Cetus constellation is located in the northern sky. Also known as the Whale, it is one of the largest constellations in the sky. Cetus is the fourth largest constellation in the sky, occupying an area of 1231 square degrees. It lies in the first quadrant of the southern hemisphere (SQ1) and can be seen at latitudes between +70° and -90°. The neighboring constellations are Aquarius, Aries, Eridanus, Fornax, Pisces, Sculptor, and Taurus.
[http://www.constellation-guide.com/constellation-list/cetus-constellation/]
Although Cetus is not generally considered part of the zodiac, the ecliptic passes less than a quarter of a degree from its constellation boundary, and thus the moon, planets, and even part of the sun may be in Cetus for brief periods of time. This is all the more true of asteroids, since their orbits usually have a greater inclination to the ecliptic than the moon and planets. For example, the asteroid 4 Vesta was discovered in this constellation in 1807.
As seen from Mars, the ecliptic passes into Cetus, with the sun appearing in Cetus for around six days shortly after the northern summer solstice. Mars’s orbit is tilted by 1.85° with respect to Earth’s.
The bizarre-looking sea monster Cetus emerging from the ocean of the southern skies, illustrated in the Atlas Coelestis of John Flamsteed (1729).
When Cassiopeia, wife of King Cepheus of Ethiopia, boasted that she was more beautiful than the sea nymphs called the Nereids she set in motion one of the most celebrated stories in mythology, whose characters are commemorated in the sky. In retribution for the insult to the Nereids, the sea god Poseidon sent a fearsome monster to ravage the coast of Cepheus’s territory. That monster, a dragon of the sea, is represented by the constellation Cetus.
To rid himself of the monster, Cepheus was instructed by the Oracle of Ammon to offer up his daughter Andromeda as a sacrifice to the monster. Andromeda was chained to the cliffs at Joppa (the modern Tel-Aviv) to await her terrible fate.
Cetus was visualized by the Greeks as a hybrid creature, with enormous gaping jaws and the forefeet of a land animal, attached to a scaly body with huge coils like a sea serpent. Hence Cetus is drawn on star maps as a most unlikely looking creature, more comical than frightening, nothing like a whale although it is sometimes identified as one.
[http://www.ianridpath.com/startales/cetus.htm]
Cetus may have originally been associated with a whale, which would have had mythic status amongst Mesopotamian cultures. It is often now called the Whale, though it is most strongly associated with Cetus the sea-monster, who was slain by Perseus as he saved the princess Andromeda from Poseidon’s wrath. Cetus is located in a region of the sky called ‘The Sea’ because many water-associated constellations are placed there, including Eridanus, Pisces, Piscis Austrinus, Capricornus, and Aquarius.
Cetus has been depicted many ways throughout its history. In the 17th century, Cetus was depicted as a ‘dragon fish’ by Johann Bayer. Both Willem Blaeu and Andreas Cellarius depicted Cetus as a whale-like creature in the same century. However, Cetus has also been variously depicted with animal heads attached to a piscine body.
In Chinese astronomy, the stars of Cetus are found among two areas: the Black Tortoise of the North (Běi Fāng Xuán Wǔ) and the White Tiger of the West (Xī Fāng Bái Hǔ).
The Brazilian Tukano and Kobeua people used the stars of Cetus to create a jaguar, representing the god of hurricanes and other violent storms. Lambda, Mu, Xi, Nu, Gamma, and Alpha Ceti represented its head; Omicron, Zeta, and Chi Ceti represented its body; Eta Eri, Tau Cet, and Upsilon Cet marked its legs and feet; and Theta, Eta, and Beta Ceti delineated its tail.
In Hawaii, the constellation was called Na Kuhi, and Mira (Omicron Ceti) may have been called Kane.
[http://www.hvaastronomy.com/cetus.htm]
[http://astropixels.com/constellations/charts/Cet.html]
Image of orange giant Beta Ceti from NASA’s Chandra X-ray Observatory
Beta Ceti is the brightest star in the constellation Cetus. The traditional name Deneb Kaitos is Arabic ‘Al Dhanab al Ḳaiṭos al Janūbīyy’ for ‘southern tail of Cetus;’ it is also known as Diphda, ‘frog,’ from the Arabic ‘al-ḍifda aṯ-ṯānī,’ ‘the second frog’ (the ‘first frog’ being Fomalhaut. This orange giant is easy to identify due to its location in an otherwise dark section of the celestial sphere. It lies at an estimated distance of 96.3 light-years (29.5 parsecs) from Earth.
Deneb Kaitos has an apparent visual magnitude of 2.02, with a stellar classification K0 III, although some sources list a classification of G9.5 III indicating that it lies along the dividing line separating G-type from K-type stars. The luminosity class of ‘III’ means that it is a giant star that has consumed the hydrogen at its core and evolved away from an A-type main sequence star. After passing through the red giant stage, it underwent the helium flash event and is generating energy through the thermonuclear fusion of helium at its core. Beta Ceti will remain in this mode for over 100 million years.
The effective temperature of the star’s outer envelope is about 4,797 K, giving it the characteristic orange hue of a K-type star. In spite of its cooler temperature, Deneb Kaitos is much brighter than the Sun with a bolometric luminosity of about 145 times the luminosity of the Sun, resulting from a radius 18 times as large as the Sun and a mass that is 2.8 times the Sun’s mass.
