Saturday, November 28, 2015

Lyra - the Kepler mission

Kepler Space Observatory FOV
Image NASA/JPL via
Kepler mission

Kepler mission is currently among the most promising searches for planets orbiting a star in the habitable zone and thus candidates for finding evidence of water and extraterrestrial life. The quest for such planets is quickly developing into its own highly specialized branch of Astronomy with greatpublic interest. What and who is out there?
Kepler is a space observatory launched by NASA to discover Earth-like planets orbiting other stars. The spacecraft, named after the Renaissance astronomer Johannes Kepler, was launched on March 7, 2009. Designed to survey a portion of our region of the Milky Way to discover dozens of Earth-size extrasolar planets in or near the habitable zone and estimate how many of the billions of stars in the Milky Way have such planets,

Kepler‍‍ '​‍s sole instrument is a photometer that continually monitors the brightness of over 145,000 main sequence stars in a fixed field of view. This data is transmitted to Earth, then analyzed to detect periodic dimming caused by extrasolar planets that cross in front of their host star.

Exoplanets in the region of Lyra

The Kepler planet candidates
image NASA Kepler: a search for habitable planets
Lyra is one of three constellations (along with neighboring Cygnus and Draco) to be in the Kepler Mission's field of view, and as such it contains many more known exoplanets than most constellations.

Kepler 7-b
One of the first discovered by the mission is Kepler-7b, an extremely low-density exoplanet with less than half the mass of Jupiter, yet nearly 1.5 times the radius.

Kepler 8-b
Almost as sparse is Kepler-8b, only slightly more massive and of a similar radius.

The Kepler-20 system contains five known planets; three of them are only slightly smaller than Neptune, and two while the other two are some of the first Earth-sized exoplanets to be discovered.

Kepler-37 is another star with an exoplanet discovered by Kepler; the planet is the smallest known extrasolar planet known as of February 2013.

In April 2013, it was announced that of the five planets orbiting Kepler-62, at least two—Kepler-62e and Kepler-62f—are within the boundaries of the habitable zone of that star, where scientists think liquid water could exist, and are both candidates for being a solid, rocky, earth-like planet. The exoplanets are 1.6 and 1.4 times the diameter of Earth respectively, with their star Kepler-62 at a distance of 1,200 light-years.

Kepler Orrery IV
Video Credit & Copyright: Ethan Kruse (University of Washington)

Read the original wiki articles linked here under the text snippets for much more information, references and links.

Friday, November 27, 2015

Cheshire Cat, Einstein and God

"Chester Cat" Galaxy group
Image Credit: X-ray - NASA / CXC / J. Irwin et al. ; Optical - NASA/STScI
Smiling mysteriously like the Chester Cat in Alice in Wonderland these galaxies are really far away in the deep space some 4.5 billion light-years from Earth. That is close to the current estimation of the birth of the Solar system. The image combines X-ray and optical image data from the Chandra and Hubble space telescopes.
The arcs are optical images of distant background galaxies lensed by the foreground group's total distribution of gravitational mass dominated by dark matter.

In fact the two large elliptical "eye" galaxies represent the brightest members of their own galaxy groups which are merging. Their relative collisional speed of nearly 1,350 kilometers/second heats gas to millions of degrees producing the X-ray glow shown in purple hues.

Gravitational lens
A gravitational lens refers to a distribution of matter (such as a cluster of galaxies) between a distant source and an observer, that is capable of bending the light from the source, as it travels towards the observer.

This effect is known as gravitational lensing and the amount of bending is one of the predictions of Albert Einstein's general theory of relativity. (Classical physics also predicts bending of light, but only half that of general relativity's.

Although Orest Chwolson (1924) or Frantisek Klin (1936) are sometimes credited as being the first ones to discuss the effect in print, the effect is more commonly associated with Einstein, who published a more famous article on the subject in 1936.

Fritz Zwicky posited in 1937 that the effect could allow galaxy clusters to act as gravitational lenses.

It was not until 1979 that this effect was confirmed by observation of the so-called "Twin QSO" SBS 0957+561.

The Twin Quasar
"QSO B0957+0561" by ESA/Hubble & NASA
Licensed under Public Domain via Commons
The Twin Quasar (Twin QSO or Double Quasar or Old Faithful, also known as SBS 0957+561, TXS 0957+561, Q0957+561 or QSO 0957+561 A/B), was discovered in 1979 and was the first identified gravitationally lensed object. It is a quasar that appears as two images, a result from gravitational lensing caused by the galaxy YGKOW G1 that is located in the line of sight between Earth and the quasar. Wikipedia

Einstein and God
No - I am not going to argue here about Albert Einstein's Jewish heritage or his relationship with the God of Israel. The point I am emphasizing has nothing to do with Einstein's personal views on science and religion.

The theological significance of gravitational lensing is in the way the phenomenon was discovered. So many things in Creation have been found by careful observation, logical theory formation, testing and double-checking the observation.  But not in this case. Einstein figured out gravitational lensing by pure mathematical thinking without any experimental evidence at hand.

It is remarkable, that while Einstein suggested the bending of light already in the general theory of relativity in 1915, the conclusive proof that he was right was found 61 years later in 1976. Accordingly, there is a hiatus of more than half a century before such equipment had been engineered that could be used to test experimentally this particular aspect in Einstein's theory. But even with best instruments it was still not a simple matter to find that needle in the haystack, evidence of gravitational lensing among billions of stars and galaxies.
The quasars QSO 0957+561A/B were discovered in early 1979 by an Anglo-American team around Dennis Walsh (1933–2005), Robert Carswell and Ray Weyman, with the aid of the 2.1 m Telescope at Kitt Peak National Observatory in Arizona/USA. The team noticed that the two quasars were unusually close to each other, and that their redshift and visible light spectrum were surprisingly similar. They published their suggestion of "the possibility that they are two images of the same object formed by a gravitational lens".
In the New Testament we have a genuine letter that was written by apostle Paul to the Christians in the capital of Rome in the 50'ies AD.  Paul is inspired by the Holy Spirit while writing and scribbles down on papyrus immortal words about the God of Israel - for there is no other real God. while talking about the fallen state of humanity Paul emphasizes the deep thinking of the Creator which is obvious to all in His works. In modern terms we can express this by saying that Creation reflects the mathematical genius of the Creator.
For since the creation of the world God’s invisible qualities—his eternal power and divine nature—have been clearly seen, being understood from what has been made, so that people are without excuse.
Romans 1:20 NIV
With his extraordinary brain Einstein figured out in his mind in a radically new way gravity fields and concluded that they also bend light. Because he achieved this feast just by thinking something in his mind must have reflected the deep reality of the Creation.

