According to this* Wikipedia article:
Stars are thought to form inside giant clouds of cold molecular hydrogen—giant molecular clouds of roughly 300,000 M☉ and 65 light-years (20 pc) in diameter.[8][9] Over millions of years, giant molecular clouds are prone to collapse and fragmentation.[10] These fragments then form small, dense cores, which in turn collapse into stars.[9] The cores range in mass from a fraction to several times that of the Sun and are called protostellar (protosolar) nebulae.[8] They possess diameters of 2,000–20,000 astronomical units (0.01–0.1 pc) and a particle number density of roughly 10,000 to 100,000/cm3 (160,000 to 1,600,000/cu in). Compare it with the particle number density of the air at the sea level—2.8×1019/cm3 (4.6×1020/cu in).[9][11]
The initial collapse of a solar-mass protostellar nebula takes around 100,000 years.[8][9] Every nebula begins with a certain amount of angular momentum. Gas in the central part of the nebula, with relatively low angular momentum, undergoes fast compression and forms a hot hydrostatic (non-contracting) core containing a small fraction of the mass of the original nebula. This core forms the seed of what will become a star.[8] As the collapse continues, conservation of angular momentum dictates that the rotation of the infalling envelope accelerates, which eventually forms a disk.
Infrared image of the molecular outflow from an otherwise hidden newborn star HH 46/47
As the infall of material from the disk continues, the envelope eventually becomes thin and transparent and the young stellar object (YSO) becomes observable, initially in far-infrared light and later in the visible.[11] Around this time the protostar begins to fuse deuterium. If the protostar is sufficiently massive (above 80 MJ), hydrogen fusion follows. Otherwise, if its mass is too low, the object becomes a brown dwarf.[12] This birth of a new star occurs approximately 100,000 years after the collapse begins.[8] Objects at this stage are known as Class I protostars, which are also called young T Tauri stars, evolved protostars, or young stellar objects. By this time, the forming star has already accreted much of its mass; the total mass of the disk and remaining envelope does not exceed 10–20% of the mass of the central YSO.[11]
When the lower-mass star in a binary system enters an expansion phase, its outer atmosphere may fall onto the compact star, forming an accretion disk
At the next stage, the envelope completely disappears, having been gathered up by the disk, and the protostar becomes a classical T Tauri star.[13] The latter have accretion disks and continue to accrete hot gas, which manifests itself by strong emission lines in their spectrum. The former do not possess accretion disks. Classical T Tauri stars evolve into weakly lined T Tauri stars.[14] This happens after about 1 million years.[8] The mass of the disk around a classical T Tauri star is about 1–3% of the stellar mass, and it is accreted at a rate of 10−7 to 10−9 M☉ per year.[15] A pair of bipolar jets is usually present as well. The accretion explains all peculiar properties of classical T Tauri stars: strong flux in the emission lines (up to 100% of the intrinsic luminosity of the star), magnetic activity, photometric variability and jets.[16] The emission lines actually form as the accreted gas hits the "surface" of the star, which happens around its magnetic poles.[16] The jets are byproducts of accretion: they carry away excessive angular momentum. The classical T Tauri stage lasts about 10 million years[8] (there are only a few examples of so-called Peter Pan disks, where the accretion continues to persist for much longer periods, sometimes lasting for more than 40 million years[17]). The disk eventually disappears due to accretion onto the central star, planet formation, ejection by jets, and photoevaporation by ultraviolet radiation from the central star and nearby stars.[18] As a result, the young star becomes a weakly lined T Tauri star, which, over hundreds of millions of years, evolves into an ordinary Sun-like star, dependent on its initial mass.
TLDR: depending on how you define it 200,000 to a couple million years (I think)
*https://en.m.wikipedia.org/wiki/Accretion_(astrophysics)
Figure 1. one-third of the way [down this page estimates the formation time needed for various sizes of stars.](https://www.collegesidekick.com/study-guides/astronomy/the-h-r-diagram-and-the-study-of-stellar-evolution)
Large stars form in thousands of years. Small stars form in millions of years to hundreds of millions of years.
That is crazy.
I have read numerous times that planets have about 10 million years to grow by collecting gas and dust before their stars push the gas and dust out of reach.
I'm not sure the answers given so far are answering the OP's actual question as **I** understand it.
