11. Stellar End States


Fig11_1planetary_nebula_hs-2000-12-a-full_tif.jpg

Fig11_1planetary_nebula_hs-2000-12-a-full_tif.jpg

Fig. 11.1 . When a Sun-like star uses up its nuclear fuel, the starís center collapses into an Earth-sized white dwarf star while its outer gas layers are ejected into space. Such a planetary nebula is named after its round shape, which resembles a planet as seen visually in small telescopes, and has nothing to do with planets. The shells of gas in the planetary nebula NGC 6751, shown here, were ejected several thousand years ago. The hot stellar core, exposed by the expulsion of the material surrounding it, has a disk temperature of about 140,000 K. Its intense ultraviolet radiation causes the ejected gas to fluoresce as a planetary nebula. (A Hubble Space Telescope image courtesy NASA/STScI/AURA/Hubble Heritage Team.)


Fig11_2Formation_of_a_plantary_nebula_and_white_dwarf_star.jpg

Fig11_2Formation_of_a_plantary_nebula_and_white_dwarf_star.jpg

Fig. 11.2 . The evolutionary track of a dying Sun-like star in the Hertzsprung-Russell diagram. When the star has used up its nuclear hydrogen fuel, which makes the star shine, it expands into a red giant star, and after a relatively short time the giant star ejects its outer layers to form a planetary nebula. The ejected gas exposes a hot stellar core, which collapses to form an Earth-sized white dwarf star that gradually cools into dark invisibility. The luminosity is in units of the Sunís luminosity, denoted L?, and the effective temperature of the stellar disk is in units of degrees kelvin, designated K.


Fig11_3 Eskimo Nebula

Fig11_3 Eskimo Nebula

Fig. 11.3 . About 10,000 years ago, a dying Sun-like star began flinging material into nearby space, producing this planetary nebula that is formally designated as NGC 2392. When first observed more than two centuries ago, it was dubbed the ďEskimoĒ Nebula, because it resembled a face surrounded by the fur parka worn by Eskimos. It is located about 5,000 light-years from Earth. This detailed image, obtained with the Hubble Space Telescope, reveals several episodes of ejection from the central star, including an outer ring of objects that are shaped like tear drops that point outward, and elongated, filamentary bubbles, each about one light-year in diameter. Dense material enveloping the starís equator has blocked ejected material, while intense winds moving at about 420 km s-1 have swept material above and below the equatorial regions. The bright central region contains another wind-blown bubble. [Courtesy of NASA/Andrew Frucher/ERO Team (Slyvia Baggett/STScI/Richard Hook, ST-ECF, and Zolan Levay/STScI.]


FIg11_4 Cat's Eye Nebula

FIg11_4 Cat

Fig. 11.4 . A detailed view of the planetary nebula known as the Catís Eye Nebula, and formally designated NGC 6543. It reveals concentric rings, jets of high-speed gas, and shock-induced knots of gas. At least eleven large concentric shells mark the dense edges of spherical bubbles of gas and dust that have been ejected from a dying Sun-like star in regular, explosive pulses at 1,500-year intervals. The formation of more complex inner structures is not well understood. (A Hubble Space Telescope image courtesy of NASA/ESA/HEIC/the Hubble Heritage Team, STScI/AURA.)


Fig11_5 (left)Sirius

Fig11_5 (left)Sirius

Fig. 11.5 . The brightest star in the night sky, called the Dog Star and also designated Sirius A, has a faint companion Sirius B. This binary star system is located just 8.6 light-years from the Earth. Sirius A is a main-sequence star with twice the mass of the Sun, and a disk temperature of 9,940 K. Its white-dwarf companion, Sirius B, has 98 percent the mass of the Sun and a disk temperature of 25,200 K. Owing to its small size, comparable to that of the Earth, Sirius B is a thousand times less luminous that Sirius A. The faint white dwarf can be seen in the lower left of the Hubble Space Telescope image taken in optically visible light (left): the cross-shaped diffraction spikes and concentric rings around the much brighter Sirius A are artifacts produced by the telescope imaging system. Sirius B is somewhat brighter in X-rays due to its higher temperature, as indicated in the Chandra X-ray Observatory image (right). [Courtesy of NASA/H. E. Bond and E. Nelan, STScI/M. Barstow and M. Burleigh, U. Leicester/ and J. B. Holberg, U. Az (left) and NASA/SAO/CXO (right).]


