All About the Stars - Part 2

In the previous post, we entered to the world of stars. But it was mentioned that this was just an introduction. In reality, we barely touched the surface of the subject. There are still many important things about stars we did not see. So it makes sense to continue from where we stopped last time. This is All About the Stars, the sequel.


How is the structure of a star?

The structure of the Sun. On the left side the densities of each layers is given while on the right the corresponding temperature. Note that a photon needs 171,000 years to escape from the Sun. On the other hand a neutrino, since it does not interact with matter and moves close to the speed of light, it needs only 2.3 seconds. Image Credit: Kelvin Song

The Milky Way offers a unique opportunity to study the properties of stars, help us understand them, and classify them. Stars have differentiated layers just like planets, but the structure of a star is more complicated than a planet (i.e., crust, mantle, core). The interior of a star can be divided into six layers. This is done based on the physical characteristics of these regions. The range of every region varies from star to star and their extent depends on the size of the star. Additionally, the boundaries are not as sharp as they are on planets. From within, a star is partitioned in:
The Fusion core – This is the region of energy production in a star. Fusion takes place here (i.e., hydrogen fuses into helium, during main-sequence), releasing energy.
The radiation shell – This the region of a stellar interior where energy is transported towards its exterior. Energy travels through the radiation zone in the form of photons.
The convection shell – At this region, the energy is transported through convection (i.e., transfer of heat due to the bulk movement of material).
The Tachocline – This is an interface layer between the radiative and the convective shells. This is a thin region, and in the case of the Sun its thickness is 4% of the Solar radius.

The next layer is the stellar atmosphere, and it is divided into three distinctive regions.
The photosphere – This is the lowest and coolest layer in the atmosphere of a star, and the only visible part. The light that we receive from the Sun leaves from the photosphere. Finally, sunspots (i.e., small regions with lower temperatures, thus appear dark) form in the photosphere. The same applies to Solar flares and prominences.
The chromosphere – This is the second layer of a stellar atmosphere, and its main feature is that the temperature starts to increase. In the case of the Sun, the chromosphere has a reddish color. Above the chromosphere lies the transition region, where the temperature increases rapidly.
The corona – This is a layer of plasma that surrounds the Sun and other stars. This is the visible halo that we can observe in the Sun during a Solar eclipse. Additionally, the Solar wind (i.e., high-energy charged particles emitted from the Sun), originate in the Solar Corona.

Note that there are no figures. For example, hot stars have larger radiative zones. On the other hand, low mass stars are dominated by convective zones. The reverse is true of cool stars.



How do stars produce and release energy?

In the previous post, we saw that the moment nuclear fusion begins, a star enters its main sequence phase. Also, we saw the effect of the mass on its evolution, and what sort of elements will go under fusion in a stellar core. But how does this process take place? Let’s use our Sun as an example, and see how hydrogen fuses to form helium through a process known as the proton-proton chain reaction.

1H + 1H → 2H + e+ + νe + 0.44 MeV – Two hydrogen atoms fuse to produce deuterium. During the process positrons and neutrinos are released

e– + e+ → γ → 1.0 MeV – The positron interacts with the electron and they annihilate, releasing gamma-ray photons.

1H + 2H → 3He + γ + 5.5 MeV – The deuterium fuses with hydrogen to produce 3He. Photons in the form of gamma-rays are released.

3He + 3He → 4He + 21H + 12.9 MeV – The two 3He atoms fuse to produce the end-product (4He). Note that two additional hydrogen atoms are produced.

Where 1H is the hydrogen atom, 2H is a deuterium atom (a hydrogen isotope, wherein the nucleus there is a proton and a neutron), 3He and 4He are helium isotopes, e+ is a positron (an electron with a positive charge), e– is an electron, γ is a gamma-ray photon, and νe is a neutrino (a subatomic particle with no charge, moving at speed close to the speed of light and its mass is very close to zero). MeV is Mega-electron volt. This is an energy unit where 1 eV is equal to 1.602×10−19 Joules. One 1 eV is defined as the amount of kinetic energy a single electron gains when it accelerates from rest through an electric potential difference of one volt in vacuum.

Note that this is the energy production process during the main-sequence. We will review the energy production methods for the later stages of stellar evolution later on.


Energy transport of starsThe energy-transport mechanism of stars according to their mass. Image Credit: http://www.sun.org/encyclopedia/stars

The energy transport in stars, depends strongly on their mass and it can vary tremendously.

Stars below with mass < 0.5 M☉, have no radiation zones, thus energy transport can happen only through convection.

In stars with mass 0.5-1.5 M☉hydrogen fusion happens via the proton-proton chain. This does not establish a steep temperature gradient. Thus, radiation dominates in the inner sections of these stars. In the outer portion of such stars convection dominates. This makes our Sun, a star with a radiative cores and a convective outer envelope.

Finally, in more massive stars > 1.5 M☉, the core temperature is above about 1.8×107 K, so hydrogen fusion occurs primarily via the CNO cycle (we will review this later on). The CNO cycle has a strong temperature sensitivity, thus at the core there is a steep temperature gradient. Due to this the core becomes convective. In the outer envelope of such star, the temperature gradient is shallow. Due to this, massive stars have radiative envelopes.



Stellar populations in galaxies

In the mid-forties, Walter Baade noticed that the spiral arms of the Milky way are dominated by the presence of bluer stars. On the contrary, yellowish stars dominate the central galactic bulge and the globular clusters. Thus stars were grouped into Population I and Population II stars. Three decades later, Population III was introduced.

Stars that are in Population I class are young and they have the highest metallicity. They are commonly found in the spiral arms of galaxies, or in general in regions with recent star formation. Such stars usually have regular elliptical orbits around the Galactic center. There was a common belief in the astronomical society that Population I stars are more likely to have planets around them. This was considered mostly for terrestrial planets, since they are formed from metals. However, observations with Kepler found planets amongst stars with a range of metallicities. Furthermore, Kepler data have caused a controversy, since it discovered only gas giants around stars with high metallicity.

A sketch of the Milky Way Galaxy . Population II stars appear in the galactic bulge, within globular clusters and the Galactic halo. Population I stars appear on the spiral arms of the Galaxy (including) our Sun. Image Credit: R. J. Hall.



Population II stars are those with relatively lower abundances of the elements heavier than helium. Such stars formed earlier in the history of the Universe. Intermediate Population II stars are common near the center of the Milky Way, while those in the halo of our Galaxy are more metal-poor. Finally, these stars also dominate globular clusters.

An interesting feature of this class of stars is that although they are metal-poor, they usually have a higher ratio of elements like oxygen, silicon, and neon relative to iron when they are compared to Population I stars. Based on the current knowledge of stellar evolution, astronomers believe that type II supernovae contributed more to the clouds that led to their formation. It is commonly known that this class of supernova explosions enriches the local medium with these elements.

Finally, Population III stars contain a hypothetical population of very massive hot stars with metallicity very close to zero. The only possible contamination comes from supernovae from such stars. This kind of description fits only to stars that existed in the very early Universe, and they were responsible for the synthesis of metals in the early Universe. Despite the effort, no Population III stars have ever been observed directly. But the presence of such stars has been observed indirectly. This comes from the metal-contamination that has been detected on the spectra of quasars.

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