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In previous Guide, We were knowing about some Features and the processes that take place on a Star, Which basically Starts when star Enter its main Sequence stage. Let's say those stars were Fusing their Hydrogen and emitting all kinds of Radiation, energy etc. for Millions of years, some are still same or some have changed a lot, They are getting old you can say and Going further to become white dwarf, neutron star or Black hole But They Don't Directly change into them. yet they'll take millions of years in this period many interesting things will happen.

Like previous Guide we'll again discuss in comparative manner meaning 3 major type of stars at same time so that you can know each of them. for example, taking a red dwarf and O - Type Star, you will notice O - Star will changed completely while Dwarf is same So let's Get Started.

Common Subgiant Star - key features


Now Stars have remained few amount of hydrogen in their core, temperature will increase so fusion rate also get intense and they gradually become more brighter than ever. once they will cease hydrogen fusion in their core, it will begin to collapse. it causes increase to temperature and now it will fuse hydrogen outside the core. by fusing outside, will provide much more energy than fusing in core.

Low and Solar mass stars expand and cool until they reach at 5000 K then they become common giant.

So here in this section we are gonna discuss about the Transition of Main Sequence phase to Giant phase, in this period Star have both properties of Giant and Main Sequence That's why it is called Subgiant phase and it is indicated by IV in MK System or Yerkes System.

Subgiant Stars have their own unique signs which differ them from other classes while some are just blended with main sequence and Giant classes Nevertheless They show lines of lithium when you will see in their spectra and show many coronal activities.

Let's Briefly see What happens to each class of stars if they will enter in Subgiant phase :


If they have Less than 0.4 Ms (Mass of the Sun) they won't become Subgiant because they are fully convective so they will keep Fusing Hydrogen until entire star is helium and Theoretically speaking it won't enter in White Dwarf stage.

On the other hand, If they got 0.4 to 0.9 Ms Meaning K - Type star and Massive M Star At least 0.6 Ms choose different path. Since They got Non convective cores, when they finish hydrogen in core they will fuse the hydrogen which lies Exterior regions. this Feature keeps them into Subgiant Phase. As they continue to fuse it and make helium. the fusing outer hydrogen will cause to expand the Star's outer shell, it can be large about 2 - 10 times than its main sequence phase. Their energy production of outer shell's hydrogen will give ability to maintain star's surface temperature and it will put the Star in little lower categories of spectral classification.

They will stay at Subgiant phase for 1 to 8 Billion years. in their last stages their helium core gets too massive to support it and start to degenerate. Further their temperature and fusion rate increase, its outer layer is highly convective. Now they become Red Giants.


From Sun like stars to 8 Solar masses stars will develop more massive helium core than Previous categories. it will take larger portion of star. when their fusion will Cease entirely, core start to Collapse and temperature will increase. become hot like My High school head-master's brain. it will continue for Several Million Years until Core becomes hot enough to burn hydrogen in a shell. Temperature will decrease and star will begin to expand.That's how they enter in Sub giant phase.

Their core will less than 2 Solar mass which is below than Schönberg–Chandrasekhar limit. However Hydrogen fusion in their shell will quickly bump the temperature beyond that limit. Slightly Massive stellar cores are already above the Schönberg–Chandrasekhar limit when they enter Subgiant phase and leave the Main Sequence. once they cross the Chandrasekhar limit (1.44 Solar Mass) they will loose thermal Equilibrium with hydrogen shell. it causes outer layers of star cool and expand. Stars with 1 - 2 Solar masses would develop degenerate Helium Core. around this point expansion stops and they Reach Giant phase.

Their Core contraction and outer layer expansion period is very Short about Few millions years. in this point, when they were in main sequence Their Temperature was 30,000 K - 6000 K, but it will decrease to around 5000 K.


Massive Than 9 - 12 Solar Mass Star's fate will decided by their Metallicity, Such stars have Hot massive Convective Cores. their High rate of Fusion will shortened their Subgiant phase. B type Main Sequence Stars go through very quick subgiant phase before becoming Supergiants.

But Massive O Stars will rarely enter subgiant phase due to rate of their Fusion.

Look at this picture For comparison among 7 main Classes Subgiants:

Let's know about The Giant Stars which is denoted by III in MK System. We have seen that these stars were expanding and getting cool, their outer layer becoming fully convective and Turbulent. Except for low mass stars, their Fusion products were deep down around the core but first time they will appear at outer surface because of convection. You know that sub giant had helium cores, it will evolve in this Giant phase.

Let's know category wise how they will choose different paths. One more thing we should remember is that stars below than 0.25 solar masses will not enter this phase at all. So I'm not gonna write here about them. Let's split them into only two major groups.


Stars about 0.6 -10 Ms become Red Giants which evolved from K and M Stars. For example, Aldebaran and Arcturus. They glow in Red color and shine extremely. Mid Sized Star will have Two Different Red giant phases, first is Red-Giant-Branch (RGB) meaning an inert core made of Helium and Hydrogen Fusing outer shell. The second is Asymptotic-Giant-Branch (AGB) such stars also contain an inert core but made with Carbon, Helium Fusing Shell inside Hydrogen Burning Shell. So our Stars are still fine but no longer in Thermal Equilibrium.

