Galaxies

This is the most recent concept drawing of the Milky Way galaxy. The yellow circle represents the location where our solar system is believed to reside, on the Orion Arm. The Milky Way is visible from Earth as a hazy band of white light that is seen in the night sky, arching across the entire celestial sphere, the visual phenomenon of the Milky Way originates from stars and other material which lies within the galactic plane. The Milky Way looks brightest in the direction of the constellation of Sagittarius, toward the galactic center. Relative to the celestial equator, it passes as far north as the constellation of Cassiopeia and as far south as the constellation of Crux, indicating the high inclination of Earth's equatorial plane and the plane of the ecliptic relative to the galactic plane. The fact that the Milky Way divides the night sky into two roughly equal hemispheres indicates that our Solar System lies close to the galactic plane. The Milky Way has a relatively low surface brightness, making it difficult to see from any urban or suburban location suffering from light pollution.

milky_way_panoramic

This is a 360º degree view of the Milky Way's horizon.



A galaxy is a massive, gravitationally bound system consisting of stars, an interstellar medium of gas and dust, and dark matter. Typical galaxies range from dwarfs with as few as ten million stars up to giants with one trillion stars, all orbiting a common center of mass. Galaxies can also contain many multiple star systems, star clusters, and various interstellar clouds.

This is the most recent concept drawing of the Milky Way galaxy. The yellow circle represents the location where our solar system is believed to reside, on the Orion Arm. The Milky Way is visible from Earth as a hazy band of white light that is seen in the night sky, arching across the entire celestial sphere, the visual phenomenon of the Milky Way originates from stars and other material which lies within the galactic plane. The Milky Way looks brightest in the direction of the constellation of Sagittarius, toward the galactic center. Relative to the celestial equator, it passes as far north as the constellation of Cassiopeia and as far south as the constellation of Crux, indicating the high inclination of Earth's equatorial plane and the plane of the ecliptic relative to the galactic plane. The fact that the Milky Way divides the night sky into two roughly equal hemispheres indicates that our Solar System lies close to the galactic plane. The Milky Way has a relatively low surface brightness, making it difficult to see from any urban or suburban location suffering from light pollution. This is a 360º degree view of the Milky Way's horizon.

Types of Galaxies

Elliptical Galaxies
Messier 49 in Virgo	Elliptical galaxies are characterized by several properties that make them distinct from other classes of galaxy. The motion of stars in elliptical galaxies is predominantly radial, unlike the disks of spiral galaxies, which are dominated by rotation. Furthermore, there is very little interstellar matter, which results in low rates of star formation, few open star clusters, and few young stars; rather, elliptical galaxies are dominated by old stellar populations, giving them red colors. Large elliptical galaxies typically have an extensive system of globular clusters. The properties of elliptical galaxies and the bulges of disk galaxies are similar, suggesting that they are formed by the same physical processes, although this remains controversial. Elliptical galaxies are preferentially found in the cores of galaxy clusters and in compact groups of galaxies.

Spiral Galaxies
Messier 63 (Sunflower Galaxy) in Canes Venatici	A spiral galaxy is a galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the Nebulae” and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk of stars, gas and dust, and a central concentration of stars known as the bulge. These are surrounded by a much fainter halo of stars, many of which reside in globular clusters. Spiral galaxies are named for the (usually two-armed) spiral structures that extend from the bulge into the disk. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disk because of the young, hot OB stars that inhabit them. Roughly half of all spirals are observed to have an additional component in the form of a bar-like structure, extending from the central bulge, at the ends of which the spiral arms begin. Our own Milky Way has long been believed to be a barred spiral, although the bar itself is difficult to observe from our position within the Galactic disk.

Barred Spiral Galaxies
NGC 1300 in Eridanus	A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure comprised of stars. Bars are found in approximately half of all spiral galaxies. Bars generally affect both the motions of stars and interstellar gas within spiral galaxies and can affect spiral arms as well. Edwin Hubble classified these types of spiral galaxies as "SB" ("Spiral", "Barred") in his Hubble sequence, and arranged them into three sub-categories based on how open the arms of the spiral are. SBa types feature tightly bound arms, while SBc types are at the other extreme and have loosely bound arms. SBb type galaxies lie in between. A fourth type, SBm, was subsequently created to describe somewhat irregular barred spirals, such as the Magellanic Cloud galaxies, which were once classified as irregular galaxies, but have since been found to contain barred spiral structures.

