Red dwarfs are the coolest, smallest, faintest type of actual star. However, because they are so faint, they burn their nuclear fuel in an extremely frugal manner, and thus will live far longer than any other type of star. By some estimates, a bottom-end red dwarf of 0.1 solar masses could live for as long as 10 trillion years.
It is thus very likely (I would say, nearly-certain) that the last actual star in our galaxy will have been a red dwarf.
It's hard to know exactly when the last star will be - but, there will eventually be one. The galaxy's supply of raw hydrogen is large but finite. It only comes to us from one place, the Big Bang, and there has only ever been the one Big Bang. Like all non-renewable resources, it will eventually run out. I'm fairly sure that I've seen papers arguing that the rate of star formation is already past its peak in our galaxy; there are other galaxies where star formation has already largely-stopped. (This lack of ongoing star formation is one of the defining features of so-called elliptical galaxies, perhaps an even more important aspect than their overall shape.)
So it seems plausible that the last star will have been a red dwarf - but, it may not still be a red dwarf at the time of its end.
We don't really know what happens to red dwarfs at the end of their lives; the Universe is still too young. We've never seen an old red dwarf, and it's extremely-doubtful that any human ever will. In fact, no star with a mass below 0.8 solar masses will have had time to reach the end of its life yet (unless it has been freakishly-unlucky and either collided with another one or perhaps been disrupted by a nearby supernova, or something). So, everything we think we know is based on projections from theory. Thus, our ideas could well be wrong (theory all-too-often is wrong), and the rest of this article should be treated as speculative.
However, theory does imply something a little interesting and (dare I say it) evocative.
Most stars will end their lives as red giants. Some high-mass stars end as supernovae, but such stars are very rare. (A galaxy the size of ours might average one supernova around every 25-30 years, and that's out of a population of stars numbering around 100 billion.) But what about red dwarfs? Are they different?
It turns out that most red dwarfs will have a red giant-like phase. Perhaps not as big and bloated as you'd get from a Sunlike star, but they'll eventually produce a recognisable red giant-like object. But, for stars under 0.25 solar masses, there never will be a red giant phase. They will eventually shift to fusing helium, but they won't expand much. Instead, their surfaces will heat up (source - PDF), turning them into what might be called "blue" dwarfs. Their final end-state will be a total luminosity around one third that of the Sun, which will persist for a few billion years, after which the star will finally fade out. (The end won't be sudden or damatic - rather, after a while there just isn't enough nuclear fuel left to keep the star warm, but its low mass means it can't contract far enough to trigger carbon fusion. The star then simply radiates away its remaining heat, effectively going straight from the Main Sequence to the white dwarf sequence, without ever ascending to the Red Giant branch. You might almost say that high-mass stars quit their jobs in the most ludicrous, over-dramatic fashion possible (think like that colleague you will have known, who flamed out of the office with an obscene Reply All email to their manager), solar-type stars work out their notice period, whereas bottom-end red dwarfs simply hang around until retirement.)
So for a few billion years, what had been red dwarfs will actually roughly approximate to Sunlike stars today. If they have any planets, currently circling out in the deep freeze, some of those will be considerably warmer, perhaps even to terrestrial levels. The obvious implications of this have been discussed elsewhere. (This time-frame will presumably also be the last time when life not-wholly-unlike us will still be possible.) But I think there's an aesthetic aspect, which perhaps has not been considered before.
Consider: the brighter blue dwarfs will be visible over much longer distances.
For the first since the end of star formation, there will be a night sky again, perhaps a little resembling the night sky we have now. It won't last forever, and there will come a time when no "new" blue dwarfs enter it. But at least for a time, the galaxy will flower again.
It is rather eerie to think that just before the final thermodynamic winter begins, the galaxy will have its final Indian summer.
The article examines the transition from M100 to NGC 7331, highlighting their structural similarities and significance in the study of spiral galaxies. It emphasizes how both systems illustrate key stages in galactic morphological evolution. Additionally, they serve as natural laboratories for understanding stellar dynamics and the development of spiral arms.
Behold the Mind Boggling Discovery of Cosmic Titan, Biggest Galaxy Supercluster Hyperion in Early Universe
Behold the Mind Boggling Discovery of Cosmic Titan, Biggest Galaxy Supercluster Hyperion in Early Universe
A worldwide group of astronomers has found a titanic structure in the early Universe, only two billion years after the Big Bang. This galaxy system proto-supercluster, nicknamed Hyperion, is the biggest and most huge structure yet found at such a remote time and separation. It has a mass evaluated at a million billion Suns.