I do not want to use this "found by thinking" argument as proof that God exists as has been done by many over the centuries. Because true faith in God does not come from intelligent reasoning - it is born from the Word of God and is a gift given to some. However, gravitational lensing has spirituality and even holiness, the nobility and genius of human mind at its sharpest and the sanctity of God of Israel, who has created that mind.

Wednesday, November 25, 2015

Lyra - GSC 02652-01324 and planet TrES-1b

"Exoplanet Comparison TrES-1 b" by Aldaron, a.k.a. Aldaron - Own work,
Licensed under CC BY-SA 3.0 via Commons
GSC 02652-01324 is an orange dwarf main sequence star approximately 512 light-years away in the constellation of Lyra.
Guide Star Catalog
The Guide Star Catalog (GSC) is also known as the Hubble Space Telescope, Guide Catalog (HSTGC). It is a star catalog compiled to support the Hubble Space Telescope with targeting off-axis stars.

GSC-I contained approximately 20,000,000 stars with apparent magnitudes of 6 to 15. GSC-II contains 945,592,683 stars out to magnitude 21. As far as possible, binary stars and non-stellar objects have been excluded or flagged as not meeting the requirements of Fine Guidance Sensors. This is the first full sky star catalog created specifically for navigation in outer space.

TrES-1b - a hot Jupiter
In 2004 the extrasolar planet TrES-1b was found to be orbiting this star by the Trans-Atlantic Exoplanet Survey using the transit method. The planet was detected crossing its parent star using a small 4-inch-diameter (100 mm) telescope. The discovery was confirmed by the Keck Observatory using the radial velocity method, allowing its mass to be determined.

The Trans-atlantic Exoplanet Survey or TrES, uses three 4-inch (10 cm) telescopes located at Lowell Observatory, Palomar Observatory, and the Canary Islands to locate exoplanets.

TrES-1b mass and radius indicate that it is a Jovian planet with a similar bulk composition to Jupiter. Unlike Jupiter, but similar to many other planets detected around other stars, TrES-1 is located very close to its star, and belongs to the class of planets known as hot Jupiters.

On March 22, 2005, Astronomers using NASA's Spitzer Space Telescope took advantage of the discovery to directly capture the infrared light of two previously detected planets orbiting outside our solar system. Their findings revealed the temperatures and orbits of the planets.

Upcoming Spitzer observations using a variety of infrared wavelengths may provide more information about the planets' winds and atmospheric compositions. It enabled determination of TrES-1's temperature, which is in excess of 1000 K (1340 °F). The planet's Bond albedo was found to be 0.31 ± 0.14.
Albedo or reflection coefficient, is derived from Latin albedo "whiteness" (or reflected sunlight) in turn from albus "white", is the diffuse reflectivity or reflecting power of a surface. It is the ratio of reflected radiation from the surface to incident radiation upon it.

Its dimensionless nature lets it be expressed as a percentage and is measured on a scale from zero for no reflection of a perfectly black surface to 1 for perfect reflection of a white surface.

Albedo depends on the frequency of the radiation. When quoted unqualified, it usually refers to some appropriate average across the spectrum of visible light.

Click on the links provided under the snippets to get more details about the subjects and references.

Tuesday, November 24, 2015

Lyra - Stars with planets and Doppler spectroscopy


HD 177830
HD 177830 is a 7th magnitude star located approximately 193 light-years away in the constellation of Lyra. It is slightly more massive than our Sun, but cooler being a type K star. Therefore, it is a subgiant clearly more evolved than the Sun. In visual light it is four times brighter than the Sun, but because of its distance, about 193 light years, it is not visible to the unaided eye. With binoculars it should be easily visible.

HD 173416
Visible to the naked eye are HD 173416, a yellow giant hosting a planet over twice the mass of Jupiter discovered in 2009.

HD 176051
HD 176051 is a low-mass binary star containing a high-mass planet.

On November 14, 1999, the discovery of a planet HD 177830 b was announced by the California and Carnegie Planet Search team using the very successful radial velocity method along with two other planets. This planet is nearly 50% more massive than Jupiter (MJ) and takes 407 days to orbit the star in an extremely circular orbit.

On November 17, 2010, the discovery of a second planet HD 177830 c was announced along with four other planets. The planet has 50% the mass of Saturn and takes 111 days to orbit the star in a very eccentric orbit. This planet is in a near 4:1 resonance with the outer planet.

Doppler spectroscopy

"Doppler Shift vs Time" by Dan Wingard
Licensed under Public Domain via Commons
Doppler spectroscopy (also known as the radial-velocity method, or colloquially, the wobble method) is an indirect method for finding extrasolar planets and brown dwarfs from radial-velocity measurements via observation of Doppler shifts in the spectrum of the planet's parent star.

About half of the extrasolar planets known were discovered using Doppler spectroscopy, as of October 2012.

A series of observations is made of the spectrum of light emitted by a star. Periodic variations in the star's spectrum may be detected, with the wavelength of characteristic spectral lines in the spectrum increasing and decreasing regularly over a period of time.

Statistical filters are then applied to the data set to cancel out spectrum effects from other sources.

Using mathematical best-fit techniques, astronomers can isolate the tell-tale periodic sine wave that indicates a planet in orbit.

If an extrasolar planet is detected, a minimum mass for the planet can be determined from the changes in the star's radial velocity.

To find a more precise measure of the mass requires knowledge of the inclination of the planet's orbit. A graph of measured radial velocity versus time will give a characteristic curve (sine curve in the case of a circular orbit), and the amplitude of the curve will allow the minimum mass of the planet to be calculated.

Although radial-velocity of the star only gives a planet's minimum mass, if the planet's spectral lines can be distinguished from the star's spectral lines then the radial-velocity of the planet itself can be found and this gives the inclination of the planet's orbit and therefore the planet's actual mass can be determined. The first non-transiting planet to have its mass found this way was Tau Boötis b in 2012 when carbon monoxide was detected in the infra-red part of the spectrum

Read the articles quoted from Wikipedia for much additional information and references.

Sunday, November 22, 2015

Lyra - Abell 46 and the Abell Planetary Catalog

Abell 46
Image Steve Gottlieb
Abell 46 is a planetary nebula. The central star, V477 Lyrae, is an eclipsing post-common-envelope binary, consisting of a white dwarf primary and an oversized secondary component due to recent accretion. The nebula itself is of relatively low surface brightness compared to the central star, and is undersized for the primary's mass for reasons not yet fully understood. Wikipedia

The Abell Catalog of Planetary Nebulae was created in 1966 by UCLA astronomer George O. Abell (1927–1983) and was composed of 86 entries thought to be planetary nebulae that were collected from discoveries, about half by Albert George Wilson and the rest by Abell, Robert George Harrington, and Rudolph Minkowski.

