|
U Monocerotis
U Monocerotis (''U Mon'') is a pulsating variable star and spectroscopic binary in the constellation Monoceros. The primary star is an RV Tauri variable, a cool luminous post-AGB star evolving into a white dwarf. History U Mon was reported to be variable in 1918 by the renown German astronomer Ernst Hartwig. It was then included by Shapley in his list of Cepheid variables. In the 1950s a series of papers established the fundamental observational parameters of the star, its period, brightness range, colour changes, and spectral variation. In 1970, U Mon was discovered to have a large infrared excess. The double-peaked spectral energy distribution and polarization are strongly indicative of a dust shell around the star. Visibility U Mon can often be seen with the naked eye between Sirius and Procyon, but drops below naked eye visibility at deep minima. It lies about two degrees west of α Mon, at fourth magnitude the brightest star in Monoceros. At its brightest U M ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
White Dwarf
A white dwarf is a Compact star, stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very density, dense: in an Earth sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place in a white dwarf; what light it radiates is from its residual heat. The nearest known white dwarf is Sirius B, at 8.6 light years, the smaller component of the Sirius binary star. There are currently thought to be eight white dwarfs among the hundred star systems nearest the Sun. The unusual faintness of white dwarfs was first recognized in 1910. The name ''white dwarf'' was coined by Willem Jacob Luyten in 1922. White dwarfs are thought to be the final stellar evolution, evolutionary state of stars whose mass is not high enough to become a neutron star or black hole. This includes over 97% of the stars in the Milky Way. After the hydrogen-stellar nucleosynthesis, fusing period of a main sequence, main-sequence star of Stellar mass, lo ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Monoceros
Monoceros ( Greek: , "unicorn") is a faint constellation on the celestial equator. Its definition is attributed to the 17th-century cartographer Petrus Plancius. It is bordered by Orion to the west, Gemini to the north, Canis Major to the south, and Hydra to the east. Other bordering constellations include Canis Minor, Lepus, and Puppis. Features Stars Monoceros contains only a few fourth magnitude stars, making it difficult to see with the naked eye. Alpha Monocerotis has a visual magnitude of 3.93, while for Gamma Monocerotis it is 3.98. Beta Monocerotis is a triple star system; the three stars form a fixed triangle. The visual magnitudes of the stars are 4.7, 5.2, and 6.1. William Herschel discovered it in 1781 and called it "one of the most beautiful sights in the heavens". Epsilon Monocerotis is a fixed binary, with visual magnitudes of 4.5 and 6.5. S Monocerotis, or 15 Monocerotis, is a bluish white variable star and is located at the center of NGC ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Planetary Nebula
A planetary nebula is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. The term "planetary nebula" is a misnomer because they are unrelated to planets. The term originates from the planet-like round shape of these nebulae observed by astronomers through early telescopes. The first usage may have occurred during the 1780s with the English astronomer William Herschel who described these nebulae as resembling planets; however, as early as January 1779, the French astronomer Antoine Darquier de Pellepoix described in his observations of the Ring Nebula, "very dim but perfectly outlined; it is as large as Jupiter and resembles a fading planet". Though the modern interpretation is different, the old term is still used. All planetary nebulae form at the end of the life of a star of intermediate mass, about 1-8 solar masses. It is expected that the Sun will form a planetary nebula at the end of i ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Asymptotic Giant Branch
The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) late in their lives. Observationally, an asymptotic-giant-branch star will appear as a bright red giant with a luminosity ranging up to thousands of times greater than the Sun. Its interior structure is characterized by a central and largely inert core of carbon and oxygen, a shell where helium is undergoing fusion to form carbon (known as helium burning), another shell where hydrogen is undergoing fusion forming helium (known as hydrogen burning), and a very large envelope of material of composition similar to main-sequence stars (except in the case of carbon stars). Stellar evolution When a star exhausts the supply of hydrogen by nuclear fusion processes in its core, the core contracts and its temperature increases, causing the oute ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Circumstellar Disc
A Circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accretion disk of matter composed of gas, dust, planetesimals, asteroids, or collision fragments in orbit around a star. Around the youngest stars, they are the reservoirs of material out of which planets may form. Around mature stars, they indicate that planetesimal formation has taken place, and around white dwarfs, they indicate that planetary material survived the whole of stellar evolution. Such a disc can manifest itself in various ways. Young star According to the widely accepted model of star formation, sometimes referred to as the nebular hypothesis, a young star (protostar) is formed by the gravitational collapse of a pocket of matter within a giant molecular cloud. The infalling material possesses some amount of angular momentum, which results in the formation of a gaseous protoplanetary disc around the young, rotating star. The former is a rotating circumstellar disc of dense gas and d ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Dredge-up
