Biggest, brightest star ever

[ToSeek]

Star may be biggest, brightest yet discovered (http://www.spaceref.com/news/viewpr.html?pid=13334)

I'll have to change one of my answers to kilopi's quiz now....

[MAPNUT]

Questions for the knowledgeable: Where is the Pistol Star? Is it visible?

If there was a star as big as the new one in the Andromeda Galaxy, or another nearby galaxy, would astronomers be able to see it and measure it?

I would guess there isn't any star this big in the Magellan Clouds.

[eburacum45]

The Pistol star is 25,000 ly away in Sagittarius, near the centre of our galaxy; it is not visible to the naked eye or binoculars.
this new star is also in Sagittarius, but twice as far away- so it is practically directly opposite the Sun in the Galaxy.

It would probably be visible in large telescopes if it was in Andromeda or the Magellanic Clouds-
there is in fact a vey large LBV star visible in the greater Magellanic cloud; it used to be called S Doradus, but now that it is believed to be a multiple star I think it is called something else...

can't remember what though...

[tracer]

Star may be biggest, brightest yet discovered (http://www.spaceref.com/news/viewpr.html?pid=13334)
Hmmm ... how come the article refers to the star as LBV 1806-20, but the picture labels the star as LBV 1806-2D?

[George]

What does 150x's larger mean? Is that volume, cross section or diameter.

If it is diameter, then the drawing is almost 1/2 too small for "Big Blue Bubba".

If it is cross-section, the drawing for BBB is about 8x's too big (in dia.). 18x's too big if it is in volume.

However, I do like the drawing.

I also like another apparent fact.....we, more than likely, know it's color! :)

[Tim Thompson]

It's "120x larger", and it refers to mass. The likely upper limit on a stellar mass today is about 120 solar masses; higher than that, and a star never forms, because the radiation pressure is too high it blows itself apart before forming a stable star. It's interesting that our astronomical vision now reaches so far, and so clearly, that a star like LBV 1806-20 can go from obscurity to "brightest" in one jump.

The Pistol star, and a few other candidates for "brightest star", are described on my webpage: What is the Brightest Star in the Milky Way? (http://www.tim-thompson.com/bright-stars.html)

[Spaceman Spiff]

There seems to be some "bad astronomy" going on at that web-site (http://www.spaceref.com/news/viewpr.html?pid=13334). Mass and size are not the same thing, but they are often confused by the media (and my students). Here is an example, from the article.

"One of the mysteries about LBV 1806-20 is how it got so big. Current theories of star formation suggest they should be limited to about 120 solar masses, or 120 times as large as the sun, because the heat and pressure from such big stars' cores force matter away from their surfaces. "

I spend a lot of time trying to get my students to say "mass" when they mean mass and that "size" refers to the object's radius or diameter. The star's radius may well be 150x that of our Sun's radius, but that does not make it 150x as massive (and it may well be 150x more massive than our Sun but that does not make it 150x the diameter of our Sun). The article does not distinguish between the two (or at least I could not tell). Regardless, this star's rival, the Pistol star, is known to have a mass of at least 120x that of our Sun's mass.

This HST press release (http://hubblesite.org/newscenter/newsdesk/archive/releases/1997/33/text/) on the Pistol star (previous record holder) does distinguish between mass and size (the Pistol star is both really big and really massive, and LBV 1806-20 is likely both as well).

[Spaceman Spiff]

Questions for the knowledgeable: Where is the Pistol Star? Is it visible?

If there was a star as big as the new one in the Andromeda Galaxy, or another nearby galaxy, would astronomers be able to see it and measure it?

I would guess there isn't any star this big in the Magellan Clouds.

This HST/ACS image (http://hubblesite.org/newscenter/newsdesk/archive/releases/2003/15/) of a field in M31's halo can see and measure stars a couple of magnitudes less luminous than our Sun (at 2.5 million light year's distance). A star 6 million times more luminous than our Sun can be observed over enormous distances -- not just in our galactic backyard.

[George]

I'm still unclear what was intended in this quote from the original post...


Luminous blue variable stars are extremely large, with LBV 1806-20 probably at least 150 times larger than the sun, Eikenberry said.

Is it diameter?

[skyglow1]

I'm still unclear what was intended in this quote from the original post...


Luminous blue variable stars are extremely large, with LBV 1806-20 probably at least 150 times larger than the sun, Eikenberry said.

Is it diameter?

It's mass, as in the "amount of stuff" made out of something.

skyglow1

[sol_g2v]

Questions for the knowledgeable: Where is the Pistol Star? Is it visible?

If there was a star as big as the new one in the Andromeda Galaxy, or another nearby galaxy, would astronomers be able to see it and measure it?

I would guess there isn't any star this big in the Magellan Clouds.

I've heard the Tarantula Nebula stellar nursery in the LMC is just barely visible to the naked eye (Never been to the Southern Hemi). At the center of the Tarantula Nebula is the super star cluster R136, which crams 120,000 stars in a region only 16 light-years across. Lots of them are massive stars with incredible masses and luminosities. At least 39 are O3 stars, that's more than the total # of O3 stars previously known in the entire Local Group.

Must be a quite a wild environment.

here (http://hubblesite.org/newscenter/newsdesk/archive/releases/2001/21/image/a) is a Hubble image page I found about the place.

[George]

I'm still unclear what was intended in this quote from the original post...


Luminous blue variable stars are extremely large, with LBV 1806-20 probably at least 150 times larger than the sun, Eikenberry said.

Is it diameter?

It's mass, as in the "amount of stuff" made out of something.

skyglow1

Thanks.

