R Scuti (''R Sct'') is a star in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

Scutum The ''scutum'' (; plural ''scuta'') was a type of shield used among Italic peoples in antiquity, most notably by the army of ancient Rome starting about the fourth century BC. The Romans adopted it when they switched from the military formati ...

. It is a yellow supergiant and is a

pulsating variable A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as e ...

known as an

RV Tauri variable RV Tauri variables are luminous variable stars that have distinctive light variations with alternating deep and shallow minima. History and discovery German astronomer Friedrich Wilhelm Argelander monitored the distinctive variations in brightne ...

. It was discovered in 1795 by Edward Pigott at a time when only a few

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as ...

s were known to exist.

Observation

R Sct is the brightest of the RV Tau-type stars and the

American Association of Variable Star Observers The American Association of Variable Star Observers (AAVSO) is an international nonprofit organization, founded in 1911, focused on coordinating, analyzing, publishing, and archiving variable star observations made largely by amateur astronomers. ...

(AAVSO) contains over 110,000 observations of this star. At its brightest it is visible to the

naked eye Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnifying, light-collecting optical instrument, such as a telescope or microscope, or eye protection. Vision corrected to normal ...

, and at its dimmest can be located with good

binoculars Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. Most binoculars are sized to be held ...

. In the sky it is about 1 degree northwest of the Wild Duck Cluster (Messier 11). RV Tauri variables often have somewhat irregular light curves, both in amplitude and period, but R Scuti is extreme. It has one of the longest periods known for an RV Tau variable, and the light curve has a number of unusual features: occasional extreme minima; intermittent standstills with only small erratic variation that may last for years; and periods of chaotic brightness changes. R-Sct-Lightcurve-AAVSO

Properties

The spectrum of R Scuti is peculiar, especially at cooler temperatures near the minima. In deep minima, much of the spectrum corresponds to an early K supergiant, but the spectrum also develops TiO bands more typical of an M-class star. The recognised spectral type of G0Iae-K2p(M3)Ibe gives an idea of the complexities of this star. Both the radius and temperature change, with maximum brightness corresponding to maximum temperature. The radius changes lag and the smallest size occurs about a quarter of the period after the minimum brightness. The spectral luminosity type classes R Sct as a supergiant, a

bright supergiant In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

(class Ia) near maximum, but it is actually thought to be a highly evolved and expanded low-mass star probably no more than 10,000 times the luminosity of the sun even at its brightest. The period-luminosity relation for type II Cepheids suggests a luminosity of , similar to the spectrophotometric derivation at . Other derivations have produced much lower values, but often making unrealistic assumptions about the distance based on an obsolete Hipparcos parallax value. An extreme post-AGB star would be expected to show measurable secular changes in its temperature and period over the time that R Scuti has been closely observed. Instead a relatively low mass loss rate with an extended cool atmosphere and fairly constant temperature and period of variation is seen. One suggestion is that R Scuti is still a thermally pulsing AGB star, consistent with calculated levels of mass loss. The evolutionary status of R Scuti is uncertain and estimates of the mass vary wildly. RV Tau variables as post-AGB stars are expected to have masses near or below that of the sun and this is confirmed for a number of RV Tau stars in binary systems. Old estimates based simply on the supergiant luminosity class gave much higher masses.

References

External links

* AAVSO
Quick Look View of AAVSO Observations
(get recent magnitude estimates for R Sct) {{DEFAULTSORT:R Scuti Scutum (constellation) RV Tauri variables G-type supergiants K-type supergiants M-type supergiants Scuti, R 173819 J18472894-0542185 092202 BD−05 4760 TIC objects

Observation

Properties

See also

References

External links