White Stars
White Stars, Sirius A and Sirius B are both binary stars. They orbit each other every 5.2 days and are separated by about 7 billion miles. Sirius A is a red giant star while Sirius B is a White Stars. White Stars are formed when a massive star runs out of fuel and collapses under gravity into a dense ball of degenerate matter called a neutron star. Neutron stars are very small—about 20 kilometers across—but weigh many times more than our Sun. As such, they pack enough mass into a tiny space to become extremely dense.
The gravitational force exerted by a white dwarf is too great to allow a planet like Earth to survive around it. However, a large moon could form around the White Stars. This companion body would be pulled closer and closer to the star until it became tidally locked with it, always showing the same side to the star. Eventually, tidal forces would cause the companion to break apart. But what happens to the pieces?
If one piece becomes sufficiently close to the White Stars, it might fall into the star. If another piece gets too close, it might be torn apart by tidal forces. Either way, the debris eventually falls onto the surface of the star. The resulting collision creates a bright flash of energy visible to the naked eye.
This phenomenon occurs because the white dwarf is hot, meaning it radiates heat. When a larger object hits it, the impact causes friction and generates heat. The heated gas expands violently, producing a brilliant flash of light.
These flashes occur approximately once per century. In fact, there have been several recorded events over the past few decades. One occurred in 1980 near the town of Sutherland Springs, Texas. Another happened in 1992 in M31, the Andromeda Galaxy. And a third event took place in 2003 in the Large Magellanic Cloud, a satellite galaxy of our Milky Way.
In addition to being beautiful, these events are important. Astronomers use them to study how the cosmos works. For example, they help determine whether dark matter exists. Dark matter is invisible stuff that makes up most of the Universe. Scientists believe it holds galaxies together. Without it, galaxies wouldn’t exist.
But why do we care about dark matter? Because it helps us understand something else: black holes. Black holes are regions where gravity is so strong that nothing, including light, can escape. They’re usually found at the center of some galaxies.
Discovery
The discovery of a white dwarf in the triple star system 40 Eridani was announced by astronomer Paul G. T. Eggleton and colleagues on 12 December 2006.
40 Eridani A is a K-type main sequence star about 2.3 times as massive as our Sun. It is located approximately 30 light-years away in the constellation Eridanus. Its apparent magnitude is 9.5, making it one of the brightest stars in the night sky.
The double-star 40 Eridani B is a close binary consisting of two M-type dwarfs orbiting each other every 3.8 days. It consists of two components separated by 0.23 arcseconds. One component is much fainter than the brighter companion; it is a White Stars with a mass around 0.6 solar masses. This white dwarf was detected by the Wide Field Planetary Camera 2 aboard the Hubble Space Telescope during observations of the nearby star 40 Eridani C, which lies at a projected separation of 730 AU from the primary star.
The total mass of the three stars is almost equal to that of the Sun, although the combined luminosity is less than half of that of the Sun. The temperature of the outermost layer of the star is over 10,000 kelvin.
Composition and structure
White dwarfs are the end products of stellar evolution. They are the final stage of the lives of most stars, including our Sun. Most stars eventually become White Stars, although some explode as supernovae. White dwarfs are composed almost entirely of matter; they have no atmosphere and no magnetic field. Their surface temperature ranges from 10,000 K to over 30,000 K. A typical White Stars has a diameter of about 12 km and weighs around one solar mass.
The density of a White Stars depends on how massive it is. More massive stars collapse into neutron stars during the explosion phase of their life cycle. These stars are very compact objects with densities ranging from several times 10^15 kg/m3 to greater than 10^18 kg/m3.
A white dwarf must weigh more than about 0.6 Suns to achieve such extreme densities. Stars less massive than this do not reach sufficient temperatures to undergo electron degeneracy pressure to form a neutron star.
A White Stars is usually formed when the core of a main sequence star collapses due to nuclear burning. As the center contracts, it heats up and becomes increasingly opaque. Eventually, the heat generated by contraction exceeds the binding energy of electrons within the nucleus, causing fusion reactions to occur. When this happens, the star begins to burn helium nuclei, releasing large amounts of energy. But because the star is now too hot to fuse hydrogen, the fuel source runs out. The star continues to shrink, becoming ever hotter and denser. By the time it reaches the Chandrasekhar limit of 1.4 Msolar, the outer layers begin to fall onto the inner core. At this point, the star enters what is called the “white dwarf state.”
As the star cools, it loses mass, and gravity pulls the material closer together. Because the star is still losing mass, the gravitational force increases. This causes the star to contract even further. In fact, the gravitational attraction of the star itself is strong enough to overcome the outward pull of radiation pressure. After a few million years, the star approaches the point where the inward pull of gravity balances the outward push of radiation pressure. The star stops contracting, and the process ends. It becomes a cold, dead ball of carbon and oxygen, surrounded by a thin layer of inert gas.
This article discusses the composition of white dwarfs. For information on the formation of white dwarfs, see Formation of white dwarfs.
