Nuclear physics: the core of matter, the fuel of stars

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Although stars are inanimate objects, we tend to describe their stages of evolution as if they were alive. Just like us, they are born, live and then die. And during their lives, stars produce monumental amounts of energy through nuclear processes in their interior, giving them their characteristic shine. Where do stars come from? Matter is not distributed equally in space. In between the voids of emptiness, there are regions of gas and dust clouds, called the interstellar medium, which are denser than their surroundings.

However, if the cloud is disturbed, perhaps by a nearby supernova, the balance is broken and the cloud may become denser in certain areas. When they reach a certain critical mass , the densest parts of the cloud can contract under the influence of their own gravitational attraction, causing the cloud to fragment into smaller and denser sections.

This process takes a few million years. As the cloud contracts, the temperature and the density increases. Eventually, the new object and gains a spherical shape and becomes what is called a protostar. Due to its gravitational pull, matter from the cloud still falls into it, continuously raising the temperature and density until they become so large that nuclear reactions start where the hydrogen fusions to make helium.

A new star is born. In the center of this gas and dust cloud called Omega nebula continuously new stars are formed. Source: NASA. Most stars are born within the arms of a spiral galaxy, where there is more gas and dust. Sometimes, several stars can form within the same molecular cloud, and we have what is known as a star cluster.

Two types of fusion reactions

There are two types of cluster; open clusters , which tend to contain a few hundred relatively young, hot stars that quite spaced out, and globular clusters , that tend to contain thousands of much older stars, more densely packed together. Excepting the Sun, most stars are extremely far away.

The nearest star, Proxima Centauri , is 4. They mainly observe two things: the luminosity of the star and its colour. One can create a 2-dimensional plot with the stars' surface temperatures versus their luminosity. The plot could look like the one here on the right.

Go from Quantum to Cosmic

Interestingly, the stars on the Hertzsprung-Russell diagram are not distributed evenly. Instead, they form a very distinct pattern. Most of the stars lie on a band from the top left of the diagram large, bright, hot stars to the bottom right small, dim, cool stars. This is called the Main Sequence. At the top right of the diagram are very large, bright but cool stars which we call Red Giants and at the bottom left corner are the White Dwarfs, small, dim, but extremely hot stars.

How Does Fusion Power the Sun?

At this point in its evolution, the star is in hydrostatic equilibrium. Therefore, its position on the Hertzsprung-Russell diagram does not change. In this state, the star is a Main Sequence star: our Sun is a prime example. They range from hot and bright starts at the top left of the Hertzsprung-Russell diagram to cooler and dimmer at bottom right.

APS Physics | GHP | The force between nucleons in QCD

Stars can stay in this phase for a very long time. However, the timescales for stars are not all the same. Larger stars tend to burn their fuel much faster and, therefore, run out much quicker. The stages of a star's life are defined by how much and what type of fuel the star has. During the Main Sequence, stars use hydrogen as their fuel.

'Nuclear Pasta' in Neutron Stars: New Type of Matter Found

When the hydrogen begins to run out, the star produces less energy to support its weight and the core begins to contract. This increases the temperature and density in the core and the luminosity of the star increases as a result. Such stars are called Red Giants. On the Hertzsprung-Russell diagram, they are located in the right upper corner. With no nuclear processes to power it, the outer layers of the star become unstable and the stellar wind produced by the star blows them away. From Earth, we can see this as huge colourful clouds rapidly moving away from the star, leaving just the star core behind.

This cloud moving away from the star is called a planetary nebula. The leftover core is small but very dense and hot. White Dwarfs are located at the lower left corner of the Hertzsprung-Russell diagram. For small stars like our Sun, the leftover core will consist of carbon and a little oxygen but for bigger stars, it could be made from neon. The most obvious difference between them is size. The picture below shows a few stars from our galaxy.

There are three red giants: Antares, Betelgeuse and Aldebaran. Random Question. How critical is the Earth-Sun distance in maintaining our average temperatures on Earth? Most Recent.


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