Star Types and Star Classification – Everything You Need to Know – Part I

In our last article on stars, we learned about what is star and how is star formed. We also learned about the lifecycle of stars and learned what happens to stars at the end of their lives. What we didn’t learn there in that article was about star types and star classification. In this article we are going to do so. Heads up, this is going to be a long article (possibly longer than the previous one) and there will be too much information crammed in. So, relax and have a clear mind before you start reading this particular article on star types and star classification.

Assuming that you are ready, let us begin…

Star Classification

Before we jump on to the Star types, it is necessary that we first look into the classification that is used for categorizing the different types of stars that are present in our universe. If we are to go by very basic classification, there are actually three types based on color. They are BLUE, RED and WHITE/YELLOW.

But, did you really think that astronomers will keep it that simple? No, not really no! The problem they face is that they come across oddities of enormous proportions every now and then and they need to accommodate all what they see into a proper classification table. It is because of this, their classification system changes over time or new classification systems come into existence. For this article in particular, we will use the system known as Morgan-Keenan or MK System. It is really easy to understand and master quickly.

Under this system, stars are classified as per their hotness using a set of letters. Those letters are O, B, A, F, G, K and M. So, there are basically 7 letters with O representing the hottest stars while M represents the coolest ones.

The quick table below will tell you more about the classification system:

Type Color Hotness (Surface Temperature in Kelvin) Brightness
O Blue > 25,000 170,000 to 1,400,000 times brighter than our Sun
B Blue 11,000 – 25,000 180 to 50,000 times brighter than our Sun
A Blue 7,500 – 11,000 14 to 80 times brighter than our Sun
F Bluish-White 6,000 – 7,500 3 to 6 times brighter than our Sun
G Yellowish-White 5,200 – 6,000 0.79 to 1.5 times brighter than our Sun (Our Sun falls in this category)
K Orangey-Red 3,500 – 5,000 Less than ½ the brightness of our Sun
M Red < 3,500 Less than 1/5th the brightness of our Sun

Now, each type that is O through M can be further subdivided using 0 (zero) to 9 (nine) where the numerals 0 to 9 give temperature grading where 0 is hottest and 9 is coolest.

Thus, we can have something like G0, G1, G2, G3 … G9 etc.

Wondering why the Star Classification letters are random? Why can’t we just go as A, B, C…. etc. That is, why can’t we go in order? The thing is that the alphabetical order was once followed during early days but when more and more knowledge about the universe started pouring in, not everything could be fit in that particular order. It is because of that, the classification system had to be altered. Somehow the astronomers decided to keep the original letters but in order to accommodate newfound knowledge, the alphabetical order was just given up. Now, it is all random and jumbled up.

Now that we know a bit about star classification, it is time we take a look into star types to get a more accurate understanding of the stars that surround us in every possible direction!

Star Types

All the different types of stars that we know of or hypothesize about their existence are listed below:

  • Yellow Dwarf Stars
  • Orange Dwarf Stars
  • Red Dwarf Stars
  • Blue Dwarf Stars
  • White Dwarf Stars
  • Brown Dwarf Stars
  • Black Dwarf Stars
  • Red Giant Stars
  • Blue Giant Stars
  • Supergiant Stars (there are subdivisions)
  • Hypergiant Stars (there are subdivisions)
  • Wolf-Rayet Stars
  • Neutron Stars
  • Variable Stars (there are subdivisions)
  • Multiple Star Systems
  • Some Weird Theories You Might Want to Know

Let’s look into each type separately and try to understand them while looking at some examples as well. Sounds good? Let’s start…

Star Types: Yellow Dwarf Stars

Colloquial Name Type of Star Evolution Stage Example
Yellow Dwarf Star G Main Sequence Sun

From the table above we see that Yellow Dwarf Stars are basically G-type Main Sequence stars. In case you are wondering what a Main Sequence star is, here is a brief: It is type of star which has crossed its ‘Protostar’ phase and is in a state of hydrostatic equilibrium and is burning Hydrogen into Helium.

Our very own Sun is a G-type star.

The G-type stars are classified as per the following rule: G – A Numeral – Roman Numeral.

Starting from back, the Roman Numeral used is V, which indicates “Dwarf Main Sequence Star”

In the middle, a Numeral can be between 0 and 9 (both included) and will indicate the hotness of the star where 0 is hottest and 9 is coolest.

