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Tau Herculis
Tau Herculis, a name Latinized from τ Herculis, is a variable star in the northern constellation of Hercules. It has a blue-white hue and is visible to the naked eye at night with an apparent visual magnitude that fluctuates around 3.91. The star is located at a distance of approximately 307 light years from the Sun based on parallax, but it is drifting closer with a radial velocity of −16 km/s. The stellar classification of Tau Hercules is B5 IV, and it serves as a standard spectrum in the modern Morgan–Keenan (MK) classification. It is estimated to be just 26 million years old with a relatively low projected rotational velocity of 32 km/s. Slowly rotating B-type stars are often chemically peculiar, so the mostly normal spectra of this star suggests we may be viewing it from near pole-on. The abundance of most heavier elements in this star are about 85% of those in the Sun. The star has four times the mass of the Sun and around 3.8 times ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hercules Historical View
Hercules (, ) is the Roman equivalent of the Greek divinity, divine hero Heracles, son of Jupiter (mythology), Jupiter and the mortal Alcmene, Alcmena. In classical mythology, Hercules is famous for his strength and for his numerous far-ranging adventures. The Romans adapted the Greek hero's iconography and myths for their literature and art under the name ''Hercules''. In later Western art and literature and in popular culture, ''Hercules'' is more commonly used than ''Heracles'' as the name of the hero. Hercules is a multifaceted figure with contradictory characteristics, which enabled later artists and writers to pick and choose how to represent him. This article provides an introduction to representations of Hercules in the classical tradition, later tradition. Mythology Birth and early life In Roman mythology, although Hercules was seen as the champion of the weak and a great protector, his personal problems started at birth. Juno (mythology), Juno sent two witches to pre ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Luminosity Of The Sun
The solar luminosity (), is a unit of radiant flux (power emitted in the form of photons) conventionally used by astronomers to measure the luminosity of stars, galaxies and other celestial objects in terms of the output of the Sun. One nominal solar luminosity is defined by the International Astronomical Union to be . This does not include the solar neutrino luminosity, which would add , or , i.e. a total of (the mean energy of the solar photons is 26 MeV and that of the solar neutrinos 0.59 MeV, i.e. 2.27%; the Sun emits photons and as many neutrinos each second, of which per m2 reach the Earth each second). The Sun is a weakly variable star, and its actual luminosity therefore fluctuates. The major fluctuation is the eleven-year solar cycle (sunspot cycle) that causes a quasi-periodic variation of about ±0.1%. Other variations over the last 200–300 years are thought to be much smaller than this. Determination Solar luminosity is related to solar irradiance (the solar c ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Iota Herculis
Iota Herculis (ι Herculis, ι Her) is a fourth-magnitude variable star system in the constellation Hercules, consisting of at least four stars all about away. The brightest is a β Cephei variable, a pulsating star. Visibility Iota Herculis is dim enough that in cities with a lot of light pollution it is unlikely to be visible with the naked eye. In rural areas it will usually be visible, and for much of the Northern Hemisphere the star is circumpolar and visible year around. Pole star As a visible star, the proximity of Iota Herculis to the precessional path the Earth's North Pole traces across the celestial sphere makes it a pole star, a title currently held by Polaris. In 10,000 BCE it was the pole star, and in the future it will be again. While Polaris is only 0.5° off the precessional path Iota Herculis is 4° off. Properties Iota Herculis is a B-type subgiant star that is at the end of its hydrogen fusion stage. With a stellar classification B3IV, it ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Pole Star
A pole star or polar star is a star, preferably bright, nearly aligned with the axis of a rotating astronomical body. Currently, Earth's pole stars are Polaris (Alpha Ursae Minoris), a bright magnitude-2 star aligned approximately with its northern axis that serves as a pre-eminent star in celestial navigation, and a much dimmer magnitude-5.5 star on its southern axis, Polaris Australis (Sigma Octantis). From around 1700 BC until just after 300 AD, Kochab (Beta Ursae Minoris) and Pherkad (Gamma Ursae Minoris) were twin northern pole stars, though neither was as close to the pole as Polaris is now. History In classical antiquity, Beta Ursae Minoris (Kochab) was closer to the celestial north pole than Alpha Ursae Minoris. While there was no naked-eye star close to the pole, the midpoint between Alpha and Beta Ursae Minoris was reasonably close to the pole, and it appears that the entire constellation of Ursa Minor, in antiquity known as '':wikt:Cynosura, Cynosura'' (Greek ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Precession
Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In other words, if the axis of rotation of a body is itself rotating about a second axis, that body is said to be precessing about the second axis. A motion in which the second Euler angle changes is called ''nutation''. In physics, there are two types of precession: torque-free and torque-induced. In astronomy, ''precession'' refers to any of several slow changes in an astronomical body's rotational or orbital parameters. An important example is the steady change in the orientation of the axis of rotation of the Earth, known as the precession of the equinoxes. Torque-free Torque-free precession implies that no external moment (torque) is applied to the body. In torque-free precession, the angular momentum is a constant, but the angular velocit ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Pole Star
