffuse, nebular cloud collapses into a denser, hot "fog" the size of an entire solar system whose temperature does not vary greatly from its center to its edge. The reason for this is that the collapse itself - i.e., the gas molecules gently jostling with each other in moving toward the center of mass - is the source of the heat, and at first the center is not especially crowded. However, as the collapse proceeds, the temperature of the core increases more rapidly than that of more distant areas, and incident angular momentum in the surrounding gas causes the system to begin spinning. As a result, the material outside the star-forming core becomes compressed into a protoplanetary disk.
Eventually the core temperature was high enough to fuse hydrogen nuclei, and at that point the Sun entered the Main Sequence - the region of active stability in a plot of certain stellar characteristics. Rather than continue to infall, much of the remaining gas in the protoplanetary disk was now being blown outward by the solar wind, causing some of it to slow down in its orbit and collide more regularly with other matter, sending a hefty proportion of the gas out in random directions that would serve to progressively reduce the density of the disk. The Hertzprung-Russel diagram charts various properties of stars and illustrates the populations in which they occur:
Gradually, temperatures in the disk declined as much of the excess material blew away completely, and an important threshold swept inward from the edges: The precipitation point of metallic elements. Once the environment was cool enough, atoms of gaseous metal began sticking to each other and forming droplets - a kind of iron and silicon "rain" - and then froze into solid dust which accreted further into larger solid bodies. Because the temperature declined from the outside in, the gas giant planets were first to form, although not in the order of their orbital arrangement: Due to the relative abundance of material in its orbit, and the much smaller size of the space the orbit passes through, Jupiter (for instance) was able to form more quickly than its brethren.
As the precipitation and then freezing points of water and methane were surpassed, the cores of these planets rapidly built thick envelopes of ices and achieved sufficient mass to begin sucking up the remaining hydrogen and helium in their vicinity. The inner planets, by contrast, were never cool enough - and still aren't - to have accreted atmospheres from surrounding gas: Earth's atmosphere, for instance, came from inside its own rocks, due to volcanic venting, and once it ceases to be tectonically active the atmosphere will gradually blow away in the solar wind.
Stars are mainly classified by their spectral type and luminosity (brightness) - categories related to temperature, mass, and radius, and in the former case derived from absorption lines observed in their spectra. Stellar classifications are typically denoted by a letter and a number indicating spectral type followed by a Roman numeral indicating luminosity.
The spectral part of the classification contains one of the letters O, B, A, F, G, K, or M in order of decreasing temperature / darker color (violet is hottest, red coolest), then a single-digit number indicating tenths of the way toward the next coolest letter (e.g., K5 is cooler than K3). Roman numerals denoting the luminosity likewise decrease in magnitude as the number value increases, with I being brightest and V being dimmest. Our Sun is G2V - a yellow dwarf (even though its color is white).
It should be noted that the luminosity classifications are not based on statistical representation - i.e., the Sun is much brighter than the vast majority of stars, as the population is overwhelmingly dominated by red dwarfs. But the very brightest stars are so much brighter that most other stars are crowded together in the lowest luminosity category (astronomers have finer gradations to distinguish between them). The Sun is likewise hotter, bigger, and more massive than the vast majority of others, but the hottest, biggest, and most massive of all exceed it by such tremendous margins that it is useful to distinguish between dwarfs and giants.
Here are some size comparisons of the Sun with its five largest planets (the dot on the right being Earth), and then with other stars:
Although the Sun contains only a negligible proportion of the galactic mass (there are hundreds of billions of other stars in the galaxy, a supermassive black hole at its center, and plenty of gas and dust besides), it accounts for 99.86% of the mass in its own solar system. It orbits the galactic black hole with a period of about 240 million years, and rotates around its own center of mass roughly every 25 Earth days. If you were to begin from its "surface" (the photosphere), you would have to attain a relative speed of well over 2 million kilometers per hour to completely escape its gravity - although significantly less to reach an object like Earth that remains bound to the Sun.
