The Science of Existence – Stars

Stars

“You must have chaos within you to give birth to a dancing star.” ~ German philosopher Friedrich Nietzsche

A stellar mass forms when gravity squeezes a dense portion of a molecular cloud into a ball of plasma, consuming and releasing heat.

When temperatures approach 107 K, atomic collisions become so energetic that they spark nuclear fusion. Hydrogen molecules lock into an irreversible embrace, becoming helium while releasing prodigious energy in an exothermic reaction. A galaxy gains a shiner.

“When a star is born, it can have a mass 0.1 to 100 times that of the Sun. This property controls a star’s influence on its environment, its lifetime and even its ability to host habitable planets.” ~ English science writer Nate Bastian

As a clump of gas tries to collapse under gravity, it hots up. The heat creates radiation pressure that opposes gravity. Unless a star can shed some of this heat, collapse stalls.

“Silently, one by one, in the infinite meadows of heaven, blossomed the lovely stars, the forget-me-nots of the angels.”
~ American poet Henry Wadsworth Longfellow

The first stars were hydrogen gas ablaze, rather terrible at shedding heat. These protostars accumulated hydrogen fuel, but the high pressure prevented them from forming a dense core. This left them unable to collapse into fusion reaction, which would drive much of the surrounding gas back out into space.

Instead, early stars gorged on gas until they had built a massive, diffuse core. The first stars could have been a million times as massive as the Sun.

The above scenario is one of many that may have transpired. Feedback loops that act on hydrogen gas as it collapses could have fragmented collapsing clouds, creating stars just a few tens that of solar masses.

While gigantic stars would have lived fast and died young, smaller stars churned through their nuclear fuel more slowly. Regardless of size, the earliest stars ended their existence in fiery supernovae before collapsing into black holes. Supernova explosions seeded the interstellar medium with an initial inventory of heavier elements, including oxygen, carbon, and silicon, while leaving behind diminished, dense neutron stars.

Hundreds of millions of supernovas have come and gone in this greedy cycle of gorge and regurgitate, with the residue as starter material for further cosmic construction. Star dust is the material medium of a maturing universe.

The tug of gravity and nudge of coherence gamed the cosmos into galaxies. A homogenous beginning begat a heterogeneous universe from the earliest blips in being. Galaxies evolved in a vast variety of formations.

Likewise, stars shine with a wide range of expanse, from 10% that of the Sun to at least 100 times more massive. A star’s size depends upon how much fodder is found in the vicinity.

Star formation is an accretion ballet based upon fluid dynamics. The planetary ecosystem that evolves nearby has much to do with the feeding of a growing star, and how cosmic building material is distributed.

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A faint star in the constellation Leo (The Lion) has been shining for over 13 billion years. Based upon its scarcity of source material, the star’s longevity defies comprehension.

“A widely accepted theory predicts that stars like this, with low mass and extremely low quantities of metals, shouldn’t exist because the clouds of material from which they formed could never have condensed.” ~ French cosmologist Elisabetta Caffau

The way in which stars form depends on their gestation habitat. Star origination is but an episode in galactic evolution which resembles an organic process in its interrelated growth and decay over an astronomical time frame. The local dynamics of star-making depend upon the galactic ecosystem.

Stars do not typically form in isolation. They are instead born in batches, cradled together within a cloud of dusty gas. These clouds are surrounded by a fog of hydrogen that interacts with the clouds. Depending upon galactic dynamics, hydrogen fog may decrease cloud pressure, suppressing star formation instead of fanning its flames.

“Star systems come in myriad forms. There can be single stars, binary stars, triple stars, even quintuple star systems.” ~ American astronomer Lewis Roberts

Planets can form within all the different types of star systems, in a vast diversity of sizes and orbits. Multiple star systems produce an abundance of massive planets.

Star formation peaked when the universe was a few billion years old. It has declined steeply ever since, as the supply of molecular hydrogen gas that fuels new stars has dwindled.

70% of the original gas is locked up in white dwarfs, neutron stars, and planets. 66% of the rest is spread thin in the intergalactic medium. Only a small portion is shed by stars at various stages of their lives, or recycled wholesale by supernovae.

