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Red stars can appear red because their photosphere spectrum peaks in the red, they're embedded in or behind a cloud that transmits in the red, or are/were perceived as red.


Around 150 AD, the Hellenistic astronomer Claudius Ptolemy described Sirius as reddish, along with five other stars, Betelgeuse, Antares, Aldebaran, Arcturus and Pollux, all of which are clearly of orange or red hue.[1] The discrepancy was first noted by amateur astronomer Thomas Barker, who prepared a paper and spoke at a meeting of the Royal Society in London in 1760.[2] The existence of other stars changing in brightness gave credence to the idea that some may change in colour too; Sir John Herschel noted this in 1839, possibly influenced by witnessing Eta Carinae two years earlier.[1] Thomas Jefferson Jackson See resurrected discussion on red Sirius with the publication of several papers in 1892, and a final summary in 1926.[1] He cited not only Ptolemy but also the poet Aratus, the orator Cicero, and general Germanicus as colouring the star red, though acknowledging that none of the latter three authors were astronomers, the last two merely translating Aratus' poem Phaenomena.[1] Seneca, too, had described Sirius as being of a deeper red colour than Mars.[3] However, not all ancient observers saw Sirius as red. The 1st century AD poet Marcus Manilius described it as "sea-blue", as did the 4th century Avienus.[1] It is the standard star for the color white in ancient China, and multiple records from the 2nd century BC up to the 7th century AD all describe Sirius as white in hue.[4][5]

In 1985, German astronomers Wolfhard Schlosser and Werner Bergmann published an account of an 8th century Lombardic manuscript, which contains De cursu stellarum ratio by St. Gregory of Tours. The Latin text taught readers how to determine the times of nighttime prayers from positions of the stars, and Sirius is described within as rubeola — "reddish". The authors proposed this was further evidence Sirius B had been a red giant at the time.[6]

Red dwarfs[edit]

This is a real visual image of AZ Cancri. Credit: SDSS Data Release 6.

A red dwarf is a small and relatively cool star on the main sequence, either late K or M spectral type. Red dwarfs are by far the most common type of star in the Galaxy, at least in the neighborhood of the Sun. Proxima Centauri, the nearest star to the Sun, is a red dwarf. Due to their low luminosity, individual red dwarfs cannot easily be observed. From Earth, none are visible to the naked eye.[7]

Typical characteristics[8]
M0V 60% 62% 7.2% 3,800
M1V 49% 49% 3.5% 3,600
M2V 44% 44% 2.3% 3,400
M3V 36% 39% 1.5% 3,250
M4V 20% 26% 0.55% 3,100
M5V 14% 20% 0.22% 2,800
M6V 10% 15% 0.09% 2,600
M7V 9% 12% 0.05% 2,500
M8V 8% 11% 0.03% 2,400
M9V 7.5% 8% 0.015% 2,300

The red dwarf AZ Cancri is shown in the visual image at right.

"[O]ut [of] a sample of 3,897 red dwarfs ... [the Kepler Space Telescope ]has identified 95 exoplanet candidates circling them. Three of these candidates are roughly Earth-size and orbit within their stars' "Goldilocks zone," where liquid water (and possibly life as we know it) can exist."[9]

Red squares[edit]

The Red Square Nebula (MWC 922) is a bipolar nebula appearing as an orange square in its center with red bowl-shaped gas and dust toward the top right and bottom left of the image. Credit: Peter Tuthill & James Lloyd.

"What could cause a nebula to appear square? No one is quite sure. The hot star system known as MWC 922, however, appears to be embedded in a nebula with just such a shape. The above image combines infrared exposures from the Hale Telescope on Mt. Palomar in California, and the Keck-2 Telescope on Mauna Kea in Hawaii. A leading progenitor hypothesis for the square nebula is that the central star or stars somehow expelled cones of gas during a late developmental stage. For MWC 922, these cones happen to incorporate nearly right angles and be visible from the sides. Supporting evidence for the cone hypothesis includes radial spokes in the image that might run along the cone walls. Researchers speculate that the cones viewed from another angle would appear similar to the gigantic rings of supernova 1987A, possibly indicating that a star in MWC 922 might one day itself explode in a similar supernova."[10]

Red rectangles[edit]

The Red Rectangle is a proto-planetary nebula. Here is the Hubble Space Telescope Advanced Camera for Surveys (ACS) image. Broadband red light is shown in red. Credit: ESA/Hubble and NASA.

Band spectra are the combinations of many different spectral lines, resulting from rotational, vibrational and electronic transition.

