2. 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.
3. 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.
4. 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.
5. 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 lowmass, 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.
6. 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.
