Galaxy Classification

In the Swapping Galaxies and Stars wiki, **Galaxy Classification** is the classification of galaxies based on their morphological characteristics. LIKe the Hubble classification, glaxies are split up into ellipticals, spirals, lenticulars, and irregular/dwarf galaxies. Visual appearance from the galaxy is analyzed by its star and nebular population along with its density or it's visible characteristics. Each class indicates a particular different star or nebular population, with the class indicating the abundance of star or nebular population. The strengths of the different morphological types vary mainly due to the lumonosity, although in some cases there are true abundance differences. The *classification* of a galaxy is a short code primarily summarizing the formation state, giving an objective measure of the history of the galaxy.

Most galaxies are currently classified under the **Morgan–Keenan** (MK) system using the letters *O*, *B*, *A*, *F*, *G*, *K*, and *M*, a sequence from the most abundant (*O* type) to the least abundant (*M* type). Each letter class is then subdivided using a numeric digit with *0* being most compact and *9* being most sparse. The sequence has been expanded with classes for other galaxies and galaxy-like objects that do not fit in the classical system, such as class *D* for tiny ultra-compact galaxies \ White Dwarves \ and classes *S* and *C* for galaxies populated with younger hot stars \ Carbon \ galaxies.

In the MK system, a size class is added to the morphological class using Roman numerals. This is based on the width of certain size in the galaxy's visible characteristics, which vary with the density of the atmosphere and so distinguish giant stars from dwarfs. Luminosity class *0* or *Ia+* is used for *hypergiants*, class *I* for *supergiants*, class *II* for bright *giants*, class *III* for regular *giants*, class *IV* for *subgiants*, class *V* for *main-sequence stars*, class *sd* (or *VI*) for *subdwarfs*, and class *D* (or *VII*) for *tiny ultra compact galaxies*. The full spectral class for the Sun is then G2V, indicating a main-sequence galaxy (V) with a normal star and nebular popularion (G, 2)

**Modern Classification**

The modern classification system is known as the *Morgan–Keenan* (MK) classification. Each galaxy is assigned a morphological class (from the older Harvard classification, which did not include luminosity) and a luminosity class using Roman numerals as explained below, forming the galaxy's spectral type.

Other modern galaxy classification systems, such as the UBV system, are based on nebula color indices—the measured differences in three or more color magnitudes from starforming regions. Those numbers are given labels such as "U−V" or "B−V", which represent the colors passed by two standard filters (e.g. *U*ltraviolet, *B*lue and *V*isual).

Extended Galaxy Types

A number of new galaxy types have been taken into use from newly discovered types of galaxies.

**- Large emission-line galaxy**: sometimes have spectra of some massive galaxies exhibit marked emission lines. They are mainly seyfert galaxies hosted by active galaxtic nuclei. (examples: M77, NGC3079)

**- Class WR: Wolf–Rayet:** Once included as type O galaxies, the Wolf–Rayet galaxies of class W or WR are usually Active Galaxtic Nuclei (AGN) notable for spectra lacking hydrogen lines. Instead their spectra are dominated by broad emission lines of highly ionized helium, nitrogen, carbon, and sometimes oxygen. They are thought to mostly be dying supergiants with their hydrogen layers blown away by galactic forces, thereby directly exposing their hot shells. (Example: NGC1275)

- **Slash Galaxies:** The slash-type galaxies are O-type galaxies that exhibit WN-like lines in the spectra. (e.g. "Of/WNL"). The name "slash" comes from their printed spectral type having a slash in it. There is a secondary group found with these spectra, a cooler, "intermediate" group designated "Ofpe/WN9". These galaxies have also been referred to as WN10 or WN11, but that has become less popular with the realisation of the evolutionary difference from other Wolf–Rayet galaxies. Recent discoveries of even rarer galaxies have extended the range of slash stars as far as O2-3.5If\*/WN5-7, which are even massive than the original "slash" galaxies. (Example: FJJG 86421638)

- **Magnetic O Galaxies:** They are O galaxies with strong magnetic fields. Designation is Of?p. (Example: NGC1277)

- **Tiny irregulars and diffuse dwarf classes**

The new spectral types L, T, and Y were created to classify dwarf galaxies. This includes both irregulars, magellanic spirals, and dwarfs that are very faint in the visible spectrum. Most of them are satellite galaxies.

* **Class L dwarfs** get their designation because they are smaller than M galaxies and L is the remaining letter alphabetically closest to M. Some of these objects have masses large enough to support nebular fusion and are therefore galaxies, but most are of substellar mass and are therefore brown dwarfs. They are a very diffuse and brightest in infrared. They have stellar populations about 1 million to 1 billion stars. (Example: UGCA020)
* **Class T dwarfs** are tiny diffuse galaxies with stellar populations ranging from 100,000 to 1 million. to Their emission peaks in the infrared. (Example: Bootes Dwarf, Ursa Major II Galaxy)
* **Class Y dwarfs** are extremely, hard to detect diffuse galaxies smaller than those of class T and have population less than 100,000 stars. A total of 17 objects have been placed in class Y as of August 2013. Although such dwarfs have been modelled and detected within forty light-years by the Wide-field Infrared Survey Explorer (WISE) there is no well-defined galaxy sequence yet and no prototypes. Nevertheless, several objects have been proposed as spectral classes Y0, Y1, and Y2. (Example: Segue galaxies, Lynx Dwarf Galaxy Candidate)
* Peculiar tiny irregulars and diffuse dwarf galaxies can be peculiar sometimes.

 sd
 β
 γ
 red
 blue

- **Carbon Galaxies**

Carbon galaxies are galaxies whose stars are very young and hot, but have no star formation. The spectra of these stars become increasingly deviant from the usual late spectral classes G, K, and M. Equivalent classes for carbon-Galaxies are S and C.

**- Class C**, Originally classified as R and N stars, these are also known as *carbon galaxies*. These are massive galaxies, near the end of their lives, in which there is an excess of hot stars in the disk. (Example: NGC4424, NGC4694, NGC4606)

**- Class S.** Class S stars form a continuum between class M galaxies and carbon galaxies. (Example: NGC4388, M90, NGC4522)

**White dwarf classifications**

The class D (for Degenerate) is the modern classification used for Tiny High Compacted Galaxies \ White Dwarf \ —low-mass galaxies that are no longer undergoing nebula fusion and have shrunk to size of satellite galaxies, slowly dying down.

A different set of galaxies peculiarity symbols are used for white dwarfs than for other types of galaxies.

 E
 H
 V
 PEC