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The closest star system to the Earth is the famous Alpha Centauri group. view of the bright Alpha Centauri A (on the left) and Alpha Centauri B (on with the Universities Space Research Association in Columbia, Md., and. Hadar (Beta Centauri) is the second brightest star in the constellation Centaurus. Hadar (r) and Alpha Centauri (l) is difficult to see and study because of the brightness difference but, as a B-type dwarf, surface temperature, 25, K. An artist's rendition of Proxima Centauri as seen from the "ring" portion of size and orbital period, but not other properties like mass or temperature. Beta Centauri, nearly as bright as Alpha Centauri, is hundreds of times.
Called Ross bthis planet orbits a red dwarf star that appears much quieter than that of Proxima b. The research team said that finding out more about its atmosphere will require a next-generation telescope such as the European Extremely Large Telescope, the Giant Magellan Telescope and the Thirty Meter Telescope that all are expected to bbe active in the s. The James Webb Space Telescope, set to launch incan't do the search itself since the planet does not transit across the face of its star.
Binary stars To the naked eye, the two main stars shine as one, making them the third brightest "star" in our night sky.
Alpha Centauri: Nearest Star System to the Sun
The two separate stars can be seen through a small telescope; one of the finest binary stars that can be observed. Proxima Centauri is too faint to see unaided, and through a telescope it appears about four diameters of the full moon away from the other two.
It is a yellow star of the same type G2 as the sun, and it is about 25 percent larger. Alpha Centauri B is an orange K2-type star, slightly smaller than the sun. Proxima Centauri is a red dwarf about seven times smaller than the sun, or one-and-a-half times bigger than Jupiter. All three stars are a bit older — 4.
Our Nearest Neighbors ] The system is in the Southern sky and is not visible to observers above the latitude of 29 degrees north — a line that passes near Houston, Texas, and Orlando, Fla. Its right ascension is 14h 39m 41s and its declination is minus 60 degrees 50 minutes 7 seconds. Though they look serene and silent from our vantage on Earth, stars are actually roiling balls of violent plasma. Test your stellar smarts with this quiz. The system's location is: The Nearest Stars to Earth ] Similarities to Earth's sun Early inastronomers announced they had found a mysterious cold layer on Alpha Centauri A that looks similar to another one on Earth's sun.
Temperature does not rise sequentially in the sun's layers; the topmost layer or corona is millions of degrees Fahrenheit, but just below it is a layer called the chromosphere that is only about 7, degrees F 4, C — cooler than the layer below. A similar layer was observed on Alpha Centauri Awhich was spotted in far infrared using the European Space Agency's Herschel space observatory. Magnetic field line twists could be responsible for the high heat in the sun's corona, but it's hard to fully comprehend the sun in isolation without looking at other stars.
At the time, astronomers said looking closely at Alpha Centauri A could bring more information to light about the corona, solar flares and similar phenomena, especially because the stars are pretty close in terms of temperature, age and other properties. Planet discovery Astronomers announced in August that they had detected an Earth-size planet orbiting Proxima Centauri. The newfound world, known as Proxima b, is about 1. The planet is also in the star's habitable zonethat just-right range of distances where liquid water can exist.
Proxima b lies just 4. As a result, it's likely that the exoplanet is tidally locked, meaning it always shows the same face to its host star, just as the moon shows only one face the near side to Earth.
Hadar (Beta Centauri)
The motion along the line of sight i. Shifts of a spectral line toward the red end of the electromagnetic spectrum i. Consider a pertinent example. The proper motion of Alpha Centauri is about 3. It thus has a projected velocity in the plane of the sky of 22 km per second.
One kilometre is about 0. Light from the stars Stellar magnitudes Measuring starlight intensity Stellar brightnesses are usually expressed by means of their magnitudesa usage inherited from classical times.
A star of the first magnitude is about 2. A star of the first magnitude is therefore 2. Stars as faint as the 30th magnitude have been measured with modern telescopes, meaning that these instruments can detect stars about four billion times fainter than can the human eye alone. The scale of magnitudes comprises a geometric progression of brightness.
Magnitudes can be converted to light ratios by letting ln and lm be the brightnesses of stars of magnitudes n and m; the logarithm of the ratio of the two brightnesses then equals 0. Magnitudes are actually defined in terms of observed brightness, a quantity that depends on the light-detecting device employed.