A pole star or polar star is a star, preferably bright, nearly aligned with the axis of a rotating astronomical body. Currently, Earth's pole stars are Polaris (Alpha Ursae Minoris), a bright magnitude-2 star aligned approximately with its northern axis that serves as a pre-eminent star in celestial navigation, and a much dimmer magnitude-5.5 star on its southern axis, Polaris Australis (Sigma Octantis). From around 1700 BC until just after 300 AD, Kochab (Beta Ursae Minoris) and Pherkad (Gamma Ursae Minoris) were twin northern pole stars, though neither was as close to the pole as Polaris is now. History In classical antiquity, Beta Ursae Minoris (Kochab) was closer to the celestial north pole than Alpha Ursae Minoris. While there was no naked-eye star close to the pole, the midpoint between Alpha and Beta Ursae Minoris was reasonably close to the pole, and it appears that the entire constellation of Ursa Minor, in antiquity known as '':wikt:Cynosura, Cynosura'' (Greek ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
North Pole
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earth's axis of rotation meets its surface. It is called the True North Pole to distinguish from the Magnetic North Pole. The North Pole is by definition the northernmost point on the Earth, lying antipodally to the South Pole. It defines geodetic latitude 90° North, as well as the direction of true north. At the North Pole all directions point south; all lines of longitude converge there, so its longitude can be defined as any degree value. No time zone has been assigned to the North Pole, so any time can be used as the local time. Along tight latitude circles, counterclockwise is east and clockwise is west. The North Pole is at the center of the Northern Hemisphere. The nearest land is usually said to be Kaffeklubben Island, off the northern coast of Greenland about away, though some perhaps semi-permanent gravel banks lie slightly clos ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Celestial Sphere
In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, which may be centered on Earth or the observer. If centered on the observer, half of the sphere would resemble a hemispherical screen over the observing location. The celestial sphere is a conceptual tool used in spherical astronomy to specify the position of an object in the sky without consideration of its linear distance from the observer. The celestial equator divides the celestial sphere into northern and southern hemispheres. Introduction Because astronomical objects are at such remote distances, casual observation of the sky offers no information on their actual distances. All celestial objects seem equally far away, as if fixed onto the inside of a sphere with a large but unknown radius, which appears to rotate westward o ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Precession
Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In other words, if the axis of rotation of a body is itself rotating about a second axis, that body is said to be precessing about the second axis. A motion in which the second Euler angle changes is called ''nutation''. In physics, there are two types of precession: torque-free and torque-induced. In astronomy, ''precession'' refers to any of several slow changes in an astronomical body's rotational or orbital parameters. An important example is the steady change in the orientation of the axis of rotation of the Earth, known as the precession of the equinoxes. Torque-free Torque-free precession implies that no external moment (torque) is applied to the body. In torque-free precession, the angular momentum is a constant, but the angular velocit ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Precession N
Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In other words, if the axis of rotation of a body is itself rotating about a second axis, that body is said to be precessing about the second axis. A motion in which the second Euler angle changes is called ''nutation''. In physics, there are two types of precession: torque-free and torque-induced. In astronomy, ''precession'' refers to any of several slow changes in an astronomical body's rotational or orbital parameters. An important example is the steady change in the orientation of the axis of rotation of the Earth, known as the precession of the equinoxes. Torque-free Torque-free precession implies that no external moment (torque) is applied to the body. In torque-free precession, the angular momentum is a constant, but the angular velocity ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Photometry (astronomy)
Photometry, from Greek '' photo-'' ("light") and '' -metry'' ("measure"), is a technique used in astronomy that is concerned with measuring the flux or intensity of light radiated by astronomical objects. This light is measured through a telescope using a photometer, often made using electronic devices such as a CCD photometer or a photoelectric photometer that converts light into an electric current by the photoelectric effect. When calibrated against standard stars (or other light sources) of known intensity and colour, photometers can measure the brightness or apparent magnitude of celestial objects. The methods used to perform photometry depend on the wavelength region under study. At its most basic, photometry is conducted by gathering light and passing it through specialized photometric optical bandpass filters, and then capturing and recording the light energy with a photosensitive instrument. Standard sets of passbands (called a photometric system) are defined to allow a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
B-type Main-sequence Star
A B-type main-sequence star (B V) is a main-sequence (hydrogen-burning) star of spectral type B and luminosity class V. These stars have from 2 to 16 times the mass of the Sun and surface temperatures between 10,000 and 30,000 K. B-type stars are extremely luminous and blue. Their spectra have neutral helium, which are most prominent at the B2 subclass, and moderate hydrogen lines. Examples include Regulus and Algol A. This class of stars was introduced with the Harvard sequence of stellar spectra and published in the ''Revised Harvard photometry'' catalogue. The definition of type B-type stars was the presence of non-ionized helium lines with the absence of singly ionized helium in the blue-violet portion of the spectrum. All of the spectral classes, including the B type, were subdivided with a numerical suffix that indicated the degree to which they approached the next classification. Thus B2 is 1/5 of the way from type B (or B0) to type A. Later, however, more refined s ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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