G Type Stars - The Sun's Stellar Homies

Intermediate Low-Mass Stars are moderate UV Sources and long-lasting. Yet they burn their fuel at an enormous rate, and we call them G-type stars. Hello and welcome to our blog.

Introduction:

If you come here often, you probably notice we always start our posts with an introduction. So Let's have a quick look at this type stars. stars those bear mass between 1.04 to 0.8 Solar masses with 1.15 - 0.96 Solar radii, Surface temperature ranges around 6000 - 5200 Kelvins, they glow in yellow color and 1.5 to 0.6 times luminous than our Sun. These Star's fusion is powered by Mostly PP Chain and a Slightly CNO Cycle, That's why we said they are Intermediate Low mass stars because the CNO Cycle only takes over the whole fusion process unless the star is 1.5 times heavier than our Sun and Core's temperature crosses 15 Million Kelvin and Below this temperature, CNO only appears in a minor way.

The yellow Color of the G stars is slightly confusing Since our Sun is Also a member of this group, which doesn't appear as yellow however, it may look like orangish yellow during sunrise and sunset but this is the effect of Rayleigh scattering not by the sun itself instead when you look it at directly in space it will glow in white color. The yellow color is only a term for spectral studies of the G stars, Mostly Early G stars appear white and cool Stars look yellow.

G stars burn their fuel at a rapid rate but are not like powerful stars whereas they are not so slow as K and M stars, That's why Sun-like Stars last 10 Billion years. Life around such stars has always been a topic of research because there is one life-bearing planet around a G star. They have longer life spans than more massive stars so they give enough time to develop life forms, also moderate UV radiation doesn't interrupt planetary formation that's why scientists have found so many planets near G-type stars. Many of them reside in large habitable zones so Life on Exoplanets around such stars are center of attraction. However, there is a vast majority of Gas giants but later on, some Rocky planets have also been discovered it was because of small and weak telescopes, they were able to detect large gas giant planets, but improved technology helps us to find small earth-sized rocky planets.

Let's Explore some more of Yellow Star's variants.

Yellow Main sequence Stars or Dwarf (V):

When a star leaves the protostar phase and enters the Main sequence phase Star is lit by the Hydrogen fusion reaction, we say the Star is a Main sequence star. In this phase, the star is mostly stable, productive and balanced by gravity and nuclear forces, this is the longest period of the star's life.

Our Sun-like star Burn around 4 Billion Kg of Hydrogen each second in the core but don't worry our Sun is not going to disappear any second, The Reasons behind this are that the sun is 2 × 10³⁰ Kg in mass and it will only convert 4 % of hydrogen into helium, that is enough to shine for 10 Billion years main sequence time. The entire lifespan of the Sun, So after 10 Billion years it can only convert 0.07 % of total mass.

Isn't it interesting? Stars use small amounts of fuel and last several million years but hey! it's not that small, only 0.1% of any element's abundance will be several thousand times larger than the entire earth.

After the hydrogen starts to decrease and helium increases, the star's luminosity grows and The core shrinks due to gravity and the outer portion expands in response. It moves towards the subgiant phase. Before we move to Subgiants, you can look at pictures that show some stable stars.

Yellow Subgiant (IV):

If you read about stars in astronomy, you have probably thought why don't the star burn its all of hydrogen? Well, Only the Core has enough temperature and pressure can fuse hydrogen into helium along with other factors. Therefore, the outer portion has nothing to do with Nuclear fusion, except in special conditions.

The G-type star loses hydrogen and creates helium you can think of it as we burn wood and ash remains, this type of helium can't be fused so we call it degenerate helium, it mainly adds more mass to the core.

Density increases and the core begins to contract and The outer envelope expands. This is called the subgiant phase and the star will keep growing in size until it becomes a common red giant. One more thing, When the outer portion of the star expands it gets cool while the contracting core's temperature increases and it causes the outer portions to look orange hue. You can look at this image by noting their radius than the previous Main sequence Stars.

Yellow Giant (III):

This is also called the RGB (Red Giant Branch) phase, where the star is dangerously stable due to gravity's compression and weak nuclear fusion energy output so gravity seems here she is winning, tries to crush the star's core and it does but something strange happens as the gravity crushes the core, atoms come close to each other it causes enormous heat and it ignites the Hydrogen that was not used yet, that is located outside the core and Compression gives them enough temperature and pressure to initiate another chance to survive the star. This is called Hydrogen Shell burning because it takes place in border regions where the convection zone and Core meet. This Fusion is so rapid and varies with the time and chemical composition, at this time star's photosphere's temperature stays between 2000 to 4000 K So it appears red. we will know about them in the M-type Stars post but here we will continue to know about Yellow Giants.

