# Astroboffins spot tiniest star yet – we guess you could call it... small fry

The tiniest star, similar in size to Saturn, has been discovered as part of an eclipsing binary system by a group of astronomers. Codenamed EBLM J0555-57Ab, researchers believe the star is teetering on the edge of how small a star can possibly be. It has a mass (0.081M☉) and radius (0.084R☉) less than a tenth of our Sun’s mass …

1. #### "Does the Sun Have a Surface" by Dr Pierre Robitaille, on YouTube

The Hydrogen-Fusion Ball Sun hypothesis has a number of defects, explained by

Dr Robitaille, director if Radiology, Ohio State University

https://youtu.be/Ergl613GO_k

2. #### The radius is wrong.

Or maybe the mass. But if you tell me the object has about a twelfth the mass of the Sun, and a twelfth of its radius, then you're also telling me it has a density 144 times that of the Sun, or about 200 times that of water.

An object the size of Saturn (which _would_ be about a twelfth of a solar diameter) isn't big enough to cause fusion to happen; you've got to be a fair bit bigger than Jupiter to do that. I think a twelfth of a solar mass is in the right ballpark, enough bigger than a brown dwarf to produce at least some light, but perhaps at low enough density to not show up when transiting a larger, more energetic star.

1. #### "then you're also telling me it has a density 144 times that of the Sun, "

When I ran the numbers I get a figure 136x the density of the sun

Or maybe they are saying that the Sun's mass and radius are being used as reference points for those parameters?

We know the gross composition of stars varies quite a lot.

Assuming this stars composition matches that of the Sun is unwise.

1. #### Re: "then you're also telling me it has a density 144 times that of the Sun, "

It may not have a composition identical to that of the sun, but if it's evolved in place as a red dwarf it must be hydrogen and helium (with just a trace of what astronomers call 'metals'), as it's far too small to fuse helium. So a density more than 100x greater is hard to explain. Unless it's some sort of stellar remnant from a much larger object?

1. #### Re: "then you're also telling me it has a density 144 times that of the Sun, "

To answer my own question, yes you can get a density 193x water from a tiny red dwarf. There's little radiation pressure to expand the size, as would happen with a larger star. So you get the apparently contradictory result of an object smaller than Jupiter, but 80x more massive; while having a similar composition.

1. #### Re: "then you're also telling me it has a density 144 times that of the Sun, "

The numbers quoted match that in the original article: https://arxiv.org/abs/1706.08781

They note explicitly that the density is very high: "... and is one of the densest non-stellar-remnant objects currently known. These measurements are consistent with models of low-mass stars. "

So apparently this is correct and expected.

2. #### Re: "density 144 times that of the Sun" - It's gravity not rocket science!

The average density of the whole Sun is about 1.4 g/cc but the core's density is about 162 g/cc. The temperature of the core (and radiation pressure) is what keeps the outer layers inflated; without this effect the Sun would collapse to about the same radius as the Earth but a lot denser.

This borderline red dwarf would have started out much bigger than Saturn while collapsing under gravity to the point where ignition started. It would have been smaller at that point until the temperature expanded it a bit up to Saturn's size.

Gravity's attraction falls off as the inverse square so given the right starting mass it will compress gas into a higher density than say Jupiter and thus a smaller relative volume.

2. #### Re: The radius is wrong.

But if you tell me the object has about a twelfth the mass of the Sun, and a twelfth of its radius, then you're also telling me it has a density 144 times that of the Sun, or about 200 times that of water.

No, it's reasonable and known behavior for brown dwarfs and large gas giants. Jupiter is about as big as a gas giant gets by diameter (with exceptions for thermally-inflated fluffy Jupiters or newly formed gas giants still hot from accretion.) Additional mass simply results in them compressing as gravity overcomes Coulomb pressure. The next density limit is electron-degeneracy pressure, like white dwarfs.

So, a red dwarf star that does have ongoing fusion will be bigger as heat inflates it, but the smallest red dwarf stars are still near Jupiter in size. Which makes for some extreme densities.

Gravitational compression also makes a difference between Mercury and Earth. Mercury would be denser than Earth by raw composition, but Earth is big enough to compress its core a bit.

