Phys/Geog 182 – Week 7 The Deaths and Remnants of Stars The evolution of intermediate-mass stars like the Sun. Planetary nebulae and the formation of white dwarf stars. Supernova explosions: two types Type I: due to “carbon detonation” of an accreting white dwarf in a binary. Type II: due to “core collapse” in a high-mass star. Compact objects: neutron stars, pulsars, and black holes.

In an old star on the main sequence, hydrogen burning eventually builds up an “ash” core, which is mostly helium. The core temperature is about T = 10 million K and the star expands to become a red giant.

Helium shell burning begins, and a core of carbon ash forms.

FIGURE 13-1 Post–Main-Sequence Evolution of Low-Mass Stars (a) A typical evolutionary track on the H-R diagram as a star makes the transition from the main sequence to the giant phase. The asterisk (*) shows the helium flash occurring in a low-mass star. (b) After the helium flash, the star converts its helium core into carbon and oxygen. While doing so, its core re-expands, decreasing shell fusion. As a result, the star’s outer layers recontract. (c) After the helium core is completely transformed into carbon and oxygen, the core recollapses, and the outer layers reexpand, powered up the asymptotic giant branch by hydrogen shell fusion and helium shell fusion.

FIGURE 13-1 Post–Main-Sequence Evolution of Low-Mass Stars (a) A typical evolutionary track on the H-R diagram as a star makes the transition from the main sequence to the giant phase. The asterisk (*) shows the helium flash occurring in a low-mass star. (b) After the helium flash, the star converts its helium core into carbon and oxygen. While doing so, its core re-expands, decreasing shell fusion. As a result, the star’s outer layers recontract. (c) After the helium core is completely transformed into carbon and oxygen, the core recollapses, and the outer layers reexpand, powered up the asymptotic giant branch by hydrogen shell fusion and helium shell fusion.

FIGURE 13-2 The Structure of an Old Low-Mass Star Near the end of its life, a low-mass star, like the Sun, travels up the AGB and becomes a supergiant. (The Sun will be about as large as the diameter of Earth’s orbit.) The star’s inert core, the hydrogen-fusing shell, and the helium-fusing shell are contained within a volume roughly the size of Earth. The inner layers are not shown to scale here.

FIGURE 13-1 Post–Main-Sequence Evolution of Low-Mass Stars (a) A typical evolutionary track on the H-R diagram as a star makes the transition from the main sequence to the giant phase. The asterisk (*) shows the helium flash occurring in a low-mass star. (b) After the helium flash, the star converts its helium core into carbon and oxygen. While doing so, its core re-expands, decreasing shell fusion. As a result, the star’s outer layers recontract. (c) After the helium core is completely transformed into carbon and oxygen, the core recollapses, and the outer layers reexpand, powered up the asymptotic giant branch by hydrogen shell fusion and helium shell fusion.

FIGURE 13-4 Some Shapes of Planetary Nebulae The outer shells of dying low-mass stars are ejected in a wonderful variety of patterns. (a) An exceptionally spherical remnant, this shell of expanding gas, in the globular cluster M15 in the constellation Pegasus, is about 7000 ly (2150 pc) away from Earth. (b) The Helix Nebula, NGC 7293, located in the constellation Aquarius, about 700 ly (215 pc) from Earth, has an angular diameter equal to about half that of the full Moon. Red gas is mostly hydrogen and nitrogen, whereas the blue gas is rich in oxygen. (c) NGC 6826 shows, among other features, lobes of nitrogen rich gas (red). The process by which they were ejected is as yet unknown. This planetary nebula is located in Cygnus. (d) MZ 3 (Menzel 3), in the constellation Norma (the Carpenter’s Square), is 3000 ly (900 pc) from Earth. The dying star, creating these bubbles of gas, may be part of a binary system. (a: WIYN/NOAL/NSF; b–d: NASA, NOAO, ESA, The Hubble Helix Nebula Team, M. Meixner/STScI, and T. A. Rector/ NRAO; b: Howard Bons, STScI/Robin Ciardullo, Pennsylvania State University/ NASA; c: AURA/STScI/NASA)

Planetary Nebulae form when the core can’t reach 600 million K, the minimum needed for carbon burning.

