Stars are categorized by their spectral properties. You know, when a star shines in space, it emits all kinds of radiation under and maybe beyond the Electromagnetic spectrum. When a star's light comes to earth or anywhere that is observable by scientists, they analyze the star's light through a Prism, grating, or spectroscope, thus they observe the star in general. off course, scientists have a lot of advanced instruments that they can use to separate the radiation of a particular star from other Stellar lights and stuff.

In most cases, a Stellar category Symbol consists of things:



(1) Letter

In chemistry, each element has its Spectral characteristics. Since stars are giant nuclear stations that generally burn their Hydrogen as a fuel and convert it into helium or other elements, and also produce enormous energy as light and heat. However, producing energy isn't possible; it just changes one form into another form.

So when a star emits its radiation from the photosphere, it glows in all regions of the electromagnetic spectrum (maybe beyond it), and astronomers observe it through a spectroscope. They find at what wavelength the Star is emitting the most energy or light, thus they assign a Letter for it.



It Can be :
{ O B A F G K M }

This is also known as the Morgan-Keenan (MK) System.





You can remember it By This mnemonic: Oh Boy Always Find Good Ketchup and Mango
Of course, you can make your own.


In General, letters represent their Temperature Class. Although these colors are in their spectral terms, inrealityl you will see Sun-type Yellow Stars are white, Red Dwarfs are Red/orange, and Brown Dwarfs are not brown, but they hypothetically appear in Gray or Dim Red.

(2) Number

Each letter Class is divided into numbers from 0 (hottest) to 9 (coolest) in that category.

For Example

An F6 Star is hotter than an F8 star.

In case any star's properties are between 2 and 3, then Scientists can say it x2.5
Where x = O, B, A, F, G, K, M.


(3) Roman Numerals

Stellar Luminosity and sometimes its size (Because The Giants are Usually Bright) are indicated by Roman Numbers, and this is determined by their width of Certain Absorption lines in the stellar Spectrum. It may vary with the atmosphere's density, and we can Distinguish Giants and dwarfs.
It is divided as follows:

0 or Ia+ = Hypergiants

I = Supergiants

II = Bright Giants

III = Regular Giants

IV = Sub-Giants

V = Main-sequence stars

VI or sd = Sub Dwarfs

VII or D = White Dwarfs

So our Sun is a "G2 V" Star means it is a main-sequence star , and its Temperature is around 5800K.

However, besides those letters, there are many other classes also made for Black holes, White dwarfs, Wolf-Rayet stars, carbon stars, etc. But we'll discuss them in another guide.


Some other Guides That can have a Relation to this guide :
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3049943844
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3049024150
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3306139352

These stars are rarer than any other main-sequence star. They are extremely hot and bright.
Their mass is 16 or more Ms (mass of the sun). Their Radii are a minimum of 6.6 times larger than the sun.
These stars emit radiation in the Ultraviolet range in tremendous amounts. Some of the most massive stars come in this category.

This group is known for very luminous and blue stars. These types of stars contain 16 to 2.1 Ms and 6.6 to 1.8 Solar radii. Indeed, they are very hot. This category has neutral helium lines in its spectra.
They are less rare compared to O-type stars.

These are white and some bluish stars, their masses are between 2.1 - 1.4 Ms, and the Radius range is
almost 1.8 - 1.4 Ms. These are a bit more common naked eye stars, and they have strong Hydrogen Lines in their spectra.

Their spectra are known for weaker Hydrogen Lines and ionized metals. The Color is white.
In this spectrum, We see the dominance of Hydrogen, Potassium and Calcium lines. They are very common stars.

This type of star's mass lies between 1.4 - 1.04 Ms and Radii is 1.4 - 1.15 Rs (Radius of the sun)

These are a bit smaller Stars than the previous group and they emit yellowish white light. Their radii are between 1.15 - 0.96 Rs and their mass is 1.04 - 0.8 Ms.. Their hydrogen lines are very weak and have prominent Hydrogen, Potassium and Calcium lines in the spectrum. They are much common than the F-type stars. As a matter of fact, our Sun also comes in this category.

Their Color is Orange and pale yellow, Mass is between 0.6 - 0.9 Ms and radius is 0.96 - 0.7 Rs.
They are slightly cooler than the Sun and there are almost 12 % of main-sequence stars found in the solar
neighborhood. There are also K-type Giant Stars, such as RW Cephei is Hyper hypergiant, and Arcturus is a Giant.
Orange Dwarf like Alpha Centauri B. These are main-sequence stars. They Have Extremely Weak
Hydrogen Lines. Scientists suggest these stars' planets can support life due to a large habitable zone
and also these types of stars have lower harmful radioactive.

This is a group of Coolest Stars, but not that cool, you can walk its surface.
They shine in Orange-Red Color, Mass stays between 0.8 - 0.45 Ms and the Radius is Less than 0.7 Rs.

These stars are very Common, about 76 % of neighboring main-sequence stars come in this group. However, due to low luminosity, We Can't see any of them except in uncommon situations like No-Pollution sky or Clear Weather, etc.

In fact Brightest Known M Type Star is Lacaille 8760 its magnitude is about 6.7 (Human eyes can't see more than 6).


Although in this class there are mostly Red Dwarf stars, most of the largest known stars are also categorized in this group, E.g, VV Cephei, Antares, Betelgeuse, etc.



spectrum of this class, lines are mostly made of oxide molecules (especially Titanium oxide ). But
Hydrogen Absorption lines are usually Absent.

Despite those groups, there are many other Classes assigned for various kinds of objects.
But these are not true stars; instead, they are star-like objects.
These categories are filled with many interesting objects like White dwarfs, Neutron stars, Quasi Star, Black Holes, protostars, Carbon stars, Brown dwarfs, supernovae and much more.

But we'll discuss them in other guides. So stay tuned for the upcoming guide, Because You will know
a lot of splendid facts about space and then you can search it on Space Engine.

Good bye
Have a nice Day.