"P48 1994 Jean Large" by Picture scanned from the original by Michael Vergara
Licensed under CC BY-SA 3.0 via Commons
The planetary nebulae were discovered before August 1955 as part of the National Geographic Society – Palomar Observatory Sky Survey on photographic plates created with the 48-inch (1.2 m) Samuel Oschin telescope at Mount Palomar.

Observing notes by Steve Gottlieb
Planetaries on the Abell list are best viewed with a large aperture telescope (e.g. 18-inch (0.46 m)) and an OIII filter.
Summer: 17.5: at 200x and 140x using an OIII filter appears faint, moderately large, ~50" diameter, round. Can hold continuously with averted vision and visible with direct vision. Did not look unfiltered for the mag 15 central star.

13: at 79x with OIII filter appears extremely faint, moderately large, 1.0' diameter, almost round, can barely hold steadily. Just visible using a UHC visible although appears near the visual threshold.
Steve Gottlieb

NGC and John Dreyer

"John Dreyer" by Unknown
Licensed under Public Domain via Commons
The New General Catalogue of Nebulae and Clusters of Stars (abbreviated as NGC) is a well-known catalogue of deep-sky objects in astronomy compiled by John Louis Emil Dreyer in 1888, as a new version of John Herschel's Catalogue of Nebulae and Clusters of Stars.

The NGC contains 7,840 objects, known as the NGC objects. It is one of the largest comprehensive catalogues, as it includes all types of deep space objects and is not confined to, for example, galaxies.

Dreyer published two supplements to the NGC, known as the Index Catalogues (abbreviated as IC). The first was published in 1895 and contained 1,520 objects, while the second was published in 1908 and contained 3,866 objects, for a total of 5,386 IC objects.

Objects in the sky of the southern hemisphere are catalogued somewhat less thoroughly, but many were observed by John Herschel or James Dunlop.

The NGC had many errors, but a serious if not complete attempt to eliminate them has been initiated by the NGC/IC Project in 1993, after partial attempts with the Revised New General Catalogue (RNGC) by Jack W. Sulentic and William G. Tifft in 1973, and NGC2000.0 by Roger W. Sinnott in 1988.

The Revised New General Catalogue and Index Catalogue was compiled in 2009 by Wolfgang Steinicke.

John Dreyer
John Louis Emil Dreyer (1852 – 1926) was a Danish-Irish astronomer.

During 1878 he moved to Dunsink, the site of the Trinity College Observatory of Dublin University to work for Robert Stawell Ball.

In 1882 he relocated again, this time to Armagh Observatory, where he served as Director until his retirement in 1916.

In 1885 he became a British citizen. In 1916 he and his wife Kate moved to Oxford where Dreyer worked on his 15 volume edition of the works of Tycho Brahe, the last volume of which was published after his death.

He won the Gold Medal of the Royal Astronomical Society in 1916 and served as the society's president from 1923 until 1925.

History of the Planetary Systems from Thales to Kepler (1905), his survey of the history of astronomy, while dated in some respects, is still a good introduction to the subject. It is currently printed with the title A History of Astronomy from Thales to Kepler (available on-line).

A crater on the far side of the Moon is named after him.

Lyra - NGC 6745 Irregular galaxy

"NGC 6745" by NASA Goddard Space Flight Center NASA-GSFC
The Goddard Library. Licensed under Public Domain via Commons
NGC 6745 (also known as UGC 11391) is an irregular galaxy about 206 million light-years (63.5 mega-parsecs) away in the constellation Lyra. It is actually a triplet of galaxies in the process of colliding.

The three galaxies have been colliding for hundreds of millions of years. After passing through the larger galaxy (NGC 6745A), the smaller one (NGC 6745B) is now moving away. The larger galaxy was probably a spiral galaxy before the collision, but was damaged and now appears peculiar.

The collision created a region filled with young, hot, blue stars visible in the photo above.

It is unlikely that any stars in the two galaxies collided directly because of the vast distances between them. The gas, dust, and ambient magnetic fields of the galaxies, however, do interact directly in a collision. As a result of this interaction, the smaller galaxy has probably lost most of its interstellar medium to the larger one.

Friday, November 20, 2015

Lyra - Alathfar and spectral class A

Fomalhaut,(Alpha Piscis Austrini) is an A3 main-sequence star
"Heic0821f" by Davide De Martin
Licensed under Public Domain via Commons

Mu Lyrae (μ Lyr, μ Lyrae) is main-sequence dwarf star with apparent magnitude 5.12. It belongs to the spectral class A0IV.  Located around 439 light-years distant, it shines with a luminosity approximately 125 times that of the Sun and has a surface temperature of 8190 K.

The star has the traditional name Alathfar, from the Arabic الأظفر al-’uz̧fur "the talons (of the swooping eagle)", a name it shares with Eta Lyrae (usually spelled with d instead of th: Aladfar).

Let me repeat: luminosity of 125 times that of the Sun.

A-type stars 
These are among the more common naked eye stars and are white or bluish-white.

They have strong hydrogen lines, at a maximum by A0, and also lines of ionized metals (Fe II, Mg II, Si II) at a maximum at A5. The presence of Ca II lines is notably strengthening by this point.

About 1 in 160 (0.625%) of the main-sequence stars in the solar neighborhood are A-type stars.

Spectral standards:
  • A0Van: Gamma Ursae Majoris
  • A0Va: Vega
  • A0Ib: Eta Leonis
  • A0Ia: HD 21389
  • A2Ia: Deneb
  • A3Va: Fomalhaut

Thursday, November 19, 2015

Lyra - Aladfar η Lyrae Blue subgiant

Alcyone, Pleiades, is a typical blue giant
NASA, ESA, AURA/Caltech, Palomar Observatoryderivative work:
Roberto Segnali all'Indiano - Pleiades_large.jpg. Licensed under Public Domain via Commons

Eta Lyrae is a blue subgiant with nearly similar metal abundance to Sun. The star belongs to spectral class B2.5IV and has apparent magnitude of +4.40.  It is approximately 1390 light years from Earth.

The originally Arabic name Aladfar الأظفر al-’uz̧fur means "the talons (of the swooping eagle)" with Arab astronomers association of Lyra with an eagle.

Blue giants
Blue giant Bellatrix compared to Algol B, the Sun, a red dwarf, and some planets
Image by User:84user, User:Paul Stansifer and others
Licensed under GPLv2 via Commons
Blue giant is a hot star with a luminosity class of III (giant) or II (bright giant). In the standard Hertzsprung–Russell diagram, these stars lie above and to the right of the main sequence.

