Star Formation and Stellar EvolutionWe are all pretty familiar with stars. We see them on clear nights as tiny, twinkling pinpricks of light in the sky. But what are stars?
This article is part of a series of articles. Please use the links below to navigate between the articles.
We are all pretty familiar with stars. We see them on clear nights as tiny, twinkling pinpricks of light in the sky. They have intrigued mankind since the beginning of time, but what is a star?
A star is essentially a super-massive ball of gas which has contracted under gravity and begun the process of nuclear fusion. The nuclear reactions superheat the gas to temperatures of over 6 million degrees Celcius.
What is a Star Made Of?
A star is made of gas which consists mostly of hydrogen and helium, the most commonly found elements in the Universe. The gas cloud is so dense it is held together by its gravity.
Like the Earth, stars are made of many layers, each having its distinctive properties.
The central core of a star is where the energy from nuclear fusion is generated. Because of the enormous amount of gravity compression from all of the layers above it, the core is very hot and dense; nuclear fusion requires extremely high temperatures and densities. The Sun's core is about 16 million K and has a density around 160 times the density of water; 20 times denser than iron.
The radiative zone is where the energy is transported from the super hot interior to the colder outer layers by photons.
At cooler temperatures, more ions can block the outward flow of photon radiation more effectively and convection takes over to transport of energy from the very hot interior to the cold space. This part of the Sun just below the surface is called the convection zone.
The deepest visible layer of a star is the photosphere (light sphere). It forms the visible surface of the Star and is generally the coolest part. Our Sun has a surface temperature of only 5840 K compared with the 16 million K of the core. Sunspots are visible in the photosphere.
Moving away from the surface, a star also has an "atmosphere" consisting of the chromosphere and corona. The chromosphere has a low density and emission lines of hydrogen are visible in this region. Its temperature rises the further away from the photosphere you travel.
Finally, the upper atmosphere of a star is called the corona. It has a very high temperature, but despite this, it has a low amount of heat because it is so tenuous (thin). Prominences are bright clouds of gas forming above the sunspots that follow the magnetic field line loops through the corona.
Life of a Star
Protostars are formed from the collapse of giant molecular clouds of gas and dust in the local interstellar medium. This collapse generates energy through gravitational contraction. Upon reaching a critical density, energy generation comes from an exothermic nuclear fusion process that converts hydrogen into helium.
The gas is mostly made from hydrogen, some helium, trace amounts of other elements and dust, which is composed of tiny grains the size of smoke particles. Gas fills most of the volume of the cloud but the dust is what makes them opaque. Clouds also contain small amounts of the element lithium which is destroyed very quickly in stars which have begun nuclear fusion. This makes lithium a good indicator of the age of a star. Not all gas clouds are stellar nurseries because they need a trigger to start the gas cloud collapsing. This could be a nearby supernova shockwave, the solar wind from a nearby massive star or a gravitational body passing nearby. Whatever the cause, once the process has started it will continue until it either runs out of momentum or forms a star.
Gravitational Collapse
In a process lasting tens of thousands of years, the gas starts to clump together and form larger clumps. Small gravitational forces bring the small clumps of gas together, forming larger forces, which in turn lead to larger clumps. The process will speed up the more clumps group together and eventually, you will end up with a self-sustaining gravitational contraction, and a protostar. This process only takes a few tens of thousands of years to complete.
What is a Protostar?
Young stars are called protostars and consist of a central condensation that will become the star, an accretion disk, and a circumstellar envelope. This envelope is opaque to optical light and thus protostars are generally invisible to ordinary telescopes, although it does glow in the infrared and millimetre wavelengths which allow us to detect them. A few protostars have also been observed at X-ray wavelengths. X-ray emission may be an important source of ionization, allowing the star, disk and outflow to be coupled by magnetic fields.
Another important property of protostars is their outflows. In addition, to matter falling onto the star from the accretion disk, there are also large amounts of material being ejected from the star in the form of powerful bipolar jets. These jets can be seen in Hubble Space Telescope images and are travelling at hundreds of kilometres per second. Bipolar jets are believed to be important for transporting excess angular momentum away from the star.
What are T Tauri Stars?
When a protostar reaches the T Tauri stage it emerges from its opaque, dusty, envelope and becomes visible at optical wavelengths. T Tauri stars are pre-main sequence stars - the youngest visible F,G,K,M spectral type stars (<2 Solar mass). Their surface temperatures are similar to those of main sequence stars of the same mass, but they are significantly more luminous because their radii are larger. T Tauri stars are divided into two categories: Classical and Weak-lined. Classical T Tauri stars were first discovered by the presence of strong chromospheric spectroscopic lines, especially the Hydrogen alpha line. This line is believed to be produced by an interaction between the disk and the stellar surface. The infrared emission from the object is from the disk since the envelope has mostly dissipated. Classical T Tauri stars are strong X-ray emitters and can also produce powerful winds. Once the disk has dissipated enough so that it no longer interacts with the star, these lines are no longer present or are very weak. These "Weak-lined" T Tauri stars are primarily found because they are bright X-ray sources. T Tauri stars produce X-rays in hot plasma trapped in magnetic fields above the stellar surface. T Tauri stars still have no nuclear fusion, their energy comes from the rapid contraction of the gasses, causing higher temperatures and ever-increasing pressure in the core.
Fusion and the Main Sequence
As the star collapses the temperature and pressure at the core increase. After about 10 million years or so the core gets hot and dense enough for fusion reactions to begin. These reactions convert hydrogen into helium and liberate energy in the process. This energy, in turn, heats the star and halts the collapse (heat causes objects to expand, including stars!). The rate of collapse caused by gravitational contraction is balanced out by the expansion due to the temperature rise and held in equilibrium.