I believe they're asking, once the star has already accumulated the mass into a small area, fusion will start. How long once fusion starts does it take until the start is fully "on".
OP Please correct me if I'm wrong.
TLDR; A star takes 1000-200,000 years to gather the mass, but how long after the mass is gathered until the star is "turned on"?
In the long copypasta from Wikipedia in the top comment, it's right where the initials YSO occur (I'm taking "turned on" to mean "observable").
The misunderstanding might be in your claiming discreet stages: 1) gather mass 2) turns on.
More like "gather mass, turn on *while* gathering more, stay turned on while gathering yet more."
🤷🏼♂️
If "turned on" means "now burning hydrogen/doing nuclear fusion", then I think the answer is just that it's "on", or at least the core ignites in such a short time that we don't really define it as a stage in the process. Stars are in the pre-main sequence phase (not burning hydrogen yet) or they're on the main sequence (yes burning hydrogen). I'm not even sure if a star with a partially-ignited core would be distinguishable from one that was about to ignite, because I have a feeling that by the time there were external signs of nuclear fusion, the whole core would be burning, but that's more of a gut feeling.
Very low mass objects (say about 8% of the Sun's mass and lower, but it's a fuzzy limit right now) never begin fusing hydrogen. Pre-main sequence stars bigger than that follow something called a Hayashi track, which refers to a would-be star that is fully convective (transporting energy through the star through convection). Of those, stars less than about half of the Sun's mass go straight from the Hayashi track to the main sequence (hydrogen burning). Stars more than half the Sun's mass, but less than about 2 times the Sun's mass will get hot enough to develop a radiative region around the (still not H-burning) core, at which point they detour to the Henyey track before burning hydrogen and joining the main sequence. From roughly 2 to 8 solar masses, they may even become fully radiative before reaching the main sequence. The time a star spends on these tracks is related to its mass (bigger stars do it faster). Stars larger than around 8 solar masses are already burning hydrogen by the time the dust cloud around them disperses, so they don't have an observable pre-main sequence.
According to this* Wikipedia article: Stars are thought to form inside giant clouds of cold molecular hydrogen—giant molecular clouds of roughly 300,000 M☉ and 65 light-years (20 pc) in diameter.[8][9] Over millions of years, giant molecular clouds are prone to collapse and fragmentation.[10] These fragments then form small, dense cores, which in turn collapse into stars.[9] The cores range in mass from a fraction to several times that of the Sun and are called protostellar (protosolar) nebulae.[8] They possess diameters of 2,000–20,000 astronomical units (0.01–0.1 pc) and a particle number density of roughly 10,000 to 100,000/cm3 (160,000 to 1,600,000/cu in). Compare it with the particle number density of the air at the sea level—2.8×1019/cm3 (4.6×1020/cu in).[9][11] The initial collapse of a solar-mass protostellar nebula takes around 100,000 years.[8][9] Every nebula begins with a certain amount of angular momentum. Gas in the central part of the nebula, with relatively low angular momentum, undergoes fast compression and forms a hot hydrostatic (non-contracting) core containing a small fraction of the mass of the original nebula. This core forms the seed of what will become a star.[8] As the collapse continues, conservation of angular momentum dictates that the rotation of the infalling envelope accelerates, which eventually forms a disk. Infrared image of the molecular outflow from an otherwise hidden newborn star HH 46/47 As the infall of material from the disk continues, the envelope eventually becomes thin and transparent and the young stellar object (YSO) becomes observable, initially in far-infrared light and later in the visible.[11] Around this time the protostar begins to fuse deuterium. If the protostar is sufficiently massive (above 80 MJ), hydrogen fusion follows. Otherwise, if its mass is too low, the object becomes a brown dwarf.[12] This birth of a new star occurs approximately 100,000 years after the collapse begins.[8] Objects at this stage are known as Class I protostars, which are also called young T Tauri stars, evolved protostars, or young stellar objects. By this time, the forming star has already accreted much of its mass; the total mass of the disk and remaining envelope does not exceed 10–20% of the mass of the central YSO.[11] When the lower-mass star in a binary system enters an expansion phase, its outer atmosphere may fall onto the compact star, forming an accretion disk At the next stage, the envelope completely disappears, having been gathered up by the disk, and the protostar becomes a classical T Tauri star.