Slashdot Mirror
Researchers Discover a Star's Minimum Possible Mass
paulmac84 writes "Stars that don't have enough mass never shine, dying billions of years before their bigger counterparts. But astronomers have never been able to measure the exact mass limit, because the lightest stars that do shine can be simply too faint to detect. Now, new images show for the first time how big a star must be to avoid impending doom. The long-awaited new images finally lay this question to rest, say the authors. The dimmest stars were measured as being 8.3% of the Sun's mass. All protostars that are smaller than this are headed for life as a brown dwarf."
... that's 87 Jupiters.
Cute. Though people could just go ahead and read the article. To wit:
Although the telescope would have been able to detect fainter stars, none could be found- so it appears that they simply don't exist. "We checked the instruments over and over again" said Professor Richer "but we don't see any stars fainter than this".
If there were dimmer stars present there, the Hubble's main camera would have been sensitive enough to have seen them...they're pretty sure of this because they were able to notice some very dim white dwarfs (a white dwarf is the remenant stellar core of a bigger star which went nova; they are very hot [initally] but also very tiny), which are dimmer than the smallest M-class stars still in the main sequence.
Basicly, this observation is in reasonably close accordance with the theories about stellar fusion; basicly, an potential star needs to have about ten or fifteen times Jupiter's mass before deuterium fusion is possible, and about 70 times Jupiter's mass before normal hydrogen fusion happens (according to the models).
Jupiter weighs 1.899 * 10^29kg; Sol weighs 1.989 * 10^32 kg (or about 1050 times what Jupiter weighs).
8.4% of Sol's mass is 1.65 * 10^30, or 87 times what Jupiter weighs.
"The human race's favorite method for being in control of the facts is to ignore them." -Celia Green
True enough, but both back-of-the-envelope calculations and the best models give you an answer that's spot on, to within something less than a factor of two. It's not as though there's some great debate within the community about whether the minimum mass for pop-II stars is significantly different from
I'm a great fan of observational confirmations, and of giving Hubble time to people doing this sort of work, but it's hard to imagine why anyone who isn't a specialist in stellar modeling looking to test their code to within a few percent would care about this particular result.
It hardly seems like press release material. What's more, dressing up the article to make it seem like some great mystery has been solved is disingenuous.
But, I suppose, "this just in: astronomers have confirmed something that they've been rather confident is true for decades" doesn't sell papers.
I'd add chemical composition (metallicity, namely), too, to your list.
Not really. You can't nail down the mass threshhold that well without a huge statistical sample. And even if you could, the luminosity of the star probably would vary a lot around it (ie, it might change a lot if you added or lost a bit of mass). Then there's the matter of rotation. A high rotation rate probably would increase the mass threshhold for fusion. And the luminosity probably changes over time, certainly the mass balance does as the stellar wind blows mass away. Finally, you need some way to calibrate instruments in your lab using this standard. I doubt anyone has a star that they can use as a standard in their lab. Not even the fancy scientists that get to play with the black holes at the center of galaxies!