What is “Burning” of the Hydrogen in the Stars?
Krs Murthy
All of us have seen the stars burning and produce heat, plasma and a lot of electromagnetic radiation. Our Sun as a star is an example of our own witness every day, so are other stars in our galaxy and stars in other galaxies of our universe. We have also know that the stars burn their hydrogen and fuse the hydrogen to form helium in its core, where the heat is in millions of degrees. The cores of stars are like oven of fusion, the fusion of hydrogen to produce helium. We know that hydrogen has one proton in the nucleus at its center and one electron orbiting the proton in the nucleus. Upon fusion, the result is the creation of a nucleus with two protons, and two electrons orbiting around the larger nucleus, thus making the helium atom.
Hydrogen Burning on the Earth Versus on the Stars
The earth has an atmosphere of oxygen among other gases. The burning of hydrogen on the earth includes mixing of the hydrogen with the oxygen in the atmosphere, producing the by-product of water. This is aerobic combustion, meaning participation of oxygen in the combustion process. Oxygen participates in all combustion processes on the earth.
However, the stars, including our sun, may not have 'manufactured' the oxygen and may have only hydrogen or helium. Therefore, the burning in stars like our sun is anaerobic. The heat is produced by the gas pressure of hydrogen or helium as an example. In these type of anaerobic combustion processes, gaseous pressure and heat are two expressions of the same property of the gaseous activities.
If the hydrogen is “burning” in the anaerobic mode in a star, where do the heat radiation and also other electromagnetic waves come from? You may note that hydrogen atoms have only one proton in the nucleus with only one electron orbiting the nucleus. As we know in the physics of electromagnetic radiation from atoms require the electrons to gain energy and move to a higher orbit and be an excited state, to later drop back to the original energy state, thus giving out the energy difference in the form of an energy quantum. The differential energy quantum has an associated frequency or wavelength expressed by Planck-Einstein relation, and it looks like this: E = hf. Here, E is the energy of each packet (or 'quanta') of light, measured in Joules; f is the frequency of light, measured in hertz; and h is the Planck's constant.
Where does the energy come from for the electrons to get to an energized state?
The only energy that drives the whole process, the sequence of processes, starting from a hydrogen cloud to the formation of a star, star burning, production of heavier elements, the full life cycle, is the gravitational forces. This force could act in the cohesion of the hydrogen atoms in the hydrogen cloud bringing the atoms together, especially suddenly, rather than slowly. This is like a chain of reactions that accelerates as the hydrogen atoms are drawn to each other with increasing force, the force increased by a square law with decreasing distance from each other. Once a critical density of the hydrogen atoms is reached, there is no turning back, as the different phases of star formation, it temperature and pressure increasing starting from the surface towards the core.
Higher the mass of the overall hydrogen cloud at the starting of the chain of reactions leading to the star formation, the higher the resulting temperature/pressure at the core, which increases progressively with time.
Once the surface reaches a critical temperature it starts glowing with the emission of electromagnetic waves, including infrared, visible light spectrum, ultraviolet and X rays. The emission spectrum is a reflection of the spectrum of vibrational modes of the energy of the hydrogen atoms on the surface of the star.
What is really “burning” referred to in this context?
“Burning” is associated with the production of heat, flame, plasma, and electromagnetic radiation.
When atoms collide with each other they exchange kinetic energy. The atoms may bounce off each other, bounce between multiple other atoms, with collision and increased collision. Once a critical rate of collision is reached, electromagnetic radiation results.
The electromagnetic radiation may contain many frequencies and associated wavelengths. Electromagnetic radiation in the infrared wavelengths is heat. Wavelengths in smaller wavelengths from red to violet is seen as light, and associated colors, by humans. The different wavelengths of electromagnetic radiation of light create different sensations of colors, and white light, in human beings and animals. Light is an experience, as is the heat, in human brains, perceived and processed with and through the eyes, the full network of sensory components and especially understood by our brain. Heat radiation is sensed by other organs and their components in our body, finally perceived by our brain.
