…to start fusion and make a star?
New question: what percentage of a solar mass does it take…
by CJ | Mar 29, 2017 | Journal | 18 comments
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There are variables in this question, and experts differ a bit, but give it a guess.
Well, according to various online sources, it takes between 5 and 10% of Sol’s mass to achieve hydrogen fusion. My SWAG was .1, so hooray for pulling numbers out of one’s bum!
Heck, I should know this as I have taught astronomy (although not for 4-5 or more years now). I tend to think of it as “how many Jupiters” does it take to achieve fusion, in the sense of Jupiter being a (very) failed star. Memory is telling me “40 or so” Jupiters… which does not answer your question.
Wikipedia says Jupiter is 0.001 solar masses, so 40 would be 0.04 – you need maybe 50, given that 0.08 for the red dwarf.
https://en.wikipedia.org/wiki/2MASS_J0523-1403 is the smallest known red dwarf and is at most 0.08 solar masses. It’s 2074 K, so the color temperature is closer to that of a candle flame than an incandescent bulb.
Just read about this:
http://www.ras.org.uk/news-and-press/2967-astronomers-identify-purest-most-massive-brown-dwarf
“An international team of astronomers has identified a record breaking brown dwarf (a star too small for nuclear fusion) with the ‘purest’ composition and the highest mass yet known. The object, known as SDSS J0104+1535, is a member of the so-called halo – the outermost reaches – of our Galaxy, made up of the most ancient stars. The scientists report the discovery in Monthly Notices of the Royal Astronomical Society.
“Brown dwarfs are intermediate between planets and fully-fledged stars. Their mass is too small for full nuclear fusion of hydrogen to helium (with a consequent release of energy) to take place, but they are usually significantly more massive than planets.
“Located 750 light years away in the constellation of Pisces, SDSS J0104+1535 is made of gas that is around 250 times purer than the Sun, so consists of more than 99.99% hydrogen and helium. Estimated to have formed about 10 billion years ago, measurements also suggest it has a mass equivalent to 90 times that of Jupiter, making it the most massive brown dwarf found to date.”
So this puts a lower bound for fusion. It must be greater than 90 Jupiters.
Oh, for numbers, 1 Jupiter = 0.00095 Sols. 90 Jupiters = 0.08595 Sols. So that’s in the ballpark of an M8 or M9 main sequence red dwarf. And then we might ask about the precision of any of these estimates, what is the inherent margin of error for the techniques being used.
Wikipedia has some good pages on Red Dwarves and Brown Dwarves, q.v. https://en.wikipedia.org/wiki/Brown_dwarf
I was ever so pleased when they found brown dwarf objects. I’d theorized the existence of points of mass that I was using as gravity anchors for my star drive—it never seemed sensible to me that a universe so rife with visible real estate did not have ‘unignited’ objects out there, too. I mean, there’s no sign on a cloud saying ‘this is a protostar’ and ‘this one will not form’…form it may, but if it’s out of ‘stuff’, it just doesn’t light up.
Like Meetpoint? Oh, I think you were on rock-solid ground there. I still reserve judgement about the amount of “exotic” forms of “dark matter”. I keep reading about discoveries of more than anticipated cold, neutral hydrogen surrounding galaxies. Our own solar system isn’t believed to have evolved this way–somebody left the party early, i.e. Nemesis. Just this week, about a SMBH that’s being ejected from its galaxy by gravity waves. The assumption that virtually all baryonic/normal matter in the Universe could be accounted for by the starlight we could see should be reopened. Nobody back then anticipated so many stars had planets, and as we did, probably shed some to the interstellar medium.
One of the things I like about your stories is I can’t complain about your use of Science. You obviously try and you get that right, plausibly so in extrapolation. You’re good.
(However I could demur from your accounting for distances in Foreigner. Your “continent” is too small, travel times too short, especially on grass-invaded double-track roads, e.g. Najida to Tanaja in the Marid, and population too small. Take Eurasia, for example, how long is a train trip from Paris to Istanbul, St Petersburg to Irkutsk, Moscow to Vladivostok?)
And more to this last point, “if it’s out of ‘stuff’, it just doesn’t light up”, I read just the day before yesterday that when one star in a gas-cloud stellar nursery lights up (and these are oftem the big, live fast die young, blue giants) astronomers find its “solar wind” tends to blow gas away from its nearby siblings, starving them out. This likely accounts for some 3/4 of stars being in the red dwarf categories (and less).
How much mass does a red dwarf lose as it burns for its lifetime? Every reaction loses a minute bit of mass converting it to light. The star obviously starts with enough mass to sustain fusion, but if it throws off matter a la coronal mass ejections, or varies due to inhomogeneity of elements within (swallowed a planet or such) could it burn out more quickly than expected? There has to be a slowest speed at which nuclear ignition can occur and if you have less ignitible matter than necessary to provide that speed, the star would “burn out” or collapse on itself without enough matter to form a neutron star by its gravity.
