The stellar disk of the Milky Way galaxy is approximately 100,000 light-years
km) in diameter, and is considered to be, on average, about 1,000 ly (0.3 kpc) thick.
It is estimated to contain at least 100 billion stars
and possibly 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 distance of more than 300 ly
(90 pc) from the Sun. Hence, current estimates of the total number
remain highly uncertain, though it is often speculated to be around 250
This can be compared to the one trillion (1012
) stars of the neighboring Andromeda Galaxy
The disk of stars in the Milky Way does not have a sharp edge; a
radius beyond which there are no stars. Rather, the concentration of
stars drops smoothly with distance from the center of the galaxy. Beyond
a radius of roughly 40,000 ly (12 kpc), the number of stars per cubic
parsec drops much faster with radius, for reasons that are not understood. Recent estimates give the galaxy a population of at least 50 billion planets, 10 billion of which could be located in the habitable zone of their parent star. New data suggests there may be up to twice as many free-floating planets in the Milky Way as there are stars. In 2011, new evidence obtained via gravitational microlensing
indicated an average of at least one bound planet per star in the Milky
Way, with Earth-sized planets being more numerous than gas giants.[13
Filling the place between the stars within and around the stellar disk is a disk of gas called the interstellar medium. The disk of gas 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.
As a guide to the relative physical scale of the Milky Way, if the Solar System out to the orbit of Pluto were reduced to the size of a US quarter
(approximately 2.5 cm in diameter) the Milky Way would span some 2,045
km across covering an area of 3.286 million square kilometers, an area
equivalent to about a third of the United States (3.276 million square
km), the total area of India (3.287 million square km), or the combined
areas of Great Britain, France, Spain, Germany, Italy, Poland, Sweden,
Finland and Norway (3.378 million square km).
The Galactic Halo
extends outward, but is limited in size by the orbits of two Milky Way satellites, the Large and the Small Magellanic Clouds
, whose perigalacticon
is at about 180,000 ly (55 kpc).
At this distance or beyond, the orbits of most halo objects would be
disrupted by the Magellanic Clouds, and the objects would likely be
ejected from the vicinity of the Milky Way