At a certain depth within any active star, the temperature of the plasma is high enough to drive collisions of nuclei with sufficient energy to overcome repulsive forces and cause fusion - a process known as nucleosynthesis. From this depth inward is the stellar core - the environment dominated by fusion processes - and in our Sun's case the core extends outward from the center to about one-fifth to one-quarter of its radius. Presently hydrogen continues to dominate the composition of the core, but over time helium (the nucleonic output of hydrogen fusion) and progressively heavier elements will do so. A rendering of the solar interior via the University of Northern Colorado:
Beyond the core is the Radiative Zone, where primarily non-fusing nuclei are packed so densely they transfer energy from the core outward largely by radiation rather than motion. In other words, the matter is unable to move fast enough to deal with the amount of energy it receives, so instead it just releases it and passes it along like a relay system - analogous in some ways to what happens when you heat metal. This region extends out to about 70% of the solar radius, followed by the Convective Zone - the region where matter is free enough to move that the heat from beneath drives convective cycles to escape, much like air currents in the terrestrial atmosphere. Hotter gas cycles upward toward higher currents, releases its energy, then cycles back down toward the border of the radiative layer, and eventually it reaches currents that extend into the photosphere. At that point, the energy is released as light and high-energy particles.
Above the photosphere is a region about 2,000 km thick that is cool enough to form simple molecules, which causes absorption lines in the spectra of light emitted from below - for this reason, this layer is called the chromosphere. It is this layer in every star which makes spectral classification possible, and gives rise to the information used in determining so many stellar properties. Beyond the chromosphere, gases become much more diffuse and dim, yet radically increase in temperature as a result - they are once again ionized, and move in thin streamers that follow the contours of the solar magnetic field or else escape into space as the solar wind. This is the corona. Examples of coronal loops and the filamentary structure of the Sun's corona:
Beyond the corona is the heliosphere, which encompasses all of the planets and extends well into the Kuiper Belt. It is the volume of the solar system through which the solar wind flows unimpeded before encountering the termination shock - the point at which the particles radically slow down due to the pressure of incoming gases from the interstellar medium - followed by the heliopause, where the competing pressures are equal, and the bow shock, where interstellar gases moving in the opposite direction begin to slow down. The definition of interstellar space is usually set either at the heliopause or the bow shock, although there are thought to be a multitude of long-period comets (the Oort Cloud) orbiting the Sun beyond both limits.
The Voyager and Pioneer probes (comprising a total of four unmanned spacecraft) are all either approaching, past, or within the boundary regions of the heliosheath, so within a few years we are expected to have the first direct data on the interstellar medium. A brief NASA report on some of the probes' findings from the environs of the heliosheath, particularly as it concerns the shape of the extended solar magnetic field:
To summarize the above report, the boundary where the Sun's magnetic field changes polarity from North to South moves in ripples that increase in frequency as it extends outward, creating the heliospheric current sheet (HCS). At the heliosheath, the HCS becomes frothy and chaotic, forming magnetic "bubbles" rather than a distinct surface. Cosmic rays move randomly through these bubbles and then follow the contours of the HCS inward. A visualization of the HCS:
As the Sun is comprised of gas and plasma, its rotation is not consistent across latitudes - material at the equator
orbits revolves more rapidly than that near the poles, helping drive convection and creating the dynamo that generates the solar magnetic field. However, the same process causes the lines of the magnetic field to become increasingly twisted over time until they are briefly chaotic and then "reset" about every 11 terrestrial years. The following clip illustrates the process in action:
The movements and contortions of the magnetic field are responsible for a number of stellar phenomena, including the coronal loops and 11-year cycles already mentioned. Additional phenomena caused by the field include sunspots, solar flares, and coronal mass ejections (CMEs). The following images and videos depict solar behavior, surface characteristics, and dynamic structure:
Here's one that shows a sungrazing comet followed by a CME:
In essence, what these dramatic events and features show are suppression and then sudden release of built-up heat. Sunspots, for instance, are cool regions that occur not because there is less heat beneath them, but because the convective currents beneath them are being obstructed due to random confluences of the magnetic field. The region of the obstruction will become significantly hotter than its surroundings, and once the field ceases to obstruct, that additional energy will explode outward. Large events of this kind can send tsunamis speeding across the surface of the Sun at upwards of 700,000 km/h.