This star displays flaring activity that results in random outbursts that increase the luminosity of the star over intervals lasting several days. This is a much longer duration than for comparable solar flare activity on the Sun, which typically last for periods measured in hours. In 2005, a relatively high rate of X-ray emission was detected with the XMM-Newton space observatory. It is emitting about 2,000 times the X-ray luminosity of the Sun, allowing the star to be imaged with the Chandra X-ray Observatory.
[https://en.wikipedia.org/wiki/Beta_Ceti]
Alpha Ceti, aka Menkar, appears as a double star in small telescopes, but in fact the two stars lie at very different distances.
[http://earthsky.org/brightest-stars/menkar-sea-monsters-alpha-star]
Alpha Ceti has the traditional name Menkar or Menkab. The name Menkar derives from the Arabic word ‘manħar,’ ‘nostril’ (of Cetus).
Menkar is a red giant with a stellar classification of M1.5 IIIa. It has more than twice the mass of the Sun and, as a giant star has expanded to about 89 times the Sun’s radius. The large area of the photosphere means that it is emitting about 1,455 times as much energy as the Sun, even though the effective temperature is only 3,795 K (compared to 5,778 K on the Sun). The relatively low temperature gives Menkar the red hue of an M-type star.
Menkar has evolved from the main sequence after exhausting the helium at its core. As it begins to burn its carbon core it will probably become a highly unstable star like Mira before finally shedding its outer layers and forming a planetary nebula, leaving a relatively large white dwarf remnant.
[https://en.wikipedia.org/wiki/Alpha_Ceti]
Mira as seen from the Earth
Mira
A is a well-known example of a category of variable stars known as Mira
variables, which are named after it. The 6–7,000 known stars of this
class are all red giants whose surfaces oscillate in such a way as to
increase and decrease in brightness over periods ranging from about 80
to more than 1,000 days.
In the particular case of Mira, its increases in
brightness take it up to about magnitude 3.5 on average, placing it
among the brighter stars in the Cetus constellation. Individual cycles
vary too; well-attested maxima go as high as magnitude 2.0 in brightness
and as low as 4.9, a range almost 15 times in brightness, and there are
historical suggestions that the real spread may be three times this or
more. Minima range much less, and have historically been between 8.6 and
10.1, a factor of four times in luminosity. The total swing in
brightness from absolute maximum to absolute minimum (two events which
did not occur on the same cycle) is 1,700 times:
[https://en.wikipedia.org/wiki/Mira]
A Star with a Comet’s Tail
Astronomers using a NASA space telescope, the Galaxy Evolution Explorer, have spotted an amazingly long comet-like tail behind a star streaking through space. The star, named Mira after the Latin word for ‘wonderful,’ has been a favorite of astronomers for about 400 years, yet this is the first time the tail has been seen.
Galaxy Evolution Explorer- ‘GALEX’ for short- scanned the popular star during its ongoing survey of the entire sky in ultraviolet light. Astronomers then noticed what looked like a comet with a gargantuan tail. In fact, material blowing off Mira is forming a wake 13 light-years long, or about 20,000 times the average distance of Pluto from the sun. Nothing like this has ever been seen before around a star.
Mira’s comet-like tail stretches more than 13 light years
Astronomers say Mira’s tail offers a unique opportunity to study how stars like our sun die and ultimately seed new solar systems. Mira is an older star called a red giant that is losing massive amounts of surface material. As Mira hurtles along, its tail sheds carbon, oxygen and other important elements needed for new stars, planets and possibly even life to form. This tail material, visible now for the first time, has been released over the past 30,000 years.
Billions of years ago, Mira was similar to our sun. Over time, it began to swell into what’s called a variable red giant- a pulsating, puffed-up star that periodically grows bright enough to see with the naked eye. Mira will eventually eject all of its remaining gas into space, forming a colorful shell called a planetary nebula. The nebula will fade with time, leaving only the burnt-out core of the original star, which will then be called a white dwarf.
Compared to other red giants, Mira is traveling unusually fast, possibly due to gravitational boosts from other passing stars over time. It now plows along at 130 kilometers per second, or 291,000 miles per hour. Racing along with Mira is a small, distant companion thought to be a white dwarf. The pair, also known as Mira A (the red giant) and Mira B (the white dwarf), orbit slowly around each other as they travel together through the constellation Cetus 350 light-years from Earth.
In addition to Mira’s tail, GALEX also discovered a bow shock, a type of buildup of hot gas, in front of the star, and two sinuous streams of material coming out of the star’s front and back. Astronomers think hot gas in the bow shock is heating up the gas blowing off the star, causing it to fluoresce with ultraviolet light. This glowing material then swirls around behind the star, creating a turbulent, tail-like wake. The process is similar to a speeding boat leaving a choppy wake, or a steam train producing a trail of smoke.
The fact that Mira’s tail only glows with ultraviolet light might explain why other telescopes have missed it. GALEX is very sensitive to ultraviolet light and also has an extremely wide field of view, allowing it to scan the sky for unusual ultraviolet activity.
“It’s amazing to discover such a startlingly large and important feature of an object that has been known and studied for over 400 years,” says James D. Neill of Caltech. “This is exactly the kind of surprise that comes from a survey mission like the Galaxy Evolution Explorer.”
[http://science.nasa.gov/science-news/science-at-nasa/2007/15aug_mira/]
[http://phys.org/news/2012-12-tau-ceti-sun-like-star-twelve.html]
Tau Ceti is a star in the constellation Cetus that is spectrally similar to the Sun, although it has only about 78% of the Sun’s mass. At a distance of just under 12 light-years from the Solar System, it is a relatively nearby star, and is the closest solitary G-class star. The star appears stable, with little stellar variation, and is metal-deficient.
