Introduction

Has anybody ever wondered how is a star self-sufficient or a better question would be how are humans able to gather intel on a particular star? This blog is going to revolve around the study of stars including our sun and a brief on quasi stars.

About

On Stars

In my previous blog, I discussed the life cycle of a star, there I penned down how a cosmic dust cloud ionizes to form a star followed by a star’s death. There are two distinct pathways, which path a star follows depending on its mass. A star with a mass lower than the Chandrashekhar Limit (i.e. below 1.4 solar mass) is bound to transform into a White Dwarf. On the other hand, if a star with a mass above 1.4 solar mass can end up as a Black Hole or as a Neutron star. Thus, we can conclude that a mass of a star defines its path in the life cycle.

The color of stars is determined by their surface temperature however, there is a common myth in people that the mass of a star decides the color of the star. Astronomers have classified stars into various types on the basis of their color. The tabular data given below gives the classification of stars based upon their temperature along with their color from a lower temperature range to a higher temperature range. It can be seen that blue colored stars are one of the hottest in the Universe.

The Temperature values are in (°C)

One can find millions of stars of different brightness in the dark blanket surrounding us. Common folks often have a misconception regarding the brightness of a star when they observe it from the Earth. The brightness of a star depends on the distance it is away from the Earth and also it’s surface temperature. Some stars are brighter but they are far away from the Earth thus appear fainter whereas some stars are fainter in reality they are very close to Earth hence appear brighter. We use the term ‘magnitude’ to define the brightness of a star. We will look into three types of magnitudes - Apparent magnitude, Absolute magnitude, and Bolometric magnitude. They are defined as follows:

• Apparent magnitude is the luminosity of a star as observed from Earth.
• Absolute magnitude is equal to the apparent magnitude of a celestial body if it were located at a distance of 10 parsecs.
• Bolometric magnitude is the luminosity of a star based on the total radiation it emits across all wavelengths.

It is a thought-provoking matter that how can humans study stars which are hundreds of light-years away (Light years - distance traveled by light in a year; 1 light-year = 9.46 x 10^15). Astronomers don’t study stars physically instead they try to analyze the different types of radiations which they are receiving from distant stars. We can study various characteristics such as redshifts, luminosities, and various different types of waves emitted because of the different types of reactions taking place in a star. Researchers have worked upon and derived mathematical relations to calculate distance, mass, velocity, etc. of any celestial body just by studying the electromagnetic waves received from the stars.

Our Sun

Our sun is 4.6 billion years old and it is classified as a type G star which is yellow-white in color and its mass is 1.99 x 10^30 kg. On a new scale when we consider the mass of our sun to be one unit then the scale is called a Solar Mass. Since our sun consists of 1 Solar Mass that is much lower than the Chandrashekhar Limit thus, it won’t end as a Black Hole or a Neutron Star. Moreover, the surface temperature of our sun is 5600°C and is found near the Orion Arm of the Milky Way Galaxy. Our sun around which eight planets and several other celestial bodies revolve still remains a mystery to us. There are many problems that astrophysicists are facing regarding our sun. Firstly, the temperature of Corona (sun’s atmosphere) is two million kelvins whereas, the surface temperature of the sun is slightly less than 6000°C, one can easily say it is a violation of the second law of thermodynamics. For others who aren’t familiar with the laws of thermodynamics can understand it in this way that the atmosphere of a light bulb (surrounding air) is less hot than the surface of the light bulb, this is common logic. Secondly, we don’t know the reason why spicules occur in our sun i.e. why a jet of dense gas is ejected from the surface of the sun up to 10,000 km in space. Lastly, reasons for disturbance in the magnetic field of the sun is also unclear, since it is connected with active regions on the Sun’s surface which are responsible for Field Reversal in the sun periodically after every 11 years (i.e. Poles in the Sun flips at the peak of every solar activity cycle - 11 years).

Quasi-Stars

Quasi-stars are a variety of star that was supposed to exist in the early universe. They are extremely luminous and very different from modern stars. Modern stars are powered by nuclear fusion, whereas a quasi star gains energy from its substances falling in the black hole that is situated at its core.

Quasi stars are formed in a very unique manner. When a very huge protostar (equal to or more than 1,000 Solar Masses) collapses a black hole is generated at its core, whereas its outer layers are strong enough to absorb the burst during a black hole formation at its core. This is how a quasi star is formed. There is another explanation for the formation of quasi-stars. Dark Matter Halos, a region devoid of cosmic expansion that contains gravitationally bound matter, are also considered to be responsible for its existence.

When a black hole is formed at the core of the massive protostar then it would release huge amounts of energy in the form of radiations whenever a stellar material falls into the black hole. And this energy is fed to quasi-star so that it continues to exist. Quasi stars have a short life span as compared to modern stars as it cools down until it dissipates and leaves behind the black hole at their core.

Conclusion

This blog was concerned with the study of stars including our sun. In the concluding section, I just want to mention that there are still many unknown variables for humans to claim that we know more about the Universe since we know too little of a star that is closest to us let alone millions of stars far away from us. Moreover, basic knowledge on quasi-star was provided and I find this topic interesting.