ANDROMEDA GALAXY

The Andromeda Galaxy  is a spiral galax approximately 780 kiloparsecs (2.5 million light-years; 2.4×1019 km) from Earth in the Andromeda constellation. Also known as Messier 31, M31, or NGC 224, it is often referred to as the Great Andromeda Nebula in older texts. The Andromeda Galaxy is the nearest spiral galaxy to our Milky Way galaxy, but not the nearest galaxy overall. It gets its name from the area of the sky in which it appears, the constellation of Andromeda, which was named after the mythological princess Andromeda. The Andromeda Galaxy is the largest galaxy of the Local Group, which also contains the Milky Way, the Triangulum Galaxy, and about 44 other smaller galaxies.
The Andromeda Galaxy is probably the most massive galaxy in the Local Group as well, despite earlier findings that suggested that the Milky Way contains more dark matter and could be the most massive in the grouping. The 2006 observations by the Spitzer Space Telescope revealed that M31 contains one trillion (1012) stars: at least twice the number of stars in the Milky Way galaxy, which is estimated to be 200–400 billion.

The Andromeda Galaxy is estimated to be 1.5×1012 solar masses while the mass of the Milky Way is estimated to be 8.5×1011 solar masses. In comparison a 2009 study estimated that the Milky Way and M31 are about equal in mass, while a 2006 study put the mass of the Milky Way at ~80% of the mass of the Andromeda Galaxy. The two galaxies are expected to collide in 3.75 billion years, eventually merging to form a giant elliptical galaxy or perhaps a large disk galaxy.
At 3.4, the apparent magnitude of the Andromeda Galaxy is one of the brightest of any Messier objects,making it visible to the naked eye on moonless nights even when viewed from areas with moderate light pollution. Although it appears more than six times as wide as the full Moon when photographed through a larger telescope, only the brighter central region is visible to the naked eye or when viewed using binoculars or a small telescope.
In 2005 Ignasi Ribas (CSIC, Institute for Space Studies of Catalonia (IEEC)) and colleagues announced the discovery of an eclipsing binary star in the Andromeda Galaxy. The binary star, designated M31VJ00443799+4129236,has two luminous and hot blue stars of types O and B. By studying the eclipses of the stars, which occur every 3.54969 days, astronomers were able to measure their sizes. Knowing the sizes and temperatures of the stars, they were able to measure their absolute magnitude. When the visual and absolute magnitudes are known, the distance to the star can be measured. The stars lie at a distance of 2.52 ± 0.14 Mly (773 ± 43 kpc) and the whole Andromeda Galaxy at about 2.5 Mly (770 kpc).This new value is in excellent agreement with the previous, independent Cepheid-based distance value.

M31 is close enough that the Tip of the Red Giant Branch (TRGB) method may also be used to estimate its distance. The estimated distance to M31 using this technique in 2005 yielded 2.56 ± 0.08 Mly (785 ± 25 kpc).
Averaged together, all these distance measurements give a combined distance estimate of 2.54 ± 0.11 Mly (779 ± 34 kpc).[a] Based upon the above distance, the diameter of M31 at the widest point is estimated to be 220 ± 3 kly (67,450 ± 920 pc). Applying trigonometry (arctangent), that figures to extending at an apparent 3.18° angle in the sky.

A Galaxy Evolution Explorer image of the Andromeda Galaxy. The bands of blue-white making up the galaxy's striking rings are neighborhoods that harbor hot, young, massive stars. Dark blue-grey lanes of cooler dust show up starkly against these bright rings, tracing the regions where star formation is currently taking place in dense cloudy cocoons. When observed in visible light, Andromeda’s rings look more like spiral arms. The ultraviolet view shows that these arms more closely resemble the ring-like structure previously observed in infrared wavelengths with NASA’s Spitzer Space Telescope. Astronomers using Spitzer interpreted these rings as evidence that the galaxy was involved in a direct collision with its neighbor, M32, more than 200 million years ago.

