What is characteristic of a main sequence star

The most massive of them will move away from the main sequence first. Low-mass stars will leave the main sequence last. Red dwarf stars are an example of low-mass stars that stay for a long time in the main-sequence. When a red dwarf cannot sustain hydrogen burning anymore, it will sink and become a blue dwarf.

It becomes more luminous as seen in the blue hue. It will then become a white dwarf, getting cooler over time and expel its outer envelope. At the end of its life, it will become the theoretical remnant called a black dwarf. No star has reached this stage yet. Being the smallest stars in the galaxy, they will outlive all the others. They can live hundreds of billions or even trillions of years. That characteristic makes them good candidates in supporting life.

The Sun is bigger than a red dwarf so its life is shorter. Bigger stars have higher core temperatures. They need a larger amount of energy to continue burning. Because of this, they ran out of fuel earlier than less massive stars.

The Sun will remain a main-sequence star for about 10 billion years. That is still about five billion years considering our yellow star is 4. Stars with about 10 times the mass of the Sun will only be in the main sequence for about 20 million years. The course of stellar evolution depends on the mass of the star. After the main sequence stage they will either become red giants or red supergiants. Stars of intermediate-mass enter the giant branch.

When this happens, the helium core contracts, and a shell is formed around it. The outer layers of the star are expelled which results in a planetary nebula. It becomes a white dwarf. This is a sign that the star has reached the end of its stellar life. Red dwarfs do not pass through the red giant stage.

After the main sequence, stars with a high mass become red supergiants. The cores of these stars become so hot that helium and eventually heavier elements are fused together. At some point, their cores will eventually collapse, resulting in a bright supernova explosion. A neutron star or black hole will be left as a result of this explosion. Generally, astronomers divide the main sequence into two parts: the upper and lower parts.

The proton-proton chain is primarily responsible for energy generation in the lower main sequence. Hydrogen is directly fused together to form helium in this nuclear fusion reaction. The lower limit to sustain this process equals the mass of 80 Jupiters. The other reaction or the upper main sequence is known as the CNO cycle. The letters in its name stand for carbon, nitrogen, and oxygen.

They serve as the intermediates that help fuse hydrogen into helium. A main-sequence star like our Sun has three important parts. These are the core, the radiative zone, and the outer region. Energy in a Sun-like star is transported because of the difference in temperature in the core and the surface.

This is done through the process of radiation and convection. It is in a state of balance or hydrostatic equilibrium. Plasma does not mix that much in the radiation zone as compared to where convection happens.

Convection carries energy through the movement of plasma. Everything in the outer region of the star is utilized, making it a more efficient process than radiation. This process happens because the hotter material moves upward while the cooler elements move downward.

The Sun is our natural source of light and heat here on Earth. Without it, we would not be here and life will not be possible. It is a main-sequence star which means that it generates energy through nuclear fusion in its core. At each stage of the reaction, the combined mass of the products is less than the total mass of the reactants.

This is better expressed as:. In conditions such as those on Earth, if we try to bring two protons hydrogen nuclei together the electrostatic interaction tends to cause them to repel. This coulombic repulsion must be overcome if the protons are to fuse. The actual process whereby two protons can fuse involves a quantum mechanical effect known as tunneling and in practice requires the protons to have extremely high kinetic energies.

This means that they must be traveling very fast, that is have extremely high temperatures. Nuclear fusion only starts in the cores of stars when the density in the core is great and the temperature reaches about 10 million K.

There are two main processes by which hydrogen fusion takes place in main sequence stars - the proton-proton chain and the CNO for carbon, nitrogen, oxygen cycle.

The main process responsible for the energy produced in most main sequence stars is the proton-proton pp chain. It is the dominant process in our Sun and all stars of less than 1. The net effect of the process is that four hydrogen nuclei, protons, undergo a sequence of fusion reactions to produce a helium-4 nucleus.

