Galaxy (Remastered) Sleep Facts

Even sleep has a center of gravity. Galaxies, those sprawling cosmic suburbs of stars and dust, come in convenient spiral, elliptical, and irregular shapes. They’re big, old, and mostly indifferent—perfect for a little insomnia relief. Enjoy this new and longer edition about galaxies.

Transcript

Welcome to the I Can't Sleep podcast,

Where I help you drift off one fact at a time.

I'm your host,

Benjamin Boster,

And today's episode is about galaxies.

A galaxy is a system of stars,

Stellar remnants,

Interstellar gas,

Dust,

And dark matter bound together by gravity.

The word is derived from the Greek galaxias,

Literally milky,

A reference to the Milky Way galaxy that contains the solar system.

Galaxies,

Averaging an estimated 100 million stars,

Range in size from dwarfs with less than a thousand stars to the largest galaxies known,

Supergiants,

With 100 trillion stars,

Each orbiting its galaxy's center of mass.

Most of the mass in a typical galaxy is in the form of dark matter,

With only a few percent of that mass visible in the form of stars and nebulae.

Supermassive black holes are a common feature of the centers of galaxies.

NGC 4414,

A typical spiral galaxy in the constellation Coma Berenices,

Is about 55,

000 light years in diameter,

And approximately 60 million light years from Earth.

Galaxies are categorized according to their visual morphology as elliptical,

Spiral,

Or irregular.

The Milky Way is an example of a spiral galaxy.

It is estimated that there are between 200 billion,

2 x 10 to the 11th,

To 2 trillion galaxies in the Milky Way galaxy.

Galaxies are between 3,

000 to 100,

000 parsecs in diameter,

Approximately 3,

000 to 300,

000 light years,

And are separated by distances in the order of millions of parsecs,

Or megaparsecs.

For comparison,

The Milky Way has a diameter of at least 26,

800 parsecs,

87,

400 light years,

And is separated by the Andromeda galaxy,

Its nearest large neighbor,

By just over 750,

000 parsecs.

The space between galaxies is filled with a tenuous gas,

The intergalactic medium,

With an average density of less than one atom per cubic meter.

Most galaxies are gravitationally organized into groups,

Clusters,

And superclusters.

The Milky Way is part of the local group,

Which it dominates along with the Andromeda galaxy.

The group is part of the Virgo supercluster.

At the largest scale,

These associations are generally arranged into sheets and filaments,

Surrounded by immense voids.

Both the local group and the Virgo supercluster are contained in a much larger cosmic structure,

Named Laniakea.

The word galaxy was borrowed via French and Medieval Latin from the Greek term for the Milky Way,

Galaxias Kiklos,

Milky Circle.

Named after its appearance as a milky band of light in the sky.

In the astronomical literature,

The capitalized word galaxy is often used to refer to the Milky Way galaxy,

To distinguish it from other galaxies in the universe.

Galaxies were initially discovered telescopically and were known as spiral nebulae.

Most 18th to 19th century astronomers considered them as neither unresolved star clusters or extragalactic nebulae.

But their true composition and natures remained a mystery.

Observations using large 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.

For this reason,

They were popularly called island universes.

Harlow Shapely began to advocate for the term galaxy and against using universes in nebulae for the objects,

But the very influential Edwin Hubble stuck to nebulae.

The nomenclature did not fully change until Hubble's death in 1953.

Millions of galaxies have been catalogued,

But only a few have well-established names,

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 MGC,

New General Catalogue,

The IC,

Index Catalogue,

The CGCG,

Catalogue of Galaxies and of Clusters of Galaxies,

The MGC,

Morphological Catalogue of Galaxies,

The UGC,

Upsilon General Catalogue of Galaxies,

And the PGC,

Catalogue of Principal Galaxies,

Also known as LIDA.

All the well-known galaxies appear in one or more of these catalogues,

But each time under a different number.

For example,

Messier 109,

Or M109,

Is a spiral galaxy having the number 109 in the catalogue of Messier.

