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THE SUN

Similar in many languages, "Sun," related to Latin's "sol," wasrepresented in ancient Greece as Helios, god of the Sun. Giver ofwarmth and life, we hardly think of the Sun as a star, the term"Sun and stars" in constant use, though the surmise that it is astar goes back to ancient times. It is different because it is OURstar, the one that belongs to us, the one we can see most closely,and the one we know most about. (ThoughAT NO TIME ATTEMPT TOVIEW THE SUN, as it is so bright it can burn the eye; leavethat to professionals.) With the Sun only 150 million kilometers(93 million miles) away, astronomers can detect incredible detail. The next nearest star of similar brightness,Alpha Centauri, is 271,000 times farther, each ofits two components (it is adouble star) appearing as merepoints. The Sun is the reference to which all other stars arecompared, their diameters almost always expressed in solardiameters, their brightnesses in solar luminosities.

To understand the stars better, to make sense of their realcharacteristics, we need to know thesolarcharacteristics which, while the Sun is hardly the brightestand biggest star in the sky, are still astounding. It is 1.4million kilometers across, the equivalent of 109 Earths set side byside, and has a mass of two million trillion trillion kilograms, or330,000 Earths. Most astonishing perhaps is its luminosity of 400trillion trillion watts. To put that in perspective, it would costthe gross national product of the United States for millions ofyears for a local power company to run the Sun for one second. This immense energy, pouring from a body with a yellow-white"surface" (a highly opaque gas called thephotosphere) of5800 Kelvin, is generated by thermonuclear fusion (ofhydrogen into helium) in the Sun'sdeep core, where the temperature reaches more than 15 millionKelvin and the density hits 14 times that of lead. No matter thedensity, however, the core, like the rest of the Sun, is entirelygaseous. Taking up about half the Sun's mass and a quarter of itsradius, the core is surrounded by an inert envelope whose outerthird or so is in a state of roiling convection (hotter gases rising,cooler ones falling). The outer layers are made of 91.5 percenthydrogen, 8.5 percent helium (the element named after Helios, sincehelium was discovered there first), and a bit over a tenth of apercent of everything else, oxygen dominating these followed bycarbon, neon, and nitrogen (known from analysis of thesolar spectrum). In the heat andpressure of the core, atoms of hydrogen are slowly being convertedinto those of helium (four H into one of He in a three-stepprocess), a small amount of mass lost and converted to energy inthe process (via Einstein's famed equation E = mc**2, wherec is the velocity of light). After 4.5 billion years (asfound from the ages of meteorites), the core of the Sun is nowabout half helium, and there remains enough hydrogen to last foranother five or so billion years.

A model of the Sun shows its nuclear fusing core (where hydrogen isturned into helium, resulting in the conversion of mass intoenergy), an envelope that extends 71 percent of the way out, whereenergy is transferred by radiation, and an outer layer whereconvection (the rising of hot gases, falling of cool gases)rules. At the surface (thephotosphere, where the bulk ofthe solar spectrum is formed), rising magnetic fields locally coolthe solar gases to makesunspots. Surrounding the wholeaffair is the magnetically heated solarcorona, which isconfined by great magnetic loops. At a typical temperature of twomillion degrees Kelvin, the corona is the seat of thesolarwind.Prominences are threads of cool gas that lie inthe corona and are supported by magnetic fields. Not shown,between the corona and photosphere is the thin reddishchromosphere. (FromStars, J. B. Kaler, ScientificAmerican Library, Freeman, NY, 1992, copyright © J. B. Kaler.)

SOLAR EVOLUTION

When the core hydrogen finally runs out, the Sun will temporarilyspike in brightness by up to a thousand times its currentluminosity, expand, and cool at the surface. Under increasingcompression, the helium created earlier within the nuclear furnacewill begin to fuse to carbon and oxygen, causing the future Sun todim back some to become a modestred giant star like so many ofthose that populate the naked-eye sky. When the helium is gone,the Sun will brighten even more, to some 5000 times its presentluminosity, expand to nearly the size of Earth's orbit, and becomeeven cooler and redder. Varying in brightness as anadvanced giant (rather like thestarMira), it will slough off its outerhydrogen layers, exposing the core. The core in turn willilluminate the expanding debris to briefly create aplanetary nebula (a misnomer having nothing todo with planets), and will then die and cool as an ultradense,dimming carbon-oxygenwhitedwarf about the size of Earth with somewhat over half the Sun'scurrent mass (showing that stars finish their lives with alotless mass than they start out with).

