http://www.daviddarling.info/encyclopedia/R/RR_Lyrae_star.html wrote:
<<A short-period, yellow or white giant pulsating variable; RR Lyrae stars belong to Population II and are often found in globular clusters (hence one of their older names – cluster variables) or elsewhere in the galactic halo. They have periods of 0.2 to 2 days, amplitudes of 0.3 to 2 magnitudes, and spectral types of A2 to F6. Some of them have similar light curves to those of Cepheid variables and, like Cepheids, obey a period-luminosity relation that enables them to serve as reliable distance indicators. RR Lyrae variables, however, are older, less massive, and fainter (
with luminosities typical around 45 L{sun}) than Cepheids.>>
RR Lyrae stars and the Hertzsprung-Russell diagram
------------------------------------------------------------
http://en.wikipedia.org/wiki/RR_Lyrae_variable wrote:
<<RR Lyrae variables are periodic variable stars, commonly found in globular clusters, and often used as standard candles to measure galactic distances. RR Lyraes are pulsating horizontal branch stars of spectral class A (and rarely F), with a mass of around half the Sun's. They are thought to have previously shed mass and consequently, they were once stars with similar or slightly less mass than the Sun, around 0.8 solar masses.
RR Lyrae stars pulse in a manner similar to Cepheid variables, so the mechanism for the pulsation is thought to be similar, but the nature and histories of these stars is thought to be rather different. In contrast to Cepheids, RR Lyraes are old, relatively low mass, metal-poor "Population II" stars. They are much more common than Cepheids, but also much less luminous. (The average absolute magnitude of an RR Lyrae is 0.75, only 40 or 50 times brighter than our Sun.) Their period is shorter, typically less than one day, sometimes ranging down to seven hours.
The relationship between pulsation period and absolute magnitude of RR Lyraes makes them good standard candles for relatively near objects, especially within the Milky Way. They are extensively used in globular cluster studies, and also used to study chemical properties of older stars.>>
RR Lyrae stars were formerly called "cluster variables" because of their strong (but not exclusive) association with globular clusters; conversely, about 90% of all variables known in globular clusters are RR Lyraes. RR Lyrae stars are found at all galactic latitudes, as opposed to classical Cepheid variables, which are strongly associated with the galactic plane.
Several times as many RR Lyraes are known as all Cepheids combined; in the 1980s, about 1900 were known in globular clusters. Some estimates have about 85000 in the Milky Way.
From 1915 to the 1930s, the RR Lyraes became more accepted as a distinct class of star from classical Cepheids, on account of their shorter periods, different location within the galaxy, and finally, they are chemically different from classical Cepheids, being mostly metal-poor, Population II stars.
RR Lyraes have proven difficult to observe in external galaxies, because of their intrinsic faintness. (In fact, Walter Baade's failure to find them in the Andromeda galaxy led him to suspect that the galaxy was much farther away than predicted, and to re-consider the calibration of Cepheid variables and to propose stellar populations.) They were finally found in the 1980s by Pritchet & van den Bergh in the halo of the Andromeda galaxy, and more recently in its globular clusters by the Hubble Space Telescope.
Ann wrote:
Wikipedia says this about Walter Baade:
He took advantage of wartime blackout conditions during World War II, which reduced light pollution at Mount Wilson Observatory, to resolve stars in the center of the Andromeda galaxy for the first time, which led him to define distinct "populations" for stars (Population I and Population II). The same observations led him to discover that there are two types of Cepheid variable stars. This discovery led him to recalculate the size of the known universe, doubling the previous calculation made by Hubble in 1929. He announced this finding to considerable astonishment at the 1952 meeting of the International Astronomical Union in Rome.
Polaris is a classic Population I Cepheid variable (although it was once thought to be Population II due to its high galactic latitude). Since Cepheids are an important standard candle for determining distance, Polaris (as the closest such star) is heavily studied. Around 1900, the star luminosity varied ±8% from its average (0.15 magnitudes in total) with a 3.97 day period; however, the amplitude of its variation has been quickly declining since the middle of the 20th century. The variation reached a minimum of 1% in the mid 1990s and has remained at a low level. Over the same period, the star has brightened by 15% (on average), and the period has lengthened by about 8 seconds each year.
http://asterisk.apod.com/viewtopic.php? ... 91#p137482
http://www.skyandtelescope.com/news/15548412.html wrote:
Light Echoes Give Accurate Cepheid Distance
RS Puppis and surrounding nebula
http://apod.nasa.gov/apod/ap080212.html
<<For nearly a century now, astronomers have used Cepheid variable stars as "standard candles" whose apparent brightnesses tell how far away they are. Starting in 1912 Cepheids provided the first good distances to nearby galaxies. One of the reasons for building the Hubble Space Telescope was to measure Cepheids in galaxies farther out than could be done through Earth's fuzzy atmosphere. Indeed, "Hubble" was something of a double-entendre. The name honored the late Edwin Hubble, but the telescope was also intended to pin down the Hubble constant — the expansion rate of the universe — by comparing the redshifts of key galaxies to their distances found using Cepheid variables.
