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Stars: How big is big? 12 December 2011, By Steve Nerlich, Universe Today The Wolf-Rayet star WR 124 and its wind nebulae (actually denoted M1-67). The mass of WR 124 is estimated at a moderate 20 solar masses, although this is after it has already lost much of its initial mass to create the wind nebula around it. Credit: ESO. You may have seen one of these astronomical scale picture sequences, where you go from the Earth to Jupiter to the Sun, then the Sun to Sirius - and all the way up to
    Stars: How big is big? 12 December 2011, By Steve Nerlich, Universe Today  The Wolf-Rayet star WR 124 and its wind nebulae(actually denoted M1-67). The mass of WR 124 isestimated at a moderate 20 solar masses, although thisis after it has already lost much of its initial mass tocreate the wind nebula around it. Credit: ESO. You may have seen one of these astronomicalscale picture sequences, where you go from theEarth to Jupiter to the Sun, then the Sun to Sirius -and all the way up to the biggest star we know ofVY Canis Majoris. However, most of the stars atthe big end of the scale are at a late point in theirstellar lifecycle - having evolved off the mainsequence to become red supergiants. The Sun will go red giant in 5 billion years or so -achieving a new radius of about one AstronomicalUnit - equivalent to the average radius of the Earth's orbit (and hence debate continues aroundwhether or not the Earth will be consumed). In anycase, the Sun will then roughly match the size ofArcturus, which although voluminously big, onlyhas a mass of roughly 1.1 solar masses. So,comparing star sizes without considering thedifferent stages of their stellar evolution might notbe giving you the full picture.Another way of considering the 'bigness' of stars isto consider their mass, in which case the mostreliably confirmed extremely massive star is NGC3603-A1a - at 116 solar masses, compared with VYCanis Majoris' middling 30-40 solar masses.The most massive star of all may be R136a1, whichhas an estimated mass of over 265 solar masses -although the exact figure is the subject of ongoingdebate, since its mass can only be inferredindirectly. Even so, its mass is almost certainly overthe 'theoretical' stellar mass limit of 150 solarmasses. This theoretical limit is based onmathematically modelling the Eddington limit, thepoint at which a star's luminosity is so high that itsoutwards radiation pressure exceeds its self-gravity. In other words, beyond the Eddington limit,a star will cease to accumulate more mass and willbegin to blow off large amounts of its existing massas stellar wind.It's speculated that very big O type stars might shedup to 50% of their mass in the early stages of theirlifecycle. So for example, although R136a1 isspeculated to have a currently observed mass of265 solar masses, it may have had as much as 320solar masses when it first began its life as a mainsequence star.So, it may be more correct to consider that thetheoretical mass limit of 150 solar massesrepresents a point in a massive star's evolutionwhere a certain balancing of forces is achieved. Butthis is not to say that there couldn't be stars moremassive than 150 solar masses - it's just that theywill be always declining in mass towards 150 solarmasses.  1 / 3    A scale comparison chart showing lots of big stars - butnote that after Rigel (frame 5) they are all red giants.When the Sun goes red giant it will become about thesize of Arcturus (frame 4) - so maybe this kind of'snapshot in time' comparison is misleading? Credit:Wikimedia. Having unloaded a substantial proportion of theirinitial mass such massive stars might continue assub-Eddington blue giants if they still havehydrogen to burn, become red supergiants if theydon't - or become supernovae.Vink et al model the processes in the early stagesof very massive O type stars to demonstrate thatthere is a shift from optically thin stellar winds, tooptically thick stellar winds at which point thesemassive stars can be classified as Wolf-Rayetstars. The optical thickness results from blown offgas accumulating around the star as a windnebulae - a common feature of Wolf-Rayet stars.Lower mass stars evolve to red supergiant stagethrough different physical processes - and since theexpanded outer shell of a red giant does notimmediately achieve escape velocity, it is stillconsidered part of the star's photosphere. There's apoint beyond which you shouldn't expect bigger redsupergiants, since more massive progenitor starswill follow a different evolutionary path.Those more massive stars spend much of theirlifecycle blowing off mass via more energeticprocesses and the really big ones becomehypernovae or even pair-instability supernovaebefore they get anywhere near red supergiantphase.So, once again it appears that maybe size isn'teverything.  More information:  Further reading: Vink et al., Wind Models for Very Massive Stars in the LocalUniverse.Provided by Universe Today  2 / 3    APA citation: Stars: How big is big? (2011, December 12) retrieved 11 October 2014 from This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only. Pwred byTCD  3 / 3    What is the upper limit for massive stars? 7 October 2014, by Brian Koberlein, One Universe At A Time  Credit: ESO/M. Kornmesser Yesterday I mentioned that hypernovae (super-supernovae) are the result of the explosion of astar that's about as massive as a star can be(about 150-200 solar masses). But how exactly dowe know that this is an upper limit? The first clue comes from a derivation by ArthurEddington. In 1916, Eddington demonstrated thatthere was a limit to how bright a stable star couldbe. The basic idea is that the atmosphere of a staris being gravitationally attracted by the mass of thestar (giving it weight), and this weight is balancedby the pressure of the deeper layer of the star. Fora star to be stable, the weight and pressure mustbe equal, so the star doesn't collapse inward orpush the atmosphere outward.We typically think of pressure as being due to gasand such, but light can also exert pressure on amaterial. We don't notice light pressure in our dailylives because it is so small. Even in our Sun, thepressure on the atmosphere is relatively small, sothe weight of our Sun's atmosphere is mostlybalanced by the pressure of the plasma in thelayer underneath it. But if the Sun were brighter,the light it emits would push harder against theparticles of the atmosphere. What Eddingtonshowed is that there is a limit where the pressureof a star's light on the atmosphere is large enoughto balance the gravitational weight of the stellaratmosphere entirely, known as the Eddingtonluminosity limit. If the star were any brighter, thelight of the star would push away the outer layers ofthe atmosphere, thus causing the star to lose mass.When Eddington first derived this limit, he foundthat the maximum luminosity (brightness) of a starwas proportional to the mass of a star. This meantthat more massive stars could be brighter than lessmassive stars, but it didn't say anything about anupper limit on mass. Then in 1924, Eddingtondiscovered a relationship between the mass of astar and its luminosity, specifically that thebrightness of a star is roughly proportional to themass cubed.This meant the brightness of a star increased withmass faster than the luminosity limit, so there mustbe an upper limit on a star's mass. Stars with largermasses would be so bright that they would burnaway their outer layers. With Eddington'scalculation, this limit is around 65 solar masses.Later, more detailed calculations put this limit ataround 150 solar masses, which is generallyconsidered an upper limit for stable stars.In 2007, a research team made a study of theAches cluster, which is the densest known starcluster in our galaxy. Looking at the brightest starsin this cluster, they found no stars greater thanabout 120 solar masses. Using their observationsto make a statistical extrapolation, they found thatthe upper limit for stars is likely 150 solar masses.But recently new evidence has questioned thatlimit. Theoretical work has shown that it is possibleto have stable stars with a brightness greater thanthe Eddington luminosity limit. Effects such asturbulence within the atmosphere and photonbubbles, where light could pass through the stellaratmosphere more easily would allow super-luminous stars to remain stable. Then there arecalculations from hypernova explosions thatestimate the progenitor (the star that exploded) hada mass of about 200 solar masses. Finally, there isa star known as R136a1. Discovered in 2010 which  1 / 2


Jul 23, 2017
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