How else can a star form, other than gravitational collapse?

Question

I read this paragraph on the Sun's page on Wikipedia:

[The Sun] formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core. It is thought that almost all stars form by this process.

Notice how it says, "almost all stars form by this process"? It says "almost all". Does it mean that there are other ways by which a star can form? I mean, other than the gravitational collapse of gas and dust?

Answer 1

About 15ish years ago, this was still a heated and pressing open question: what is the dominant mechanism by which most stars (i.e., low-mass stars) form? This came to the fore in a showdown between the two main hypotheses for stellar formation: gravitational collapse and competitive accretion.

Under gravitational collapse, star-forming molecular clumps, of typically hundreds to thousands of solar masses (M$_{\odot}$), fragment into gaseous cores that subsequently collapse to make individual stars or small multiple systems. In contrast, competitive accretion theory suggests that at birth all stars are much smaller than the typical stellar mass (∼0.5M$_{\odot}$), and that final stellar masses are determined by the subsequent accretion of unbound gas from the clump

For a concise yet in-depth discussion of these controversies, see this paper by Clarke (2006).

Comparing updated simulations (for examples, including more realistic radiation effects and turbulence) with observations essentially ruled out the models of competitive accretion in favor of gravitational collapse models. Since then, this result has advanced further, with some apparent exceptions. For example, a recent study of the Orion Nebula found that accretion may play a role in determining the final stellar masses, but that is not necessarily news - we've known for a while now that accretion can play an important role in stellar evolution. But regarding the question of stellar formation itself, this recent study does not seem to give hope for competitive accretion models, despite the pop sci headlines (or maybe I've misunderstood something here?).

In molecular clouds, accretion may play a role in aiding gravitational collapse, as molecular cloud dynamics is a complicated multi-scale process.

For high mass stars, the situation is ever more complicated, as accretion disks can cause a star or protostar to become gravitationally unstable and fragment into smaller stars. Keep in mind, however, that the IMF for high mass stars is more uncertain than low mass stars, and so we can expect theory of high mass stellar formation to be more uncertain. Note, that the vast majority of stars are not considered to be "high mass."

To the OP's question:

Does it mean that there are other ways by which a star can form? I mean, other than the gravitational collapse of gas and dust?

I interpret the language of the Wikipedia article as an attempt to being fair, in the sense that, gravitational collapse is by far the most likely formation scenario for most stars, especially for stars like our Sun, while leaving some room for unlikely yet possible alternatives in, perhaps finely tuned, regions of the parameter space. Another possibility that is considered are stellar mergers, but this also has issues when confronted with observations, and requires some fine tuning of binary hardening in clusters and interactions with tertiary components and is really only feasible in clusters with just the right density.

Answer 2

"Almost all" is likely just a writing mistake. However, I know of at least one other way to get stars that is not due to gravitational collapse and has been discussed in serious papers.

In (Adams & Laughlin 1997) it is discussed how dim red dwarf stars can form if two heavy brown dwarfs merge. This is an exceedingly rare and slow process, but the paper deals with the seriously long-term future of the universe where even rare events eventually come to pass. The timescale for converting free-floating brown dwarves to stars (that then burn out in a mere trillion years into white dwarfs) is on the order of $10^{16}$ years.

Answer 3

One possible mechanism to form (low-mass) stars, but more likely, brown dwarfs, is to have them form within the disk of another forming star.

i.e. Rather than form from a collapsing gas cloud they form in a disk of gas that is surrounding another forming star and which becomes susceptible to an instability that leads to the formation of another gravitationally compact object.

I think this is different only in a technical sense. One still expects such formation to be occurring along with star formation taking place in the "normal" way.

Answer 4

You can make a new star by collision from smaller objects. For example, if two brown dwarves collide.

In fact, the closest brown dwarf to us, the system Luhman 16, is a binary system, while other brown dwarfs have been known to have giant planets orbiting them. In the specific case of Luhman 16, the masses of the two brown dwarfs are determined to be:

• Between 8.0 × 10^28 kg and 1.0 × 10^29 kg, for the primary, and
• between 6.0 × 10^28 kg and 1.0 × 10^29 kg, for the secondary.

In other words, there’s an excellent chance that if these two failed stars, orbiting at about three times the Earth-Sun distance from one another, were to merge, they would form an actual star. In fact, any addition of mass that takes a failed star over that mass threshold to begin burning hydrogen in its core ought to do it.

The problem is that this takes a very long time even by astronomical standards. One could still have random collisions between brown dwarves, but that also takes a long time. Still, it is possible, hence we say "almost all" stars form by gravitational collapse.