The "ERE manifests itself through a broad, featureless emission band of 60 < FWHM < 100 nm, with a peak appearing in the general wavelength range 610 < λp < 820 nm."[11]

In the Red Rectangle Nebula, diffraction-limited speckle images of it in visible and near infrared light reveal a highly symmetric, compact bipolar nebula with X-shaped spikes which imply toroidal dispersion of the circumstellar material. The central binary system is completely obscured, providing no direct light.[12]

"The star HD 44179 is surrounded by an extraordinary structure known as the Red Rectangle. It acquired its moniker because of its shape and its apparent colour when seen in early images from Earth. This strikingly detailed new Hubble image reveals how, when seen from space, the nebula, rather than being rectangular, is shaped like an X with additional complex structures of spaced lines of glowing gas, a little like the rungs of a ladder. The star at the centre is similar to the Sun, but at the end of its lifetime, pumping out gas and other material to make the nebula, and giving it the distinctive shape. It also appears that the star is a close binary that is surrounded by a dense torus of dust — both of which may help to explain the very curious shape. Precisely how the central engine of this remarkable and unique object spun the gossamer threads of nebulosity remains mysterious. It is likely that precessing jets of material played a role."[13]

The Red Rectangle is an unusual example of what is known as a proto-planetary nebula. These are old stars, on their way to becoming planetary nebulae. Once the expulsion of mass is complete a very hot white dwarf star will remain and its brilliant ultraviolet radiation will cause the surrounding gas to glow. The Red Rectangle is found about 2 300 light-years away in the constellation Monoceros (the Unicorn).

The High Resolution Channel of the NASA/ESA Hubble Space Telescope’s Advanced Camera for Surveys captured this view of HD 44179 and the surrounding Red Rectangle nebula — the sharpest view so far. Red light from glowing Hydrogen was captured through the F658N filter and coloured red.

Red giants[edit]

This is a real visual image of the red giant Mira by the Hubble Space Telescope. Credit: Margarita Karovska (Harvard-Smithsonian Center for Astrophysics) and NASA.
This is a visual or optical image of Aldebaran. Credit: Aladin at SIMBAD.{{fairuse}}
This image shows the Hyades star cluster, the nearest cluster to us with Aldebaran the bright star on the left. Credit: NASA, ESA, and STScI.{{free media}}

A red giant is a luminous giant star The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower. The appearance of the red giant is from yellow orange to red, including the spectral types K and M, but also class S stars and most carbon stars. The most common red giants are the so-called red giant branch stars (RGB stars). Another case of red giants are the asymptotic giant branch stars (AGB). To the AGB stars belong the carbon stars of type C-N and late C-R. The stellar limb of a red giant is not sharply-defined, as depicted in many illustrations. Instead, due to the very low mass density of the envelope, such stars lack a well-defined photosphere. The body of the star gradually transitions into a 'corona' with increasing radii.[14]

In Hindu astronomy Aldebaran is identified as Rohini ("the red one"), one of the twenty-seven daughters of Daksha and the wife of the god Chandra (the Moon).

Aldebaran shown on the left is a red giant on the red giant branch (RGB) at 44 times the diameter of the Sun.[15] equivalent to approximately 61 million kilometres.

Aldebaran is a slow irregular variable star, type LB, varying by about 0.2 in apparent magnitude from 0.75 to 0.95.[16] With a near-infrared J band magnitude of −2.1, only Betelgeuse (−2.9), R Doradus (−2.6), and Arcturus (−2.2) are brighter.[17]

Its photosphere shows abundant carbon, oxygen, and nitrogen.[18] With its slow rotation, Aldebaran may lack a corona and may not be a source of hard X-ray emission. However, small scale magnetic fields may still be present in the lower atmosphere, resulting from convective turbulence near the surface. (The measured strength of the magnetic field on Aldebaran is 0.22 Gauss (G).[19]) Any soft X-ray emissions from this region may be attenuated by the chromosphere, although ultraviolet emission has been detected in the spectrum.[20] The star is currently losing mass at a rate of (1–1.6) × 10−11 M yr−1 (about one Earth mass in 300,000 years) with a velocity of 30 km s−1.[18] This stellar wind may be generated by the weak magnetic fields.[20]

