Friday, September 12, 2008

The Milky Way Galaxy and our SUN.

Question: How many Suns are located within the Milky Way Galaxy besides our very own beloved Sun?
Answer: The Milky Way is estimated to contain 200 billion, or up to 400 billion, suns (if small-mass stars predominate). As a guide to the
relative physical scale of the Milky Way, if it were reduced to 130 km (80 mi)
in diameter, the Solar System would be a mere 2 mm (0.08 inches) in width. TheGalactic Halo extends outward but is limited in size by the orbits of the twoMilky Way satellites, the Large and the Small Magellanic Clouds, whose
perigalacticon is at ~180,000 light-years. New discoveries indicate that the
disk extends much farther than previously thought.

The Milky Way as it appears in a night sky time exposure from a dark location.

Question: How old is our Galaxy?
Answer: It is extremely difficult to define the age at which theMilky Way formed, but the age of the oldest stars in the Galaxy is now estimated to be about 13.6 billion years, nearly as old as the Universe itself.[8]

This estimate is based on research done in 2004 by astronomers Luca Pasquini, Piercarlo Bonifacio, Sofia Randich, Daniele Galli, and Raffaele G. Gratton. The team used the UV-Visual Echelle Spectrograph of the Very Large Telescope to measure, for the first time, the beryllium content of two stars in globular cluster NGC 6397. This allowed them to deduce the elapsed time between the rise of the first generation of stars in the entire Galaxy and the first generation of stars in the cluster, at 200 million to 300 million years. By including the estimated age of the stars in the globular cluster (13.4 ± 0.8 billion years), they estimated the age of the oldest stars in the Milky Way at 13.6 ± 0.8billion years.

Question: How old is our Sun?
Answer: The Sun's current main sequence age, determined using computer models of stellar evolution and nucleocosmochronology, is thought to be about 4.57 billion years.

The Sun is about halfway through its main-sequence evolution, during which nuclear fusion reactions in its core fuse hydrogen into helium. Each second, more than 4 million tonnes of matter are converted into energy within the Sun's core, producing neutrinos and solar radiation; at this rate, the Sun will have so far converted around 100 Earth-masses of matter into energy. The Sun will spend a total of approximately 10 billion years as a main sequence star.

The Sun does not have enough mass to explode as a supernova. Instead, in 4–5billion years, it will enter a red giant phase, its outer layers expanding as the hydrogen fuel in the core is consumed and the core contracts and heats up.Helium fusion will begin when the core temperature reaches around 100 MK, and will produce carbon and oxygen, entering the asymptotic giant branch of a planetary nebula phase in about 7.8 billion years, during which instabilities in interior temperature lead the surface of the sun to shed mass. While it is likely that the expansion of the outer layers of the Sun will reach the current position of Earth's orbit, recent research suggests that mass lost from the Sun earlier in its red giant phase will cause the Earth's orbit to move further out,preventing it from being engulfed. However, Earth's water will be boiled away and most of its atmosphere will escape into space. The increase in solar temperatures over this period is sufficient that by about 500-700 million years into the future, the surface of the Earth will become too hot for the survival of life as we know it.

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