What's the largest star?

For many years this title in mainstream media has gone to UY Scuti; before that started in 2014 it was VY Canis Majoris. I'm going with the old view that VY CMa is the largest; here's my reasons
Additionally, the list on Wikipedia currently has Nubecula Major's WOH G64 at the top, but it seems that no one's reported on that yet.

(PLEASE PROVIDE RELIABLE SOURCES IN YOUR REPLIES)

Edit: Obviously this question refers to the largest known star, and um... I'm talking about radius/diameter/volume here, not mass

 

How are you defining the boundary of the gaseous envelope for these objects that don't have a solid surface? In addition, your list is of the largest known stars. There are many more stars (and that's an obvious understatement) that we haven't examined. This is an arbitrary classification problem and to some extent a product of the human mind. Perhaps some find it more interesting why others feel it's important to know this.

Anyway, the largest star is the sun. It looks way bigger than any of those tiny points of light.

 

Size does matter. Don't let anyone tell you different.

 

"What is the largest known star by mass?" might be a more interesting question. Observations of massive stars indicate that there is an upper limit to their luminosity, the Humphreys–Davidson limit. In practice, this is lower than the theoretical Eddington limit, which is the maximum luminosity a star can reach, assuming a balance between the outward force due to radiation and the inward gravitational force. Stellar outflows are observed for massive stars that exceed this limit during outbursts.

 

And if the Father Ted clip doesn't work for you:

[youtube] [/youtube]

Mark

 

The largest known by mass is R136a1. It's also the most luminous.

 

Yes, I can use Wikipedia as well. I just consider it a more interesting question. A star's properties and its life cycle are mostly determined by its mass and composition (including its metallicity). Considerations such as rotation rate, magnetic field and interaction with other stars also have some bearing.

 

umm... we're talking about radius/diameter/volume here, not mass

 

Radius is an arbitrary concept for a star, which has no solid surface. How are you defining it?

More interesting to me are theoretical explanations of the Humphreys–Davidson limit that causes rapid and unsteady mass loss if exceeded. For example:
https://www.tandfonline.com/doi/pdf/10.1080/21672857.2012.11519705
https://www.aanda.org/articles/aa/abs/2020/03/aa37374-19/aa37374-19.html (abstract only)

 

One can define an area A of radiative emission for a star of effective temperature T with luminosity L. Assuming the star to be a spherical black body with emissivity ε, one can calculate its effective radius R from the Stefan-Boltzmann law L = AσεT^4 = 4πR^2σεT^4 = 4πR^2σT^4, where σ is the Stefan-Boltzmann constant (5.67...×10^−8 Wm^−2K^−4). This is problematic for several reasons, including:

• It assumes that stars are spherical (like cows - a well-known physicists' joke)
• It assumes the emissivity ε = 1 (black body) and doesn't take into account obscuring material

These factors are unlikely to be equally biased for stars with different ages and metallicities. Add in the uncertainties in measurement and it's no wonder that it's difficult to state definitively which star has the largest radius. A very few large stars are close enough that their angular diameter can be measured directly, which depends on the opacity of their atmospheres and so varies with wavelength. Other ways of measuring a star's radius use occultation by the earth's moon or binary system eclipses - methods that are possible for only a few stars. Again, opacity considerations mean that the deduced radius depends on wavelength.

 

And you're not confining your quest to our galaxy alone, then?

 

In some respects the largest Star was whatever was the first seed of a galactic core black hole—burning very brightly on its way to the pit, as it were

 

We now have a new largest: Stephenson 2-18 with a diameter of three billion kilometers or 2,158 solar diameters.

 

My top 10 largest stars in terms of the Sun's diameter:
1. Stephenson 2-18 (2,150) - 2,993,660,000 km
2. VY Canis Majoris (2,000) - 2,784,800,000 km
3. HV 888 (1,869.5)* - 2,603,091,800 km
4. WOH G64 (1,788) - 2,489,611,200 km
5. NML Cygni (1,642) - 2,286,320,800 km
6. Westerlund 1-26 (1,546.5)* - 2,153,346,600 km
7. S Persei (1,379) - 1,920,119,600 km
8. Mu Cephei/Garnet Star (1,357.3)* - 1,889,950,929 km
9. VX Sagittarii (1,343)* - 1,869,993,200 km
10. VV Cephei A (1,283.3)* - 1,786,913,329 km

* mean of various values given by different sources

 

The size of these is staggering. I think Stephenson 2 would stretch to beyond the orbit of Saturn.

 

Lifespan of a 40 Solar Mass behemoth: 1 million years
Lifespan of the sun: 11 billion years
Projected lifespan of a red dwarf 8% the mass of the sun: 5 trillion years

Size might matter, but there's something to be said for endurance.

 

It would. Saturn's perihelion is 1.3479 billion km; Stephenson 2-18's radius is 1.497 billion km.

 

Terrifying and brilliant at the same time.

 

when it collapses Stephenson 2-18 could become a neutron star or black hole.
(edited to add: or even, because of its extreme properties something exotic like a quark matter star)

 

I can believe the quark matter part - we could a quark star form for the very first time!