Fig11_5 (right)Sirius_X-ray

Fig11_5 (right)Sirius_X-ray

Fig. 11.5 . The brightest star in the night sky, called the Dog Star and also designated Sirius A, has a faint companion Sirius B. This binary star system is located just 8.6 light-years from the Earth. Sirius A is a main-sequence star with twice the mass of the Sun, and a disk temperature of 9,940 K. Its white-dwarf companion, Sirius B, has 98 percent the mass of the Sun and a disk temperature of 25,200 K. Owing to its small size, comparable to that of the Earth, Sirius B is a thousand times less luminous that Sirius A. The faint white dwarf can be seen in the lower left of the Hubble Space Telescope image taken in optically visible light (left): the cross-shaped diffraction spikes and concentric rings around the much brighter Sirius A are artifacts produced by the telescope imaging system. Sirius B is somewhat brighter in X-rays due to its higher temperature, as indicated in the Chandra X-ray Observatory image (right). [Courtesy of NASA/H. E. Bond and E. Nelan, STScI/M. Barstow and M. Burleigh, U. Leicester/ and J. B. Holberg, U. Az (left) and NASA/SAO/CXO (right).]


Fig11_6 Hot white dwarf revealed

Fig11_6 Hot white dwarf revealed

Fig. 11.6 . The expanding material in planetary nebula NGC 2440 has been cast off by a dying Sun-like star in episodic outflows and in different directions, revealing a central white-dwarf star. It is one of the hottest stars known, with a disk temperature of about 200,000 K that is more than 30 times hotter than our own Sunís photosphere. Ultraviolet radiation from the hot star has excited oxygen and nitrogen ions in the nebular gas, making them glow and fluoresce. Clouds of dust form long dark streaks pointing away from the central star. A much larger, cooler cloud of gas and dust remains dark and unseen in this visible-light image; it can be detected by its infrared radiation. (Courtesy of NASA/Hubble Heritage Team/AURA/STScI.)


Fig11_7 Nova

Fig11_7 Nova

Fig. 11.7 . A classical nova is a thermonuclear explosion that occurs on the surface of a white dwarf star that is in a close orbit with a main-sequence star. The strong gravitational attraction of the white dwarf pulls its nearby companion into an elongated shape, whose outer edge is designated the Roche lobe. Some of the hydrogen in the outer atmosphere of the main-sequence star spills over at the inner Lagrangian point, denoted L1, where the gravitational pull of the two stars is equal. This hydrogen spirals into a rotating accretion disk and down to the white dwarf, igniting an explosion, like a colossal hydrogen bomb.


Fig11_8 Type II supernova

Fig11_8 Type II supernova

Fig. 11.8 . In this type of supernova explosion, an isolated star blows up and its shattered remains are propelled into surrounding space. Radio and x-ray radiation from the expanding supernova remnant can be observed for thousands of years after the explosion. The core of the star is compressed by gravitational contraction into a neutron star or a black hole. Neutrinos emitted from the collapsing core remove most of the supernova energy and assist shock waves in pushing the stellar remains into an expanding remnant.


Fig11_9 A light echo

Fig11_9 A light echo

Fig. 11.9 . Two complete rings of light surround the exploded star SN 1987A in this negative image taken with the 3.9-m (153.5-inch) Anglo-Australian Telescope on 15 July 1988. The initial flash of light from the supernova explosion has been reflected off clouds of interstellar dust, and observed 14 months after the explosion was brightest, somewhat like an echo of sound. These light echoes arise in two thin sheets of microscopic dust grains located about 470 light-years (inner ring) and 1,300 light-years (outer ring) in front of the supernova. The rings have been made more prominent by photographically subtracting an image taken three years before the supernova exploded, canceling much that existed previously. Stars, however, are still visible as faint haloes. (Courtesy of David Malin and the Anglo-Australian Observatory.)


Fig11_10 tycho supernova remnant

Fig11_10 tycho supernova remnant

Fig. 11.10 . The expanding remains from a Type Ia supernova that occurred in 1572. It is named after the Danish astronomer Tycho Brahe (1546-1601)) who recorded observations of its brightness in that year. The circular supernova remnant is located at a distance of about 13,000 light-years and is about 20 light-years across. It is bounded by an expanding shock wave, and consists of ejected material moving away from the explosion and the interstellar material it sweeps up and heats along the way. The explosion has left a hot cloud of expanding debris (green and yellow). The location of the blastís outer shock wave is seen as a blue sphere of very energetic electrons. Newly synthesized dust in the ejected material and heated pre-existing dust from the area radiate at infrared wavelengths (red). Foreground and background stars in the image are white. This image is a composite of an x-ray image (blue, green and yellow) taken from the Chandra X-ray Observatory, an infrared image (red) taken from the Spitzer Space Telescope, and a visible light image (white) taken with the 3.5-m (138-inch) Calar Alto telescope located in southern Spain. (Courtesy of MPIA/NASA/Calar Alto Observatory.)


