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
) 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 !
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. 
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. "
LOVE IT