It also has the designations NGC 3992,

UGC 6937,

CGCG 269-023,

MCG plus 09-20-044,

And PGC 37617,

Or LIDA 37617,

Among others.

Millions of fainter galaxies are known by their identifiers in sky surveys,

Such as the Sloan Digital Sky Survey.

Greek philosopher Democritus,

450-370 BCE,

Proposed that the bright band on the night sky known as the Milky Way might consist of distant stars.

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.

Neoplatonist philosopher Olympiodorus the Younger,

495-570 CE,

Was critical of this view,

Arguing that if the Milky Way was sublunary,

Situated between Earth and the Moon,

It should appear different at different times and places on Earth,

And that it should have parallax,

Which it did not.

In his view,

The Milky Way was celestial.

According to Mohamed,

Arabian astronomer Ibn al-Haytham,

965-1037,

Made the first attempt at observing and measuring the Milky Way's parallax,

And he thus determined that because the Milky Way had no parallax,

It must be remote from the Earth,

Not belonging to the atmosphere.

Persian astronomer Al-Biruni,

974-1048,

Proposed the Milky Way galaxy was a collection of countless fragments of the nature of nebulous stars.

Andalusian astronomer Avampes 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.

In the 14th century,

Syrian-born Ibn Kayyum al-Jaziyya proposed the Milky Way galaxy was a myriad of tiny stars packed together in the sphere of the fixed stars.

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.

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 disc of stars could be seen as a band on the sky from a perspective inside it.

In his 1755 treatise,

Immanuel Kant elaborated on Wright's idea about the Milky Way's structure.

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.

He produced a diagram of the shape of the galaxy with the solar system close to the center.

Using a refined approach,

Kapteyn in 1920 arrived at the picture of a small ellipsoid galaxy with the sun close to the center.

A different method by Harlow Shapley,

Based on the cataloging of globular clusters,

Led to a radically different picture,

A flat disc with diameter approximately 70 kiloparsecs,

And the sun far from the center.

Boe's analyses failed to take into account the absorption of light by interstellar dust present in the galactic plane,

But after Robert Julius Trumbler quantified this effect in 1930 by studying open clusters,

The present picture of the Milky Way galaxy emerged.

A few galaxies outside the Milky Way are visible on a dark night to the unknown.

The Andromeda Galaxy,

Large Magellanic Cloud,

Small Magellanic Cloud,

And the Triangulum Galaxy.

In the 10th century,

Persian astronomer Abd al-Rahman al-Sufi made the earliest recorded identification of the Andromeda Galaxy,

Describing it as a small cloud.

In 964,

He probably mentioned the Large Magellanic Cloud in his Book of Fixed Stars,

Referring to al-Bakr of the Southern Arabs,

Since at a declination of about 70 degrees south,

It was not visible where he lived.

It was not well known to Europeans until Magellan's voyage in the 16th century.

The Andromeda Galaxy was later independently noted by Simon Marius in 1612.

In 1734,

Philosopher Immanuel Swedenborg,

In his Principia,

Speculated that there might be other galaxies outside that were formed into galactic clusters,

That were minuscule parts of the universe,

That extended far beyond what could be seen.

Swedenborg's views are remarkably close to the present-day views of a cosmos.

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 Avelius' view that the bright spots were massive and flattened due to their rotation.

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 Milky Ways.

Toward the end of the 18th century,

Charles Messier compiled a catalog containing the 109 brightest celestial objects having nebulous appearance.

Subsequently,

William Herschel assembled a catalog of 5,

000 nebulae.

In 1845,

Lord Ross examined the nebulae catalogued by Herschel,

And observed the spiral structure of Messier object M51,

Now known as the Whirlpool Galaxy.

In 1912,

Vesto M.

Slipher made spectrographic studies of the brightest spiral nebulae to determine their composition.

Slipher discovered that the spiral nebulae have high Doppler shifts,

Indicating that they are moving at a rate exceeding the velocity of the stars he had measured.

He found that the majority of these nebulae are moving away from us,

And that they are moving at a rate exceeding the velocity of the stars he had measured.