Observations have detected more than ten times as much dust surrounding Tau Ceti as is present in the Solar System. Since December 2012, there has been evidence of possibly five planets orbiting Tau Ceti, with two of these being potentially in the habitable zone. Because of its debris disk, any planet orbiting Tau Ceti would face far more impact events than Earth. Despite this hurdle to habitability, its solar analog (Sun-like) characteristics have led to widespread interest in the star. Given its stability, similarity and relative proximity to the Sun, Tau Ceti is consistently listed as a target for the Search for Extra-Terrestrial Intelligence (SETI), and it appears in some science fiction literature.
It can be seen with the unaided eye as a third-magnitude star. As seen from Tau Ceti, the Sun would be a third-magnitude star in the constellation Boötes.
Tau Ceti does not have a widely-recognized traditional name, and is usually simply referred to as Tau Ceti. In the catalogue of stars in the Calendarium of Al Achsasi al Mouakket, written at Cairo about 1650, this star was designated ‘Thālith al Naʽāmāt,’ which was translated into Latin as Tertia Struthionum, meaning the third of the ostriches. This star, along with η Cet (Deneb Algenubi), θ Cet (Thanih Al Naamat), ζ Cet (Baten Kaitos), and υ Cet, were ‘Al Naʽāmāt,’ the Hen Ostriches.
The proper motion of a star is its amount of movement across the celestial sphere, determined by comparing its position relative to more distant background objects. Tau Ceti is considered to be a high-proper-motion star, although it only has an annual traverse of just under two arc seconds. It will require about two thousand years before the location of this star shifts by more than a degree. A high proper motion is an indicator of closeness to the Sun. Nearby stars can traverse an angle of arc across the sky more rapidly than the distant background stars and are good candidates for parallax studies. In the case of Tau Ceti, the parallax measurements indicate a distance of 11.9 ly. This makes it one of the closest star systems to the Sun, and the next-closest spectral class-G star after Alpha Centauri A.
The Sun (left) is both larger and somewhat hotter than the less active Tau Ceti (right)
The Tau Ceti system is believed to have only one stellar component. A dim optical companion has also been observed with magnitude 13.1. As of 2000, it was 137 arcseconds distant from the primary. It may be gravitationally bound, but it is considered more likely to be a line-of-sight coincidence.
Most of what is known about the physical properties of Tau Ceti and its system has been determined through spectroscopic measurements. By comparing the spectrum to computed models of stellar evolution, the age, mass, radius and luminosity of Tau Ceti can be estimated. However, using an astronomical interferometer, measurements of the radius of the star can be made directly to an accuracy of 0.5%. Through such means, the radius of Tau Ceti has been measured to be 79.3 ± 0.4% of the solar radius. This is about the size that is expected for a star with somewhat lower mass than the Sun.
The chemical composition of a star provides important clues to its evolutionary history, including the age at which it formed. The interstellar medium of dust and gas from which stars form is primarily composed of hydrogen and helium with trace amounts of heavier elements. As nearby stars continually evolve and die, they seed the interstellar medium with an increasing portion of heavier elements. Thus younger stars will tend to have a higher portion of heavy elements in their atmospheres than do the older stars. These heavy elements are termed metals by astronomers and the portion of heavy elements is the metallicity. The amount of metallicity in a star is given in terms of the ratio of iron (Fe), an easily observed heavy element, to hydrogen. A logarithm of the relative iron abundance is compared to the Sun. In the case of Tau Ceti, the atmospheric metallicity is equivalent to about a third the solar abundance.
This lower abundance of iron indicates that Tau Ceti is almost certainly older than the Sun. Its age had previously been estimated to be about 10 Ga but is now thought to be around half that at 5.8 Ga. This compares with 4.57 Ga for the Sun. However, computed age estimates for Tau Ceti can range from 4.4–12 Ga, depending on the model adopted.
The luminosity of Tau Ceti is equal to only 55% of the Sun’s luminosity. A terrestrial planet would need to orbit this star at a distance of about 0.7 AU in order to match the solar-insolation level of Earth. This is approximately the same as the average distance between Venus and the Sun.
The chromosphere of Tau Ceti- the portion of a star’s atmosphere just above the light-emitting photosphere- currently displays little or no magnetic activity, indicating a stable star. One nine-year study of temperature, granulation, and the chromosphere showed no systematic variations; Ca II emissions around the H and K infrared bands show a possible 11-year cycle, but this is weak relative to the Sun. Alternatively it has been suggested that the star could be in a low-activity state analogous to a Maunder minimum- a historical period, associated with the Little Ice Age in Europe, when sunspots became exceedingly rare on the Sun’s surface. Spectral line profiles of Tau Ceti are extremely narrow, indicating low turbulence and observed rotation. The amplitude of the star’s oscillations are about half those of the Sun, and have a lower mode lifetime.
The habitable zone for this star, defined as the locations where liquid water could be present on an Earth-size planet, is at a radius of 0.55-1.16 AU, where 1 AU is the average distance from the Earth to the Sun.
On December 19, 2012, evidence was presented that suggest a system of five planets orbiting Tau Ceti. The planets’ estimated minimum masses are between two and six times the mass of Earth and their orbital periods range from 14 to 640 days.
