MILKY WAY

José Miguel Pérez Hita Moreno

ORIGINS

• The Milky Way is the galaxy that contains our Solar System. Its name "milky" is derived from its appearance as a dim glowing band arching across the night sky in which the naked eye cannot distinguish individual stars. The term "Milky Way" is a translation of the Classical Latin via lactea.

HALO• El halo es una estructura esferoidal que envuelve la galaxia. En el

halo la concentración de estrellas es muy baja y apenas tiene nubes de gas, por lo que carece de regiones con formacion estelar. En cambio, es en el halo donde se encuentran la mayor parte de los cúmulos globulares. Estas formaciones antiguas son reliquias de la formación galáctica. Estas agrupaciones de estrellas se debieron de formar cuando la galaxia era aún una gran nube de gas que colapsaba y se iba aplanando cada vez más. Otra característica del halo es la presencia de gran cantidad de materia oscura. Su existencia se dedujo a partir de anomalías en la rotación galáctica. Los objetos contenidos en el halo rotan con una componente perpendicular al plano muy fuerte, cruzando en muchos casos el disco galáctico.

SIZE AND MASS

• The stellar disk of the Milky Way Galaxy is approximately 100,000 ly (30 kpc) in diameter, and is, on average, about 1,000 ly (0.3 kpc) thick. As a guide to the relative physical scale of the Milky Way, if it were reduced to 100 m (110 yd) in diameter, the Solar System, including the hypothesized Oort cloud, would be no more than 1 mm (0.04 in) in width, about the size of a grain of sand. The nearest star, Proxima Centauri, would be 4.2 mm (0.2 in) distant.Alternatively visualized, if the Solar System out to Pluto were the size of a US quarter (25 mm or 1.0 in in diameter), the Milky Way would have a diameter of 2,000 kilometers, an area approximately one-third the size of the United States.

STARS AND PLANETS• The Milky Way contains at least 100 billion stars and may have up to

400 billion stars. The exact figure depends on the number of very low-mass, or dwarf stars, which are hard to detect, especially at distances of more than 300 ly (90 pc) from the Sun. As a comparison, the neighboring Andromeda Galaxy contains an estimated one trillion (1012) stars. Filling the space between the stars is a disk of gas and dust called the interstellar medium. This disk has at least a comparable extent in radius to the stars, while the thickness of the gas layer ranges from hundreds of light years for the colder gas to thousands of light years for warmer gas.Both gravitational microlensing and planetary transit observations indicate that there may be at least as many planets bound to stars as there are stars in the Milky Way,] while microlensing measurements indicate that there are more rogue planets not bound to host stars than there are stars. The Milky Way Galaxy contains at least one planet per star, resulting in 100–400 billion planets, according to a January 2013 study of the five-planet star system Kepler-32 with the Kepler space observatory.

STRUCTURE

• The Galaxy consists of a bar-shaped core region surrounded by a disk of gas, dust and stars. The gas, dust and stars are organized in roughly logarithmic spiral arm structures (see Spiral arms below). The mass distribution within the Galaxy closely resembles the type SBc in the Hubble classification, which represents spiral galaxies with relatively loosely wound arms. Astronomers first began to suspect that the Milky Way is a barred spiral galaxy, rather than an ordinary spiral galaxy, in the 1990s. Their suspicions were confirmed by the Spitzer Space Telescope observations in 2005] that showed the Galaxy's central bar to be larger than previously suspected.