User:Omarseid2011/List of Most Massive Stars

This is a list of the most massive stars so far discovered, in solar masses.

Uncertainties and caveats
Most of the masses listed below are contested and, being the subject of current research, remain under review and subject to revision. Indeed, many of the masses listed in the table below are inferred from theory, using difficult measurements of the stars’ temperatures and absolute brightnesses. All the masses listed below are uncertain: both the theory and the measurements are pushing the limits of current knowledge and technology. Either measurement or theory, or both, could be incorrect. For example, VV Cephei could be between, or , depending on which property of the star is examined.

Massive stars are rare; astronomers must look very far from the Earth to find one. All the listed stars are many thousands of light years away and that alone makes measurements difficult.

In addition to being far away, many stars of such extreme mass are surrounded by clouds of outflowing gas created by powerful stellar winds; the surrounding gas interferes with the already difficult-to-obtain measurements of stellar temperatures and brightnesses and greatly complicates the issue of estimating internal chemical compositions.

Both the obscuring clouds and the great distances make it difficult to judge whether the star is just a single supermassive object or, instead, a multiple star system. A number of the "stars" listed below may actually be two or more companions orbiting too closely to distinguish, each star being massive in itself but not necessarily “supermassive”. Other combinations are possible – for example a supermassive star with one or more smaller companions or more than one giant star – but without being able to see inside the surrounding cloud, it is difficult to know the truth of the matter. More globally, statistics on stellar populations seem to indicate that the upper mass limit is in the 100–200 solar mass range.

Rare reliable estimates
Eclipsing binary stars are the only stars whose masses are estimated with some confidence. However note that almost all of the masses listed in the table below were inferred by indirect methods; only a few of the masses in the table were determined using eclipsing systems.

Amongst the most reliable listed masses are those for the eclipsing binaries NGC 3603-A1, WR 21a, and WR 20a. Masses for all three were obtained from orbital measurements. This involves measuring their radial velocities and also their light curves. The radial velocities only yield minimum values for the masses, depending on inclination, but light curves of eclipsing binaries provide the missing information: inclination of the orbit to our line of sight.

Relevance of stellar evolution
Some stars may once have been heavier than they are today. It is likely that many have suffered significant mass loss, perhaps as much as several tens of solar masses, expelled by the process of superwind, where high velocity winds are driven by the hot photosphere into interstellar space. This process is similar to superwinds generated by asymptotic giant branch (AGB) stars in form red giants or planetary nebulae. The process forms an enlarged extended envelope around the star that interacts with the nearby interstellar medium and infusing the region with elements heavier than Hydrogen or Helium.

There are also – or rather were – stars that might have appeared on the list but no longer exist as stars, or are supernova impostors; today we see only the debris. The masses of the precursor stars that fueled these cataclysms can be estimated from the type of explosion and the energy released, but those masses are not listed here (see § Black holes below).

Mass limits
There are two related theoretical limits on how massive a star can possibly be: the accretion limit and the Eddington mass limit. The accretion limit is related to star formation: After about 120 have accreted in a protostar, the combined mass should have become hot enough for its heat to drive away any further incoming matter. In effect, the protostar reaches a point where it evaporates away material as fast as it collects new material. The Eddington limit is based on light pressure from the core of an already-formed star: As mass increases past ~150, the intensity of light radiated from a Population I star's core will become sufficient for the light-pressure pushing outward to exceed the gravitational force pulling inward, and the surface material of the star will be free to float away into space.

Accretion limits
Astronomers have long hypothesized that as a protostar grows to a size beyond 120, something drastic must happen. Although the limit can be stretched for very early Population III stars, and although the exact value is uncertain, if any stars still exist above 150–200 they would challenge current theories of stellar evolution.

Studying the Arches Cluster, which is currently the densest known cluster of stars in our galaxy, astronomers have confirmed that stars in that cluster do not occur any larger than about 150.

Rare ultramassive stars that exceed this limit – for example in the R136 star cluster – might be explained by the following proposal: Some of the pairs of massive stars in close orbit in young, unstable multiple-star systems must occasionally collide and merge where certain unusual circumstances hold that make a collision possible.

Eddington mass limit
A limit on stellar mass arises because of light-pressure: For a sufficiently massive star the outward pressure of radiant energy generated by nuclear fusion in the star's core exceeds the inward pull of its own gravity. This effect is called the Eddington limit.

Stars of greater mass have a higher rate of core energy generation, and heavier stars' luminosities increase far out of proportion to the increase in their masses. The Eddington limit is the point beyond which a star ought to push itself apart, or at least shed enough mass to reduce its internal energy generation to a lower, maintainable rate. The actual limit-point mass depends on how opaque the gas in the star is, and metal-rich Population I stars have lower mass limits than metal-poor Population II stars, with the hypothetical metal-free Population III stars having the highest allowed mass, somewhere around 300.

In theory, a more massive star could not hold itself together because of the mass loss resulting from the outflow of stellar material. In practice the theoretical Eddington Limit must be modified for high luminosity stars and the empirical Humphreys–Davidson limit is used instead.

List of the most massive stars
The following two lists show a few of the known stars with an estimated mass of 25 or greater, including the stars of Arches Cluster, Cygnus OB2 cluster, Pismis 24 cluster, and R136 cluster.

The first list gives stars that are estimated to be 80 or larger. The majority of stars thought to be more than 100 are shown, but the list is incomplete.

The second list gives examples of stars 25–79, but is far from a complete list. Note that all O-type stars have masses greater than 15 and catalogs of such stars (GOSS, Reed) list hundreds of cases.

In each list, the method used to determine the mass is included to give an idea of uncertainty: Binary stars being more securely determined than indirect methods such as conversion from luminosity, extrapolation from stellar atmosphere models, ... . The masses listed below are the stars’ current (evolved) mass, not their initial (formation) mass.





A few examples of mass less than 80.

Black holes
Black holes are the end point evolution of massive stars. Technically they are not stars, as they no longer generate heat and light via nuclear fusion in their cores.
 * Stellar black holes are objects with approximately 4–15.
 * Intermediate-mass black holes range from 100–10 000.
 * Supermassive black holes are in the range of millions or billions.