One of them, tentatively named Tau Ceti e, appears to orbit about half as far from Tau Ceti as Earth does from the Sun. With Tau Ceti’s luminosity of 52% that of the Sun and a distance from the star of 0.552 AU, the planet would receive 1.71 times as much stellar radiation as Earth does, slightly less than Venus with 1.91 times Earth’s. It has a minimum mass of 4.3 Earth masses. Because the minimum mass of a super-Earth is 5 Earth masses, Tau Ceti e may be Earth sized. If it possesses an Earth-like atmosphere, the surface temperature would be around 68 °C (154 °F).
Tau Ceti f orbits Tau Ceti at a distance of 1.35 AU (near Mars’s orbit in the Solar System) with an orbital period of 642 days and has a minimum mass of 6.6 Earth masses, which means it may be a super-Earth. Assuming an Earth-like atmosphere, the surface temperature would be approximately −40°C (233 K). With a denser atmosphere able to produce a stronger greenhouse effect it could have a much higher temperature, between 0 °C and 50 °C; warm enough for liquid water to exist on the surface. The Planetary Habitability Laboratory has estimated that Tau Ceti f, which would receive 28.5% as much starlight as Earth, would be narrowly within the habitable zone of the star as well.
In 2004, a team of UK astronomers led by Jane Greaves discovered that Tau Ceti has more than ten times the amount of cometary and asteroidal material orbiting it than does the Sun. This was determined by measuring the disk of cold dust orbiting the star produced by collisions between such small bodies. This result puts a damper on the possibility of complex life in the system, because any planets would suffer from large impact events roughly ten times more frequently than Earth. Greaves noted at the time of her research that it is likely that any planets will experience constant bombardment from asteroids of the kind believed to have wiped out the dinosaurs. Such bombardments would inhibit the development of biodiversity between impacts. However, it is possible that a large Jupiter-sized gas giant could deflect comets and asteroids.
The debris disk was discovered by measuring the amount of radiation emitted by the system in the far infrared portion of the spectrum. The disk forms a symmetric feature that is centered on the star, and the outer radius averages 55 AU. The lack of infrared radiation from the warmer parts of the disk near Tau Ceti imply an inner cut-off at a radius of 10 AU. By comparison, the Solar System’s Kuiper belt extends from 30-50 AU. To be maintained over a long period of time, this ring of dust must be constantly replenished through collisions by larger bodies. The bulk of the disk appears to be orbiting Tau Ceti at a distance of 35-50 AU, well outside the orbit of the habitable zone. At this distance, the dust belt may be analogous to the Kuiper belt that lies outside the orbit of Neptune in the Solar System.
Tau Ceti shows that stars need not lose large disks as they age and such a thick belt may not be uncommon among Sun-like stars. Tau Ceti’s belt is only 1⁄20th as dense as the belt around its young neighbor, Epsilon Eridani. The relative lack of debris around the Sun may be the unusual case: one research team member suggests the Sun may have passed close to another star early in its history and had most of its comets and asteroids stripped away. Stars with large debris disks have altered astronomical thinking about planet formation; debris disk stars, where dust is continually generated by collisions, appear to form planets readily.
Tau Ceti could have been a search target for the canceled Terrestrial Planet Finder
The most optimistic search project to date was Project Ozma, which was intended to ‘search for extraterrestrial intelligence’ (SETI) by examining selected stars for indications of artificial radio signals. It was run by the astronomer Frank Drake, who selected Tau Ceti and Epsilon Eridani as the initial targets. Both are located near the Solar System and are physically similar to the Sun. No artificial signals were found despite 200 hours of observations. Subsequent radio searches of this star system have also turned up negative.
This lack of results has not dampened interest in observing the Tau Ceti system for bio-signatures. In 2002, astronomers Margaret Turnbull and Jill Tarter developed the Catalog of Nearby Habitable Systems (HabCat) under the auspices of Project Phoenix, another SETI endeavour. The list contained more than 17,000 theoretically habitable systems, approximately 10% of the original sample. The next year, Turnbull would further refine the list to the 30 most promising systems out of 5000 within one hundred light-years of the Sun, including Tau Ceti; this will form part of the basis of radio searches with the Allen Telescope Array. She also chose Tau Ceti for a final shortlist of just five stars suitable for searches by the (indefinitely postponed) Terrestrial Planet Finder telescope system, commenting that “these are places I’d want to live if God were to put our planet around another star.”
[https://en.wikipedia.org/wiki/Tau_Ceti]
PSR J0108-1431 is a solitary pulsar located at a distance of about 130 parsecs (424 light years) in the constellation Cetus. This pulsar was discovered in 1994 during the Parkes Southern Pulsar Survey. It is considered a very old pulsar with an estimated age of 166 million years and a rotation period of 0.8 seconds. The rotational energy being generated by the spin-down of this pulsar is 5.8 × 10^23 W and the surface magnetic field is 2.5 × 10^7 T. As of 2008, it is the second faintest known pulsar.
An X-ray emission with an energy flux of (9 ± 2) × 10^12 W m−2 was detected in the 0.3–8 keV band using the Chandra X-ray Observatory. This X-ray energy is generated from the conversion of 0.4% of the pulsar’s spin-down power. As of 2009, PSR J0108-1431 is the least powerful of the ordinary pulsars that have been detected in the X-ray range.
