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|22.5||Evolution of Stars|
In this Physics tutorial, you will learn:
What type of celestial bodies are stars? Do they produce energy by themselves or do they obtain it from other sources?
Do you think the Sun will continue to exist forever? Why do you think so?
What kind of stars do you see when observing the sky at night? Is there any star that emits its light with interruption?
Is it appropriate to consider a star as similar to a living organism? Why?
This tutorial aims to explain how different stages of a stars existence develops. All stages in which a star passes from its creation to the end of its life are explained for two seperate categories of stars: those with dimensions similar to Sun and those that are much bigger than the sun. It is important to consider the two types seperately as the evolution of the two-aforementioned types of stars differ at a certain point of their existence.
The evolution of a star is very similar to that of a human being. A star is "conceived" from the compression of a large gas cloud, then it is "born" when thermo-nuclear reactions of fusion are activated. At this point, the star begins a long period of stable state which is part of its main sequence. In a certain sense, this stage represents the "adulthood" of a star. Then, the star gets old during the red giant stage and eventually terminates its life at the white dwarf stage or at another stage we will discuss later on in this tutorial. Now, let's take a closer look at each of these stages in more detail.
The formation (birth) of a star may occur inside any gas cloud. There are many such clouds in the Universe as ithe Universe contains very large amounts of hydrogen and helium in gaseous states not currently associated to any star or other celestial body. When there is enough gas available, it begins to collect in smaller volumes due to gravitational (attracting) forces. The more that gas collects around this "condensation" zone, the more gravitational force increases, attracting more gas from around. This process is similar to clouds formation in the sky.
The process of star formation is very long; it includes a number of factors where the most relevant are gravitation (which tends to attract matter and compress it inside the newly created sphere, and gas pressure (which increases with the increase in temperature of gas and tends to oppose the further compression of matter).
During the process of star formation the gas sphere becomes so large that the gravitational force at its centre increases the temperature up to several million Kelvin degrees. This is essential process that activates the thermonuclear fusion reaction during which hydrogen burns out converting into helium. This moment marks the formation or the birth of a star.
The position of star in the main sequence graph after its formation is determined by its mass; the most massive stars lie in the upper part of the main sequence graph in the H-R diagram. The figure below shows all possible stages of the Sun (past, present and future) where the process of formation has lasted for 30 million years.
All stars in the main sequence involve the thermonuclear reaction of hydrogen fusion at their centre. This process is very slow and balanced; hence, this is the longest stage of a stars life. For example, the Sun is estimated to exist in this state for approximately 10 billion years. The actual age of the Sun is about 4.5 billion years, so it is nearly at half of its life cycle. The rate of mass loss is small however; this makes the Sun very stable, i.e. its displacement rate in the main sequence graph is very close to zero.
The stars of the main sequence that are bigger than the Sun have a higher temperature in their core due to the larger pressure exerted on their core by the weight of star. This accelerates the rate of nuclear reactions. Hence, they radiate more energy than the Sun and are it in the main sequence H-R graph. (Sirius is an example of such stars.) They consume their inner sources of energy much faster and as a result, they have a shorter lifespan than smaller stars. For example, a main sequence star 50 times heavier than Sun has a lifespan of only a few million years, i.e. about 1000 times less than the Sun.
On the other hand, stars that have a mass of about one tenth of the Sun can live up to 20-30 billion years. This is because they have a lower temperature and pressure than the Sun due to the smaller weight they have to carry. This temperature however must be sufficient to initiate thermo-nuclear reactions of hydrogen fusion. Such reactions occur at a slower rate and as a result, the stars live longer.
When hydrogen (that acts as a nuclear fuel) in the central part of a star begins to drain out, the gas pressure starts to decrease gradually. As a result, the star starts shrinking (figure 1) as pressure (as an opposing effect to shrink) is smaller than before. Therefore, gravitational force prevails over the resistive forces and the equilibrium breaks down. Matter starts a very fast process of compression known as gravitational collapse. In other words, gravitational collapse is the contraction of an astronomical object due to the influence of its own gravity, which in turn, tends to draw matter inward toward the centre of gravity.
On the other hand, the new values of pressure created after the collapse bring an increase in temperature and as a result, hydrogen starts to burn out again in a spherical layer that surrounds the star core (figure 2). The energy produced in this process causes an expansion of external layers of the star as shown in the figure 3 below.
We can identify three zones in the second and third figure where the star is in red giant and at the red super giant stage. They are the red core (where helium is burned out), the spherical surrounding layer (where there is still some hydrogen burning out) and the outer (surrounding) layer.
Due to this process, the star magnifies in dimension in a very short time. When this occurs to our Sun, its outer layer will have reached the Earths' orbit and, as a direct result, melting everything on its surface. This occurs because the surface temperature will reach 2000-3000 K. The Sun eventually comes out of the stable phase and turns into a red giant. As explained earlier, helium will convert to carbon, oxygen, etc. In the case of our Sun, this stage will last for about 2 billion years, until it eventually burns out the entire reserve of helium.
From the processes described above, it is clear that the Sun is in the mature stage - it still has 71% hydrogen and 28% helium. When all hydrogen turns to helium, the stage of red giant will start. Then the red super giant stage will follow it until all helium reserves are exhausted. At this point the core of the Sun will only contain oxygen, carbon and helium. Due to the high gravitation, the core has a very high temperature and pressure. They, in turn, cause the Sun to eject outer layers into interstellar space. This mass leaving the star (and the Sun in the future) is called planetary nebulae (it has nothing to do with planets though). Consequently, the Sun will eventually lose an important part of its mass to the point where only the central core, made up mainly by carbon and oxygen, remains. These need much higher temperatures than the core temperature at this stage to burn out carbon and oxygen. As a result, thermonuclear reaction cannot occur anymore and eventually, the Sun turns into a white dwarf. It continues to illuminate and loses energy without having any source to replace it. Hence, it turns into a cold and is no longer an observable object. This marks the end of Sun and all the other similar stars in the main sequence which have a mass up to 10 times heavier than the Sun but not more. The life of heavier stars experiences other stages, which we will discuss in the next paragraph.
