The Life Cycles of Stars: Meaning and Example

Key Concepts

• Stars
• Analysis of starlight
• Composition of stars
• Stars’ temperature
• Size and mass of stars
• Stages of life cycle of a star

Introduction

Stars are huge, shining balls of extremely hot gas (known as plasma) in space. The Sun is our nearest star. During the nighttime, many other stars are visible to the naked eye from Earth and appear as glowing dots due to their huge distance from the Earth. 

Though stars look tiny in the sky, they are actually huge objects, millions of times bigger than the Earth. They look tiny since they are very far away. The Sun is the nearest star, and after that, Proxima Centauri is the nearest star. It is 4.3 light-years away, which is around 41 million, million km away. 

A star is a shiny ball made up of gases that releases a huge amount of electromagnetic energy. This energy comes from the nuclear fusion that takes place within the star. Nuclear fusion is the combination of fusion of light nuclei to form heavy nuclei. 

When we see from the Earth, many stars look like tiny sparkling objects of white light. But if we look closely at the stars, then we can see that they differ in color. For example: the star Antares glow with a somewhat reddish color. Rigel star shines with a blue-white color, and the Arcturus star shines with an orange color tint. The star, Sun glows with a yellow color. 

Analysis of starlight 

Astronomers study stars basically by examining the emission of starlight. Astronomers study starlights by using spectrographs. Spectrographs are instruments that separate light into various colors or wavelengths.  

The light of a star passing through a spectrograph gives out colors and lines known as spectrum. There are three types of spectra:  

1. Emission spectra or bright line spectra    
2. Absorption spectra or dark line spectra   
3. Continuous spectra 

All the stars have got dark line spectra, i.e, bands of color crossed by dark lines where the color gets faded. A star’s composition and temperature can be shown by star’s dark line spectrum. 

Stars are made up of various types of elements present in the gaseous state. The inner layers of star’s photosphere is very hot, whereas the outer layers are relatively cool. Elements present in the outer layer absorb a certain amount of light emitted by elements present in the lower photosphere. Different elements absorb various wavelengths of light. By studying the spectrum and elements that form a star, scientists can find out the temperature of the stars. 

The Composition of Stars 

Each chemical element has a standard spectrum in a given range of temperatures. The color and lines of the spectrum of a star show the elements that make the star. By using spectrum analysis, scientists have studied that stars are made up of similar elements that comprise Earth. The most common element on Earth is oxygen, the most common element in the star is hydrogen, and the second most common element is helium. Elements like carbon, oxygen and nitrogen are generally present in small quantities. 

Star’s temperature 

The surface temperature of a star is shown by its color. The temperature of the majority of stars varies from 2,800 ﹾC to 24,000ﹾC. Usually, blue color stars have a normal surface temperature of 35,000ﹾC. The red stars are the coolest stars with anormal temperature of less than 3,500ﹾC. Yellow stars like the sun and Capella have a surface temperature in the range of 5,000 to 6,000 ﹾC. 

Stars: Size and Mass 

Stars differ in size and mass. The smallest stars are somewhat bigger than the planet Jupiter, around 1/7^th the size of the Sun. The majority of the stars are small and less massive than the sun. The Sun is a medium-sized star with a diameter of around 1,390,000 km. Some of the huge stars have diameters that are 1,000 times bigger than the diameter of the Sun. The mass of many stars is the same as that of the Sun. However, some stars may be more or less huge. The stars that are very dense may have higher mass as compared to the sun. The less dense stars may have a large diameter as compared to the sun but still have less mass as compared to the sun. 

Stages of Life Cycle of a Star 

A star remains stable due to the energy created by nuclear reactions on the inside, which balances the inwardly directed gravitational force. Let us learn the life cycle of a star. 

Stage 1: Huge cloud of gas 

A star starts its life as a huge cloud of gas which is normally an accumulation of dust, gas, and plasma. 

From inside, stars form comparatively dense concentrations of interstellar gas and dust called molecular clouds. Gases become molecular at these temperatures; it means that atoms attach together. CO and H₂ are the very common molecules in interstellar gas clouds. 

Stage 2: Protostar -Baby star 

A very young star called protostar is still in the process of collecting mass from its parent molecular cloud. The protostar stage is the initial step in the process of stellar evolution. The protostar stage starts when the fragment of molecular cloud first disintegrates under the force of self-gravity, and an opaque, pressure supported core forms within the collapsing fragment. In the beginning, these fragments consist of ~0.01 solar masses of material but slowly increase in mass as the adjacent material is accumulated through accumulation.

A protostar looks like a star, but still, its core is not hot enough for the process of fusion to take place. The luminosity comes entirely from the heating of the protostar as it contracts. They are generally encircled by dust that blocks the light that protostar emit, so they are hard to see in the visible spectrum. 

