The stars and night sky have captivated human’s throughout history. And the fascination with how stars are formed is one that most of us have probably had at some point in our lifetime.
Stars form from astrological giant gas clouds. The creation of main-sequence stars follows a seven-stage process. Different events trigger low-mass star formation and high-mass star formation. The progression of a star’s life cycle will result in the formation of different star types.
Historically, our understanding of what stars are and how they are formed has been somewhat conflicted and difficult to study objectively. Stars live for millions and even billions of years. No human will be able to experience both the birth and death of a single star.
Our study of stars is primarily based on extrapolation of observed data and behavioral patterns to theoretical models of a star’s life cycle and internal processes. This article will tell what we do know about stars and how they are formed…
What are Stars?
The history of human understanding of star formation has been a varied one full of theory, myth and legend, where the narrative of stars has been entwined in people’s search for meaning.
But new technology and improved scientific understanding have allowed us to strip away the mythology and legend surrounding stars and study them objectively as they are born, live, and die. The definition of stars holds some important clues as to the formation of stars.
Astrologists formally define stars as astrological bodies of luminous gas with an approximately spherical shape held together by an internal gravitational core.
The stars must have sufficient mass to generate an internal gravitational field to drive the contraction and nuclear fusion at the star’s core. The production of energy by fusion is a foundational criterion for the classification of stars.
A brown dwarf cannot be considered a true star as the mass is insufficient to maintain consistent nuclear fusion at its core and, as such, they are sometimes referred to as failed stars.
Observations & Studies of Star Formation
In 1584, Giordano proposed the formations of millions of new stars and planetary systems similar to our own. Without the aid of modern technology, he correctly theorized the existences and dynamic interactions between stars and planetary systems.
Unfortunately for Giordano, the Roman Inquisition burned him at the stake due to his “heretical” belief about stars! Star science has made giant leaps forward as modern technology has allowed humans to see and even hear the evidence of stars.
Radical theories are more readily accepted if the evidence can be re-produced within a human’s sensory capabilities. This brings me to the study of interstellar clouds and nebulae (nebulae are clouds of gas and dust in outer space)…
Studying Gaseous Interstellar Clouds & Nebulae
Perhaps you’re wondering why we need to study interstellar clouds. After all, these clouds are not stars; they don’t emit light, they only reflect it, and there is no nuclear activity within a cloud’s core!
Scientists have long been fascinated with these important galactic structures. While not stars themselves, interstellar clouds are the origin of many billions of stars. Most of the scientific studies of nebulae and interstellar clouds focus on understanding and predicting the fluctuating movement of interstellar matter within the cloud and calculating the material’s density.
By studying movement and density, scientists can predict when and where stars will form and the potential number of star formations that can occur within a cloud.
The gaseous cloud structures that form nebulae can be studied according to:
- far-infrared wavelength thermal emissions
- near-infrared wavelength dust absorption
- line emissions
Let’s take a look at each one…
Far-Infrared Wavelength: Thermal Emissions
The Herschel satellite was launched over a decade ago and has provided valuable data based on thermal dust emissions. When calculating thermal dust emissions, it is essential to know the cloud’s dust opacity curve and dust temperature to calculate its column density.
Near-Infrared Wavelength: Dust Absorption
A related technique calculates dust absorption of background starlight in the near-infrared spectrum. However, this technique is only viable if the cloud mass does not entirely block out the background stars. It also requires that the cloud mass be located in an area with sufficient star presence to calculate light absorption within the dust and gas mass.
In this technique, the extinction of background stars is measured, and the measurement is divided by the dust’s opacity to get the cloud’s interstellar matter density.
The study of line emissions results in the most information-rich modeling of star formation within interstellar clouds. However, this the most complex formulation to use and understand.
Line emissions are the emissions studied when different atoms and molecules collide with hydrogen and helium molecules and, in so doing, transition from a non-excited ground zero state, N0, to a state of excitement N1.
During the transition of molecules from N0 to N1, photons of light are released. Studying the relative luminosity of the light emissions is the foundation of the study of interstellar cloud activity based on line emissions.
Studying Young Stars and Star Formation
When studying star formation, scientists usually indirectly calculate the formation rate, class or phase of formation, mass, and star temperature based on the star’s luminosity and color.
Very young stars are usually obscured by a dust cloud and cannot be observed. These stars’ presence is generally identified by detecting ejection of very high-velocity molecular emissions from the gaseous cloud.
Studying Single Stars
As the young star continues to evolve, the star’s mass and relative density increase while the surrounding dust envelope begins to disperse, and the opacity is decreased. As this occurs, infrared waves of the dust photosphere and eventually the stellar photosphere can be detected and measured.