In 1917,

Heber Doust Curtis observed Nova S.

Andromedae within the Great Andromeda Nebula,

As the Andromeda Galaxy,

Messier object M31,

Was then known.

Searching for the photographic record,

He found 11 more novae.

Curtis noticed that these novae were,

On average,

10 magnitudes fainter than those that occurred within this galaxy.

As a result,

He was able to come up with a distance estimate of 150,

000 parsecs.

He became a proponent of the so-called Island Universes Hypothesis,

Which holds that spiral nebulae are actually independent galaxies.

In 1920,

A debate took place between Harlow Shapely and Heber Curtis,

The Great Debate,

Concerning the nature of the Milky Way,

Spiral nebulae,

And the dimensions of the universe.

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 a significant Doppler shift.

In 1922,

The Estonian astronomer Ernst Oerpeg gave a distance determination that supported the theory that the Andromeda Nebula is indeed a distant extragalactic object.

Using the new 100-inch Mount 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 1926,

Hubble produced a classification of galactic morphology that is used to this day.

Advances in astronomy have always been driven by technology.

After centuries of success in optical astronomy,

Recent decades have seen major progress in other regions of the electromagnetic spectrum.

The dust present in the interstellar medium is opaque to visual light.

It is more transparent to far infrared,

Which can be used to observe the inferior regions of giant molecular clouds and galactic cores in great detail.

Infrared is also used to observe distant red-shifted galaxies that were formed much earlier.

Water vapor and carbon dioxide absorb a number of useful portions of the infrared spectrum,

So high-altitude or space-based telescopes are used for infrared astronomy.

The first non-visual study of galaxies,

Particularly active galaxies,

Was made using radio frequencies.

The Earth's atmosphere is nearly transparent to radio between 5 MHz and 30 GHz.

The ionosphere blocks signals below this range.

Large radio interferometers have been used to map the active jets emitted from active nuclei.

Ultraviolet and X-ray telescopes can observe highly energetic galactic phenomena.

Ultraviolet flares are sometimes observed when a star in a distant galaxy is torn apart from the tidal forces of a nearby black hole.

The distribution of hot gas in galactic clusters can be mapped by X-rays.

The existence of supermassive black holes at the cores of galaxies was confirmed through X-ray astronomy.

In 1944,

Hendrik van der Holst predicted that microwave radiation with wavelength of 21 cm would be detectable from interstellar atomic hydrogen gas.

In 1951,

It was observed.

This radiation is not affected by dust absorption,

And so its Doppler shift can be used to map the motion of the gas in this galaxy.

These observations led to the hypothesis of a rotating bar structure in the center of this galaxy.

With improved radio telescopes,

Hydrogen gas could also be traced in other galaxies.

In the 1970s,

Vera Rubin uncovered a discrepancy between observed galactic rotation speed and that predicted by the visible mass of stars and gas.

Today,

The galaxy rotation problem is thought to be explained by the presence of large quantities of unseen dark matter.

Beginning in the 1990s,

The Hubble Space Telescope yielded improved observations.

Among other things,

Its data helped establish that the missing dark matter in this galaxy could not consist solely of inherently faint and small stars.

The Hubble Deep Field,

An extremely long exposure of a relatively empty part of the sky,

Provided evidence that there are about 125 billion galaxies in the observable universe.

Improved technology in detecting the spectra invisible to humans,

Radio telescopes,

Infrared cameras,

And x-ray telescopes,

Allows detection of other galaxies that are not detected by Hubble.

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.

A 2016 study published in the Astrophysical Journal,

Led by Christopher Kinsellas of the University of Nottingham,

Analyzed many sources of data to estimate that the observable universe contained at least two trillion galaxies,

A factor of 10 more than are directly observed in Hubble images.

However,

Later observations with the New Horizons space probe from outside the zodiacal light observed less cosmic optical light than Kinsellas,

While still suggesting that direct observations are missing galaxies.

Galaxies come in three main types,

Ellipticals,

Spirals,

And irregulars.

A slightly more extensive description of galaxy types,

Based on their appearance,

Is given by the Hubble sequence.

