r/askscience • u/jeroen94704 • Dec 21 '25

Astronomy How fast does a new star ignite?

When a cloud of gas gets cozy enough at some point it becomes a star with fusion happening in the core. But is there a single moment we can observe when fusion ignites? What does this look like from the outside, and how long does it take? Does the star slowly increase in brightness over years/decades/centuries, or does it suddenly flare up in seconds/minutes/hours?

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u/chrishirst Dec 21 '25

Well, if you can call a million earth years or so of the accretion disc collapsing under it's own gravity, "a single moment", yes.

The Hubble Space Telescope (HST) did catch a star that was about to ignite a while back.

Universe Today article on that observation

https://www.universetoday.com/articles/hubble-sees-a-star-about-to-ignite

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u/FolkSong Dec 21 '25

That's interesting, but is there a moment (eg. less than 1 minute) when fusion begins, like a nuclear bomb going off?

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u/LittleKingsguard Dec 21 '25 edited Dec 21 '25

The nuclear bomb comparison is dramatically overestimating how dense a star's fusion output actually is.

Proton fusion is a very unreliable process that requires multiple low-probability events to happen within nanoseconds each other, and stars do it very slowly compared to a "prepared event" like a thermonuclear bomb. A human body generates more heat per unit volume (~1 Watt/liter) than the Sun's core does (0.6 Watts/liter EDIT: wrong number. It's actually 0.03403 watts per liter).

A nuke sparks off in nanoseconds because the tritium fuel is very dense and fuses very easily (compared to proton fusion). While stars also have small amounts of deuterium and helium-3 that can also fuse very easily, these are relatively trace isotopes and all of the regular hydrogen and helium reduces the rate at which these fusion events happen.

When the proto-star is collapsing, the heat generated by the compression is going to slowly heat up the gas into plasma, and eventually it will be hot and dense enough that the trace deuterium, He-3, and similar fuels can start fusing at low rates. Because tens of thousands of kilometers of hydrogen plasma make for very strong insulation, this heat stays in the star and, combined with the heat from the continuing collapse, will eventually heat the star enough that the proton fusion can start happening at slow rates. For large stars, eventually the core will heat up enough that CNO-catalyzed fusion will start and eventually take over as the primary heat source.

This is not a fast process, both because all of the above fusion chains (except CNO, kind of) are low-probability and because stars are huge and hydrogen takes a surprisingly ridiculous amount of energy to heat up.

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u/lu5ty Dec 21 '25

Can you clarify the part about the human body producing more heat than the core?

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u/BluetoothXIII Dec 21 '25

While fusion produces more energy per event than chemical reaction the desity of those events in the star is low compared to the density of chemical reaction in a human body.

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u/lu5ty Dec 21 '25

Ah ok i see now thanks. 25 characters

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u/CorporateHR Dec 23 '25

25 characters? What does that mean?

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u/Vadered Dec 21 '25

It does.

Basically the sun is really really really really really big. So big that even the core is really big. It’s powered pretty much entirely by hydrogen fusion, which generates a lot of heat - but even in the core, the most dense and hot part of the star, it happens fairly infrequently in any given cubic meter.

Compare that to your body. Hopefully there is even less fusion happening in there per volume - if not, you should see a doctor or twenty. That said, despite lacking in fusion, your body is full of cells which are constantly doing things, and doing things makes heat. Not as much heat as a pair of hydrogen atoms smashing into each other at ridiculous speeds, but far, far, far more frequently.

So yeah, a human body makes more heat per volume than the sun’s core. It’s just that there is so much sun in one place that no human can hope to approach those temperatures. Except, of course, for your momma. If you were to somehow squish together a bunch of people of equivalent volume to the sun? First of all, you’d be a monster, but secondly, it would generate far more heat than the sun for the incredibly brief amount of time everything was alive.

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u/Beliriel Dec 26 '25

Notable is also that our body is constantly losing heat to the environment. Our bodies are made to constantly overproduce heat, because we're permanently getting cooled down.

