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Planet-Gobbling Star
crymeph0 writes "BBC is carrying a story about a star that mysteriously brightened three times last year. Scientists now know why. It's been eating gas-giant planets that orbit it! I'm just glad Earth isn't a tasty gas planet, or else we'd have to start making sacrifices to Sol to play it safe." It's hard to prove things from 20,000 light years away, but this explanation is interesting.
Star devours planets
By Dr David Whitehouse
BBC News Online science editor
The mystery of an erupting star may be explained by the realisation that it has been engulfing planets.
Last year, the usually well behaved star V838 Monocerotis dramatically brightened three times, and astronomers were at a loss to explain why.
The latest suggestion is that the expanding red-giant star was swallowing three gas planets that were orbiting it.
Astronomers are looking carefully at the observations as what happened to those planets may one day also happen to the Earth.
Previously unexplained
V838 Monocerotis is located about 20,000 light-years away in the constellation Monoceros (the Unicorn).
The outbursts were detected last year by Australian amateur astronomer Nicholas Brown.
The star was seen to brighten to more than 600,000 times our Sun's luminosity.
Astronomers had previously been unable to explain what transformed a dim star into the brightest, cool, supergiant star in the Milky Way.
The Hubble Space Telescope's Advanced Camera for Surveys had earlier recorded a dramatic image detecting a burst of light spreading into space and reflecting off shells of dust around the troubled star.
Now, in research soon to be published in the journal Monthly Notices of the Royal Astronomical Society, Dr Alon Retter and Dr Ariel Marom, from Sydney University, suggest the activity can be explained by the expanding star swallowing nearby planets.
Three planets, three meals
The researchers say that V838 Monocerotis flared because it was fuelled as it engulfed three orbiting planets. It could be the first evidence for an event that had been predicted but not knowingly observed.
Support for this assessment, say the astronomers, is provided by the study of the shape of the light curve and comparison between the observed properties of the star and several theoretical studies.
In addition to the gravitational energy generated by the process, there may also have been a rapid release of nuclear energy as fresh hydrogen was driven into the hydrogen-burning region of the star.
Some researchers believe that planet swallowing may be common and may explain why so many stars have enhanced levels of metals in their surface regions. The metals may have come from engulfed planets.
CMDRTACO CHECK YOUR EMAIL!
Troll? Granted, it was a little obscure, but troll? Was that comment really going to spark an argument?
Orson Wells was the voice of Unicron in the Transformers Movie. Unicron was a planet eater. Orson Wells was a star of the movie. It's a joke.
"Derp de derp."
If it's 20,000 LY away it didn't brighten 3 times last year (that we know of)... rather it brightened 3 times in one year approximately 20,000 years ago.
Read Niven's A World Out Of Time (multiple meanings in the title) for a similar idea. It's one if his first "State" books.
SPOLIERS BELOW
Basically, something else gets dropped into Jupiter. And there's some fascinating ideas on how to move a planet around.
You cannot apply a technological solution to a sociological problem. (Edwards' Law)
1) See, I asked about how much it *brightened*. Not how bright it *got*. I noted that line, too., but knowing that it is now 600,000 L_sun isn't really helpful in telling us how much brighter it is now than before. We would need to also know what its starting point was. This makes a difference: if the star brightened by 0.1%, the possible mechanisms are quite different from the star brightening by a factor of 100.
2) Um, no. When a star gets to Fe (and only very large stars do), it makes a nice little explosion adn we enrich the interstellar medium. Which is where pretty much all of the "metals" (anything heavier than helium, according to astrophysicsists) in your body, Earth, the Sun, etc. come from. So the question about metal-rich stars isn't "are they producing the metals", they would have had to leave the main sequence for one thing. The question is did the cloud that formed them have an more metals than the average, or did the metals get preferentially introduced by, say, planets smacking in to them.
No, see, as star is WAY bigger than a planet. (By definition, almost.) So a planet, particular a gas giant which is in large part hydrogen and helium (10s of percent and up, by mass) smack into the star, unless the material stays right near the surface, all of those metals will basically be so thinly spread throughout the volume of the star that you'll never see a real enrichment to within error bars.
