Smallest star found so far.

[Scotius]

Astronomers keep finding all sorts of weird and unusual things in space This one is a star - just a bit bigger than Saturn is.

https://en.wikipedia.org/wiki/EBLM_J0555-57

[Bill Phil]

Nice. Stars can be tiny, but they can also be ginormous...

[YNM]

Quite interesting when talking objects of this size - you can't eyeball the mass. Jupiter is larger than the star, but much less massive; Saturn is about the same size, but even much less massive (magnitude-wise); Other red dwarfs are larger and heavier.

Edited July 20, 2017 by YNM

[Snark]

1 hour ago, YNM said:

Quite interesting when talking objects of this size - you can't eyeball the mass.

Yup.  I've heard it speculated that Jupiter may be about as large (i.e. in diameter) as it's possible for an object to be without having internal fusion processes to boost the temperature:  i.e. if you were to take a Jupiter-sized planet and dump more hydrogen into it, it would actually get smaller due to the increased gravitation.  It just gets denser.

Once fusion ignites, then larger sizes are possible because the higher temperatures raise the pressure.  In the case of EBLM J0555-57Ab, it's presumably relatively cool (as stars go) so hasn't boosted its radius much.

[_Augustus_]

6 minutes ago, Snark said:

Yup.  I've heard it speculated that Jupiter may be about as large (i.e. in diameter) as it's possible for an object to be without having internal fusion processes to boost the temperature:  i.e. if you were to take a Jupiter-sized planet and dump more hydrogen into it, it would actually get smaller due to the increased gravitation.  It just gets denser.

Unless it's superheated by being close to a star, in which case it can balloon to around 2-2.5 Jupiter radii.

[Spaceception]

That star will be around for awhile, wonder if there's any planets, I heard it was like, what, over 100 ly out? But since it's so small, transits/radial velocity shouldn't be too difficult with next gen telescopes.

[Scotius]

8 hours ago, Spaceception said:

That star will be around for awhile, wonder if there's any planets, I heard it was like, what, over 100 ly out? But since it's so small, transits/radial velocity shouldn't be too difficult with next gen telescopes.

Doubtful. Orbital period of this tiny dwarf is just under 8 days. It's hard to imagine a planet circling the secondary, able to retain stable orbit in such proximity to a primary star.

[kerbiloid]

This thing orbits round a normal, full-featured 1.13 M_Sun star.
It has a shape of hydrostatic equilibrium.
I believe, it has cleared neighbourhood.

Is it now a planet?

[Spaceception]

3 hours ago, kerbiloid said:

This thing orbits round a normal, full-featured 1.13 M_Sun star.
It has a shape of hydrostatic equilibrium.
I believe, it has cleared neighbourhood.

Is it now a planet?

But it can sustain fusion, which makes it a star

[Green Baron]

...and companion in a binary system :-)

[kerbiloid]

None of these condintions are mentioned in the IAU definition of planet.

So, scientifically, it's just a massive hot planet with its own energy source.
Yes, its orbit barycenter is placed outside of the reference body, but this mostly calls into question its companion status.

Btw. Sensation! Planets can fuse!

Edited July 21, 2017 by kerbiloid

[Bill Phil]

28 minutes ago, kerbiloid said:

None of these condintions are mentioned in the IAU definition of planet.

So, scientifically, it's just a massive hot planet with its own energy source.
Yes, its orbit barycenter is placed outside of the reference body, but this mostly calls into question its companion status.

Btw. Sensation! Planets can fuse!

That only determines planets in the Solar System. It doesn't have any sway (as of now) on extrasolar bodies.

Quote

The definition of planet set in Prague, Czech Republic, in August 2006 by the International Astronomical Union (IAU) states that, in the Solar System, a planet is a celestial body which:

1. is in orbit around the Sun,
2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
3. has "cleared the neighborhood" around its orbit.

That's from your link. Notice how it says "in the Solar System," which implies that the following only applies for objects in our solar system.

[YNM]

31 minutes ago, kerbiloid said:

None of these condintions are mentioned in the IAU definition of planet.

So, scientifically, it's just a massive hot planet with its own energy source.
Yes, its orbit barycenter is placed outside of the reference body, but this mostly calls into question its companion status.

Btw. Sensation! Planets can fuse!

Well, Earth have fusion reaction too, everytime some creature on it push a unique button...

[KerikBalm]

but that fusion is not caused by gravitational compression

[YNM]

@KerikBalm But on the subject that "Planets can fuse !", it's as true as any fusion reaction. Well, if you see humans and their equipment as part of the planet they're on...

 

 

Anyway, please someone get this back on topic.

