Milky Way

Milky Way

The Milky Way is by far the largest object in the night sky, which it occupies an important fraction. It is observable preferably through a night without moon and especially without light pollution. The Milky Way appears as a long white trail that runs through the night sky 360 degrees. This is due to the very large number of stars that make up the Milky Way, their lights superimposed to give this whitish appearance. It is by observing with a telescope, Galileo was able to resolve individual stars. All the stars we can see with the naked eye are part of the Milky Way.

 The solar system is part of the Milky Way. Our Galaxy is dominated by his record, a huge "plate". Being inside the disk we see it from the side, like a long trail of stars, the Milky Way. It contains from 150 to 250 billion stars, gas in different phases (hot and molecular) and dust that absorbs visible light, which explains the dark streaks in the most central parts of the Galaxy.

The structure of our Galaxy

Our Galaxy is a type of galaxy "spiral". It is dominated by a disc with the stars revolve around the center, which consists of a bulb and a bar. Between 70% to 80% of stars are in the disc which has a diameter of 90,000 light years to a thickness of just 2,500 light years. The disc consists of several spiral arms in which the majority of stars concentrated. It is believed that the arms are formed naturally by gravitational instabilities induced during rotation of the disk. The two spiral arms dominate the galaxy are those of Scutum-Centaurus and Perseus, the Sun lying in a secondary arm called the Orion Arm.

Our position in the Galaxy

our sun's position in the galaxy

The Sun is one of the many stars of the galaxy disk. It is almost exactly in the plane of the disc, with the Galactic Centre distance of 28,000 light years. The Sun orbits the Galactic center with a speed of 230 km / s.
To know exactly our position in the Galaxy, it is necessary to determine the distances of stars. The distance to nearby stars is determined by the effect called parallax. They use their changes in position on the sky when viewed from Earth at two different positions around the sun. The GAIA satellite to be launched by ESA in 2013 can observe the position of almost a billion stars! The distance to the most distant stars is determined by the specific properties of certain variable stars, Cepheids, whose variability period depends on their intrinsic luminosities.
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Halley's Comet

Halley's Comet

The appearance in the sky of this beautiful comet is listed for centuries. But it is only since his last visit in 1986 that knows more about this exotic solar system.
Halley's Comet is one of the known comets of the public. His spectacular pass in 1910 has long marked the spirits.

History of comet

In 1705, the English astronomer Edmond Halley announced that comets that had appeared in 1531, 1607 and 1682 were in fact one and the same comet. He predicted, thanks to the laws of Newton, his return in 1759. The comet actually came back in 1758 with a few months in advance, 16 years after his death. The comet was named in his honor, even if now, comets are baptized in the name of their discoverer.
This triumph of the late astronomer allowed to permanently sit laws of Newtonian mechanics.
  
 famous passages 

The most famous appearance of Halley's comet was undoubtedly at the Battle of Hastings in 1066, which marked the invasion of England by the Normans. This famous event is represented on the 32nd stage of the famous Bayeux Tapestry. This appearance on the eve of the battle was interpreted as a bad omen. The English defeat that ensued gave them ample reason.



More recently, last appearances dating from 1910, which was an impressive sight and winter 1986 when several probes went visits to the comet. The Giotto probe afforded some impressive photos of the comet nucleus and learn more about these strange bodies of the solar system.
The accurate prediction of the next passage of the comet is difficult because it undergoes gravitational influence of the giant of the solar system that may cause it to radically change its trajectory. However, with the data collected, astronomers estimate that the comet will return in 2061 near the Earth.

Specifications

Orbit of the comet: The comet's orbital period varies around 76 years. With perihelion located 88 million kilometers from the Sun and an aphelion located 5240 million kilometers, its orbit is highly elongated. The comet orbits in retrograde direction (east to west) and is inclined 18 ° to the plane of the ecliptic. Only when the comet is less than 200 million kilometers that we can begin to see on Earth.

With all these features, astronomers think it would come from the large tank, located in the outer solar system called the Oort Cloud. However, comets coming from this reservoir has a period greater than 200 years. Now Halley's Comet to a period of 76 years. This anomaly could be explained by the gravitational perturbations of the giant that would have shortened its orbit.
A dark core: With an albedo of only 3%, Comet Halley is even darker than coal. With the Giotto probe, astronomers have seen surprised by the darkness of the sun so bright when it first appeared. This albedo because of Comet Halley one of the darkest planets of the solar system.
Low density: The density of the comet nucleus of Halley's comet is much lower than previously thought: 0.1 mg / cm3. We must therefore imagine a porous foam made of water ice, dry ice and hydrocarbons rather than a block of ice. Astronomers have in fact estimated the mass of this star range only between 20 and 100 billion tons. The comet, with each pass, losing about 0.1% of its mass which represents some 100 billion kilos! It was estimated that the icy geysers of Halley's Comet expelled at their maximum activity around 30 000 liters of water per second!
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