Visual magnitudes were originally measured with the eye, which is most sensitive to yellow-green light, while photographic magnitudes were obtained from images on old photographic plates, which were most sensitive to blue light. Today, magnitudes are measured electronically, using detectors such as CCDs equipped with yellow-green or blue filters to create conditions that roughly correspond to those under which the original visual and photographic magnitudes were measured.
Yellow-green magnitudes are still often designated V magnitudes, but blue magnitudes are now designated B. The scheme has been extended to other magnitudes, such as ultraviolet Ured Rand near- infrared I. Other systems vary the details of this scheme. All magnitude systems must have a reference, or zero, point. In practice, this is fixed arbitrarily by agreed-upon magnitudes measured for a variety of standard stars. The actually measured brightnesses of stars give apparent magnitudes.
These cannot be converted to intrinsic brightnesses until the distances of the objects concerned are known. The absolute magnitude of a star is defined as the magnitude it would have if it were viewed at a standard distance of 10 parsecs This is the magnitude that the Sun would have if it were at a distance of 10 parsecs—an object still visible to the naked eye, though not a very conspicuous one and certainly not the brightest in the sky.
Many astronomers suspect that large numbers of such faint stars exist, but most of these objects have so far eluded detection. Stellar colours Stars differ in colour. Most of the stars in the constellation Orion visible to the naked eye are blue-white, most notably Rigel Beta Orionisbut Betelgeuse Alpha Orionis is a deep red.
In the telescope, Albireo Beta Cygni is seen as two stars, one blue and the other orange. One quantitative means of measuring stellar colours involves a comparison of the yellow visual magnitude of the star with its magnitude measured through a blue filter. Hot, blue stars appear brighter through the blue filter, while the opposite is true for cooler, red stars. In all magnitude scales, one magnitude step corresponds to a brightness ratio of 2.
The zero point is chosen so that white stars with surface temperatures of about 10, K have the same visual and blue magnitudes. OrionThe constellation Orion is one of the easiest to recognize because of a group of three stars. The three stars form a straight line that is often called Orion's Belt. The Orion Nebula can be seen as a pink fuzzy light below the line of three stars. The red star Betelgeuse is in the upper left, and the bright star Rigel is in the lower right.
Astronomers have overcome these difficulties by measuring the magnitudes of the same stars through three or more filters, often U ultravioletB, and V see UBV system. Observations of stellar infrared light also have assumed considerable importance. In addition, photometric observations of individual stars from spacecraft and rockets have made possible the measurement of stellar colours over a large range of wavelengths.
These data are important for hot stars and for assessing the effects of interstellar attenuation. Bolometric magnitudes The measured total of all radiation at all wavelengths from a star is called a bolometric magnitude. The corrections required to reduce visual magnitudes to bolometric magnitudes are large for very cool stars and for very hot ones, but they are relatively small for stars such as the Sun.
A determination of the true total luminosity of a star affords a measure of its actual energy output. Bright, cool stars can be observed at infrared wavelengths, however, with special instruments that measure the amount of heat radiated by the star.
A star whose surface temperature is 20, K or higher radiates most of its energy in the inaccessible ultraviolet part of the electromagnetic spectrum. To compare the true luminosities of two stars, the appropriate bolometric corrections must first be added to each of their absolute magnitudes.
The ratio of the luminosities can then be calculated. Spectrograms secured with a slit spectrograph consist of a sequence of images of the slit in the light of the star at successive wavelengths. Adequate spectral resolution or dispersion might show the star to be a member of a close binary system, in rapid rotation, or to have an extended atmosphere.
Quantitative determination of its chemical composition then becomes possible. Inspection of a high-resolution spectrum of the star may reveal evidence of a strong magnetic field. Line spectrum Spectral lines are produced by transitions of electrons within atoms or ions. As the electrons move closer to or farther from the nucleus of an atom or of an ionenergy in the form of light or other radiation is emitted or absorbed.
The yellow D lines of sodium or the H and K lines of ionized calcium seen as dark absorption lines are produced by discrete quantum jumps from the lowest energy levels ground states of these atoms. The visible hydrogen lines the so-called Balmer series ; see spectral line serieshowever, are produced by electron transitions within atoms in the second energy level or first excited statewhich lies well above the ground level in energy. Only at high temperatures are sufficient numbers of atoms maintained in this state by collisions, radiations, and so forth to permit an appreciable number of absorptions to occur.
