Which main sequence stars are the least massive

As this process continues, the core temperatures of newly forming stars steadily increase until they glow and eventually shine as hydrogen undergoes nuclear fusion forming yet more helium and releasing vast amounts of energy. At this point, the point of hydrogen burning , stars are said to join the Main Sequence. The life cycle of Main Sequence stars is determined by their mass:. High mass stars stars with masses greater than three times the mass of the Sun are the largest, hottest and brightest Main Sequence stars and blue, blue-white or white in colour.

High mass stars use up their hydrogen fuel very rapidly and consequently have short lives. High mass stars pass through a Red Supergiant stage before dying catastrophically in supernovae explosions.

More massive stars have a stronger gravitational force acting inwards so their core gets hotter. The higher temperatures mean that the nuclear reactions occur at a much greater rate in massive stars. They thus use up their fuel much quicker than lower mass stars. This is analogous to the situation with many chemical reactions, the higher the temperature the faster the reaction rate.

Lifespans for main sequence stars have a vast range. Whilst our Sun will spend 10 billion years on the main sequence, a high-mass, ten solar-mass 10 M Sun star will only last 20 million years 2. A star with a only half the mass of Sun can spend 80 billion years on the main sequence. This is much longer than the age of the Universe which means that all the low-mass stars that have formed are still on the main sequence - they have not had time to evolve off it.

Although there are 92 naturally occurring elements and a few hundred isotopes, the composition of stars is remarkably similar and simple. Stars are composed almost entirely of hydrogen and helium. Historically astronomers termed these elements with atomic numbers greater than two as metals. These include elements such as carbon and oxygen.

The use of "metals" is not to be confused with the more common chemical meaning of the term. Metallicity is a measure of the abundance of elements heavier than helium in a star and is expressed as the fraction of metals by mass. It can be determined or at least inferred from spectroscopic and photometric observations. In general stars with higher metallicities are inferred to be younger than those with very low values.

This is due to the fact that elements heavier than helium are made inside stars by nucleosynthesis and released into interstellar space by mass-loss events such as supernova explosions in the late stages of stellar evolution.

Early generations of stars. Stars found in the spiral arms of galaxies, including our Sun, are generally younger and have high metallicities. They are referred to as Population I stars. Population II stars are older, red stars with lower metallicities and are typically located in globular clusters in galactic halos, in elliptical galaxies and near the galactic centre of spiral galaxies.

Nucleosynthesis simply refers to the production of nuclei heavier than hydrogen. This occurs in main sequence stars through two main processes, the proton-proton chain and the CNO cycle carbon, nitrogen, oxygen. Primordial nucleosynthesis occurred very early in the history of the Universe, resulting in some helium and small traces of lithium and deuterium, the heavy isotope of hydrogen. Fusion processes in post-main sequence stars are responsible for many of the heavier nuclei.

Other mechanisms such as neutron capture also occur in the last stages of massive stars. Both discussed in later pages. Main sequence stars fuse hydrogen into helium within their cores. This is sometimes called "hydrogen burning" but you need to be careful with this term. The nuclear fusion in the cores of main sequence stars involves positive hydrogen nuclei, ionised hydrogen atoms or protons, to slam together, releasing energy in the process.

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. Skip to content. Like this: Like Loading 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. At this point, it leaves the main sequence. Stars smaller than a quarter the mass of the sun collapse directly into white dwarfs. White dwarfs no longer burn fusion at their center, but they still radiate heat. Eventually, white dwarfs should cool into black dwarfs , but black dwarfs are only theoretical; the universe is not old enough for the first white dwarfs to sufficiently cool and make the transition.

Larger stars find their outer layers collapsing inward until temperatures are hot enough to fuse helium into carbon. Then the pressure of fusion provides an outward thrust that expands the star several times larger than its original size, forming a red giant. The new star is far dimmer than it was as a main sequence star. Eventually, the sun will form a red giant, but don't worry — it won't happen for a while yet. If the original star had up to 10 times the mass of the sun, it burns through its material within million years and collapses into a super-dense white dwarf.

More massive stars explode in a violent supernova death , spewing the heavier elements formed in their core across the galaxy.