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# Classification of Stars

A star is a celestial body made of a brilliant spheroid of plasma glued together by its gravity. The closest star to the Earth is the Sun. Numerous stars are observable to the naked eye at night time. As they are very far away from our location, they always appear as fixed light points in the night sky. The significant stars have been grouped into asterisms and constellations. Astronomers have developed star catalogues that recognise and give standardised star names. The entire detectable Universe has about

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stars. Due to the optical restrictions, most are not visible to the human eye from the Earth.

## Star Formation

Stars are created within the clouds of matter and stellar dust scattered in most galaxies. These dust clouds are mostly called Nebulae. One of the most known examples of such clouds is the Orion Nebula. Compressed disturbances within these bodies generate regions with concentrated matter and energy. In those regions, dust and gas can start to collapse under the influence of their own inherent gravitational attraction. As the dust and gases collapse, matter at the centre starts to heat up. The earliest form of a star is called a protostar. It is this molten core of the collapsing cloud that eventually becomes a star. Computer simulation of star formation shows that the rotating clouds of collapsing dust and gas may split into multiple blobs of matter. This might be the reason why most of the stars in our galaxy are in groups or pairs.

## Classification of Stars

There are numerous types of stars in the visible Universe, from protostars to blackholes.

They are classified based on their temperature, mass, spectra absorption, and stages of evolution. Here let’s go into the classification of stars based on their spectral class and brightness.

According to the spectral characteristics, stars are classified into O, B, A, F, G, K, and M. B and I are the most common ones. Group B and O are very rare. They are immensely bright and hot. M stars are the most abundant type of stars. They are much dimmer and cooler. In this classification, dying stars are not considered at all. An updated system of classification was developed in the 1920s, which consists of about 225,300 stars. It was known as the Henry Draper Catalogue.

In the case of the MK system, a factor called the luminosity class is considered with the spectral class using Roman symbols. This model is built around particular absorption lines in the spectrum of stars. These values change with the atmosphere’s density. It differentiates gigantic stars from dwarf stars. Hypergiant stars are grouped into the luminosity class 0 or Ia+, supergiants in class I, bright giants II, regular giants in class III, subgiants in class IV, main-sequence stars in class V, subdwarfs in class VI (sd), and white dwarfs in class VII (D). Our Sun comes under the class of main-sequence stars as it has a surface temperature of about 5,800 K.

The video explains the fundamentals of a star.

### Different Types of Stars Based on Star Evolution

Protostar

A protostar is the earliest compact form of a star. It is an accumulation of gas that has been collapsed and compressed down from a huge dust-gas cloud. This early phase of a star has a time period of about 100,000 years. As time passes, pressure and gravity rise, which pushes the baby star to collapse into a denser form. Almost all the energy emitted by the protostar arises from the heat generated from the gravitational force. In this state, there is enough energy to start the nuclear reactions.

T Tauri Star

A T Tauri star is an intermediate stage just before a star transforms into a mature state. This phase begins after the end of the protostar stage when the gravitational force glueing the star together is the reason for its entire energy. These stars don’t possess enough temperature and pressure at their cores to kickstart a nuclear fusion. In spite of this, these stars look similar to main-sequence stars. They both have identical temperatures. T Tauri stars have strong X-ray flares and intense stellar winds. Stars stay in the T Tauri phase for about 100 million years.

Main Sequence Star

Most stars in the visible Universe are main-sequence stars. In fact, the Sun, Alpha Centauri A, and Sirius are the most familiar examples of this class of stars. Different main sequence stars can vary in mass, brightness, and size. The fundamental similarity is the existence of a nuclear fusion core, converting hydrogen atoms into helium atoms. A huge amount of energy is emitted along with this process. A star in the main sequence phase is usually in hydrostatic equilibrium. Gravity pulls that star’s matter inward. Outward forces cancel out one another, thus continuing to exist as a spherical shape. The size of these stars directly depends on their mass. In other words, the strength of gravity pulling the star inward determines the size of the star.

Red Giant Star

As the hydrogen depletes in the core of a star, the fusion reaction slows down. Once the star consumes the entire reserve of hydrogen, fusion ceases to exist, and the star fails to produce the outward pressure to counterbalance the inward pressure of its gravity. Ignition of the remaining hydrogen in the star continuously drives the star and causes it to enlarge its size drastically. The star transforms into a red giant star. The ageing star can be 100 times bigger than what it was as a main-sequence star. When the entire stock of hydrogen fuel is consumed, helium and heavier elements can be used in nuclear fusion. This phase only lasts a few hundred million years until it runs out of essential fuel. In the end, it turns into a white dwarf.

Red Dwarf Star

A red dwarf star is the coolest and smallest type of star in the main-sequence phase. They are the most common class of stars in the Milky Way. As per the latest estimation, they make up 75% of the stars in our galaxy. Even though they are very common, they are not easily detectable due to their relatively low luminosity. Proxima Centauri is a red dwarf, just like the majority of our nearest stars. They are much cooler and dimmer than the Sun. Even though they are exhausted from a lack of adequate fuel, they are still able to squeeze available hydrogen into their core. They are able to conserve fuel for a much longer time than other stars. The smallest red dwarf stars are 0.075 times the Sun’s mass.

White Dwarf Star

When a star loses its entire reserve of hydrogen fuel in the core and lacks enough mass to push heavier elements into a nuclear fusion reaction, it transforms into a white dwarf star. The outward push from the fusion reaction stops, and the stellar core collapses inward due to its own overwhelming gravitational pull. As it is hot enough to emit light, it shines to a mediocre extent. On the other hand, there is no nuclear fusion happening inside the core. It will continue to lose its heat and energy until it reaches the Universe’s background temperature. This phenomenon takes about hundreds of billions of years to complete.

Neutron Stars

Stars with a mass between 1.35 and 2.1 times that of the Sun don’t transform into white dwarfs when their core collapses. In this scenario, stars do not disintegrate into a supernova explosion; instead, they become extremely dense celestial bodies called neutron stars. The stellar core collapses into a much denser form with extremely high gravitational pull. As the name suggests, they are entirely made of neutrons. Gravitational force is so strong that the matter collapses to the level of a neutron. Gravity crushes protons and electrons together to form neutrons. If stars are much more massive, then the stars explode as supernovas and ultimately turn into black holes.

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## Frequently Asked Questions – FAQs

### What is a star?

A star is a celestial body made of a brilliant spheroid of plasma glued together by its gravity. The closest star to the Earth is the Sun. Numerous stars are observable to the naked eye at night time. As they are very far away from our location, they always appear as fixed light points in the night sky.

### How are stars generally classified?

There are numerous types of stars in the visible Universe, from protostars to blackholes.
They are classified based on their temperature, mass, spectra absorption, and stages of evolution.

### Explain the formation of a star.

Stars sprout within the clouds of matter, and stellar dust is dispersed in most galaxies. Such dust clouds are usually called Nebulae. One of the most known examples of such clouds is the Orion Nebula. Compressed disturbances within these bodies generate regions with concentrated matter and energy. In those regions, dust and gas can start to collapse under the influence of their own inherent gravitational attraction. As the dust and gases collapse, matter at the centre starts to heat up. This kickstarts the early stage of a star called a protostar.

### How are stars classified according to their spectral class?

According to the spectral characteristics, stars are classified into O, B, A, F, G, K, and M. B and I are the most common ones. Group B and O are very rare. They are immensely bright and hot. M stars are the most abundant type of stars. They are much dimmer and cooler. In this classification, dying stars are not considered at all.

### Which is the most common type of star in the Milky Way?

Red dwarf stars are the most common type of stars in the Milky Way.

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