The Most Massive Star Yet?

[glenc]

These 2 stars can be seen in Carina near NGC 3532.
"Looking deep into NGC 3603, one of the youngest and most massive star clusters in the Milky Way, the scientists studied five extremely luminous stars. One is an only child, and the others are in two binary pairs. For one of the binaries, the team used shifts in the two stars’ spectral lines caused by the stars’ speeding around each other to calculate a rough mass for each companion. They came up with 85 to 145 solar masses for one, 75 to 105 solar masses for the other..."
http://www.skyandtelescope.com/news/20606684.html
http://arxiv.org/abs/0806.2815

[ngcles]

Hi Glen,

Interesting stuff -- I missed that one on the website and also missed the papers. Thanks for that.

Yep, they are certainly right up there among the candidates for the most massive/most luminous stars of the Milky Way. NGC 3603 is a very, very massive and bright cluster -- one of the brightest and most massive open clusters known in the Milky Way. Seemingly inconspicuous Westerlund 1 (on the border between Ara & Scorpius) is also of a similar nature and maybe an even more massive OC. There is a paper on Westerlund 1 here: http://arxiv.org/PS_cache/astro-ph/pdf/0504/0504342v2.pdf



Several other contenders for Milky Way's brightest star include:

Eta Carinae (of course)

HD 93129AB which is very close to Eta, at the heart of Trumpler 14 within the Eta Carinae complex. This is a binary where the combined mass is estimated at 200 solar-masses and the larger component is likely >120 solar-masses.

The Pistol Star

LBV 1806-20

Cygnus OB 2-12

At the moment, the last mentioned is apparently ahead of the others by a whisker, though it is a really difficult thing to pin-down the exact brightness, magnitudes and masses of these "megastars" given the uncertainties in distances and extinction due to dust and other "variables" in the modelling of "top-shelf" really massive stars and their evolution.

All these stars including the ones you mentioned are in the same cricket-ground weighing in with somewhere between about 100 and 140 solar-masses and bolometric luminosities (ie across all wavelengths) approaching 6,000,000 solar. One study suggests LBV 1806-20 may be as bright as 40,000,000 solar, though that is in at least some doubt -- other papers suggest it is binary or multiple and not a single source.

Interestingly, HD 93129A which is currently classified as O3 Iab has also in some literature been classified as O2 Ia star -- the only one known (if that is a correct classification -- it is disputed). It is one of my favourite stars in the sky. Secretly, I hope it wins the contest! It has the extraordinary surface temperature of 51,000 deg K!

I hope Cygnus OB 2-12 doesn't win -- it's a (the only) northern hemisphere (Booo ... ) contender.

There is some further info on all these exotic beasties (and a couple of others) here:

http://www.tim-thompson.com/bright-stars.html

Best,

Les D

[erick]

I can think of three measures:-

Biggest - as in greatest number of solar radii
Most massive - as in greatest number of solar masses
Most luminous - as in highest absolute magnitude

These three are fairly closely related? Sorry I forget what I've learnt of stellar evolution.

Are there different candidates for each from amongst relatively stable stars?

[ngcles]

Hi Eric & All,

The above stuff Glen raised and I added further comment to relates to the most massive/most luminous stars, not necessarily biggest in diameter/volume. Those ultra-bloated ones are nearly all red giant or red supergiant stars probably of somewhat less mass.

Quick and dirty, this page on Wikipedia lists a number of stars which appear to have very or exceptionally large diameters/volumes:

http://en.wikipedia.org/wiki/List_of_largest_known_stars

Can't vouch for the origin/accuracy of the material there, but it seems okay.

You can probably uncover other lists elsewhere with a good search engine interrogation.

To put that list on Wiki into a scale we can appreciate more easily, 1 A.U (ie the Earth-Sun distance) _approximately_ equals 108 solar-diameters. Therefore a star like Mu Cephei (in the list at Wikipedia and near the top) is about 13 AU diameter -- bigger than the orbit of Saturn and well on the way to Uranus. If the estimate is accurate as to it's diameter, VY Canis Majoris (top of the list) would pretty much reach Uranus -- though there is considerable doubt as to whether the estimate of it's diameter is accurate and whether the star is actually in a state of hydrostatic equilibrium (ie where outward pressure by radiation is equalled by gravity pushing inward).

But, ultra big diameters at the "massive end of town" (ie +25 solar-masses) does not necessarily correlate to brightest/most massive. I'm no astrophycist but as I understand it, the most massive "top-shelf" stars (ie +50 solar masses up to maybe 120 solar-masses) probably don't go through the red supergiant stage before they meet their end, because they can't sustain hydrostatic equilibrium as a red supergiant. Many of these most extreme stars therefore evolve very rapidly off the main-sequence from their starting points as early O-type main sequence stars into O and B type supergiants. From there, they become one of the several finely distinguished types of Wolf-Rayet stars that have blown off their outer layers with fierce winds as they loose mass, before finally exploding as Type 1b or 1c supernovae and/or short-duration gamma ray bursts.

If _you_ (the reader -- any reader please) know more about this please, please comment and clarify so we all know better. But, I think a lot of those red supergiants in the list are very massive stars in the 20-40 solar-mass range (and truly very, very rare stars) and will end their lives as Type II supernovae and neutron stars, but are no match for the Eta Carinae types of megastars (exceptionally rare) that follow these dramatic, very very unstable life paths, by-passing the middle age spread completely.

Live fast, die young, leave a good lookin' corpse -- to quote someone, though I can't remember who. James Dean? Fonzie??

Best,

Les D

[renormalised]

You've pretty much hit the nail on the head there, Les, except that the most massive of stars don't go through the WR stage either. There seems to be a cutoff at around 70 solar masses where stars can't expel enough mass via stellar wind to become WR stars. The progenitors of WR stars weigh in between 40-70 solar masses and usually form from stars around 50-60 solar masses...at least it appears these stars are the most likely to become WR stars. Smaller stars around the 40 solar mass mark can either go WR or become red supergiants, depending on the initial conditions within the stars as they leave the main sequence...it's all dependent on mass loss rates during the giant/supergiant phases of the star's evolution. The largest of stars (LBV's...hypergiants)...the eta Carina's, Pistol Star's and the ilk....tend to go off as hypernovae and create short period GRB's.

As you've said, red supergiants aren't always necessarily the most massive or brightest of stars. They usually form from stars between 10-40 solar masses as these stars can maintain hydrostatic equilibrium as red supergiants.

[ngcles]

Hi Renormalised,

Thanks very much -- great stuff!

No matter what their shape or size, all stars are interesting in some way.


Best,

Les D

[gaa_ian]

This is an a great thread guys !
Everyone wants to know "What is the biggest, brightest etc ..."
So, I now have another topic for my science show today !
Which star is the biggest and brightest in our galaxy ?
This will certainly be a worthy Podcast for inclusion on my site:
http://www.askthescienceguru.com

[Ian Robinson]

You might be interested in http://rainman.astro.uiuc.edu/ddr/stellar/archive/highwidetrackson.mpg for high mass stars to 80 Sols.

This simulation shows the evolution of 7 high mass stars ranging from 20 to 80 solar masses.

You can do your own sim : http://rainman.astro.uiuc.edu/ddr/stellar/intermediate.html

[ngcles]

Hi Ian & All,

Gotta say that the DIY simulation is one of the coolest things I've seen for a long, long time.

I've already played with it for about an hour changing parameters and I'm still fascinated!

Best,

Les D