The name is applied to a wide variety of different types of stars with a moderate increase in size and luminosity compared to main-sequence stars of the same mass or temperature, and are hot enough to be called blue, meaning spectral class O, B, and sometimes early A.

Stars found in the blue giant region of the HR diagram can be in very different stages of their lives, but all are evolved stars that have largely exhausted their core hydrogen supplies.

Blue giants have temperatures from around 10,000 K upwards, ZAMS masses greater than about twice the Sun (M☉), and absolute magnitudes around 0 or brighter. These stars are only 5–10 times the radius of the Sun (R☉), compared to red giants which are up to 100 R☉.

Blue giants are much rarer than red giants, because they only develop from more massive and less common stars, and because they have short lives in the blue giant stage.

Hertzsprung–Russell diagram
by User:Rursus. Licensed under CC BY-SA 3.0 via Commons

Tuesday, November 17, 2015

Lyra - M57 Planetary nebula

Messier 57 Ring Nebula by Hubble Space Telescope
"M57 The Ring Nebula" by The Hubble Heritage Team (AURA/STScI/NASA)
Licensed under Public Domain via Commons
 M57, also known as the Ring Nebula and NGC 6720, is a planetary nebula classified as a bipolar nebula. It is between 6,000 and 8,000 years old and approximately one light-year in diameter. The diameter is about one light-year and is at a distance of 2,000 light-years from Earth.

"Antoine Darquier de Pellepoix" by G. Vidal
Licensed under Public Domain via Commons
Ring Nebula is one of the best known planetary nebulae and the second to be discovered; its integrated magnitude is 8.8. It was discovered in January 1779 by French astronomer Antoine Darquier (1718 – 1802) fifteen years after Charles Messier discovered the Dumbbell Nebula in the constellation Vulpecula. The planetary nucleus, a white dwarf, was discovered by Hungarian astronomer Jenő Gothard on September 1, 1886.
  • The central star is a white dwarf with a temperature of 120,000 Kelvin.  
  • The outer part of the nebula appears red in photographs because of emission from ionized hydrogen
  • The middle region is colored green; doubly ionized oxygen emits greenish-blue light
  • The hottest region, closest to the central star, appears blue because of emission from helium

Location of M57
Sky map Wikimedia

Planetary nebulae

Computer simulation of the formation of a planetary nebula from a star with a warped disk,
showing the complexity which can result from a small initial asymmetry.
Credit: Vincent Icke Huygens Laboratory, Leiden University, Netherlands
Licensed under CC BY-SA 3.0 via Commons
A planetary nebula  is a kind of emission nebula consisting of an expanding glowing shell of ionized gas ejected from old red giant stars late in their lives. They are a relatively short-lived phenomenon, lasting a few tens of thousands of years, compared to a typical stellar lifetime of several billion years.

At the end of the star's life, during the red giant phase, the outer layers of the star are expelled by strong stellar winds. Eventually, after most of the red giant's atmosphere is dissipated, the exposed hot, luminous core emits ultraviolet radiation to ionize the ejected outer layers of the star. Absorbed ultraviolet light energises the shell of nebulous gas around the central star, appearing as a bright coloured planetary nebula at several discrete visible wavelengths.

Planetary nebulae may play a crucial role in the chemical evolution of the Milky Way, returning material to the interstellar medium from stars where elements, the products of nucleosynthesis (such as carbon, nitrogen, oxygen and neon), have been created.

Planetary nebulae are also observed in more distant galaxies, yielding useful information about their chemical abundances.

In recent years, Hubble Space Telescope images have revealed many planetary nebulae to have extremely complex and varied morphologies. About one-fifth are roughly spherical, but the majority are not spherically symmetric. The mechanisms which produce such a wide variety of shapes and features are not yet well understood, but binary central stars, stellar winds and magnetic fields may play a role.

Click on the links provided for detailed discussions on the subjects and for references.

Monday, November 16, 2015

Lyra - Globular cluster Messier 56 and Dwarf galaxies

M56 Glubar cluster by Hubble Space Telescope
"Messier 56 Hubble WikiSky" by en:NASA, en:STScI
icensed under Public Domain via Commons

Messier 56 (NGC 6779) is a globular cluster at a distance of about 32,900 light-years from Earth and measures roughly 84 light-years across, with a combined mass some 230,000 times that of the Sun. It is about 31–32 kly (9.5–9.8 kpc) from the Galactic Center and 4.8 kly (1.5 kpc) above the galactic plane. The cluster has an estimated age of 13.70 billion years and is following a retrograde orbit through the Milky Way.

"Charles Messier at Age of 40" by Ansiaume (1729—1786)
Licensed under Public Domain via Commons
M56 was discovered by the French Astronomer Charles Messier (1730 – 1817) on January 19, 1779.
The properties of this cluster suggest that it may have been acquired during the merger of a dwarf galaxy, of which Omega Centauri forms the surviving nucleus. The abundance of elements other than hydrogen and helium (metallicity) has a very low value of [Fe/H] = –2.00 dex. This is equivalent to 1% of the abundance in the Sun.

The brightest stars in M56 are of 13th magnitude, while it contains only about a dozen known variable stars, such as V6 (RV Tauri star; period: 90 days) or V1 (Cepheid: 1.510 days); Other variable stars are V2 (irregular) and V3 (semiregular).

In 2000, a diffuse X-ray emission was tentatively identified coming from the vicinity of the cluster. This is most likely interstellar medium that has been heated by the passage of the cluster through the galactic halo. The relative velocity of the cluster is about 177 km s−1, which is sufficient to heat the medium in its wake to a temperature of 940,000 K..

Globular clusters
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite.

Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers.  They are found in the halo of a galaxy and contain considerably more stars and are much older than the less dense galactic, or open clusters, which are found in the disk.

Every galaxy of sufficient mass in the Local Group has an associated group of globular clusters, and almost every large galaxy surveyed has been found to possess a system of globular clusters. There are about 150 to 158 currently known globular clusters in the Milky Way, with perhaps 10 to 20 more still undiscovered. They orbit the Galaxy at radii of 40 kiloparsecs (130,000 light-years) or more. Larger galaxies can have more: Andromeda, for instance, may have as many as 500. Some giant elliptical galaxies, particularly those at the centers of galaxy clusters, such as M87, have as many as 13,000 globular clusters.

The Sagittarius Dwarf galaxy and the disputed Canis Major Dwarf galaxy appear to be in the process of donating their associated globular clusters (such as Palomar 12) to the Milky Way.  This demonstrates how many of this galaxy's globular clusters might have been acquired in the past.

