APOD: Habitable Worlds (2014 Mar 03)

[Chris Peterson]

In one documentary, it was stated that, in our galaxy, red dwarfs outnumber main sequence stars by 3 to 1, so they represent numerous opportunities for things to not be so easy for life to arise.

Not sure what they meant by that. Red dwarfs are main sequence stars. Because of their low mass, they are very long lived; while they may be any age, the oldest are probably nearly as old as the Universe, and still haven't left the main sequence. Many red dwarfs have planets. Habitability, of course, is largely a matter of definition at this point. Planets around red dwarfs are likely to be very different from Earth, which suggests that any life is likely to be quite different, as well.

[chuckster]

Aren't there certain conditions under which a star collapses but doesn't lose enough mass to change the orbits of its planets ?
Is that true of a red dwarf ? Or will things get rearranged in that star system when the star goes dwarf, as in some planets flying away and others finding new, closer orbits ? I suppose you could postulate a former frozen planet thawing out now and providing the liquid water to get things going. And how much heat could circulate to the cold side via, say, atmospheric movement ? Probably some nasty winds.
I wonder if life that developed around deep sea volcanic geysers could survive such a radical change in the parent star whose light never penetrated to their depth. Since the whole star system got old, would a red dwarf star have planets that still have molten centers, or would they be so old that they've cooled off, and their magnetic shields are gone ? Oh well, some of this is going on somewhere out there. If a red dwarf lives so much longer than it's former yellow self, I guess you' would have to consider it part of the main sequence.

[Ann]

chuckster wrote:
Aren't there certain conditions under which a star collapses but doesn't lose enough mass to change the orbits of its planets ?

I think you are talking about a Type II (a core-collapse) supernova. A core-collapse supernova is a product of a massive star, many times more massive than a red dwarf, whose core has built up heavier and heavier elements until no more energy can be extracted from further fusion, and therefore the core collapses and the star explodes.

Typically a Type II supernova explosion will leave behind a neutron star or a pulsar with a mass a little more than 1.4 solar masses. This is still a lot more than the typical mass of a red dwarf.

It isn't known what happens to the planets that were in orbit around the star that later went supernova. Maybe the planets survived, maybe not. New planets may form from the massive amounts of dusty debris that are produced by the supernova explosion.

Or will things get rearranged in that star system when the star goes dwarf, as in some planets flying away and others finding new, closer orbits ?

Our own Sun became a so called "yellow dwarf" at the point when it got hydrogen fusion going in its core. At that point, the Sun entered the main sequence, which is the time in the life of a star when it produces energy by fusing hydrogen to helium in its core.

Red dwarf stars are also main sequence stars, because they, too, fuse hydrogen to helium in their cores. But red dwarfs are always less massive than the Sun. A typical mass of a red dwarf may be half the mass of the Sun, but it may contain as little as about 8% the mass of the Sun.

Because red dwarf stars don't contain a lot of mass, they accrete mass slowly and grow slowly. It takes a long time for them to get their hydrogen fusion going.

Planets, which are even less massive than red dwarfs, probably form even more slowly than the red dwarfs themselves. To my knowledge, the onset of hydrogen fusion in a red dwarf isn't likely to upset the orbits of any planets, for two reasons. I don't think that the onset of hydrogen fusion is a very violent process (certainly not in comparison with the titanic explosion of a supernova), and in any case, the newborn red dwarf may not yet have any planets.

However, planetary scientists believe that planets may migrate so that they end up closer to or farther away from the star than the position in the proto-planetary disk where they were born. It is also true that planets may be kicked out of the solar system where they were born.

I suppose you could postulate a former frozen planet thawing out now and providing the liquid water to get things going.

There has been speculation that, say, Saturn's largest moon Titan might become nice and warm after the Sun has become a red giant. When that happens, the Earth will be burnt to a crisp. But when we are talking about red dwarfs, the stars themselves are not going to grow much larger or become a lot more energetic for hundreds of billions of years. Of course a planet orbiting a red dwarf may migrate inside its own solar system, so that it ends up in the red dwarf's habitable zone. But remember that the habitable zone of the red dwarf (where water may become liquid) is so small that a planet moving in there will become tidally locked.