But in the absence of anything dramatic, we see that the surface of the Sun takes on a granular texture. These "grains" are more or less stable convective cells that efficiently move hot plasma up to the surface, emit energy, and then resubmerge in relatively smooth currents. Here is a simulation of solar convection:
V. Past Relevance to Humanity
Our star has been of the utmost importance to life on Earth since the evolution of photosynthesis, which was the first time that organisms could survive without the heat of geological activity to drive its chemistry. The Sun freed our microbial ancestors from the seafloor vents, geothermal pools, and geysers by driving the evolution of plant life and thereby the animal life that would feed on it. Its magnetic field has also served as a first line of defense against lethal cosmic rays, even to this day, leaving less to chance on the strength of the Earth's planetary magnetic field. Its daily rising and setting have been fundamental in shaping the diurnal rhythms of animal life, and seasonal variations in its apparent strength due to the Earth's axial tilt are responsible for countless physiological adaptations among both plants and animals.
With the coming of agriculture, the Sun came to inspire innumerable myths and religious rituals in many civilizations throughout the history of mankind. The Egyptian sun-god Ra dominated the religion of that society in its various individual and dual incarnations (e.g., Amun-Ra). Later, pharaoh Akhenaten (with his famous queen Nefertiti) would form an exclusive, monotheistic religion around Sun-worship and unsuccessfully attempt to suppress the other gods. In Aztec society, elaborate rituals including mass-human-sacrifice were undertaken to ensure that the Sun would continue to rise. But throughout history, solar eclipses were often regarded as omens (usually negative) in most pre-scientific civilizations.
It is entirely possible - though likely unprovable - that evolving in a single-star system may have lent some kind of physiological predisposition to monotheism, once it became socially adaptive. Could it be that a theology like Zoroastrianism with its dual godhead would have found enduring purchase in a civilization that developed in a binary star system? I realize this is speculation, but I think it would be difficult to overstate the profound influences of the Sun in every aspect of evolution - including human psychology.
Transitioning into the scientific era, the Maunder Minimum - a period of extremely low sunspot activity from 1645 to 1715 - was correlated with (though not proven a cause of) the coldest period of the Little Ice Age. This was a period famously so cold in Northern Europe that the Thames regularly froze in winter. It may be that such periods are indicators of reduced solar output, or they may be entirely conditional nth-degree consequences of events that only sometimes result in colder periods on Earth. Nevertheless, the correlation is significant.
VI. Current Relevance to Humanity
As we come to grips with global warming and climate change, few scientific subjects are as urgently relevant as the Sun, solar energy, and how it relates to the changing atmospheric gas composition and albedo characteristics of planet Earth. The overwhelming majority of current and potential energy, both sustainable and fossil fuels, comes from the Sun: Coal and natural gas are time-compressed versions of the solar energy absorbed by plant matter over millions of years, and oil is likewise the plant-mediated solar energy eaten by dinosaurs. Wind power also comes directly from the Sun - without its energy, our atmosphere would precipitate and freeze on to the surface far more quickly than seismic activity could replace it.
As wind and the two forms of solar technology, photovoltaic and solar thermal, account for thousands of times more available energy than total world consumption today - all with zero emissions - the Sun holds a great deal of promise for humanity in avoiding climate disaster.
However, the Sun continues to present dangers in the form of the solar flares and CMEs already discussed - threats which become ever more significant the more radically human civilization comes to depend on electronics. HoundDog recently gave us a rousing warning about potential (and imminent!) dangers in his Rec Listed diary, Huge Solar Flares In Next Decade Could Cause Year Long Blackouts, and Nuclear Crises Says NOAA. Presumably efforts are being made to mitigate both the immediate and long-term danger, but it is worth monitoring.
There is, additionally, an intriguing supposition - not quite having attained the rank of theory - that the Sun's periodic migration into and out of the galactic plane may be correlated with a greater intensity of cosmic rays, more frequent close encounters with other stars, and thereby a greater occurrence of gravitational perturbations: The sort that send asteroids and comets flying on perilously random trajectories. The hypothesis is that this may have something to do with periodic mass extinctions on Earth, although it has not been rigorously supported or refuted. An exaggerated illustration of the solar system's periodic motion:
VII. Future Relevance to Humanity
I make no secret of my unabashedly utopian views of the future with respect to solar energy and human colonization of space. Specifically, I consider it a matter of inexorable destiny that humanity will become directly economically tied to the Sun, and that all future developments will occur within that context. Even with the (equally inevitable, IMHO) advent of fusion power, I do not think it will ever prove cheaper or more convenient this close to the Sun (it will be quite economical at further distances) than just passively harvesting what our star already gives us in abundance.