A Planetary Nebula shaped like a sphere, about 1. 5 pc across A Planetary Nebula shaped like a sphere, about 1.5 pc across. The white dwarf is in the center.

FIGURE 13-4 Some Shapes of Planetary Nebulae The outer shells of dying low-mass stars are ejected in a wonderful variety of patterns. (a) An exceptionally spherical remnant, this shell of expanding gas, in the globular cluster M15 in the constellation Pegasus, is about 7000 ly (2150 pc) away from Earth. (b) The Helix Nebula, NGC 7293, located in the constellation Aquarius, about 700 ly (215 pc) from Earth, has an angular diameter equal to about half that of the full Moon. Red gas is mostly hydrogen and nitrogen, whereas the blue gas is rich in oxygen. (c) NGC 6826 shows, among other features, lobes of nitrogen rich gas (red). The process by which they were ejected is as yet unknown. This planetary nebula is located in Cygnus. (d) MZ 3 (Menzel 3), in the constellation Norma (the Carpenter’s Square), is 3000 ly (900 pc) from Earth. The dying star, creating these bubbles of gas, may be part of a binary system. (a: WIYN/NOAL/NSF; b–d: NASA, NOAO, ESA, The Hubble Helix Nebula Team, M. Meixner/STScI, and T. A. Rector/ NRAO; b: Howard Bons, STScI/Robin Ciardullo, Pennsylvania State University/ NASA; c: AURA/STScI/NASA)

A Planetary Nebula with the shape of a ring, 0 A Planetary Nebula with the shape of a ring, 0.5 pc across, called the “Ring Nebula”.

FIGURE 13-4 Some Shapes of Planetary Nebulae The outer shells of dying low-mass stars are ejected in a wonderful variety of patterns. (a) An exceptionally spherical remnant, this shell of expanding gas, in the globular cluster M15 in the constellation Pegasus, is about 7000 ly (2150 pc) away from Earth. (b) The Helix Nebula, NGC 7293, located in the constellation Aquarius, about 700 ly (215 pc) from Earth, has an angular diameter equal to about half that of the full Moon. Red gas is mostly hydrogen and nitrogen, whereas the blue gas is rich in oxygen. (c) NGC 6826 shows, among other features, lobes of nitrogen rich gas (red). The process by which they were ejected is as yet unknown. This planetary nebula is located in Cygnus. (d) MZ 3 (Menzel 3), in the constellation Norma (the Carpenter’s Square), is 3000 ly (900 pc) from Earth. The dying star, creating these bubbles of gas, may be part of a binary system. (a: WIYN/NOAL/NSF; b–d: NASA, NOAO, ESA, The Hubble Helix Nebula Team, M. Meixner/STScI, and T. A. Rector/ NRAO; b: Howard Bons, STScI/Robin Ciardullo, Pennsylvania State University/ NASA; c: AURA/STScI/NASA)

FIGURE 13-4 Some Shapes of Planetary Nebulae The outer shells of dying low-mass stars are ejected in a wonderful variety of patterns. (a) An exceptionally spherical remnant, this shell of expanding gas, in the globular cluster M15 in the constellation Pegasus, is about 7000 ly (2150 pc) away from Earth. (b) The Helix Nebula, NGC 7293, located in the constellation Aquarius, about 700 ly (215 pc) from Earth, has an angular diameter equal to about half that of the full Moon. Red gas is mostly hydrogen and nitrogen, whereas the blue gas is rich in oxygen. (c) NGC 6826 shows, among other features, lobes of nitrogen rich gas (red). The process by which they were ejected is as yet unknown. This planetary nebula is located in Cygnus. (d) MZ 3 (Menzel 3), in the constellation Norma (the Carpenter’s Square), is 3000 ly (900 pc) from Earth. The dying star, creating these bubbles of gas, may be part of a binary system. (a: WIYN/NOAL/NSF; b–d: NASA, NOAO, ESA, The Hubble Helix Nebula Team, M. Meixner/STScI, and T. A. Rector/ NRAO; b: Howard Bons, STScI/Robin Ciardullo, Pennsylvania State University/ NASA; c: AURA/STScI/NASA)

