People are saying that The Pillars Of Creation have been destroyed, and will take 400-500 Million light years to reach us. Can someone explain how do we know it when the light hasn’t reached us?
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Looking around the galaxy we can see many examples of the sequence of events as stars form from these sorts of clouds, and have worked out an appropriate timeline for how how long it takes.
When we (well, scientists in the respective fields) apply this knowledge to something like the Pillars of Creation we see the state shown is a temporary condition, with a fairly defined lifespan before it turns from a dust cloud into a star field.
We then compare that length of time against how far away they are we can know that the actual events have already occurred, just the light from those events won’t reach us for a very long time.
Edit: As others have pointed out, the specific demise of the Pillars is an impending supernova. For a star to go supernova it needs to be pretty decently sized, and perhaps counterintuitively, larger stars actually have much shorter lifespans. For very large stars this can be a few million years, vs the billions of years our Sun has left.
As a star matures it starts burning different fuels in its core – first hydrogen, then helium, then carbon, then oxygen, then finally silicon. The duration of each state shortens, dramatically. It can fuse hydrogen for millions of years, helium for tens of thousands, carbon for a few hundred, oxygen for a matter of months, and silicon for about a day.
Once it starts fusing silicon the product (Iron) sucks energy out of the reaction, so the force from the ongoing nuclear explosion in the center can no longer hold back the enormous force of gravity, and the star collapses in on itself, resulting in a supernova.
If we see evidence; or a timeline that suggests, a star 6000 light years away is in the carbon burning stage we know it’s already gone through a supernova and some time in the next ~500 years we’ll see the result.
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