A dredge-up is any one of several stages in the evolution of some stars. By definition, during a ''dredge-up'', a convection zone extends all the way from the star's surface down to the layers of material that have undergone fusion. Consequently, the fusion products are mixed into the outer layers of the star's atmosphere, where they can be seen in stellar spectra. Multiple stages *;''The first dredge-up'': The first dredge-up occurs when a main-sequence star enters the red-giant branch. As a result of the convective mixing, the outer atmosphere will display the spectral signature of hydrogen fusion: The C/ C and C/ N ratios are lowered, and the surface abundances of lithium and beryllium may be reduced. The counter-intuitive existence of lithium-rich red giant stars that have gone through first dredge-up may be explained by scenarios such as mass transfer. *;''The second dredge-up'': The second dredge-up occurs in stars with 4–8 solar masses. When helium fusion co ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
S-process
The slow neutron-capture process, or ''s''-process, is a series of nuclear reactions, reactions in nuclear astrophysics that occur in stars, particularly asymptotic giant branch stars. The ''s''-process is responsible for the creation (nucleosynthesis) of approximately half the Atomic nucleus, atomic nuclei Heavy metal (chemical element), heavier than iron. In the ''s''-process, a seed nucleus undergoes neutron capture to form an isotope with one higher atomic mass. If the new isotope is stable nuclide, stable, a series of increases in mass can occur, but if it is unstable nucleus, unstable, then beta decay will occur, producing an element of the next higher atomic number. The process is ''slow'' (hence the name) in the sense that there is sufficient time for this radioactive decay to occur before another neutron is captured. A series of these reactions produces stable isotopes by moving along the valley of stability, valley of beta-decay stable isobars in the table of nuclides. ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Supergiant
Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram, with absolute visual magnitudes between about −3 and −8. The temperatures of supergiant stars range from about 3,400 K to over 20,000 K. Definition The title ''supergiant'', as applied to a star, does not have a single concrete definition. The term ''giant star'' was first coined by Hertzsprung when it became apparent that the majority of stars fell into two distinct regions of the Hertzsprung–Russell diagram. One region contained larger and more luminous stars of spectral types A to M, which received the name ''giant''. Subsequently, as they lacked any measurable parallax, it became apparent that some of these stars were significantly larger and more luminous than the bulk, and the term ''super-giant'' arose, quickly adopted as ''supergiant''. Supergiants with spectral classes of O to A are typically referred to as ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Alpha Monocerotis
Alpha Monocerotis, Latinised from α Monocerotis, is the Bayer designation for the brightest star in the equatorial constellation of Monoceros. It can be viewed with the naked eye, having an apparent visual magnitude of 3.94. Based upon an annual parallax shift of as seen from Earth, it is located 146 light-years away from the Sun. The star is moving away from the Sun with a radial velocity of +11.7 km/s. The stellar classification of indicates this is an evolved giant star of type G, which means the hydrogen has been depleted at its core and the outer envelope has expanded and cooled. The 'Fe−0.5' notation indicates the spectrum displays a slight underabundance of iron relative to other stars of this temperature. It is a red clump giant, which means it is generating energy through helium fusion at its core. At the age of 890 million years, this yellow-hued star has an estimated 2.2 times the mass of the Sun and 10 times the Sun's radius Solar radius is ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Procyon
Procyon () is the brightest star in the constellation of Canis Minor and usually the list of brightest stars, eighth-brightest star in the night sky, with an apparent visual magnitude of 0.34. It has the Bayer designation α Canis Minoris, which is Latinisation of names, Latinized to Alpha Canis Minoris, and abbreviated α CMi or Alpha CMi, respectively. As determined by the European Space Agency ''Hipparcos'' astrometry satellite, this system lies at a distance of just , and is therefore one of Earth's List of nearest stars, nearest stellar neighbors. A binary star system, Procyon consists of a white-hued main-sequence star of spectral type F5 IV–V, designated component A, in orbit with a faint white dwarf companion of spectral type DQZ, named Procyon B. The pair orbit each other with a orbital period, period of 40.84 years and an orbital eccentricity, eccentricity of 0.4. Observation Procyon is usually the eighth-brightest star in the night sky, Culmina ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Sirius
Sirius is the brightest star in the night sky. Its name is derived from the Greek word (Latin script: ), meaning 'glowing' or 'scorching'. The star is designated Canis Majoris, Latinized to Alpha Canis Majoris, and abbreviated CMa or Alpha CMa. With a visual apparent magnitude of −1.46, Sirius is almost twice as bright as Canopus, the next brightest star. Sirius is a binary star consisting of a main-sequence star of spectral type A0 or A1, termed Sirius A, and a faint white dwarf companion of spectral type DA2, termed Sirius B. The distance between the two varies between 8.2 and 31.5 astronomical units as they orbit every 50 years. Sirius appears bright because of its intrinsic luminosity and its proximity to the Solar System. At a distance of , the Sirius system is one of Earth's nearest neighbours. Sirius is gradually moving closer to the Solar System and it is expected to increase in brightness slightly over t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