Mass–radius relationship
The mass–radius relation is one of several relations describing the structure of compact objects such as neutron stars and white dwarfs. A typical equation relating the mass and radius of a spherical object is:
M 4πρR3
where ρ is the density of the material comprising the sphere, M is the total mass, R is the radius, and 3 is used because the sphere is assumed to be isotropic; that is, the same properties apply throughout the entire sphere. This formula is valid for both solid spheres and hollow spheres. For example, the Earth and the Sun each have a mass of about 5×1010 kg and a radius of roughly 6371 km. Using this formula, one finds that the masses of the Earth and the Sun are about 5.97 × 1030 kg and 5.98 × 1030 kg, respectively.
Radiation and cooling
A white dwarf is a stellar remnant composed primarily of carbon and oxygen, and thus appears mostly white in color. White dwarfs are formed when stars run out of fuel and collapse under their own gravity. They are extremely dense—about one gram per cubic centimeter. Because of their extreme density, white dwarfs have almost no internal motion except for some rotation caused by the spin of the original star. The surface temperature of a typical white dwarf is around 10,000K, although some are much hotter. The surface temperature is determined by how far away the star is from the center, where there is less gravitational pull. For example, a star near the center of a galaxy is cooler than a star farther out.
The interior of a white dwarf consists mainly of electrons bound together by electrostatic forces into a crystalline lattice. Electrons are negatively charged particles, and the positively charged nuclei of atoms form the “lattice”. In the core of most white dwarfs, the electrons become highly ionized, forming a plasma. The ions move freely inside the crystal lattice, and the electrons drift along in response to electric fields set up by the moving ions. The electrons
Atmosphere and spectra
The composition of the atmosphere of a white dwarf is determined mainly by the history of mass loss during its life. White dwarfs lose matter via stellar winds and planetary nebulae ejections. In particular, the atmospheres of old white dwarfs show evidence of having been enriched by metals produced by nucleosynthesis processes occurring in previous stages of evolution of massive stars.
A typical spectrum of a DAZ white dwarf contains narrow absorption lines due to species such as H, He, C, N, O, Mg, Si, Ca, Fe, Ni, Al, Ti, Cr, Mn, Co, V, and others, whose abundances reflect those present in the photosphere. These features are very similar to those found in the atmospheres of hot subdwarf stars of spectral type BHB/O, although the latter do not possess magnetic fields strong enough to produce Zeeman splitting. Spectral analysis of white dwarfs provides information about the chemical compositions of their progenitors.
Sirius A and Sirius B are both binary stars. They orbit each other every 5.2 days and are separated by about 7 billion miles. Sirius A is a red giant star while Sirius B is a White Stars. White Stars are formed when a massive star runs out of fuel and collapses under gravity into a dense ball of degenerate matter called a neutron star. Neutron stars are very small—about 20 kilometers across—but weigh many times more than our Sun. As such, they pack enough mass into a tiny space to become extremely dense.
The gravitational force exerted by a white dwarf is too great to allow a planet like Earth to survive around it. However, a large moon could form around the White Stars. This companion body would be pulled closer and closer to the star until it became tidally locked with it, always showing the same side to the star. Eventually, tidal forces would cause the companion to break apart. But what happens to the pieces?
If one piece becomes sufficiently close to the White Stars, it might fall into the star. If another piece gets too close, it might be torn apart by tidal forces. Either way, the debris eventually falls onto the surface of the star. The resulting collision creates a bright flash of energy visible to the naked eye.
This phenomenon occurs because the white dwarf is hot, meaning it radiates heat. When a larger object hits it, the impact causes friction and generates heat. The heated gas expands violently, producing a brilliant flash of light.
These flashes occur approximately once per century. In fact, there have been several recorded events over the past few decades. One occurred in 1980 near the town of Sutherland Springs, Texas. Another happened in 1992 in M31, the Andromeda Galaxy. And a third event took place in 2003 in the Large Magellanic Cloud, a satellite galaxy of our Milky Way.
In addition to being beautiful, these events are important. Astronomers use them to study how the cosmos works. For example, they help determine whether dark matter exists. Dark matter is invisible stuff that makes up most of the Universe. Scientists believe it holds galaxies together. Without it, galaxies wouldn’t exist.
But why do we care about dark matter? Because it helps us understand something else: black holes. Black holes are regions where gravity is so strong that nothing, including light, can escape. They’re usually found at the center of some galaxies.
Discovery
The discovery of a white dwarf in the triple star system 40 Eridani was announced by astronomer Paul G. T. Eggleton and colleagues on 12 December 2006.
40 Eridani A is a K-type main sequence star about 2.3 times as massive as our Sun. It is located approximately 30 light-years away in the constellation Eridanus. Its apparent magnitude is 9.5, making it one of the brightest stars in the night sky.
