Novae:
A Point-Form Summary.
This section of the course notes, and the associated PowerPoint presentation, makes the following critical points:
not all stars live alone; some are in close binary pairs. This can have an effect on the way a star evolves
in particular, as the more massive star in the pair uses up its fuel, it expands. If it has a companion star nearby, the outer parts of the expanding star can intrude on the second star, and mass can be transferred to it
the mass transfer is not straightforward. Atoms do not simply fall directly from one star to the other, since the stars are in mutual orbit and have a lot of 'sideways' motion. Because of the conservation of angular momentum, the infalling material winds up moving in a swirling orbit and a disk of material forms around the recipient star
after the material orbits the star, it runs into yet more material following its path (like a snake running into its own tail). The collisions lead to heating which can be extreme enough to lead to the emission of X-rays
friction and interactions slow some of the material in the orbiting disk, and it can thus fall into the recipient star, which grows in mass as it accretes material
as the recipient star grows in mass, it feels a stronger inward pull of gravity, and gets hotter at the core, so the nuclear reactions increase and the star gets hotter and consumes its fuel faster. As a result, its life is shortened despite the gain of total fuel supply
the infalling material is mainly hydrogen and helium, and it does not much have immediate impact if it falls onto an ordinary star (although the cumulative amount can matter)
if the recipient star has gone through its life and turned into a white dwarf, the incoming material falls a long way onto a very dense surface. The impact causes a lot of heating, and it can in fact lead to local thermonuclear reactions and a bright flaring up. This is a nova
only a tiny amount of material is consumed in a nova, and they do not get as fantastically bright as supernovae. In fact, novae are known to repeat in fairly regular fashion, consuming just a tiny fraction of a star's hydrogen in each episode
if the star which is the recipient of the incoming material is a white dwarf with a mass close to the Chandrasekhar limit, the new material can overwhelm the electron degeneracy pressure and cause the whole star to collapse inwards. This gives rise to a supernova which is different in detail from the kind of supernova produced when a single massive star dies
the sun is not in a binary system, and will not become a nova, although many people seem to assume that is our inevitable fate
Associated Readings from the Text.Please look at: Chapter 17, pages 566-567 Chapter 18, pages 576-579Back to Binary Stars.We have now explored the birth, life, and death of stars of a great variety of masses. But our story is not yet complete, in one important respect. We have not considered the behaviour of stars which pass their lives in close proximity to others. I noted earlier that stars are typically very small relative to the separations between them, and that their structure and properties are not much affected by the stars around them. There does exist, however, a class of binary stars in which the two components are so close to one another that the evolution of each star is materially affected, in ways we will now explore. The importance of this is underscored by our recognition of the fact that fully half of the stars you see in the night sky are in binary systems (or more complicated systems, like the triple system of Alpha Centauri). But this somewhat overstates the situation, since it is only the very closest binaries which are affected. Two stars in a large mutual orbit may have minimal interaction at all stages of their lives. Still, the effects can be important and, in the case of the novae, spectacular.Degrees of Connectedness.There are, roughly speaking, three kinds of binaries: 1 A detached binary, a system in which two stars orbit about their mutual center of mass but which are so widely separated that the internal structure of each one is essentially unaffected by the presence of the other. Among other things, this means that the tidal influences are negligible, and the stars are spherical in shape. For example, you might have a system in which a star like the sun has a partner which is like itself but very far away - perhaps as far away as Jupiter is from the sun, for instance, a distance which is several hundred solar diameters. 2 A semi-detached binary, a system in which one of the stars is large enough that some of its outermost atoms feel nearly as strong a gravitational pull from the other star as they do from the 'parent' star. This may sound unlikely, but remember that stars expand when they run out of fuel and become red giants. In this way, an originally detached pair of stars can reach the sort of situation shown in the middle panel of the figure on page 567 as one of the stars 'puffs up.' (If this happens, the tidal forces of the other star lead to the expanded star becoming somewhat elongated.) To understand what happens then, consider an individual atom which belongs to the puffy star but which is on the edge nearest the other star. It feels the gravitational tug from both of them, and as the parent star continues to expand the atom actually finds itself more strongly attracted by the gravity of the other star than that of its original parent. (The atom does not need not be exactly halfway between them for this to happen, of course: it depends on the relative masses of the stars.) Such an atom can become completely detached from its parent and move over to join the other star - but only in a complicated way which I will describe in a moment. 