Inert Core doesn't mean they completely lacking processes but they show very minimal Activities and not so remarkable at current period of Astronomical Era.



Stars with more than 12 Solar masses already have high luminosity, they become Blue giants. their core starts Helium Fusion before it degenerates.

Stars with 8 - 12 Solar masses have intermediate properties and are often called Super AGB Stars. Such stars mainly take the path of Lighter Stars, they become Red Giant and AGBs.
but they have enough mass to start Carbon Burning and sometimes Neon. Such stars will form Oxygen-Magnesium-Neon Cores. These stars may leave an oxygen-neon White Dwarf.


Since O type Stars are Highly luminous since birth, Their giant phase will be very short and quick, in which they slightly increase luminosity and size. Such giants can Thousands of times Brighter than the Sun and supergiants. Sometimes while they burn hydrogen, they can reveal features like Giants stars due to powerful stellar winds and mixing of heavier elements in their surface. It expands their atmosphere.

Let's know some more about RGBs and AGBs in upcoming Sections.

In this giant phase, fusion stops in the core and contracts due to gravity, it increases temperature. then Start to fuse hydrogen which lies outside of the core, since this is limited to its appropriate temperature and pressure zones so it is called Shell burning.

Due to radiation pressure, outer layers expand and we know when Gas expands when heated. convection delivers the energy from Fusion zones to outer portions of star. during this time its luminosity reaches thousands of times. G Star can become K or low mass M Type star of class III..

Its outer layers are held weakly by gravity and can easily be ejected from star thus they lose their mass. its outer envelope has very low density and Cool, while its contracted core is incredibly dense it becomes one third of its original size. electrons of core form degenerate electron gas in which they are tightly packed in a volume and don't behave like normal gasses.

Hydrogen fusion creates more helium and deposits some of its helium towards the core, in this way they get mass and heat. when its temperature touches 100 Million Kthen, it is able to Start Triple alpha process meaning it will fuse Helium and produce carbon and Oxygen. Stars with 2 or 3 Solar masses can do it. once somewhere in the core this process starts, it spreads quickly due to Electron Degenerate gas. This kind of sudden Break out of Helium fusion is called Helium Burning or Helium Flash.

When Helium Flash occurs in stellar core, it again raises the Temperature to the point where it will no longer degenerate.

A solar Mass Star can Burn Helium for about 100 Million Years.

Now we have Seen that Red giant star’s core was creating Carbon and oxygen isotopes Through Helium Fusion. We should remember that these elements are hard to fuse due to their high proton numbers.

Thus No Fusion can Sustain in their core, but Core contraction can cause heating up surrounding outer portions of Helium and it starts to Fuse it. Now heat energy generated by Helium fusion will reach unused Hydrogen layers and initiate Hydrogen Fusion in the shell.

Star Expands again, this time it can become as large as 1.5 AU, almost Mars's orbit. This phase is called Asymptotic giant branch Star (AGB). its luminosity would be 10,000 times stronger than the Sun.

Like common Giants, its outer envelope is also held weakly by Gravity. Its high reaction rate releases enormous energy. It also shows thermal pulses for about hundreds of years. Meanwhile, nuclei of helium burning shell can synthesise heavier elements By Beta Decay and Neutron capture process.
This process is called S - Process (slow in lower mass stars) and R - Process (Rapid in higher mass stars) in this way they can produce elements like Bismuth. Thermal pulses in AGB Stars can occur once 10,000 - 100,000 years.

Helium Fusion Products can be Carried by Convection to the stars surface this is detectable in their spectra. As the AGB Star evolves its material ejection rate also increases, which contains carbon and oxygen. The carbon particles often tend to surround the star and create dust and gas clouds.

When the cloud expands and cools it absorbs and emits again the radiation from the star at longer wavelengths, that’s Why AGB stars are more luminous in Infrared regions. Some radio wavelengths have also been detected.

The Giant Stars are abundant in the universe and they have a wide variety. Let's look at some of them.

01) Bright giants :

These stars have been large since birth and have evolved while maintaining their mass and radius.



02) Red giants :

These are evolved stars from K, M, and G Type Stars. A brief description has been given above. They have two main phases named Red giant (RGB) and Asymptotic giant(AGB)

03) Yellow Giants :

They evolve from F and A Stars, this group contains several Variable Stars. They are also rarer than red giants because they form from massive intermediate stars and they pass this phase quickly. But here’s one thing that if a stellar phase is at least 1000 years it will be enough to locate and study because in human timescale thousand years is a very long time. Since technology is forwarding at light year speed (You know what I mean)

04) Blue and white giants :

Hot Stars like O, B, and Massive A Type Stars and sometimes low mass O and B Stars also called White giants. Blue giant group contains diverse features and shows various endings.