Fig11_11 Cassiopeia A_radio VLA image

Fig11_11 Cassiopeia A_radio VLA image

Fig. 11.11 . This Type II supernova remnant is the brightest radio source in the sky, other than the Sun, and is designated 3C 461 from its number in the third Cambridge catalogue of bright radio objects. It is also known as Cassiopeia A, the most intense radio source in the direction of the constellation of Cassiopeia. Also called Cas A for short, it is the remnant of a supernova explosion that occurred about 1680, or roughly 330 years ago as observed from Earth, at a distance of about 10,000 light-years. It has a radius of about 8.6 light-years and is expanding at a velocity of roughly 5,000 km s-1. (A VLA image courtesy of NRAO/AUI.)


Fig11_12 Cassiopeia A x-ray

Fig11_12 Cassiopeia A x-ray

Fig. 11.12 . The expanding supernova remnant Cassiopeia A has a temperature of about 30 million K, and is therefore a luminous x-ray source seen in this image from the Chandra X-ray Observatory. Still visible in x-rays, the tiny point-like source near the center of Cas A is a neutron star, the collapsed core of the star that exploded about 330 years ago, as observed from Earth, and gave rise to the expanding debris. (Courtesy of NASA/CXC/MIT/U. Mass. Amherst/M. S. Stage et al.)


Fig11_13 Crab Nebula

Fig11_13 Crab Nebula

Fig. 11.13 . The optically visible light of the Crab Nebula, designated as M 1 and NGC 1952, consists of two distinct parts. A system of expanding filaments forms an outer envelope in which emission lines occur at well-defined wavelengths; an inner amorphous region emits continuum radiation at all wavelengths. A Type II supernova explosion observed nearly 1,000 years ago, in 1054, ejected the filaments. The expanding filaments shine mostly in the light of hydrogen (orange), but also include the light of neutral oxygen (blue), singly ionized sulfur (green) and doubly ionized oxygen (red). The blue-white continuum glow that is concentrated in the inner parts of the nebula is the non-thermal radiation of high-speed electrons spiraling in magnetic fields. This continuum emission is powered by a spinning neutron star, the south westernmost (bottom right) of the two central stars. The neutron star is the crushed, ultra-dense core of the exploded star. It is also a radio pulsar that acts like a lighthouse spinning 30 times a second. (Courtesy of NASA/ESA/J. Hester and A. Loll, ASU.)


Fig11_14 Pulsar radio

Fig11_14 Pulsar radio

Fig. 11.14 . A spinning neutron star has a powerful magnetic field whose axis intersects the north and south magnetic poles. The rotating fields generate strong electric currents and accelerate electrons, which emit an intense, narrow beam of radio radiation from each magnetic polar region. Because the magnetic field axis can be inclined to the neutron starís rotation axis, these beams can wheel around the sky as the neutron star rotates. If one of the beams sweeps across the Earth, a bright pulse of radio emission, called a pulsar, is observed once per rotation of the neutron star.


Fig11_15 crab xrayopt

Fig11_15 crab xrayopt

Fig. 11.15 . A neutron star, or radio pulsar, at the center of the Crab Nebula is spinning at 30 times a second, accelerating particles up to the speed of light, and flinging high-speed electrons out into the Crab Nebula. This image combines optically visible light (red) from the Hubble Space Telescope and an x-ray image (blue) taken from the Chandra X-ray Observatory. It shows tilted rings and waves of high-energy particles that appear to have been flung outward from the central pulsar, as well as high-energy jets of particles in a direction perpendicular to the rings. The ring-like structures are x-ray regions where high-energy particles slam into the nebular material. The innermost ring is about one light-year across. (Courtesy of NASA/HST/CXC/SAO/J. Hester, ASU.)


Fig11_16 Pulsar Xray

Fig11_16 Pulsar Xray

Fig. 11.16 . The outer atmosphere of an ordinary star, detected in optically visible light, spills onto its companion, an invisible neutron star. The flow of gas is diverted by the powerful magnetic fields of the neutron star, which channels the in-falling material onto the magnetic polar regions. The impact of the gas on the star creates a pair of x-ray hotspots aligned along the magnetic axis at each magnetic cap. Because the magnetic field axis can be inclined to the neutron starís rotation axis, the x-ray radiation from the hot spots can sweep across the sky once per rotation, which is observed as periodic x-rays if one of the hot spots intersects the observerís line of sight.