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Comment by PeterCorless | Hacker News Reader

We can spot the orphan stars at this distance. However, Euclid needs to turn to closer targets than the Pegasus cluster to spot rogue planets.

https://www.theguardian.com/science/article/2024/may/23/eucl....

In this case, it found dozens of rogue planets in the Orion nebula, which is only 1,500 ly away.

I am presuming there are also going to be great numbers of rogue planets in deep space, not tied to any star or galaxy. But there would be no way for Euclid to spot them at that distance. Stars were hard enough.

I presume, in due time, there will be some sort of calculable estimation or projection of orphan stars and rogue planets per cubic parsec or kilosparsec.

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It would be interesting to calculate how much these stars contribute to the mass deficit of the universe.

Probably not that much relatively.

Specifically the Perseus cluster reported on here has a mass of around 10^15 solar masses in total. Of that ~10% is in the form of stars, 20% is intergalactic gas and the remainder is dark matter.

The report here says 1.5 trillion of these stars in this cluster which is ~10^12 solar masses (assuming average star mass is similar to the sun's.)

So the newly observed stars reported here are around 0.1% of the total cluster mass, or around 1% of the total mass of stars in the cluster. All very approximate of course.

Not just stars, it implies there’s a lot of planets, asteroids, etc tossed out there.

I suspect in any solar system, the mass of the non-stars are a rounding error compared to the whole system's mass.

I suspect the current contents of a solar system is a poor proxy for everything that’s been ejected from it. Pluto is 0.0006 light-years from the sun, there’s a great deal of space between stars we don’t know that much about.

Further we dramatically underestimated the number of expo planets for decades. Quite reasonably we don’t put a lot of weight on stuff we can’t detect.

The sun accounts for 99.9+% of the solar system mass if I recall correctly

Close it’s ~99.85% excluding the ort cloud. However there’s a major bias towards smaller stars because they last longer though conversely they are harder to detect.

> I suspect in any solar system, the mass of the non-stars are a rounding error compared to the whole system's mass.

A brown dwarf orbiting a red dwarf would beg to differ.

Which are both "low mass" compared to other stars out there

It might even be possible to have this combination weigh less than our sun

Is the brown dwarf not considered a star? I don't know.

“A brown dwarf is an object which is made of the same stuff as stars, but does not have enough mass for hydrogen fusion (combining hydrogen atoms into helium atoms). Hydrogen fusion is what makes stars glow. Brown dwarfs are not massive enough to do this, so they are not stars. On the other hand, they are not regular giant planets, because they do glow.”

https://simple.wikipedia.org/wiki/Brown_dwarf

While very young they generate heat from both deuterium fusion but that only lasts millions to tens of millions of years. Energy from radioactive decay and their slow contraction lasts for much longer.

The nebulous space the Brown dwarf occupies in terms of classification is fascinating to say the least. I'm a passing not even hobbyist on the subject but that they glow but not like that is really interesting.

When I think of the brown dwarf I am also reminded of the hypothesized black dwarf. This would be the stage after white dwarf. It's hypothesized because it takes longer for the white dwarf bodies to cool than exists presently in the history of the universe. It's only a handful of sentences and will largely repeat the previous paragraph but there's the wiki page for black dwarf.

https://simple.wikipedia.org/wiki/Black_dwarf

Very interesting stuff all around. Space is so fucking cool, y'all.

>We can spot the orphan stars at this distance.

Can we resolve these individual orphan stars? Or just see the cumulative glow from a lot of them?

Note that there are two different announcements today, and I need to clarify my comment above.

• One piece cited viewing 1.5 trillion stars in the Perseus cluster of galaxies, which is 240 million ly distance: https://www.nottingham.ac.uk/news/first-pictures-from-euclid...

• The other cited spotted stars in galaxy cluster Abell 2390, which is in the Pegasus constellation, which is 2.7 billion ly away — an order of magnitude further away: https://www.esa.int/Science_Exploration/Space_Science/Euclid...

If I am reading this correctly, it seems like Euclid was able to see the light from individual stars ripped from their galaxies in Abell 2390. Which is quite the accomplishment.

Please let me know if I am reading too much into this, or reading it incorrectly.

Abell clusters are really cool to image if you can see some of the closer ones. The deer tick galaxies are really neat

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