A Cepheid variable or Cepheid is a member of a particular class of variable stars, notable for a fairly tight correlation between their period of variability and absolute luminosity. Messier 100 (also known as NGC 4321) is a Spiral galaxy about 52 For the astronomical object see Variable star. Variable Star is a 2006 novel written by Spider Robinson The Instability strip is a nearly vertical region in the HR diagram which is occupied by pulsating Variable stars (including RR Lyrae variable, Cepheid Luminosity has different meanings in several different fields of science The namesake and prototype of these variables is the star Delta Cephei, discovered to be variable by John Goodricke in 1784. Namesake (sometimes "name's sake" is a term used to characterize a person place thing quality action state or idea that is called after or named out of regard to A prototype is an original type form or instance of something serving as a typical example basis or standard for other things of the same category Delta Cephei (δ Cep / δ Cephei is a Binary star system approximately 891 Light-years away in the Constellation of Cepheus (the King John Goodricke FRS ( September 17 1764 &ndash April 20 1786) was an eminent and profoundly Deaf amateur Astronomer
Because of this correlation (discovered and stated by Henrietta Swan Leavitt in 1908[1] and given precise mathematical form by her in 1912[2]), a Cepheid variable can be used as a standard candle to determine the distance to its host cluster or galaxy. Henrietta Swan Leavitt ( July 4 1868 &ndash December 12 1921) was an American Astronomer, and the Deaf daughter of a A standard candle is an astronomical object that has a known Luminosity. Star clusters are groups of Stars which are gravitationally bound A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter Since the period-luminosity relation can be calibrated with great precision using the nearest Cepheid stars, the distances found with this method are among the most accurate available.
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Description
A Cepheid is usually a population I giant yellow star, pulsing regularly by expanding and contracting, resulting in a regular oscillation of its luminosity. In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen In Astronomy, stellar classification is a classification of Stars based initially on photospheric temperature and its associated Spectral characteristics The luminosity of cepheid stars range from 103 to 104 times that of the Sun. The Sun (Sol is the Star at the center of the Solar System. Because Cepheids are from population I, they are sometimes called Type I Cepheids, while the similar (but belonging to population II) W Virginis variables are known as Type II Cepheids. In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen A W Virginis variable is a Variable star similar to a Cepheid variable.
The exact mass of Cepheids with given brightness or oscillations is not known to any great precision, but astronomers hope to gather data for this from the newly-discovered third star of the Polaris system [1]. Polaris (α UMi / α Ursae Minoris / Alpha Ursae Minoris commonly North(ern Star or Pole Star, and sometimes Lodestar
The variation in luminosity is caused by a cycle of ionization of helium in the star's atmosphere, followed by expansion and deionization. While ionized, the atmosphere is more opaque to light. This cycle has a period equal to the star's dynamical time scale, therefore giving information on the mean density of the body as well as its luminosity. The dynamical time scale, sometimes known as the freefall time scale, is in general the length of time over which changes in one part of a body can be communicated to the rest
Use as a "standard candle"
The relationship between a Cepheid variable's luminosity and variability period is quite precise, and has been used as a standard candle for almost a century and is part of the Extragalactic Distance Scale. A standard candle is an astronomical object that has a known Luminosity. The extragalactic distance scale is a series of techniques used today by astronomers to determine the distance of cosmological bodies (beyond our own galaxy not easily obtained with This connection was discovered in 1912 by Henrietta Swan Leavitt. Henrietta Swan Leavitt ( July 4 1868 &ndash December 12 1921) was an American Astronomer, and the Deaf daughter of a She measured the brightness of hundreds of Cepheid variables and discovered a distinct period-luminosity relationship. A three-day period Cepheid has a luminosity of about 800 times that of the Sun. The Sun (Sol is the Star at the center of the Solar System. A thirty-day period Cepheid is 10,000 times as bright as the Sun. The scale has been calibrated using nearby Cepheid stars, for which the distance was already known. This high luminosity, and the precision with which their distance can be estimated, makes Cepheid stars the ideal standard candle to measure the distance of clusters and external galaxies. Of course, a small error will be present because we do not know the precise location of the Cepheid variable within the cluster or galaxy. The word error has different meanings and usages relative to how it is conceptually applied This error is typically small enough to be irrelevant in these kinds of measurements.
