Which spectral class is most common

Types of Stars. Stars of Orion's Belt. Proxima Centauri. Supergiants are consuming hydrogen fuel at an enormous rate and will consume all the fuel in their cores within just a few million years. Supergiant stars live fast and die young, detonating as supernovae; completely disintegrating themselves in the process.

Stars with luminosity classifications of III and II bright giant and giant are referred to as blue giant stars. The term applies to a variety of stars in different phases of development. They are evolved stars that have moved from the main sequence but have little else in common. Therefore blue giant simply refers to stars in a particular region of the HR diagram rather than a specific type of star. Blue giants are much rarer than red giants, because they only develop from more massive and less common stars, and because they have short lives.

Some stars are mislabelled as blue giants because they are big and hot. Blue supergiant stars are scientifically known as OB supergiants, and generally have luminosity classifications of I, and spectral classifications of B9 or earlier.

Blue supergiant stars are typically larger than the Sun, but smaller than red supergiant stars, and fall into a mass range of between 10 and solar masses. Typically, type-O and early type-B main sequence stars leave the main sequence in only a few million years, since they burn through their supply of hydrogen very quickly due to their high masses. When a star has consumed its stock of hydrogen in its core, fusion stops and the star no longer generates an outward pressure to counteract the inward pressure pulling it together.

A shell of hydrogen around the core ignites continuing the life of the star but causes it to increase in size dramatically. In these stars, hydrogen is still being fused into helium, but in a shell around an inert helium core. The aging star has become a red giant star and can be times larger than it was in its main sequence phase.

When this hydrogen fuel is used up, further shells of helium and even heavier elements can be consumed in fusion reactions. Red supergiant stars are stars that have exhausted their supply of hydrogen at their cores, and as a result, their outer layers expand hugely as they evolve off the main sequence. Stars of this type are among the biggest stars known in terms of sheer bulk, although they are generally not among the most massive or luminous. Antares , in the constellation Scorpius , is an example of a red supergiant star at the end of its life.

An artists rendering of Antares, a red supergiant star Inverse. When a star has completely run out of hydrogen fuel in its core and it lacks the mass to force higher elements into fusion reaction, it becomes a white dwarf star. The outward light pressure from the fusion reaction stops and the star collapses inward under its own gravity.

A white dwarf will just cool down until it becomes the background temperature of the Universe. This process will take hundreds of billions of years, so no white dwarfs have actually cooled down that far yet. Neutron stars are the collapsed cores of massive stars between 10 and 29 solar masses that were compressed past the white dwarf stage during a supernova explosion. A simulated view of a neutron star Wikipedia.

The remaining core becomes a neutron star. A neutron star is an unusual type of star that is composed entirely of neutrons; particles that are marginally more massive than protons, but carry no electrical charge.

The intense gravity of the neutron star crushes protons and electrons together to form neutrons. If stars are even more massive, they will become black holes instead of neutron stars after the supernova goes off.

While smaller stars may become a neutron star or a white dwarf after their fuel begins to run out, larger stars with masses more than three times that of our sun may end their lives in a supernova explosion.

The dead remnant left behind with no outward pressure to oppose the force of gravity will then continue to collapse into a gravitational singularity and eventually become a black hole , with the gravity of such an object so strong that not even light can escape from it.

There are a variety of different black holes. Stellar-mass black holes are the result of a star around 10 times heavier than the Sun ending its life in a supernova explosion, while supermassive black holes found at the center of galaxies may be millions or even billions of times more massive than a typical stellar-mass black hole.

Brown Dwarfs are also known as failed stars. This is due to the result of their formation. Brown Dwarfs form just like stars. This may nicely explain several stellar types which seem analogous with K and M stars temperature-wise, but show some other spectral features as if their outer atmospheres had been enriched with heavier element.

These types are the R, N, and S types. R and N type stars A number of giant stars appear to be K or M type stars, but also show significant excess spectral features of carbon compounds. They are often referred to as "carbon stars" and many astronomers collectively refer to them as C type stars. The abundance of carbon to oxygen in these stars is four to five times higher than in normal stars.

The presence of these carbon compounds will tend to absorb the blue portion of the spectrum, giving R and N type giants a distinctive red colour. R stars are those with hotter surfaces which otherwise more closely resemble K type stars.

S type stars have cooler surfaces and more closely resemble M stars. S type stars S type stars have photospheres with enhanced abundances of s -process elements. These are isotopes of elements which have been formed from the capture of a free neutron changing the isotope of the element followed by a beta decay a neutron decays into a proton and an electron, thus changing the element to one with a higher atomic number and an isotope with one less neutron.

The s -process is one of the mechanisms by which elements with atomic numbers higher than 56 Iron can be made. The s stands for slow. By way of contrast, its partner r -process for rapid takes place when there are a sufficient supply of free neutrons for additional neutrons to be acquired in the atomic nucleus before the captured neutron has a chance to beta decay. Instead of or in addition to the usual lines of titanium, scandium, and vanadium oxides characteristic of M type giants, S type stars show heavier elements such as zirconium, yttrium, and barium.

A significant fraction of all S type stars are variable. Peculiar Stars Wolf-Rayet Stars WR Wolf-Rayet stars are similar to O type stars, but have broad emission lines of hydrogen and ionized helium, carbon, nitrogen, and oxygen with very few absorption lines.

Current theory holds that these stars exist in binary systems where the companion star has stripped away the Wolf-Rayet star's outer layers. Thus the spectra observed is from the exposed stellar interior rather than the normal surface material. The broadness of the lines also indicates that the material observed may be from high velocity gases streaming away from the star, with the range of velocities smearing out the observed lines. T Tauri Stars T T Tauri stars are very young stars, typically found in bright or dark interstellar clouds from which they have presumably just formed.

Typically T Tauri stars are irregular variable stars, with unpredictable changes in their brightness. Their spectra contains bright emission lines and a number of "forbidden lines" so-called because they are not observable in typical laboratory conditions which indicate extremely low densities. Why do different stars have different lines? This question is the key to helping us classify stars.

If we compare an O-class star with and M-class star they have very different lines. The key factor at work here is temperature.

By temperature we really mean the effective temperature of the star sometimes called the surface temperature. This is the temperature of a black body having the same size and luminosity as the star and is determined by Stefan's Law.

The variations in spectral lines for different stars are due primarily to the difference in temperature of the outer layers of gas in the star. In very hot stars, helium can be ionised so we can expect to see spectral lines due to absorption by helium ions. In most stars the temperature is too cool for helium to ionise so no such lines can form in the spectrum. Even though spectral lines due to helium are not found in cool stars it does not mean that helium is missing from the star.

In fact helium is the second most abundant element in the Universe and in stars. The absence of helium lines simply means that the conditions are not right for helium lines to form or be abundant in that star. Some stars are cool enough that molecules can exist in outer layers without being ripped apart. As the number of possible electron transitions is much greater in molecules than single atoms there are many possible spectral lines that can form hence cool stars typically have many lines.

The standard spectral class classification scheme is thus based on temperature. Most stars fit into one of the following types or spectral classes :.