Stellar Evolution

“Then there was a star danced, and under that was I born.’’
— William Shakespeare, Much Ado About Nothing, II, I

A rose of galaxies.

How often have we gazed at the little flecks of light illumining the night sky. Since time immemorial, from the ancient Egyptians to the early Arabs, astronomers have observed stars in all phases of their life and have chartered down the cycle of life that all stars appear to go through. Today with the aid of state-of-the-art technologies, cosmologists have recorded stellar evolution in chapter and verse.

Nebula

The Orion Nebula: The Orion Nebula is a picture book of star formation, from the massive, young stars to the pillars of dense gas that may be the homes of budding stars. Credit: NASA, ESA, M. Robberto ( Space Telescope Science Institute/ESA) et al

A nebula is a cloud of dust and gas, mostly hydrogen and helium, that resembles shimmering mist in the interstellar. Matter distribution within a nebula is highly uneven. With the passage of time, hydrogen begins collapsing in different regions across the nebula as gravity begins to pull the cosmic dust and gas together. This is possibly triggered by shock waves from the explosion of a massive star in the vicinity or from the gravitational influence of a whirling star nearby. Nebulae are star formation factories, i.e., regions where new stars are born. A nebula may lie dormant for millions or billions of years as it waits for just the right conditions. The Orion nebula, which is the closest to the Earth, offers the best views of stars forging right before our very eyes.

The Crab Nebula: The colours correspond to various elements blown away after the explosion. Orange is hydrogen; green is sulphur and red is oxygen. The eerie blue is produced by the neutron star left behind. Credit: FORS Team, 8.2-meter VLT, ESO

Nebulae could also be formed from the stardust surrounding dying red giant stars or from the vestiges of a supernova explosion like the Crab Nebula, which is the remnant of a neutron star. Thus, nebulae are not just the nurseries of stellar evolution, but can also be the graveyards of stars.

Star Cluster Pismis 24: The gas and dust of the nebula hide huge baby stars nested in the nebula from telescopes. Credit: NASA, ESA and Jesús Maíz Apellániz

It is a happy notion that stars aren’t lonely creations. Nebulae birth hundreds, even thousands of stars together within a cosmologically short time frame. Stars spun of the same fabric of stardust form clusters called Star Clusters. The stars are roughly of the same age, but each develops in its own way.

Protostar

A Protostar is essentially a star in its infancy. As matter continues to coalesce into denser clumps, their gravity increases. The enhanced gravity in these parts attracts more gas and dust, and they morph into a spiralling disk around the dense core to conserve its angular momentum. As gravitational attraction increases, the luminous core gets progressively hotter because the atoms collide with each other more frequently and their kinetic energy increases. This clump formed is a protostar, a star in the making. Within its deep interior, the protostar has a lower temperature than an ordinary star and is lambent in mesmerising colours inside out.

Protostars remain enshrouded by swirling stardust and are hence not detectable at optical wavelengths, but can be detected in the infrared by astronomers using infrared radiation detectors. It is essential to remember that a protostar is continually collapsing upon itself under gravity and isn’t a star just yet.

Protostar IRAS 20324+4057: This protostar is contracting to forge a new star. Vigorous winds are blowing and eroding away its gas and dust. Credit: NASA, ESA, Hubble Heritage Team (STScI/AURA), and IPHAS

Main Sequence Star

One Solar Mass is equal to approximately 2×10³⁰ kg and is equivalent to the mass of the sun.

The protostar is still collapsing inward, although far more slowly now. Its heart is becoming denser and hotter creating an outward pressure within the core. The feud between gravity, that keeps the protostar collapsing inward, and internal pressure, that rages outward in the hopes of becoming a star, now begins tipping in favour of the internal core. When the core of the protostar eventually reaches 10 million Kelvin, the gravitational in-fall of the hydrogen envelope is balanced by the pressure from the interior and the entire envelope of cosmic dust is blown off unclothing a fully-fledged main-sequence star. The nascent star begins to glow brilliantly and contracts, then becomes stable. The core is said to be in hydrostatic equilibrium and nuclear fusion begins in its centre.