The Very Large Telescope at the European Southern Observatory in Northern Chile observed a possible optical counterpart of this neutron star. The object has an apparent magnitude that is (X ≤ 27.8). No companions have been discovered in orbit around this object:
[https://en.wikipedia.org/wiki/PSR_J0108-1431]
PSR J0108-1431: Geriatric Pulsar Still Kicking
The composite image on the left shows an image from NASA’s Chandra X-ray Observatory in purple and an optical image from the European Southern Observatory’s Very Large Telescope (VLT) in red, blue and white. The Chandra source in the center of the image is the ancient pulsar PSR J0108-1431 (J0108 for short), located only 770 light years from us. The elongated object immediately to its upper right is a background galaxy that is unrelated to the pulsar. Since J0108 is located a long way from the plane of our galaxy, many distant galaxies are visible in the larger-scale optical image.
The position of the pulsar seen by Chandra in this image from early 2007 is slightly different from the radio position observed in early 2001, implying that the pulsar is moving at a velocity of about 440,000 miles per hour, in the direction shown by the white arrow. The detection of this motion allowed an estimate of where J0108 should be located in the VLT image taken in 2000. The faint blue star just above the galaxy is a possible optical detection of the pulsar.
The artist’s impression on the right shows what J0108 might look like if viewed up close. Radiation from particles spiraling around magnetic fields is shown along with heated areas around the neutron star’s magnetic poles. Both of these effects are expected to generate X-ray emission. Most of the surface of the neutron star is expected to be too cool to produce X-rays, but it should produce optical and ultraviolet radiation. Thus, multi-wavelength observations are important for providing a complete picture of these exotic objects.
At an age of about 200 million years, this pulsar is the oldest isolated pulsar ever detected in X-rays. Among isolated pulsars- ones that have not been spun-up in a binary system- it is over 10 times older than the previous record holder with an X-ray detection. This pulsar is slowing down as it ages and converting some of the energy that is being lost into X-rays. The efficiency of this process for J0108 is found to be higher than for any other known pulsar.
[http://chandra.harvard.edu/photo/2009/j0108/]
[https://en.wikipedia.org/wiki/NGC_246]
NGC 246 and the Dying Star
Appropriately nicknamed ‘the Skull Nebula,’ planetary nebula NGC 246 really does surround a dying star some 1,600 light-years away in the constellation Cetus. Expelled over a period of thousands of years, the lovely, intricate nebula is the outer atmosphere of a once sun-like star. The expanding outer atmosphere is interacting with the gas and dust in the interstellar medium, while the star itself, the fainter member of the binary star system seen at the nebula’s center, is entering its final phase of evolution, becoming a dense, hot white dwarf. Star and nebula are moving rapidly toward the top of the detailed view, as suggested by the nebula’s brighter, upper, leading edge. The sharp image spans just over 2.5 light-years at the estimated distance of NGC 246 and also reveals distant background galaxies, some visible right through the nebula along the bottom.
[http://apod.nasa.gov/apod/ap060418.html]
IC 1613 (also known as Caldwell 51) is an irregular dwarf galaxy in the constellation Cetus near the star 26 Ceti, and it belongs to the Local Group. It has played an important role in the calibration of the Cepheid variable period luminosity relation for estimating distances. Other than the Magellanic Clouds, it is the only Local Group dwarf irregular galaxy where RR Lyrae-type variables have been observed; this factor, along with an unusually low abundance of interstellar dust both within IC 1613 and along the line of sight enable especially accurate distance estimates:
[https://en.wikipedia.org/wiki/IC_1613]
The Milky Way’s Clean and Tidy Galactic Neighbor
Many galaxies are chock-full of dust, while others have occasional dark streaks of opaque cosmic soot swirling in amongst their gas and stars. However, the subject of this new image, snapped with the OmegaCAM camera on ESO’s VLT Survey Telescope in Chile, is unusual- the small galaxy, named IC 1613, is a veritable clean freak! IC 1613 contains very little cosmic dust, allowing astronomers to explore its contents with great clarity. This is not just a matter of appearances; the galaxy’s cleanliness is vital to our understanding of the Universe around us.
IC 1613 is a dwarf galaxy in the constellation of Cetus (The Sea Monster). This VST image shows the galaxy’s unconventional beauty, all scattered stars and bright pink gas, in great detail.
German astronomer Max Wolf discovered IC 1613’s faint glow in 1906. In 1928, his compatriot Walter Baade used the more powerful 2.5-metre telescope at the Mount Wilson Observatory in California to successfully make out its individual stars. From these observations, astronomers figured out that the galaxy must be quite close to the Milky Way, as it is only possible to resolve single pinprick-like stars in the very nearest galaxies to us.
Astronomers have since confirmed that IC 1613 is indeed a member of the Local Group, a collection of more than 50 galaxies that includes our home galaxy, the Milky Way. IC 1613 itself lies just over 2.3 million light-years away from us. It is relatively well-studied due to its proximity; astronomers have found it to be an irregular dwarf that lacks many of the features, such as a starry disc, found in some other diminutive galaxies.
However, what IC 1613 lacks in form, it makes up for in tidiness. We know IC 1613’s distance to a remarkably high precision, partly due to the unusually low levels of dust lying both within the galaxy and along the line of sight from the Milky Way- something that enables much clearer observations.
The second reason we know the distance to IC 1613 so precisely is that the galaxy hosts a number of notable stars of two types: Cepheid variables and RR Lyrae variables. Both types of star rhythmically pulsate, growing characteristically bigger and brighter at fixed intervals.
