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  1. Not too long ago, Pluto was "demoted" because it no longer met the criteria for being a full-fledged planet. Similarly, in order to be classified as a star (and not, say, a proto-star, or ... etc.), a cosmic body must meet a number of requirements.

  2. Imagine that your field binoculars are way out of focus and all you can see is undifferentiated blur. You start adjusting them, turning the knob ever so slowly, until suddenly there comes a moment when you can see shapes. They're still blurry, but they're definitely shapes: trees, houses, the sky, etc. Only a second ago you couldn't tell which was which, but now you can.

In that sense, has the birth of a new star ever been registered?

It is theorized that approximately three new stars are formed in the Milky Way each year. That is the official, approved, paradigm-friendly view.

Three new stars. Every year. That's 300 stars in the last hundred years.

No matter how slow the process of star formation may be, there must always come a moment when a budding star enters a registerable new phase.

The distance between this new star and the Sun and the appropriate time lapse are irrelevant, since three stars per year has been the standard rate for billions of years. It is therefore of no consequence whatsoever whether the star entered said phase in Aristotle's time or Moses' time or even before the Earth-as-a-planet became, cosmically speaking, a thing. Formation phase changes remain formation phase changes no matter how far in the past they occurred. They don't get depreciated with time.

A theory (as opposed to a hypothesis) requires a healthy evidence/conjecture ratio.

With all of the above in mind: has the birth of a new star ever been actually directly observed and registered? (I.e. does direct evidence in favor of the process actually exist?)

And if not, is it safe to say, that for all we know, star formation is no longer a thing in the Milky Way, and/or, for that matter, in the rest of the Universe?

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Connor Garcia
    Jul 9, 2022 at 17:46
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    $\begingroup$ @ConnorGarcia How very noble, courteous, and helpful of you! $\endgroup$
    – Ricky
    Jul 9, 2022 at 17:48

2 Answers 2

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Stellar evolution is not a fast process. And the Galaxy is HUGE and we see a tiny fraction of it.

Even once the protostar is formed, the evolution from the protostar to the main sequence star takes hundret of thousand of years and until it reaches the main sequence. As the star moves during its evolution over millenia along the Hayashi track, it changes luminosity gradually. Along this path there is not one single point where you can say "now it is main sequence". In terms of stellar evolution in the pre-main sequence a "moment" lasts thus several hundret to thousands of years - such stars are known.

Even if it was not such gradual and very slow process (like asking for "when did the tree grow between last year and now?), it would need constant observation. The Milkyway is huge, and even with Gaia's observations we only know one percent, thus a tiny fraction of its assumed 100 billion stars. So if there is 3 per year new and even if it were a descrete moment in time... we would need to look at that time in that direction with a sufficiently powerful telescope and the right instrumentation to actually detect these changes: we'd need to measure the brightness, and for correct classification also obtain spectra, thus our set of data of reasonably continuously observed objects is even much much smaller than the Gaia dataset.

Given this very slow evolution, and the extremely limited data set in terms of the overall ensemble, and the very low star birth rate of 300 / century, it's unlikely we know a single star which we observed often and detailed enough so that we can say "this was pre-main sequence back then and is main sequence now". It's similar difficult and unlikely as looking at a group of high-school students at the beginning of the year and end of the year and tell which of those became an adult in course of the year.

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    $\begingroup$ Thank you for the detailed answer. However, in the OP, I didn't even mention main sequence. The scenario is that during the million years or so when a star is forming, it remains undetectable by our instruments, and then suddenly someone trains the telescope on one particular spot, detects a very faint something, and goes, "Hey, that one wasn't there before. What gives?" $\endgroup$
    – Ricky
    Jul 8, 2022 at 8:41
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    $\begingroup$ Which is to say that, so far as the scientific method goes, there is no ACTUAL, empirical, palpable EVIDENCE that star formation is still very much a thing, only conjecture. $\endgroup$
    – Ricky
    Jul 8, 2022 at 8:43
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    $\begingroup$ @Ricky But star formation doesn't work like that! Stars don't suddenly start shining. Young protostars emit a lot of heat and light. It's hard to see the details of what's going on inside a big cloud of hot gas & dust, but infrared telescopes can get a bit more detail than optical scopes. As I said in the comments on your previous question, we know about numerous stellar nurseries in the Milky Way. And in those nurseries we can detect protostars at various stages of formation. $\endgroup$
    – PM 2Ring
    Jul 8, 2022 at 10:16
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    $\begingroup$ In order to underline what @PM2Ring says: there is no switch, identically as there is no switch when you are suddenly a grown up compared to being an adolescent. It transitions slowly from one to the other. $\endgroup$ Jul 8, 2022 at 11:11
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    $\begingroup$ Indeed, protostars get less luminous as they develop. $\endgroup$
    – ProfRob
    Jul 8, 2022 at 14:53
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We Can See Protoplanetary Disks

Numerous star forming regions have 'Proplyds' visible in them. These are the protoplanetary disks from which the star and its planets form. Here is an image of some Proplyds inside the Orion Nebula:

Protoplanetary Disk in the Orion Nebula

Here is an image of a very young star system showing a planet in the process of being formed out of the protoplanetary disk:

SPHERE image of PDS70 planetary formation. Image credit: ESO/A. Müller et al.

The young star is blacked out by a coronagraph so the planets can be imaged.

Also, there appears to be a very young star that has formed out of the remnants of Supernova 1987A, making it the youngest star we've yet seen.

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  • $\begingroup$ The images are computer-enhanced (i.e. photoshopped), aren't they? $\endgroup$
    – Ricky
    Jul 8, 2022 at 23:57
  • $\begingroup$ They are certainly processed, but not in a way that removes or creates any new data. If they were taken using monochrome cameras with filters, they would be combined to make a color image. But this is real imagery you are seeing, It's not like the black hole 'photograph' created out of radio data - these are real images, and the only processing would be color combination and stacking to remove noise. $\endgroup$
    – Dan Hanson
    Jul 9, 2022 at 0:36
  • $\begingroup$ If you are suggesting that these are being faked by some conspiracy, Proplyds have been imaged by many amateur astronomers. Here's an example: deepsky-visuell.de/Projekte/PPN.htm A 12" to 36" amateur scope will resolve them in Orion, And I can tell you that the processing done in those images is primarily just noise reduction from stacking multiole images together. No fakery. $\endgroup$
    – Dan Hanson
    Jul 9, 2022 at 0:37
  • $\begingroup$ @DanHanson I wanted to point out that the link in your last comment doesn't contain pictures of protoplanetary disks, but of protoplanetary nebulae, which is a misnomer that doesn't have anything to do with planets or young stars. Apart from that, great answer and beautiful pictures of protoplanetary disks! $\endgroup$
    – Prallax
    Jul 9, 2022 at 10:09

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