FIGURE 13-5 Formation of a Bipolar Planetary Nebula Bipolar planetary nebulae may form in two steps. Astronomers hypothesize that (a) first, a doughnut-shaped cloud of gas and dust is emitted from the star’s equator, (b) followed by outflow that is channeled by the original gas to squirt out perpendicularly to the plane of the doughnut. (c) The Hourglass Nebula appears to be a textbook example of such a system. The bright ring is believed to be the doughnut- shaped region of gas lit by energy from the planetary nebula. The Hourglass is located about 8000 ly (2500 pc) from Earth. (c: R. Sahai and J. Trauer, JPL; WFPC-2 Science Team; and NASA)

FIGURE 13-5 Formation of a Bipolar Planetary Nebula Bipolar planetary nebulae may form in two steps. Astronomers hypothesize that (a) first, a doughnut-shaped cloud of gas and dust is emitted from the star’s equator, (b) followed by outflow that is channeled by the original gas to squirt out perpendicularly to the plane of the doughnut. (c) The Hourglass Nebula appears to be a textbook example of such a system. The bright ring is believed to be the doughnut- shaped region of gas lit by energy from the planetary nebula. The Hourglass is located about 8000 ly (2500 pc) from Earth. (c: R. Sahai and J. Trauer, JPL; WFPC-2 Science Team; and NASA)

FIGURE 13-5 Formation of a Bipolar Planetary Nebula Bipolar planetary nebulae may form in two steps. Astronomers hypothesize that (a) first, a doughnut-shaped cloud of gas and dust is emitted from the star’s equator, (b) followed by outflow that is channeled by the original gas to squirt out perpendicularly to the plane of the doughnut. (c) The Hourglass Nebula appears to be a textbook example of such a system. The bright ring is believed to be the doughnut- shaped region of gas lit by energy from the planetary nebula. The Hourglass is located about 8000 ly (2500 pc) from Earth. (c: R. Sahai and J. Trauer, JPL; WFPC-2 Science Team; and NASA)

Cat’s Eye Nebula, 0.1 pc across, may be from a pair of binary stars that both shed envelopes.

M2-9 has twin lobes leaving the central star at 300 km/sec, reaching 0 M2-9 has twin lobes leaving the central star at 300 km/sec, reaching 0.5 pc end-to-end.

(See the slide show of planetary nebulae (See the slide show of planetary nebulae.) Many more examples of planetary nebulae are known: NOAO: http://www.noao.edu/image_gallery/planetary_nebulae.html http://www.noao.edu/jacoby/pn_gallery.html AAO: http://203.15.109.22/images/general/planetary_frames.html ESO: http://www.eso.org/public/images/archive/category/nebulae/ Hubble: http://hubblesite.org/gallery/album/nebula/planetary/ ESO: http://www.eso.org/public/images/archive/category/nebulae/ And for a list of the Messier Catalog, see the SEDS Messier database: http://messier.seds.org/

Sirius Binary System: Sirius B is a white dwarf (this is also known as Alpha Canis Majoris B)

White Dwarf formation on the H–R Diagram Some heavier elements are formed in the last years of the burning in the shells surrounding the carbon core. H, He, C, O, and some Ne and Mg are expelled from the star as a “planetary nebula”

Sirius B has a high mass for a white dwarf, and probably came from a mass 4 Msolar star. Compare with Earth’s radius, 6371 km.

