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The telescope photograph of the Great Andromeda Nebula is taken around 1899. Credit: Isaac Roberts.

Currently, the Universe remains relatively unexplored at submillimetre wavelengths, for example, so astronomers expect to uncover many new secrets about star formation, as well as the origins of galaxies.


Astronomy is the study of stars and other celestial bodies. It has been practiced since ancient times.


The Hubble Space Telescope image shows four high-velocity, runaway stars plowing through their local interstellar medium. Credit: NASA - Hubble's Advanced Camera for Surveys.

A star is a massive, luminous sphere of plasma held together by gravity. This is a traditional definition of a star. The term "luminous" relates to light, specifically visible light, as it is perceived by the human eye.

From a dictionary:


1.a: "any natural luminous body visible in the sky [especially] at night",
1.b: "a self-luminous gaseous celestial body of great mass whose shape is [usually] spheroidal and whose size may be as small as the earth or larger than the earth's orbit".[1]

is called a star.

Def. "any object forming on a dynamical timescale, by gravitational instability", is called a star.[2]

Def. a star that exists alone, is secluded or isolated from other stars, a reclusive or hermitary star, is called a solitary star.

Def. a separate, distinct, or individual star from others in a group is called a single star.

A solitary star differs from a single star in that the former exists alone, secluded or isolated from other stars. For example, Psi2 Aquarii (93 Aquarii) is a solitary star. Radial velocity measurements have not yet revealed the presence of planets orbiting it.

18 Scorpii is another solitary star.

Def. a star that shares a barycenter with one or more astronomical substellar objects is called a unary star.

A unary star contrasts with a binary star, trinary (three stars), and a multiple star. It is not necessarily alone like the solitary star.

Def. a star moving faster than 65 km/s to 100 km/s relative to the average motion of the stars in the Sun's neighbourhood is called a high-velocity star.

Def. a high-velocity star moving through space with an abnormally high velocity relative to the surrounding interstellar medium is called a runaway star.

Def. a star whose elliptical orbit takes it well outside the plane of its galaxy at steep angles is called a halo star.

Star clusters[edit]

Messier 92 is a star cluster in the constellation Hercules. Credit: Daniel Bramich (ING) and Nik Szymanek.
This is a Hubble Space Telescope image of the spiral galaxy NGC 1672. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration.

Def. two stars that appear to be one when seen with the naked eye is called a double star.

Def. a star that appears as a double due to an optical illusion; in reality, the stars may be far apart from each other is called an optical double.

Def. two and only two stars orbiting around their apparent barycenter is called a binary star.

Def. a binary star whose components can be visually resolved is called a visual binary.

Def. a group of gravitationally bound stars is called a star cluster.

Def. a more or less irregular star cluster containing tens to thousands of stars is called an open cluster.

Def. a spherical star cluster containing thousands to millions of stars is called a globular cluster.

Def. a large group of many stars spread over a very many light-years of space a region of greater than average stellar density is called a star cloud.

Def. a group of stars moving together through space is called a stellar association.

Def. an association of stars stretched out along its orbit of a galaxy is called a stellar stream.

Def. a stellar association drifting through the galaxy as a somewhat coherent assemblage is called a moving group.

Def. any of the collections of many millions of stars existing as independent and coherent systems is called a galaxy.

Def. any galaxy, considerably smaller than the Milky Way, that has only several billions of stars is called a dwarf galaxy.

Def. a small group of stars that forms a visible pattern but is not an official constellation is called an asterism.

Def. any of the 89 officially recognized regions of the sky, including all stars is called a constellation.

Galactic astronomy[edit]

"The space distribution of stars and the chemical elements in the Milky Way Galaxy are discussed along with the large-scale structure and stellar content of galaxies, the solar motion, the stellar residual-velocity distribution, and the rotation of galaxies."[3]

Galactic sciences[edit]

The composite image shows a classification of galaxies. Credit: Ville Koistinen.