As we know from our daily lives on Earth, shining objects such as light bulbs or candle flames appear dimmer the further they are away from us. Astronomers can use this simple piece of logic to figure out exactly how far away things are in the Universe- so long as they know how bright they really are, referred to as their intrinsic brightness.
Cepheid and RR Lyrae variables have the special property that their period of brightening and dimming is linked directly to their intrinsic brightness. So, by measuring how quickly they fluctuate astronomers can work out their intrinsic brightness. They can then compare these values to their apparent measured brightness and work out how far away they must be to appear as dim as they do.
Stars of known intrinsic brightness can act like standard candles, as astronomers say, much like how a candle with a specific brightness would act as a good gauge of distance intervals based on the observed brightness of its flame’s flicker.
Using standard candles- such as the variable stars within IC 1613 and the less-common Type Ia supernova explosions, which can be seen across far greater cosmic distances- astronomers have pieced together a cosmic distance ladder, reaching deeper and deeper into space.
Decades ago, IC 1613 helped astronomers work out how to utilize variable stars to chart the Universe’s grand expanse. Not bad for a little, shapeless galaxy.
[https://www.eso.org/public/news/eso1603/]
NGC 247 is an intermediate spiral galaxy (although it is sometimes classified as a dwarf spiral galaxy) about 11.1 Mly away in the constellation Cetus. This distance was confirmed in late February 2011.
NGC 247 is marred by an unusually large void on one side of its spiral disk. This void contains some older, redder stars but no younger, bluer stars.
NGC 247 is one of several galaxies that is gravitationally bound to the Sculptor Galaxy (NGC 253). These galaxies form a small core in the center of the Sculptor Group, which is one of the nearest groups of galaxies to the Milky Way. Most other galaxies associated with the Sculptor Group are only weakly gravitationally bound to this core:
[https://en.wikipedia.org/wiki/NGC_247]
The Dusty Disc of NGC 247
This image of NGC 247, taken by the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile, reveals the fine details of this highly inclined spiral galaxy and its rich backdrop. Astronomers say this highly tilted orientation, when viewed from Earth, explains why the distance to this prominent galaxy was previously overestimated.
The spiral galaxy NGC 247 is one of the closest spiral galaxies of the southern sky. In this new view from the Wide Field Imager on the MPG/ESO 2.2-metre telescope in Chile large numbers of the galaxy’s component stars are clearly resolved and many glowing pink clouds of hydrogen, marking regions of active star formation, can be made out in the loose and ragged spiral arms.
NGC 247 is part of the Sculptor Group, a collection of galaxies associated with the Sculptor Galaxy (NGC 253). This is the nearest group of galaxies to our Local Group, which includes the Milky Way, but putting a precise value on such celestial distances is inherently difficult.
To measure the distance from the Earth to a nearby galaxy, astronomers have to rely on a type of variable star called a Cepheid to act as a distance marker. Cepheids are very luminous stars, whose brightness varies at regular intervals. The time taken for the star to brighten and fade can be plugged into a simple mathematical relation that gives its intrinsic brightness. When compared with the measured brightness this gives the distance. However, this method isn’t foolproof, as astronomers think this period- luminosity relationship depends on the composition of the Cepheid.
Another problem arises from the fact that some of the light from a Cepheid may be absorbed by dust en route to Earth, making it appear fainter, and therefore further away than it really is. This is a particular problem for NGC 247 with its highly inclined orientation, as the line of sight to the Cepheids passes through the galaxy’s dusty disc.
However, a team of astronomers is currently looking into the factors that influence these celestial distance markers in a study called the Araucaria Project. The team has already reported that NGC 247 is more than a million light-years closer to the Milky Way than was previously thought, bringing its distance down to just over 11 million light-years.
Apart from the main galaxy itself, this view also reveals numerous galaxies shining far beyond NGC 247. In the upper right of the picture three prominent spirals form a line and still further out, far behind them, many more galaxies can be seen, some shining right through the disc of NGC 247.
This color image was created from a large number of monochrome exposures taken through blue, yellow/green and red filters taken over many years. In addition exposures through a filter that isolates the glow from hydrogen gas have also been included and colored red. The total exposure times per filter were 20 hours, 19 hours, 25 minutes and 35 minutes, respectively.
[https://www.eso.org/public/news/eso1107/]
Messier 77 (also known as NGC 1068) is a barred spiral galaxy about 47 million light-years away in the constellation Cetus. Messier 77 is an active galaxy with an Active Galactic Nucleus (AGN), which is obscured from view by astronomical dust at visible wavelengths. The diameter of the molecular disk and hot plasma associated with the obscuring material was first measured at radio wavelengths by the VLBA and VLA. The hot dust around the nucleus was subsequently measured in the mid-infrared by the MIDI instrument at the VLTI. It is the brightest Seyfert galaxy and is of type 2. Messier 77’s diameter is 170,000 light-years:
[https://en.wikipedia.org/wiki/Messier_77]
Hubble observes the hidden depths of Messier 77
The NASA/ESA Hubble Space Telescope has captured this vivid image of spiral galaxy Messier 77, one of the most famous and well-studied galaxies in the sky. The patches of red across this image highlight pockets of star formation along the pinwheeling arms, with dark dust lanes stretching across the galaxy’s energetic center.
Messier 77 is a galaxy in the constellation of Cetus, some 45 million light-years away from us. Also known as NGC 1068, it is one of the most famous and well-studied galaxies. It is a real star among galaxies, with more papers written about it than many other galaxies put together!