Provided we achieve any level of space-based solar infrastructure, it is probably inevitable that we will continue to scale and extend that infrastructure indefinitely until such improbable-sounding things as Dyson spheres and Matrioshka brains occur naturally, with no greater forethought than strings of settlements forming on a river bank merging into a contiguous civilization. Of course, even stars are finite.
VIII. Future of The Sun
As the Sun progresses through the stages of stellar evolution, its core will become dominated by increasingly heavier elements, pushing the lighter ones outward into increasingly distant shells where their fusion processes continue. This extension of fusion will cause the Sun to bloat greatly over billions of years, increasing its surface area and luminosity while decreasing surface temperature. About 5 billion years from now, its radius will have extended out to the orbit of Mars, long ago having swallowed Mercury, Venus, and Earth, and the intense solar wind from the Sun in its Red Giant state will strip Jupiter of its atmosphere,. The light from our luminous red Sun will provide a balmy environment to the organics-rich Saturnian moon Titan, and perhaps some of the large moons of Uranus and Neptune to a lesser extent. A highly explanatory video illustration of our Sun's future via the Max Planck Institute:
However, it will be continuously throwing off large amounts of mass in huge, devastating waves, and will finally throw off the vast majority of its remaining mass into a planetary nebula. The core, however, having ceased to sustain fusion, will collapse into a dense state of matter called electron-degeneracy: A state where the nuclei are compressed virtually solid (close to the limits of the Pauli Exclusion Principle), and electrons move freely among them. It shines quite brightly, but only because it has trapped an enormous amount of heat - so it is essentially just passively radiating energy generated in the past. Images of planetary nebulae:
This stage in stellar evolution is called a white dwarf star, and when the Sun has become one, it will have diminished to the size of the Earth (though retaining a stellar-sized mass). White dwarfs are beneath the mass threshold required for neutron stars, which are so dense the electrons that move freely in a white dwarf are forced into the nuclei and turn protons into neutrons (a state known as neutron degeneracy), and that in turn is beneath the mass threshold for black holes. So the "stellar corpse" of the Sun - its own brightly-shining memorial, illuminating the planetary nebula around it - will be weird, but not as weird as others will be. A white dwarf star:
IX. Catalog of Exploration
1. Major ground-based solar observatories: (locations in parentheses)
McMath-Pierce Solar Telescope (Arizona)
Richard B. Dunn Solar Telescope (New Mexico)
GREGOR solar telescope (Spain)
Big Bear Solar Observatory (California)
Swedish 1-m Solar Telescope (Spain)
Praerie View Solar Observatory (Texas)
Dutch Open Telescope (Spain)
THÉMIS Solar Telescope (Spain)
Vacuum Tower Telescope (Spain)
2. Solar space probes: (can't guarantee this list is exhaustive - many probes are multitasked)
Solar and Heliospheric Observatory (SOHO) (1996-)
Global Geospace Science WIND (2004-)
Advanced Composition Explorer (ACE) (1997-)
Solar Terrestrial Relations Observatory A & B (STEREO) (2006-)
Helios A & B (1975-1985, 1976-1979)
Solar Dynamics Observatory (2010-)
Solar Radiation and Climate Experiment (SORCE) (2003-)
Solar Maximum Mission (1980-1989)
Transition Region and Coronal Explorer (TRACE) (1998-2010)
Coronal Solar Magnetism Observatory (COSMO) (proposed, Hawaii)
European Solar Telescope (proposed, Canary Islands)
National Large Telescope (proposed, India)
Advanced Technology Solar Telescope (ATST) (approved, Hawaii)
Aditya (approved space probe, India 2012)
Solar Probe Plus (approved space probe, USA 2015)
Solar Orbiter (SOLO) (proposed space probe, Europe 2017)
Solar Sentinels (six proposed space probes, USA 2017)
Planned future entries:
5. The Moon
6. Mars + Phobos & Deimos
8. Jupiter (primary)
9. Jupiter (moons)
10. Saturn (primary)
11. Saturn (rings)
12. Saturn (Titan)
13. Saturn (other major moons)
14. Uranus system
15. Neptune system
16. Kuiper Belt
18. The stellar neighborhood
Sat Aug 13, 2011 at 6:10 AM PT: I felt that I rushed a couple of the later sections a bit, so I've added an explanatory video to the Future of The Sun section and added planned/proposed future telescopes and space probes to the Catalog of Exploration section.