Thursday, October 27, 2011
Stellar Nuclear Fusion
Hydrogen (75% of the Sun) furnishes nuclear fule for stars like the Sun. Because the core of a typical star is so violent and hot, hydrogen nuclei are separated from their electrons. in the star's core, the great pressure of overlying material forces the protons to collide so violently that the nuclei fuse together. The nuclear reactions fuse the nuclei of four hydrogen atoms into a single helium nucleus, liberating energy in the process and producing a star's light and heat. In this fashion, more than 4 million tons of the Sun's mass are converted into energy every second.
Tuesday, October 25, 2011
Proton Proton Chain Reaction
The proton–proton chain reaction is one of several nuclear fusion reactions by which stars convert hydrogen to helium. The Sun which consits mainly of Hydrogen (75%) and Helium (24%) is emiting its light energy due to fusion of protons into helium.
Proton–proton fusion can occur only if the temperature (i.e. kinetic energy) of the protons is high enough to overcome their mutual electrostatic or Coulomb repulsion.
In the Sun, deuterium-producing events are rare. The fact that the Sun is still shining is due to the slow nature of this reaction; if it went more quickly, the Sun would have exhausted its hydrogen long ago.
The first step involves the fusion of two hydrogen nuclei 1H (protons) into deuterium, releasing a positron and a neutrino as one proton changes into a neutron.
This first step is extremely slow, both because the protons have to tunnel through the Coulomb barrier and because it depends on weak interactions.
The positron immediately annihilates with an electron, and their mass energy, as well as their kinetic energy, is carried off by two gamma ray photons.
After this, the deuterium produced in the first stage can fuse with another hydrogen to produce a light isotope of helium, 3He:
From here there are three possible paths to generate helium isotope 4He.
The complete proton-proton chain reaction releases a net energy of 26.7 MeV.
Proton–proton fusion can occur only if the temperature (i.e. kinetic energy) of the protons is high enough to overcome their mutual electrostatic or Coulomb repulsion.
In the Sun, deuterium-producing events are rare. The fact that the Sun is still shining is due to the slow nature of this reaction; if it went more quickly, the Sun would have exhausted its hydrogen long ago.
The first step involves the fusion of two hydrogen nuclei 1H (protons) into deuterium, releasing a positron and a neutrino as one proton changes into a neutron.
This first step is extremely slow, both because the protons have to tunnel through the Coulomb barrier and because it depends on weak interactions.
The positron immediately annihilates with an electron, and their mass energy, as well as their kinetic energy, is carried off by two gamma ray photons.
After this, the deuterium produced in the first stage can fuse with another hydrogen to produce a light isotope of helium, 3He:
From here there are three possible paths to generate helium isotope 4He.
The complete proton-proton chain reaction releases a net energy of 26.7 MeV.
Saturday, October 22, 2011
Relative distance of Earth and Venus to the Sun
When Venus is at its highest in the sky at sunset or sunrise, it subtends an angle of approximately 45° (47.8°) to the horizon. At that time of year (the next time will be on January 3rd 2012), the distance from Earth to Venus is equal to the distance from Venus to the Sun. Using pythagorus theorem the distance from Earth to the Sun is then equal to Earth to Venus multiplied by 1.414.
At that time if a radar pulse is sent from Earth to Venus and if the time is taken to record the echo, it will be found to take about 12 minutes and 6 seconds. Halving the time and multiplying by the speed of light, leads to the determination of the distance from the Earth to the Sun as 152 million km.
Tuesday, October 18, 2011
Stellar Parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις (parallaxis), meaning "alteration". Nearby objects have a larger parallax than more distant objects when observed from different positions, so parallax can be used to determine distances. Astronomers use the principle of parallax to measure distances to celestial objects including to the Moon, the Sun, and to the Stars beyond the Solar System. For example, the Hipparcos satellite took measurements for over 100,000 nearby stars within 2000 light years from Earth. This provides a basis for other distance measurements in astronomy, the cosmic distance ladder. Here, the term "parallax" is the angle or semi-angle of inclination between two sight-lines to the star.
Stellar parallax is the effect of parallax on distant stars in astronomy. It is parallax on an interstellar scale, and it can be used to determine the distance of Earth to another star directly with accurate astrometry. It was the subject of much debate in astronomy for hundreds of years, but was so difficult it was only achieved for a few of the nearest stars in the early 19th century. Even in the 21st century, stars with parallax measurements are relatively close on a galactic scale, as most distance measurements are calculated by red-shift or other methods.
The parallax is usually created by the different orbital positions of the Earth, which causes nearby stars to appear to move relative to more distant stars. By observing parallax, measuring angles and using geometry, one can determine the distance to various objects in space, typically stars, although other objects in space could be used.
The parallax is usually created by the different orbital positions of the Earth, which causes nearby stars to appear to move relative to more distant stars. By observing parallax, measuring angles and using geometry, one can determine the distance to various objects in space, typically stars, although other objects in space could be used.
Subscribe to:
Posts (Atom)