Observation data Epoch J2000.0 Equinox J2000.0 | |
---|---|
Constellation | Corona Borealis |
Right ascension | 15h 48m 34.4147s [1] |
Declination | +28° 09′ 24.295″ [1] |
Apparent magnitude (V) | 5.71 - 14.8 [2] |
Characteristics | |
Spectral type | G0Iep [3] |
U−B color index | 0.13 [4] |
B−V color index | 0.60 [4] |
V−R color index | 0.45 [4] |
J−H color index | 0.275 [5] |
J−K color index | 0.800 [5] |
Variable type | R CrB [6] [2] |
Astrometry | |
Radial velocity (Rv) | 27.83 [7] km/s |
Proper motion (μ) | RA: -2.10 [8] mas/yr Dec.: −11.52 [8] mas/yr |
Parallax (π) | 0.73 ± 0.27 mas [1] |
Distance | 1,400 [9] pc |
Absolute magnitude (MV) | −5 [9] |
Details | |
Mass | 0.8-0.9 [10] M☉ |
Radius | 85 [9] R☉ |
Luminosity | ~10,000 [11] L☉ |
Surface gravity (log g) | 0.5 [12] cgs |
Temperature | 6,750 [11] K |
Other designations | |
Database references | |
SIMBAD | data |
R Coronae Borealis is a low-mass yellow supergiant star in the constellation of Corona Borealis. It is the prototype of the R Coronae Borealis variable of variable stars, which fade by several magnitudes at irregular intervals. R Coronae Borealis itself normally shines at approximately magnitude 6, just about visible to the naked eye, but at intervals of several months to many years fades to as faint as 15th magnitude. Over successive months it then gradually returns to its normal brightness, giving it the nickname "reverse nova", after the more common type of star which rapidly increases in brightness before fading.
R Coronae Borealis is a faint naked eye star, but does not have any traditional names. Johann Bayer did not give it a Greek letter designation although it is marked on his map. John Flamsteed numbered all the Bayer stars but did not add any additional designations for fainter stars, so R Coronae Borealis does not appear in either of these two catalogues. [13]
At its discovery it was described simply as "the variable in the Northern crown". [13] It was later referred to as Variabilis Coronae, "Variable (star) of Corona (Borealis)". [14] It has also been called a "reverse nova" because of its habit of fading from sight. [15] The variable star designation R Coronae Borealis was introduced, as "Coronae R" by Friedrich Wilhelm Argelander in 1850. [16]
The variability of R Coronae Borealis was discovered by English astronomer Edward Pigott in 1795. [13] In 1935 it was the first star shown to have a different chemical composition to the Sun via spectral analysis. [17]
R Coronae Borealis is the prototype of the R Coronae Borealis class of variable stars. It is one of only two R Coronae Borealis variables bright enough to be seen with the naked eye, along with RY Sagittarii. [18] Much of the time it shows variations of around a tenth of a magnitude with poorly defined periods that have been reported as 40 and 51 days. These correspond to the first overtone and fundamental radial pulsation modes for an extreme helium star slightly under one M☉. [10]
At irregular intervals a few years or decades apart R Coronae Borealis fades from its normal brightness near 6th magnitude for a period of months or sometimes years. There is no fixed minimum, but the star can become fainter than 15th magnitude in the visual range. [19] The fading is less pronounced at longer wavelengths. Typically the star starts to return to maximum brightness almost immediately from its minimum, although occasionally this is interrupted by another fade. The cause of this behaviour is believed to be a regular build-up of carbon dust in the star's atmosphere. The sudden drop in brightness may be caused by a rapid condensation of carbon-rich dust similar to soot, resulting in much of the star's light being blocked. The gradual restoration to normal brightness results from the dust being dispersed by radiation pressure. [20]
In August 2007, R Coronae Borealis began a fade to an unprecedented minimum. It fell to 14th magnitude in 33 days, then continued to fade slowly, dropping below 15th magnitude in June 2009. It then began an equally slow rise, not reaching 12th magnitude until late 2011. This was an unusually deep and exceptional long minimum, longer even than a deep five year minimum which had occurred in 1962–7. It then faded again to near 15th magnitude, and by August 2014 it had been below 10th magnitude for 7 years. In late 2014, it brightened quickly to 7th magnitude but then began to fade again. [20] By mid-2017, it had been below its "normal" brightness for ten years. It also reached a new record faintest at magnitude 15.2. [19]
R Coronae Borealis at maximum light shows the spectrum a late F or early G yellow supergiant, but with marked peculiarities. Hydrogen lines are weak or absent, while carbon lines and molecular bands of cyanogen (CN) and C2 are exceptionally strong. Helium lines and metals such as calcium are also present. [17] The spectrum is variable, most obviously during the brightness fades. The normal absorption spectrum is replaced by emission lines, especially HeI, CaII, NaI, and other metals. The lines are typically very narrow at this stage. Helium emission lines sometimes show P Cygni profiles. In deep minima, many of the metal lines disappear although the Ca doublet remains strong. Forbidden "nebular" lines of [OI], [OII], and [NII] can be detected at times. [20]
The spectrum at maximum indicates that hydrogen in R Coronae Borealis is strongly depleted, helium is the dominant element, and carbon is strongly enhanced. At minimum, the spectrum shows the development of carbon clouds that obscure the photosphere, leaving chromospheric lines visible at times.
