Most Stars Are Single

An anonymous reader writes to tell us Space.com is reporting that 'for more than 200 years, astronomers thought that most of the stars in our galaxy had stellar companions. But a new study suggests the bulk of them are born alone and never have stellar company.' The key difference seems to come from the difference between the highly turbulent clouds that produce massive stars in groups and the less active smaller clouds that produce red dwarfs."

True, not true

[ChrisDolan]

The article goes on to explain that red dwarfs (tiny stars, much smaller than the sun) are much more common that sun-like and larger stars, and that red dwarfs are much less likely to be binaries. So, in total stars are more likely to be single.

However, from my reading it seems that the conventional wisdom that most sun-like stars are binaries is still true. I once learned the humorous mnemonic "Three out of every two stars is a binary".

Re:It's disappointing...

[wanerious]

As an astronomy educator, believe me, I understand. But in realizing that scientific theories are really only our best models right now, it's actually invigorating that the more we study and investigate the universe the more beautiful, complex, and subtle it is.

It's probable that *everything* you have been taught will some day have to be "unlearned". In this case, as with many others, it's not so much that what you were taught was wrong, only that it was imprecise. The article reaffirms that it is still true that most of the bright stars in the sky are members of multiple-star systems. Just the previously unobserved swarm of very dim, red stars seem to be largely isolated. Still consistent with previous observations.

Re:It's disappointing...

[Anonymous Coward]

...a scientific fact is nothing more than a theory with solid evidence to support it, and nothing to contradict it.

Old News

[Einer2]

We've known for about a decade that the binary frequency among low-mass (early M) stars is only 30-35%. We've also known for at least that long that the general shape of the field mass function is weighted in favor of low-mass stars. It's a very short leap to draw the corresponding conclusion, and it's been done in plenty of other papers that actually present useful results at the same time.

For those who care about the background, the binary frequency has been shown pretty clearly to depend on mass. Solar-mass stars have binary frequencies of at least 60%, stars of 0.5 solar masses have binary frequencies of ~35%, and very low-mass stars and brown dwarfs (under 0.2 solar masses) have binary frequencies of around 10-20%. The binary frequency among more massive stars appears to be even higher than for solar-mass stars.

The popular reason to care about binary frequencies is to determine the frequency with which planetary systems could occur. If you're interested in habitable planets around solar-type stars, the higher binary frequency is one to care about. The frequency with which planets could form around lower-mass stars is intrinsically interesting since they're so common, but they're also much harder to detect any of these planets using existing indirect methods, so it's a harder question to actually answer. Once we have the ability to directly image planets, the problem will invert itself since it's easier to see planetary companions to faint stars than bright stars.

Re:It's disappointing...

[maxwell demon]

Well, it's true that science only knows facts. As does everyone else. However, science isn't just about what we know, but mostly about what we predict from what we know. That is, we search for patterns, and if a pattern seems to work reasonably well (i.e. we have tried it quite often and it worked on all our tests), we have a theory. Those theories are not facts, the only fact involved is that they describe the past observations well. It is a prediction that they will also describe future observations well, however that is something you cannot know, just expect. This expectation is the more reasonable, the more similar the situation is to those you already have observed.

For example, if you have seen many things fall down, and never have seen anything that raises, then a reasonable theory is that all things fall down. Now say, your experience is mostly on stones and bananas (where you have tested hundreds of them). Now if you find any new, unknown object, it's quite reasonable to expect that it will fall down as well. It's unreasonable to assume you know in advance that it will fall down. If it is a stone or a banana, then it's so likely that you'll again see falling it down that you can treat it almost like a fact. If it is another object, then you shouldn't be too sure. After all, up to now you only observed the pattern with stones and bananas. And indeed, if the object turns out to be a helium balloon, then you'll find that your expectation is not met. That is, you'll find that your theory that everything falls down is in its generality wrong. It is, however, not completely wrong, because it still accurately describes your experience with stones and bananas. So the old theory is not invalid, but only has a limited validity. Now you can look at things which fall down, and things which raise up, and try to find a new pattern which lets you predict if a thing falls down or raises. If you find that pattern (which turns out to be "things which have a greater mass density than air ("are heavier than air") fall, things which are "lighter than air" rise), then you have a new, better theory, which contains your old theory (all things fall down) as special case.

Ok, in this case, it's that most of the stars observed so far (which were the bright stars, because those are the stars you can easily see) were not alone. So the theory was "most stars are not alone" (although in this case, I wouldn't really call it a theory, more an assumption or hypothesis). Now, stars which are sufficiently different from those which were observed before are found to be mostly alone, and there are also more of them. So we find that the old theory's validity is limited to those stars we previously observed (i.e. the bright ones), and a new, better theory replaces it (brighter stars usually have companions, dimmer stars usually don't).

So the fact (most stars previously observed have companions) didn't change, only the conjecture (therefore it probably is true for all other stars).

Note that the working of science isn't really too different from the everyday way of thinking. The main difference is that in everyday life, we often take our conjectures as facts without ever questioning them, while in science, the questioning of theories is institutionalized.