THE SUN

EVOLUTION ACTIVITY PROPERTIES OTHER SUNS

Similar in many languages, "Sun," related to Latin's "sol," was represented in ancient Greece as Helios, god of the Sun. Giver of warmth 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 a star goes back to ancient times. It is different because it is OUR star, the one that belongs to us, the one we can see most closely, and the one we know most about. (Though AT NO TIME ATTEMPT TO VIEW THE SUN, as it is so bright it can burn the eye; leave that 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 of its two components (it is a double star) appearing as mere points. The Sun is the reference to which all other stars are compared, their diameters almost always expressed in solar diameters, their brightnesses in solar luminosities.

To understand the stars better, to make sense of their real characteristics, we need to know the solar characteristics which, while the Sun is hardly the brightest and biggest star in the sky, are still astounding. It is 1.4 million kilometers across, the equivalent of 109 Earths set side by side, and has a mass of two million trillion trillion kilograms, or 330,000 Earths. Most astonishing perhaps is its luminosity of 400 trillion trillion watts. To put that in perspective, it would cost the gross national product of the United States for millions of years 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 the photosphere) of 5800 Kelvin, is generated by thermonuclear fusion (of hydrogen into helium) in the Sun's deep core, where the temperature reaches more than 15 million Kelvin and the density hits 14 times that of lead. No matter the density, however, the core, like the rest of the Sun, is entirely gaseous. Taking up about half the Sun's mass and a quarter of its radius, the core is surrounded by an inert envelope whose outer third or so is in a state of roiling convection (hotter gases rising, cooler ones falling). The outer layers are made of 91.5 percent hydrogen, 8.5 percent helium (the element named after Helios, since helium was discovered there first), and a bit over a tenth of a percent of everything else, oxygen dominating these followed by carbon, neon, and nitrogen (known from analysis of the solar spectrum). In the heat and pressure of the core, atoms of hydrogen are slowly being converted into those of helium (four H into one of He in a three-step process), a small amount of mass lost and converted to energy in the process (via Einstein's famed equation E = mc**2, where c is the velocity of light). After 4.5 billion years (as found from the ages of meteorites), the core of the Sun is now about half helium, and there remains enough hydrogen to last for another five or so billion years.

model A model of the Sun shows its nuclear fusing core (where hydrogen is turned into helium, resulting in the conversion of mass into energy), an envelope that extends 71 percent of the way out, where energy is transferred by radiation, and an outer layer where convection (the rising of hot gases, falling of cool gases) rules. At the surface (the photosphere, where the bulk of the solar spectrum is formed), rising magnetic fields locally cool the solar gases to make sunspots. Surrounding the whole affair is the magnetically heated solar corona, which is confined by great magnetic loops. At a typical temperature of two million degrees Kelvin, the corona is the seat of the solar wind. Prominences are threads of cool gas that lie in the corona and are supported by magnetic fields. Not shown, between the corona and photosphere is the thin reddish chromosphere. (From Stars, J. B. Kaler, Scientific American Library, Freeman, NY, 1992, copyright © J. B. Kaler.)

SOLAR EVOLUTION

When the core hydrogen finally runs out, the Sun will temporarily spike in brightness by up to a thousand times its current luminosity, expand, and cool at the surface. Under increasing compression, the helium created earlier within the nuclear furnace will begin to fuse to carbon and oxygen, causing the future Sun to dim back some to become a modest red giant star like so many of those that populate the naked-eye sky. When the helium is gone, the Sun will brighten even more, to some 5000 times its present luminosity, expand to nearly the size of Earth's orbit, and become even cooler and redder. Varying in brightness as an advanced giant (rather like the star Mira), it will slough off its outer hydrogen layers, exposing the core. The core in turn will illuminate the expanding debris to briefly create a planetary nebula (a misnomer having nothing to do with planets), and will then die and cool as an ultradense, dimming carbon-oxygen white dwarf about the size of Earth with somewhat over half the Sun's current mass (showing that stars finish their lives with a lotless mass than they start out with).

SOLAR ACTIVITY

The Sun spins slowly with a period of 25 days at its equator, the spin and churning outer gases producing a magnetic field about five times the strength of Earth's. The rotation wraps the internal field into powerful ropes that rise upward to break through the surface, where they chill local areas by inhibiting convection, and thus create the famed sunspots. Sunspots come in pairs, one where the field goes out of the solar surface, the other where it re-enters. Numerous spots commonly gather into packs within centers of activity, the tangled fields making it difficult, even impossible, to see which ones belong together. The magnetic fields are unstable, so the individual spots do not last long, just days to perhaps a month. The magnetism heats a tenuous outer layer, the corona, to around two million Kelvin. The corona's thinness makes it dim and visible to us only during a total eclipse (when the bright surface is blotted out by the Moon) or from space. Controlled by magnetism and luminosity, from the opaque hot corona flows a thin but fast wind that blasts past the Earth and makes comet tails point away from the Sun. Collapsing solar magnetic fields produce localized powerful flares and release coronal gases that fly down the solar wind. If one of these coronal mass ejections hits Earth, it can massively disturb its magnetic field to produce the northern and southern lights (the aurora), can wreck satellite systems, and have even been know to bring down power grids on the ground. Solar magnetic activity, seen most vividly in the number of sunspots, is cyclic, coming and going over an average of 11 years. At minimum, there may be no spots at all, while at maximum the Sun can be covered with them. Many of the same phenomena are seen in the stars around us, the Sun providing a way to understand them, the stars in turn allowing us better to understand the Sun, our own personal star.

TABLES

The following two tables give a summary of solar properties and a list of stars similar to the Sun, both taken from Stars and their Spectra (J. B. Kaler, Cambridge University Press, 2011). A primary source for the first table is Allen's Astrophysical Quantities, 4/ed, AIP Press/Springer, 1999. Exponents are expressed by "**." The data in the second table may differ some from those in the individual stellar essays and in the table of Brightest Stars. Distances are in parsecs; multiply by 3.26 to get 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 10**4 cm/sec**2 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 of full Moon
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 by Jim Kaler 4/23/99; last revised 7/12/11. Return to STARS.