And remember, the volume of a shell goes like the radius or the star squared, so the thickness of the shell has to be pretty thin to keep an appreciable fraction of the metals. Say we want to spread the metals out over a volume roughly equal to the volume of the original core. Uranus is mostly core, so let us use its radius as the radius of the core. (Note: much of Uranus's core is hygrogren compounds, as are all giant planet cores. This means that we're *over*estimating the volume of metals.) And lets spread it over a spherical shell on the Sun's surface.
V_Uranus = 4/3 pi r_u^3
V_shell = 4 pi r_s^2 deltaR
where r_u is Uranus's radius (2.62E9 cm) and r_s is the Sun's (6.9E10 cm), deltaR is the thickness of the spherical shell, and the Vs are volumes. Equating and cancelling, we get that deltaR = r_u^3/3 r_s^2. Plugging in numbers, that's a thickness of about 1.3E6 cm, or about 0.0018 % of the Sun's diameter. Which, when you consider that the Sun is fluid and convection does happen (although the most convective part is a bit lower down below the surface), isn't a whole lot. Confining the metals to that region would be very difficult.
This would probably be why current thinking tends more towards the "the clouds that formed star with planets were unusally rich in metals." Also, it makes sense: more metals, more stuff to actually *build* planets with.
To answer my own question (woo for Google!), the star has brightened from about magnitude 11 or 12 to about 6.5. That's around 5 magnitudes of brightening, or a factor of 100 in the overall luminosity. AAVSO's site talks about it: http://www.aavso.org/vstar/vsots/1202.shtml
I have to say, I'll be interested to see their paper when it hits press, but I'm really skeptical.
2) Um, no. When a star gets to Fe (and only very large stars do), it makes a nice little explosion adn we enrich the interstellar medium. Which is where pretty much all of the "metals" (anything heavier than helium, according to astrophysicsists) in your body, Earth, the Sun, etc. come from. So the question about metal-rich stars isn't "are they producing the metals", they would have had to leave the main sequence for one thing. The question is did the cloud that formed them have an more metals than the average, or did the metals get preferentially introduced by, say, planets smacking in to them.
:)
Ah, nitpicking.
When the core of the star gets to iron it CAN blow up, because, of course, iron is king when it comes to nuclear stability. Can't fuse it, can't fission it, it's just... iron. Hence the reason that cosmic rays are generally considered to be either particles, or iron.
Anyway, the star still won't blow up if it does start to burn stuff heavier than carbon - it'll only blow up if it has enough mass to overcome the electron degeneracy pressure. Most people merge these two - "duh, if the star has enough mass to burn up to iron, it'll have enough mass to overcome the electron degeneracy pressure!"
This isn't *completely* clear (and as far as I know, it's still ambiguous) because you don't know how much matter the star's going to slough during its helium burning/carbon burning dying phase. The planetary nebulae around planets contain a huge fraction of the star's mass.
Anyway, blah, this is nitpicking. Any star which honestly seriously gets to silicon burning is going to supernova, except for some really really weirdo situations.
Other thing is that the materials that were likely formed completely in supernovae are actually only elements past iron. Anything below iron could have formed in a star, and been sloughed off, or shredded away somehow. Anyway, the point that you made was that everything past helium comes from a supernova - that's definitely not true. Probably a majority of the elements past about, oh, oxygen comes from a supernova (but probably not virtually all). Carbon, nitrogen, and oxygen can, and probably do, come from other sources than supernovae.
Being a TOTAL nitpick, only everything past iron is formed in a supernova. Elements higher than oxygen are probably spread via a supernova. The reason that C, N, O can be spread easily is because, as with the triple-alpha process, stars balloon when they manage to reach a temperature where they start burning new elements, and they slough off a ton of material, and the stages near C,N,O last long enough that they probably can shove a significant amount of material off (hundreds/thousands of years - still virtually no time cosmologically).
No, see, as star is WAY bigger than a planet. (By definition, almost.) So a planet, particular a gas giant which is in large part hydrogen and helium (10s of percent and up, by mass) smack into the star, unless the material stays right near the surface, all of those metals will basically be so thinly spread throughout the volume of the star that you'll never see a real enrichment to within error bars.