Can a good guess at it's radius be determined? I want to get a feel of how big this baby is not how much it weighs. :)

[Spaceman Spiff]

I'm still unclear what was intended in this quote from the original post...


Luminous blue variable stars are extremely large, with LBV 1806-20 probably at least 150 times larger than the sun, Eikenberry said.

Is it diameter?

It's mass, as in the "amount of stuff" made out of something.

skyglow1

Thanks.

Can a good guess at it's radius be determined? I want to get a feel of how big this baby is not how much it weighs. :)

If they know the total luminosity (have to know the distance, and corrections by foreground extinction of light by dust), and we measure the surface temperature from its spectrum, then we can use the following law of blackbody radiators:

L = surface area x sigma x T^4

Surface area of a sphere is 4 x pi * Radius^2, sigma is a physical constant (stefan-boltzmann constant), L is the star's total luminosity, and T is the surface (effective blackbody) temperature. So we can simply solve for the star's effective radius.

[AstroSmurf]

Except that a star is not a blackbody. But you probably knew that :P

The derivation of a star's radius and surface temperature is rather complex, and involves solving for the entire intensity spectrum. Whoever said reality had to be simple... And this is the easy part - finding the mass of the star, now there's a problem to keep astronomers busy 8)

[tracer]

Mass and size are not the same thing, but they are often confused by the media (and my students).
Not the least of which confusions happened in the movie Independence Day, when one of the minor characters is describing the alien mothership to U.S. Space Command:

"In terms of mass, it's 1/4 the size of the Earth's moon."

(It was about 500 km in diameter. If it really was 1/4 the mass of the Earth's moon, that would mean that, on average, it was 20 times as dense as solid lead!)

[Tim Thompson]

Re-reading the University of Florida Press Release (http://www.napa.ufl.edu/2004news/bigbrightstar.htm), it's evident that they are talking about mass. Eikenberry thinks the star's mass is about 150 solar masses (despite the fact that all of the press sources call this "150 times bigger than the sun", as Spaceman Spiff has already pointed out). But since current theory pegs the maximum mass of a stable star at about 120 solar masses, the problem is explaining how LBV 1806-20 made it to 150. Eikenberry suggests some kind of compression mechanism related to a supernova, the remnants of which surround LBV 1806-20. But I think the real issue is that both of these numbers (theory's 120, Eikenberry's 150) are somewhat "plastic" (neither is cited with an uncertainty). There won't be anything real to worry over until we know that neither of them is ±50.


The CNN report (http://www.cnn.com/2004/TECH/space/01/06/stellar.giant.ap/index.html) says that, according to Eikenberry, the star's diameter is 200 solar diameters. one Astronomical Unit, the average distance from the sun to Earth is 215 solar radii, so this star, at 200 solar radii, would be a bit under a couple of AU across. That's big, but typical of supermassive stars. The largest known star, according to James Kaler (http://www.astro.uiuc.edu/~kaler/), is VV cephei (http://www.hposoft.com/Astro/PEP/VVCephei.html), 1670 solar radii according to the webpage, but 1720 accrding to Kaler (that's 8.0 AU). Some folks say that epsilon Aurigae (http://www.hposoft.com/Astro/PEP/EpsilonAurigae.html) is bigger, but Kaler only gives it a radius of 2 AU, and most sources don't quote a radius at all. Betelgeuse (http://www.astro.uiuc.edu/~kaler/sow/betelgeuse.html) comes in at 2.9 AU, but is close enough for its disk to be imaged by the HST (http://hubblesite.org/newscenter/newsdesk/archive/releases/1996/04/).

Try Kaler's books (http://www.astro.uiuc.edu/~kaler/books.html), especially The Hundred Greatest Stars or Extreme Stars for some well written material on the largest & brightest stars.

[George]

Thanks all.

This means the artist drew the star about 1/2 of it's described size. Wow.

Are there larger stars which are also blue giant?

[Spaceman Spiff]

Except that a star is not a blackbody. But you probably knew that :P

The derivation of a star's radius and surface temperature is rather complex, and involves solving for the entire intensity spectrum. Whoever said reality had to be simple... And this is the easy part - finding the mass of the star, now there's a problem to keep astronomers busy 8)

I know what you're saying, but let me just nitpick a bit...
A star is a blackbody. It's just that near the surface that spectrum is modified by a variety of processes -- the primary origin of which is that the photons are decoupling from the thermal bath of the matter (i.e., a star emit photons into the vacuum of space). Astronomers can still define an effective temperature, which measures the flux from an equivalent perfect BB equal to the total flux observed from the star. And indeed, this is the temperature that goes into that expression I mentioned.

As for measuring the total (or bolometric) flux...this isn't as much of a problem as it used to be. Today we have detectors sensitive to nearly all wavelengths and orbiters to measure those wavelengths that do not penetrate Earth's atmosphere. The surface temperature is generally not so difficult, at least to get pretty darned near. The shape of the underlying blackbody spectrum coupled with the spectral absorption lines (and other features) is generally sufficient to get one close without working all that hard. Cool supergiant stars, on the otherhand, are nasty beasts due to their very low density atmospheres, their ability to form lots of grains, their huge convective zones (and accompanying starspots), their pulsational instabilities, etc.

[Zero Signal]

Here's another article: http://www.space.com/scienceastronomy/brightest_star_040106-1.html

According to this article, LBV 1806-20 is about 150 solar masses, not 150 solar radii. The article Tim posted also said 150 solar masses.

So does anyone think it is a single star, or do you suppose it might be a binary system like Eta Carinae?