When nuclear fusion occurs, the star joins the main sequence where it will remain relatively stable for a very long time (a star like our sun has the Main Sequence phase lasting 10 billion years.)
The disk that was formed early in the star's life and began to dissipate in the Weak-lined T Tauri star phases may have formed into planets by now. Perhaps as many as one-half of all pre-Main Sequence stars have circumstellar disks and this may mean that half of all Main Sequence stars possess planets.
Death of a Star
When a star leaves the main sequence it is said to be a dying star. This process can take another few million years to eventually die, and may in the process trigger the birth of a new star.
The death of a star can take several forms, all depending on its mass.
In general:
- Stars <1.5 solar masses, the [link pageid="1379"]Chandrasekhar limit[/link], will swell to a Red Giant before exploding to a Planetary Nebula, shrinking to a White Dwarf before finally resting as a Black Dwarf
- Stars 1.5 - 3 solar masses (the Tolman-Oppenheimer-Volkoff limit) will swell to a Red Super Giant before blowing themselves apart in a Supernova and resting as a Neutron Star
- Stars over 3 solar masses will swell to a Red Super Giant, explode in a Supernova and collapse to a Black Hole
As the amount of Hydrogen fuel in the core goes down so does the fusion rate and the amount of energy generated. Since the star was in thermal equilibrium (energy generated = energy radiated = star stable) a drop in fusion causes lower energy (temperature) in the core and a pressure drop. This decrease in pressure causes the star to contract slightly and with the contraction an increase in core pressure. This will cause the temperature to rise and a new hydrogen-burning shell in a region previously too cold for fusion. In this new hydrogen-burning shell the fusion rate increases as does temperature. This is the star's last gasp as almost all the hydrogen is gone by now. As this happens the star becomes a little brighter and a little cooler and makes a jump on the HR Diagram.
Once the last of the Hydrogen is used up, fusion stops and the temperature drops and the star collapses. This converts gravitational energy (potential energy) into thermal energy (kinetic energy). This energy is directed into the hydrogen-burning shell, which expands to consume more fuel in the star's interior.
The hydrogen-burning shell generates more energy than the core did (it has access to a much larger volume of the star's mass) and the star increases sharply in luminosity and expands in size to become a red giant (or supergiant depending on its mass). Even though the star is brighter and produces more energy, its pressure has increased such that its surface area has become very large, and the surface temperature of the star drops into the K and M spectral type regions.
This process can take several million years, after which different mass stars will do their own thing.
Sun-like Stars
A Sun-like star is classified as a star below 1.5 times the mass of the Sun. These will shed the outer layers via pulsations and strong stellar winds. Without these opaque outer layers, the remaining core of the star shines very brightly and is very hot. The ultraviolet radiation emitted by this core ionises the ejected outer layers of the star which radiate as a planetary nebula.
Eventually, the star will cool down to a point where it cannot radiate enough ultraviolet radiation to ionize the expanding gas cloud. The star becomes a white dwarf, and the gas cloud recombines becoming invisible.
A white dwarf's mass is around the same mass as when it was on the main sequence, however, its volume has shrunk to that resembling the size of the Earth, and as such, it is very dense.
The material in a white dwarf no longer undergoes fusion reactions, so the star has no source of energy, nor is it supported against gravitational collapse by the heat generated by fusion. It is supported only by electron degeneracy pressure, causing it to be extremely dense. The physics of degeneracy yields a maximum mass for a non-rotating white dwarf (the Chandrasekhar limit - approximately 1.4 solar masses) beyond which it cannot be supported by degeneracy pressure.
Over the next few million years, the star will cool to a temperature at which it is no longer visible and become a cold black dwarf. Since no white dwarf can be older than the Universe, even the oldest white dwarfs still radiate at a few thousand kelvins, and no black dwarfs are thought to exist yet.
Huge Stars
Like smaller stars, large stars of up to 3 solar masses will burn their fuel and collapse. Unlike small stars which can regain an equilibrium, the huge stars collapse in on themselves under gravitational pressure. Since the mass of the star is greater than the Chandrasekhar limit, the outer layers of the star collapse in on the core, the forces holding atomic nuclei apart in the innermost layer of the core suddenly give way. The core implodes under its mass, and no further fusion process can ignite or prevent collapse.
Under these great pressures, a process known as photodisintegration, gamma rays decompose iron into helium nuclei and free neutrons, which absorb energy, and electrons and protons merge via electron capture, producing neutrons and electron neutrinos which escape. During the collapse, a new "neutron-rich" core is created from the newly created neutrons which is 6000 times the temperature of the Sun's core.
The inner core eventually collapses to around 30km in diameter with a density comparable to that of an atomic nucleus. Further collapse is abruptly stopped by strong force interactions and by the degeneracy pressure of neutrons. The infalling matter is suddenly halted, rebounds, and produces a violent shock wave that propagates outward - a supernova.
After the supernova, all that is left is a neutron star - a very small, dense and hot mass of neutrons and sometimes a supernova remnant. The gravitational field on the surface is about 2x1011 times stronger than on Earth. Such a strong gravitational field acts as a gravitational lens and bends the radiation emitted by the star such that parts of the normally invisible rear surface become visible.
Massive Stars
Massive stars above 3 solar masses share the same fate as a huge star, however, the huge masses involved do not produce supernovae. The strong force interactions cannot stop the collapse and the core becomes so dense it forms a Black Hole. Black Holes range from 30km in size for a 10 solar mass star to 10AU for supermassive black holes 109 times the mass of the Sun.
