sillyworm 2 wrote: ↑
Wed Nov 13, 2019 1:04 pm
So much dust from so many dying stars.Wrapping my mind around this.Is this a steady accumalitive process or could there have been more stars dying earlier when the galaxy was just forming?
Well, astronomers talk about "cosmic noon, when star formation peaked in the Universe. This happened when the Universe was around 3.5 billion years old, or about 10 billion years ago.
Joanna Bridge of Astrobites
The last two decades of galaxy research have made it very clear that star formation in galaxies peaked at a redshift of z ~ 2, which occurred about 3.5 billion years after the Big Bang. In the approximately 10 billions years since then, the number of stars forming per year, or star formation rate, has been universally decreasing. This peak in star forming activity at z ~ 2 is often referred to as “cosmic noon.”
Star formation in the early Universe.
(Image: © NASA/ESA/ESO/Wolfram Freudling et al. (STECF))
What this means is that most galaxies, including NGC 3717 and the Milky Way, have most certainly formed most of their stars in the distant past. Of course, star formation didn't stop 3.5 billion years ago - not by any means! But for the Universe as a whole, the rate of star formation has declined during the last ten billion years. And for galaxies with overall red colors and little obvious star formation, we can be sure that they formed almost all their stars in the past.
As for gas and dust, that is an interesting question. The amount of gas has most certainly steadily decreased in the Universe.
That is because star formation robs the Universe of "free gas", as gas gets locked up inside stars. Massive stars give back most of their gas to the Universe as they explode as supernovas. Even stars a little more massive than the Sun, and stars no more massive than the Sun, give back a lot of gas to the Universe as they swell into red giants and then "slough off" much of their gaseous outer atmospheres, which are briefly lit up as shimmering planetary nebulas.
But most of the gas is locked up inside little red dwarfs, which are as common as dirt in the cosmos, and which jealousy guard their supply of gas without letting the Universe recycle it. To the best of our knowledge, not a single M-type red dwarf has ever "died a natural death" since the Universe was born in the Big Bang. A few may have fallen into black holes, others may have merged with other stars and become massive enough to actually evolve into red giants. But left on their own, the red dwarfs may live as long as the Universe.
So, as more stars form, less and less of the gas that was created in the Big Bang is available for star formation. Dust, by contrast, accumulates in the Universe, as various "metals" (elements more massive than hydrogen and helium) are formed inside stars, in supernova explosions and in processes going on inside M-type red giants.
Galaxy cluster Abell 2218. NASA, ESA, and Johan Richard (Caltech, USA).
Acknowledgement: Davide de Martin & James Long
So why is it that some galaxies appear to lack dust?
Take a look at galaxy cluster Abell 2218. You can see, if you look carefully, that while some of the galaxies do show clear signs of dust, many of the galaxies appear to be dust free. Yet we know that they must once have contained dust, because dust is a by-product of star formation and also of star death.
So where did the dust go?
The way I understand it, the dust is still there, but it is widely scattered and not gathered into clumps or dust lanes like they are in almost all spiral galaxies. The way I understand it, hot and violent processes related to black holes, particularly supermassive black holes, prevent the dust (and gas) from "settling down" and cooling, which is necessary for star formation. For myself, I picture the gas and dust in elliptical and lenticular galaxies as being constantly battered by a relentless hot wind.