Observations using larger telescopes of a few nearby bright galaxies, like the Andromeda Galaxy, began resolving them into huge conglomerations of stars, but based simply
on the apparent faintness and sheer population of stars, the true distances of these objects placed them well beyond the Milky Way.
These structures are thought to develop when a larger galaxy absorbs a smaller companion galaxy—that as the two galaxy centers approach, they start to oscillate around a center
point, and the oscillation creates gravitational ripples forming the shells of stars, similar to ripples spreading on water.
Since the Hubble sequence is entirely based upon visual morphological type (shape), it may miss certain important characteristics of galaxies such as star formation rate in
starburst galaxies and activity in the cores of active galaxies.
Though the stars and other visible material contained in such a galaxy lie mostly on a plane, the majority of mass in spiral galaxies exists in a roughly spherical halo of
dark matter which extends beyond the visible component, as demonstrated by the universal rotation curve concept.
 Many dwarf galaxies may orbit a single larger galaxy; the Milky Way has at least a dozen such satellites, with an estimated 300–500 yet to be discovered.
Wilson telescope, Edwin Hubble was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables, thus allowing
him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way.
 In 1750, English astronomer Thomas Wright, in his An Original Theory or New Hypothesis of the Universe, correctly speculated that it might be a rotating body of a
huge number of stars held together by gravitational forces, akin to the Solar System but on a much larger scale, and that the resulting disk of stars could be seen as a band on the sky from our perspective inside it.
They can grow to enormous sizes (compared to spiral galaxies, for example), and giant elliptical galaxies are often found near the core of large galaxy clusters.
 Starburst Main article: Starburst galaxy M82, a starburst galaxy that has ten times the star formation of a “normal” galaxy Stars are created within galaxies
from a reserve of cold gas that forms giant molecular clouds.
Particularly, surveys in the Zone of Avoidance (the region of sky blocked at visible-light wavelengths by the Milky Way) have revealed a number of new galaxies.
 The Hubble Deep Field, an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about 125 billion (1.25×1011) galaxies in the
 Such an event may have affected the Andromeda Galaxy, as it displays a multi-ring-like structure when viewed in infrared radiation.
Extremely luminous, they were first identified as high redshift sources of electromagnetic energy, including radio waves and visible light, that appeared more similar to stars
than to extended sources similar to galaxies.
To support his claim that the Great Andromeda Nebula is an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well
as the significant Doppler shift.
About one-tenth of elliptical galaxies have a shell-like structure, which has never been observed in spiral galaxies.
 They are strong enough to be dynamically important, as they: • Drive mass inflow into the centers of galaxies • Modify the formation of spiral arms • Can affect the rotation
of gas in the galaxies’ outer regions • Provide the transport of angular momentum required for the collapse of gas clouds, and hence the formation of new stars The typical average equipartition strength for spiral galaxies is about 10 μG (microgauss)
or 1 nT (nanotesla).
 Both analyses failed to take into account the absorption of light by interstellar dust present in the galactic plane; but after Robert Julius Trumpler quantified this
effect in 1930 by studying open clusters, the present picture of our host galaxy emerged.
Seen in visible light, most look like normal spiral galaxies; but when studied under other wavelengths, their cores’ luminosity is equivalent to the luminosity of whole galaxies
the size of the Milky Way.
 Modern research Rotation curve of a typical spiral galaxy: predicted based on the visible matter (A) and observed (B).
Radio-faint galaxies like M 31 and M33, our Milky Way’s neighbors, have weaker fields (about 5 μG), while gas-rich galaxies with high star-formation rates, like M 51, M 83
and NGC 6946, have 15 μG on average.
This suggests that galaxies are largely formed by dark matter, and that the minimum size may indicate a form of warm dark matter incapable of gravitational coalescence on
a smaller scale.
Instead, they are dominated by generally older, more evolved stars that are orbiting the common center of gravity in random directions.
“ Andalusian astronomer Ibn Bâjjah (“Avempace”, d. 1138) proposed that it was composed of many stars that almost touched one another, and appeared to be a continuous image
due to the effect of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars as evidence of this occurring when two objects were near.
Many elliptical galaxies are believed to form due to the interaction of galaxies, resulting in a collision and merger.
 The first project to describe the shape of the Milky Way and the position of the Sun was undertaken by William Herschel in 1785 by counting the number of stars in different
regions of the sky.
 Hoag’s Object, an example of a ring galaxy Barred spiral galaxy A majority of spiral galaxies, including our own Milky Way galaxy, have a linear, bar-shaped band of stars
that extends outward to either side of the core, then merges into the spiral arm structure.
It may be the same size as the Milky Way, but have a visible star count only one percent of the Milky Way’s.
There are multiple classification and naming schemes for AGNs, but those in the lower ranges of luminosity are called Seyfert galaxies, while those with luminosities much
greater than that of the host galaxy are known as quasi-stellar objects or quasars.
They are relatively small when compared with other galactic formations, being about one hundredth the size of the Milky Way, with only a few billion stars.
If one of the galaxies is much more massive than the other, the result is known as cannibalism, where the more massive larger galaxy remains relatively undisturbed, and the
smaller one is torn apart.
 In 1750, Thomas Wright correctly speculated that the Milky Way was a flattened disk of stars, and that some of the nebulae visible in the night sky might be separate
Starburst galaxies were more common during the universe’s early history, but still contribute an estimated 15% to total star production.