The Sun consumes 600 million tons per second of hydrogen (actual rate). Would it continues burning hydrogen at this rate when it become a red giant? Take MSun = 1.989 × 1031 kg.
We know that 71% of Solar mass is hydrogen. This value corresponds to
Hydrogen is consumed at an actual rate of R = 600 000 000 tons/s = 6 × 1011 kg/s. Hence, the time required to burn out the entire hydrogen at the actual rate is
One year has 365.2422 days. When converted to seconds, this value becomes
Hence, the duration of the Sun in years in these conditions would be
This value corresponds to 745.6 billion years.
For stars that are more than 10 times heavier (and bigger) than the Sun, the first two stages are similar to those of the Sun. The evolution process starts to differ at the red supergiant stage. At this stage, all helium resources are exhausted and as a result, the gravitational pressure has no more opponents and as a consequence, gravitational collapse starts again. Let's discuss this situation in more detail.
Due to the large mass, pressure in the stars core increases too much and as a result, the temperature at this region may reach 500 million Kelvin degree. The conditions for thermonuclear fusion reactions of carbon and oxygen in order to form heavier elements are ideal. The energy produced is so large that the light flux of these stars is thousands of times higher than the Sun flux. The result is that a red supergiant is formed.
There are various stages of thermonuclear reaction and these depend on the mass of the specific star. The heaviest stars have the possibility to burn heavier chemical elements but not heavier than iron. This is because iron is the most stable element in nature; fusion reactions cannot produce heavier elements than iron.
After consuming all reserves of nuclear reactions of fusion, the star will, once again, experience the process of gravitational collapse, which now becomes even more intense. When compressing, the stars core may reach very high densities up to thousand billion times the density of water. Electronic pressure cannot overcome the high gravitational pressure. As a result, protons in the nuclei catch the electrons around them and turn into neutrons. In this way, the star core becomes a neutron star that is a star made only of neutrons. Pressure exerted by neutrons is much higher than that of electrons in white dwarfs, so it can bear the star's weight. When the core of the shrinking star turns into a neutron star, its outer layers moving towards the centre, collide with the strong core and turn back at very high energy. This collision represents a supernova. It is accompanied with a powerful explosion that is easily observable in the sky. If the star experiencing this event was previously invisible from Earth, it now becomes luminous. The intense brightness lasts for several days or even months. The amount of illumination a supernova produces is comparable to that of millions of stars combined together, as seen in the photo below.
After the explosion, only the neutron star remains at the centre and it is surrounded by the cloud of matter that is leaving the star. We call this cloud "planetary nebula". As explained earlier, it is made up by outer layers the star ejects during explosions.
Neutron stars rotate at very high speeds around their own axis. During this rotation, they radiate EM waves in a specific direction, which in many cases does not fit the axis of rotation. As a result, when radiation emitted by a neutron star reaches Earth, it comes in regular intervals determined by its period of rotation. In this instance, we are observing a pulsar star (or simply pulsar). Look at the figure.
The mass of neutron stars may reach three solar masses. However, this mass in concentrated in a very small volume, in a sphere of radii equal to a few kilometres. This is because there are more empty space between neutrons; the density of neutron stars is equal to that of atomic nuclei, i.e. about 1021 kg/m3. Like white dwarfs, neutron stars do not produce energy. They are not eligible to be part of H-R diagram, hence we cannot see them appear there. They are observable only through the pulses they emit.
When the core of a collapsing star is heavier than three solar masses, the pressure produced by neutrons cannot withstand to gravitational force that "guzzles" everything around it. Nothing is able to resist such a high-energy elan. In this way, a spatial black hole is created. Black holes have triple the mass of the Sun and radius not more than a few kilometres. Only at the cente of galaxies may black holes of greater dimensions (millions of km) occur and obviously have much heavier (millions of one solar mass). There are also some black holes of very small dimensions and very high densities as well.
A black hole does not radiate EM waves; it only absorbs, everything from matter to radiation. Scientists can identify them only by the effects (mainly of gravitational nature) they cause in the surrounding space.
In the current (but also in the previous) tutorial, we described planetary nebulae as large gas clouds, once parts of a star that are moving away from its core. This process may occur during three stages: red giant, white dwarf and neutron star. When moving in the interstellar space, planetary nebulae provide it with elements such as hydrogen, helium and other chemical elements once contained in the parent star. We can compare a star with a factory that produces heavy chemical elements by processing hydrogen and helium as raw material.
We explained that chemical elements lighter than iron (including it) are formed through thermonuclear fusion reactions. But we have not explained yet how heavier elements are produced. The answer is: they are still produced in stars but through more complicated processes than thermonuclear fusion. Hence, planetary nebulae are the distributors of stellar output throughout the rest of the Universe.
The Cancer Nebulae is made of the outer layer of a neutron star ejected during a supernova explosion in 1054 AD. People could see the brightness of this supernova in the sky for three weeks, even during the day when the Sun was present. If the moving speed of material ejected from the parent neutron star was 1500 km/s, calculate the diameter of the nebula at the end of the explosion process.
The diameter d of nebula at the given time is double its radius r, which on the other hand represents the distance travelled by the nebula during the 3-week period, assuming that this nebula has a spherical shape where the neutron star is located at centre. Thus, we have
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