Zeta Ophiuchi: This stunning portrait is of the runaway main-sequence star Zeta Oph. It was once believed to have been part of a binary star system and escaped the gravity when its partner exploded. Credit: NASA, JPL-Caltech, Spitzer Space Telescope

The fusion involves hydrogen nuclei to collide and fuse into heavier helium nuclei, releasing energy in the process. This fusion cycle is a proton-proton chain and is rampant in stars having masses less than 1.5 solar masses. Heavier stars use a different process called the CNO (carbon-nitrogen-oxygen) cycle; as a massive star uses up a substantial fraction of its hydrogen rapidly, it begins to synthesize heavier elements. In this process, hydrogen nuclei fuse into helium using carbon, nitrogen and oxygen as catalysts — substances that increase the reaction rate without affecting the rudimentary reaction.

The law of conservation of mass states that matter can neither be created nor destroyed. It can only be transformed from one state to another.

At each stage of the reaction, the mass of the helium nucleus formed is less than the mass of the constituent hydrogen nuclei. But by the law of conservation of mass, mass cannot be destroyed. What happens is that the mass lost is transformed into energy which accounts for the source of stellar energy released during nuclear fusion. Its value is obtained using Einstein’s famous equation:

E = m

where E is the energy, m the mass and c the speed of light in a vacuum. This is better written as

E = Δm

where E is the stellar energy released and Δm is the change in mass of the constituent hydrogen and helium nuclei.

The main-sequence is the longest phase of a star’s life. For this reason more than 90% of the stars in the universe are main sequence stars. These young stars, relative to the human lifespan, enjoy eternal youth. The timespan of each star on the main sequence depends on its mass and temperature. Tiny cool stars take billions of years to expend their core hydrogen. Contrarily, large hot stars consume their hydrogen very rapidly as higher temperatures facilitate a faster rate of the nuclear reactions and only remain stable on the main sequence for thousands of years. The sun is in its main-sequence phase and will continue to be so for another 10 billion years. The lower mass limit for a main sequence star is about 0.08 solar masses. Limits on the upper mass of stars has been theoretically fixed to be somewhere between 150 and 200 solar masses. Such stars are extremely rare and short-lived.

Alpha Centauri (left) and its surrounding stars. This is a main sequence star. Credit: Skatebiker

When the hydrogen at the core is consumed, the star evolves into a more luminous star transmogrifying into either a red giant, a supergiant, or directly to a white dwarf.

Brown Dwarf

Protostars less massive than 0.08 solar mass never become hot enough for fusion to start; they become brown dwarfs. Brown dwarfs have an amalgam of features of small stars and large planets. Unlike stars, they have very little mass so can be easily mistaken them for humongous planets but, unlike planets, they are nursed in nebulae like any other star. Ironically, some brown dwarfs seem radiant in a dim magenta glow. This faint light isn’t from nuclear reactions raging in the core but simply from the residual energy of the collapsed nebula in its initial stages.

“Brown dwarfs are so elusive”, said Ian McLean, an astronomer at the University of California, Los Angeles. Since they give off so little energy and heat, it is difficult to detect them. Even using infrared, which is the most efficient way to detect them, it can be a struggle as they have to be relatively close by — within 100 light years — for us to detect the heat signature.

Gliese 229B was the first brown dwarf, with a mass of 0.05 solar masses, to be discovered in 1995.

Red Dwarf

In the main-sequence phase, stars acquire different personas based on their masses. Stars weighing between 0.08 and 0.5 solar masses, are called red dwarfs. A red dwarf is an M-type main sequence star which are smaller, cooler, and dimmer than our Sun. The cooler a star the redder it is, just as a dying ember fades from yellow-orange to cherry-red. Red dwarf stars make up the largest population of stars in the Milky Way. Their dim luminosity helps to extend their lifetimes. Their low temperature also means that they burn through their supply of hydrogen less rapidly. This stretches out the lifetime of red dwarfs to trillions of years; due to universe being a mere 13.7 billion years old, no red dwarfs exist at advanced stages of evolution.

In the quest of extraterrestrial life, Parke Loyd of ASU’s School of Earth and Space Exploration says:

If the genesis of life on a planet is more or less a roll of the dice, then M stars are rolling those dice far more than any other type of star.