The sequence shown below is the most common form of this chain and is also called the ppI chain. If you study the diagram above you will note that six protons are used in the series of reactions but two are released back. Other products include the He-4 nucleus, 2 neutrinos, 2 high-energy gamma photons and 2 positrons. Each of these products carries some of the energy released from the slight reduction in total mass of the system.

The overall reaction can be summarised as:. The neutrinos are neutral and have extremely low rest masses. They essentially do not interact with normal matter and so travel straight out from the core and escape from the star at almost the speed of light.

Positrons are the antiparticle of electrons. Although the pp chain involves the fusion of hydrogen nuclei, the cores of stars still contain electrons that have been ionised or ripped off from their hydrogen or helium nuclei. When a positron collides with an electron, an antimatter-matter event occurs in which each annihilates the other, releasing yet more high-energy gamma photons.

In the ppII chain, a He-3 nucleus produced via the first stages of the ppI chain undergoes fusion with a He-4 nucleus, producing Be-7 and releasing a gamma photon. The Be-7 nucleus then collides with a positron, releasing a neutrino and forming Li This in turn fuses with a proton, splitting to release two He-4 nuclei. A rarer event is the ppIII chain whereby a Be-7 nucleus produced as above fuses with a proton to form B-8 and release a gamma photon.

B-8 is unstable, undergoing beta positive decay into Be-8, releasing a positron and a neutrino. Be-8 is also unstable and splits into two He-4 nuclei. This process only contributes 0. These forms are summarised as:. Stars with a mass of about 1. CNO stands for carbon, nitrogen and oxygen as nuclei of these elements are involved in the process. As its name implies, this process is cyclical. It requires a proton to fuse with a C nuclei to start the cycle.

The resultant N nucleus is unstable and undergoes beta positive decay to C This then fuses with another proton to from N which in turn fuses with a proton to give O Being unstable this undergoes beta positive decay to form N When this fuses with a proton, the resultant nucleus immediately splits to form a He-4 nucleus and a C nucleus. This carbon nucleus is then able to initiate another cycle. Carbon thus acts like a nuclear catalyst, it is essential for the process to proceed but ultimately is not used up by it.

As with the various forms of the pp chain, gamma photons and positrons are released in the cycle along with the final helium and carbon nuclei. All these possess energy. Why does the CNO cycle dominate in higher-mass stars? The answer has to do with temperature. The first stage of the pp chain involves two protons fusing together whereas in the CNO cycle, a proton has to fuse with a carbon nucleus.

As carbon has six protons the coulombic repulsion is greater for the first step of the CNO cycle than in the pp chain.

The nuclei thus require greater kinetic energy to overcome the stronger repulsion. This means they have to have a higher temperature to initiate a CNO fusion. Born in , the son of a Presbyterian minister, Russell showed early promise. When he was 12, his family sent him to live with an aunt in Princeton so he could attend a top preparatory school. He lived in the same house in that town until his death in interrupted only by a brief stay in Europe for graduate work.

He was fond of recounting that both his mother and his maternal grandmother had won prizes in mathematics, and that he probably inherited his talents in that field from their side of the family. Before Russell, American astronomers devoted themselves mainly to surveying the stars and making impressive catalogs of their properties, especially their spectra as described in Analyzing Starlight.

Russell began to see that interpreting the spectra of stars required a much more sophisticated understanding of the physics of the atom, a subject that was being developed by European physicists in the s and s.

Russell embarked on a lifelong quest to ascertain the physical conditions inside stars from the clues in their spectra; his work inspired, and was continued by, a generation of astronomers, many trained by Russell and his collaborators. Russell also made important contributions in the study of binary stars and the measurement of star masses, the origin of the solar system, the atmospheres of planets, and the measurement of distances in astronomy, among other fields.

He was an influential teacher and popularizer of astronomy, writing a column on astronomical topics for Scientific American magazine for more than 40 years. He and two colleagues wrote a textbook for college astronomy classes that helped train astronomers and astronomy enthusiasts over several decades.

