Stars are bright objects that we can see in the night sky. During the day, we see the brightest object in the sky is also a star. Of course, I am referring to our Sun. Our Sun is a very ordinary star. Due to the brightness of stars (including our Sun), you may wonder how do stars shine.
NASA image from the Solar Dynamics Observatory showing the effects of energy produced in the Sun.
In this article, we will ponder that subject. I will first discuss some of the early ideas before briefly discussing what causes stars to shine. For some, this may be enough. For those who would like to gain a deeper understanding, the later sections will go into more detail. I encourage you to read the whole article to gain a good understanding of how stars produce energy.
Early Idea of How the Sun Shines
Obviously, because of its significance, the Sun’s energy source was the first thing studied.
To put things into context, the Sun has 1000 times the mass of all other objects in the Solar system combined. It has a surface temperature of 5800K (9980 degrees Fahrenheit, 5527 Degrees Celsius). The Sun’s energy output is calculated at 3.9 x 1026 Watts. That is 39 followed by 38 zeros! We will discuss the significance of the energy produced in a later section.
Because we can produce energy by burning things like wood, perhaps the energy is produced by regular burning. Burning occurs due to the rapid oxidation of materials. If this idea was correct, the Sun could not be any older than 10,000 years. This is because the fuel would have been consumed by that time.
The idea was dismissed because geology and biology indicated that the Earth was much older than this.
As a body of gas contracts, it releases heat. Two very famous scientists (Lord Kelvin and Hermann von Helmholtz) proposed that this may have been the source of the Sun’s energy. However, closer inspection determined that the Sun was only large enough to produce energy for about 25 million years using this mechanism. This was still not long enough to allow for the observed geological and biological processes.
Scientists were at an end to explain how stars produce energy until a young scientist provided the clue in 1905.
Stars Shine by Producing Energy in Their Cores
In 1905, Albert Einstein published his paper detailing the Special Theory of Relativity. In it, he demonstrated that matter can be converted into energy. Not only did it convert matter to energy, but it showed that a small amount of matter produces huge amounts of energy.
Of course, this is Einstein’s famous equation E=mc2. In this equation, E denotes energy, m is the amount of mass converted to energy and c is the speed of light. Because the speed of light is huge and the value is squared, it is a humongous number. Thus, very small amounts of matter release vast amounts of energy. If you wish to know more about this read the later sections.
Astronomers quickly realized that Einstein may have given them an answer on how stars shine. The low density and spectrography of the Sun showed that it was composed of mainly hydrogen and some helium. Furthermore, it was determined that the pressure and temperature in the Sun’s core were both high. These conditions produce ionized gas (referred to as plasma).
Under high temperatures and pressures, hydrogen can fuse to form helium. During the multi-step process, four hydrogen nuclei (i.e. a single proton) are fused to form a helium nuclei. A helium nucleus contains two protons and two neutrons. As part of the fusion process, two of the original protons are converted to neutrons. If you wish to explore this process you can refer to Proton–proton chain on Wikipedia.
Therefore, it was determined that the fusion of hydrogen to helium in the Sun’s core is its energy source. As the Sun is a star, it was inferred that all stars produce energy similarly.
The process of hydrogen fusion is referred to as hydrogen burning. More scientifically, it is called thermonuclear fusion. Thermo denotes high temperature, nuclear as the atoms of elements are involved and fusion because more than one nucleus is combined.
We will now explore the thermonuclear fusion process in stars more closely. We will also look at how the energy travels from their cores to their surface.
How Do Stars Shine: Getting into the Physics
To truly discover how stars shine, we must understand some nuclear physics. Don’t be overly concerned as the principles are easy to understand. Don’t try to understand the mathematics behind it as that is truly baffling.
The key to understanding thermonuclear fusion is understanding two of the four fundamental natural forces.