In the last sub-section, we knew they had to become Red Giants but here we see them as Yellow Giants Why is that? That's because The Main sequence G-type star otherwise called Yellow dwarf expands and becomes a Red giant due to temperature loss in the outer layers, but If you read our other posts, especially about the A and F types of stars you will know that such massive stars are not immune to temperature loss either they evolve when they hydrogen fusion stops for sometimes and they become yellow giant when they lose heat but it doesn't happen in one day rather it takes around thousands of years, in this phase, They will sit here If they glow Yellow. That's why you see here 5 Yellow balls most of these were not G Main Sequence stars instead can be evolved from other higher classes.

Yellow Bright Giant (II):

We know that massive stars change their color as their fusion slows or gets intense Since it all doesn't happen in one day, we have a few such stars. They were massive Blue or White Bright giants, They lost their temperature and glowing in yellow color.

Yellow Supergiant (I):

Now the star is working hard for existence, it has to do something to cope with gravity. In this phase, Compression of the core stops when Helium gets enough temperature to Start the Fusion. when this fusion initiates, the Whole System of the core feels a powerful wave of energy and enormous light comes out almost like a supernova but most of the light is soon absorbed or driven away by the surrounding layers and their interactions. Now Helium fusion starts in degenerate conditions in an appropriate region hence this tremendous heat and density removes degeneracy and acts like the next fuel of the Star. Therefore, the Supergiant star has two Sources of Energy. Thus, the Star becomes brighter yet unstable we see it as a Flashing light. Many Supergiant stars pulsate which is an indication of the ongoing battle between Star and its Gravity. hence it is no longer on its balance point instead it tries to maintain this point.

AGB Star is losing plasma which was a Sun-like star

The energy and pressure of two burning shells in the core exert strong pressure outwards that expels the outer portions of plasma of the Supergiant star and it stops Several AU's away, creating a nebula-like structure. that's why such stars are surrounded by dense clouds of dust and gas. They will keep losing plasma hence their mass because the core doesn't have enough fuel to fight gravity which requires fuel to burn at an enormous rate so The core will crushed by gravity. Now, this time it seems the core will become a black hole but the degeneracy pressure of electrons manages to prevent that further collapse we can discuss this process in another post. For now, you should know that The core becomes a White dwarf due to degeneracy pressure. Thus, All the processes that provide energy to the star stop and cool over time to become brown then black dwarf.

You can see some examples of Supergiant Yellow Stars that have Sob story of becoming Blue-white stars to Yellow Supergiant. keep in mind These stars were supergiant since Birth.

Let's Explore some Exoplanets:

When topic comes around habitable zones around G type stars, There are a vast number of planets have been found and this number is still growing. We know That Earth orbits a G star so there is more chance of life on exoplanets. Let's explore some Interesting Exoplanets that are habitable or considered at least.

An exoplanet with Green Sky and G type Star

Tau Ceti e:

This system has 5 known planets but this planet has some similarities with Earth, It orbits a G8V Star which is slightly cooler than the sun and this planet was detected in 2012 but its properties were defined in 2017. Tau Ceti e orbits its star in 168 days which means it is close to the star and may have a temperature around 65 degrees Celsius, some studies also suggest it may have turned into a Venus-like planet but after 2022 it came up that this planet receives Powerful Stellar Flares hence life is not possible there.

Gliese 139 d or HD :

This planet orbits a G6 Yellow dwarf star and completes an orbit around 647 days. 20 years of observation revealed that it could be 6 times more massive than our earth and maybe a Neptune-like gas giant or can be a rocky planet because its composition is not cleared yet but it is High priority target for Goldilocks planet hunters.

Kepler 22b:

This system is 500 Light years away in Cygnus Constellation, This planet orbits a G5V Star and completes in 290 days. This is a super-earth Kepler-452 b. The Sun of this planet is 25 % dimmer than our sun. The striking feature of this planet is its highly elliptical orbit which will cause massive temperature differences. However, despite these obstacles, Scientists think this can be next our home.

Kepler 452b:

It is 1400 light-years far in the Cygnus constellation and This is a famous exoplanet that has tremendous analogies with Our Earth, It revolves around its star in 385 Days, Where our Earth has a Radius of 6378 Km this planet has 9567 Km which is only 1.5 times larger than Earth, Average Temperature stays at -8 degrees Celsius and earth has 15 and this planet's escape velocity is around 20 Km/s. This means we won't be able to walk too easily on its surface it would be a little bit difficult, If you lie down you will have more force to get up, If you weigh 72 Kilograms on Earth, Your weight will be 136 Kgs there and If your monthly Ration is about 10 Kg then on this planet it will 19 Kg everything heavy on earth will even heavier on Kepler 452b. which makes it hard to survive here.

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