Although it appears that globular clusters contain some of the first stars to be produced in the galaxy, their origins and their role in galactic evolution are still unclear. It does appear clear that globular clusters are significantly different from dwarf elliptical galaxies and were formed as part of the star formation of the parent galaxy rather than as a separate galaxy. However, recent conjectures by astronomers suggest that globular clusters and dwarf spheroidals may not be clearly separate and distinct types of objects.

Dwarf Galaxies

GALEX. artist's view
image Licensed under Public Domain via Commons
A dwarf galaxy is a small galaxy composed of up to several billion stars, a small number compared to our own Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is sometimes classified as a dwarf galaxy; others consider it a full-fledged galaxy.

Dwarf galaxies' formation and activity are thought to be heavily influenced by interactions with larger galaxies. Astronomers identify numerous types of dwarf galaxies, based on their shape and composition.

Current theory states that most galaxies, including dwarf galaxies, form in association with dark matter, or from gas that contains metals. However, NASA's Galaxy Evolution Explorer space probe identified new dwarf galaxies forming out of gases lacking metals. These galaxies were located in the Leo Ring, a cloud of hydrogen and helium around two massive galaxies in the constellation Leo.

Because of their small size, dwarf galaxies have been observed being pulled toward and ripped by neighbouring spiral galaxies, resulting in galaxy merger.

Click on the given links for more detailed discussions and references.

Blazing fireball! An amazing astrophoto by Ivo Sheggia

Meteorite in fire!
Image Copyright Ivo Shaggia via APOD
Astrophotography is demanding business as everyone who has tried it knows. It requires excellent equipment, high technical skills and lots of experience. The ratio between failures and successes can be rather discouraging especially for the beginner. But occasionally true wonders happen to the patient photographer such as in this photo that was intended to show Orion Nebula but went wonderfully wrong. The lucky photo was taken by Ivo Sheggia in Swiss Alps.

Personally, I have never before seen pictures of such "flames". This must be a once in a lifetime photo, the kind of which cannot be planned nor prepared - it just happened like a winning ticket in lottery. The camera was pointing to the right direction and the shutter was open just in the right moment - not a minute too early nor a minute too late.

Click the link below to see this exquisite photo in all its glory and to learn details about it. There is written, among other things that
the persistent train's glow emanated from atoms in the Earth's atmosphere in the path of the meteor -- atoms that had an electron knocked away and emit light during reacquisition. Persistent trains often drift, so that the long 3-minute exposure actually captured the initial wind-blown displacement of these bright former ions.
Congratulations Ivo Shaggia, what a great photo!

Sunday, November 15, 2015

Lyra - HR Lyrae classical nova White dwarf

"Making a Nova" by NASA/CXC/M.Weiss
Licensed under Public Domain via Commons
HR Lyrae flared in 1919 to a maximum magnitude of 6.5, over 9.5 magnitudes higher than in quiescence. Some of its characteristics are similar to those of recurring novae. Wikipedia

A nova is a cataclysmic nuclear explosion on a white dwarf, which causes a sudden brightening of the star. Novae are not to be confused with other brightening phenomena such as supernovae or luminous red novae.

Novae are thought to occur on the surface of a white dwarf in a binary system when they are sufficiently near to one another, allowing material (mostly hydrogen) to be pulled from the companion star's surface onto the white dwarf. The nova is the result of the rapid fusion of the accreted hydrogen on the surface of the star, commencing a runaway fusion reaction.

White dwarf
Image of Sirius A and Sirius B taken by the Hubble Space Telescope
"Sirius A and B Hubble photo.editted" by Bokus
Licensed under Public Domain via Commons
A white dwarf, also called a degenerate dwarf, is a stellar remnant composed mostly of electron-degenerate matter.

A white dwarf is very dense: its mass is comparable to that of the Sun, and its volume is comparable to that of Earth.

A white dwarf's faint luminosity comes from the emission of stored thermal energy.

The nearest known white dwarf is Sirius B, at 8.6 light years, the smaller component of the Sirius binary star.

There are currently thought to be eight white dwarfs among the hundred star systems nearest the Sun.

The unusual faintness of white dwarfs was first recognized in 1910. The name white dwarf was coined by Dutch-American astronomer Willem Luyten (1899 – 1994) in 1922.

White dwarfs are thought to be the final evolutionary state of stars (including our Sun) whose mass is not high enough to become a neutron star—over 97% of the stars in the Milky Way.

After the hydrogen–fusing period of a main-sequence star of low or medium mass ends, a star will expand to a red giant during which it fuses helium to carbon and oxygen in its core by the triple-alpha process.

If a red giant has insufficient mass to generate the core temperatures required to fuse carbon, around 1 billion K, an inert mass of carbon and oxygen will build up at its center. After shedding its outer layers to form a planetary nebula, it will leave behind this core, which forms the remnant white dwarf.

Lyra - AY and V344 Lyrae SU Ursae Majoris dwarf novae

Dwarf nova HT Cas seen in outburst (mag ~13.4) on November 2, 2010
"HTCas-LB1-2010Nov12" by Kevin Heider. Licensed under CC BY-SA 3.0 via Commons

AY Lyraw ia an SU Ursae Majoris-type dwarf nova that has undergone several superoutbursts.

V344 Lyrae is notable for an extremely short period between superoutbursts coupled with one of the highest amplitudes for such a period.

Dwarf nova
A U Geminorum-type variable star, or dwarf nova (pl. novae) is a type of cataclysmic variable star consisting of a close binary star system in which one of the components is a white dwarf that accretes matter from its companion.

The first one to be observed was U Geminorum in 1855; however, the mechanism was not known till 1974, when Brian Warner showed that the nova is due to the increase of the luminosity of the accretion disk.

They are similar to classical novae in that the white dwarf is involved in periodic outbursts, but the mechanisms are different: classical novae result from the fusion and detonation of accreted hydrogen, while current theory suggests that dwarf novae result from instability in the accretion disk, when gas in the disk reaches a critical temperature that causes a change in viscosity, resulting in a collapse onto the white dwarf that releases large amounts of gravitational potential energy.

Dwarf novae are distinct from classical novae in other ways; their luminosity is lower, and they are typically recurrent on a scale from days to decades. The luminosity of the outburst increases with the recurrence interval as well as the orbital period; recent research with the Hubble space telescope suggests that the latter relationship could make dwarf novae useful standard candles for measuring cosmic distances.