Beyond the chromosphere of Aldebaran is an extended molecular outer atmosphere where the temperature of 1,000−2,000 K is cool enough for molecules of gas to form. This region lies between 1.2 and 2.8 times the radius of the star. The spectrum reveals lines of carbon monoxide, water, and titanium oxide.[18] Past this radius, the modest outflow of the stellar wind itself declines in temperature to about 7,500 K at a distance of 1  AU. The wind continues to expand until it reaches the termination shock boundary with the hot, ionized interstellar medium that dominates the Local Bubble, forming a roughly spherical astrosphere with a radius of around 1,000 AU, centered on Aldebaran.[21]

Red supergiants[edit]

Red supergiants (RSGs) are supergiant stars (luminosity class I) of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive. Betelgeuse and Antares are the best known examples of a red supergiant. These stars have very cool surface temperatures (3500–4500 K), and enormous radii. The five largest known red supergiants in the Galaxy are VY Canis Majoris, VV Cephei A, V354 Cephei, RW Cephei and KW Sagittarii, which all have radii about 1500 times that of the [S]un (about 7 astronomical units, or 7 times as far as the Earth is from the [S]un). The radius of most red giants is between 200 and 800 times that of the [S]un. Absolute luminosities may reach -10 magnitude compared to +5 for our [S]un.

Red hypergiants[edit]

Wide Field and Planetary Camera 2 (WFPC2) Hubble Space Telescope (HST) image shows the asymmetric nebula surrounding VY CMa, which is the central star. Credit: Judy Schmidt.{{free media}}

Astrometric "results of phase-referencing very long baseline interferometry observations of 43 GHz SiO maser emission toward the red hypergiant VY Canis Majoris (VY CMa) [are from] using the Very Long Baseline Array (VLBA)."[22]

VY CMa is a single star with a large infrared (IR) excess, making it one of the brightest objects in the sky at wavelengths of between 5 and 20 µm and indicating a dust shell or disk heated by the star.[23][24]

VY CMa is embedded within the large molecular cloud Sharpless 310 (Sh2-310), one of largest star-forming H II regions with a diameter of 480 ' or 681 ly (209 pc).[25][26]

Since 1847, VY Canis Majoris has been described as a crimson star.[27] Visual observations in 1957 and high-resolution imaging in 1998 showed that there are no companion stars.[27][28]

VY CMa was also discovered to be a strong source of OH (1612 MHz), H
(22235.08 MHz), and SiO (43122 MHz) masers emission, which is typical of an OH/IR star.[29][30] [31]

Many molecules, such as HCN, NaCl, PN, CH, CO, CH
, TiO, and TiO
, have also been detected.[32][33][34][35][36]

The variation in VY CMa's brightness was first described in 1931 when it was listed (in German) as a long period variable with a photographic magnitude range of 9.5 to 11.5.[37]

It was given the variable star designation VY Canis Majoris in 1939, the 43rd variable star of the constellation Canis Major.[38]

Red clumps[edit]

This Hertzsprung-Russell diagram shows the evolution of stars of different masses. The red clump is marked RC on the green line showing the evolution of a star of 2 solar masses. Credit: .
The image shows orange and red stars in NGC 416. Credit: Andrzej Udalski, Peter Garnavich and Krzysztof Stanek.
This is the Hipparcos color-magnitude diagram. Credit: Hipparcos.
This image shows the variance in the I-band. Credit: Stanek and Garnavich (1998).

The red clump is a feature in the Hertzsprung-Russell diagram of stars. The red clump is considered the metal-rich counterpart to the horizontal branch. Stars in this part of the Hertzsprung-Russell diagram are sometimes called clump giants. These stars are more luminous than main sequence stars of the same surface temperature (or colder than main sequence stars of comparable luminosity), or above and to the right of the main sequence on the Hertzsprung-Russell diagram.

"Stellar cluster NGC 416 [in the image at the right is] located in the nearby Small Magellanic Cloud galaxy."[39]

"The cluster [in the image at the right] contains many red clump stars, allowing for accurate distance measurement to the host galaxy. This photograph was made with the 1.3-meter (51-inch) Warsaw University Telescope at Las Campanas Observatory, near La Sarena, Chile."[39]

"The ideal distance indicator would be a standard candle abundant enough to provide many examples within reach of parallax measurements and sufficiently bright to be seen out to local group galaxies."[39]

Red "clump stars (shown in the Hipparcos color-magnitude diagram [second image at the left]) precisely fit this description. These stars are the metal rich equivalent of the better known horizontal branch stars, and theoretical models predict that their absolute luminosity fairly weakly depends on their age and chemical composition. Indeed the absolute magnitude-color diagram of Hipparcos clearly shows how compact the red clump is."[39]