However, this only works if you know the distances to local Cepheids in our own galaxy well enough to calibrate the Cepheid distance scale as a whole. They're rather unusual super giant stars, so none of them lie close enough to the solar system for really accurate parallax measurements of their distances. Accordingly, astronomers have been expending great efforts to deduce local Cepheids' distances accurately in any way they can.
The best such measurements recently attained an accuracy of just a few percent. Now a group of astronomers has broken that record — by using a unique method to get a range on the bright Cepheid RS Puppis good to about 1.4 percent. They did it by measuring "light echoes" of the star's pulsations on a surrounding reflection nebula, combined with the star's accurately known pulsation period, the speed of light, and some simple geometry.
RS Puppis varies in brightness (from magnitude 6.5 to 7.6) every 41.4 days. It is 10 times more massive than the Sun, 200 times larger, and on average 15,000 times more luminous. Pierre Kervella and his colleagues used the European Southern Observatory's New Technology Telescope at La Silla, Chile, to record the faint reflections of these light pulses moving across the nebula. The speed at which they appeared to move, combined with the known speed of light, gave the distance to the nebula and star: 6,500 light years plus or minus 90.
RS Pup is the only Cepheid embedded in a large nebula. "Light that travels from the star to a dust grain to the telescope arrives a bit later than light that comes directly from the star to the telescope," explains Kervella. "As a consequence, if we measure the brightness of a particular, isolated dust blob in the nebula, we obtain a brightness curve that has the same shape as the variation of the Cepheid, but shifted in time." The delay is called a "light echo," by analogy with a sound echo off, say, a canyon wall.
"Knowing the distance to a Cepheid star with such an accuracy proves crucial to the calibration of the period-luminosity relation of this class of stars," says Kervella. RS Pup is especially important because it's one of the longest-period nearby Cepheids, and few of these have been well measured. The new result should help firm up the entire cosmic distance scale.>>
[quote=" http://www.daviddarling.info/encyclopedia/R/RR_Lyrae_star.html"]
<<A short-period, yellow or white giant pulsating variable; RR Lyrae stars belong to Population II and are often found in globular clusters (hence one of their older names – cluster variables) or elsewhere in the galactic halo. They have periods of 0.2 to 2 days, amplitudes of 0.3 to 2 magnitudes, and spectral types of A2 to F6. Some of them have similar light curves to those of Cepheid variables and, like Cepheids, obey a period-luminosity relation that enables them to serve as reliable distance indicators. RR Lyrae variables, however, are older, less massive, and fainter ([color=#FF0000][b]with luminosities typical around 45 L[sub]{sun}[/sub][/b][/color]) than Cepheids.>>[/quote]
[img]http://www.daviddarling.info/images/RR_Lyrae_stars.jpg[/img][img]http://zebu.uoregon.edu/~soper/MilkyWay/cepheid.gif[/img]
[b]RR Lyrae stars and the Hertzsprung-Russell diagram[/b]
------------------------------------------------------------
[quote=" http://en.wikipedia.org/wiki/RR_Lyrae_variable"]
<<RR Lyrae variables are periodic variable stars, commonly found in globular clusters, and often used as standard candles to measure galactic distances. RR Lyraes are pulsating horizontal branch stars of spectral class A (and rarely F), with a mass of around half the Sun's. They are thought to have previously shed mass and consequently, they were once stars with similar or slightly less mass than the Sun, around 0.8 solar masses.
RR Lyrae stars pulse in a manner similar to Cepheid variables, so the mechanism for the pulsation is thought to be similar, but the nature and histories of these stars is thought to be rather different. In contrast to Cepheids, RR Lyraes are old, relatively low mass, metal-poor "Population II" stars. They are much more common than Cepheids, but also much less luminous. (The average absolute magnitude of an RR Lyrae is 0.75, only 40 or 50 times brighter than our Sun.) Their period is shorter, typically less than one day, sometimes ranging down to seven hours.
The relationship between pulsation period and absolute magnitude of RR Lyraes makes them good standard candles for relatively near objects, especially within the Milky Way. They are extensively used in globular cluster studies, and also used to study chemical properties of older stars.>>
[img]http://www.caresa.com.au/astrophysicsdp_files/image028.gif[/img]
RR Lyrae stars were formerly called "cluster variables" because of their strong (but not exclusive) association with globular clusters; conversely, about 90% of all variables known in globular clusters are RR Lyraes. RR Lyrae stars are found at all galactic latitudes, as opposed to classical Cepheid variables, which are strongly associated with the galactic plane.
