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Therefore, later generations of stars formed in this spiral disk. Most younger stars, including the Sun, are observed to be in the disk. [251] [252] Since the first stars began to form, the Milky Way has grown through both galaxy mergers (particularly early in the Milky Way's growth) and accretion of gas directly from the Galactic halo. [252]
Halo stars may be acquired from small galaxies which fall into and merge with the spiral galaxy—for example, the Sagittarius Dwarf Spheroidal Galaxy is in the process of merging with the Milky Way and observations show that some stars in the halo of the Milky Way have been acquired from it.
When a galaxy forms, it has a disk shape and is called a spiral galaxy due to spiral-like "arm" structures located on the disk. There are different theories on how these disk-like distributions of stars develop from a cloud of matter: however, at present, none of them exactly predicts the results of observation.
The Milky Way started out small and grew in size as it merged with other galaxies, gaining stars as well as hydrogen to form more stars. Each galaxy has hydrogen gas that aids in the birth of stars.
2005 – Spitzer Space Telescope data confirm what had been considered likely since the early 1990s from radio telescope data, i.e., that the Milky Way Galaxy is a barred spiral galaxy. [14] [15] 2012 – Astronomers report the discovery of the most distant dwarf galaxy yet found, approximately 10 billion light-years away. [16]
In the dense nebulae where stars are produced, much of the hydrogen is in the molecular (H 2) form, so these nebulae are called molecular clouds. [4] The Herschel Space Observatory has revealed that filaments, or elongated dense gas structures, are truly ubiquitous in molecular clouds and central to the star formation process.
Baade observed that bluer stars were strongly associated with the spiral arms, and yellow stars dominated near the central galactic bulge and within globular star clusters. [2] Two main divisions were defined as Population I star and population II , with another newer, hypothetical division called population III added in 1978.
The most powerful telescope to be launched into space has made history by detecting a record number of new stars in a distant galaxy. NASA's James Webb Space Telescope, history's largest and most ...