Credit: Luis Calçada and Olga Cucciati/ESO
Elsewhere on the Internet, I got asked a question that touched on galactic structure. This is probably a high-risk question around me, really :) Since I wrote it, I figured I might as well share the resulting essay.
So, here are some comments on the strucutre of the Milky Way...
Stars in the galaxy can be split into several statistical groups. These groups are referred to as "populations". The populations are defined in three main ways. The first is velocity - the stars in a population will have some similar velocities. The second is chemistry - stars in populations have notable differences in their chemical composition. The third characteristic, and actually by far the least important, is the age of the stars. (The physical location is not really very important for population purposes, and in fact it can be actively-deceptive, so this isn't used for classification purposes.)
The reason why these populations exist is simple - the populations are an artifact of how the galaxy formed, and that in turn influenced the formation of the stars in them. The populations are best thought of as 'generations' of stars, if you will.
These classes are separate from the more-famous OBAFGKM(LTY) spectral classification sequence - that's essentially a proxy for stellar surface temperature.
So, the galactic components are as follows...
THE DISK STARS - POPULATION I
The most well-known grouping is the galactic disk (this is the one to which our Sun belongs). Disk stars all orbit in roughly the same plane, move in roughly the same direction and also have similar orbital speeds. (If they didn't do these three things, the disk would disintegrate as the stars scatter off in all sorts of random directions.)
Disk stars are the youngest of the three components of the galaxy. The galactic disk formed between 8-9 billion years ago. It is the site of ongoing star formation, and it also has the highest metallicity of all the populations.
('Metallicity' refers to the amount of matter inside a star which has an atomic number higher than helium. The significance of this is that [almost] anything heavier than helium definitely came from stellar fusion, and not from the Big Bang. Well, okay, there's a niggle - the Big Bang also made some lithium. In fact, the existence of lithium is a very powerful piece of evidence for the Big Bang, as it's next to impossible to make in stars. But, we usually ignore lithium as there's almost nothing of it.)
((As to how the metallicity is computed, it's done as a logarithmic fraction of the mass of metals as compared against solar abundances. Yes, this definition has its problems - non-solar abundances, anyone? do we even have the Sun properly-calibrated? signal-to-noise ratios? line-obscuration? etc. etc. - but we have to start somewhere. And this method does sort-of work, so it'll do for now.))
(((Also, the Americans really should have spoken to the astronomers before they tried putting Lithium-7 as a neutron absorber inside the Castle Bravo nuclear test. Yes, technically it did work - the lithium was quite happy to accept all those neutrons - but it didn't stop there, and they got several more megatons of 'bang' than they were expecting. We actually don't know for sure how many, because the oversized explosion destroyed a lot of the instruments. Lithium is not a toy, folks!)))
So, you can be reasonably sure that something is a disc star if;
1. its orbit is such that it stays in the galactic disk;
2. it's younger than 9 billion years;
3. it has a metallicity that's more than a few percent that of the Sun's.
THE GALACTIC BULGE - POPULATION IIish (SORT OF, MAYBE, KINDA - IT'S COMPLICATED!)
If you look up on a clear, dark night, you'll be able to see the Milky Way running through the sky. You're actually looking into the plane of the galaxy - the blurry fog-effect is because the star density is highest there. If you look toward Sagittarius, you'll see a fatter blob.
This is the galactic bulge.
The bulge is the fat bit in the middle of the galaxy, essentially.
You may sometimes hear talk about a supermassive black hole; the bulge is where it lives. (The presence of the supermassive black hole has been proven by the orbits of nearby stars - they're too fast, so there has to be an enormous mass inside a very small space.)
Incidentally, our supermassive black hole isn't actually that impressive, as these things go. It weighs in at a fairly-boring 3 million solar masses or so.
The bulge is actually ovoidal in shape. We see it at an angle of somewhere between 12 and 42 degrees from side-on (there's a lot of uncertainty about the exact geometry of the bulge). The bulge stars tend to older and lower-metallicity than the disk stars. There doesn't seem to be much star formation inside the bulge, although there are some puzzlingly-young stars close to the central black hole.
The bulge is probably the least well-understood of the three components. The difficulty comes from the direction it's in - to study it, we have to look through the disk of our own galaxy, and all those stars, gas and dust get in the way. It's literally a 'can't see the wood for the trees' problem!
As for how we know that a star is a bulge star, well, the defining characteristic is basically that it’s located in the bulge. Yes, there is some circular logic in this definition, and no, to my knowledge no-one’s really happy with it. But, because of the data-quality problems associated with studies of the bulge, we don’t really have a better definition :(
THE STELLAR HALO, A.K.A POPULATION II
The oldest category of star is found inside the galaxy's stellar halo.