Young stars (T-Tauri stars) who are not yet main-sequence stars will also show dense lithium absorption lines, magnetic activity, and X-ray emissions which can be measured and studied. The magnetic activity and lithium absorption lines are essential parameters in the differentiation between young stars and main-sequence stars. Both of these characteristics are absent in main-sequence stars.
Studying Star Systems
When studying young stars, it is essential to look at both individual stars and star systems, e.g. binary, triples quadruples, and large star systems.
Young stars are usually born in the presence of other stars. It is rare for a single isolated star to form. When studying star systems, we can calculate the system’s initial mass formation by either counting the number of stars within the system and estimating the entire system’s mass as a product of individual masses or by measuring the relative luminosity of neighboring star systems.
Alternatively, young star systems can be measured by studying recombination lines, infrared and ultraviolet emissions. Young stars produce ionizing radiation, which creates areas of ionized hydrogen gas. Within these regions, the ionized hydrogen atoms will recombine to form unstable hydrogen molecules, transitioning from a ground state (no-excitation) to an excited state. These molecules begin to decay, producing hydrogen line emissions which can be detected.
Studying recombination lines does not work well with dusty star systems dominated by young stars, as the hydrogen emission lines are obscured. Instead, the preferred measurement technique is far-infrared wavelength measurement. Light is absorbed by the dust particles and re-emitted in the infrared spectrum.
The GALEX Satellite has allowed astrophysicists to measure and study ultraviolet rays emitted by young star systems. Broadband ultraviolet rays are practical information-rich parameters used to study star systems approximately fifty million years old (this is relatively young for a star system) that are not obscured by dust. Utilizing a combination of techniques will yield the most accurate and comprehensive data sets for analyses and interpretation.
A Brief Overview of Star Formation
There are seven stages in the formation of a star like our sun:
- Atoms in giant gaseous clouds begin to form molecules. Molecular formation can happen spontaneously or occur due to a passing shockwave caused by a neighboring star’s supernova or solar flare.
- The cloud begins to collapse and fragment.
- Fragmentation ceases, the core becomes opaque, and the core temperature increases.
- A protostar is formed.
- It evolves into a T-Tauri star.
- Thermonuclear fusion begins.
- The main-sequence star achieves hydrostatic equilibrium.
Existing between stars is a mixture of gas and dust known as interstellar matter. Ninety-nine percent of interstellar matter is made up of atoms and molecules in a gaseous state. The overwhelming majority of these gases are hydrogen and helium, which eventually become essential components of a star’s nuclear core.
The remaining one percent of interstellar matter is made up of interstellar dust, also known as interstellar grains. These grains are atoms or molecules existing in a solid-state. The core of these dust particles is made of graphite or a rock-like substance called silicates.
Surrounding the solid body is a collar of ice. The most common ice collars are formed from mixtures of water, methane, and ammonia. Collectively, the interstellar dust particles and gases are known as the interstellar matter, which comprises the whole of the interstellar medium or ISM.
The volume of galaxies in space is enormous, and so while the density of interstellar matter is relatively tiny, its mass is not. Just to give you an example, it takes light traveling at a speed of 299,792,458 m/s, only 4 seconds to cross the sun’s diameter, which is 1,199,169,832m.
However, it takes light FOUR YEARS to travel from the sun to the nearest star! Within the Milky Way, the smallest distance between neighboring stars in densely populated areas is three light-years apart from each other, but most neighboring stars are much further apart from each other.
Earth’s closest neighboring star is approximately 4.3 light-years away. A light-year is equal to nine trillion kilometers. Space is BIG; our human minds cannot comprehend our Galaxy’s magnitude, never mind, the entirety of space!
To calculate the mass of interstellar matter, we first need to know the volume of the Galaxy. Our Galaxy is approximately cylindrically shaped, and so the volume can be calculated with the following equation:
V is the volume, R is the radius, and H is the cylinder’s height. However, obtaining these numerical values requires some fancy analyses of complex data sets and an intimate understanding of advanced physics. Once the volume has been calculated, you can calculate the interstellar matter’s mass and the potential number of star formations.
Number of Stars=Galaxy Volume x Density of Atoms x Mass per AtomStar Mass
The equation predicting potential star numbers is an integral equation to understand. Even though the mass of interstellar matter in our Galaxy accounts for only fifteen percent of the Galaxy’s entire mass, this is still enough matter to form approximately seven billion new stars the size of our sun!