The "variance in the I-band is only about 0.15 mag (see the [second figure at the right])."[39]

"Red clump giants can provide a very precise estimate of the distances to metal-rich Galactic globular clusters and nearby galaxies."[39]

Tip of the red giant branch[edit]

Tip of the red-giant branch (TRGB) is a primary distance indicator used in astronomy. It uses the luminosity of the brightest red giant branch stars in a galaxy to gauge the distance to that galaxy. It has been used in conjunction with observations from the Hubble Space Telescope to determine the relative motions of the Local Cluster of galaxies within the Local Supercluster. There is a sharp discontinuity in the evolutionary track of the star on the HR diagram.[40] This discontinuity is called the tip of the red giant branch. When distant stars at the TRGB are measured in the I-band, their magnitude is somewhat insensitive to their composition of elements with more mass than helium (metallicity) and their mass. This makes the technique especially useful as a distance indicator. The TRGB indicator uses stars in the old stellar populations (Population II).[41]

Red novas[edit]

V838 Monocerotis in this real visual image from the Hubble Space Telescope is a prototypic luminous red nova. Credit: NASA, ESA and H.E. Bond (STScI).

A luminous red nova (abbr. LRN, pl. luminous red novae, pl.abbr. LRNe) is a stellar explosion thought to be caused by the merger of two stars. They are characterised by a distinct red colour, and a light curve that lingers with resurgent brightness in the infrared. Luminous red novae are not to be confused with standard novae, explosions that occur on the surface of white dwarf stars. The visible light lasts for weeks or months, and is distinctively red in colour, becoming dimmer and redder over time. As the visible light dims, the infrared light grows and also lasts for an extended period of time, usually dimming and brightening a number of times. Some astronomers believe it to be premature to declare a new class of stellar explosions based on such a limited number of observations. For instance, Pastorello et al. 2007[42] explained that the event may be due to a type II-p supernova and Todd et al. 2008[43] pointed out that supernovae undergoing a high level of extinction will naturally be both red and of low luminosity.

Red supernovae[edit]

"Supernovae [especially Type Ia (SNe Ia)], as extremely luminous (MB ~ -19.5) point sources, offer an attractive route to extragalactic distances. [...] Type II supernovae have a wide range in peak absolute magnitude and can not be treated as standard candles. Distances to individual SNe II can be estimated by means of the expanding photosphere (Baade-Wesselink) and the expanding radiosphere methods, but only elementary applications based on simplifying assumptions have been made to SNe II beyond the Local Group [...]. [Applications] of the method to SN 1987A in the Large Magellanic Cloud [have been] based on detailed calculations [...]. Supernovae of Type Ib, Type Ic, or Type II-L may turn out to be good standard candles but the present samples are small and all three subtypes have the disadvantage of being less luminous than Type Ia."[44]

"Supernovae of Type Ia lack hydrogen lines and helium lines in their optical spectra; during the first month after maximum light they do have a strong absorption feature produced by the red doublet (λ6347, λ6371 Å) of singly ionized silicon. [... One model is that] Type Ia supernovae are the result of the nuclear detonation of a white dwarf which is at or near the Chandrasekhar mass limit [...]. Since such stars are present in the old stellar populations of all galaxies (but see Foss et al. 1991), there is good reason to believe that Type Ia supernovae behave as standard candles."[44]

"Numerous analyses of unrestricted samples of SNe Ia, involving various assumptions about relative distances and interstellar extinction, have produced values of the dispersion in peak Mpg or MB that are generally consistent with [early results having a] determined σ = 0.6 mag. Smaller dispersions of 0.3-0.5 mag have been obtained by restricting the SN Ia samples to those beyond the Local Supercluster [...], in elliptical galaxies [...], in the Virgo cluster [...], and in the Coma cluster [...]. The restriction to remote samples lowers the dispersion by a combination of two effects: (1) the avoidance of the problem of uncertain relative distances for the nearby galaxies, and (2) the tendency to select against SNe Ia that are observationally subluminous (whether they are intrinsically subluminous or are highly extinguished)."[44]

A "small intrinsic dispersion for ordinary SNe Ia that may be ≲ 0.3 mag. Most SNe Ia that are observationally subluminous tend to be red and in inclined disk galaxies, and probably just suffer high interstellar extinction. The peculiar, intrinsically subluminous SN 1991bg also was red. If observationally faint events enter into samples of remote SNe Ia, in spite of the selection against them, they can be recognized by their colors, and, in the case of those that are intrinsically abnormal, by their spectra. There is not yet any solid evidence for anomalously bright SNe Ia."[44]

"From the Hubble diagram for [a] sample of 35 SNe Ia"[44]

with an error on the first constant of ± 0.08, "where h is the Hubble constant in units of 100 km s-1 Mpc-1."[44]

"From a sample of 40 SNe"[44]

with an error on the first constant of ± 0.04.