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.

Many galaxies are thought to contain a supermassive black hole at their center.

This includes the Milky Way,

Whose core region is called the galactic center.

The Hubble classification system rates elliptical galaxies on the basis of their ellipticity,

Ranging from E0,

Being nearly spherical,

Up to E7,

Highly elongated.

These galaxies have an ellipsoidal profile,

Giving them an elliptical appearance regardless of the viewing angle.

Their appearance shows little structure,

And they typically have relatively little interstellar matter.

Consequently,

These galaxies also have a low portion of open clusters,

And a reduced rate of new star formation.

Instead,

They are dominated by generally older,

More evolved stars,

That are orbiting the common center of gravity in random directions.

The stars contain low abundances of heavy elements,

Because star formation ceases after the initial burst.

In this sense,

They have some similarity to the much smaller globular clusters.

The largest galaxies are the type CD galaxies.

First described in 1964 by a paper by Thomas A.

Matthews and others,

They are a subtype of the more general class of D galaxies,

Which are giant elliptical galaxies,

Except that they are much larger.

They are popularly known as the supergiant elliptical galaxies,

And constitute the largest and most luminous galaxies known.

These galaxies feature a central elliptical nucleus with an extensive faint halo of stars extending to megaparsec scales.

The profile of their surface brightnesses as a function of their radius,

Or distance from their cores,

Falls off more slowly than their smaller counterparts.

The formation of these CD galaxies remains an active area of research,

But the leading model is that they are the result of the mergers of smaller galaxies in the environments of dense clusters,

Or even those outside of clusters with random overdensities.

These processes are the mechanisms that drive the formation of fossil groups or fossil clusters,

Where a large,

Relatively isolated supergiant elliptical resides in the middle of a cluster and are surrounded by an extensive cloud of x-rays as the residue of these galactic collisions.

Another older model posits the phenomenon of cooling flow,

Where the heated gases in clusters collapses towards their centers as they cool,

Forming stars in the process,

A phenomenon observed in clusters such as Perseus,

And more recently in the Phoenix cluster.

A shell galaxy is a type of elliptical galaxy where the stars in its halo are arranged in concentric shells.

About one-tenth of elliptical galaxies have a shell-like structure,

Which has never been observed in spiral galaxies.

These structures are thought to develop when a larger galaxy absorbs a smaller companion galaxy,

That is,

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.

For example,

Galaxy NGC 3923 has over 20 shells.

Spiral galaxies resemble spiraling pinwheels,

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.

Spiral galaxies consist of a rotating disc of stars and interstellar medium,

Along with a central bulge of generally older stars.

Extending outward from the bulge are relatively bright arms.

In the Hubble classification scheme,

Spiral galaxies are listed as type S,

Followed by a letter,

A,

B,

Or C,

Which indicates the degree of tightness of the spiral arms and the size of the central bulge.

An SA galaxy has tightly wound,

Poorly defined arms,

And possesses a relatively large core region.

At the other extreme,

An SC galaxy has open,

Well-defined arms,

And a small core region.

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 spiral arms.

The speed in which a galaxy rotates is thought to correlate with the flatness of the disc,

As some spiral galaxies have thick bulges,

While others are thin and dense.

In spiral galaxies,

The spiral arms do have the shape of approximate logarithmic spirals,

A pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars.

Like the stars,

The spiral arms rotate around the center,

But they do so with constant angular velocity.

The spiral arms are thought to be areas of high-density matter,

Or density waves.

As stars move through an arm,

The space velocity of each stellar system is modified by the gravitational force of the higher density.

The velocity returns to normal after the stars depart on the other side of the arm.

This effect is akin to a wave of slowdowns moving along a highway full of moving stars.

The arms are visible because the high density facilitates star formation,

And therefore they harbor many bright and young stars.

A majority of spiral galaxies,

Including the 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.

In the Hubble classification scheme,

These are designated by an SB,

Followed by a lowercase letter,

A,

B,

Or c,

Which indicates the form of the spiral arms in the same manner as the categorization of normal spiral galaxies.