3. FTA: "The smallest stars provide optimal conditions for the discovery of Earth-like planets..."

I thought that red dwarfs were not optimal, because their goldilocks zones are so close as to force tidal locking for any earth-like planets, thus rendering them non-earth-like.

Is it just that the relative mass ratio between the star and the planet is smaller, making it easier to detect them via solar wobbling?

1. "I thought that red dwarfs were not optimal, because their goldilocks zones are so close as to force tidal locking for any earth-like planets, thus rendering them non-earth-like."

I suppose it depends on what they mean by Earth-like. It might simply be Earth-sized and rocky irrespective of habitability.

2. The paper says, "Stars with masses below 0.25M are the most common stellar objects and prove to be excellent candidates for the detection of Earth-sized planets and their atmospheric characterization."

It doesn't say much more than that on the topic, only referencing you to other papers.

1. #### habitability of tidally-locked planets

Newer climate models seem to suggest that tidally-locked planets around red dwarfs could have liquid surface water and are thus potentially habitable:

http://newatlas.com/tidally-locked-exoplanets-potentially-habitable/39407/

1. #### Re: habitability of tidally-locked planets

I'm thinking that its down to this:

Odd of spotting Small planet transiting Small star > Odds of spotting Small planet transiting Large star.

As a larger percentage of the star's output would be blocked.

2. #### Re: habitability of tidally-locked planets

@stepharsh - your statement is totally logical. Just for argument's sake though. One advantage of our earth is that the core spins (at a retrograde I suspect) to the spin of our earth. This creates the Van Allen belt attributed to protection of our atmosphere. We need another model to protect the atmosphere of your theoretical planet - no? How will such a slow moving planet generate a magnetosphere?

4. Very interesting. More of a starlet than a star, but it does further constrain the border between red and brown dwarfs. I wonder if that boundary is influenced by helium content or metallicity. It is bound to, I suppose, given that the CNO cycle will depend on carbon, nitrogen, and oxygen (all metals to astronomers) abundances

1. #### Red Dwarf Metallicity, CNO cycle

The CNO cycle doesn't have much of an effect until you reach F stars in the main sequence and is insignificant in M dwarfs. M dwarfs with high metallicities should be smaller and denser, but have more planets around them.

5. #### Hmmn.. interesting..

I did a quick search for the star by name and found this: https://arxiv.org/abs/1706.08781 where it remarks that the star " is one of the densest non-stellar-remnant objects currently known. These measurements are consistent with models of low-mass stars. "

Which is perfectly true. Starting with a planet the size of Saturn and piling extra mass onto it does not result in size growth to teh extent that you;d imagine, due to gravitational compression of the matter in the object. Similarly, one of the reasons larger stars are less dense than smaller ones is because the energy generated by the fusion proceses within make the thing swell like a balloon until the point is reached where radiation pressure and gravity balance out. A star only just massive enough to have any fusion going on at all would be very dense indeed, not generating enough energy to develop a greatly swollen photosphere, and probably the core would be the densest that matter can get short of being crushed into neutronium.

1. #### Re: Hmmn.. interesting..

the densest that matter can get short of being crushed into neutronium

White dwarf (degenerate) matter is ~1,000,000x density of water. They're much more massive than this object, of course.

1. #### Re: Hmmn.. interesting..

White dwarf (degenerate) matter is ~1,000,000x density of water.

So one tonne per cubic centimetre? Cool. Could somebody get me a single cubic millimetre, I reckon that would be perfect for a whole range of practical jokes.

1. #### Re: Hmmn.. interesting..

> So one tonne per cubic centimetre? Cool. Could somebody get me a single cubic millimetre, I reckon that would be perfect for a whole range of practical jokes.

Youd have to be quick. Paper: hyperdense quantum nuggets detected in seismograph data

https://arxiv.org/pdf/astro-ph/0205089.pdf

2. #### Re: Hmmn.. interesting..

~200 g/cubic cm is still far far short of white dwarf densities.

6. #### Hmmm...

Any large black monoliths spotted?

7. #### the star would instead have transformed into a brown dwarf

You mean like the guy that played Marcus in 'Bad Santa'?

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