3 A contact binary, a system in which the outer parts of both stars are literally in contact. In this case, the stars are distinctly elongated or `egg-shaped' because of the tidal forces which they exert on one another.Mass Exchange, and Its Inevitability.Let us consider the semi-detached binary again, the middle panel in the figure on page 567. It will clarify things, though, if we start by visualising the situation as it used to be, as shown in the top panel. Suppose that the star on the left is the more massive one. Since the nuclear fuel is used up more quickly in such a star, it is the first to expand as a red giant. Now consider an atom at the very edge of that star, on the side nearest the lower-mass companion star. As the envelope expands and the atom is puffed up to higher and higher levels in its parent star, the gravitational tug it feels from the other star becomes stronger. Eventually, there will come a time when it feels exactly equal pulls from both members of the binary pair. The atom will not remain poised there, at rest! There is no way it can fall freely back towards the bigger star, because there are other atoms in the way: it is held up by the gas pressure. But if there is even the slightest tendency for it to move to the right, towards the smaller star -- perhaps because it is given a random bump from some other atom, but more obviously because that star continues to expand, pushing all of its atoms upwards -- it will start to feel the gravity of the smaller star more and more dominantly. Since its motion in that direction is not impeded, the atom will literally start to fall towards the smaller star. This holds true for many atoms, of course, not just the one I have been describing. Consequently, there may be a considerable flow of matter from the evolving star towards the other. (This is shown quit nicely in the figure.) I should re-emphasise that binary stars are not made as semi-detached binaries, but if they are not too widely separated to begin with, they can readily evolve to this stage, thanks to the enormous expansion of of a star as it becomes a red giant.The Details: Slow Accumulation.When an atom crosses the point described above, so that it is now more obviously under the influence of the second star, you might think that it would simply fall straight onto it. This does not happen, however, for the same reason that the Earth does not fall straight into the sun. Since the stars are in mutual orbit, each atom which makes its way across the boundary has some `sideways' motion with respect to the small star. It therefore follows a curved path as it moves inward, a path which called a `Mass-transfer stream.' You can see it, in schematic form, on the middle panel of the figure on page 567. As any infalling atoms draw closer and closer to the target star, the speed of the sideways motion increases, thanks to the Conservation of Angular Momentum. Just like Halley's Comet, then, the very first atoms will whiz around the star in an extremely elongated orbit and completely avoid hitting the star. They are still doomed, however, because the end of their first complete orbit brings them back to near where they started. You might be excused for concluding that we would wind up with a whole bunch of material orbiting the star in a big elliptical orbit, like the dust particles which are left by fragmented comets in the solar system (those which give rise to meteor showers). This is not the case. The myriads of atoms find themselves rushing back into a now-crowded region of space, since many other atoms have followed their lead! The stream of infalling material loops back to collide with its own tail, in a sense, rather like the snakes in some well-known computer games. ( A more realistic representation would be to visualise two fire hoses playing streams of water against one another.) As they collide, the various atoms lose some of their orbital speed: the process is said to be dissipative (because some of the energy of ordered motion is dissipated). Of course the energy does not vanish: it gives rise to rapid random motions. In other words, the infalling material gets very hot!! The net result of all this action, therefore, is the formation of a disk of hot material swirling around the second star, rather like the swirl of water which forms around the drain in the sink when the plug is pulled. Collisions between the particles slow some of them down so much that they fall almost directly onto the surface of the smaller star, which experiences a more-or-less steady rain of infalling gas. Since the star accretes (gains) material from this disk, it is called, sensibly enough, an accretion disk. Quite a large fraction of the first star's mass can be transferred to the second star over many tens or hundreds of millennia. An artist's impression of this is shown on page 577.The Effect of the Infall on Ordinary Stars.What effect will the infalling gas have on the star which is the unexpected recipient of this stream of material? Well, remember that the outer parts of a star remain pretty nearly unchanged by the nuclear reactions which go on deep within it. The gas which leaves the expanding star and falls onto the smaller one is thus very much as it was when the star first formed, and the recipient star is simply having fresh hydrogen and helium dumped onto its surface. The effect is rather like dumping a pail of water into a lake - not much immediate impact. But in the long run, this is not without consequences, because a large fraction of the mass of the donating star may be transferred to the recipient. In other words, the smaller star grows in mass, and thus develops a higher central pressure thanks to the extra weight of the new material. This compresses the core a little, raising the temperature a little and thus increasing the rate of nuclear reactions a lot (thanks to the great temperature sensitivity). The net result, then, is that the star winds up consuming its fuel more rapidly, and its life is shortened, not lengthened, by the addition of fresh fuel. (The reasoning is exactly that which explains why isolated massive stars don't live as long as low-mass stars.) Natually enough, this process also changes the appearance of the stars involved in ways that one would not have happened had they been isolated. In the binary star