The Evolution of Binary-Star Systems If the stars in a binary-star system are relatively widely separated, their evolution proceeds much as it would have if they were not companions... If they are closer, it is possible for material to transfer from one star to another, leading to unusual evolutionary paths!

The Evolution of Binary-Star Systems Each star is surrounded by its own Roche lobe; particles inside each lobe belong to that lobe’s star… The Lagrangian point is where the gravitational forces are equal. Figure 20-21. Stellar Roche Lobes Each star in a binary system can be pictured as being surrounded by a “zone of influence,” or Roche lobe, inside of which matter may be thought of as being “part” of that star. The two teardrop-shaped Roche lobes meet at the Lagrangian point between the two stars. Outside the Roche lobes, matter may flow onto either star with relative ease.

The Evolution of Binary-Star Systems There are different types of binary-star systems, depending on how close the stars are… In a detached binary, each star has its own Roche lobe: Figure 20-22. Close Binary-Star Systems (a) In a detached binary, each star lies within its respective Roche lobe.

The Evolution of Binary-Star Systems In a contact binary, much of the mass is shared between the two stars: Figure 20-22. Close Binary-Star Systems (c) In a contact, or common-envelope, binary, both stars have overflowed their Roche lobes, and a single star with two distinct nuclear-burning cores results.

The Evolution of Binary-Star Systems In a semidetached binary, one star can transfer mass to the other: “Accretion” Figure 20-22. Close Binary-Star Systems (b) In a semidetached binary, one of the stars fills its Roche lobe and transfers matter onto the other star, which still lies within its own Roche lobe.

The Evolution of Binary-Star Systems As the stars evolve, their binary system type can evolve as well. This is the Algol system: It is thought to have begun as a detached binary Figure 20-23. Algol Evolution The evolution of the binary star Algol. (a) Initially, Algol was probably a detached binary made up of two main-sequence stars: a relatively massive blue giant and a less massive companion similar to the Sun.

The Evolution of Binary-Star Systems As the blue-giant star entered its red-giant phase, it expanded to the point where mass transfer occurred (b). Eventually enough mass accreted onto the smaller star that it became a blue giant, leaving the other star as a red subgiant (c). Figure 20-23. Algol Evolution The evolution of the binary star Algol. (b) As the more massive component (star 1) left the main sequence, it expanded to fill, and eventually overflow, its Roche lobe, transferring large amounts of matter onto its smaller companion (star 2). (c) Today, star 2 is the more massive of the two, but it is on the main sequence. Star 1 is still in the subgiant phase and fills its Roche lobe, causing a steady stream of matter to pour onto its companion.

Life after Death for White Dwarfs A white dwarf that is part of a semidetached binary system can undergo repeated explosions! Figure 21-2. Close Binary System If a white dwarf in a semidetached binary system is close enough to its companion (in this case, a main-sequence star), its gravitational field can tear matter from the companion’s surface. (Cf. Figure 20.23.) Notice that, unlike the scenario shown in the earlier figure, the matter does not fall directly onto the white dwarf’s surface. Instead, it forms an accretion disk of gas spiraling down onto the dwarf.

Life after Death for White Dwarfs Material falls onto the white dwarf from its main-sequence companion. When enough material has accreted, fusion can reignite very suddenly, burning off the new material. Material keeps being transferred to the white dwarf, and the process repeats, as illustrated here: Figure 21-3. Nova Explosion In this artist’s conception, a white-dwarf star (actually faraway at upper left) orbits a cool red giant (a). As the dwarf swings around in an elliptical orbit, coming closer to the giant, material accretes from the giant to the dwarf and accumulates on the white dwarf’s surface (b and c). The dwarf star then ignites in hydrogen fusion as a nova outburst (d). (D. Berry)

Life after Death for White Dwarfs A Nova is a star that flares up very suddenly and then returns slowly to its former luminosity: “Nova” = “New Star” (…well, that’s what they called it…) Figure 21-1. Nova A nova is a star that suddenly increases enormously in brightness, then slowly fades back to its original luminosity. Novae are the result of explosions on the surfaces of faint white dwarf stars, caused by matter falling onto their surfaces from the atmosphere of larger binary companions. Shown is Nova Herculis 1934 in (a) March 1935 and (b) May 1935, after brightening by a factor of 60,000. (c) The light curve of a typical nova. The rapid rise and slow decline in the light received from the star, as well as the maximum brightness attained, are in good agreement with the explanation of the nova as a nuclear flash on a white dwarf’s surface. (UC/Lick Observatory)