At the very beginning, G represents the ‘Spectral Type’ and in case of G-type stars, the spectral type is ‘yellow’.

So, a good example will be G2V (represents our own Sun). So, our Sun is a G-type star with hotness of 2 and is a Dwarf Main Sequence star.

What really is ‘Spectral Type’?

Stars emit a full spectrum of light. Astronomers study this light spectrum to understand what chemicals are present in the photosphere of the star. This helps scientists to determine the surface temperature of the stars they study.

So, G ‘Spectrum Type’ indicates Yellow Dwarf Stars with surface temperature that ranges between 5,200 Kelvin and 6,000 Kelvin.

What about our Sun?

Our Sun is classified as G2V. This means:

Spectrum Type: G with surface temperature ranging between 5,200 Kelvin and 6,000 Kelvin. The actual surface temperature of our Sun is around 5,700 Kelvin.

2: It represents that our Sun is pretty hot.

V: It is the brightness factor of our Sun representing that it is Dwarf Main Sequence Star.

In short:

Yellow Dwarf Stars are categorized or classified as ‘G V’. The numeric subdivision ranges from 0 to 9. For each subdivision, one star is considered to be standard for studying other stars that are discovered in that same subdivision. So, the main standard G V stars or Yellow Dwarf Stars are:

  • G0V: beta Cvn
  • G1V: HD 115043
  • G2V: Our Sun
  • G3V: 16 Cygni B
  • G4V: 70 Virginis
  • G5V: Kappa1 Ceti
  • G6V: 82 Eridani
  • G8V: 61 Ursae Majoris

A G-type stars always Yellow?

Good question. The answer is no! The color Yellow used in this case is a bit misleading. The truth is that all G-type stars can have color range from White to light Yellow. Possibly ‘Yellow’ is used for differentiating the G-type stars from ‘White Dwarf Stars’.

General Characteristics of Yellow Dwarf Stars or G-Type Stars

  • They have a mass of anywhere between 0.84 Solar Mass and 1.15 Solar Mass (Solar Mass means the mass of our own Sun).
  • Surface temperature of G-type stars range between 5,200 and 6,000 Kelvin.
  • G-type stars will have a radius range of 0.96 Solar Radius to 1.4 Solar Radius (Solar Radius is the radius of our Sun).
  • G-type stars are all Main Sequence Stars.
  • G-type stars have life span of anywhere between 4 billion years and 17 billion years.
  • At one point in time, G-type stars will evolve in Red Giant stars after they run out of their Hydrogen fuel.
  • Approximately 3.5% of all stars in universe are G-type stars.

Star Types: Orange Dwarf Stars

Colloquial Name Type of Star Evolution Stage Example
Orange Dwarf Star K Main Sequence Alpha Centauri B

These stars are classified as ‘K V’ with K being the spectral type and V is the Roman Numeral signifying Dwarf Main Sequence Star.

These are subdivided in terms of hotness by inserting a numeral between K & V just as in case of Yellow Dwarf Stars above. So, K-type stars can range from K0V to K9V with 0 being the hottest and 9 being the coolest.

The Spectral Type ‘K’ denotes surface temperature range of 3,500 Kelvin to 5,000 Kelvin.

The Orange Dwarf Stars that are used as standard for study of stars that fall in K V category are:

  • K0V – 70 Ophiuchi
  • K1V – 107 Piscium
  • K2V – Epsilon Eridani
  • K3V – HD 219134
  • K4V – TW Piscis Austrini
  • K5V – 61 Cygni A
  • K6V – HD 120467
  • K7V – 61 Cygni B
  • K8V – HIP 111288
  • K9V – HIP 3261

General Characteristics of Orange Dwarf Stars or K-Type Stars:

  • They have mass ranging between approximately 0.45 Solar Mass and 0.8 Solar Mass. Yes, these stars are smaller than our Sun.
  • Surface temperature of K-type stars range between 3,500 and 5,000 Kelvin.
  • K-type stars will have a radius range of approximately 0.7 Solar Radius and 0.96 Solar Radius.
  • K-type stars are all Main Sequence Stars
  • K-type stars have a lifespan of anywhere between 15 billion years and 30 billion years.
  • Around 8% of all stars in universe are K-type stars.