Two Fundamental Forces of Fusion
The electromagnetic force exists between charged particles. The effect of this force is that like charges repel and particles charge differently are attracted. Electromagnetism operates over large distances. As particles approach each other, the force gets stronger. While not crucial for our understanding of the subject, photons mediate electromagnetism. For a description of what a photon is, see the Q&A section below.
The strong nuclear force holds the constituents of the nucleus of atoms together. It is much stronger than the electromagnetic force, so can hold similarly charged particles together. Unlike electromagnetism, the strong nuclear force only operates at very small distances. The range of this force is only about twice that of the radius of a proton or neutron. The gluon particle mediates the strong nuclear force.
A normal hydrogen nucleus comprises a single proton with a positive charge. As such, the electromagnetic force pushes hydrogen nuclei apart. This is problematic as for fusion to occur hydrogen nuclei must get very close to each other for the strong nuclear force to overcome electromagnetism.
We will now temporarily deviate to describe the conditions in stars’ cores before returning to this discussion.
Conditions in the Cores of Stars
In the same way that pressure increases as you dive deeper in water, the pressure in stars increases with depth. With depth, the temperature also increases.
For example, the temperature at the Sun’s core is about 27 million degrees Fahrenheit (15 million degrees Celsius). The pressure is about 3.84 trillion psi.
Under these conditions, hydrogen is in an ionized state. In this state, hydrogen nuclei are independent of electrons.
Thermonuclear Fusion in the Cores of Stars Produce Energy
So we now understand that there are two important forces to consider and that the core of stars are hot and dense. With this understanding, we can proceed with explaining how nuclear fusion occurs.
High temperatures mean hydrogen nuclei (prominently a single proton) move around at very high speeds and are packed closely together. Under these conditions, two nuclei can get sufficiently close together for the strong nuclear force to overcome the electromagnetic force.
Two protons fuse when two hydrogen nuclei get close enough for the strong nuclear force to take effect. This fusion releases gamma radiation and a new particle that quickly escapes the star (neutrino). Gamma radiation is the primary form of energy produced in stars.
As with physics, that simple explanation is only part of what occurs. Let’s delve a bit deeper.
Hydrogen Fusion Process: the Proton-Proton Chain
As two protons in a nucleus are not stable, one of the protons emits a positron and becomes a neutron. A positron is an electron with a positive charge and is a fancy name for an anti-electron. The positron quickly encounters an electron, annihilating both particles and producing two photons of gamma radiation. Also emitted with the positron is a particle called a neutrino. A neutrino is a very light particle that does not interact with other matter very often. As such, it escapes the star very quickly.
Those of you with some physics or chemistry may have noticed that this process produces a heavier form of hydrogen. So, there is more of the story. In fact, there are two more steps before a stable helium nucleus is produced.
During the second step of the process, the particle produced in the first step is fused to a normal hydrogen nuclei. In the process, a further photon of gamma radiation is produced. We now have a nucleus of helium with one neutron.
A helium atom with only one neutron is not stable. A stable form of helium is produced in the last step. Two unstable helium nuclei fuse to form a stable helium atom containing two protons and two neutrons. The process releases two protons (i.e. hydrogen nuclei).
This process of fusing hydrogen to helium is called the proton-proton chain and is illustrated below. In larger stars hydrogen fusion can also occur more readily via the CNO cycle, but this will not be covered here.
If you look at the illustration above, you may have noticed that neutrinos are released in the process. It also mentions that they are massless. The idea that they were massless led to what was called the Solar Neutrino Problem when it was found that we detected far less of them than expected. It turns out that they do have a small mass. If you wish to know more about this, you can read Solving the Solar Neutrino Problem.
How Does the Energy Get from the Core of Stars to the Surface?
Due to the fusion, we have three photons of gamma radiation in addition to the helium and neutrino produced. Gamma radiation is not what we see emitted from the surface of stars. In the case of the Sun, the vast majority of the emitted light is in the visible range, so something else must happen.
That process occurs as the photons travel from the core to the surface. As the photons travel, they encounter atoms. If the conditions are suitable, atoms absorb the photons. As a result, electrons are elevated into an ‘excited’ state. Electrons don’t like to remain in an excited state for long and spontaneously return to a less excited state.