However, in spite of their great numbers and long lifespans, a planet in the star’s system would likely be tidally locked to the parent star meaning perpetual daylight on one side leaving it stark and scorched, while the other side would exist in an eternal night, forgotten, frigid, and frozen. Also, red dwarfs are often flare stars. The echoes from the heart may make it difficult for life to develop and persist near a red dwarf.

Orange Dwarf

K-type main-sequence stars, simply known as orange dwarfs are stars a little less massive than the sun, but more massive than red dwarfs. They have masses between 0.5 and 0.8 solar masses. These stars live much longer than sun-like stars, and have safer habitable zones — where liquid water can exist — than those of lighter red dwarf stars. Alpha Centauri B is a well-known orange dwarf star.

Yellow Dwarf

The Sun is a yellow dwarf. The tiny black dot at the upper left corner is the planet Venus passing in front of it. Credit: NASA/SDO, AIA

Stars weighing between 0.8 and 1.15 solar masses become G-type main sequence stars known as yellow dwarfs. They are bright yellow, approaching white, sun-like stars with sweltering surface temperatures. Being main-sequence stars, sustenance happens through nuclear fusion of hydrogen. About 10% of stars in the Milky Way are yellow dwarfs. A prime example is our very own star, the Sun.

Red Giant

U Camelopardalis: This red giant star imitating the human iris, is ejecting gas, visible in this picture as a faint bubble of gas surrounding the star. Credit:ESA/Hubble, NASA and H. Olofsson (Onsala Space Observatory)

As stars age, they become giants. Eventually the core of the star runs out of hydrogen. Energy flow from the core decreases and gravity, ultimately, triumphs. The inner envelops of the star begin collapsing, compressing the helium-filled core, inciting fusion of helium into carbon in the core. Powered by the heat from the core, hydrogen fusion commences in the outer shell and the energy outflux causes the atmosphere of the star to expand greatly and it becomes a red giant. For most stars this is the beginning of the end.

Red giants evolve out of main-sequence stars that have masses in the range from around 0.3 solar masses to around 8 solar masses. The Sun, in about 5 billion years, is fated to morph into a red giant.

White Dwarf

White dwarfs are born when a star — those up to eight times as massive as our own sun — shuts down.

As the red giant expands, stellar winds begin eroding the star’s outer layer forming a planetary nebula. Albeit its name, a planetary nebula is merely stardust around the star that looks like the fuzzy disk around a planet like Uranus. Once the star evaporates, the remnant core is a much smaller, newly formed white dwarf. A white dwarf officially trumpets the death of the star as fusion is doused.

The Butterfly Nebula: This pretty picture is the planetary nebula of a dying red giant. The dainty butterfly wings are actually roiling cauldrons of gas. Credit: NASA, ESA and the Hubble SM4 ERO Team

Another theoretical formation of the white dwarf is through the evolution of a red dwarf. Red dwarfs — those too small to evolve into red giants — have been prophesied to simply burn through their hydrogen stash and punctuate into a dim-lit white dwarf. However, since red dwarfs have billions of years of existence and since the universe is too young, such a phenomenon is yet to transpire.

The constituents of a white dwarf are a blend of helium, carbon, and oxygen nuclei swimming in a sea of electrons. The long dead star has no energy source due to the absence of fusion. What combats gravitational collapse is the electron degeneracy pressure, a clandestine play of quantum mechanics, which occurs when electrons are compressed into a very small volume thus creating a pressure. The Chandrasekhar limit — approximately 1.44 solar masses — sets the maximum mass of a white dwarf as beyond this it cannot be supported by electron degeneracy pressure.

White Dwarf Stars: These shining stellar diamonds, as seen by Hubble, turn out to be 12 to 13 billion years old. Credit: NASA and H. Richer (University of British Columbia)

The white dwarf has a long, quiet life paved ahead of it. Initially fiery and hot, it begins emanating the heat and quells into calm bubble of faint luminosity. Over billion years under intense pressure, the carbon core begins to crystallize into a diamond. A single diamond about 300,000 times the mass of the entire Earth!

Black Dwarf

As the white dwarf radiates its energy, it slips into a hushed languor. When it cools to nearly absolute zero after quadrillions of years, its lights go out and it goes still into the night. The dark diamond carcass is now known as a black dwarf. The universe isn’t old enough yet for black dwarfs to exist.

