Spiral galaxy

[4] The most convincing evidence for the stars forming a bar in the Galactic Center comes from several recent surveys, including the Spitzer Space Telescope.

In contrast, the bulges of Sc and SBc galaxies are much smaller[9] and are composed of young, blue Population I stars.

In our own galaxy, for instance, the object called Sagittarius A* is a supermassive black hole.

Bar-shaped elongations of stars are observed in roughly two-thirds of all spiral galaxies.

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.

Due to their irregular movement around the center of the galaxy, these stars often display unusually high proper motion.

[dubious – discuss] The galaxy has a redshift of 4.4, meaning its light took 12.4 billion years to reach Earth.

Computer models based on that assumption indicate that BX442's spiral structure will last about 100 million years.

[24][25] The pioneer of studies of the rotation of the Galaxy and the formation of the spiral arms was Bertil Lindblad in 1925.

He realized that the idea of stars arranged permanently in a spiral shape was untenable.

Since the angular speed of rotation of the galactic disk varies with distance from the centre of the galaxy (via a standard solar system type of gravitational model), a radial arm (like a spoke) would quickly become curved as the galaxy rotates.

Measurements in the late 1960s showed that the orbital velocity of stars in spiral galaxies with respect to their distance from the galactic center is indeed higher than expected from Newtonian dynamics but still cannot explain the stability of the spiral structure.

[27] The first acceptable theory for the spiral structure was devised by C. C. Lin and Frank Shu in 1964,[28] attempting to explain the large-scale structure of spirals in terms of a small-amplitude wave propagating with fixed angular velocity, that revolves around the galaxy at a speed different from that of the galaxy's gas and stars.

They suggested that the spiral arms were manifestations of spiral density waves – they assumed that the stars travel in slightly elliptical orbits, and that the orientations of their orbits is correlated i.e. the ellipses vary in their orientation (one to another) in a smooth way with increasing distance from the galactic center.

It is clear that the elliptical orbits come close together in certain areas to give the effect of arms.

Stars therefore do not remain forever in the position that we now see them in, but pass through the arms as they travel in their orbits.

[27] As stars move through an arm, the space velocity of each stellar system is modified by the gravitational force of the local higher density.

[31] The stars in spirals are distributed in thin disks radial with intensity profiles such that[32][33][34] with

[35] The question of whether such objects were separate galaxies independent of the Milky Way, or a type of nebula existing within our own galaxy, was the subject of the Great Debate of 1920, between Heber Curtis of Lick Observatory and Harlow Shapley of Mount Wilson Observatory.

[38][39] Their suspicions were confirmed by Spitzer Space Telescope observations in 2005,[40] which showed that the Milky Way's central bar is larger than what was previously suspected.