There are three subtypes of U Geminorum star (UG):

  • SS Cygni stars (UGSS), which increase in brightness by 2-6 mag in V in 1–2 days, and return to their original brightnesses in several subsequent days.
  • SU Ursae Majoris stars (UGSU), which have brighter and longer "supermaxima" outbursts, or "super-outbursts," in addition to normal outbursts. Varieties of SU Ursae Majoris star include ER Ursae Majoris stars and WZ Sagittae stars. 
  • Z Camelopardalis stars (UGZ), which temporarily "halt" at a particular brightness below their peak. 

Text snippets are provided for your convenience. Click on the links for much addition information, links and references.

Lyra . MV Lyrae nova-like star

VY Sculptor star observation
Graph Astro Keel, UK

MV Lyrae a nova-like star consisting of a red dwarf and a white dwarf.

Originally classified as a VY Sculptoris star due to spending most time at maximum brightness, since around 1979 the system has been dominantly at minimum brightness, with periodic outbursts. Its nature is still not fully understood. Wikipedia

For an example of a scientific report on observing MV Lyrae see
Pavlenko, E. P. (Crimean Astrophysical Observatory, Nancy, Ukraine)
Shugarov, S. Yu. (Sternberg State Astronomical Institute, Moscow, Russia)
Photometric study of the nova-like variable MV Lyrae during an enormous outburst in 1997. Astronomy and Astrophysics 343, 909–915 (1999).  Harvard link PDF
VY Sculptoris star is a nova-like variable that occasionally shows a sudden drop in brightness, reminiscent of a *Z Camelopardalis star. Oxford Dictionary of Astronomy

Nova-like stars
Cataclysmic (Explosive or Nova-like) Variables
These are variable stars that show outbursts which are caused by thermonuclear burst processes on the surface (novae) or deep in the interior of the star (supernovae).

The majority of explosive and nova-like systems are close binaries, which are often made up of a white dwarf and normal star companion.

The stars have strong mutual effects on each others development. A normal star, similar to our Sun, will lose mass onto the white dwarf by accretion because the white dwarf is incredibly dense and has huge gravitational potential.

Classic nova outbursts are caused by sudden nuclear fusion of hydrogen-rich material on the surface of the white dwarf. White Dwarfs are the cinders of our stars like the sun, hydrogen fusion is only possible when fresh fuel is accreted onto its surface, and the normal star in close binary system does this.
Astro Keele UK

Lyra - Gliese 758 and Subaru Telescope

The Star
Gliese 758 is a 6th magnitude Sun-like star with 97% of the Sun's mass and 88% of the radius of the Sun. Parallax measurements from the Hipparcos mission give it an estimated distance of around 51.4 light-years (15.8 parsecs) from Earth

The spectrum matches a stellar classification of G8V, identifying it as a G-type main sequence star that is generating energy through the nuclear fusion of hydrogen at its core. It is radiating this energy into space from its outer envelope at an effective temperature of 5425 K.

Estimates of its age put it at about 7.7–8.7 billion years old, although some measurements give it an age as low as 720 million years. The abundance of elements other than hydrogen and helium, what astronomers term the star's metallicity, are 51% higher than in the Sun.

Are Gliese 759 b and c planets?
In November 2009, a team using the HiCIAO instrument of the Subaru Telescope imaged a substellar companion orbiting the star. This object, designated Gliese 758 b, was estimated to be of approximately 10-40 Jupiter masses.

A second candidate object was also detected, which was given the designation Gliese 758 C. Follow-up studies of the system suggested the mass range of Gliese 758 b, indicating it to be a companion with approximately 30 to 40 Jupiter masses and revealed that Gliese 758 C is a background star which is not physically associated with the Gliese 758 system.

On the other hand, a younger age was suggested from the kinematic stellar grouping.

Subaru Telescope

"MaunaKea Subaru" by Denys (fr) - Own work. Licensed under CC BY 3.0 via Commons
Subaru Telescope  is the 8.2 meter flagship telescope of the National Astronomical Observatory of Japan, located at the Mauna Kea Observatory on Hawaii. It is named after the open star cluster known in English as the Pleiades. It had the largest monolithic primary mirror in the world from its commission 1999 until 2005. Wikipedia

The large single mirror for Subaru was cast by Corning and polished at Contraves Brashear Systems in Pennsylvania.

One of the many instruments is High-Contrast Coronographic Imager for Adaptive Optics (HiCIAO) infrared camera for hunting planets around other stars.

Subaru Telescope official website

List of worlds largest reflecting telescopes

Saturday, November 14, 2015

Lyra - 19 Lyrae and Alpha2 Canum Venaticorum variables

19 Lyrae, is a small-amplitude variable, an Alpha2 Canum Venaticorum variable with a period of just over one day. Wikipedia

Alpha2 Canum Venaticorum variable
An Alpha2 Canum Venaticorum variable (or α2 CVn variable) is a type of variable star. 
  • These stars are chemically peculiar main sequence stars of spectral class B8p to A7p.
  • They have strong magnetic fields and strong silicon, strontium, or chromium spectral lines.
  • Their brightness typically varies by 0.01 to 0.1 magnitudes over the course of 0.5 to 160 days.
In addition to their intensities, the intensities and profiles of the spectral lines of α2 CVn variables also vary, as do their magnetic fields. The periods of these variations are all equal and are believed to equal the period of rotation of the star. It is thought that they are caused by an inhomogeneous distribution of metals in the atmospheres of these stars, so that the surface of the star varies in brightness from point to point.

The type-star which this class is named after is α² Canum Venaticorum, a star in the binary system of Cor Caroli, which is located in the northern constellation of Canes Venatici. Its brightness fluctuates by 0.14 magnitudes with a period of 5.47 days.

Lyra - W and S Lyrae Carbon stars and Mira variables

W and S Lyrae are two of the many Mira variables in Lyra. W varies between 7th and 12th magnitudes over approximately 200 days,while S, slightly fainter, is a silicate carbon star, likely of the J-type. Wikipedia

Carbon stars
 A carbon star is a late-type star similar to a red giant (or occasionally to a red dwarf) whose atmosphere contains more carbon than oxygen; the two elements combine in the upper layers of the star, forming carbon monoxide, which consumes all the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds, giving the star a "sooty" atmosphere and a strikingly ruby red appearance.

In normal stars (such as the Sun), the atmosphere is richer in oxygen than carbon. Ordinary stars not exhibiting the characteristics of carbon stars but cool enough to form carbon monoxide are therefore called oxygen-rich stars.

Carbon stars have quite distinctive spectral characteristics, and they were first recognized by their spectra by Angelo Secchi in the 1860s, a pioneering time in astronomical spectroscopy.

