Milky Way's graveyard of dead stars found | Hacker News Reader

Milky Way's graveyard of dead stars found

phys.org

104 points8bitsrule20 comments

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The title uses the word "found", which is an exaggeration.

This is a _theoretical_ result, as the very last paragraph suggests.

From the first paragraph of the actual article [1]:

> We chart the expected Galactic distribution of neutron stars and black holes.

(emphasis mine)

[1] https://academic.oup.com/mnras/article-abstract/516/4/4971/6...

But we repeat these theoretical results until they take on "factual" weight, until that awkward moment when the theoretical results go under the bus of empirical observations.

Great model, though.

Agreed, by figuring out where to look, we are far more likely to actually find (or not find) instances and then update the model accordingly.

Could such dead matter account for any of the missing mass attributed to Dark Matter? Very vaguely, the dead matter distribution looks vaguely like a DM halo around the galaxy.

Unlikely, dead dark stars being dark matter candidates is a theory known as MACHOs (massive compact halo objects)

If they exist we would expect to detect them as they pass between another star and earth creating a gravitational lens. You can estimate the number of these MACHOs by looking at a bunch of stars for a long period of time and counting the number of microlensing events you see.

A team did this in 2000 and found that while there were some events, there weren't enough to explain all of the dark matter around the Milky Way. https://arxiv.org/abs/astro-ph/0001272

Could also be some mix of some of the proposed answers, and possibly even things that haven't yet been considered. To contrast, sort of like performance improvements in new CPUs aren't any one single thing but a lot of little incremental improvements.

The graveyard is outside traditional boundaries of Milky Way. So the stars there do not contribute to microlensing.

That's not how gravity works. The "kicks" from star death aren't shooting them to the outside of the galazy where they just park.

The "kicks" are just tranforming the galactic orbital path from the typical "visible star path" to a different set of paths. The objects still would pass through the visible layer at some point, so the lensing events would still occur.

You know, assuming the described calculated results are true.

When the remains form surely they contribute to microlensing. But after they out of the visible Galaxy boundary, they stop. And given the end up 2-3 times from the boundary, they spent the vast majority of the time there without contributing to lensing.

Now consider that according to the paper 30% is going to leave the Galaxy. Those after the initial travel will never contribute to anything.

Dark matter is probably a grouping of several phenomena like this. Should not discount MACHOs just because it doesn't explain all the dark matter observations. I think I remember reading in Carroll's Astrophysics Intro that MACHO's can explain about 10-20% of dark matter. Dark matter could be potentially be explained fully by multiple dark matter explanations, each adding to the overall phenomena.

As fn-mote mentioned in another comment, they didn't "find" anything. This is a theoretical result that supposedly determines where something is that they already knew must exist somewhere. No new mass has been discovered.

The title is very misleading - I'd say deliberately so.

This was my first question.

Fun exercise: calculate the mass of 'missing dark matter' in the proposed mass of the Milky Way. Divide by the volume of a sphere of the same radius. It doesn't take much invisible matter - space is big.

> Supernova explosions are asymmetric, and the remnants are ejected at high speed—up to millions of kilometers per hour—and, even worse, this happens in an unknown and random direction for every object.

Doesn't this influence their usefulness as standard candles for measuring long distances? Or is luminosity of the supernova not affected by this?

Hopefully someone more knowledgeable about this chimes in but I believe calculating the redshift by knowing the expected spectrum and comparing to the observed spectrum gives us the info needed for distance (since along the way the light is constantly stretched by the expansion of the universe). From Wikipedia on supernovae:

> Because of the expansion of the universe, the distance to a remote object with a known emission spectrum can be estimated by measuring its Doppler shift (or redshift); on average, more-distant objects recede with greater velocity than those nearby, and so have a higher redshift.

And a bit on Hubble's law / the early discoveries around cosmological redshift: https://en.m.wikipedia.org/wiki/Hubble%27s_law

Only certain supernovas are usable as standard candles, like type 1A.

https://en.wikipedia.org/wiki/Type_Ia_supernova

Just in case, since your statement is ambiguous.[1]

[1] https://en.wikipedia.org/wiki/Category:Standard_candles

Does this change the numbers we get from our own galaxy for the existence of dark matter and/or dark energy ?

All we need now are grave robbers. /s

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