Wikipedia: “Because low-mass red dwarfs are fully convective, helium does not accumulate at the core, and compared to larger stars such as the Sun, they can burn a larger proportion of their hydrogen before leaving the main sequence. As a result, red dwarfs have estimated lifespans far longer than the present age of the universe, and stars less than 0.8 M☉ have not had time to leave the main sequence. The lower the mass of a red dwarf, the longer the lifespan. It is believed that the lifespan of these stars exceeds the expected 10 billion year lifespan of our Sun by the third or fourth power of the ratio of the solar mass to their masses; thus a 0.1 M☉ red dwarf may continue burning for 10 trillion years.”
But they’re cranky little things and prone to snit-fits. If not for—or perhaps in spite of—that bad habit, they might have life-bearing planets.
Oh yeah! One might think a little red dwarf wouldn’t have the energy to get uppity. Remember, with a red dwarf the habitable zone is in close? Many red dwarves are “flare stars”. Flare stars are likely to have sterilized their planets.
https://en.wikipedia.org/wiki/Flare_star
“Flare stars are intrinsically faint, but have been found to distances of 1,000 light years from Earth. On April 23, 2014, NASA’s Swift satellite detected the strongest, hottest, and longest-lasting sequence of stellar flares ever seen from a nearby red dwarf. The initial blast from this record-setting series of explosions was as much as 10,000 times more powerful than the largest solar flare ever recorded.
“The Sun’s nearest stellar neighbor Proxima Centauri is a flare star that undergoes occasional increases in brightness because of magnetic activity. The star’s magnetic field is created by convection throughout the stellar body, and the resulting flare activity generates a total X-ray emission similar to that produced by the Sun.”
You’ve got a stake in Barnard’s Star, don’t you?
“In 1998, astronomers observed an intense stellar flare, surprisingly showing that Barnard’s Star is a flare star.”
It doesn’t preclude life at a red dwarf, but it would likely have to be subterranean or benthic.
Barnard’s, yes, is one of those.
Goldilocks zone has been in the news a lot—but nobody has mentioned ‘flare star’ and that little problem.
Jane and I do mention the problem in Alliance Rising.
As I recall, Tripping is made up of three masses that didn’t coalesce, but are close enough that together, they form a dimple, a pothole, in the rubber sheet that is either the gravity well surface sheet, or the hyperspace sheet (even though it’s really a 3D volume, not a 2D surface/plane). (The rubber sheet is just the analogy I’ve seen for gravity or hyperspace.)
I don’t recall if Tripoint has one or more of its three objects a brown dwarf. Seems like it’s said that it’s either not lit up or not enough of a tar in any of the three masses to be more than the bump in the road the anchor, useful for a jump point.
On the other hand, whatever mass Finity parks at briefly in Finity’s End, is also not mentioned (that I recall) as a star but a failed star or just a mass big enough to again be a useful point. Seems like there are one or two in Compact space that the Pride or Hilfy’s ship use besides.
A really big ball of stuff that didn’t ignite fusion, between a gas giant or a “big snowball” or a Hot Jupiter or a brown or red dwarf, seems entirely plausible, even likely. Why wouldn’t there be things at several points on the scale in between, after all?
It’s really exciting what they’re finding in exoplanets, in star system makeup. I do wonder if, as we find out more and get more detail, we’ll discover the actual arrangement of orbiting objects and cloud of gases will be rather different than what’s been estimated. The observation of how many objects make a star flicker and wobble, and thus show up exoplanets, should give number and mass and position pretty well, but I’d guess the finer points are still estimates. But it’s turning up some odd extreme planetary and moon conditions.
Thing is, if we do find a really very Earth-like planet suitable for habitation / colonization, doesn’t it seem likely it will already have residents, even if they are not sapient / sentient, anywhere from primordial soup (viruses, bacteria, Protista) on up to complex lifeforms? Or, hmm, evidence that civilization was there but died out Or that they left the cradle and didn’t come back.
the biggest thing seems to be how commonplace planets are, and how commonplace life might be out there.
One of the things that got me most about Downbelow Station and Pride of Chanur, the first two books of yours I read, was that the science was careful, plausible, while the characters, the aliens and humans, their cultures and languages, were obviously from someone who knew how foreign languages and history worked. Having both the “soft” and “hard” science so on point at the same time, and having such a complex and visual feel to thebooks wowed me, really drew me in and hooked me.
Having raised the spectre of Dark Matter, this week there was this challenge to Dark Energy.
http://www.ras.org.uk/news-and-press/2968-explaining-the-accelerating-expansion-of-the-universe-without-dark-energy
“Einstein’s equations of general relativity that describe the expansion of the universe are so complex mathematically, that for a hundred years no solutions accounting for the effect of cosmic structures have been found. We know from very precise supernova observations that the universe is accelerating, but at the same time we rely on coarse approximations to Einstein’s equations which may introduce serious side-effects, such as the need for dark energy, in the models designed to fit the observational data.” explains Dr László Dobos, co-author of the paper. … “The theory of general relativity is fundamental in understanding the way the universe evolves. We do not question its validity; we question the validity of the approximate solutions. Our findings rely on a mathematical conjecture which permits the differential expansion of space, consistent with general relativity, and they show how the formation of complex structures of matter affects the expansion. These issues were previously swept under the rug but taking them into account can explain the acceleration without the need for dark energy.”
This is fascinating stuff, Paul! Thanks very much for the posting.