At the low surface temperatures of a red dwarf starfew electrons populate the second level of hydrogen, and thus the hydrogen lines are dim. By contrast, at very high temperatures—for instance, that of the surface of a blue giant star—the hydrogen atoms are nearly all ionized and therefore cannot absorb or emit any line radiation.
Consequently, only faint dark hydrogen lines are observed. The characteristic features of ionized metals such as iron are often weak in such hotter stars because the appropriate electron transitions involve higher energy levels that tend to be more sparsely populated than the lower levels. The main source of light absorption in the hotter stars is the photoionization of hydrogen atoms, both from ground level and from higher levels.
Spectral analysis The physical processes behind the formation of stellar spectra are well enough understood to permit determinations of temperatures, densities, and chemical compositions of stellar atmospheres.
- Rigel Kentaurus (Alpha Centauri): Third-Brightest Star
- Alpha Centauri
- Alpha Centauri: Nearest Star System to the Sun
The star studied most extensively is, of course, the Sun, but many others also have been investigated in detail. The general characteristics of the spectra of stars depend more on temperature variations among the stars than on their chemical differences. Dwarf starswith great surface gravities, tend to have high atmospheric densities ; giants and supergiantswith low surface gravities, have relatively low densities.
Hydrogen absorption lines provide a case in point. Normally, an undisturbed atom radiates a very narrow line. If its energy levels are perturbed by charged particles passing nearby, it radiates at a wavelength near its characteristic wavelength. In a hot gas, the range of disturbance of the hydrogen lines is very high, so that the spectral line radiated by the whole mass of gas is spread out considerably; the amount of blurring depends on the density of the gas in a known fashion.
Classification of spectral types Most stars are grouped into a small number of spectral types. The Henry Draper Catalogue and the Bright Star Catalogue list spectral types from the hottest to the coolest stars see stellar classification. This group is supplemented by R- and N-type stars today often referred to as carbonor C-type, stars and S-type stars. The R- N- and S-type stars differ from the others in chemical composition; also, they are invariably giant or supergiant stars.
With the discovery of brown dwarfs —objects that form like stars but do not shine through thermonuclear fusion —the system of stellar classification has been expanded to include spectral types L, T, and Y. The spectral sequence O through M represents stars of essentially the same chemical composition but of different temperatures and atmospheric pressures.
This simple interpretation, put forward in the s by the Indian astrophysicist Meghnad N. Sahahas provided the physical basis for all subsequent interpretations of stellar spectra. The spectral sequence is also a colour sequence: In the case of cool stars of type M, the spectra indicate the presence of familiar metals, including ironcalciummagnesiumand also titanium oxide molecules TiOparticularly in the red and green parts of the spectrum.
Rigel Kentaurus (Alpha Centauri): Third-Brightest Star
In the somewhat hotter K-type starsthe TiO features disappear, and the spectrum exhibits a wealth of metallic lines. A few especially stable fragments of molecules such as cyanogen CN and the hydroxyl radical OH persist in these stars and even in G-type stars such as the Sun. The spectra of G-type stars are dominated by the characteristic lines of metals, particularly those of iron, calciumsodiummagnesium, and titanium.
The behaviour of calcium illustrates the phenomenon of thermal ionization. At low temperatures a calcium atom retains all of its electrons and radiates a spectrum characteristic of the neutral, or normal, atom; at higher temperatures collisions between atoms and electrons and the absorption of radiation both tend to detach electrons and to produce singly ionized calcium atoms. At the same time, these ions can recombine with electrons to produce neutral calcium atoms.
At high temperatures or low electron pressures, or both, most of the atoms are ionized. At low temperatures and high densities, the equilibrium favours the neutral state. The concentrations of ions and neutral atoms can be computed from the temperature, the density, and the ionization potential namely, the energy required to detach an electron from the atom.
In stars of spectral type Fthe lines of neutral atoms are weak relative to those of ionized atoms.
Star Facts: Hadar
The hydrogen lines are stronger, attaining their maximum intensities in A-type starsin which the surface temperature is about 9, K. Thereafter, these absorption lines gradually fade as the hydrogen becomes ionized. The hot B-type starssuch as Epsilon Orionis, are characterized by lines of helium and of singly ionized oxygennitrogenand neon.
In very hot O-type stars, lines of ionized helium appear. Other prominent features include lines of doubly ionized nitrogen, oxygen, and carbon and of triply ionized siliconall of which require more energy to produce.
In the more modern system of spectral classification, called the MK system after the American astronomers William W. Morgan and Philip C.