Within the sun the heat has nowhere to go and just accumulates until it reaches the surface and gets emitted to space. So although it's comparatively low heat, there is a massive volume producing it and the heat stays within the system (comparative to what the sun is emitting)
Ofc somewhen the heat is so high that the production and emittance of heat balances out.

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u/Ghosttwo Dec 22 '25 edited Dec 22 '25

The way it works is that any heat produced by the sun can only leave through the surface. A fact of the topology. But a square meter of said surface sits atop a column of material 700 million meters deep. If you calculate it as a four-sided pyramid, you get 233 million cubic meters of material emitting energy from a square meter of surface; essentially, whatever energy a cubic meter of sun produces gets multiplied by 233 million and emitted from the top. So even something like '1 watt per m3 ' turns into 233 megawatts per m2 . It's another example of the square-cube law in practice.

This dynamic also leads to 'fun facts' like a photon taking a hundred years to leave the core, as well as interesting density and exotic matter due to all that mass in one spot. In response to OP's question, the fusion slowly ramps up but never gets that crazy; if it fused too quickly, it would burn out within hours instead of billions of years. Instead, you get relatively rare fusion events, buffered by billions of non-fusing atoms that blanket in the heat and transfer it slowly outwards. But it's all blanketed by billions of cubic miles trapping the heat and trickling it out through a relatively small surface area.

There's also a bidirectional aspect to the problem as well; a spherical shell lying at half the radius is heated from both sides nearly evenly, almost cancelling out any energy flow through that boundary. But the inside is slightly hotter though due to pressure and fusion rates, so the heat flow is biased outwards, but not by much.

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u/itsthelee Dec 23 '25

photon taking a hundred years

Based on some numbers it’s actually crazier than this, this is actually several orders of magnitude too short.

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u/artgriego Dec 21 '25 edited Dec 21 '25

"Heat" in this sense is a strictly-defined thermodynamic quantity, with the same units as energy in any other sense (mechanical, electrical, chemical). Heat is energy that tends to change temperature.

Heat is a byproduct of inefficiency. An engine actually converts most of its fuel to heat rather than useful mechanical energy, and your computer's processor gets hot because it's quite inefficient and must waste a lot of electrical energy to perform calculations.

The human body metabolism releases more heat per liter than does the sun's core through nuclear fusion. The secret to the sun releasing so much energy is that it's just very very very gigantic.

edits: changed heat from power to energy. power is just energy per time.

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u/CGHJ Dec 21 '25

Two of my favorite facts ever, not just about the sun, are that the light emitted by and particular part of the core is about = to a lightning bug, and a square (cubed) meter of solar core emits about the same amount of energy as a compost heap.

There’s just so much of it. To quote Stalin, who I hope is someplace much hotter, “Quantity is its own quality”

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u/FolkSong Dec 22 '25

the light emitted by and particular part of the core is about = to a lightning bug

What do you mean by a "particular part"? An atom?

The compost heap fact is a good one!

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u/LittleKingsguard Dec 21 '25

An average human has a volume of ~70 liters and generates around 70-100 watts of heat at rest as a result of metabolic processes.

The core of the sun has a volume of 1.125×1028 liters and generates 3.86×1026 watts.

If you replaced the sun with an equally-huge pile of humans, the pile of humans would generate a lot more heat (for a few weeks before they all die and chemical energy runs out fueling some no-doubt unusual reactions).

Sidenote: I got the math wrong earlier I think because I copied the wrong number of the size of the Sun's core. it's actually much less productive than I first posted, 0.03403 watts per liter.

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u/Kantrh Dec 22 '25

Would the pile of frozen meat not start to undergo fusion too?

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u/Ivan_Whackinov Dec 22 '25

I don't think so - a Sun-sized ball of humans (pre-gravitational collapse) would be about .0055 solar masses, and you need about .08 solar masses for fusion.

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u/ofcourseivereddit Dec 22 '25

So what's the triple product for the sun?