Despite its current fame and striking swirling appearance, the galaxy has been a victim of mistaken identity a couple of times; when it was initially discovered in 1780, the distinction between gas clouds and galaxies was not known, causing finder Pierre Méchain to miss its true nature and label it as a nebula. It was misclassified again when it was subsequently listed in the Messier Catalogue as a star cluster.
Now, however, it is firmly categorized as a barred spiral galaxy, with loosely wound arms and a relatively small central bulge. It is the closest and brightest example of a particular class of galaxies known as Seyfert galaxies- galaxies that are full of hot, highly ionized gas that glows brightly, emitting intense radiation.
Strong radiation like this is known to come from the heart of Messier 77- caused by a very active black hole that is around 15 million times the mass of our Sun. Material is dragged towards this black hole and circles around it, heating up and glowing strongly. This region of a galaxy alone, although comparatively small, can be tens of thousands of times brighter than a typical galaxy.
Although no competition for the intense center, Messier 77’s spiral arms are also very bright regions. Dotted along each arm are knotty red clumps- a signal that new stars are forming. These baby stars shine strongly, ionizing nearby gas which then glows a deep red color as seen in the image above. The dust lanes stretching across this image appear as a rusty, brown-red color due to a phenomenon known as reddening; the dust absorbs more blue light than red light, enhancing its apparent redness.
A version of this image won second place in the Hubble’s Hidden Treasures Image Processing Competition, entered by contestant Andre van der Hoeven.
[https://www.spacetelescope.org/news/heic1305/]
NGC 1073 is a barred spiral galaxy in the constellation Cetus. Unlike the Milky Way NGC 1073 does not have well- formed symmetrical arms and the center bar is larger:
[https://en.wikipedia.org/wiki/NGC_1073]
Barred Spiral Galaxy NGC 1073
Many spiral galaxies have bars across their centers. Even our own Milky Way Galaxy is thought to have a modest central bar. Prominently barred spiral galaxy NGC 1073, pictured above, was captured in spectacular detail in this recently released image taken by the orbiting Hubble Space Telescope. Visible are dark filamentary dust lanes, young clusters of bright blue stars, red emission nebulas of glowing hydrogen gas, a long bright bar of stars across the center, and a bright active nucleus that likely houses a supermassive black hole. Light takes about 55 million years to reach us from NGC 1073, which spans about 80,000 light years across. NGC 1073 can be seen with a moderately-sized telescope toward the constellation of the Sea Monster (Cetus), Fortuitously, the above image not only caught the X-ray bright star system IXO 5, visible on the upper left and likely internal to the barred spiral, but three quasars far in the distance.
[http://apod.nasa.gov/apod/ap120220.html]
NGC 34 (or NG 17) is the result of a merger between two disk galaxies, resulting in a recent starburst in the central regions and continuing star-forming activity. The galaxy is still gas-rich, and has a single galactic nucleus. Due to the major merger event NGC 34 has no defined spiral arms like the Milky Way galaxy, and the center bar nucleus is also distorted. The merger destroyed any galactic habitable zone that may have been there before. For the Milky Way, the galactic habitable zone is commonly believed to be an annulus with an outer radius of about 10 kilo-parsecs and an inner radius close to the Galactic Center, both of which lack hard boundaries:
[https://en.wikipedia.org/wiki/NGC_34]
NGC 17
This galaxy features a single nucleus, a containing a blue central disc with delicate fine structure in the outer parts and tidal tails indicative of two former disc galaxies. At present these galaxies appear to have completed their merger. The remnant shows clear signs that the merger was gas-rich and accompanied by a starburst. NGC 17 is gas-rich and can sustain its strong central starburst and present mild central activity for some time to come.
NGC 17 is located about 250 million light-years away in the constellation of Cetus, the Whale.
This image is part of a large collection of 59 images of merging galaxies taken by the Hubble Space Telescope and released on the occasion of its 18th anniversary on 24th April 2008.
[https://www.spacetelescope.org/images/heic0810an/]
Arp 147 (also known as IC 298) is an interacting pair of ring galaxies in the constellation Cetus. The system was originally discovered in 1893 by Stephane Javelle and is listed in the Atlas of Peculiar Galaxies:
[https://en.wikipedia.org/wiki/Arp_147]
Arp 147: Giant Ring of Black Holes
This composite image of Arp 147, a pair of interacting galaxies located about 430 million light years from Earth, shows X-rays from the NASA’s Chandra X-ray Observatory (pink) and optical data from the Hubble Space Telescope (red, green, blue) produced by the Space Telescope Science Institute (STScI) in Baltimore, Md.
Arp 147 contains the remnant of a spiral galaxy (right) that collided with the elliptical galaxy on the left. This collision has produced an expanding wave of star formation that shows up as a blue ring containing an abundance of massive young stars. These stars race through their evolution in a few million years or less and explode as supernovas, leaving behind neutron stars and black holes.
A fraction of the neutron stars and black holes will have companion stars, and may become bright X-ray sources as they pull in matter from their companions. The nine X-ray sources scattered around the ring in Arp 147 are so bright that they must be black holes, with masses that are likely ten to twenty times that of the Sun.
An X-ray source is also detected in the nucleus of the red galaxy on the left and may be powered by a poorly-fed supermassive black hole. This source is not obvious in the composite image but can easily be seen in the X-ray image. Other objects unrelated to Arp 147 are also visible: a foreground star in the lower left of the image and a background quasar as the pink source above and to the left of the red galaxy.