SOLAR ACTIVITY

The Sun spins slowly with a period of 25 days at its equator, thespin and churning outer gases producing a magnetic field about fivetimes the strength of Earth's. The rotation wraps the internalfield into powerful ropes that rise upward to break through thesurface, where they chill local areas by inhibiting convection, andthus create the famedsunspots. Sunspots come in pairs, onewhere the field goes out of the solar surface, the other where itre-enters. Numerous spots commonly gather into packs withincenters of activity, the tangled fields making it difficult,even impossible, to see which ones belong together. The magneticfields are unstable, so the individual spots do not last long, justdays to perhaps a month. The magnetism heats a tenuous outerlayer, thecorona, to around two million Kelvin. Thecorona's thinness makes it dim and visible to us only during atotal eclipse (when the bright surface is blotted out by the Moon)or from space. Controlled by magnetism and luminosity, from theopaque hot corona flows a thin but fastwind that blastspast the Earth and makes comet tails point away from the Sun. Collapsing solar magnetic fields produce localized powerful flaresand release coronal gases that fly down the solar wind. If one ofthesecoronal mass ejections hits Earth, it can massivelydisturb its magnetic field to produce the northern and southernlights (theaurora), can wrecksatellite systems, and have even been know to bring down powergrids on the ground. Solar magnetic activity, seen most vividly inthe number of sunspots, is cyclic, coming and going over an averageof 11 years. At minimum, there may be no spots at all, while atmaximum the Sun can be covered with them. Many of the samephenomena are seen in the stars around us, the Sun providing a wayto understand them, the stars in turn allowing us better tounderstand the Sun, our own personal star.

TABLES

The following two tables give a summary of solar propertiesand a list of stars similar to the Sun, both taken fromStars and their Spectra (J.B. Kaler, Cambridge University Press, 2011). A primary source forthe first table isAllen's Astrophysical Quantities, 4/ed,AIP Press/Springer, 1999. Exponents are expressed by "**." Thedata in the second table may differ some from those in theindividual stellar essays and in the table ofBrightest Stars. Distances are inparsecs; multiply by 3.26 toget light-years. Where they are given in the Remarks, ages are in"Gyr," billions of years.

SOLAR PROPERTIES
Property	Physical Units	Relative to Earth/Remarks
Mean distance	149,597,871 km	1 AU; 389 X distance to Moon
Diameter	1.391 X 10**6 km	109.1 (equatorial)
Mass	1.989 X 10**33 grams	3.329 X 10**5
Non-core composition	91.5% H, 8.5% He, 0.15% other	Nearly 100% "other"
Average density	1.407 gm/cubic cm	0.2551
Surface gravity	2.740 X 104 cm/sec2	27.94
Escape velocity	617.7 km/s	55.2
Polar magnetic field	...	A few times
Age	4.6 billion years	1.00
Luminosity	3.845 X 10**26 watts	1367 watts/square meter at Earth
Spectral class	G2 V	...
Apparent visual magnitude	-26.75	420,000 times that offullMoon
Absolute visual magnitude	4.83	...
Color index (B-V)	0.65	...
Color index (U-B)	0.19	...
Bolometric correction	-0.08 magnitudes	Visual-to-total magnitude
Effective temperature	5777 Kelvin	23
Equatorial rotation period	25.1 days	27.1 days rel. to Earth
Depth of convection zone	0.29 solar radius	...
Main oscillation period	5 minutes	...
Central temperature	15.7 million Kelvin	2200
Central density	162 gm/cubic centimeter	12; 14.3 X lead
Central pressure	2.5 X 10**11 Earth atmospheres	...
Solar wind rate	5 X 10**-14 Msun/yr	Speed at Earth, 200-700 km/s
Sunspot cycle	11 years	...
Coronal temperature	2 million Kelvin	...

OTHER SUNS
Star	Class	Vis mag	Dist (pc)	Abs mag	Temp (K)	Lum (Suns)	Mass (Suns)	Rotation (days)	Age/Remarks
Sun	G2 V	-26.75	...	4.83	5777	1.00	1.00	25.1	...
53 Aqr A	G1 V	6.35	20	4.75	5830 K	1.07	1.0	<5.4	...
53 Aqr B	G2 V	6.57	20	4.85	5790	0.99	1.0	<6.3	Binary mass 2.25 Msun
Alpha Cen A	G2 V	-0.01	1.3	4.34	5780	1.57	1.10	25	Except for Proxima, nearest star
9 Cet	G2 V	6.39	20.9	4.79	5760	1.04	1.0	7.8	5.5 Gyr, metal-rich
Rho CrB	G2 V	5.41	17.2	4.16	5822	1.72	1.0	...	10 Gyr;planet
16 Cyg A	G1.5 V	5.96	21	4.33	5750	1.58	1.0	<32	8 Gyr
16 Cyg B	G2.5 V	6.20	21	4.57	5770 K	1.27	1.01	<19	Planet
18 Sco	G2 V	5.50	13.9	4.78	5789	1.05	1.01	23	Mag cycle 9-13 yr
Zeta-2 Ret	G2 V	5.24	12.1	4.84	5795	0.98	0.96	...	Zeta-1 G3-5, mass 0.9

Written byJim Kaler 4/23/99; last revised7/12/11. Return toSTARS.

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