Scientists have theorised that when the curtains do fall on the universe, the last dazzling event would likely be the explosion of these black dwarfs. Called black dwarf supernovae, these will ensue shortly after pycnonuclear fusions process the dwarfs composition to iron. Although black dwarfs are burnt stars, fusion reactions can still happen because of a phenomenon known as quantum tunnelling, only much slower. Once the black dwarf is mostly iron, it would collapse triggering a huge implosion that ejects the remains of the black dwarf. These brilliant bursts will be the harbingers of the universe’s sink into frigid dormancy. The first explosion is calculated to occur in about 10¹¹⁰⁰ years. Humankind would have been reduced to dust way before that! The universe will seemingly end in ice after all.

“Some say the world will end in fire,

Some say in ice.”

— Robert Frost, Fire And Ice

Nova

Not all white dwarfs go so quiet into the night. If a white dwarf exists in a binary or multiple star system, it may experience a more eventful demise as a nova. If a white dwarf is close enough to a companion star, its gravity may drag matter from the outer layers of that star onto itself, building up its surface layer. When enough hydrogen has accumulated on the surface, a burst of nuclear fusion occurs, causing the white dwarf to brighten substantially in a flash and expel the remaining material. Within a few days, the glow subsides and the cycle starts again. Sometimes, particularly massive white dwarfs may accrete so much mass that it surpasses the Chandrashekar limit that they collapse and explode completely, becoming what is known as a supernova.

The Classical Nova: GK Persei as an example of a “classical nova”, an outburst produced by a thermonuclear explosion on the surface of a white dwarf star. Credit: NASA/CXC/RIKEN/D.Takei et al

Red Supergiant

While the smaller stars become red giants and die uneventful deaths, stars after the main-sequence phase with masses between 8 to 40 solar masses become red supergiants and are fated for a more spectacular ending.

Expanding halo of light around a distant red supergiant star, V838 Monocerotis. Credit: NASA, the Hubble Heritage Team (AURA/STScI) and ESA

When hydrogen is extinguished from the heart, it collapses and helium fusion begins. The outer layers swell out into a giant star, but even bigger, forming a red supergiant. In the core, fusion of helium into carbon is followed by the subsequent fusion of heavier and heavier elements until iron builds up in the core. The fusion of iron actually requires more energy than it dispenses. At this stage, matter recoils out from the heart of the star in an explosive shock wave spewing off into the abyss of space.

Red supergiants are physically the most massive stars known.

Supernova

The magnificent stellar life of a colossal star only warrants a more theatrical death. In one of the most spectacular events in the universe, gas and stardust is strewn across interstellar space away from the star in a catastrophic explosion called a supernova.

Cassiopeia A: This remnant of a supernova explosion 325 years ago, beams a resemblamce with a flirty floral bouquet. A sharp turquoise dot in the center of the shimmering shell is the left-behind neutron star. Credit: NASA/JPL-Caltech/STScI/CXC/SAO

A supernova is preceded by fiery upheavals in the iron-filled heart of the star. Although the exterior is expanding into a humongous scarlet bubble, the interior is shrinking, making a supernova imminent. Iron atoms are crushed so compactly, they recoil and detonate, leading to the stellar demise. The star loses about 75% of its mass in the supernova. The shock heats the matter it traverses inducing an explosive nuclear burning for a short span of time. Known as supernova nucleosynthesis, it enriches the cosmos with rarer elements like copper, gold, silver. The fate of the left-over core depends on its mass.

Neutron Star

Rightly known as a stellar phoenix, a neutron star is the ancient fossil of a once formidable star of about 8 to 20 solar masses that has disembarked on its evolutionary journey and mellowed into a super-dense city-sized celestial body. After going supernova, if the residual core is above the Chandrashekar limit but below 3 solar masses, it will collapse into a neutron star. Under the rage and wrath of gravity, protons and electrons combine to make neutrons, yielding its name.