P. Angelo Secchi SJ:  Theologian and Astronomer
"Angelo Secchi". Licensed under Public Domain via Commons

Italian Jesuite and astronomer Pietro Angelo Secchi SJ (1818 – 26 February 1878) was Director of the Observatory at the Pontifical Gregorian University (then called the Roman College) for 28 years. He was a pioneer in astronomical spectroscopy, and was one of the first scientists to state authoritatively that the Sun is a star.

Mira variables

"A Wide-field view of the sky around a field studied in the MASSIV survey"
by DSS 2/ESO - .Licensed under CC BY 4.0 via Commons
Mira variables, named after the prototype star Mira, a red giant star estimated 200–400 light years away in the constellation Cetus.

These are a class of pulsating variable stars characterized by very red colours, pulsation periods longer than 100 days, and amplitudes greater than one magnitude in infrared and 2.5 magnitude at visual wavelengths.

They are red giants in the very late stages of stellar evolution, on the asymptotic giant branch, that will expel their outer envelopes as planetary nebulae and become white dwarfs within a few million years.

Mira variables are stars massive enough that they have undergone helium fusion in their core but are less than two solar masses, stars that have already lost about half their initial mass. However, they can be thousands of times more luminous than the Sun due to their very large distended envelopes.

They are pulsating due to the entire star expanding and contracting. This produces a change in temperature along with radius, both of which factors cause the variation in luminosity. The pulsation depends on the mass and radius of the star and there is a well-defined relationship between period and luminosity (and colour).

The very large visual amplitudes are not due to large luminosity changes, but due to a shifting of energy output between infra-red and visual wavelengths as the stars change temperature during their pulsations.

Lyra - V473 Lyrae Classical Cepheid variable

"HR-vartype" by Rursus - Own work. Licensed under CC BY-SA 3.0 via Commons 
V473 Lyrae is an unusual Classical Cepheid variable with a range of 5.99 to 6.35 and a period of 1.49078 days. It is unique in that it is the only known Cepheid in the Milky Way to undergo periodic phase and amplitude changes, analogous to the Blazhko effect in RR Lyrae stars  At 1.5 days, its period was the shortest known for a classical Cepheid at the time of its discovery.  Wikipedia

Classical Cepheids
Classical Cepheids (also known as Population I Cepheids, Type I Cepheids, or Delta Cephei variables) are a type of Cepheid variable star. They are population I variable stars that exhibit regular radial pulsations with periods of a few days to a few weeks and visual amplitudes from a few tenths of a magnitude to about 2 magnitudes.

There exists a well-defined relationship between a classical Cepheid variable's luminosity and pulsation period, securing Cepheids as viable standard candles for establishing the Galactic and extragalactic distance scales.

Hubble Space Telescope observations of classical Cepheid variables have enabled firmer constraints on Hubble's law.

Classical Cepheids have also been used to clarify many characteristics of our galaxy, such as the Sun's height above the galactic plane and the Galaxy's local spiral structure.

Around 800 classical Cepheids are known in the Milky Way Galaxy, out of an expected total of over 6,000. Several thousand more are known in the Magellanic Clouds, with more known in other galaxies.  The Hubble Space Telescope has identified classical Cepheids in NGC 4603, which is 100 million light years distant.

Click on the links under the snippets to get much more information about the subjects discussed.

Friday, November 13, 2015

Lyra - HP and EP Lyrae, RV Tauri variables

Example of a RVa star variation in magnitude
image Atlas of Variable Stars
 HP Lyrae is a post-asymptotic giant branch (AGB) star that shows variability.

The reason for its variability is still a mystery: first cataloged as an eclipsing binary, it was theorized to be an RV Tauri variable in 2002, but if so, it would be by far the hottest such variable discovered.

EP Lyrae, a faint RV Tauri variable and an "extreme example" of a post-AGB star. It and a likely companion are surrounded by a circumstellar disk of material.

RV Tauri variables
RV Tauri variables are luminous variable stars that have distinctive light variations with alternating deep and shallow minima.

These are not typical supergiants. These are post-AGB objects with low masses and they have lower luminosities than the bulk of the supergiants, so the spectral luminosity class is indicative of an evolved expanded star undergoing mass loss rather than an exceptionally luminous star.

They have ceased fusion and are rapidly losing their atmospheres on their way to becoming a white dwarf, Although this should happen in a period measured in thousands of years, even hundreds for the more massive examples, the known RV Tau stars have not shown the secular increase in temperature that would be expected.

The main sequence progenitor of this type of star has a mass near to that of the sun, although they have already lost about half of that during red giant and AGB phases. They are also thought to be mostly binaries surrounded by a dusty disc.  Such stars are clearly metal-deficient Population II stars since it takes around 10 billion years for stars of that mass to evolve beyond the AGB.

RV Tauri stars are further subclassified into two types:
  • RVa variables:these do not vary in mean brightness
  • RVb variables: these show periodic variations in their mean brightness, so that their maxima and minima change on 600 to 1500 day timescales
The prototype of these variables, RV Tauri is a RVb type variable which exhibits brightness variations between magnitudes +9.8 and +13.3 with a formal period of 78.7 days.

The brightest member of the class, R Scuti, is an RVa type, with an apparent magnitude varying from 4.6 to 8.9 and a formal period of 146.5 days.  AC Herculis is an example of an RVa type variable.

The luminosity of RV Tau variables is typically a few thousand times the sun, which places them at the upper end of the W Virginis instability strip.

Therefore RV Tau variables along with W Vir variables are sometimes considered a subclass of Type II Cepheids. They exhibit relationships between their periods, masses, and luminosity, although not with the precision of more conventional Cepheid variables. Although the spectra appear as supergiants, usually Ib, occasionally Ia although this may just need better measurements, the actual luminosities are only a few thousand times the sun which would make them bright giants.

RV Tau variables exhibit changes in luminosity which are tied to radial pulsations of their surfaces. Their changes in brightness are also correlated with changes in their spectral type. While at their brightest, the stars have spectral types F or G. At their dimmest, their spectral types change to K or M. The difference between maximum and minimum brightness can be as much as four magnitudes.

The period of brightness fluctuations from one deep minimum to the next is typically around 30 to 150 days, and exhibits alternating primary and secondary minima, which can change relative to each other.

The approximate division between W Vir variables and RV Tau variables is at a fundamental pulsation period of 20 days, RV Tau variables are typically described with periods of 40-150 days.

