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?
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We are all familiar with stars. We see them as tiny, twinkling pinpricks of light in the sky on clear nights. They have intrigued humankind since the beginning, but what is a star?
A star is a supermassive ball of gas that has contracted under gravity and begun the process of nuclear fusion. The nuclear reactions superheat the gas to over 6 million degrees Celcius temperatures.
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 its gravity holds it together.
Like the Earth, stars are made of many layers, each with distinctive properties.
The central core of a star is where the energy from nuclear fusion is generated. The core is very hot and dense because of the enormous gravity compression from the layers above; nuclear fusion requires extremely high temperatures and densities. The Sun's core is about 16 million K and is around 160 times the density of water, 20 times denser than iron.
In the radiative zone, 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 energy transport 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 is low-density, and hydrogen emission lines 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, and it is composed of tiny grains the size of smoke particles. Gas fills most of the volume of the cloud, but the dust makes them opaque. Clouds also contain small amounts of lithium, which is destroyed very quickly in stars that 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, leading to larger clumps. The process will speed up the more clumps grouped, and eventually, you will end up with an autonomous 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 they 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 ionisation source, 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, large amounts of material are also being ejected from the Star in the form of powerful bipolar jets. These jets can be seen in Hubble Space Telescope images and travel 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 produce powerful winds. Once the disk has dissipated enough to no longer interact with the Star, these lines are neither present nor 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, 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 is held in equilibrium.
When nuclear fusion occurs, the Star joins the main sequence, which 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 formed early in the Star's life and dissipated in the Weak-lined T T Tauri star phases, which may have formed into planets by now. Perhaps as many as one-half of all pre-Main Sequence stars have circumstellar disks, which 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 dying. This process can take another few million years to die eventually 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 three 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 decreases, so does the fusion rate and energy generated. Since the Star was in thermal equilibrium (energy generated = energy radiated = star stable), a drop in fusion caused 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. The fusion rate and temperature increase in this new hydrogen-burning shell. This is the Star's last gasp, as almost all the Hydrogen is gone now. As this happens, the Star becomes brighter and cooler and jumps 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 (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 Star's surface temperature 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 Star's remaining core shines brightly and is very hot. The ultraviolet radiation emitted by this core ionises the Star's ejected outer layers, which radiate as a planetary nebula.
Eventually, the Star will cool down to a point where it cannot radiate enough ultraviolet radiation to ionise the expanding gas cloud. The Star becomes a white dwarf, and the gas cloud recombines, becoming invisible.
A white dwarf's mass is the same as in 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 energy source, 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 no longer visible temperature 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 that can regain an equilibrium, huge stars collapse 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; 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 three 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 ten solar mass star to 10AU for supermassive black holes 109 times the mass of the Sun.