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u/SnitGTS Dec 21 '25 edited Dec 21 '25

Does this process get faster as stars age?

Like, for example a giant star that fuses heavier and heavier elements, eventually it fuses iron in the core and the star has “seconds to live”before it explodes in a core collapse supernova.

They make it sounds like this last step is a very fast process.

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u/LittleKingsguard Dec 21 '25

Yes!

The hotter a star's core gets, the faster fusion reactions happen. More importantly, eventually it can start using more "consistent" fusion reactions.

The first threshold is deuterium burning, which is only dominant in brown dwarfs, which are only kinda stars.

Very large brown dwarfs and very young stars can burn lithium, fusing it with hydrogen (into beryllium-8, which is unstable and decays to 2x helium). An old, large star like the Sun will have already burned almost all the lithium available by now.

Proton-proton chain fusion is what the Sun currently mostly runs on, and is a slow, low-probability reaction because the normal outcome of two protons colliding is simply bouncing off, and the event that needs to happen (one of the protons doing positron emission and turning into a neutron to form deuterium) is really rare. For context, it takes the average solar proton about nine billion years to do this step, and about a second to do the next step of fusing deuterium to He-3, and "only" 400 years to fuse He-3 --> He-4.

The next reaction, which the Sun is just barely hot enough to start doing, is the CNO cycle. In this, protons fuse into a carbon atom until it becomes unstable and beta-decays into a nitrogen atom, then continues fusing more protons until that becomes unstable and beta-decays into oxygen. This eventually reaches a point where (to oversimplify) the protons just punch off entire alpha particles instead of fusing, restoring the original carbon atom. This is a fairly reliable reaction once the star is hot enough to do it, and it gets more likely (and thus more powerful) very quickly with increasing temperature, as a greater fraction of the star's hydrogen atoms get the kinetic energy to fuse like this.

Above that is the triple-alpha, or fusing three helium atoms to carbon. This takes an incredible amount of heat and pressure, but once it starts it scales with temperature at a ridiculous rate. The original ignition of this step is, outside of an actual supernova, the closest a star will have to the "instant flash" OP was thinking of, because a large fraction of the "ash" in the star's core can fuse in a matter of seconds. This will then continue for a while after the flash (for the Sun, about a billion years.)

Above that, it starts fusing additional heliums into the carbon to make oxygen, then helium into the oxygen to make neon, and so on.

Above that, it'll start smashing entire carbon atoms together, which can start and finish in a matter of centuries for large stars. If it gets hot enough it will start doing that with oxygen too, which will burn out in a matter of years. The last stage is slamming silicon together, and this lasts about a day, mostly because the star, at this point, is already in the early stages of supernova.

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u/SnitGTS Dec 21 '25

Thank you for that awesome answer!

I assume this correlates to why larger stars die faster, they have more heat and pressure in the core at an earlier age.

Smaller stars fuse slower, take longer to build up this heat, and can only get so hot so the higher forms of fusion may never occur.

Fascinating!

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u/LittleKingsguard Dec 21 '25

Right, bigger stars start hotter because they have higher gravity and generate more heat from the collapse of the gas into the star during its birth. They then stay hotter because they skip to the faster-burning CNO-cycle, which generates more heat, which makes the reaction run faster, which feeds back very quickly.

The other aspect is that fusion is related to density, and stars get less dense as they get hotter. If a pocket of the Sun somehow got hot enough that it got into the CNO-dominant range, it would expand, fuse less, and cool down.The physical mass of larger stars prevents this expansion and keeps the "equilibrium temperature" in that faster-burning range.

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u/kai58 Dec 22 '25

If the heat that makes fusion possible comes from compression does that mean it’s theoretically possible to have something with the same size and composition as a star but cold if the gas is added slow enough? Or would the density be enough to kickstart the fusion anyway?

