In this section, we will focus on stars.
Stars
“The interstellar medium is filled with gas and dust. In some regions the density of gas and dust is much higher than the mean value in the medium. Stars are formed due to the collapse of such clouds or regions of high density. The temperature of these clouds is normally quite small and hence they do not emit any visible radiation. However, they are often illuminated by light due to stars in their neighborhood. Such illuminated clouds are called nebula. Due to their high density, they cause considerable extinction of star light. Hence the density of stars appears much reduced in the direction of these clouds.
The gas in the interstellar medium is predominantly hydrogen and helium. Hydrogen may be in the atomic, ionized, or molecular state and contributes roughly 70% by mass. Helium is predominantly in the atomic state and approximately 28% by mass. The mean gas density is about 10% of the total density of the Milky Way. […] Dust is composed of complex molecules formed out of atoms such as carbon, oxygen, nitrogen, silicon, and hydrogen. Its mean density is about 0.1% of the total density of the galaxy (Jain 189).
“We find several different types of clouds in the interstellar medium. We observe diffuse hydrogen clouds, where hydrogen is found in its atomic state. The temperature in these clouds ranges from 30K to 80K. […] Due to their low temperature, hydrogen is found in its ground state. We also observe molecular hydrogen clouds. These are regions of high density of molecular hydrogen and dust. Molecular hydrogen would normally be broken down by star light. However, these clouds are shielded by regions of high-density atomic hydrogen and dust. These cause considerable extinction of star light and hence allow molecular hydrogen to exist. We observe many enormous complexes of dust and gas, called Giant Molecular Clouds (GMCs). […] These are the sites of active star formation. Many such clouds are found in the Milky Way. They exist predominantly in the spiral arms” (Jain 189-90).
Early Star Formation
“During the initial stage of star formation, a compact object called a protostar is formed. This is the stage before the nuclear reactions start. A molecular cloud will collapse if it is gravitationally unstable. […] Normally a cloud remains in equilibrium until some trigger, such as a nearby supernova explosion, makes it unstable to collapse. […] The trigger may be some disturbance such as a supernova explosion in their neighborhood which sends out a shock wave into the interstellar medium compressing the gas with which it comes in contact. During the initial stages, the cloud is nearly in free-fall under gravitational attraction, with the pressure gradient being almost negligible. The cloud has sufficiently low density that heat is efficiently radiated out. Hence its temperature does not change substantially during collapse and remains the same throughout the medium. The collapse at this stage is isothermal. One can solve Newton’s equation of motion to determine the typical time scale of collapse.
As the cloud contracts, its density increases. In time, its core becomes sufficiently dense so that the heat generated due to the release of gravitational potential energy does not get radiated away efficiently. The temperature of the core starts to increase. The pressure gradient can no longer be neglected and the core contracts at a slower rate. The surrounding medium, however, is still in free-fall. The central object is called a protostar. […] The temperature of this object is such that it radiates at infrared frequencies. The energy transport occurs dominantly by convection. This object is surrounded by dense molecular clouds. Hence observational evidence for such an object is the existence of small IR sources embedded within molecular clouds. Their detection is difficult because these objects are very short lived due to the smallness of the free-fall time scale. […]
Stars often form in clusters. This is due to the fact that as the cloud collapses, it undergoes fragmentation, wherein the different fragments collapse independently to form stars. The fragmentation occurs because the original cloud is not exactly homogeneous. It has some regions of higher density. During the initial stages of collapse, the temperature of a cloud does not change. However, its density increases. Hence in time, some dense regions might themselves become unstable to collapse and start contracting independently of the original cloud, leading to fragmentation. This region may undergo further fragmentation due to its inhomogeneous density distribution. The process of fragmentation would continue until our assumption that the collapse is isothermal breaks down. At this point, the cloud becomes very dense and the gravitational potential energy released cannot be efficiently transported out. The temperature starts to rise and the process of fragmentation stops. […]
When these stars reach the main sequence, their UV radiation raises the temperature of the medium. This inhibits further star formation. Furthermore, their radiation pressure drives out the outer layer of the cloud. This may disperse the remaining cloud. Due to the increase in temperature and reduction in cloud size, stars that were gravitationally bound earlier may no longer remain bound and may start to drift apart (Jain 189-192).
Evolution on the Main Sequence
“A star of mass less than 0.26 MSun becomes a white dwarf at the end of the main sequence phase. If the mass of a star is greater than 0.26 MSun , helium burning starts in the core. The star becomes a red giant or a supergiant. If the mass of a star lies between 0.26 MSun and 2 MSun , the helium fusion starts explosively. For higher mass stars, it starts non-explosively. This explosion is called a helium flash.
The explosion occurs because the fusion starts when the core is degenerate. In this situation the gravitational attraction is balanced by the pressure of the degenerate electron gas. The nuclear fusion reactions generate heat, leading to an increase in temperature. However the pressure does not change much as long as the temperature is sufficiently small so that degeneracy is maintained to a good approximation. Hence, in contrast to the non-degenerate matter, the core is unable to expand with increasing temperature. Meanwhile the nuclei get heated up. The reaction rate of helium fusion, that is, the triple α process, is very sensitive to temperature. Hence an increase in temperature leads to a rapid increase in the energy production rate. This leads to a further increase in temperature and triggers a runaway nuclear fusion reaction.