Sirius B is at the 5 o’clock position. FIGURE 13-6 Sirius and Its White Dwarf Companion (a) Sirius, the brightest-appearing star in the night sky, is actually a double star. The smaller star, Sirius B, is a white dwarf, seen here at the five o’clock position in the glare of Sirius. The spikes and rays around the bright star, Sirius A, are created by optical effects within the telescope. (b) Since Sirius A (11,000 K) and Sirius B (30,000 K) are hot blackbodies, they are strong emitters of X rays. (a: R. B. Minton; b: NASA/SAO/CXC)

X-ray picture of Sirius A 11,000K and Sirius B 24,000K FIGURE 13-6 Sirius and Its White Dwarf Companion (a) Sirius, the brightest-appearing star in the night sky, is actually a double star. The smaller star, Sirius B, is a white dwarf, seen here at the five o’clock position in the glare of Sirius. The spikes and rays around the bright star, Sirius A, are created by optical effects within the telescope. (b) Since Sirius A (11,000 K) and Sirius B (30,000 K) are hot blackbodies, they are strong emitters of X rays. (a: R. B. Minton; b: NASA/SAO/CXC)

A Nova is an explosion on a white dwarf, but only a small amount of material on the surface of the white dwarf explodes. Nova Herculis 1934 a) in March 1935 b) in May 1935, after brightening by a factor of 60,000

Nova Persei - matter ejection seen 50 years after the 1901 flash (it brightened by a factor of 40,000).

Nova light curve – due to a nuclear flash on a white dwarf

Supernova explosions There are two types of supernova explosions: Type I: due to “carbon detonation” of an accreting white dwarf in a binary star system. Type II: due to “core collapse” in a high-mass star, forming a neutron star or black hole. They have distinctive light curves (next slides).

Artist’s drawing of a Close Binary System

FIGURE 13-8 The Light Curve of a Nova This graph shows the history of Nova Cygni 1975, a nova that was observed to blaze forth in the constellation of Cygnus in September 1975. The rapid rise in magnitude followed by a gradual decline is characteristic of many novae, although some oscillate in intensity as they become dimmer.

Type Ia supernovae When enough carbon accumulates on the close binary white dwarf, it can suddenly start carbon fusion, and with no outer layers, it will completely explode. This has about the same brightness for each explosion because it happens at a particular limit of the star’s mass (1.4 solar masses). These can therefore be used to estimate distances to remote galaxies (109 ly away).

FIGURE 13-9 Supernova Light Curves A Type Ia supernova, which gradually declines in brightness, is caused by an exploding white dwarf in a close binary system. A Type II supernova is caused by the explosive death of a massive star and usually has alternating intervals of steep and gradual declines in brightness.

High-Mass stellar evolutionary tracks (the top track) are quite different from the lower-mass stellar evolution tracks. Notice that the core can heat up so fast that the envelope of the star tends to lag behind. Carbon fusion can start before the red giant phase.

Heavy Element Fusion - shells like an onion

Supernova 1987A seen near nebula 30 Doradus

SN2005cs in M51(Whirlpool galaxy) discovered June 27, 2005

A type II Supernova is a “core collapse” and occurs when the core is finally pure iron, which cannot be fused to other elements. The core collapses to a big ball of neutrons (a neutron star), which causes a shock wave to bounce back outward, which blows off the entire envelope of the red giant, to form a supernova remnant.

Supernova Remnants Crab nebula Vela supernova remnant Other examples: Cassiopeia A (link) (link) N63A (link)

M1 – the Crab Nebula is from a supernova seen in year A.D. 1054 The remnant is 1800 pc away and the diameter is currently 2 pc.

The Crab Nebula contains a pulsar: The Crab Pulsar is due to a spinning neutron star that rotates 30 times per second.

The Crab Pulsar also blinks ON and OFF in X-rays. The Chandra observatory has seen some detail in the accretion disk of the Crab pulsar.

Neutron Star: extremely compact and dense solid sphere, made of neutrons, about 25 km across, about one solar mass, density over 1018 kg/m3 spins rapidly

The Cycle of Stellar Evolution