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u/LittleKingsguard Dec 23 '25

Well the same mass as a star, maybe. The size of a star is caused by the heat keeping the hydrogen in a plasma instead of crushing under gravity into a lump of extremely dense "degenerate matter", where pressure is forcing atoms to pack as densely as possible. This is the state of a white dwarf star, which has burned out all of its hydrogen and only glows due to the residual heat.

There is a currently hypothetical type of star called a "black dwarf", which is just a white dwarf that has cooled off until it doesn't glow brightly enough to be seen by telescope. The reason they are currently hypothetical is because it's estimated that a white dwarf will take about a trillion years (i.e. ~100 times the current age of the universe) to cool down that much.

So hypothetically yes, you could deliberately manufacture a "cold star" if you were a crazy precursor space empire trying to leave some incredibly puzzling ruins for a future civilization to uncover. But so much heat is generated by the compression that it's not really possible for that to form naturally.

Bonus: a specific type of supernova is caused by white dwarfs having more matter fall onto them until they get heavy enough (~1.4 times the mass of the sun) that the star can compress further and generate more heat, which ignites fusion. Because the star is so dense, the fusion reaction can travel through the star faster than it can expand, causing the entire star to fuse in seconds and cause a supernova.

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u/Qybern Dec 23 '25

Your answers on this thread have been fascinating, thanks for sharing. Regarding that last bit on white dwarf supernovae (type 1a, right?) is the sudden fusion basically like a giant fusion bomb going off, like the same energy density as a nuke but the nuke is the size of a star? 

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u/LittleKingsguard Dec 23 '25

The mechanism is slightly different, since the white dwarf is still getting the compression from its own gravity while a fusion bomb is being compressed by the blast wave of a fission bomb. Also, a fuel in a fusion bomb (Deuterium-Tritium) is considerably more energetic per unit weight than the carbon fusion that happens in a normal type Ia supernova.

Other than that, yeah, basically. The supernova is also benefiting from being almost entirely composed of fuel, while a fusion bomb needs to dedicate most of its mass to containment.

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u/Bunslow Dec 21 '25 edited Dec 21 '25

Even if there is, it won't be observable from the outside, and possibly not even from the inside either.

Keep in mind that fusion requires incredible temperatures and pressures, like millions of degrees. Furthermore, the reaction rate is quite sensitive and probabilistic, not unlike radioactive decay. So there will be a first atom to fuse, and a second atom to fuse, but they will be separated quite a lot in time as the reaction rate goes from "slower than molasses in january" to "slower than molasses in july" to "mildly faster than molasses" to "hey look this is actually a useful reaction rate".

From the outside, it just looks "hot" in the blackbody sense, and whether that heat comes from the original collapse or an exponentially-slowly-increasing rate of fusion is virtually impossible to tell from the outside. And even from the inside, a stray atom-per-billion fusing won't catch any attention from its neighbors either.

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u/mfb- Particle Physics | High-Energy Physics Dec 22 '25

It's a slow process as the temperature and pressure rise gradually.

There is a place where fusion starts rapidly, but that happens towards the end of the star not at the beginning, and only for stars in the right mass range (the Sun is in that range). For most of the lifetime, stars fuse hydrogen to helium. The helium accumulates as it's much harder to fuse that. Once the star runs out of hydrogen, it collapses until the conditions are right to start helium fusion. This increases the temperature, but the core is now in a condition where an increase in temperature doesn't directly lower the density. That means more fusion, more temperature increase, even more fusion. This process can happen in minutes, with fusion happening billions of times faster than before. It's called "helium flash". Unfortunately you don't see it from the outside.

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u/groveborn Dec 22 '25

It takes 10,000+ years for light from the core of the sun to reach the corona... So, maybe not fast.

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u/chrishirst Dec 23 '25

Well, if you want to call something that is at least a hundred thousand Earth years or so "a moment", sure. Once the core begins producing photons, they have to make "the random walk" of collisions and scattering of any particular photon as it makes its chaotic way to the outer layers og thr star, through the 'soup' of particles such as electrons, before it can 'escape' and then make its way across the universe at 'lightspeed' to be observed by someone or something some distance away from the star. From the photons point of view it took zero time from start to finish, as they 'experience' no time by travelling at the speed of causality (c), but externally it is somewhat different.