The halo stars were the very first still-existing stars to form. They formed in a single, rapid spasm of star formation, between 12-10 billion years ago. This spasm happened because of the collapse of the primordial gas cloud that preceded the galaxy itself.
The halo is a rather odd beast.
First of all, it's much bigger than the galactic disk. You can find halo stars out to dozens of kiloparsecs above/below the disk. There is no distinct cut-off point. The disc has fairly-clear edges; the halo just gradually fades out into the intergalactic void.
Secondly, unlike the disc, the halo has no overall rotation. That is, there are roughly as many stars orbiting 'backwards' as there are stars orbiting 'forwards'.
Thirdly, relative to the speeds of disk stars, halo stars move very fast. They can have speeds into the hundreds of kilometres per second. (I'm glossing over a very confusing subtlety about velocity components here - briefly, disk stars actually move very fast in one single direction, and that's with the spin of the disk itself. Halo stars can, and do, move very fast along any axis.)
In fact, some halo stars have orbital speeds so high that they are close to escaping from the galaxy entirely.
Fourthly, there are no young halo stars. There is no active star formation in the halo. You can see this on the Hertzprung-Russell Diagram. The sequence that corresponds to the halo stars, the so-called 'subdwarfs' (because they lie a little below the Main Sequence in luminosity), cuts off at spectral type F. Brighter stars lead shorter lives, and factoring in the age of the halo, any stars brighter than about type F will have evolved off into red giants by now.
Because the halo stars are so ancient, they preserve the chemistry of the primordial, pre-galactic gas-cloud. The early universe had yet to experience very many supernovae or red giants, and consequently it contained very little in the way of heavy elements. Consequently, halo stars have extremely low metallicities. These can range from maybe a percent or so of the Sun's, down to thousandths of the Sun's.
(The last time I checked, the record-holder was a star called HE 1327-2326 -see Frebel et al. 2008, ApJ, vol. 684 - which may have had as little as less than a 1/100,000th the heavy-element content of the Sun.)
Halo stars spend most of their lives a long way out from the galactic disk. (If you lived on a planet orbiting one, the view would be quite amazing.) However, their orbits do swing down through the disc - to be an orbit, it has to be a complete loop! So, at any one time, there are thousands of halo stars passing through the galactic disk. In fact the closest one to us right now is Kapteyn's Star, at a distance of 12.76 LY.
The ratio of disc stars to halo stars is, as I recall, something like 3,000 to 1 - there are a lot more disc stars than there are halo stars. (That's what you get for having several billion more years of ongoing star formation!)
So, here's how you know a star is a halo star;
1. Low metallicity;
2. It doesn't orbit in the plane of the disc, and may even orbit 'backwards';
3. It's extremely old (>10 GYr).
POPULATION III
Lastly, a quick word about Population III, which you may have heard references to.
Population III is a hypothetical population of stars. They don't still exist, and they are not directly-observable (though there have been some claims about detection of scattered light from Pop III stars at high redshift).
Population III would literally have been the very first stars, ever. These would have been the ones that formed as soon as the Universe was cool enough/low-pressure enough to support star formation. They would have been quite different from today's stars - this is notable because there are no Population III red dwarfs.
(Red dwarfs - small, faint, cool stars of spectral class M - are actually by far the most common. Amongst both the disc and the halo, they account for something like 75-80% of all stars. Our Sun is actually in the top 5% for luminosity, believe it or not.)
While we don't know much about them, the best guess about what Pop III stars were like is essentially bloated, ravening, unstable monstrosities that all underwent supernova collapses within just a few million years.
We know that Population III stars must have existed because Population II stars have some metallicity. While they don't contain much metal, the Pop II stars do have some, and those metals must have come from somewhere.
SUMMARY
Well, long post is long, isn't it?
Okay, there are three points to take away from this unexpected astronomy lesson.
1. Stars can be categorised into several distinct groups.
2. These groups are differentiated by the orbits and the chemical compositions of the stars.
3. The galaxy is composed of three such groups - the halo, the disk and the bulge.
The Pan-STARRS1 Science Consortium has carried out a multi-color survey of the entire sky north of declination -30 degrees for more than 3 years using the Pan-STARRS1 telescope in Hawaii. The observing mission formally ends in March 2014 and the project is planning to release the data publicly in 2015 through the STScI archive. This meeting will present the breadth of science from Pan-STARRS1,…