Bars are thought to be temporary structures that can occur as a result of density wave radiating outward from the core,

Or else due to a tidal interaction with another galaxy.

Many barred spiral galaxies are active,

Possibly as a result of gas being channeled into the core along the arms.

Our own galaxy,

The Milky Way,

Is a large,

Dark-shaped barred spiral galaxy,

About 30 kiloparsecs in diameter and a kiloparsec thick.

It contains about 200 billion stars,

And has a total mass of about 600 billion times the mass of the Sun.

Recently,

Researchers described galaxies called superluminous spirals.

They are very large,

With an upward diameter of 437,

000 light-years,

Compared to the Milky Way's 87,

400 light-year diameter.

With a mass of 340 billion solar masses,

They generate a significant amount of ultraviolet and mid-infrared light.

They are thought to have an increased star formation rate,

Around 30 times faster than the Milky Way.

Other morphologies Peculiar galaxies are galactic formations that develop unusual properties due to tidal interactions with other galaxies.

A ring galaxy has a ring-like structure of stars and interstellar medium surrounding a bare core.

A ring galaxy is thought to occur when a smaller galaxy passes through the core of a spiral galaxy.

Such an event may have affected the Andromeda galaxy,

As it displays a multi-ring-like structure when viewed in infrared radiation.

A lenticular galaxy is an intermediate form that has properties of both elliptical and spiral galaxies.

These are categorized as Hubble type S0,

And they possess ill-defined spiral arms with an elliptical halo of stars.

Barred lenticular galaxies receive Hubble classification Sb0.

Irregular galaxies are galaxies that cannot be readily classified into an elliptical or spiral morphology.

An Ir1 galaxy has some structure,

But does not align cleanly with the Hubble classification scheme.

Ir2 galaxies do not possess any structure that resembles a Hubble classification and may have been disrupted.

Nearby examples of dwarf irregular galaxies include the Magellanic clouds.

A dark or ultra-diffused galaxy is an extremely low-luminosity galaxy.

It may be the same size as the Milky Way,

But have a visible star count only 1% of the Milky Way's.

Multiple mechanisms for producing this type of galaxy have been proposed,

And it is possible that different dark galaxies formed by different means.

One candidate explanation for the low-luminosity is that the galaxy lost its star-forming gas at an early stage,

Resulting in old stellar populations.

Despite the prominence of large elliptical and spiral galaxies,

Most galaxies are dwarf galaxies.

They are relatively small when compared with other galactic formations.

Being about one-hundredth the size of the Milky Way was only a few billion stars.

Blue,

Compact dwarf galaxies contain large clusters of young,

Hot,

Massive stars.

Ultra-compact dwarf galaxies have been discovered that are only 100 parsecs across.

Many dwarf galaxies may orbit a single larger galaxy.

The Milky Way has at least a dozen such satellites,

With an estimated 300 to 500 yet to be discovered.

Most of the information we have about dwarf galaxies come from observations of the local group containing two spiral galaxies,

The Milky Way and Andromeda,

And many dwarf galaxies.

These dwarf galaxies are classified as either irregular or dwarf elliptical dwarf spheroidal galaxies.

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 of stars.

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.

Interactions between galaxies are relatively frequent,

And they can play an important role in galactic evolution.

Near misses between galaxies result in warping distortion due to tidal interactions,

And may cause some exchange of gas and dust.

Collisions occur when two galaxies pass directly through each other and have sufficient relative momentum not to merge.

The stars of interacting galaxies usually do not collide,

But the gas and dust within the two forms interacts,

Sometimes triggering star formation.

A collision can severely distort the galaxy's shapes,

Forming bars,

Rings,

Or tail-like structures.

At the extreme of interactions are galactic mergers,

Where the galaxy's relative momentums are insufficient to allow them to pass through each other.

Instead,

They gradually merge to form a single larger galaxy.

Mergers can result in significant changes to the galaxy's original morphology.

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.

The Milky Way galaxy is currently in the process of cannibalizing the Sagittarius dwarf elliptical galaxy and the Cana's major dwarf galaxy.