In 1920, the Great Debate between Harlow Shapley and Curtis took place, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that the Great Andromeda Nebula was an external galaxy, Curtis also noted the appearance of dark lanes resembling the dust clouds in our own Galaxy, as well as the significant Doppler shift. In 1922 Ernst Öpik presented a method to estimate the distance of M31 using the measured velocities of its stars. His result put the Andromeda Nebula far outside our Galaxy at a distance of about 450,000 parsecs (1,500,000 ly).[28] Edwin Hubble settled the debate in 1925 when he identified extragalactic Cepheid variable stars for the first time on astronomical photos of M31. These were made using the 2.5-metre (100-in) Hooker telescope, and they enabled the distance of Great Andromeda Nebula to be determined. His measurement demonstrated conclusively that this feature was not a cluster of stars and gas within our Galaxy, but an entirely separate galaxy located a significant distance from our own.

COMET GALAXY

                                                                           COMET GALAXY                                                                                                                                                           The Comet Galaxy is a spiral galaxy located 3.2 billion light-years from Earth, in the galaxy cluster Abell 2667, was found with the Hubble Space Telescope. This galaxy has a little more mass than our Milky Way. It was detected on 2 March 2007.

he comet galaxy is currently being ripped to pieces. Moving through a cluster at speeds of greater than 2 million mph, is one of the main reasons the gas and stars of the galaxy are being stripped away by the tidal forces of the cluster. Other factors adding to the damage of the galaxy are the cluster's scorching gas plasma. As the galaxy speeds through, its gas and stars are still being stripped away by the tidal forces exerted by the cluster - just as the tidal forces exerted by the moon and Sun push and pull the Earth's oceans. Also contributing to this destructive process is the pressure of the cluster's hot gas plasma reaching temperatures as high as 10-100 million degrees. Scientists estimate that the total duration of the transformation process is close to one billion years. What is seen now in the Hubble's image is roughly 200 million years into the process. Even though the Comet Galaxy’s mass is slightly greater than the Milky Way, it will lose all its gas and dust, and so not be able to generate stars later in life. It will become a gas-poor galaxy with an old population of red stars.

During the ram pressure stripping process, the charged particles strip and push away the infalling galaxy’s gas, just as the solar wind of charged particles pushes ionized gas away from a comet to create a gas tail. For this reason the scientists have nicknamed the stretched spiral the "comet galaxy."

"This unique galaxy, situated 3.2 billion light-years from Earth, has an extended stream of bright blue knots and diffuse wisps of young stars driven away by the tidal forces and the ram pressure stripping' of the hot dense gas," said Jean-Paul Kneib, a study collaborator from the Laboratoire d'Astrophysique de Marseille.

Even though its mass is slightly larger than that of the Milky Way, the spiral will inevitably lose all its gas and dust as well as its chance of generating new stars later, and become a gas-poor galaxy with an old population of red stars. The finding sheds light on the process by which gas-rich galaxies might evolve into gas-poor galaxies over billions of years. The new observations also reveal one mechanism for forming of “homeless” stars seen scattered throughout galaxy clusters.

The strong gravitational pull exerted by the galaxy cluster's collective mass has bent the light of other, more distant galaxies and distorted their shapes - an effect called gravitational lensing. The giant bright banana-shaped arc seen just to the left of the cluster centre corresponds to the magnified and distorted image of a distant galaxy located behind the cluster's core.

The unique galaxy, which is situated 3.2 billion light-years from the Earth, has an extended stream of bright blue knots and diffuse wisps of young stars. This spiral galaxy rushes with 3.5 million km/h through the cluster Abell 2667 and thereby like a comet shows a tail with a length of 600,000 light-years.

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The “Comet Galaxy” is actually just one galaxy located in a distant galaxy cluster known as Abell 2667, located about 3.2 billion light-years away. A recent photograph captured by the Hubble Space Telescope showed this galaxy being torn apart into a comet shape by the intense gravity of galaxy cluster – and that’s how it got the nickname as the Comet Galaxy.