Because of relatively high luminosity, Cepheid stars are visible from great distances. Edwin Hubble first identified some Cepheids in the Andromeda galaxy, thus proving its extragalactic nature (not known at that time). Edwin Powell Hubble ( November 20, 1889 – September 28, 1953) was an American astronomer. The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda More recently, the Hubble Space Telescope succeeded in identifying some Cepheid stars in the Virgo cluster, at a distance of 60 million light years. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into The Virgo Cluster is a cluster of galaxies at a distance of approximately 59 ± 4 Mly (18 A light-year or light year (symbol ly) is a unit of Length, equal to just under ten trillion Kilometres As defined by
Period-luminosity relationship
The relationship between a Type I Cepheid's period P, and its absolute magnitude Mv has been empirically derived by many astronomers throughout the Twentieth Century. In Astronomy, absolute magnitude (also known as absolute visual magnitude) is the Apparent magnitude an object would have if it were at a standard The twentieth century of the Common Era began on The relationship is calibrated using data collected from Cepheids whose distances are determined by other means. Ejnar Hertzsprung attempted the first calibration in 1913,[3] but, due to his ignorance of interstellar absorption, his results were highly inaccurate. Ejnar Hertzsprung ( October 8, 1873, Copenhagen &ndash October 21, 1967, Roskilde) was a Danish chemist Year 1913 ( MCMXIII) was a Common year starting on Wednesday (link will display the full calendar of the Gregorian calendar (or a Common The accuracy of the period-luminosity relationship has remained essentially unchanged since the calibration given in 1968 by Allan Sandage and Gustav Tammann. Year 1968 ( MCMLXVIII) was a Leap year starting on Monday (link will display full calendar of the Gregorian calendar. Allan Rex Sandage (born June 18 1926 in Iowa City Iowa) is an American Astronomer. Dr Gustav Tammann was the director of the Astronomical Institute of the University of Basel, Switzerland, European Space Agency member of the Space [4]
A more recent notable calibration was published by Michael Feast and Robin Catchpole in 1997. Year 1997 ( MCMXCVII) was a Common year starting on Wednesday (link will display full 1997 Gregorian calendar Using data from the Hipparcos satellite, Feast and Catchpole calculated the distances to many Galactic Cepheids via trigonometric parallax. Hipparcos (an Acronym for Hi gh P recision Par allax Co llecting S atellite) was an Astrometry mission Parallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between The resultant period-luminosity relationship was:
with P measured in days. [5][6]
Notes
- Some Cepheid stars (for example Polaris), have shown a decrease in their oscillation over a period of a few tens of years, and now are virtually constant. Polaris (α UMi / α Ursae Minoris / Alpha Ursae Minoris commonly North(ern Star or Pole Star, and sometimes Lodestar
- In 2008, astronomers announced they had measured the distance to the Cepheid RS Puppis to high accuracy using light reflected off a large nebula surrounding the star. RS Puppis (or RS Pup) is a Cepheid variable Star in the Constellation of Puppis. [7]
Examples
Some Cepheid variables with fairly bright apparent magnitudes and variations in brightness large enough to easily distinguish with the naked eye include Eta Aquilae, Zeta Geminorum, Beta Doradus, as well as the prototype Delta Cephei. The apparent magnitude ( m) of a celestial body is a measure of its Brightness as seen by an observer on Earth, normalized to the value The naked eye is a Figure of speech referring to human Visual perception that is unaided by enhancing equipment such as a Telescope or Eta Aquilae (η Aql / η Aquilae is a Star in the Constellation Aquila. Zeta Geminorum (&zeta Gem / &zeta Geminorum is a Star in the Constellation Gemini. Delta Cephei (δ Cep / δ Cephei is a Binary star system approximately 891 Light-years away in the Constellation of Cepheus (the King
See also
References
- ^ Leavitt, Henrietta S. The Instability strip is a nearly vertical region in the HR diagram which is occupied by pulsating Variable stars (including RR Lyrae variable, Cepheid Hubble's law is the statement in Physical cosmology that the Redshift in light coming from distant galaxies is proportional to their distance RR Lyrae variables are Variable stars often used as Standard candles RR Lyrae are pulsating Horizontal branch stars with a mass of around half the A W Virginis variable is a Variable star similar to a Cepheid variable. "1777 Variables in the Magellanic Clouds". Annals of Harvard College Observatory. LX(IV) (1908) 87-110.
- ^ Miss Leavitt in Pickering, Edward C. "Periods of 25 Variable Stars in the Small Magellanic Cloud". Harvard College Observatory Circular 173 (1912) 1-3.
- ^ Hertzsprung, Ejnar. "Uber die raumliche Verteilung der Verlandlichen vom Delta Cephei – typus". Astronomische Nachrichten. 196 (1913) 201-210.
- ^ Sandage, Allan & Gustav A. Tammann. " A composite period-luminosity relation for Cepheids at mean and maximum light". Astrophysical Journal. The Astrophysical Journal (abbreviated to ApJ or Astrophys J) is a Scientific journal covering Astronomy and Astrophysics. 151 (1968) 531-545.
- ^ Feast, Michael W. & Robin M. Catchpole. "The Cepheid period-luminosity zero-point from Hipparcos trigonometrical parallaxes". Monthly Notices of the Royal Astronomical Society. Monthly Notices of the Royal Astronomical Society (MNRAS is one of the world's leading Scientific journals in Astronomy and Astrophysics. 286 (1997) L 1-5.
- ^ Allen, Nick. "The Cepheid Distance Scale: A History". Aug 2005. http://www.institute-of-brilliant-failures.com.
- ^ Kervella, Pierre: Light echoes whisper the distance to a starhttp://www.eso.org/public/outreach/press-rel/pr-2008/pr-05-08.html
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