High mass Stars will become Blue giants then Blue supergiants then supernova and neutron stars or black hole remains.

Lower mass helium burning stars can evolve from Red giants then AGB, depending on mass and metallicity they can become Blue giants.



Now Here are some other types of stars Which you may have seen in SpcaeEngine, If you are a Star hunter. They are Different from other stars which we have mentioned earlier.

01) C - Type Star :

These are basically Classified as R and N Stars, These Red Giant Stars have reached the end of their lives, their abundance of carbon in their spectra makes them special from Common Giants. Although most known Carbon Stars or C - Stars are Giants or Supergiants but few main sequence carbon stars have been found too.

Here are some of the C type Stars and their specifications



Some Carbon Stars.


02) S - Type Star :

They are closely connected with Carbon Stars and M type Stars, their special feature is Zirconium (Zr) and other elements which are produced by S - Process. They have similar amounts of Carbon and Oxygen like M Stars. These are Also AGB Stars. In SpaceEngine They are called Zirconium Star

Their spectral classifications are similar to what we've seen in my previous guide. They also consist of numbers based on temperature and they almost follow the Temperature scale of M Type Stars. S3 to S5 Stars are common S Stars.

However, some intermediate stars are also possible eg. SC or MS.


Some S - Type Stars (aka Zirconium stars in SpaceEngine)


03) Wolf - Rayet Stars or WR Stars :

These kinds of stars were members of O - Class, But scientists found they had slightly different features in their spectra. Unlike Common O-Type Spectra, They had not enough hydrogen, instead these spectra were showing lines of highly Ionized helium, Nitrogen, Carbon and sometimes oxygen. These were thought to be dying Massive and hot O - Type Supergiant Stars because they were ejecting material at very high speed which was exposing their Helium shells. Therefore they are located mostly in centers of Some Planetary Nebulae. Before this group was known as W Stars.

If you wanna know some more details of these Stars You can Go Here

Let’s Discuss on Largest Types of stars now, in MK System They have various denoting methods. The most common is, Ia for Luminous Supergiants, Ib for Supergiants and 0 or Ia+ for Hypergiants. Some intermediate Supergiants can also be described as Iab.

The Supergiant can belong to every Spectral class from O to M if they have enough specs. Sometimes O type Blue supergiants could be Young Stars whereas M Supergiants can be Highly Evolved Stars although it's not clear.

Supergiants have produced a lot of Elements in their Life times, You remember while reading previous sections they were creating Carbon and Oxygen, these elements also found in our bodies that’s why it is said Stars are Creator of Chemical Elements when they become supernova or loose their outer envelope and Finally create a New Star and planets, they take consist these element.

Such Stars are 8 to 12 times massive than our Sun, thousands of times more luminous. They Fuse all the way to Iron. due to higher mass They often explode as Supernova.

According to Stefan - Boltzmann Law, The Cool surface of Red supergiant will radiate much less energy per unit area than Blue Supergiant. At a certain luminosity the Red Supergiant star will be larger than Blue.

They aren’t stable either, they shrink and grow called pulsation. Their temperature varies from O type about 40,000 K to M type 3400 K. Supergiant doesn’t exist Cooler than Mid - M Type stars.

Although Supergiants exist in every Spectral Class, the B class has the majority of them while G Contains few Yellow Supergiants. Most Type II Supernovae occur from Red Supergiants.

Those were the stages and processes that take place on Post Main Sequence Stars If you want to know their deaths then Refer to this guide.

Even though scientists have gathered enormous information and are still gathering, we can’t say anything with certainty because one important question comes to mind: did we solve one of the most difficult mysteries, which is How stars are born and die?

Just think about it, the MK system uses IV, V, III etc. numerals for stellar luminosity that’s why some Blue giants are main sequence stars while mostly stars become giant due to ageing. Roman numerals can denote the evolutionary stage and luminosity of a star but age is defined by spectral features and complex maths equations.

We still don’t know whether the supergiant, subgiant or giant phase is the effect of age or they were such since birth, but scientists interpret something like that.

You can also understand this phenomenon. Just imagine you are an alien and want to know about human life on earth. When you come to earth and go to the city, you’ll see people, men, women, boys, girls, children, toddlers, teenagers, young and old people. I mean whole individuals who contribute to society. You may be wondering how such beings are here and why, where did they come from? If all these guys and gals are human then why do they look different ? why do some are smaller, shorter, taller, paler, fat, bald, long haired, hipsters etc. do they such were by birth or got some changes over time?

After some little research you will probably know how they are born and die, but from toddler to old guy will still be a mystery for you unless you have an advanced technical device that can show you such changes or evolutionary phases of human beings or maybe you will have to live among them and watch them growing.

So the stellar life cycle is the same kind of mystery, but science and technology is developing. In the future we will know more about them and these will be proved solidly not by just theories and speculations.

Well, I hope you enjoy this guide while reading and your knowledge increases. We’ll see soon in another guide.

Thanks again for reaching and reading this guide.

Have a nice day and night

Goodbye.