The observation of the Comet Galaxy and the rest of the galaxies in Abell 2667 helped astronomers understand why many galaxies are “gas poor”. Our own Milky Way has tremendous stores of gas and dust which are used for star formation. But other galaxies out there have very little gas which can be used for star formation.






The image of the Comet Galaxy by Hubble helped show that huge gravitational interactions between galaxies in massive clusters cause tremendous damage to the structure of a galaxy, and the amount of gas they have. Galaxies near the center of clusters experience the most damage of all, which galaxies at the outskirts are relatively unharmed. The galaxy collisions can distort the shape of galaxies, and even fling out “homeless stars” into intergalactic space.

Even though the Comet Galaxy’s mass is slightly greater than the Milky Way, it will lose all its gas and dust, and so not be able to generate stars later in life. It will become a gas-poor galaxy with an old population of red stars.

Because the Comet Galaxy is 3.2 billion light-years away, it can only really be seen with the Hubble Space Telescope. Even a powerful backyard telescope wouldn’t be able to locate it.

IRREGULAR GALAXY

                                                                               IRREGULAR GALAXY                                                                                                                                                                                                              An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy. The shape of an irregular galaxy is uncommon – they do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure.

Collectively they are thought to make up about a quarter of all galaxies. Some irregular galaxies were once spiral or elliptical galaxies but were deformed by disorders in gravitational pull. Irregular galaxies may contain abundant amounts of gas and dust. This is not necessarily true for Dwarf Irregulars.

The Magellanic Cloud galaxies were once classified as irregular galaxies. The Large Magellanic Cloud has since been re-classified as type SBm  a type of barred spiral galaxy, the barred Magellanic spiral type. The Small Magellanic Cloud remains classified as an Irregular galaxy of type Im under current Galaxy morphological classification, although it does contain a bar structure. Therefore, newer classification schemes place the SMC outside the irregular class as well.

Irregular galaxies have no particular shape. They are among the smallest galaxies and are full of gas and dust. Having a lot of gas and dust means that these galaxies have a lot of star formation going on within them. This can make them very bright. The Large and Small Magellanic Clouds are examples of irregular galaxies. They are two small galaxies which orbit around our own Milky Way Galaxy. About 20% of all galaxies are irregulars.

Environment and Galaxies

The type of galaxy one finds at a given location depends on the density of galaxies. This suggests that the interaction between galaxies over the evolution of the universe plays a central role in the evolution of individual galaxies. In the low-density regions of our universe, where galaxies are not bound gravitationally to one another, the dominant type of galaxy is the spiral galaxy. Of the field galaxies, 80% are spiral galaxies, 10% are elliptical galaxies, and less than 10% are lenticular galaxies. In the highest-density regions of our universe, where galaxies form gravitationally-bound clusters, the dominant type of galaxy is the lenticular galaxy, composing up to 50% of the cluster, followed by the elliptical galaxy, composing up to 40% of the cluster. Spiral galaxies compose only 10% of the core of a dense galaxy cluster.

                                            Galaxy Notation

The class of a galaxy is given by a letter and number notation. An elliptical galaxy is noted as En, where n is a digit ranging from 0 to 7 that gives the ellipticity of the galaxy: b/a = 1 - n/10, where a is the semimajor axis and b is the semiminor axis of the ellipse. A galaxy that appears purely isotropic is therefore an E0 elliptical galaxy, while a galaxy with b = 0.3 a is an E7 elliptical galaxy. The lenticular galaxy has a single notation: S0. Spiral galaxies have four symbols: Sa, Sb, Sc, and Sd, with the notation progressing from Sa galaxies, which have tightly-wound spiral arms and a spheroid of stars that is relatively bright compared to the disk stars, to the Sd galaxies, which have loosely-wound and clumpy spiral arms and a relatively dim spheroidal stellar component. The final type, the irregulars, are in general noted by Irr, although those that resemble the Magellanic clouds are noted by Sm or Im. The progression of galaxies from gas poor to gas rich is E0→E7→S0→Sa→Sd→Irr.