Ring Galaxies
Hoag's Object in Serpens	A ring galaxy is a galaxy with a ring-like appearance. The ring consists of massive, relatively young blue stars, which are extremely bright. The central region contains relatively little luminous matter. Astronomers believe that ring galaxies are formed when a smaller galaxy passes through the center of a larger galaxy. Because most of a galaxy consists of empty space, this "collision" rarely results in any actual collisions between stars. However the gravitational disruptions caused by such an event could cause a wave of star formation to move through the larger galaxy.

Lenticular Galaxies
NGC 5866 (Spindle Galaxy) in Draco	A lenticular galaxy is a type of galaxy which is intermediate between an elliptical galaxy and a spiral galaxy in galaxy morphological classification schemes. Lenticular galaxies are disc galaxies, like spiral galaxies, which have used up or lost their interstellar matter, like elliptical galaxies. Because of their ill-defined spiral arms, if they are inclined face-on it is often difficult to distinguish between them and elliptical galaxies. Although lenticular galaxies do not vary in shape as much as spiral galaxies, they may still be divided into a series of subclasses based on their appearance. In the de Vaucouleurs classification system, lenticular galaxies may be split into three subclasses based on the presence or absence of a central bar structure. The SA0 designation is used for lenticular galaxies with no apparent bars. The SB0 designation is used for galaxies with a strong bar. The SAB0 designation is an intermediate class that may be referred to as weakly barred. Some lenticular galaxies are also given inner ring (S0(r)) and S-shaped (S0(s)) designations as well as an intermediate designation (S0(rs)). However, these designations were defined primarily for spiral galaxies, and finding objects that represent some of these classes is very difficult.


Irregular Galaxies
Sextans A in Sextans	An irregular galaxy is a galaxy that does not fall into any of the regular classes of the Hubble sequence. These are galaxies that feature neither spiral nor elliptical morphology. They are often chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure. Collectively they are thought to make up about a quarter of all galaxies. Most irregular galaxies were once spiral or elliptical galaxies but were deformed by gravitational action.

Dwarf Galaxies
I Zwicky 18 in Ursa Major	A dwarf galaxy is a small galaxy composed of up to several billion stars, a small number compared to our own Milky Way's 200-400 billion stars. The Large Magellanic Cloud, containing over 30 billion stars, is sometimes classified as a dwarf galaxy while others consider it a full-fledged galaxy going around the Milky Way galaxy. There are many dwarf galaxies in the Local Group: these small galaxies frequently orbit around larger galaxies, such as the Milky Way, the Andromeda Galaxy and the Triangulum Galaxy. The Milky Way has 14 known dwarf galaxies orbiting it.

Starburst Galaxies
Messier 82 (Cigar Galaxy) in Ursa Major	A starburst galaxy is a galaxy in the process of an exceptionally high rate of star formation, compared to the usual star formation rate seen in most galaxies. Galaxies are often observed to have a burst of star formation after a collision or close encounter between two galaxies. The rate of star formation is so great for a galaxy undergoing a starburst that, if the rate was sustained, the gas reservoirs from which stars are formed would be used up on timescales much shorter than the dynamical lifetime of the galaxy. For this reason, it is presumed that starbursts are temporary.

Interacting Galaxies
Messier 51 (Whirlpool Galaxy) shows satellite interaction with NGC 5195 in Ursa Major	Interacting galaxies are the result of one galaxy's gravity disturbing another galaxy. An example of minor interaction is a satellite galaxy disturbing the primary galaxy's spiral arms. An example of major interaction is a galactic collision. There are three kinds of interacting galaxies. Satellite interaction - A giant galaxy interacting with its satellites is common. A satellite's gravity could attract one of the primary's spiral arms. Or even the satellite could dive in to the primary. This could trigger a small amount of star formation. The satellite could be a vacuum cleaner and suck up some of its primary's stars or vice versa. Galaxy collision - Colliding galaxies are common in galaxy evolution. Colliding may lead to merging. Merging is the most violent of all galaxy interactions. This occurs when two galaxies collide and do not have enough momentum to continue traveling after the collision. Instead, they fall back into each other and eventually merge together, forming one galaxy. If one of the colliding galaxies is much larger than the other, it will remain largely intact after the merger; that is, the larger galaxy will look much the same while the smaller galaxy will be stripped apart and become part of the larger galaxy. Collisions are less violent than mergers in that both galaxies remain separate after the collisions. Galactic cannibalism - Refers to the process by which a large galaxy, through tidal gravitational interactions with a companion, merges with that companion, resulting in a larger, often irregular galaxy. The most common result of the gravitational merger of two or more galaxies is an irregular galaxy of one form or another, although elliptical galaxies may also result.

NGC 2207 (left) and IC 2163 (right) in Canis Major are colliding galaxies