Life after Death for White Dwarfs This series of images shows ejected material expanding away from a star after a Nova explosion: Figure 21-4. Nova Matter Ejection (a) The ejection of material from a star’s surface can clearly be seen in this image of Nova Persei, taken some 50 years after it suddenly brightened by a factor of 40,000 in 1901. This corresponds approximately to Figure 21.3(d). (b) Nova Cygni, imaged here with a European camera on the Hubble Space Telescope, erupted in 1992. At left, more than a year after the blast, a rapidly billowing bubble is seen; at right, 7 months after that, the shell continued to expand and distort. The images are fuzzy because the object is more than 10,000 light-years away. (Palomar Observatory; ESA)

T PYXIDIS OUTBURST CBET No. 2700 issued on April 15, 2011 reports that the recurrent nova T Pyxidis has been discovered in outburst. It was detected by by Mike Linnolt at visual magnitude 13.0 on 2011 April 14, and the outburst has been visually confirmed by several observers.

“The End” for a White Dwarf with a close binary companion… A “Carbon-Detonation” (Type Ia) Supernova!

Computer Simulations of a Type Ia SN Explosion“Detonation”!

Two Alternative Roads to Type Ia White Dwarf Explosion…?

Type Ia Supernovae make great “Standard Candles”!

The End of a High-Mass Star A high-mass star can continue to fuse elements in its core right up to Iron (after which the fusion reaction is energetically unfavored). As heavier elements are fused, the reactions go faster and faster! Figure 21-5. Heavy-Element Fusion Cutaway diagram of the interior of a highly evolved star of mass greater than 8 solar masses (not to scale). The interior resembles the layers of an onion, with shells of progressively heavier elements burning at smaller and smaller radii and at higher and higher temperatures.

The End of a High-Mass Star This graph shows the relative stability of nuclei. On the left, nuclei gain energy through fusion; on the right they gain it through fission: Iron is the crossing point; when the core has fused to iron, no more (energetically profitable) fusion can take place… Figure 21-6. Nuclear Masses This graph shows how the masses (per nuclear particle—proton or neutron) of most known nuclei vary with nuclear mass. It contains an enormous amount of information about nuclear structure and stability. When light nuclei fuse (left side of the figure), the mass per particle decreases and energy is released. (Sec. 16.2) Similarly, when heavy nuclei split apart (right side), the total mass again decreases and energy is again released. The nucleus with the smallest mass per nuclear particle—the most stable element—is iron. It can be neither fused nor split to release energy. The difference in mass between a given nucleus and the value for hydrogen represents the amount of mass lost (or energy released) if free neutrons and protons were combined to form that nucleus—or equivalently, the amount of energy that must be provided to split the nucleus into its component particles.

“The End” for a High-Mass (M > 8 x MSun) Star… A “Core-Collapse” (Type II) Supernova!

As Seen Around the World in 1054 AD …!

The Crab Nebula in Motion This second set of images, focused in on the central pulsar, shows ripples expanding outward at half the speed of light:

Supernovae This is the Vela supernova remnant: Extrapolation shows it exploded about 9000 BCE : Figure 21-12. Vela Supernova Remnant The glowing gases of the Vela supernova remnant are spread across 6° of the sky. The inset shows more clearly some of the details of the nebula’s extensive filamentary structure. (The long diagonal streak was caused by the passage of an Earth-orbiting satellite while the photo was being exposed.) (D. Malin/AAT)

The 2 General Types of Supernova Explosions…

Supernovae A supernova is incredibly luminous—as can be seen from these curves—and more than a million times as bright as a nova: Figure 21-8. Supernova Light Curves The light curves of typical Type I and Type II supernovae. In both cases, the maximum luminosity can sometimes reach that of a billion suns, but there are characteristic differences in the falloff of the luminosity after the initial peak. Type I light curves somewhat resemble those of novae (see Figure 21.1), but the total release of energy is much larger. Type II curves have a characteristic plateau during the declining phase.

A Supernova can outshine its Entire Galaxy!

The End of a High-Mass Star… Q: Does anything survive the Type II SN Explosion? The inward pressure is enormous, due to the high mass of the star. There is nothing stopping the star from collapsing further; it does so very rapidly, in a giant implosion. As it continues to become more and more dense, the protons and electrons react with one another to become neutrons: p + e → n + neutrino The neutrinos escape; the neutrons are compressed together until the whole star has the density of an atomic nucleus, about 1015 kg/m3.

“Neutron Stars”! Neutron stars, although they have 1–3 solar masses, are so dense that they are very small. This image shows a 1-solar-mass neutron star, ~10 km in diameter, compared to Manhattan: Figure 22-1. Neutron Star Neutron stars are not much larger than many of Earth’s major cities. In this fanciful comparison, a typical neutron star sits alongside Manhattan Island. (NASA)

The Crab Nebula in Motion The Crab Nebula is complex; its expansion is detectable and there is a Pulsar at its center…