R Coronae Borealis is about 90% helium and less than 1% hydrogen. The majority of the remainder is carbon. [21] This classifies it as a carbon-enhanced extreme helium star. Modelling the pulsations suggests that the star's mass is 0.8-0.9 M☉. The temperature at maximum is reasonably well known at 6,900K and appears to decrease during the fades as the photosphere is obscured by condensing dust.
The distance of R Coronae Borealis is not known exactly, but is estimated at 1.4 kiloparsecs from assumptions about its intrinsic brightness. The absolute magnitude of −5 is calculated by comparison with R CrB variables in the Large Magellanic Cloud whose distances are known quite accurately. The luminosity is estimated from helium star models to be 19,000 L☉ and the star has a radius around 100 R☉. [22] The Gaia data release 1 parallax also gives a distance of 1.4 kpc although with a considerable margin of error. [1]
There is a fainter star 3" away from R Coronae Borealis, but it is believed to be a distant class K dwarf. Its colour and apparent magnitude are not consistent with being at the same distance as R Coronae Borealis. [20]
There are two main models for the formation of R CrB stars: the merger of two white dwarfs; or a very late helium flash in a post-AGB star. Models of post-AGB stars calculate that a star with the appearance of R CrB would have a mass around 0.6 M☉ so it is thought to have formed by the merger of a carbon-oxygen white dwarf and a helium white dwarf. [23] The detection of significant lithium in the atmosphere is not easily explained by the merger model, but is a natural consequence of a late helium flash. [22] Evolutionary models of post-AGB stars give a mass of 0.66 M☉ for R CrB, but with a considerable margin of error. [24]
Direct imaging with the Hubble Space Telescope shows extensive dust clouds out to a radius of around 2000 astronomical units from R Coronae Borealis, corresponding to a stream of fine dust (composed of grains about 5 nm in diameter) associated with the star's stellar wind, and coarser dust (composed of grains with a diameter of around 0.14 μm) ejected periodically. [25] The obscuration appears to happen closer to the star as clouds of carbon condense at shock regions in an expanding front. "Puffs" of dust emitted from the star condense at about 85 R☉ from the surface, and are visible as cometary knots when they lie to side of the star. [20] There is also a 2 M☉ shell about 4 pc wide containing dust at 25 K, which may be a fossil planetary nebula. [22]
Corona Borealis is a small constellation in the Northern Celestial Hemisphere. It is one of the 48 constellations listed by the 2nd-century astronomer Ptolemy, and remains one of the 88 modern constellations. Its brightest stars form a semicircular arc. Its Latin name, inspired by its shape, means "northern crown". In classical mythology Corona Borealis generally represented the crown given by the god Dionysus to the Cretan princess Ariadne and set by her in the heavens. Other cultures likened the pattern to a circle of elders, an eagle's nest, a bear's den or a smokehole. Ptolemy also listed a southern counterpart, Corona Australis, with a similar pattern.
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 temperature range of supergiant stars spans from about 3,400 K to over 20,000 K.
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.
La Superba is a strikingly red giant star in the constellation Canes Venatici. It is faintly visible to the naked eye, and the red colour is very obvious in binoculars. It is a carbon star and semiregular variable.
