The A-Type Star - The Cosmic Showstoppers

Bright, Massive and Whitish-blue stars, Sources of powerful Ultraviolet Radiation. Hello and Welcome to our blog CosmicWisdom where we discuss astronomical bodies and structures with simple comprehensive language. In this post, we're going to know about A Type Stars. So, without wasting any time let's dig in.

An A Star Surrounded by Debris disk

Introduction:

These stars are Very hot and shine in whitish-blue color in space, when they are in the main sequence phase, they have a common temperature of about 7500 - 10,000 Kelvins whereas our sun has surface temperatures of about 5800 K. However, When we say surface temperature, we mean its photosphere, the area where the Light emanates from the star. Their radius is 1.2 to 1.8 times and Luminosity is 5-25 times greater than our sun. some A Stars are surrounded by debris disks which are building blocks for planet formation hence we can expect some planets around such stars. These stars last about a few 100s of Million years, the perfect timescale for planets. Their UV Radiation is not so strong for at least planets that can disperse the gas molecules of the stellar nursery into space.

This planet is a Jupiter like Gas giant which has Raging Storms on its outer layers

Imaginary view of an Orbiting Gas giant orbiting a binary A Stars

This image describes, There are 9 A Stars within Light years Vicinity of our Solar system.

Let's see their different types and stages:

Evolutionary phases:

1) Main sequence:

After the Protostar phase, when the star generates its energy by Hydrogen Fusion and creates helium in the core, we call it Main sequence phase and stars spend most of their lives although we have repeated this fact so many times in our posts. we are just mentioning it here just because you know what we are talking about. So, The Fusion of hydrogen atoms gives enormous energy to the star but the Helium starts to deposit. this time depends on how fast or slow the star is fusing. The faster the process shorter the lifetime. Massive A Star can last about 1 billion years in this phase, while the least massive A star will spend around some billion years. like many other massive stars, Their nuclear fusion is also powered by CNO Cycle. you can also look at the image below to see some common A main sequence stars.

This Chart contains some common features about 5 A type Main sequence stars

This image tells about the Castor Moving group of stars. These stars have similar properties. Some of its members are Castor, Fomalhaut, Vega, and Alpha Librae.

2) Subgiants:

According to stellar evolution theory, when the star's hydrogen supply in the core is about to exhaust, it starts to expand and become Giant but this takes quite a long time depending on the mass of the star. There is so much degenerate or inactive helium which is the ash of the hydrogen fusion, however, Degenerate or inactive doesn't mean that they are useless rather they are less important in the case of fusion. when the fusion rate slows down a little bit, the core contracts and the outer part of the star begins to expand. As the core becomes shorter due to gravity, the particles inside the core are forced to get closer than before, this event increases the heat and it touches more than 13 Million Kelvins, it ignites the Hydrogen that was lying just outside the core. fusion begins but in a small region where the outer layers aka envelope and degenerate helium's sphere meet, this is called shell burning and the star now becomes a subgiant. luminosity gets stronger than the main sequence phase because shell burning provides much more energy than burning in the center. you can see in the image below.

This Chart contains some common features about 5 Subgiant stars from A class.

3) Giants:

Let's talk about the Giant phase. As the Hydrogen burning in the shell keeps adding more helium and inactive helium core mass increases, once it meets the required conditions meaning 100 Million K temperature, the core becomes half a solar mass and Density gets around 100,000 grams/cc, meaning If you put this one hundred thousand grams into just one cubic centimeter along all those parameters. star will be able to fuse that inactive helium and make Carbon and oxygen. At this time, Star's core was contracting and the outer envelope expanding we call this the mirror effect. Star now enters the giant phase. However, most of the A-type stars won't be White any longer because as they expand their temperature drops and become Red giants. we can say some of the current white giants are evolved O or B-type low Main sequence stars that have lost their temperatures and become white but the MK system looks for their mass and color That's why we are treating them as white A-type giants. you can look at the picture below.

This star is surrounded by a large debris disk containing dust and gas, and this area extends up to several astronomical units.

Here's the comparison and Basic properties of 5 Common White Giants

4) Bright giants:

When Giants Fuse Helium and create Carbon and oxygen which deposits under the Helium burning zone. stars slowly move toward the Supergiant phase and this transition is called the Bright Giant phase. like the subgiant stage, this is a very short period compared to the other phases.

Sirius is a Binary Star system whose companion is a White dwarf called Sirius B.

Some White Bright Giant Stars with Their Basic Properties

5) Supergiants:

As we said earlier, Most of the white main sequence stars can't sustain their color due to temperature loss. so they become white to red supergiants. here we see some white supergiants, they can also belong to the A V class unless they are very hot like A0-A3 stars otherwise, they are supergiants from Cool B type stars or Late O stars that have fallen here because of enormous mass and temperature loss through stellar winds. they are also burning helium and creating Carbon and oxygen along with a minority of other elements but they are not capable of Burning the Carbon and Oxygen. Therefore, their life ends here. They will eject their outer layers in massive amounts, creating a planetary nebula and Star's core will become a White dwarf.

This image shows the Properties and Comparison of 5 White Supergiants

Cultural Significance:

Let's see some visible A type stars with cultural importance:

Sirius:

This is the brightest star in Earth's night sky, in most cultures a dog represents it hence it is called Dog-star.

Vega:

In Chinese, this star refers to a weaver girl called Zhinü who is connected with the Qixi Festival which is celebrated one day on 7th day of the 7th month of the Chinese lunar year. this festival is similar to the Valentine's day.

Merak:

In Hindu culture, this star is Pulah, one of the 7 great Rishis.

Castor:

In Greek culture, This star refers to one of the twin sons of Zeus and Leda.

Denebola:

This star is known as The Changer as denoted by The Irani astronomer Ulugh beg.

The Sun as an A Star:

It could be possible If the sun was formed such strong, it would age quickly but not as fast as O and B types. A star has relatively longer life than more Massive stars that's why some exoplanets have been found around these stars. Let's know what would happen If Our Life giver star turns into a gentle white, Life taker.

Effect on Life and Earth:

The entire landmass will convert into the Sahara Desert, Herbivores will starve, Carnivore animals first kill and eat other animals and extinct later thus, the Food chain of every ecosystem will disappear soon, enormous Heat and radiation will fry everything and everyone. As for humans, Nobody can see anything due to the intense flash of Sunlight unless you have good dark goggles. The Sea's water will vaporize and reach the sky and cover the earth then it will disappear soon in about a few hundred years. Life will be completely erased from The Earth.

Effect on Solar system:

As soon as the Sun becomes an A2 V Star it will Burn 100 Billion Kg of fuel instead of 4 Billion Kg each second, Sun will shine brighter than ever, radiation output will increase in tremendous amounts. it will destroy the inner planets and asteroid belts with heat and Radiation in some thousand years, whereas the outer planets will become hot Gas giants and the outer solar system will Fry in that Stellar oven.

At the end:

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