Stars are created within galaxies from a reserve of cold gas that forms giant molecular clouds.

Some galaxies have been observed to form stars at an exceptional rate,

Which is known as a starburst.

If they continued to do so,

They would consume their reserve of gas in a time span less than the galaxy's lifespan.

Hence,

Starburst activity usually lasts only about 10 million years,

A relatively brief period in a galaxy's history.

Starburst galaxies were more common during the universe's early history,

But still contribute an estimated 15% to total star production.

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 H2 regions These stars produce supernova explosions,

Creating expanding remnants that interact powerfully with the surrounding gas.

These outbursts trigger a chain reaction of star building that spreads throughout the gaseous region.

Only when the available gas is nearly consumed or dispersed does the activity end.

Starbursts are often associated with the merging or interacting galaxies.

The prototype example of such a starburst forming interactions is M82,

Which experienced a close encounter with a larger M81.

Irregular galaxies often exhibit spaced knots of starburst activity.

A radiogalaxy is a galaxy with giant regions of radio emission extending well beyond its visible structure.

These energetic radiolobes are powered by jets from its active galactic nucleus.

Radiogalaxies are classified according to their Phaneroff-Riley classification.

The FR1 class have lower radioluminosity and exhibit structures which are more elongated.

The FR2 class are higher radioluminosity.

The correlation of radioluminosity and structure suggests that the sources in these two types of galaxies may differ.

Radiogalaxies can also be classified as giant radiogalaxies,

GRGs,

Whose radio emissions can extend to scales of megaparsecs.

While Cyaneus is an FR2 class low-exhitation radiogalaxy,

Which has the largest observed radio emission,

With lobed structures spanning 5 megaparsecs.

For comparison,

Another similarly sized giant radiogalaxy is 3C236 with lobes 15 million light years across.

It should however be noted that radio emissions are not always considered part of the main galaxy itself.

A giant radiogalaxy is a special class of objects characterized by the presence of radiolobes generated by relativistic jets powered by the central galaxy's supermassive black hole.

Giant radiogalaxies are different from ordinary radiogalaxies in that they can extend to much larger scales,

Reaching upwards to several megaparsecs across,

Far lighter than the A normal radiogalaxy does not have a source that is a supermassive black hole or monster neutron star.

Instead the source is synchrotron radiation from relativistic electrons accelerated by supernovae.

These sources are comparatively short-lived,

Making the radio spectrum from normal radiogalaxies an especially good way to study star formation.

Some observable galaxies are classified as active if they contain an active galactic nucleus,

AGN.

A significant portion of the galaxy's total energy output is emitted by the active nucleus instead of its stars,

Dust,

And interstellar medium.

There are multiple classification and naming schemes for AGNs,

But those in the lower ranges of luminosity are called ciphered galaxies,

While those with luminosities much greater than that of the host galaxy are known as quasi-stellar objects or quasars.

Models of AGNs suggest that a significant fraction of their light is shifted to far infrared.

Models of AGNs suggest that a significant fraction of their light is shifted to far infrared frequencies because optical and UV emission in the nucleus is absorbed and remitted by dust and gas surrounding it.

The standard model for an active galactic nucleus is based on an accretion disk 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 disk.

The AGN's luminosity depends on the SMBH's mass and the rate at which matter falls into it.

In about 10% of these galaxies,

A diametrically opposed pair of energetic jets ejects particles from the galaxy core at velocities close to the speed of light.

The mechanism for producing these jets is not well understood.

Ciphered galaxies are one of the two largest groups of active galaxies along with quasars.

They have quasar-like nuclei,

Very luminous,

Distant,

And bright sources of electromagnetic radiation with very high surface brightnesses.

But unlike quasars,

Their host galaxies are clearly detectable.

Seen through a telescope,

A ciphered galaxy appears like an ordinary galaxy with a bright star superimposed atop the core.

Ciphered galaxies are divided into two principal subtypes based on the frequencies observed in their spectra.

Quasars are the most energetic and distant members of active galactic nuclei.

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.