Our Sun as a star also produces ultraviolet, X rays and higher frequencies that even reach the earth. In the first satellite built and launched by India of which program I was fortunate to play a primary part, diurnal variation solar X rays were measured using scintillation counters on the satellite.
Burning is nothing but the increased collision of hydrogen atoms, resulting in the expulsion of radiation, which is perceived on earth as light, heat and electromagnetic radiation. However, it should be noted that only a small part of the electromagnetic radiation leaves the star like our Sun, expanding in all directions away from the Sun, out of which only a very insignificant part travels towards the earth, while many parts of the radiation is absorbed in their traveling path, and only the remaining fractional part reaches the earth.
What we receive on the earth is only a waste lost from the Sun, similar concepts being true for other stars also.
While radiation escapes the stars from their surface only, the remaining burning intensity inside and all the way to the core perform different functions. If we virtually travel from the surface of the stars towards its center, the intensity of collision increases. In other words, the intensity of collision of the hydrogen atoms translates to decreasing mean free path between the collision of the hydrogen atoms, which also translates to a density of hydrogen atoms per unit volume. Very close to the center of the star, the hydrogen atoms are pressured so close together that the electrons in orbit around the proton in the hydrogen atoms rip each others away from their nucleus, called degenerate electrons, giving the nuclei and their protons to come so very close to each other that protons join to form a larger nuclei of two protons; the electrons that were torn away from their original single proton nucleus find themselves orbiting around nuclei of two protons. This is the genesis of “Helium” atoms in the innermost core surrounded by hot “soup of hydrogen atoms.
Smaller stars may fuse hydrogen atoms in their core into helium atoms, whereas the larger stars can produce even more heat and pressure in their cores to do a further fusion of helium atoms into heavier atoms. This is because larger stars have even longer radius than the smaller stars, thus able to produce increased heat and atomic pressure harboring conditions for the fusion of hydrogen atoms to helium atoms, and further to heavier atoms. The heat and atomic pressure is proportional to the size of the stars.
Helium is the second most abundant element in the universe and is a major component of main sequence stars such as the Sun. Helium accumulates in the core of stars as a result of hydrogen nuclear fusion. Helium accounts for approximately 27 percent of the Sun's mass
Chemical composition. When stars form in the present Milky Way galaxy they are composed of about 71% hydrogen and 27% helium, as measured by mass, with a small fraction of heavier elements
27 million degrees Fahrenheit
At the core of the sun, gravitational attraction produces immense pressure and temperature, which can reach more than 27 million degrees Fahrenheit(15 million degrees Celsius). Hydrogen atoms get compressed and fuse together, creating helium. This process is called nuclear fusion.
Once the temperature reaches 15,000,000 degrees Celsius, nuclear fusion takes place in the center, or core, of the cloud. The tremendous heat given off by the nuclear fusion process causes the gas to glow creating a protostar. This is the first step in the evolution of a star.
The incredible mass of stars creates intense heat and pressure in the core, triggering the fusion process, so it makes sense that the more mass, and therefore gravity, that a star has, the greater the pressure, and the more fusion is going to be driven.
Luminosity is a measure of the power of a star. Since fusion is the source of energy in a star, we should expect the luminosity to increase as we increase the rate of fusion. Radius and temperature, on the other hand, are better understood empirically.
As a star ages, however, it begins to run out of hydrogen in its core. Since fusion provides the force to hold the star up against gravity, as fusion slows down, the core becomes denser and heats up. As it does so, the outer layers of the star expand and cool, and the star moves to the right of the diagram where we find the red giant and supergiant stars.
Radius, therefore, depends more on the age of the star than anything else, however, more massive stars will ultimately make for larger stars in the long run.
Stars are classified according to their physical characteristics. Characteristics used to classify stars include color, temperature, size, composition, and brightness. Stars vary in their chemical composition.
The Sun is a G2V type star, a yellow dwarf and a main sequence star. Stars are classified by their spectra (the elements that they absorb) and their temperature. There are seven main types of stars. In order of decreasing temperature, O, B, A, F, G, K, and M.