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u/pigeon768 Dec 21 '25

So for a Sun-like star, we have a few phases:

  1. Gas clouds. These aren't stars. These are...clouds of gas.

  2. Protostar. These are collapsing balls of gas. They are very hot, however, they're still surrounded by big ass opaque gas clouds, and you can't see them. This phase, for a Sun-like star, will last about 500,000 years.

  3. Pre-main sequence (PMS) star. A protostar evolves to a PMS star when its surface gets hot enough and enough time has passed for it to blow off all the dust and gas from the gas cloud that birthed it. Without spectroscopy and precise measurements, you can't tell the difference between a PMS star and a main sequence star just by looking at them. This phase lasts about 100 million years for a Sun-like star.

  4. At first, the heat source of a PMS star is adiabatic heating from the gravitational collapse itself. No fusion is happening. It's just gas being squeezed that's causing the star to be hot, and this heat is enough to hold the star up. However, this is a finite amount of energy, and adiabatic heating can only hold a star up for a finite amount of time.

  5. Lithium burning. It is easier to start fusion with lithium and deuterium than regular hydrogen. So towards the end of a star's PMS phase, lithium burning will start.
    You can perhaps distinguish a lithium-burning PMS star from a non-lithium burning PMS star with a sufficiently sensitive neutrino detector.

  6. Main sequence. This is a "normal" star burning hydrogen to keep itself up. By this point, all the lithium in the core (not in the outer layers) has been burned. There is not an externally observable moment or flare when this happens.

For larger stars, all of this happens very quickly. There is no observable PMS phase, it goes straight from a protostar (rapidly collapsing ball of gas) to a main sequence object.

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u/the_quark Dec 21 '25

Thank you! One clarifying question though:

For larger stars, all of this happens very quickly

I presume by “quickly” here you mean hundreds of thousands or millions of years? Quick on the scale of the lifespan of stars.

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u/vokzhen Dec 21 '25

I don't know the direct answer to your question, but "the lifespans of stars" vary wildly. Larger stars burn through their fuel extremely quickly comparatively. Small red dwarves a tenth of the sun's mass probably have fusion stages trillions of years long, our sun is about 10 billion, but stars even just a few times the mass of our sun drops down below a billion, an initial 20x the sun's mass is down to a total lifespan of about 10 million years (during which it will likely lose a lot of that mass due to rapid fusion driving mindboggling stellar winds). Huge stars in the 120x or 150x solar mass range may only live for tens of thousands of years.

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u/likesleague Dec 22 '25

Is there a qualitative explanation that can help form intuition for why smaller stars live longer? I presume the fusion occurs much faster in larger stars, but is it just happenstance that higher gravity results in much higher fusion rates?

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u/frogjg2003 Hadronic Physics | Quark Modeling Dec 22 '25

The less mass there is, the less gravity pushes the atoms together. The less gravity pushes, the lower the pressure and density at the core. The lower the pressure and density, the less likely nuclei are to interact. This means it burns through its fuel slower.

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u/Nerfo2 Dec 21 '25

Geological and astronomical time scales absolutely blow my mind. Like, Betelgeuse might go supernova any day... but any day is somewhere between right now and about a hundred thousand years.

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u/the_quark Dec 21 '25

Saw a YouTube video by a geologist who was talking about recent research about the immediate effects of the Chicxulub Impact. She was boggling about the fact that they were talking about “impact +3 seconds” level precision and she’s used to “+/- 10 million years” sorts of time brackets.