If a red dwarf lives so much longer than it's former yellow self, I guess you' would have to consider it part of the main sequence.

A star that is a red dwarf has never been a yellow dwarf in the past, and it will never become a yellow dwarf in the future, unless it, for some reason, stars to accrete a lot of mass.

But certainly, a red dwarf star fuses hydrogen to helium in its core, and therefore it does, indeed, belong to the main sequence.

Ann

[Anthony Barreiro]

...
In one documentary, it was stated that, in our galaxy, red dwarfs outnumber main sequence stars by 3 to 1, ... .

This statement doesn't make sense to me. Red dwarves are main sequence stars, happily fusing hydrogen into helium for tens of billions of years. I.e., they're going to be on the main sequence long after bigger stars like our Sun have run out of hydrogen and died. Just as ants outnumber elephants, red dwarves outnumber larger main sequence stars. But ants and elephants are both animals, and red dwarves and stars like our Sun are both main sequence stars.

[chuckster]

Got it, and thanks. Main Sequence refers to H to He conversion, which can initiate at red dwarf levels or blue supergiant, etc levels. I've spent many a lunch hour following the links in APOD writeups, but some basics seem to get lost on me in the rush. I'd thought that red dwarfs were always just the remains of bigger, hotter stars that died. But I guess all gas clouds aren't created equal, and the collapse, if and when it comes, doesn't always produce a nice yellow (or hotter) sun.

Count me among those who believe life is everywhere out there. I just hope the universe leaves us alone a little longer, and lets us catalogue our extrasolar microbes, slimes and invertebrates, before we have to contend with another technical civilization !

[Ann]

Got it, and thanks. Main Sequence refers to H to He conversion, which can initiate at red dwarf levels or blue supergiant, etc

A main sequence star fuses hydrogen to helium in its core.

A red giant star fuses hydrogen to helium in a shell around its helium core. This star is not on the main sequence.

I'd thought that red dwarfs were always just the remains of bigger, hotter stars that died. But I guess all gas clouds aren't created equal, and the collapse, if and when it comes, doesn't always produce a nice yellow (or hotter) sun.

The collapse of a star doesn't ever produce a star like the Sun. Yes, it might do so, if the collapse and the ensuing explosion compresses gas clouds in the vicinity and triggers another round of star formation in those gas clouds. If that happens, a new star like the Sun might form. But the star that collapsed never turns into a star like the Sun after collapsing.

Click to view full size image

This is an illustration of the life cycle of a star like the Sun. It begins forming inside a gas cloud, grows in mass until it becomes hot enough to start fusing hydrogen to helium in its core, and keeps on fusing hydrogen to helium in its core for billions of years until it runs out of core hydrogen. Then it leaves the main sequence forever. It fuses hydrogen to helium in a shell around its core, and after a while it starts fusing helium to oxygen and carbon in its core. After a while it becomes unstable, casts off its swollen atmosphere, exposes its hot inner core, and ionizes its cast-off atmosphere, which glows gloriously as a planetary nebula. Then the planetary nebula fades, and only the small, hot but cooling stellar core remains. It is called a white dwarf.

Ann

[rstevenson]

That's a good chart showing the evolution of the Sun, but note the "not to scale" under the red giant stage. Here's an illustration (from Wikipedia) of the approximate size of the red giant stage compared to the Sun today, drawn to scale. Sol_now is the little yellow dot in the lower left; Sol_RedGiant is about 200 times larger.


Rob

[chuckster]

Wow, thanks for the great, focused, subject-specific research and graphics (that I should've done for myself ) I get interested in this stuff, then get distracted, then come back and try to pick up where I left off, and maybe a little noise creeps into my knowledge base !

