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The Earth may not be engulfed by the expanding fireball of the dying Sun, which has long been assumed to be our home planet’s ultimate fate, according to scientists. Don’t worry: this is not expected to happen for another five billion years, long after all life on Earth has been wiped out. When the Sun burns through all of the hydrogen in its core, it will go through two immense expansion phases: first becoming a red giant, then, when its helium is spent, an “AGB” star……..Continue reading….
Source: Malay Mail
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Within 10 light-years of the Sun there are relatively few stars, the closest being the triple star system Alpha Centauri, which is about 4.4 light-years away and may be in the Local Bubble’s G-Cloud. Alpha Centauri A and B are a closely tied pair of Sun-like stars, whereas the closest star to the Sun, the small red dwarf Proxima Centauri, orbits the pair at a distance of 0.2 light-years. In 2016, a potentially habitable exoplanet was found to be orbiting Proxima Centauri, called Proxima Centauri b, the closest confirmed exoplanet to the Sun.
The Solar System is surrounded by the Local Interstellar Cloud, although it is not clear if it is embedded in the Local Interstellar Cloud or if it lies just outside the cloud’s edge. Multiple other interstellar clouds exist in the region within 300 light-years of the Sun, known as the Local Bubble. The latter feature is an hourglass-shaped cavity or superbubble in the interstellar medium roughly 300 light-years across. The bubble is suffused with high-temperature plasma, suggesting that it may be the product of several recent supernovae.
The Local Bubble is a small superbubble compared to the neighboring wider Radcliffe Wave and Split linear structures (formerly Gould Belt), each of which are some thousands of light-years in length. All these structures are part of the Orion Arm, which contains most of the stars in the Milky Way that are visible to the unaided eye.
The radiative zone is the thickest layer of the Sun. It starts above the core at about 0.25 solar radii and out to about 0.7 solar radii. The zone is so named because thermal radiation is the primary means of energy transfer: photons scatter from dense gas so often that they take a million years to cross this zone. The temperature drops from approximately 7 million to 2 million kelvins with increasing distance from the core.
This temperature gradient is less than the value of the adiabatic lapse rate and hence cannot drive convection, which explains why the transfer of energy through this zone is by radiation instead of thermal convection. The density drops a hundredfold (from 20,000 kg/m3 to 200 kg/m3) between 0.25 solar radii and 0.7 radii, the top of the radiative zone.
The radiative zone and the convective zone are separated by a transition layer, the tachocline. This is a region where the sharp regime change between the uniform rotation of the radiative zone and the differential rotation of the convection zone results in a large shear between the two—a condition where successive horizontal layers slide past one another. Presently, it is hypothesised that a magnetic dynamo, or solar dynamo, within this layer generates the Sun’s magnetic field.
Energy from the Sun supports life on Earth by photosynthesis, allows vision in animals, and drives Earth’s climate and weather. The Sun is by far the brightest object in the Earth’s sky, with an apparent magnitude of −26.74. This is just less than 13 billion times brighter than the next brightest star, Sirius, which has an apparent magnitude of −1.46.
The solar constant is the amount of power that the Sun deposits per unit area that is directly exposed to sunlight. The solar constant is equal to approximately 1,368 W/m2 (watts per square metre) at a distance of one astronomical unit (au) from the Sun (that is, at or near Earth’s orbit). Sunlight on the surface of Earth is attenuated by Earth’s atmosphere, so that less power arrives at the surface, closer to 1,000 W/m2 (in clear conditions when the Sun is near the zenith).
Sunlight at the top of Earth’s atmosphere is composed (by total energy) of about 50% infrared light, 40% visible light, and 10% ultraviolet light. The atmosphere filters out over 70% of solar ultraviolet, especially at the shorter wavelengths. The Sun’s colour is white, with a CIE colour-space index near (0.3, 0.3), when viewed from space or when the Sun is high in the sky. The Solar radiance per wavelength peaks in the green portion of the spectrum when viewed from space.
When the Sun is very low in the sky, atmospheric scattering renders the Sun yellow, red, orange, or magenta, and in rare occasions even green or blue. Some cultures mentally picture the Sun as yellow and some even red; the cultural reasons for this are debated. An optical phenomenon, known as a green flash, can sometimes be seen shortly after sunset or before sunrise. The flash is caused by light from the Sun just below the horizon being bent (usually through a temperature inversion) towards the observer.
Light of shorter wavelengths (violet, blue, green) is bent more than that of longer wavelengths (yellow, orange, red) but the violet and blue light is scattered more, leaving light that is perceived as green. The brightness of the Sun can cause pain from looking at it with the naked eye; however, doing so for brief periods is not hazardous for normal non-dilated eyes. Looking directly at the Sun, known as sungazing, causes phosphene visual artefacts and temporary partial blindness.
It also delivers about 4 milliwatts of sunlight to the retina, slightly heating it and potentially causing damage in eyes that cannot respond properly to the brightness. Viewing of the direct Sun with the naked eye can cause UV-induced, sunburn-like lesions on the retina beginning after about 100 seconds, particularly under conditions where the UV light is intense and focused.
Viewing the Sun through light-concentrating optics such as binoculars may result in permanent damage to the retina without an appropriate filter. Some improvised filters that pass UV or IR rays can harm the eye at high brightness levels. Brief glances at the midday Sun through an unfiltered telescope can cause permanent damage.
During sunrise and sunset, sunlight is attenuated because of Rayleigh scattering and Mie scattering from a particularly long passage through Earth’s atmosphere, and the Sun is sometimes faint enough to be viewed comfortably with the naked eye or safely with optics (provided there is no risk of bright sunlight suddenly appearing through a break between clouds). Hazy conditions, atmospheric dust, and high humidity contribute to this atmospheric attenuation.
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