An abundance of small stars

Dec 10, 2010
An optical image of the elliptical galaxy NGC 4472 (Messier 49). New studies find that it has nearly five to ten times as many stars, mostly smaller than the sun, than previously known. Credit: McDonald Observatory

(PhysOrg.com) -- Stars form from giant clouds of gas and dust in space, as the matter in these clouds comes together under the influence of gravity.

A long-standing goal of astronomy has been to determine the population of produced in a cloud; that is, how many stars of various sizes form, and how does this depend on the physical properties of the particular cloud? The initial mass function (IMF) describes this distribution when averaged over the galaxy, and is currently based on observations of stars in our Milky Way.

The observed IMF has relatively few (i.e., ones more massive than the sun). Sun-sized stars are comparatively abundant. Stars somewhat smaller than the sun are even more common, but then stars of decreasing mass (down to one-tenth of the sun's mass or even less) decrease in numbers. The precise statistics for low mass stars are uncertain because they are faint and hard to detect. The theoretical assumptions for the IMF are also being debated. Meanwhile astronomers wonder if the IMF of the Milky Way is representative of the IMF elsewhere in the universe.

Apparently not. CfA astronomer Charlie Conroy, together with a colleague, studied the population of low mass stars (smaller than about 0.3 solar-masses) in a set of nearby elliptical galaxies. Although one such star in another galaxy is too far away and faint to be detectable, collectively they were detected by the astronomers because of their diffuse, faint red glow. That starlight has spectral features characteristic of low-mass stars, enabling the scientists to reach a firm conclusion about the stellar masses.

The conclusion was dramatic: nearly 80% of all the stars in these galaxies must be small -- a much larger fraction of small stars than astronomers think exists for the Milky Way. This means that they account for at least 60% of the mass of these galaxies. If these elliptical galaxies are typical, the total number of stars in the universe must be about three times larger than previously estimated. And, not least, the result implies that the IMF for the is not representative of the IMF elsewhere, and so the star forming processes must likewise differ in some important ways.

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geokstr
2.2 / 5 (6) Dec 10, 2010
Let's see - five or ten times as many stars as previously known. Just a few days ago, three times more red dwarfs were found than previously known. Last year, we learned that brown dwarfs may be much more numerous than previously known. A couple years ago, it was discovered that 10% of one of the largest galaxy superclusters were orphan stars torn out during collisions and tidal pull between galaxies, which were previously unknown. In the last ten years, it looks like we've found that planets are far more numerous than previously known. Given that there is no particular reason that bodies smaller than brown dwarfs can't form without an attendant star when a cloud of dust collapses, there is likely a lot more of these than previously known as well. Giant gas and dust clouds surrounding galaxies are more common than previously known.

Pretty soon, we may actually discover that "dark matter" is a lot less necessary to make the equations work than was - "previously known".
Quantum_Conundrum
2 / 5 (4) Dec 10, 2010
Given that there is no particular reason that bodies smaller than brown dwarfs can't form without an attendant star when a cloud of dust collapses, there is likely a lot more of these than previously known as well


That's actually a very good point that a lot of people don't give much attention to.

There's no reason why the universe shouldn't be teeming with rogue planets and other black bodies significantly smaller than a star. In fact, we might expect that such objects should be at least several orders of magnitude more common than actual "stars", because it seems obvious that it's easier to make something the size of earth or some other planet than to make something the size of a star.

Quite a few of these objects might even be very rich in resources that would prove useful to a type 2 or type 3 civilization.
geokstr
1.6 / 5 (5) Dec 10, 2010
...it seems obvious that it's easier to make something the size of earth or some other planet than to make something the size of a star.

Didn't we used to think that the space between the solar system and the next star was empty too? Then the Oort Cloud came along and it's estimated that there are trillions of bodies in it, many of them Pluto sized or larger. There still seems to be conjecture that there is a Jupiter-to-brown dwarf size body out there that dislodges comets. If there's one, and we can't see it because it's too dark and too small, it's more than likely that there may be LOTS of them. And if our solar system has an Oort Cloud, it only ends where the next star's own Oort Cloud starts.