Another important aspect of the phenomenon is that the conductivity of a degenerate electron gas is very high. Hence the temperature in this medium is uniform and the fusion process starts in the entire core almost at the same time. At sufficiently high temperature, the degeneracy is lifted and the core expands explosively. This is called a helium flash. The outer envelope is pushed out and the star settles into the red giant phase. Despite the explosion in the core, it does not lead to an overall disruption of the star. […]
If the mass of a star is less than 3MSun , the core never gets hot enough to burn carbon. As carbon accumulates in the core, helium and hydrogen fusion takes place in the surrounding shells […]. Once the carbon core gets big enough, it becomes unstable to collapse. The collapse produces energy, which expels the outer envelope into the interstellar medium, forming, what is called a planetary nebula. We point out that, despite the name, this has nothing to do with planets. The core of this system forms a white dwarf. […]
In the case of stars of mass larger than 3MSun , the nuclear reactions proceed beyond helium fusion. The temperature in the core gets sufficiently high to burn carbon and heavier elements. […] Its mass is dominated by carbon at the lower end of this mass range and by oxygen at the upper end. Hence, in analogy with a helium flash, the fusion of carbon (or oxygen) starts explosively. This is a very intense process and probably destroys the entire star in a supernova explosion.
If the mass of a star is greater than 15MSun , the carbon fusion in the core starts when it is nondegenerate. Hence the process is nonexplosive. In this case, all the allowed fusion reactions take place, eventually forming iron in the core. During its final stages, the star has an iron core, with silicon, oxygen, carbon, helium, and hydrogen fusion occurring in shells […]. Fusion reactions cannot occur in equilibrium in the iron core. As its mass becomes sufficiently large, it becomes unstable to collapse. During the collapse, the gravitational energy released breaks down iron into lighter nuclei, which further break down, eventually forming protons and neutrons. This collapse occurs very rapidly, within a fraction of a second. The outer layer explodes as a supernova. The core collapse eventually stops when it reaches neutron degeneracy. In this case, the free neutron gas becomes degenerate. The star becomes a neutron star. If the mass of the star is sufficiently large, even the neutron degeneracy pressure is not enough to stop the collapse. The star becomes a black hole (Jain 196-198).
Population I and II Stars
“The Big Bang model of the Universe proposes that the Universe originated at some time and has been expanding since then. In very early times, it was very hot and filled with plasma composed of photons, electrons, protons, neutrons, and neutrinos. Due to very high temperatures, neither nuclei nor atoms could exist at that time. As it cooled, a few light nuclei, predominantly helium, formed. As it cooled further, hydrogen and helium atoms formed. Eventually, galaxies and galaxy clusters formed. Hence the first stars in the Universe formed out of material that was predominantly hydrogen and helium. The percentage of other elements was negligible. These stars are called population II stars. These are the oldest stars in the Universe. Within the Milky Way, they are found in globular clusters, which populate the galactic halo. These clusters probably formed at the same time as the formation of the Milky Way. The surface composition of population II stars is predominantly hydrogen and helium. In the interior, of course, heavier elements are present due to fusion.
As the Universe evolves, some of the population II stars end their life in a supernova explosion, which replenishes the interstellar medium with metals […]. Hence the next and subsequent generation of stars is formed in a medium that is much richer in metals. These are called population I stars. For example, the Sun is a population I star. These have a much larger abundance of metals on their surface in comparison with population II stars. They are found predominantly in the disks of spiral galaxies, in particular the spiral arms, and are generally younger, brighter, and hotter in comparison with population II stars. Because a higher mass star has a smaller lifetime, we expect that population II stars, in general, have lower masses in comparison with population I stars. The higher mass population II stars would have reached the end of their life cycle by the current time” (Jain 198-199).
Up next: The Solar System
Works Cited
“Brightest galaxy and first-generation stars.” EarthSky. Earthsky Communications. 18 June 2015. Web. 19 July 2015.
Bryd, Deborah. “Newly forming star stretched a light-year long.” EarthSky. Earthsky Communications. 1 Sept. 2013. Web. 19 July 2015.
“Earth-sized diamond in space is coolest white dwarf star.” EarthSky. Earthsky Communications. 24 June 2014. Web. 19 July 2015.
Jain, Pankaj. An Introduction to Astronomy and Astrophysics. Boca Raton: CRC Press, 2015. Print.
MacPhee, Martin. “Exploring the Trifid Nebula.” EarthSky. Earthsky Communications. 18 Aug. 2014. Web. 19 July 2015.
“This black hole outgrew its galaxy.” EarthSky. Earthsky Communications. 13 July 2015. Web. 19 July 2015.
The Renaissance Girl's Guide to Life 