Depends on the kind of star. The core of the star - that is, the "dense" part - is not that inconsistent with the size of a gas giant. After the triple-alpha process ignites, the outer regions of a star are so incredibly not dense that they probably wouldn't produce ANY drag, and you could get significant enrichment when the planet actually encountered the star's core, if the resultant nova didn't blow the contents of the star out to kingdom come (i.e. escape velocity) because you'd probably isotropize the shell, and get a locally "metal-heavy" shell about the star which would show up in spectroscopy.
Note that this, of course, would only result in an enrichment of red giants, and again, isn't applicable for main sequence stars.
Note: much of Uranus's core is hygrogr
No - you don't fuse iron. You neutron-stuff them. During the supernova, the outbound shock wave carries so much energy (and neutrinos) that neutrons are literally "shoved" into nuclei. You get ridiculous things like iron with hundreds of neutrons, which then decay down into normal elements by alpha emission and beta decay. This is r-process stellar nucleosynthesis. (There's also s-process stellar nucleosynthesis, which is also neutron stuffing, but on a much longer timescale. Essentially everything past iron is formed by r-process stellar nucleosynthesis. Check Carroll & Ostlie pp 527-528 for more info.
Stars die when they hit the iron stage because they can generate no outward pressure from fusing iron. They can't even fuse iron at all! It's actually a really complex procedure - basically, the iron starts to lose all of its electrons (from proton capture and other mechanisms) so the core rapidly loses electron degeneracy pressure, which is what was (briefly) supporting it. The inner core collapses very uniformly to a little neutron star, and the outer core decouples from the inner core, and the outer core rushes inwards at extreme velocities. The collision of the two is one of many explosions in a supernova. (Again, see Carroll & Ostlie's section on the Death of Massive Stars)
Anyway, the fuel isn't insignificant depending on what stage the star is in, and also depending on how fast the planet's orbit would decay once it's inside the photosphere. If it meets with the star's core without significantly losing mass, that could cause a VERY large brightening. Functionally it's equivalent to a nova, or the pulsing of Wolf-Rayet stars (without the mass shell shielding it).
Depends on the kind of star. The core of the star - that is, the "dense" part -
In this case, it's an F-class star. And you missed my point entirely, which was that if the star can convect the planet's hydrogen into the shell-burning zone, it can damn well convect its own hydrogren reserves down there, which are vastly in excess of what the planet could provide. So the star should never notice the miniscule addition of the planet's hydrogen.
This is all theory, of course, but unfortunately, theory doesn't quite bear out the "hydrogen compounds = gas giant planet cores".
See, that's where you're amazingly wrong. Let's review out giant planets, shall we? (If you want, I suggest you crack open Protostars and Planets IV; it's always good to actually do a bit of research.)
Jupiter May or may not have a core in the first place. If it does, it's at most around 10 Earth-masses (maybe as high as 15, but that's at the outer edge of the error bars). Mostly, it'll be hydrogen compounds with some rock and metal (real metals, not in the astrophysical sense). You need that core in standard formation models before you can accrete the hydrogen and helium gas. The metallic hydrogen is a layer right above the core, not the core itself.
Saturn Has a core, around 10-15 Earth-masses. Same as Jupiter in composition. This is easier to work out in theory because the equation of state is better understood for the interior pressures within Saturn. (Jupiter's higher pressures make things more dicey.)
Uranus and Neptune Definately have cores. Also icey with a bit of metals and rock thrown in. Again, need said core to hold on to the gas in the first place. Cores are pretty well constrained in size at around 15 Earth-masses in both planets. Given that both planets are around 18 Earth-masses in size, you bet your ass that this means that they are both mostly core. In fact, it's this that has lead some leading researchers to dub them "ice giants", in contrast to Jupiter and Saturn, the "gas giants."
I don't know where you got your "facts", but they're pretty much uniformly wrong. See Wuchterl et al. in P&P IV for more details on constraints on the present structures of these planets.