The Milky Way suddenly stopped birthing stars after it formed its thick, saucer-like disc ~9 billion years ago. The galaxy resumed forming stars after this sudden die-off, but at a much slower rate.

“Star formation boils down to a battle between gravity and other things, like turbulence.” ~ American astronomer Katherine Alatalo

The Milky Way’s bulging disc and bar-like concentration of stars at its center stir galactic gas, injecting energy that keeps gas from coalescing, thus arresting star formation. The mass of stars in the Milky Way’s central bulge is ~20 billion times the mass of the Sun.

Half the stars in the Milky Way have a companion, traveling as a binary system. Infant triplets are not unusual.

Partnered stars are often torn apart by a collision on the galactic dance floor sometime during their lives, often in their infancy. By this, the population of binaries is diminished before the stars spread out into the wider galaxy.

Fusion

Fusion is chemical fury. Even the lightest element, hydrogen (H), does not easily give up its molecular independence (as H2). H2 is formed by sharing electrons; easily done because solitary electrons like the company of another.

Atomic protons are anti-social: they naturally repel each other. It takes enormous energy to cajole their union and bang on fusion.

Once stellar fusion starts it sustains itself. The feast isn’t over until the food runs out.

Stars prefer light eating: a steady diet of hydrogen is imminently combustible. At the extreme pressures during star formation, and as an active star, hydrogen exists in a unique phase: as a honeycombed 3d matrix interlaced with free-floating molecules. Some of the hydrogen slurry fuses into helium because of the intense gravity inside a star.

As a star evolves, the number of atoms in its core decreases, but its luminosity increases. The Sun has gained 30% in brightness in the past 4.5 billion years.

Hydrogen consumption typically lasts for billions of years, with helium accumulating along the way. This is a star’s stable period. Then the contraction that birthed the star pauses.

Stars are insatiable consumers. A star has to do what a star has to do – stay hot. Once the hydrogen stock is depleted, contraction commences again. The star heats up more.

At about 108 K, helium burns. Helium atoms do not normally bond, but within stars their electrons interactively dance in a way to form an attraction, thanks to hellish temperatures and ferocious magnetic fields.

The pressure in a stellar core is relentless. Heavier elements form, including carbon (12C) and 2 isotopes of oxygen (16O & 8O).

Helium doesn’t have the kick of hydrogen. Stars eat through their helium supply within a few hundred million years.

Some stars burn out after consuming their helium, making molten masses of carbon and oxygen; leaving themselves a legacy as a white dwarf. A white dwarf may be the size of Earth, or as massive as the Sun.

The carbon–nitrogen–oxygen (CNO) cycle is 1 of the 2 sets of fusion reactions by which stars combust; converting hydrogen to helium as a pathway to even heavier elements. The proton–proton chain reaction is the other star combustion process. The proton–proton chain reaction predominates in smaller stars, while CNO feeds the fire in stars more massive than 1.3 times the mass of the Sun.

Small stars in binary systems dance to a different dynamic. A white dwarf may pull in its partner, thereby gathering enough energy to collapse before erupting in a stellar explosion.

Bulkier stars burn on: the ones that are at least 8 times more massive than the Sun. Increasingly heavier elements collect through fusion, up to the point of iron. Iron is the final hurrah in a normal star’s natural life.

Massive Stars

“The most massive stars reach the highest core temperatures because they can release the most gravitational potential energy.” ~ American astronomer Jennifer Johnson

The heavyweight stars that drive galactic evolution are truly massive and bright.

“These stars are absolute behemoths. They have 15 or more times the mass of our Sun and can be up to a million times brighter. These stars are so hot that they shine with a brilliant blue-white light.” ~ Dutch astronomer Hughes Sana

75% of these high-mass stars exist in pairs. Vampirism is common: the smaller star sucks matter off the surface of its larger neighbor. 1/3rd of these stars eventually merge.

A supersized star’s afterlife is spectacular. Stars at least 12 times the size of the Sun burn through an increasingly heavy chemical cocktail, including such spicy elements as silicon, sulfur, and scandium, to a manganese tang, right down to the iron core, all in about a day.