"The difference is primarily due to the fact that [for the first equation no] corrections for parent-galaxy extinction, while [for the second] an inclination-dependent correction to those SNe Ia in spirals that appear to be subluminous [has been applied]. Perhaps the most accurate available estimate for the intrinsic absolute magnitude is [for] for nine SNe Ia in ellipticals:"[44]

with an error on the first constant of ± 0.11.

"The standard model for a Type Ia supernova is the thermonuclear disruption of a carbon-oxygen white dwarf that has accreted enough mass from a companion star to approach the Chandrasekhar mass [...]. The nuclear energy released in the explosion unbinds the white dwarf and provides the kinetic energy of the ejected matter, but adiabatic expansion quickly degrades the initial internal energy and the observable light curve is powered by delayed energy input from the radioactive decay of 56Ni and 56Co. This model brings with it a self-calibration of the peak luminosity. Arnett (1982a) predicted on the basis of an analytical model that the SN Ia peak luminosity would be equal to the instantaneous decay luminosity of the nickel and cobalt, in which case the peak luminosity follows directly from the ejected nickel mass and the rise time to maximum light. The rise time can be inferred from observation but owing to uncertainties in the physics of the nuclear burning front [...] the amount of synthesized and ejected 56Ni cannot yet be accurately predicted by theory. [The] nickel mass can be estimated indirectly from spectra and light curves. The more nuclear burning, the more 56Ni and kinetic energy, and the greater the blueshifts in the spectrum and the faster the decay of the light curve. [From] the blueshifts in the spectra [...] the nickel mass must be in the range 0.4 to 1.4 M [with] a value of 0.6 M (as in the particular carbon deflagration model W7 [...]). Adopting a rise time to maximum of 17 ± 3 days and distributing the luminosity according to the observed ultraviolet-deficient flux distribution of SNe Ia, [provides an] estimated MB = -19.5+0.4-0.9) at bolometric maximum, which corresponds to MB = -19.6 with limits of -19.2 and -20.5 at the time of maximum blue light a few days earlier."[45]

Starburst galaxy[edit]

This mosaic image taken by the Hubble Space Telescope of Messier 82 combines exposures taken with four colored filters that capture starlight from visible and infrared wavelengths as well as the light from the glowing hydrogen filaments. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA).

"The presence of ERE has been established spectroscopically in ... the starburst galaxy M82 (Perrin, Darbon, & Sivan 1995)."[11]

"This mosaic image [at right] is the sharpest wide-angle view ever obtained of M82. The galaxy is remarkable for its bright blue disk, webs of shredded clouds, and fiery-looking plumes of glowing hydrogen blasting out of its central regions."[46]

"Throughout the galaxy's center, young stars are being born 10 times faster than they are inside our entire Milky Way Galaxy. The resulting huge concentration of young stars carved into the gas and dust at the galaxy's center. The fierce galactic superwind generated from these stars compresses enough gas to make millions of more stars."[46]

"In M82, young stars are crammed into tiny but massive star clusters. These, in turn, congregate by the dozens to make the bright patches, or "starburst clumps," in the central parts of M82. The clusters in the clumps can only be distinguished in the sharp Hubble images. Most of the pale, white objects sprinkled around the body of M82 that look like fuzzy stars are actually individual star clusters about 20 light-years across and contain up to a million stars."[46]

"The rapid rate of star formation in this galaxy eventually will be self-limiting. When star formation becomes too vigorous, it will consume or destroy the material needed to make more stars. The starburst then will subside, probably in a few tens of millions of years."[46]

"Located 12 million light-years away, M82 appears high in the northern spring sky in the direction of the constellation Ursa Major, the Great Bear. It is also called the "Cigar Galaxy" because of the elliptical shape produced by the oblique tilt of its starry disk relative to our line of sight."[46]

"The observation was made in March 2006, with the Advanced Camera for Surveys' Wide Field Channel. Astronomers assembled this six-image composite mosaic by combining exposures taken with four colored filters that capture starlight from visible and infrared wavelengths as well as the light from the glowing hydrogen filaments."[46]

See also[edit]


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