DY Persei variables are a subclass of R Coronae Borealis variables. They are carbon-rich asymptotic giant branch (AGB) stars that exhibit pulsational variability of AGB stars and irregular fades similar to R CrB stars.
An R Coronae Borealis variable is an eruptive variable star that varies in luminosity in two modes, one low amplitude pulsation, and one irregular, unpredictably-sudden fading by 1 to 9 magnitudes. The prototype star R Coronae Borealis was discovered by the English amateur astronomer Edward Pigott in 1795, who first observed the enigmatic fadings of the star. Only about 150 RCB stars are currently known in our Galaxy while up to 1000 were expected, making this class a very rare kind of star.
S Coronae Borealis is a Mira variable star in the constellation Corona Borealis. Its apparent magnitude varies between 5.3 and 13.6, with a period of 360 days—just under a year. Within the constellation, it lies to the west of Theta Coronae Borealis, and around 1 degree southeast of the eclipsing binary star U Coronae Borealis.
A yellow supergiant (YSG) is a star, generally of spectral type F or G, having a supergiant luminosity class. They are stars that have evolved away from the main sequence, expanding and becoming more luminous.
UX Antliae is a post-AGB and R Coronae Borealis variable star that has a base apparent magnitude of around 11.85, with irregular dimmings down to below magnitude 18.0.
FG Sagittae is a supergiant star in the constellation Sagitta at a distance of 4000 light-years. When first noted in 1943, it was identified to be a variable star, and it was found to be a hot, blue star of stellar spectral type B in 1955. Since then it has expanded and cooled, becoming a yellow G-type star by 1991, and then further cooling to become an orange K-type star. It started to pulsate when becoming an A-type star with a period of 15 days. This period later increased to over 100 days.
TX Piscium is a variable red giant star in the constellation Pisces. It is amongst the reddest naked eye stars, with a significant reddish hue when seen in binoculars. It is approximately 800 light years from Earth. It is close to—and sometimes considered part of—the asterism on the western end of the constellation called the circlet of Pisces.
RY Sagittarii is a yellow supergiant and an R Coronae Borealis type variable star in the constellation Sagittarius. Although it ostensibly has the spectrum of a G-type star, it differs markedly from most in that it has almost no hydrogen and much carbon.
RS Telescopii, abbreviated RS Tel, is a variable star in the southern constellation of Telescopium. It is a dim star with an apparent visual magnitude of 10.67, which is much too faint to be visible without a telescope. The variability of this star was discovered by Evelyn F. Leland and announced by Edward C. Pickering in 1910. It was first studied by Cecilia H. Payne in 1928 at the Harvard College Observatory.
V Coronae Australis is a R Coronae Borealis variable (RCB) star in the constellation Corona Australis. These are extremely hydrogen-deficient supergiants thought to have arisen as the result of the merger of two white dwarfs; fewer than 100 have been discovered as of 2012. V Coronae Australis dimmed in brightness from 1994 to 1998.
WX Coronae Australis is an R Coronae Borealis star in the constellation Corona Australis, one of the brightest examples of this extremely rare class of variable star. Despite the rarity, Corona Australis hosts another R CrB star, V Coronae Australis.
V Coronae Borealis is a Mira-type long period variable star and carbon star in the constellation Corona Borealis. Its apparent magnitude varies between 6.9 and 12.6 over a period of 357 days.
S Apodis, also known as HD 133444 is a variable star located in the southern circumpolar constellation Apus. It has an apparent magnitude ranging from 9.6 to 17, which is below the limit for naked eye visibility. The object is located relatively far at a distance of approximately 15,000 light years based on Gaia DR3 parallax measurements, but it is drifting closer with a heliocentric radial velocity of −75 km/s.
IK Tauri or NML Tauri is a Mira variable star located about 280 parsecs (910 ly) from the Sun in the zodiac constellation of Taurus.
DY Centauri is a variable star in the constellation Centaurus. From its brightness, it is estimated to be 7000 parsecs (23000 light-years) away from Earth.
V348 Sagittarii is a peculiar variable star in the southern constellation of Sagittarius, abbreviated V348 Sgr. It ranges in brightness from an apparent visual magnitude of 11.2 down to 18.4, requiring a telescope to view. Based on parallax measurements, it is located at an approximate distance of 3,700 light years from the Sun.