Several times as many RR Lyraes are known as all Cepheids combined; in the 1980s, about 1900 were known in globular clusters. Some estimates have about 85000 in the Milky Way.
From 1915 to the 1930s, the RR Lyraes became more accepted as a distinct class of star from classical Cepheids, on account of their shorter periods, different location within the galaxy, and finally, they are chemically different from classical Cepheids, being mostly metal-poor, Population II stars.
RR Lyraes have proven difficult to observe in external galaxies, because of their intrinsic faintness. (In fact, Walter Baade's failure to find them in the Andromeda galaxy led him to suspect that the galaxy was much farther away than predicted, and to re-consider the calibration of Cepheid variables and to propose stellar populations.) They were finally found in the 1980s by Pritchet & van den Bergh in the halo of the Andromeda galaxy, and more recently in its globular clusters by the Hubble Space Telescope.[/quote]
[quote="Ann"]
Wikipedia says this about Walter Baade:
[quote]He took advantage of wartime blackout conditions during World War II, which reduced light pollution at Mount Wilson Observatory, to resolve stars in the center of the Andromeda galaxy for the first time, which led him to define distinct "populations" for stars (Population I and Population II). The same observations led him to discover that there are two types of Cepheid variable stars. This discovery led him to recalculate the size of the known universe, doubling the previous calculation made by Hubble in 1929. He announced this finding to considerable astonishment at the 1952 meeting of the International Astronomical Union in Rome.[/quote][/quote]
Polaris is a classic Population I Cepheid variable (although it was once thought to be Population II due to its high galactic latitude). Since Cepheids are an important standard candle for determining distance, Polaris (as the closest such star) is heavily studied. Around 1900, the star luminosity varied ±8% from its average (0.15 magnitudes in total) with a 3.97 day period; however, the amplitude of its variation has been quickly declining since the middle of the 20th century. The variation reached a minimum of 1% in the mid 1990s and has remained at a low level. Over the same period, the star has brightened by 15% (on average), and the period has lengthened by about 8 seconds each year.
http://asterisk.apod.com/viewtopic.php?f=31&t=22095&p=137491#p137482
[quote=" http://www.skyandtelescope.com/news/15548412.html"]
Light Echoes Give Accurate Cepheid Distance
RS Puppis and surrounding nebula
http://apod.nasa.gov/apod/ap080212.html
<<For nearly a century now, astronomers have used Cepheid variable stars as "standard candles" whose apparent brightnesses tell how far away they are. Starting in 1912 Cepheids provided the first good distances to nearby galaxies. One of the reasons for building the Hubble Space Telescope was to measure Cepheids in galaxies farther out than could be done through Earth's fuzzy atmosphere. Indeed, "Hubble" was something of a double-entendre. The name honored the late Edwin Hubble, but the telescope was also intended to pin down the Hubble constant — the expansion rate of the universe — by comparing the redshifts of key galaxies to their distances found using Cepheid variables.
However, this only works if you know the distances to local Cepheids in our own galaxy well enough to calibrate the Cepheid distance scale as a whole. They're rather unusual super giant stars, so none of them lie close enough to the solar system for really accurate parallax measurements of their distances. Accordingly, astronomers have been expending great efforts to deduce local Cepheids' distances accurately in any way they can.
The best such measurements recently attained an accuracy of just a few percent. Now a group of astronomers has broken that record — by using a unique method to get a range on the bright Cepheid RS Puppis good to about 1.4 percent. They did it by measuring "light echoes" of the star's pulsations on a surrounding reflection nebula, combined with the star's accurately known pulsation period, the speed of light, and some simple geometry.
RS Puppis varies in brightness (from magnitude 6.5 to 7.6) every 41.4 days. It is 10 times more massive than the Sun, 200 times larger, and on average 15,000 times more luminous. Pierre Kervella and his colleagues used the European Southern Observatory's New Technology Telescope at La Silla, Chile, to record the faint reflections of these light pulses moving across the nebula. The speed at which they appeared to move, combined with the known speed of light, gave the distance to the nebula and star: 6,500 light years plus or minus 90.
RS Pup is the only Cepheid embedded in a large nebula. "Light that travels from the star to a dust grain to the telescope arrives a bit later than light that comes directly from the star to the telescope," explains Kervella. "As a consequence, if we measure the brightness of a particular, isolated dust blob in the nebula, we obtain a brightness curve that has the same shape as the variation of the Cepheid, but shifted in time." The delay is called a "light echo," by analogy with a sound echo off, say, a canyon wall.
"Knowing the distance to a Cepheid star with such an accuracy proves crucial to the calibration of the period-luminosity relation of this class of stars," says Kervella. RS Pup is especially important because it's one of the longest-period nearby Cepheids, and few of these have been well measured. The new result should help firm up the entire cosmic distance scale.>>[/quote]