Friedrich Wilhelm Argelander (1799 – 1875)
Image Wikimedia
German astronomer Friedrich Wilhelm Argelander  monitored the distinctive variations in brightness of R Scuti from 1840 to 1850. R Sagittae was noted to be variable in 1859, but it was not until the discovery of RV Tauri by Russian astronomer Lydia Ceraski in 1905 that the class of variable was recognised as distinct.
Lydia Ceraski discovered 219 variable stars (or 180 – the literature disagrees somewhat). Among them are important ones such as SU UMa, RV Tauri and T UMi. Despite that, not much is known about her. She graduated from the Petersburg Teachers’ Institute and was married to the astronomer Witold Ceraski.
Gustav Holmberg

Lyra - V361 Lyrae a contact binary

Contact binary
Image David Darling, The Internet Encyclopedia of Science 
V361 Lyrae is a faint star at the very northernmost edge of the constellation. It can be found less than a degree away from the naked-eye star 16 Lyrae,

V361 us a 5th-magnitude A-type subgiant located around 120.6 ly (37 parsecs) distant. It is an eclipsing binary that does not easily fall into one of the traditional classes, with features of Beta Lyrae, W Ursae Majoris, and cataclysmic variables.

It may be a representative of a very brief phase in which the system is transitioning into a contact binary.

Contact binaries
Contact binary is a binary star system whose component stars are so close that they touch each other or have merged to share their gaseous envelopes.

A binary system whose stars share an envelope may also be called an overcontact binary.

Almost all known contact binary systems are eclipsing binaries; eclipsing contact binaries are known as W Ursae Majoris variables, after their type star, W Ursae Majoris.

Contact binaries are sometimes confused with common envelopes. However, whereas the configuration of two touching stars in a contact binary has a typical lifetime of millions to billions of years, the common envelope is a dynamically unstable phase in binary evolution that either expels the stellar envelope or merges the binary in a timescale of months to years.

Lyra studies

Image Top
Studies concentrated on one of the smallest of constellations, Lyra, are one way to approach the truly astronomic amount of information astronomers and cosmologists have so patiently gathered and organized about stars, deep sky objects, exoplanets and other objects in God of Israel's magnificent creation - the Universe.

These Lyra pages hopefully assist beginning students of astronomy to penetrate into this challenging world of Aristotelian classifications, scientific definitions and explanations of the observed light and other forms of radiation reaching planet Earth from the perimeters of Lyra. The texts listed below discuss only a fraction of the richness revealed by professional astronomers during centuries in that region of night sky.

The pages listed below contain mostly snippets taken from Wikipedia and other reliable sources. Each page has links to the original articles. These usually contain significantly more information and important references to sources and scholarly works the links serving as gateways to further learning.

Lyra constellation Cultural Astronomy

16 Lyrae and dwarf stars
19 Lyrae Alpha2 Canum Venaticum
Aladfar Blue subgiant
Alathar spectral class A0IV
AY and V344 Lyrae SU Ursae majoris type variables
Delta Lyrae 1 and 2 
FL Lyrae Algol variable 
Gliese 758 and Subaru Telescope
HP Lyrae and RV Tauri variables
HR Lyrae Classical nova, White dwarf
R Lyrae and other red giants 
RR Lyrae variable and distances to stars
Small triangle
V361 Contact binary
W and S Lyrae - Carbon stars and Mira variables

Stars with planets and Doppler spectroscopy
Kepler Mission

Deep sky objects
Abell 46 and the Abell Planetary CatalogMessier 56 Globular cluster and Dwarf galaxies
Messier 57 Planetary nebula
NGC 6745 Irregular galaxy

"Lyra IAU" by IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)
Licensed under CC BY 3.0 via Commons

Thursday, November 12, 2015

Lyra - RR Lyrae variables and distances to stars

RR Lyrae is a variable star, located near the border with the neighboring constellation of Cygnus.

As the brightest star in its class, it became the eponym for the RR Lyrae variable class of stars and it has been extensively studied by astronomers. 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. Wikipedia

RR Lyrae variables
RR Lyrae variables serve as important standard candles that are used to measure astronomical distances.

The period of pulsation of an RR Lyrae variable depends on its mass, luminosity and temperature, while the difference between the measured luminosity and the actual luminosity allows its distance to be determined via the inverse square law. Hence, understanding the period-luminosity relation for a set of local RR Lyrae-type variable stars allows the distance of more distant stars of this type to be determined.
Williamina Fleming (1857-1911)
Found Horsehead Nebula 1888
image Wikimedia
The variable nature of RR Lyrae was discovered by the Scottish astronomer Williamina Fleming at Harvard Observatory in 1901.

The distance of RR Lyrae remained uncertain until 2002 when the Hubble Space Telescope's Fine Guidance Sensor was used to determine the distance of RR Lyrae within a 5% margin of error, yielding a value of 854 light-years (262 parsecs).

When combined with measurements from the Hipparcos satellite and other sources, the result is a distance estimate of 860 ly (260 pc).

Pulsating horizontal branch star
"HR-diag-instability-strip" by Rursus - Own work. Licensed under CC BY-SA 3.0 via Commons

RR Lyrae type of low-mass star has consumed the hydrogen at its core, evolved away from the main sequence, and passed through the red giant stage. Energy is now being produced by the thermonuclear fusion of helium at its core, and the star has entered an evolutionary stage called the horizontal branch (HB).

The effective temperature of an HB star's outer envelope will gradually increase over time. When its resulting stellar classification enters a range known as the instability strip—typically at stellar class A—the outer envelope can begin to pulsate.

RR Lyrae shows just such a regular pattern of pulsation, which is causing its apparent magnitude to vary between 7.06–8.12 over a short cycle lasting 0.56686776 days (13 hours, 36 minutes). Each radial pulsation causes the radius of the star to vary between 5.1 and 5.6 times the Sun's radius.

This star has a low abundance of elements other than hydrogen and helium—what astronomers term its metallicity. It belongs to the Population II category of stars that formed during the early period of the Universe when there was a lower abundance of metals in star-forming regions.

The trajectory of this star is carrying it along an orbit that is close to the plane of the Milky Way, taking it no more than 680 ly (210 pc) above or below this plane. The orbit has a high eccentricity, bringing RR Lyrae as close as 6.80 kly (2.08 kpc) to the Galactic Center at periapsis, and taking it as far as 59.9 kly (18.4 kpc) at apapsis.

Blazhko effect
"Rr lyrae ltcrv en" by RJHall - Own work. Licensed under CC BY-SA 3.0 via Commons

RR Lyrae belongs to a subset of RR Lyrae-type variables that show a characteristic behavior called the Blazhko effect, named after Russian astronomer and member of Soviet Academy of Sciences, Sergey Blazhko (1870 - 1956).

This effect is observed as a periodic modulation of a variable star's pulsation strength or phase; sometimes both. It causes the light curve of RR Lyrae to change from cycle to cycle. As of 2009, the cause of this effect is not yet fully understood. The Blazhko period for RR Lyrae is 39.1 ± 0.3 days.