Their luminosity can be 100 times that of the Milky Way.

The nearest known quasar,

Markarian 231,

Is about 581 million light-years from Earth,

While others have been discovered as far away as UHZ-1,

Roughly 13.

2 billion light-years distant.

Quasars are noteworthy for providing the first demonstration of the phenomenon that gravity can act as a lens for light.

Blazars are believed to be active galaxies with a relativistic jet pointed in the direction of Earth.

A radio galaxy emits radio frequencies from relativistic jets.

A unified model of these types of active galaxies explains their differences based on the observer's position.

Possibly related to active galactic nuclei,

As well as starburst regions,

Are low ionization nuclear emission line regions,

Liners.

The emission from liner-type galaxies is dominated by weakly ionized elements.

The excitation sources for the weakly ionized lines include post-AGB stars,

AGN,

And shocks.

Approximately one-third of nearby galaxies are classified as containing liner nuclei.

Luminous infrared galaxies,

LIRGs,

Are galaxies with luminosities,

The measurement of electromagnetic power output,

Above 10 to the 11th solar luminosities.

In most cases,

Most of their energy comes from large numbers of young stars,

Which heat surrounding dust,

Which re-radiates the energy in the infrared.

Luminosity high enough to be a LIRG requires a star formation rate of at least 18 solar masses per year.

Ultra-luminous infrared galaxies,

ULIRGs,

Are at least 10 times more luminous still,

And form stars at rates greater than 180 solar masses per year.

Many LIRGs also emit radiation from an AGN.

Infrared galaxies emit more energy in the infrared than all other wavelengths combined,

With peak emission typically at wavelengths of 60 to 100 microns.

LIRGs are believed to be created from the strong interaction and merger of spiral galaxies.

While uncommon in the local universe,

LIRGs and ULIRGs were more prevalent when the universe was younger.

Galaxies do not have a definite boundary by their nature,

And are characterized by a gradually decreasing stellar density as a function of increasing distance from their center,

Making measurements of their true extents difficult.

Nevertheless,

Astronomers over the past few decades have made several criteria in defining the sizes of galaxies.

As early as the time of Edwin Hubble in 1936,

There have been attempts to characterize the diameters of galaxies.

The earliest efforts were based on the observed angles subtended by the galaxy and its estimated distance,

Leading to an angular diameter,

Also called metric diameter.

The isophotal diameter is introduced as a conventional way of measuring a galaxy's size based on its apparent surface brightness.

Isophotes are curves in a diagram,

Such as a picture of a galaxy,

That adjoins points of equal brightnesses and are useful in defining the extent of the galaxy.

The apparent brightness flux of a galaxy is measured in units of magnitudes per square arc second,

Which defines the brightness depths of the isophote.

The half-light radius,

Also known as effective radius,

Is a measure that is based on the galaxy's overall brightness flux.

This is the radius upon which half or 50% of the total brightness flux of the galaxy was emitted.

This was first proposed by Gérard de Vauquelure in 1948.

The choice of using 50% was arbitrary,

But proved to be useful in further works by R.

A.

Fish in 1963,

Where he established a luminosity concentration law that relates to the brightnesses of elliptical galaxies in their respective R-sub-e,

And by José Luis Sércic in 1968 that defined a mass-radius relation in galaxies.

In defining R-sub-e,

It is necessary that the overall brightness flux galaxy should be captured with a method employed by Bershade in 2000,

Suggesting to measure twice the size where the brightness flux of an arbitrary chosen radius,

Defined as the local flux,

Divided by the overall average flux equals 2.

2.

Using half-light radius allows a rough estimate of a galaxy's size,

But is not particularly helpful in determining its morphology.

Variations of this method exist.

In particular,

In the ESO Apsala Catalog of Galaxies,

Values of 50%,

70%,

And 90% of the total blue light,

The light detected through a B-band specific filter,

Had been used to calculate a galaxy's diameter.

First described by Vahe-Petrosian in 1976,

A modified version of this method has been used by the Sloan Digital Sky Survey,

SDSS.