This 2,000 year-old-remnant of a cosmic explosion occurred about 10,000 light years from Earth. At the center, the bright blue dot is aneutron star that astronomers believe formed when the star exploded. Credit: X-ray: NASA/CXC/University of Amsterdam/N.Rea et al

Neutron stars come with a tag of diversity attached. A neutron star that has an abnormally strong magnetic field is known as a magnetar, and is able to pull keys out of pockets from as far away as the moon. A rapidly spinning neutron star that emits rhythmic pulses of energy like the beam of light from a turning lighthouse, is known as a pulsar.

An interesting speculation about these neutron-rich bodies is that if they housed life, it would be two-dimensional. Gravity on a neutron star is 2 billion times stronger than gravity on Earth. Thus the surface of a neutron star is exceedingly smooth; gravity does not permit anything tall to exist.

Blackhole

For a gargantuan star with more than 20 solar masses, what is written in fate’s book eminently has been one of the greatest enigmas to humankind — a black hole. These past years have promisingly served to narrowing the knowledge gap about them.

Black holes begin life with more than the mass of three suns and feed on interstellar gases and visiting stars that accrete around it like a halo. In fact, astronomers use radiations from this disk to try and detect black holes which themselves like to play cloak-and-dagger and evade prying eyes of telescopes. Black holes formed from the fall of stars after going supernova are knighted with the title of “stellar”. A different kind that astronomers have labelled “supermassive” black holes have been proved to be residing in the eye of every galaxy since the dawn of the universe. And befitting their title, they are humongous having masses of more than a million suns. The supermassive black hole at the center of the Milky Way galaxy is called Sagittarius A. It has a mass equal to about 4 million suns.

Humankind caught its first glimpse of a black hole on April 10, 2019.

M87*: The infamous image of the first ever black hole M87* captured by scientists. At the center of galaxy M87, it is outlined by emission from hot gas swirling around it under the influence of strong gravity near its event horizon. Credits: Event Horizon Telescope collaboration et al.

Skirting the black hole is the event horizon — the boundary of the black hole in space-time — that marks the end of the familiar. Bearing the likeness of a sinkhole, matter and light entering the event horizon is imprisoned inside forever. One would have to be superluminal — faster than light — to escape it. Near the event horizon a time-warping phenomenon known as gravitational time dilation slows time. To a distant observer, a clock near a black hole would appear to tick more slowly than one further away.

If a human fell into a black hole, he would be spun into spaghetti, a process commonly known as spaghettification. The human falling in feet-first, will experience the stronghold of gravity at his feet much more immensely than at his head. The fabric of space-time is so folded and crumpled that even at a meters distance gravity rises stupendously. If man ever did enter a non-rotating black hole in a gravity-defying suit per say, he would be inevitably traversing towards his bone-crushing doom. All paths lead to the very heart of the black hole– the singularity. Classical physics has deduced that inside the event horizon lies the singularity. Metaphorically akin to a tear in the fabric of space-time, everything within the event horizon irreversibly merges at this point where the space-time curvature becomes infinite. The singular region has zero volume and infinite density. So it is inevitable to assume that the person would be crushed to a dot. However, a rotating black hole could provide an escape from the clutches of death. Such a black hole forms a tunnel called a wormhole with the singularity in the centre. If the person successfully manages to pass the point of singularity, a hypothetical possibility would be that he would exit the wormhole through a white-hole into a different universe. This is merely theoretical and perhaps a supersonic jet away from intergalactic make-believe since any perturbation would destroy this possibility.

The Information Loss Paradox of black holes, the enigma of lost information inside black holes which contradicts a rudimentary quantum law implying information is sacred and never destroyed, is now in the process of being unravelled. Physicists using quantum mechanics hope to finally explain and put a close to this paradox that has left scientists baffled for decades.

From the Remains, New Stars Arise

From novae and supernovae rise new stars. Life, in the celestial realm, comes full circle. The dust and debris left behind by novae and supernovae eventually swirl in with the interstellar gas and dust, enriching it with the heavy elements produced during stellar death. These form the star beds from where new generations of stars spring accompanying planetary systems. The universe is quite efficient in this sense and is the epitome self-sustenance.

Globular cluster in the constellation Ara.

“Turn him into stars and form a constellation in his image. His face will make the heavens so beautiful that the world will fall in love with the night and forget about the garish sun.”

— William Shakespeare, Romeo and Juliet, III, II

Author | Lizzen Camelo

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