The galaxy that is our home is called the Milky Way Galaxy, or just the Galaxy. It measures about 100,000 light years across. Our local star, the Sun, is one of at least 200 billion stars in the Galaxy, and lies in one of the Galaxy’s spiral arms. We also call the faint band of light that arches across the night sky the Milky Way. This band is a just a section of our Galaxy.

THE GALAXY

                                                                                                                                                           THE GALAXY                                                                                                                                                                                                                                                                                                           A galaxy is a gravitationally bound system consisting of stars, stellar remnants, an interstellar medium of gas and dust, and dark matter. The word galaxy is

derived from the Greek galaxias  literally "milky", a reference to the Milky Way. Examples of galaxies range from dwarfs with just a few thousand (103) stars to giants with one hundred trillion (1014) stars each orbiting their galaxy's own center of mass. Galaxies have historically been categorized according to their visual morphology, including elliptical spiral, irregular and starburst. Many galaxies are believed to have black holes at their active center. The Milky Way's central black hole, known as Sagittarius has a mass four million times that of our Sun.

The word galaxy derives from the Greek term for our own galaxy, galaxias "milky one"), or kyklos galaktikos ("milky circle") due to its appearance as a "milky" band of light in the sky. In Greek mythology, Zeus places his son born by a mortal woman, the infant Heracles, on Hera's breast while she is asleep so that the baby will drink her divine milk and will thus become immortal. Hera wakes up while breastfeeding and then realizes she is nursing an unknown baby: she pushes the baby away and jet of her milk sprays the night sky, producing the faint band of light known as the Milky Way.In the astronomical literature, the capitalized word "Galaxy" is used to refer to our galaxy, the Milky Way, to distinguish it from the other galaxies in our universe. The English term Milky Way can be traced back to a story by Chaucer c. 1380:

When William Herschel constructed his catalog of deep sky objects in 1786, he used the term spiral nebula for certain objects such as M31. These would later be recognized as conglomerations of stars, when the true distance to these objects began to be appreciated, and they would later be termed island universes. However, the word Universe was understood to mean the entirety of existence, so this expression fell into disuse and the objects instead became known as galaxies.

Tens of thousands of galaxies have been catalogued, but only a few have been given a well-established name, such as the Andromeda Galaxy, the Magellanic clouds, the Whirlpool Galaxy and the Sombrero Galaxy. Astronomers work with numbers from certain catalogues, such as the Messier catalogue, the NGC (New General Catalogue), the IC (Index Catalogue), the CGCG (Catalogue of Galaxies and of Clusters of Galaxies), the MCG (Morphological Catalogue of Galaxies) and UGC (Uppsala General Catalogue of Galaxies). All of the well-known galaxies appear in one or more of these catalogues but each time under a different number. For example, the Messier 109, a spiral system which has the number 109 in the catalogue of Messier also codes NCG3992, UGC6937, CGCG 269-023, MCG +09-20-044 and PGC 37617.

Because it is customary in science to assign names to most of the studied objects, even to the smallest ones, the Belgian astrophysicist Gerard Bodifee and the classicist Michel Berger started a new catalogue (CNG-Catalogue of Named Galaxies) in which a thousand well-known galaxies are given meaningful, descriptive names in Latin (or Latinized Greek) in accordance with the binomial nomenclature that one uses in other sciences such as biology, anatomy, paleontology and in other fields of astronomy such as the geography of Mars. One of the arguments to do so is that these impressive objects deserve better than uninspired codes. For instance, Bodifee and Berger propose the informal, descriptive name Callimorphus Ursae Majoris for the well-formed barred galaxy Messier 109 in Ursa Major.