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    Categories: Stars

Star Facts: the Sun (Sol)

The Sun really does not need an introduction, except to say that it is not yellow, but pure white, and that it is not an average star. In fact, there are no average stars, as all stars are unique. However, and not to put too fine a point on it, the Sun is at best similar to other G-type main sequence stars in regards that it consists of a core that is overlaid by several layers. As with other G-type stars, the layers are reasonably well differentiated, and all layers contribute in transporting and emitting the light and energy that is produced in the star’s core.

While a full description of the Sun would take up several volumes, suffice it to say that our nearest star is unique in terms of its size, mass, composition, and as far as we know, its retinue of planets. At its current state of evolution, the Sun consists mainly of hydrogen and helium with a smattering of heavier elements in the ratio of 1,000,000 molecules of hydrogen for every:

• 98,000 molecules of helium
• 850 molecules of oxygen
• 360 molecules of carbon
• 120 molecules of neon
• 110 molecules of nitrogen
• 40 molecules of magnesium
• 35 molecules each of iron and silicon

In these ratios, hydrogen accounts for about 73% of the Sun’s total mass.

Quick Facts

• Constellation: Only star in the Universe that does not belong to any particular constellation
• Distance from Galactic Core: 27,200 light years
• Distance to Earth: 150 million km (Average)
• Star Type: G2V
• Mass: 1.98855 ± 0.00025×1030 kg ( 330,000 Earth masses)
• Radius: 695,700 km (Equatorial)
• Apparent Magnitude: -26.74
• Absolute Magnitude: 4.83
• Surface Temperature: 5,778K
• Rotational Velocity: 7.189×103 km/h
• Motion in Space:
≈ 220 km/s (orbital speed around the centre of the Milky Way)
≈ 20 km/s (relative to average velocity of other stars in stellar the neighborhood)
≈ 370 km/s (relative to the cosmic microwave background radiation)
• Age: 4.6 billion years

Physical Properties

The Sun is a G-type main sequence star that contains roughly 99.86% of all the mass in the solar system, and with an absolute magnitude of +4.83, it outshines at least 85% of all other stars in the Milky Way galaxy. As seen from Earth, the Sun has an apparent visual magnitude of -26.74, which is about 13 billion times as bright as Sirius (-1.46) as seen from Earth.

Origin

In terms of its chemical composition, the Sun is a middle-aged, metal-rich Population 1 star that formed out of the remains of a previous generation of stars. While there is some uncertainty about the actual mechanism that caused the progenitor cloud to collapse to form the Sun, the abundances of extremely heavy metals (relative to Population II stars) such as gold and uranium that is present in the solar system suggests two possible scenarios. One possibility is that these heavy elements formed in endothermic nuclear reactions during a nearby supernova event (which could have contributed to the collapse of the progenitor cloud), while another was a process of transmutation through neutron absorption in an extremely massive Population II star.

Color

While the Sun appears to be yellow when viewed from Earth, it is in fact pure white when it is viewed from outside of Earth’s atmosphere, or when it is at or near the zenith. While the exact reasons why most people perceive the Sun as yellow remain uncertain, the fact is that the Sun emits more light in the green part of the visible spectrum that any other visible color. By rights then, the Sun should appear green to our eyes, but the green light is absorbed and/or scattered by other light frequencies both in the Sun itself, and in Earth’s atmosphere.

Image Credit: NASA

Size

Like almost all other stars, the Sun does not have a definitive boundary. Its density decreases exponentially with increasing altitude above the photosphere, but for the purposes of measuring its diameter, its radius is taken to be the distance from its centre to the visible edge of its photosphere. By this standard, the Sun is an almost perfect sphere; its oblateness comes to around nine millionths, which means that the Sun’s equatorial circumference is only about 10 km (6.2 miles) larger than its polar circumference. Note that the gravitational effects of the planets have almost no discernible influence on the shape of the Sun.

Energy and Light

While the Sun’s light and energy supports (almost) all life on Earth and drives the planets’ weather and climate, and has done so for the past 3 billion years or so, the Sun’s energy output has been steadily increasing as it burns through its supply of hydrogen fuel. The Sun’s current energy output, aka the solar constant, is about 1,368 W/m2 (watts per square meter) at a distance of 1 astronomical unit. At the Sun’s current energy output, sunlight consists of about 50% infrared light, 40% visible light, and 10% ultraviolet light, with Earth’s atmosphere filtering out about 70% of the UV component’s shorter wavelengths.

Temperature

While the Sun is expected to burn hydrogen for at least another 4 billion years or so, its temperature is increasing at the rate of about 10 degrees Celsius per billion years. In practice, this means that a ten-degree increase over current temperatures will cause Earth’s water to boil off long before the next billion years have passed. In fact, the next few hundred million years will see Earth move out of the Sun’s habitable zone and by the time the Sun stops burning hydrogen in its core the planet Mars will be in the habitable zone, and Earth will be devoid of life. Note that by the time the Sun has fully evolved from a yellow dwaf into a red giant, Earth will likely have been engulfed by the Sun’s outer layers.

Visibility

While the Sun is the brightest object in the sky when seen from Earth, it will just be another (maybe bright) star when viewed from elsewhere, as can be seen in the image above. For instance, to observers in the Alpha Centauri system, which is only about 4.5 light years away, the Sun will be a part of the “W” in the constellation Cassiopeia, turning the “W” into an extended zigzag. Note the location of the Sun relative to the “W” in the top right hand corner of the image.

Image: Stellarium

History

The English proper name “Sun” is taken to have developed from the Old English word “sunne”, which may be related to “south”, where “south” relates to a cardinal compass direction. However, cognates to the English “Sun” also appear in several other Germanic languages, such as “sunne or sonne” in Old Frisian, “sonne” in Old Saxon, “sonne” in Middle Dutch, and “ sunno” in Gothic.

The Latin name for the Sun, “Sol”, is not in widespread use except among fiction writers and planetary scientists, with the latter group using the word “sol” to refer to a Martian day, which is 24 hours, 39 minutes, and 35.244 seconds long.

Peter Christoforou :

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