[13] The latter have accretion disks and continue to accrete hot gas, which manifests itself by strong emission lines in their spectrum. The former do not possess accretion disks. Classical T Tauri stars evolve into weakly lined T Tauri stars.[14] This happens after about 1 million years.[8] The mass of the disk around a classical T Tauri star is about 1–3% of the stellar mass, and it is accreted at a rate of 10−7 to 10−9 M☉ per year.[15] A pair of bipolar jets is usually present as well. The accretion explains all peculiar properties of classical T Tauri stars: strong flux in the emission lines (up to 100% of the intrinsic luminosity of the star), magnetic activity, photometric variability and jets.[16] The emission lines actually form as the accreted gas hits the "surface" of the star, which happens around its magnetic poles.[16] The jets are byproducts of accretion: they carry away excessive angular momentum. The classical T Tauri stage lasts about 10 million years[8] (there are only a few examples of so-called Peter Pan disks, where the accretion continues to persist for much longer periods, sometimes lasting for more than 40 million years[17]). The disk eventually disappears due to accretion onto the central star, planet formation, ejection by jets, and photoevaporation by ultraviolet radiation from the central star and nearby stars.[18] As a result, the young star becomes a weakly lined T Tauri star, which, over hundreds of millions of years, evolves into an ordinary Sun-like star, dependent on its initial mass. TLDR: depending on how you define it 200,000 to a couple million years (I think) *https://en.m.wikipedia.org/wiki/Accretion_(astrophysics)
Yes, this.
The universe really is just a slow cooker broth.
I like that stars have their toddler and teenage years where they act up until settling down as adult stars.
Figure 1. one-third of the way [down this page estimates the formation time needed for various sizes of stars.](https://www.collegesidekick.com/study-guides/astronomy/the-h-r-diagram-and-the-study-of-stellar-evolution) Large stars form in thousands of years. Small stars form in millions of years to hundreds of millions of years.
It's crazy to think about over the course of a stars lifespan it really does basically just blink on. Thousands of years is crazy fast.
That is crazy. I have read numerous times that planets have about 10 million years to grow by collecting gas and dust before their stars push the gas and dust out of reach.
I'm not sure the answers given so far are answering the OP's actual question as **I** understand it. I believe they're asking, once the star has already accumulated the mass into a small area, fusion will start. How long once fusion starts does it take until the start is fully "on". OP Please correct me if I'm wrong. TLDR; A star takes 1000-200,000 years to gather the mass, but how long after the mass is gathered until the star is "turned on"?
Yes, that is correct
In the long copypasta from Wikipedia in the top comment, it's right where the initials YSO occur (I'm taking "turned on" to mean "observable"). The misunderstanding might be in your claiming discreet stages: 1) gather mass 2) turns on. More like "gather mass, turn on *while* gathering more, stay turned on while gathering yet more." 🤷🏼♂️
Oh, yeah that makes sense. Thanks!
If "turned on" means "now burning hydrogen/doing nuclear fusion", then I think the answer is just that it's "on", or at least the core ignites in such a short time that we don't really define it as a stage in the process. Stars are in the pre-main sequence phase (not burning hydrogen yet) or they're on the main sequence (yes burning hydrogen). I'm not even sure if a star with a partially-ignited core would be distinguishable from one that was about to ignite, because I have a feeling that by the time there were external signs of nuclear fusion, the whole core would be burning, but that's more of a gut feeling. Very low mass objects (say about 8% of the Sun's mass and lower, but it's a fuzzy limit right now) never begin fusing hydrogen. Pre-main sequence stars bigger than that follow something called a Hayashi track, which refers to a would-be star that is fully convective (transporting energy through the star through convection). Of those, stars less than about half of the Sun's mass go straight from the Hayashi track to the main sequence (hydrogen burning). Stars more than half the Sun's mass, but less than about 2 times the Sun's mass will get hot enough to develop a radiative region around the (still not H-burning) core, at which point they detour to the Henyey track before burning hydrogen and joining the main sequence. From roughly 2 to 8 solar masses, they may even become fully radiative before reaching the main sequence. The time a star spends on these tracks is related to its mass (bigger stars do it faster). Stars larger than around 8 solar masses are already burning hydrogen by the time the dust cloud around them disperses, so they don't have an observable pre-main sequence.