Did you know?

The K-type stars are of particular interest to astronomers when it comes to searching for extraterrestrial life. There are several reasons for this. They are:

  • They radiate less UV rays and hence, are less likely to destroy DNA.
  • They are very stable with their Main Sequence stage lasting for up to 30 billion years. This means that they are more likely to develop planetary systems.
  • They are far more abundant compared to G-type stars and hence, easy to hunt.

Star Types: Red Dwarf Stars

Colloquial Name Type of Star Evolution Stage Example
Red Dwarf Star M Main Sequence Proxima Centauri

These stars also follow the same classification rules. They are:

Categorized as M V with M being the Spectral Type, and Roman numeral V representing Dwarf Main Sequence star.

They can be further subdivided into M0V to M9V with 0 to 9 representing the degree of hotness of the stars falling in M V category.

Though the M-type stars are called Red Dwarf Stars, they are far from being red. They are actually close to dirty orange.

The Red Dwarf Stars that are used as standard for study of stars that fall in M V category are:

  • M0V – GJ 270
  • M1V – GJ 229A
  • M2V – Lalande 22185
  • M3V – GJ 752A
  • M4V – GJ 402
  • M5V – GJ 51
  • M6V – Wolf 359
  • M7V – Van Biesbroeck 8
  • M8V – VB 10
  • M9V – LHS 2924

General Characteristics of Orange Dwarf Stars or M-Type Stars:

  • M-type stars have mass ranging between approximately 0.08 Solar Mass and 0.45 Solar Mass.
  • Surface temperature of M-type stars is less than approximately 4,000 Kelvin.
  • M-type stars have a radius of approximately 0.7 Solar Radius.
  • All M-type stars are Main Sequence Stars.
  • M-type stars have a lifespan of several trillion years.
  • These stars are really faint and are difficult to spot without optical aid.
  • M-type stars that have mass more than approximately 0.35 Solar Mass will be fully convective. This simply means that Hydrogen is burned not just in the core but also in outer layers.
  • M-type stars are highly abundant, making up 80% of all stars in Universe. They make up 3/4th of all stars in our Milky Way galaxy.

Did you know?

The nuclear synthesis process of these stars is so slow that it will take trillions of years to completely burn off their Hydrogen fuel and move into next stage of evolution. So, given the fact that our universe is only 13.5 billion years old, we don’t yet know what will have to M-type stars when they come out of their Main Sequence stage of life.

Star Types: Blue Dwarf Stars

THESE STARS DO NOT EXIST! Yes, you read it right! In reality, there are no such stars in universe that are categorized as Blue Dwarf Stars. They exist only and only in theory. Theoretical models have been used to postulate the existence of such stars.

It is a known fact that when stars grow in age, there is an increase in their brightness. A usual mechanism that the stars use is that they just grow in size (Yellow Dwarf Stars) and eventually become Red Giant stars.

Astronomers who have been studying the evolution of the Red Dwarf Stars have placed a hypothesis that once they come out of their Main Sequence stage several trillions of years later, those stars will increase their brightness by not increasing in size but by increasing their surface temperature while remaining roughly the same size. This theory however holds only for those Red Dwarf Stars that have a mass of 1/4th of our Sun or less than that.

If they use that rule, that is, they increase their surface temperature to become bright rather than increasing their size, they will cease to be Red. They will turn Blue. But why? The logic is simple! The hotter a star is, the bluer it is!

So, once Red Dwarf Stars turn Blue while maintaining their actual size, they will become Blue Dwarf Stars. Unfortunately, that’s only a hypothesis because our universe is only 13.5 billion years old and Red Dwarf Stars will take trillions of years to get out their Main Sequence stage. So, no one of us will be around to see that happening.

Why can’t we be around?

Brilliant question! You see, our Sun is a Yellow Dwarf and it will turn into a Red Giant some 5 billion years from now. When that happens, it will completely engulf our planet Earth because it will grow enormously big. If Earth doesn’t exist, we don’t exist until we find some other place to live and of course, we as a human race exist for that long in first place. As far as we know about evolution, humans might not exist at all in a few million years from now or we as a race may perish even earlier!

We will end our article here for now. We will continue in next part and learn about other star types. Stay tuned until then…

Sources: 1, 2, 3, 4

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