Infrared observations with NASA’s Spitzer Space Telescope and ultraviolet observations with NASA’s Galaxy Evolution Explorer (GALEX) have allowed estimates of the rate of star formation in the ring. These estimates, combined with the use of models for the evolution of binary stars have allowed the authors to conclude that the most intense star formation may have ended some 15 million years ago, in Earth’s time frame.
[http://chandra.harvard.edu/photo/2011/arp147/index.html]
NGC 1055 is an edge-on spiral galaxy located in the constellation Cetus that has a prominent nuclear bulge crossed by a wide, knotty, dark lane of dust and gas. It was discovered on December 19, 1783 by William Herschel from his home in Slough England. It is a binary system together with the bright spiral galaxy M77 (NGC 1068). These two are the largest galaxies of a small galaxy group that also includes NGC 1073, and five other small irregular galaxies:
[https://en.wikipedia.org/wiki/NGC_1055]
Cetus Duo M77 and NGC 1055
At the top right, large spiral galaxy NGC 1055 joins spiral Messier 77 in this sharp cosmic view toward the aquatic constellation Cetus. The narrowed, dusty appearance of edge-on spiral NGC 1055 contrasts nicely with the face-on view of M77’s bright nucleus and spiral arms. Both over 100,000 light-years across, the pair are dominant members of a small galaxy group about 60 million light-years away. At that estimated distance, M77 is one of the most remote objects in Charles Messier’s catalog and is separated from fellow island universe NGC 1055 by at least 500,000 light-years. The field of view is about the size of the full Moon on the sky and includes colorful foreground Milky Way stars (with diffraction spikes) along with more distant background galaxies.
[http://apod.nasa.gov/apod/ap141226.html]
JKCS 041 is a group of galaxies in the constellation Cetus with the distinction of being the farthest away group from Earth ever observed, as of 2009. There are at least 19 members to the cluster:
[https://en.wikipedia.org/wiki/JKCS_041]
JKCS041: Galaxy Cluster Smashes Distance Record
This is a composite image of the most distant galaxy cluster yet detected. This image contains X-rays from NASA’s Chandra X-ray Observatory, optical data from the Very Large Telescope (VLT) and optical and infrared data from the Digitized Sky Survey. This record-breaking object, known as JKCS041, is observed as it was when the Universe was just one quarter of its current age. X-rays from Chandra are displayed here as the diffuse blue region, while the individual galaxies in the cluster are seen in white in the VLT’s optical data, embedded in the X-ray emission.
JKCS041 was originally detected in 2006 with infrared observations from the United Kingdom Infrared Telescope (UKIRT). The distance to the cluster was then determined from optical and infrared observations from UKIRT, the Canada-France-Hawaii telescope in Hawaii and NASA’s Spitzer Space Telescope. However, scientists were not sure if it was a true galaxy cluster, rather than one that has been caught in the act of forming. The shape and extent of the X-ray emission in the Chandra data, however, provided the definitive evidence that showed that JKCS041 was, indeed, a galaxy cluster. The Chandra data also allowed scientists to rule out other possible explanations for the data, including a group of galaxies, or a filament of galaxies seen along the line of sight.
Galaxy clusters are the largest gravitationally-bound objects in the Universe. Scientists have calculated when they should start assembling in the early Universe, and JKCS041, at a distance of some 10.2 billion light years, is on the early edge of that epoch. Follow-on observations of JKCS041 will provide scientists with an opportunity to find important information about how the Universe evolved at this crucial stage.
[http://chandra.harvard.edu/photo/2009/jkcs041/index.html]
Abell 85: Dark Energy Found Stifling Growth in Universe
The composite image on the left is of the galaxy cluster Abell 85, located about 740 million light years from Earth. The purple emission is multi-million degree gas detected in X-rays by NASA’s Chandra X-ray Observatory and the other colors show galaxies in an optical image from the Sloan Digital Sky Survey. This galaxy cluster is one of 86 observed by Chandra to trace how dark energy has stifled the growth of these massive structures over the last 7 billion years. Galaxy clusters are the largest collapsed objects in the Universe and are ideal for studying the properties of dark energy, the mysterious form of repulsive gravity that is driving the accelerated expansion of the Universe.
The illustration on the right shows snapshots from a simulation by Volker Springel, representing the growth of cosmic structure when the Universe was 0.9 billion, 3.2 billion and 13.7 billion years old (now). This shows how the Universe has evolved from a smooth state to one containing a vast amount of structure. Gas is shown in these snapshots, where the yellow regions are stars and the brightest structures are galaxies and galaxy clusters. The growth of these structures was initially driven only by the attractive force of gravity, but then later there was competition with the repulsive force of dark energy.
Understanding the nature of dark energy is one of the biggest problems in science. Possibilities include the cosmological constant, equivalent to the energy of empty space, a modification in general relativity on the largest scales, or a more general physical field. To help decide between these options, Chandra was used to study the increase in mass of galaxy clusters with time over the last 7 billion years. The results are remarkably consistent with those from previous results that measure the expansion of the Universe using distance measurements, revealing that general relativity works as expected on large scales. The cluster work, in combination with other studies, also provides the strongest evidence to date that dark energy is the cosmological constant, or that ‘nothing weighs something.’
[http://chandra.harvard.edu/photo/2008/darkenergy/index.html]
[https://en.wikipedia.org/wiki/Cetus]
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