The double-star 40 Eridani B is a close binary consisting of two M-type dwarfs orbiting each other every 3.8 days. It consists of two components separated by 0.23 arcseconds. One component is much fainter than the brighter companion; it is a White Stars with a mass around 0.6 solar masses. This white dwarf was detected by the Wide Field Planetary Camera 2 aboard the Hubble Space Telescope during observations of the nearby star 40 Eridani C, which lies at a projected separation of 730 AU from the primary star.
The total mass of the three stars is almost equal to that of the Sun, although the combined luminosity is less than half of that of the Sun. The temperature of the outermost layer of the star is over 10,000 kelvin.
Composition and structure
White dwarfs are the end products of stellar evolution. They are the final stage of the lives of most stars, including our Sun. Most stars eventually become White Stars, although some explode as supernovae. White dwarfs are composed almost entirely of matter; they have no atmosphere and no magnetic field. Their surface temperature ranges from 10,000 K to over 30,000 K. A typical White Stars has a diameter of about 12 km and weighs around one solar mass.
The density of a White Stars depends on how massive it is. More massive stars collapse into neutron stars during the explosion phase of their life cycle. These stars are very compact objects with densities ranging from several times 10^15 kg/m3 to greater than 10^18 kg/m3.
A white dwarf must weigh more than about 0.6 Suns to achieve such extreme densities. Stars less massive than this do not reach sufficient temperatures to undergo electron degeneracy pressure to form a neutron star.
A White Stars is usually formed when the core of a main sequence star collapses due to nuclear burning. As the center contracts, it heats up and becomes increasingly opaque. Eventually, the heat generated by contraction exceeds the binding energy of electrons within the nucleus, causing fusion reactions to occur. When this happens, the star begins to burn helium nuclei, releasing large amounts of energy. But because the star is now too hot to fuse hydrogen, the fuel source runs out. The star continues to shrink, becoming ever hotter and denser. By the time it reaches the Chandrasekhar limit of 1.4 Msolar, the outer layers begin to fall onto the inner core. At this point, the star enters what is called the “white dwarf state.”
As the star cools, it loses mass, and gravity pulls the material closer together. Because the star is still losing mass, the gravitational force increases. This causes the star to contract even further. In fact, the gravitational attraction of the star itself is strong enough to overcome the outward pull of radiation pressure. After a few million years, the star approaches the point where the inward pull of gravity balances the outward push of radiation pressure. The star stops contracting, and the process ends. It becomes a cold, dead ball of carbon and oxygen, surrounded by a thin layer of inert gas.
This article discusses the composition of white dwarfs. For information on the formation of white dwarfs, see Formation of white dwarfs.
Mass–radius relationship
The mass–radius relation is one of several relations describing the structure of compact objects such as neutron stars and white dwarfs. A typical equation relating the mass and radius of a spherical object is:
M 4πρR3
where ρ is the density of the material comprising the sphere, M is the total mass, R is the radius, and 3 is used because the sphere is assumed to be isotropic; that is, the same properties apply throughout the entire sphere. This formula is valid for both solid spheres and hollow spheres. For example, the Earth and the Sun each have a mass of about 5×1010 kg and a radius of roughly 6371 km. Using this formula, one finds that the masses of the Earth and the Sun are about 5.97 × 1030 kg and 5.98 × 1030 kg, respectively.
Radiation and cooling
A white dwarf is a stellar remnant composed primarily of carbon and oxygen, and thus appears mostly white in color. White dwarfs are formed when stars run out of fuel and collapse under their own gravity. They are extremely dense—about one gram per cubic centimeter. Because of their extreme density, white dwarfs have almost no internal motion except for some rotation caused by the spin of the original star. The surface temperature of a typical white dwarf is around 10,000K, although some are much hotter. The surface temperature is determined by how far away the star is from the center, where there is less gravitational pull. For example, a star near the center of a galaxy is cooler than a star farther out.
The interior of a white dwarf consists mainly of electrons bound together by electrostatic forces into a crystalline lattice. Electrons are negatively charged particles, and the positively charged nuclei of atoms form the “lattice”. In the core of most white dwarfs, the electrons become highly ionized, forming a plasma. The ions move freely inside the crystal lattice, and the electrons drift along in response to electric fields set up by the moving ions. The electrons
Atmosphere and spectra
The composition of the atmosphere of a white dwarf is determined mainly by the history of mass loss during its life. White dwarfs lose matter via stellar winds and planetary nebulae ejections. In particular, the atmospheres of old white dwarfs show evidence of having been enriched by metals produced by nucleosynthesis processes occurring in previous stages of evolution of massive stars.
A typical spectrum of a DAZ white dwarf contains narrow absorption lines due to species such as H, He, C, N, O, Mg, Si, Ca, Fe, Ni, Al, Ti, Cr, Mn, Co, V, and others, whose abundances reflect those present in the photosphere. These features are very similar to those found in the atmospheres of hot subdwarf stars of spectral type BHB/O, although the latter do not possess magnetic fields strong enough to produce Zeeman splitting. Spectral analysis of white dwarfs provides information about the chemical compositions of their progenitors.