The stage of stellar evolution may go on for between 100,000 and 10 million years based on the size of the star being formed. If the final result is a protostar with more than 0.08 solar masses, then it will go on to start burning of hydrogen and will join the main cycle as a normal star. For protostars with masses less than 0.08 solar masses, temperatures are not sufficient to start the burning of hydrogen, and they become ‘brown dwarf stars.’ 

STAGE 3: The T-Tauri Phase 

When the star is still in the initial phases of formation, it does not have sufficient temperature in its core to start the fusion of hydrogen and helium. Instead, the star shines only with the continuous collapse of its gravitational energy. This pre-star is called a T Tauri star. This initial stage lasts for about 100 million years before nuclear fusion stops in, and it turns into a true star. 

In this stage, a young star starts to create strong winds, which push away the nearby gas and molecules. This permits the forming star to become visible for the first time. A star in its T-Tauri stage can be spotted without the help of infrared waves or radio waves. T-Tauri stars are glowing violent babies. 

STAGE 4: Main Sequence Stars 

If the body has enough mass, the collapsing gas and dust burn hotter, and ultimately, it reaches the temperatures that are sufficient to fuse hydrogen into helium. The star turns on and becomes a main sequence star, fuelled by hydrogen fusion. Nuclear fusion creates an outward pressure that balances with the inward pressure produced by gravity that stabilizes the star. 

Main sequence stars combine hydrogen atoms to produce helium atoms in their cores. In the universe, around 90% of stars are main sequence stars. These stars can range from around one tenth of the mass of the sun to almost up to 200 times as massive. Currently the Sun is in its main sequence phase. 

STAGE 5: Expansion into Red Giant 

When all the hydrogen present in a star is attached to helium, the core shrinks, and its temperature rises. This increased core temperature and pressure affect helium to fuse into carbon through the triple-alpha process. 

This fusion produces more energy than hydrogen-helium fusion. This causes a rise in radiation pressure. This increased radiation pressure pushes matter outward; therefore, the star expands. As the star expands, its surface gets cool and turns red — a red giant is produced. 

The surface of red giants is cooler than the sun (main sequence stars). Due to this, they look red instead of yellow. In the last stages of stellar evolution, a red giant star is a dying star. In a few billion years, Sun will turn out to be a red giant. It will expand and engulf the inner planets, maybe even Earth also. 

STAGE 6: Fusion of Heavier Elements 

Nuclear fusion in stars changes hydrogen into helium. This is the only reaction that takes place in stars that is less massive than the Sun. In stars, more gigantic than the Sun (but less gigantic than around 8 solar masses), further reactions that change helium to carbon and oxygen occur in consecutive phases of stellar evolution. 

Once helium fusion stops, the core contracts and the star starts combining carbon. This process repeats till iron starts appearing in the core. The fusion of iron absorbs energy; hence, the existence of iron causes the core to collapse. In very huge stars, the chain reaction continues to make elements like silicon upto iron.

STAGE 7: Supernova and Planetary Nebula 

The death of a star results in an explosion of luminous stellar. A supernova explosion is one of the brightest happenings in the universe. Precisely how a star dies depends on its mass. For example, the Sun doesn’t have sufficient mass to explode like a Supernova. 

A star can go to the supernova stage in one of two ways: 

Type I supernova: star collects matter from a nearby neighbour till a huge nuclear reaction explodes. 

Type II supernova: Nuclear fuel in stars will reduce, and stars will collapse under their own gravity. 

Low-mass stars turn into planetary nebulae at the end of their red giant phase. At that point, the star turns out to be highly unstable, and it begins to pulsate. The subsequent stellar winds eject the outer layers. Planetary nebulae are comparatively short-lived and take just a few tens of thousands of years.

Summary

• Stars are huge, shining balls of extremely hot gas (known as plasma) in space.
• The Sun is the nearest star, and after that, Proxima Centauri is the nearest star.
• Nuclear fusion is the combination of fusion of light nuclei to form heavy nuclei.
• Astar’s composition and temperature can be shown by the star’s dark line spectrum.
• Stars are made up of various types of elements present in the gaseous state.
• The surface temperature of a star is shown by its color.
• The Sun is a medium sized star with a diameter of around 1,390,000 km.
• Astar remains stable due to the energy created by nuclear reactions on the inside, which balances the inwardly directed gravitational force.
• Astar starts its life as a huge cloud of gas which is normally an accumulation of dust, gas, and plasma.
• The protostar stage is the initial step in the process of stellar evolution.
• T-Tauri stars are glowing violent babies.
• Main sequence stars combine hydrogen atoms to produce helium atoms in their cores.
• The surface of red giants is cooler than the sun (main sequence stars).
• Once helium fusion stops, the core contracts and the star starts combining carbon. This process repeats till iron starts appearing in the core.
• The death of a star results in an explosion of luminous stellar.