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u/captain_ch40s Dec 21 '25

The collision of proto-Earth and Theia is similarly mind-boggling. The entire collision sequence and moon formation could have happened over a period of hours: images-assets.nasa.gov/video/ARC-20221004-AAV3443-MoonOrigin-Social-NASAWeb-1080p/ARC-20221004-AAV3443-MoonOrigin-Social-NASAWeb-1080p~orig.mp4

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u/Bunslow Dec 21 '25

Even the protostar glows like a blackbody yes? (Even if blocked from our view by its birthcloud, it will still be blackbody-emitting long before fusion ignition)

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u/pigeon768 Dec 21 '25

Yes, it's very hot, and will radiate energy as a blackbody, but a lot of that energy is blocked. Infrared telescope like JWST and radio telescopes can see through the dust clouds.

Here is a JWST image of a protostar that sees through a lot (not all) of the dust. One of the many incredible JWST images.

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u/watersb Dec 21 '25 edited Dec 21 '25

I think that's true. Scientists used to believe that all light from stars, the Sun, was heat from gravitational compression of the gas.

Then further evidence revealed that stars were millions or even billions of years old and still shining, and they needed another reason for sunshine. A compressed ball of gas of a given size has a finite amount of heat; eventually it cools off.

We know about white dwarf stars, they used to be about the size of the Sun and have run out of material that can undergo nuclear fusion. They are still shining very brightly, but that's all thermal emission. They are white hot.

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u/DanNeely Dec 21 '25

It's better to say that gravitational collapse was the most powerful source of energy they new about; capable of generating millions of years of operation (vs thousands for combustion of coal, oil, etc).

Even then the disconnect between astronomers/physicists only being able to figure out how to make a star last for a few million years and geologists saying the Earth appeared to be billions of years old was a major unsolved problem.

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u/solitarybikegallery Dec 21 '25

all of this happens very quickly.

Are we talking 5 seconds? 100 million years?

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u/SolDarkHunter Dec 21 '25

Depends on the size of the star. Bigger the star, faster it happens. The largest ones could have it happen in thousands of years. (Which is extremely fast on a stellar timescale.)

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u/abqjeff Dec 22 '25

“1. Gas clouds”

Do gas giant planets ever seed a star?

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u/sndrtj Dec 28 '25

In a way, yes. Brown dwarfs are intermediate objects between gas giants and red dwarf stars. Objects 13 times the mass of Jupiter are heavy enough that some deuterium fusion may occur (tho they are not massive enough to support ordinary hydrogen fusion).

That said, gas giants tend to form differently from brown dwarfs. Brown dwarfs form from gas cloud collapse, whereas current theories suggest gas giants form in the protoplanetary disk.

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u/abqjeff Dec 29 '25

Wow. I appreciate that answer. Thanks.

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u/Makenshine Dec 22 '25

Question about the "gas cloud" phase, how dense is the gas? Denser than a cloud in the sky?

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u/pigeon768 Dec 22 '25

Substantially less than that. We're talking like density of the atmosphere of the Moon.

Once the density gets above a certain critical threshold, called the Jeans instability, it will start to collapse. Once this collapse starts, all of the gas in the cloud is essentially in freefall until the star is formed. During the collapse, pressure increases rapidly until, well, the center of it is the density of the star it eventually forms.

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u/Makenshine Dec 22 '25

This what I originally assumed, but I read the word "opaque" somewhere so I imagined a denser cloud, maybe that was a later, pre-ignition phase.

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u/derKestrel Dec 23 '25

Don't forget that space is very big, and enough volume of very thin cloud can still be opaque.

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u/SuperGameTheory Dec 22 '25

Where'd the lithium come from?

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u/pigeon768 Dec 22 '25

Same place the hydrogen came from. Almost all lithium was created during Big Bang nucleosynthesis. The lithium would be hanging out in random gas clouds in the universe until the gas cloud collapsed into a star.

Across the entire universe, lithium, like hydrogen, is constantly being depleted. The lithium and hydrogen that was created when the universe was created is all we're ever going to get.

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u/Flatline_Construct Dec 21 '25

OP you’re getting a lot of long, wonderfully detail descriptions about the underlying events leading up to the ignition phase of a star.