This method employs a mathematical model on a galaxy whose radius is determined by the azimuthally horizontal average profile of its brightness flux.

In particular,

The SDSS employed the Petrosian magnitude and the R-band to ensure that the brightness flux of a galaxy would be captured as much as possible,

While counteracting the effects of background noise.

For a galaxy whose brightness profile is exponential,

It is expected to capture all of its brightness flux,

And 80% for galaxies that follow a profile that follows de Vuccaler's law.

Petrosian's magnitudes have the advantage of being redshift and distance-independent,

Allowing the measurement of the galaxy's apparent size,

Since the Petrosian radius is defined in terms of the galaxy's overall luminous flux.

A critique of an earlier version of this method has been issued by the Infrared Processing and Analysis Center,

With the method causing a magnitude of error upwards to 10% of the values,

Then using isophotal diameter.

The use of the Petrosian magnitudes also have the disadvantage of missing most of the light outside the Petrosian aperture,

Which is defined relative to the galaxy's overall brightness profile,

Especially for elliptical galaxies,

With higher signal-to-noise ratios on higher distances and redshifts.

A correction for this method has been issued by Graham et al.

In 2005,

Based on the assumption that galaxies follow Circic's law.

The near-infrared method has been used by 2MASS as an adaptation from the previously used methods of isophotal measurement.

Since 2MASS operates in the near-infrared,

Which has the advantage of being able to recognize dimmer,

Cooler,

And older stars,

It has a different form of approach compared to other methods that normally use B-filters.

The detail of the method used by 2MASS has been described thoroughly in a document by Jarrett et al.

,

With the survey measuring several parameters.

Seifert's Sextet is an example of a compact galaxy group.

Deep-sky surveys show that galaxies are often found in groups and clusters.

Solitary galaxies that have not significantly interacted with other galaxies of comparable mass in the past few billion years are relatively scarce.

Only about 5% of the galaxies surveyed are isolated in this sense.

However,

They may have interacted and even merged with other galaxies in the past,

And may still be orbited by smaller satellite galaxies.

On the largest scale,

The universe is continually expanding,

Resulting in an average increase in the separation between individual galaxies.

Associations of galaxies can overcome this expansion on a local scale through their mutual gravitational attraction.

These associations formed early,

As clumps of dark matter pulled their respective galaxies together.

Nearby groups later merged to form larger-scale clusters.

This ongoing merging process,

As well as an influx of involving gas,

Heats the intergalactic gas in a cluster to very high temperatures of 30 to 100 megakelvins.

About 70 to 80% of a cluster's mass is in the form of dark matter,

With 10 to 30% consisting of this heated gas,

And the remaining few percent in the form of galaxies.

Most galaxies are gravitationally bound to a number of other galaxies.

These form a fractal-like hierarchical distribution of clustered structures,

With the smallest such associations being termed groups.

A group of galaxies is the most common type of galactic cluster.

These formations contain the majority of galaxies,

As well as most of the baryonic mass in the universe.

To remain gravitationally bound to such a group,

Each member galaxy must have a sufficiently low velocity to prevent it from escaping.

If there is insufficient kinetic energy,

However,

The group may evolve into a smaller number of galaxies through mergers.

Clusters of galaxies consist of hundreds to thousands of galaxies bound together by gravity.

Clusters of galaxies are often dominated by a single giant elliptical galaxy,

Known as the brightest cluster galaxy,

Which over time tidally destroys its satellite galaxies and adds their mass to its own.

Reviews

I enjoyed it so much that I had to listen to it twice in a row! 🤣🤣
After I finally fell asleep, my cat decided he needed to serenade me by singing the song of his people at 2 AM. It was joyful to hear…..🙄🙄🙄
On the other hand, this remastered version was really interesting! 😊😊😊

Slept very well and have no idea what you were saying. Yay!

Thanks for the update and extended version of my favorite episode! 💫 ✨It's the perfect topic to drift into sleep on.
😴 ✨ 💫

Phenomenal!! Thank you 🙏🏼 Ben!! Loved it being long!! Kept me sleeping 😴