 Aristotle (384–322 BCE), however, believed the Milky Way was caused by “the ignition of the fiery exhalation of some stars that were large, numerous and close together”
and that the “ignition takes place in the upper part of the atmosphere, in the region of the World that is continuous with the heavenly motions.
A study of 27 Milky Way neighbors found that in all dwarf galaxies, the central mass is approximately 10 million solar masses, regardless of whether it has thousands or millions
The stars of interacting galaxies usually do not collide, but the gas and dust within the two forms interacts, sometimes triggering star formation.
In 2021, data from NASA’s New Horizons space probe was used to revise the previous estimate to roughly 200 billion galaxies (2×1011), which followed a 2016 estimate that
there were two trillion (2×1012) or more galaxies in the observable universe, overall, and as many as an estimated 1×1024 stars (more stars than all the grains of sand on all beaches of the planet Earth).
Consequently, these galaxies also have a low portion of open clusters and a reduced rate of new star formation.
Among other things, its data helped establish that the missing dark matter in our galaxy could not consist solely of inherently faint and small stars.
He produced a diagram of the shape of the galaxy with the Solar System close to the center.
Actual proof of the Milky Way consisting of many stars came in 1610 when the Italian astronomer Galileo Galilei used a telescope to study it and discovered it was composed
of a huge number of faint stars.
All the well-known galaxies appear in one or more of these catalogs but each time under a different number.
o A ring galaxy has a ring-like structure of stars and interstellar medium surrounding a bare core.
 In 1734, philosopher Emanuel Swedenborg in his Principia speculated that there might be galaxies outside our own that were formed into galactic clusters that were minuscule
parts of the universe that extended far beyond what we could see.
Today, the galaxy rotation problem is thought to be explained by the presence of large quantities of unseen dark matter.
 Starburst galaxies are characterized by dusty concentrations of gas and the appearance of newly formed stars, including massive stars that ionize the surrounding clouds
to create H II regions.
Milky Way Main article: Milky Way Greek philosopher Democritus (450–370 BCE) proposed that the bright band on the night sky known as the Milky Way might consist of distant
The standard model for an active galactic nucleus is based on an accretion disc that forms around a supermassive black hole (SMBH) at the galaxy’s core region.
The radiation from an active galactic nucleus results from the gravitational energy of matter as it falls toward the black hole from the disc.
 A galaxy with poorly defined arms is sometimes referred to as a flocculent spiral galaxy; in contrast to the grand design spiral galaxy that has prominent and well-defined
A different method by Harlow Shapley based on the cataloguing of globular clusters led to a radically different picture: a flat disk with diameter approximately 70 kiloparsecs
and the Sun far from the center.
Bars are thought to be temporary structures that can occur as a result of a density wave radiating outward from the core, or else due to a tidal interaction with another galaxy.
Most 18th- to 19th-century astronomers considered them as either unresolved star clusters or anagalactic nebulae, and were just thought of as a part of the Milky Way, but
their true composition and natures remained a mystery.
“ In 1745, Pierre Louis Maupertuis conjectured that some nebula-like objects were collections of stars with unique properties, including a glow exceeding the light its
stars produced on their own, and repeated Johannes Hevelius’s view that the bright spots were massive and flattened due to their rotation.
The arms are visible because the high density facilitates star formation, and therefore they harbor many bright and young stars.
Ultra-luminous infrared galaxies (ULIRGs) are at least ten times more luminous still and form stars at rates >180 M☉ yr−1.
They are thought to have an increased star formation rate around 30 times faster than the Milky Way.
Observation history The realization that we live in a galaxy that is one among many parallels major discoveries about the Milky Way and other nebulae.
 In the astronomical literature, the capitalized word “Galaxy” is often used to refer to our galaxy, the Milky Way, to distinguish it from the other galaxies in our
A significant portion of the galaxy’s total energy output is emitted by the active nucleus instead of its stars, dust and interstellar medium.
Some galaxies have been observed to form stars at an exceptional rate, which is known as a starburst.
“ The shape of the Milky Way as estimated from star counts by William Herschel in 1785; the Solar System was assumed to be near the center.
A ring galaxy is thought to occur when a smaller galaxy passes through the core of a spiral galaxy.
The Milky Way’s central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun.
Galaxies range in size from dwarfs with just a few hundred million (108) stars to giants with one hundred trillion (1014) stars, each orbiting its galaxy’s center of mass.
 Blazars Main article: Blazar Blazars are believed to be active galaxies with a relativistic jet pointed in the direction of Earth.
In the 1970s, Vera Rubin uncovered a discrepancy between observed galactic rotation speed and that predicted by the visible mass of stars and gas.
 However, later observations with the New Horizons space probe from outside the zodiacal light reduced this to roughly 200 billion (2×1011).
 Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms.
The prototype example of such a starburst-forming interaction is M82, which experienced a close encounter with the larger M81.
They are very large with an upward diameter of 437,000 light-years (compared to the Milky Way’s 100,000 light-year diameter).
Formation Artist’s impression of a protocluster forming in the early universe Current models of the formation of galaxies in the early universe are based on the ΛCDM model.
[‘Galaxies to the left side of the Hubble classification scheme are sometimes referred to as “early-type”, while those to the right are “late-type”.
2. ^ The term “field galaxy” is sometimes used to mean an isolated galaxy, although the same term is
also used to describe galaxies that do not belong to a cluster but may be a member of a group of galaxies.
3. Sparke & Gallagher 2000, p. i
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Photo credit: https://www.flickr.com/photos/usfwsmtnprairie/5223643767/ ‘]