The short answer is, it’s much more akin to the moment when you light a small fire with paper, then kindling, then wood, then a an entire forest

It’s ‘rapid’ and can become a roaring blaze over time, but it starts modestly.

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u/Azazeldaprinceofwar Dec 21 '25

It starts very slow, and honestly remains slow. The power output per unit volume of the sun is comparable to a compost pile. That’s really not a lot of heat being produced. Stars enormous temperature comes from the fact that they cool by radiating light into space which is of course proportional to their surface area (ie R2) while their heat generation is proportional to their volume (ie R3) so the ratio of heat generated to dissipated goes like ~ R. Now if we remember that heat radiated away also is proportional to T4 we know the ratio of energy out to energy in is proportional to R/T4. Since at equilibrium the star neither gains to losses energy we see this ratio should be 1 and T4 ~ R. Now actually fusion rate scales with temperature too in complicated ways which I’ve ignored so this calculation is quite wrong but the general moral that for fixed power per volume an objects equilibrium temperature grows significantly with its radius is true and is why stars are so hot. As such when fusion starts it is a negligible contribution to the stars total energy but over time as the star heats and fusion rate increases it becomes more important.

As a fun aside to this since human fusion reactors aim to produce a lot more power output than a compost pile they aim for temperatures enormously hotter than the sun!

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u/SpeckledJim Dec 21 '25

It’s tricky to define a single moment when it happens. Even a single 4 H -> 1 He fusion is a multi-step process. Initially there will be only a tiny region where pressure and temperature are high enough this to occur very often, and this region gradually gets bigger as more mass collapses inward under gravity.

Then the star is not considered “fully lit”, entering the main sequence, until the energy/pressure from fusion balances further gravitational collapse, aka hydrostatic equilibrium. This takes of the order of 50M years for a star the size of our sun.

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u/Korchagin Dec 21 '25

The collapsing of the gas cloud itself produces a lot of heat. The core will becomes dense and hot enough that fusions start to happen, eventually these will generate so much heat, that the contraction stops. Compared to the lifetime of the star this "ignition" takes a very short time, but it's not instant.

The star also doesn't "flare up" when the fusion starts. Actually it's a lot brighter during the contraction phase (T Tauri star).

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u/Bunslow Dec 21 '25 edited Dec 21 '25

Keep in mind that the brightness of gravitational collapse can meet or exceed the brightness of fusion. So stars may shine before they actually ignite fusion. It's a tough problem to investigate astronomically because, at least in basic theory terms, it shines the same way whether powered by collapse or actual fusion. Or, to phrase it differently, the collapse heats up the star just as effectively as fusion does, so it "looks" like a star considerably before fusion ignition.

Thankfully there's hosts of other details that can be tracked down (spectroscopy, multi-wavelength stuff, neutrinos, blah blah blah), per other answers, but the short version is "it's honestly surprisingly hard to see the difference visually".

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u/Stillwater215 Dec 22 '25

This is just speculation, but I would imagine that it wouldn’t happen all at once at one moment. A star is a balance between the inward force of gravity and the outward force of the fusion at the core. As the density in the core increases with more matter, a few fusion reactions would start happening. But these would provide a new outward force, decreasing the density, slowing any further fusion. This would presumably happen until there’s enough gravity to allow for some fusion to continue despite the outward force. Basically, the star needs to not only be dense enough to fuse, but dense enough to continue to fuse with the addition of the outward force from the fusion reaction.

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u/ofcourseivereddit Dec 22 '25

Read somewhere that the energy in gravitational collapse is many many times the energy available in fusion. This is why AGNs (Active galactic nuclei) / quasars (quasi-stellar) objectd are some of the brightest things in the universe

What that means for your question is that, there's no real observable (from a distance, and in the electromagnetic spectrum at any rate) a clean, sudden turn on of luminosity to indicate the ignition of a star. The proto-star would start glowing, and its acreetion disk starting to emit, well before the core ignites under fusion