When an electron moves to a less excited state, it may drop to the state it was in before the gamma radiation was absorbed. If this occurs, a gamma photon will be emitted.
However, if it drops to a more excited state than the initial state, a photon containing less energy will be emitted. At a later time, another photon can be released when the electron returns to its ground state. This process converts a single gamma photon to multiple less energetic photons. Each new photon contains less energy than gamma photons, but their sum will be the same as the original gamma photon.
In summary, the energy from the initial fusion is passed from atom to atom as it moves away from the center of a star towards its surface before it is released into space. At the same time, it is converted to lower energy photons that are emitted from the star. The process changes the original gamma radiation to visible light.
It may surprise you that the energy released in the Sun’s core takes an average of 200,000 years to leave the Sun.
We will use the Sun as an example to quantify how much hydrogen is consumed in the process.
As detailed earlier the Sun emits about 3.9 x 1026 Watts of energy. This means that 3.9 x 1026 Joules of energy is produced every second.
As we now know, stars shine due to the conversion of mass to energy. To determine how much mass is converted, we need to know the original and remaining mass. As it turns out, 0.7% of the original mass is converted into energy. We can determine that every 1kg of converted hydrogen produces 6.3×1014 joules of energy.
With that information, we can calculate that the Sun consumes 600 million metric tons of hydrogen every second.
Frequently Asked Questions
Why do stars shine?
Stars shine due to the emitting energy from their surface. The energy is produced in their cores via thermonuclear fusion of hydrogen to helium.
How large does a star need to be to produce energy?
For a star to be a star it must be large enough to sustain hydrogen fusion. We believe that the smallest a star can be is 7 or 8% of that of our Sun. We have yet to find a star so small.
How old is the Sun?
Our Sun is approximately 4.6 billion years old. It is believed that the Sun will exist in its present state for another 5 billion years.
How hot is the Sun?
The Sun is a range of temperatures depending on where it is measured. Its surface is 9980 degrees Fahrenheit (5527 degrees Celius). In scientific terms, it is 5800K. The Sun’s core is 27 million degrees Fahrenheit (15 million degrees Celsius).
What are neutrinos?
Neutrinos are almost massless particles that are released in stars during fusion. The Sun releases copious Neutrinos. In fact, about 100 billion neutrinos from the Sun pass through the area of a thumbnail every second. Luckily, they rarely interact with other matter. Most neutrinos pass through the Earth unhindered.
Initially, it was believed that neutrinos were massless. However, the resolution of the Solar Neutrino Problem illustrated that neutrinos ‘oscillate’ as they travel. This means that they change from one type to another. For this to occur, they must have a small mass.
What is a photon?
A photon can be considered as a partial of electromagnetic energy. It is the smallest unit of electromagnetism. However, not all photons are the same. Some are more energetic than others. Gamma radiation is composed of very high-energy photons, while what we perceive as light is less energetic. Radio waves have photons of much lower energy photons.
Concluding Thoughts
I congratulate you on making it this far. That was one hell of a ride.
Hopefully, you have some understanding that stars like the Sun shine due to thermonuclear fusion in their cores.
The process converts a small amount of mass into a lot of energy.
The released energy in the core slowly reaches the star’s surface. Along the way, it is converted to less energetic photons.
I found astronomy while working in dark rural locations. Initially, I explored the night sky and learnt the constellations before purchasing a pair of binoculars to further my knowledge of the sky.
My first telescope was a 200 mm Newtonian reflector on an equatorial mount. I found that this telescope had a steep learning curve but was a rewarding experience.
As time progressed, I became interested in astrophotography. This resulted in purchasing a 110 mm refracting telescope and a dedicated monochrome-cooled astronomical camera. This resulted in another very rewarding steep learning curve that far surpassed the experience with my first telescope.
I have joined Telescope Guru to share my knowledge of telescopes and astronomy.