Edited July 22, 2017 by YNM

[Green Baron]

Here is the announcement. I never trust Wikipedia :-)

Planets may be en vogue these days, but this is introduced as an eclipsing binary system.

 

Edited July 22, 2017 by Green Baron

[KerikBalm]

6 hours ago, YNM said:

@KerikBalm But on the subject that "Planets can fuse !", it's as true as any fusion reaction. Well, if you see humans and their equipment as part of the planet they're on...

 

 

Anyway, please someone get this back on topic.

Well, for the time being its not Proton-Proton fusion

Its not CNO cycle fusion. The fusion we do is more comparable to what occurs in a brown dwarf. I also don't see our equipment as part of the planet we're on, given that we can send equipment to other planets.

Sending a thermonuclear weapon to Mars doesn't make Mars a star because it had fusion/ is capable of fusion.

The mechanism by which the fusion is achieved is important, despite cratercraker's disrespectful comment.

[YNM]

3 hours ago, KerikBalm said:

The mechanism by which the fusion is achieved is important, despite cratercraker's disrespectful comment.

True that, but in the spirit of having stars as planets, the fusion mechanism can be random as well.

 

For god sake back on topic !

 

One question : how do they tell that PP-burning is occuring inside the star ? Spectral lines ?

Edited July 22, 2017 by YNM

[Green Baron]

You can't look inside a star. Modelling of the conditions (temp, pressure, forces in equilibrium -> hull burning, etc.) and the conception of energy transport through radiation, convection, etc. in a star leads to the analyses of nuclear reactions in stars. Main figure is the star's mass. I don't want to cite wikipedia ... :-)

Edited July 22, 2017 by Green Baron

[UmbralRaptor]

1 hour ago, YNM said:

One question : how do they tell that PP-burning is occuring inside the star ? Spectral lines ?

Mostly modeling given the observed energy output, size, and mass. (All hail the polytropes?) In the case of the sun, neutrinos are also detectable.

[Vanamonde]

Removed a post or two. Please keep things on-topic and don't troll each other. 

[YNM]

7 hours ago, UmbralRaptor said:

Mostly modeling given the observed energy output, size, and mass. (All hail the polytropes?) ...

 

7 hours ago, Green Baron said:

You can't look inside a star. Modelling of the conditions (temp, pressure, forces in equilibrium -> hull burning, etc.) and the conception of energy transport through radiation, convection, etc. in a star leads to the analyses of nuclear reactions in stars. Main figure is the star's mass. ...

We must be very sure of the modelling then ?

I wonder if there's any experimental methods of doing so ? I heard that starquakes can fix down what model to be used, so I presume we have done it to this star ?

I am aware that modelling is the leading method today, with all the four equations for basic modelling (hydrostatic equilibrium, mass continuity, temperature gradient, energy production, additionally polytropes) but surely there have to be a method to anchor them down to reality.

Edited July 23, 2017 by YNM

[UmbralRaptor]

2 hours ago, YNM said:

I wonder if there's any experimental methods of doing so ? I heard that starquakes can fix down what model to be used, so I presume we have done it to this star ?

Not so much starquakes as pulsations, though that tends to be limited in how deep it can go (I think). Conditions can in some cases be replicated at very small scales in the lab, but the closest things to direct measurements of the core of a star come from neutrinos. (Constant low level flux from Sol, and that one-off from 1987A)

[Green Baron]

4 hours ago, YNM said:

 

We must be very sure of the modelling then ?

Morning :-),

as sure as one can be with any modeling. Models are under constant revision and update as new insights come in. Modern astronomy is a young science.

4 hours ago, YNM said:

I wonder if there's any experimental methods of doing so ? I heard that starquakes can fix down what model to be used, so I presume we have done it to this star ?

Short form: none. Plasma generated on earth is too thin and/or too hot. There are only observations via telescopes and checks against the models. These experiments are too big to conduct on earth.

4 hours ago, YNM said:

I am aware that modelling is the leading method today, with all the four equations for basic modelling (hydrostatic equilibrium, mass continuity, temperature gradient, energy production, additionally polytropes) but surely there have to be a method to anchor them down to reality.

Constant observation and revision and exchange of findings in the scientific publications and reviews.

That includes the danger of being mistaken, in which case the model will be overthrown and a new one takes its place or it stays vacant for some time until a new one takes its place.

But a model with an acknowledged foundation is far better than all the guessing that takes place, in many fields in general, especially with planet hunting and newly observed deviations from the norm in stars. That is actually harming science a lot, as can be read in almost every issue of the scientific journals (Science, Nature, Elsevier journals).

[YNM]

Quite interesting proposition for this star then. As modelling uncertainty goes, it may as well not actually fusing anything for the time being.