Suddenly lacking the energy to maintain their full volume, burned-out stars implode under their own immense gravity, collapsing thousands of kilometers in mere seconds. In their cores, they even crush protons and electrons into neutron mush. Then these stars fiercely explode.

“Neutrinos are the engine that drives the exploding star.” ~ American physicist John Cherry

Neutrinos are wispy subatomic particles that interact gravitationally among themselves and other particles but are otherwise stand-offish. During star implosion, massive numbers of neutrinos are zipping all about: colliding, changing flavors (type), and in doing so driving energetic interactions and the transformations of other particles. The least substantial subatomic particle is most impressive when other matter is most pressed.

A supernova stretches millions of kilometers, briefly shining brighter than a billion stars. The show may last but a month; but what a show. Gazillions of intensely excited particles fuse, creating a blizzard of element alphabet soup.

Such supernovae typically leave behind charred remains: a neutron star or even a black hole. Stars that are sized 20–25 suns don’t explode. Instead, they implode. The crushing cascade of density bottles up the energy into a black hole.

Neutron & Quark Stars

A neutron star is a stellar husk packed with neutrons. Neutron stars are as compressed as normal matter can be.

In their extant state, these stars can collapse no further because of quantum degeneracy pressure: a property of the Pauli exclusion principle, which states that no 2 fermions (subatomic particles of matter) can occupy the same place at the same time.

Owing to furious quantum mechanics, neutron stars stay hot. Their spinning, at up to 1,000 revolutions per second, emits intense electromagnetic radiation that is received as directional pulses. Such a star is termed a pulsar: a portmanteau of pulsating star.

Pulsars directionally pulse radiation at regular intervals, so precise that some rival atomic clocks for their accuracy in timekeeping. Sometimes these pulses become temporally distorted – what has been termed starquakes. The cause is uncertain.

“When a topological defect passes through a pulsar, its mass, radius, and internal structure may be altered, resulting in a pulsar ‘quake’.” ~ Australian physicist Victor Flambaum

A neutron is made up of 3 subatomic quarks: 2 down and 1 up. While protons and neutrons are the core of normal matter, they do not occupy the lowest net energy possible. Under intense gravity, a greater degree of subatomic stability exists.

As it rotates, a neutron star is constantly shedding magnetic field energy. In cosmological time, the star’s spin slows. Centrifugal forces that kept the utmost gravity at bay weaken, yielding further squish.

As a spinning neutron star slows down, deep within, under intensifying gravity, neutron quarks shirk to an even lower energy level than normal matter. Neutrons convert to hyperons: a soup of up, down, and strange quarks.

The initial nucleation of strange quark matter runs amok once hyperons start to form. Core density increases, melting the star from the inside out. Quarks are liberated from their normally bound state.

A neutron star evolves into a quark star. The seed of strange quark matter spreads until it reaches the iron-rich crust. It then separates from the crust, collapsing into even greater density.

An intense shock wave is generated when the collapse concludes. A spectacular explosion ejects the crust and leftover neutrons in a quark nova.

Quark nova bits slam into earlier supernova remnants, causing another outburst of light, as happened when the star first exploded. The 2nd blast can occur anywhere from seconds to years after the original supernova. Such double explosions, in rapid succession, have been observed in multiple instances.

“More conservative thinkers are just not open to the idea that free quarks exist in neutron stars.” ~ American astrophysicist Fridolin Weber

Hypernovae

Hypernovae, which are supernovae at least 140–200 solar masses, are even more explosive than their more petite sisters – leaving absolutely nothing behind as core material. Part of hypernova explosive power stems from production of matter-antimatter particle pairs, which are particularly antagonistic toward each other in such a high-energy setting.

These hypernovae are rare, but particularly potent in seeding the next generation of cosmic matter consumers and doing so with the most energetic explosion possible.

If matter becomes hot enough it can emit photons so energetic that they can collide and convert into other particles, notably pair production of an electron and a positron, the electron’s antiparticle. Pair production results in matter at much lower pressure. This deadweight intensifies the collapse of a hypernova, causing a runaway reaction that results in an energy release that exceeds the star’s entire gravitational energy. The inevitable explosion obliterates the hypernova, leaving behind only an expanding cloud of the elemental debris synthesized from the terminal fury.