6.9 x 109=8.0 x 1066cm3×(1)atom/cm31.7 x 10-27Kg2 x 1030Kg
6.9 x 109 ≈7,000,000,000 stars
Giant Molecular Clouds and Nebulae
The interstellar matter shifts and flows throughout the Galaxy as it alternates between areas of low-density concentrations and high-density concentrations. Areas in which interstellar matter has become highly concentrated into giant molecular cloud formations are called nebulae.
Like the atmospheric clouds of earth, the interstellar medium is a shape-shifting structure that constantly evolves. The changing shape can result in transient formations of massive areas with a high enough density of interstellar matter to cause the cloud to begin to collapse under its center of gravity. When this happens, a star begins the first phase of formation.
There are three types of nebulae:
- Emission nebulae
- Reflection nebulae
- Dark nebulae
Emission nebulae are luminescent due to the ionizing effects of neighboring ultraviolet O and B type stars’ radiation. The stars emit ionizing radiation into the nebulae, causing ionization of interstellar gases, mainly hydrogen, oxygen, and sulfur.
These atoms’ ionization causes them to transition from a ground state N0 to an excited N1 state. As these ionized atoms begin to decay back to the ground state, they emit photons of light which can be studied using line emission calculations. Hydrogen will emit red light, oxygen green, and sulfur blue.
Reflection nebulae are not inherently luminescent but rather reflect the light of neighboring stars as it hits the solid dust particles within the nebula. Dark nebulae are nebulae in which the interstellar matter is so dense that it completely obscures the starlight of the stars existing behind the cloud.
The extinction of background starlight means that these nebulae cannot be studied using near-infrared absorption or line emission techniques. The most helpful tool for studying dark nebulae is far-infrared thermal emission.
Dark nebulae are icy, dense structures. These nebulae are most often found within giant molecular clouds and are the birthplace of stars. The temperatures range between 10 and 20 Kelvin, while the diameter can be as large as 10light years across with 104 Solar mass.
A nearby solar flare or shockwave from a dying star can cause further compaction of the already dense interstellar matter contained with dark nebulae. This compression of existing interstellar material can result in the formation of molecules.
High-mass star formation are stars in which neighboring star activity triggered their formation. These triggered star formations occur when robust stellar wind activity causes cold interstellar atoms to move and be compressed against neighboring atoms, resulting in molecules’ formation and an increase in density.
Triggered Star Formations
A solar flare can trigger star formations, but a massive star’s death more commonly initiates it. When a massive star dies, the balance between expansion and contraction forces is destabilized as the star’s internal equilibrium is disrupted. As the end of a star’s life-cycle approaches, fuel (hydrogen and helium) for the nuclear core runs out.
The expansion effect overcomes the star’s gravitational collapse, resulting in a violent outward explosion of the star. This explosion generates enormous amounts of heat energy which rapidly transfers kinetic energy to the cold, relatively inert, and almost static interstellar matter. The transfer of kinetic energy to these particles causes the compression of matter and a critical increase in interstellar density.
Low Mass Star Formations
Unlike high-mass stars, low-mass stars are not triggered by the death of a massive star. Within a low-mass star system, a neighboring star’s death will not trigger star formation, as low-mass stars do not explode upon their death, nor do they have strong stellar winds.
A Protostar’s Contracting Core
Once the density threshold is reached, the interstellar matter begins to collapse under the weight of its gravitational field, overcoming gaseous molecules’ expansion effect.
The nebula’s core is classified as a protostar when the gravitational effect of the core overcomes the natural thermal and magnetic expansion effects of highly concentrated gaseous molecules resulting in a net contraction of the nebula’s core. Nuclear fusion has not yet started, and thus the structure is not yet classified as a true star. The majority of light produced by protostars is due to thermal luminescence and not nuclear fusion.
Accretion Discs And The Law Of Conservation Of Angular Momentum
As the core continues to contract, the gaseous molecules have less space to move even as they achieve higher and higher states of energy. This excess of kinetic energy increases the number of collisions occurring between gaseous atoms and molecules, which produces turbulence within the core of the protostar. Eventually, this turbulence begins to re-align and create a spinning motion.
Initially, this spinning motion is relatively slow, but as the core continues to contract, it begins to spin faster and faster. The phenomenon is explained by the law of conservation of angular momentum. Before we can understand the law of conservation of angular momentum, it is essential to understand linear momentum and torque concepts.
Torque, Momentum, and Conservation of Angular Momentum
Momentum is the product of an object’s mass and its velocity. Momentum is a vector quantity having both magnitude and direction. Mathematically, this is defined as:
Momentum p=mass kg×velocity(ms-1)
The momentum of a collection of particles is equal to the individual particulate’s vector sum’s momentums.