I didn't mean to suggest that a Sun-like star could be a stage of stellar death for some higher-order star. I was just trying to work out what you were saying about how a star can basically be born as a red dwarf. Also, you seem to suggest that planet formation will be slow in such a star system. I read someplace that the Milky Way is supposedly 12 billion years old (and the WMAP satellite finally pegged the Universe at 13.7 billion) which makes us a surprisingly old galaxy. If we've got a sky full of red dwarfs outnumbering other star types by three to one, and we're in an old galaxy, we must be the young punks on the scale of galactic life around here. By some analysis, tech civilizations only spend a few thousand years broadcasting in ways we're equipped to detect, and then become directly involved in their own evolution and then disappear again. Not dead, just conducting business outside or beyond the EM spectrum. If we're surrounded by old civilizations (kind of the main theme of "Bablyon Five") I don't doubt they've chosen to leave us alone, or don't even particularly take notice, other than just as a minor data point. But in the absence of intelligent alien species, it seems overwhelmingly obvious that a large variety of situations could support life, even sunless, roving planets with geothermal vents beneath their frozen oceans.

[Anthony Barreiro]

... . I read someplace that the Milky Way is supposedly 12 billion years old (and the WMAP satellite finally pegged the Universe at 13.7 billion) which makes us a surprisingly old galaxy.

More recent data from the Planck satellite suggest an age of 13.82 billion years. But hey, what's a hundred million years or so between friends? And as we build more sensitive telescopes we see galaxies at earlier and earlier ages of the universe, some that seem to have formed more than 13 billion years ago. So I wouldn't assume that the Milky Way is a remarkably old galaxy.

If we've got a sky full of red dwarfs outnumbering other star types by three to one, and we're in an old galaxy, we must be the young punks on the scale of galactic life around here. By some analysis, tech civilizations only spend a few thousand years broadcasting in ways we're equipped to detect, and then become directly involved in their own evolution and then disappear again. Not dead, just conducting business outside or beyond the EM spectrum. If we're surrounded by old civilizations (kind of the main theme of "Bablyon Five") I don't doubt they've chosen to leave us alone, or don't even particularly take notice, other than just as a minor data point. But in the absence of intelligent alien species, it seems overwhelmingly obvious that a large variety of situations could support life, even sunless, roving planets with geothermal vents beneath their frozen oceans.

At this point we know of only one planet with life, so what seems overwhelmingly obvious to you seems like an interesting but unproven hypothesis to me. But I hope we can skew the Drake equation by sticking around for a while, even if that means we'll eventually have to wear tight polyester costumes and fight the Vorlons.

[neufer]

I hope we can skew the Drake equation by sticking around for a while, even if that means we'll eventually have to wear tight polyester costumes and fight the Vorlons.

<<The Vorlon species is a member of the First Ones, a group made up of the earliest species to gain sentience in the galaxy. When in the company of aliens, Vorlons wear encounter suits to conceal their true form. The suits are large, cumbersome units with crest-like shoulders and elongated helmets and draping fabric; aside from a single illuminated iris to represent an eye/face, a Vorlon's shape cannot be deduced from the suit's design. While the official Vorlon explanation is that the suit maintains the specific environment necessary to support them, the actual reason is to hide their appearance. In actuality, Vorlons are incorporeal beings who require very little if any environment to survive; Vorlons have even managed to survive in vacuum with no ill effect.

A Vorlon's true shape, without the aid of telepathic projection, is that of a glowing cephalopod. Vorlon spaceships, being composed of the same organic foundation as their builders, illustrate this appearance in their squid-like design. The only occasion when a Vorlon's true form was viewed by humans was during the assassination of Vorlon Ambassador Ulkesh Naranek in Falling Toward Apotheosis, when his suit was shattered by weaponized electricity. Vorlons are known for speaking cryptically with brief phrases that vaguely relate to the question they are asked (e.g. "The avalanche has already started. It is too late for the pebbles to vote"). This is typically considered a limitation of their language as it is translated into English, Interlac, or other languages, but there remains speculation that they are deliberately obscure to prevent disruptions to the timeline.>>

[Beyond]

Here's another Vorlon reveal... the revealing of Kosh.

Click to play embedded YouTube video.

[chuckster]

LOVE IT
Time will tell. And I wish I could be there for the telling !