That book set the scene for the kind of textbook you are now reading, which not only lays out the facts of astronomy but also explains how they fit together. Russell gave lectures around the country, often emphasizing the importance of understanding modern physics in order to grasp what was happening in astronomy.

Today, one of the highest recognitions that an astronomer can receive is an award from the American Astronomical Society called the Russell Prize, set up in his memory. Figure 3. Along the horizontal axis, we can plot either temperature or spectral type also sometimes called spectral class. Several of the brightest stars are identified by name.

Most stars fall on the main sequence. Following Hertzsprung and Russell, let us plot the temperature or spectral class of a selected group of nearby stars against their luminosity and see what we find Figure 3.

Such a plot is frequently called the Hertzsprung—Russell diagram , abbreviated H—R diagram. It is one of the most important and widely used diagrams in astronomy, with applications that extend far beyond the purposes for which it was originally developed more than a century ago. It is customary to plot H—R diagrams in such a way that temperature increases toward the left and luminosity toward the top.

Notice the similarity to our plot of height and weight for people Figure 1. Stars, like people, are not distributed over the diagram at random, as they would be if they exhibited all combinations of luminosity and temperature. Instead, we see that the stars cluster into certain parts of the H—R diagram.

The great majority are aligned along a narrow sequence running from the upper left hot, highly luminous to the lower right cool, less luminous. This band of points is called the main sequence. It represents a relationship between temperature and luminosity that is followed by most stars.

We can summarize this relationship by saying that hotter stars are more luminous than cooler ones. A number of stars, however, lie above the main sequence on the H—R diagram, in the upper-right region, where stars have low temperature and high luminosity. How can a star be at once cool, meaning each square meter on the star does not put out all that much energy, and yet very luminous?

The only way is for the star to be enormous—to have so many square meters on its surface that the total energy output is still large. These stars must be giants or supergiants , the stars of huge diameter we discussed earlier. Figure 4. Schematic H—R Diagram for Many Stars: Ninety percent of all stars on such a diagram fall along a narrow band called the main sequence.

A minority of stars are found in the upper right; they are both cool and hence red and bright, and must be giants. Some stars fall in the lower left of the diagram; they are both hot and dim, and must be white dwarfs.

There are also some stars in the lower-left corner of the diagram, which have high temperature and low luminosity. If they have high surface temperatures, each square meter on that star puts out a lot of energy. How then can the overall star be dim? It must be that it has a very small total surface area; such stars are known as white dwarfs white because, at these high temperatures, the colors of the electromagnetic radiation that they emit blend together to make them look bluish-white.

We will say more about these puzzling objects in a moment. Figure 4 is a schematic H—R diagram for a large sample of stars, drawn to make the different types more apparent. A red dwarf , which is half as massive as the sun, can last 80 to billion years, which is far longer than the universe's age of This long lifetime is one reason red dwarfs are considered to be good sources for planets hosting life , because they are stable for such a long time.

More than 2, years ago, the Greek astronomer Hipparchus was the first to make a catalog of stars according to their brightness , according to Dave Rothstein, who participated in Cornell University's "Ask An Astronomer" website in In the early 20th century, astronomers realized that the mass of a star is related to its luminosity , or how much light it produces.

These are both related to the stellar temperature. Stars 10 times as massive as the sun shine more than a thousand times as much. The mass and luminosity of a star also relate to its color. More massive stars are hotter and bluer, while less massive stars are cooler and have a reddish appearance. The sun falls in between the spectrum, given it a more yellowish appearance. This understanding lead to the creation of a plot known as the Hertzsprung-Russell H-R diagram, a graph of stars based on their brightness and color which in turn shows their temperature.

Most stars lie on a line known as the "main sequence," which runs from the top left where hot stars are brighter to the bottom right where cool stars tend to be dimmer. Eventually, a main sequence star burns through the hydrogen in its core, reaching the end of its life cycle.