Algol, for example, we believe that the system started with one blue star of moderately high mass and a second faint red main sequence star, with a potentially long and quiet life ahead of it. But as the blue star evolved into a red giant, it dumped more and more matter onto the red dwarf, which became first a yellow star and then a blue main-sequence star, with now a fairly limited life in prospect.Back and Forth.Even more complicated sequences of events are possible, incluing one which has an important final state. Here is what happens: We start with a detached pair of stars. To be specific, imagine that the binary contains one star with a mass twice that of the sun, while the other is of exactly one solar mass. The more massive star evolves first, and becomes a red giant. By the mechanism described above, it dumps some of its mass onto the lower-mass star, which grows to (let us say) 1.5 solar masses. (Again, the precise numbers don't matter). When we look somewhat later, the originally massive star has ended its life and become a white dwarf of about 1.2 solar masses. (Remember that some mass may be ejected out into space as a planetary nebula.) The second star, now the more massive of the two, is now getting close to the end of its main-sequence life. Finally, the second star runs out of fuel in the core. It becomes a red giant and dumps some of its material back onto the first star. The fact that this star is now a white dwarf is of critical importance. Let us see why.Fuel Onto the Fire.In the first episode of mass exchange, the two-solar-mass star dumped raw hydrogen and helium onto a smaller star, the outer parts of which were at a temperature of a few thousand degrees. Not much happened except for a slow increase in the mass of the star. But the second episode is different. The target star, a white dwarf, is very compact, which means that the material raining down onto it falls a long way in a rather strong gravitational field, onto a very dense surface which does not readily give way. By the time is hits the surface, it is moving very fast, and the energy of the impact has the effect of heating up the material to very high temperature. In fact, it gets so hot that nuclear reactions can take place within this inflow of hydorgen-rich gas. In a sense, it is analogous to the behaviour of a fire into which one flings a cupful of gasoline. (Don't ever do this!) The fire flares up because of the fresh fuel. So too the resurgence of nuclear reactions, right at the surface of the white dwarf star, leads to a brief but intense output of energy. The star flares up and becomes conspicuous, partly because the release of energy drives off a hot expanding shell which develops a large radiating area. In this respect, the phenomenon is like a supernova, but it is different in other important ways. For one thing, for instance, the event does not entail the destruction of the star. Instead, only a tiny fraction of the total mass comes off in the shell -- probably less than a millionth of a solar mass. For this reason, the whole episode can be repeated again and again. Moreover, the peak luminosity reached is very much less than that attained by a supernova, and there is no important formation of heavy elements to enrich the interstellar medium. The whole event is called a nova (and the repeating ones are called recurrent novae). Quite a few recurrent novae are known.Novae Observed.On page 578 of your text, you will see a photograph of a nova, showing the expanding shell of gas which is moving out from it. On well-known recent nova took place in the constellation Cygnus in 1975. At its brightest, this nova was clearly visible to the naked eye in the night sky. If a star in that location had become a supernova, however, it would have been hundreds of thousands or even millions of times brighter.But Not the Sun.The Canadian singer Bruce Cockburn wrote a song entitled ``When the Sun Goes Nova.'' What is now clear is that the sun will not go nova, since it is not in a close binary pair. Nor will it become a supernova, since it is too low in mass. But we can forgive Bruce, a non-astronomer, his understandable ignorance of these fine details.One More Kind of Supernova.We learned earlier that there is a limit to the possible mass of a white dwarf, namely the Chandrasekhar mass. Now visualise a close binary system in which the white dwarf is having fresh fuel dumped onto it, but imagine further that the white dwarf is already very close to the Chandrasekhar limit. What will happen as mass is added to it? The answer, not surprisingly, is that the extra mass can overwhelm the resisting pressure provided by the degenerate electrons within the white dwarf, and it can collapse catastrophically to become a supernova. In other words, supernovae can be formed in this rather indirect route in addition to the more straightforward collapse of a single massive star. (There are ways of distinguishing them.) Previous chapter:Next chapter0: Physics 016: The Course Notes, spring 2005. 1: The Properties of the Sun: 2: What Is The Sun Doing? 3: An Introduction to Thermonuclear Fusion. 4: Probing the Deep Interior of the Sun. 5: The Sun in More Detail. 6: An Introduction to the Stars. 7: Stars and Their Distances: 8: The HR Diagram: 9: Questions Arising from the HR Diagram: 10: The Importance of Binary Stars: 11: Implications from Stellar Masses: 12: Late in the Life of the Sun: 13: The Importance of Star Clusters in Understanding Stellar Evolution: 14: The Chandrasekhar Limit: 15: Supernovae: The Deaths of Massive Stars, 16: Pulsars: 17: Novae: 18: An Introduction to Black Holes: 19: Gravity as Geometry: 20: Finishing Off Black Holes: 21: Star Formation: 22: Dust in the Interstellar Medium: 23: Gas in the ISM: 24: The Size and Shape of Our Galaxy: 25: The Discovery of External Galaxies: 26: Galaxies of All Kinds: 27: The Expanding Universe: 28: Quasars and Active Galaxies: 29: The Hot Big Bang: 30: The Geometry of the Universe: 31: Closing Thoughts: Part 1:Part 2:Part 3: |
(Wednesday, 22 April, 2026.)