Torque is any force that results in rotational movement. Torque occurring in a two-dimensional plane can be calculated as:
Torque N∙m=radius m × Perpendicular Force (N)
A rotational force (torque) resulting in three-dimensional rotation can be calculated as:
Torque=The moment of inertia × angular acceleration
τ N∙m= Ιkg∙m2× ∝(radians ∙s-1)
The law of conservation of angular momentum is essentially the rotational version of Newton’s first law of motion. A body at rest or moving with constant motion will remain at rest or moving with constant motion unless acted upon by an external force. The law of conservation of angular momentum states that angular momentum will remain constant in the absence of an external force acting upon the system. It can be calculated with the following formulas:
Angular momentum=Ιrotational inertia×ω(angular velocity)
L kg⋅m2s-1= Ιkg⋅m2× ω (radians ⋅s-1)
Following the mathematical process allows us to understand how the angular velocity is indirectly proportional to the distance from the axis to conserve angular momentum, i.e., the more the spinning core contracts, the faster it spins.
As the protostar core continues to contract and spin, the spherical core reshapes into a flattened elliptical disc. The particles moving near the equator will begin moving faster, while those further away from the equator near the poles will be moving slower.
Interstellar grains that fall toward the poles will remain near the poles and add matter to the core. Gaseous molecules obtained from the dust and gas envelope surrounding the core will fall into the center of the spinning disc.
At this stage, the protostar will begin emitting infrared rays but is still largely invisible as it remains shrouded in the dust envelope. Eventually, the disc starts spinning sufficiently fast that stellar winds are generated. At this point, the protostar becomes a T-Tauri star.
Before you get too excited, the T-Tauri stars are still not true stars. Like a protostar, their core is still contracting and has not yet reached sufficiently high temperatures to support nuclear fusion. Once all of the star’s matter has been accreted, the star’s mass is stabilized at or just below our sun’s mass.
Significantly smaller stars do not successfully become stars as they have insufficient mass to initiate and maintain nuclear fusion within their core. In contrast, significantly larger star forms will never go through the T-Tauri phase of star evolution.
The stellar winds generated by the spinning disc escapes at the polar points of the accretion disc, where particulate movement is slowest. These stellar winds typically comprise free electrons and hydrogen nuclei (which are essentially just protons), although they may contain some interstellar grains as they shoot out from the core.
These stellar winds can be ejected from the star’s core with such speed that they may excite clumps of gas in the neighboring nebule a few light-years away and, in doing so, trigger star formation. The domino effect of neighboring stars is why it is common to find multiple star formations in star clusters.
Eventually, these stellar winds will disperse the remnants of the protostar’s dust envelope and, in doing so, become visible once more.
Formation of Main Sequence Stars
During the formation of protostars and T-Tauri stars, the material from the surrounding dust and gas envelope is accreted into the spinning disc at the star’s center. The molecules captured within this disc begin to collide with one another and are consequently subjected to friction. The resistance generated through friction causes kinetic energy to be converted into thermal energy. Initially, this energy is dispersed outwardly through the relatively transparent core.
As the core continues contracting, the captured matter becomes increasingly densely packed within the core until the core is opaque. When this occurs, thermal energy can no longer be radiated outwardly. Instead, the core’s internal temperature and pressure begin to rise, and the core’s contraction expansion achieves equilibrium.
Once the core reaches a temperature of ten million kelvin or more, nuclear fusion begins, and the astrological body can now be officially classified as a star.
The study of stars is a truly fascinating field of science. The most prolific interstellar nurseries in the Galaxy are the giant molecular clouds that house dark nebulae. The compression of interstellar matter within the dark nebulae by neighboring stellar winds or the explosive force of dying stars can trigger star formation.
Initially, the cloud will begin to fragment and collapse into the internal core as it becomes a protostar. As a protostar’s core begins to spin, the spherical shape is converted into an elliptical accretion disc. This accretion disc draws in surrounding interstellar matter from the envelope shielding it from curious scientists.
Once the accretion disc’s mass begins to stabilize and accretion slows and stops, the protostar becomes a T-Tauri star. A T-Tauri star’s spinning core generates powerful stellar winds that blow away the obscuring dust envelop and may even be responsible for triggering star formation in neighboring nebulae.
Throughout this process, the core continues to heat up and become a highly pressurized opaque system. Once the core’s temperature exceeds ten million kelvin, nuclear fusion begins. Only once nuclear fusion has stabilized does the astrological body meet the criteria for a main-sequence star.
The starlight you are seeing is many light years old and is the product of a vast and infinitely mysterious universe that we have only just begun to understand. I think that although our minds might grapple with understanding the science of star formation, humankind’s fascination with stars has more to do with how the mystery and beauty of space resonate with the spirit inside us all.
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