I'd wager that even these "voids" we hear so much about between the filaments of galaxy clusters and superclusters are not as empty as we think. It's likely they are filled with bodies of all sizes that we will never be able to detect, with our primitive technology.
Quantum_Conundrum
3 / 5 (3) Dec 10, 2010
There still seems to be conjecture that there is a Jupiter-to-brown dwarf size body out there that dislodges comets.


That was always thought to be the case. The discovery of Pluto was a coincidence, because they were searching for a fifth gas giant to explain pertubations in the solar system that could not be explained by the known objects, and probably still can't be explained.

If a "cold Jupiter" (a gas giant or possibly an ice giant,) exists beyond Pluto out somewhere near 60 to 80 a.u., it might be almost impossible to detect with our existing technology other than by pure luck, particularly if it orbits on an abnormal plane.

However, if the original theories were even close at all, it should still be somewhere in the actual sector that Neptune and Uranus were passing through at the time Pluto was discovered, as it would be moving signifcantly slower than them and thus lagging behind, though if it's on a different plane that doesn't help much anyway.
geokstr
1 / 5 (1) Dec 10, 2010
That was always thought to be the case.

I was aware that there was always this conjecture, which is why I said "still". But the tinfoils are still obsessing over Nemesis, so I tried to preclude that. I suppose, though, that if the world is going to end in 2012, then what does all this matter?

:-)

...somewhere near 60 to 80 a.u., it might be almost impossible to detect...

You are limiting your thinking way too much. Halfway to the next star is approx 50,000 a.u. Why does this cold Jupiter, or even a red or brown dwarf, or even many of them, need to be within a mere 60-80? If the Oort Cloud extends that far, would a large black body, say, 10,000 a.u. out, have even a perceptible effect on the orbits of the outer gas giants, while still having the capability to scatter comets?
Quantum_Conundrum
not rated yet Dec 10, 2010
Inverse squared law puts certain limits on how far away the object could be, since every time you double it's distance you would need to scale up it's mass by a factor of 4 to have the same gravitational influence.

So for an example, an object at 10000 a.u. would need to be about 1563 times as massive as an object at 80 a.u. in order to have the same gravitational influence on the "eight" planets plus Pluto.

So a 1 jupiter mass object at 80 a.u. would have the same gravitational influence as a 1563 jupiter mass object at 10000 a.u., and that would already be 50% more massive than the Sun itself.

So then you would need to decide how massive an object "could" reasonably be and still escape detection after all the attempts to find it.

By the time you get to 10000 a.u. you are basicly talking about a neutron star or a small black hole, because anything else large enough to have a gravitational influence would have already been detected due to it's visible and infrared light.
geokstr
1 / 5 (1) Dec 11, 2010
Thanks for the detailed explanation. While that indicates that there might be a closer object affecting the orbits of the 8 inner planets, it certainly does not preclude the existence of lots of black bodies in the Oort Cloud much farther out, does it? If there are trillions of comet size bodies out there, they could still be dislodged by such black bodies, no?

My original point is that there is just so much ordinary matter that we keep discovering because of improving technology, that we may soon have found enough ordinary matter to significantly reduce or even eliminate the need for "dark matter" to explain why galaxies don't fly apart and where gravitational lensing is coming from. 5-10X as many stars + 3X as many red dwarves + 10% of superclusters as orphans and pretty soon you're talking lots of real matter. :-)

What would that do to the models that require "dark matter" to explain the composition of the universe? And would that affect the need for "dark energy" at all?
ThanderMAX
not rated yet Dec 11, 2010
Why that galaxy looks like a fuzzy ball of light?
Quantum_Conundrum
not rated yet Dec 11, 2010
Why that galaxy looks like a fuzzy ball of light?


Ever study "Pointelism" in an art class?

The galaxy is so far away that the light from individual stars blends together to be indistinguishable, much as the "dots" in a pointelism painting.
geokstr
1 / 5 (1) Dec 11, 2010
Why that galaxy looks like a fuzzy ball of light?

Plus it's an elliptical galaxy, which are generally composed of older stars of relatively uniform brightness. It has no arms like spiral galaxies, and no giant star-forming regions, which are what stands out in spirals, not individual stars, which are too far away to see.

Ellipticals are thought to form when spiral galaxies collide and combine. The stars in ellipticals are so densely packed that if our Solar System were inside one, it would be as bright as day at night from the combined light of the stars.