Probably the earliest classification of galaxies "is based on the forms of the photographic images."[4]

"About 3 per cent are irregular, but the remaining nebulae fall into a sequence of type forms characterized by rotational symmetry about dominating nuclei."[4]

"The sequence is composed of two sections, the elliptical nebulae and the spirals, which merge into each other."[4]

"The classification of these nebulae is based on structure, the individual members of a class differing only in apparent size and luminosity."[4]

The "forms divide themselves naturally into two groups:

  1. those [nebulae] found in or near the Milky Way and
  2. those in moderate or high galactic latitudes."[4]

For the elliptical nebulae [galaxies], the classification En, where "n=1, 2, .... , 7 indicates the ellipticity of the image without the decimal point".[4]

For example, NGC 3379 is E0, NGC 221 is E2, NGC 4621 is E5 and NGC 2117 is E7.[4]

The spirals are divided into two types:

  1. Normal spirals (S) of Early (Sa), Intermediate (Sb), and Late (Sc) and
  2. Barred spirals (SB) of Early (SBa), Intermediate (SBb), and Late (SBc).[4]

The irregular galaxies are put into that structure form with "Irr".[4]

Examples are

  1. Sa - NGC 4594,
  2. Sb - NGC 2841,
  3. Sc - NGC 5457,
  4. SBa - NGC 2859,
  5. SBb - NGC 3351,
  6. SBc - NGC 7479, and
  7. Irr - NGC 4449.[4]

Theoretical galactic astronomy[edit]

This image shows the spiral galaxy Messier 100. Credit: NASA, STScI.
The image is of a large barred spiral galaxy. Credit: Hubblesite.
The elliptical galaxy NGC 1316 has dust lanes and star clusters. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA).
A lenticular galaxy is NGC 5866. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA).

Def. a galaxy having a number of arms of younger stars that spiral out from the centre containing older ones is called a spiral galaxy.

Def. a spiral galaxy with a central bar-shaped structure composed of stars is called a barred spiral galaxy.

Def. a galaxy having a smooth, featureless light-profile is called an elliptical galaxy.

Def. a galaxy that like spiral galaxies has a flat disk but unlike them has lost most of its interstellar matter and therefore has no spirals; considered a transitional form between spirals and elliptical galaxies is called a lenticular galaxy.

Def. a galaxy which is has no spirals and is not elliptical is called an irregular galaxy.

Def. a faint galaxy, devoid of gas, having a higher than normal proportion of dark matter; especially those that orbit the Milky Way and Andromeda is called dwarf spheroidal galaxy.


"Most of the sources are resolved in [Hubble Space Telescope] HST F814W imaging so they are certainly galaxies and not M stars."[5]


This is a Hubble space telescope image of Mayall's Object. Credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University).

"We know that within 1.5 billion years after the Big Bang, some galaxies had formed; this is evidenced by galaxies observed to z ∼6. Even at this epoch most of the intergalactic medium was ionized as evidenced by the lack of continuum absorption redward of Lyman α (the Gunn-Peterson effect, Gunn & Peterson 1965). It must be concluded that some objects must have existed earlier that produced sufficient UV flux to ionise nearly all the baryonic matter in the Universe."[6]

Mayall's Object (also classified under the Atlas of Peculiar Galaxies as Arp 148) is the result of two colliding galaxies located 500 million light years away within the constellation of Ursa Major. When first discovered, Mayall's Object was described as a peculiar nebula, shaped like a question mark. Originally theorized to represent a galaxy reacting with the intergalactic medium,[7] it is now thought to represent the collision of two galaxies, resulting in a new object consisting of a ring-shaped galaxy with a tail emerging from it. It is thought that the original collision between the two original galaxies created a shockwave that initially drew matter into the center which then formed the ring.[8]

"The nature of extremely red objects (EROs) remains an open question in understanding the faint galaxy population at z > 1."[5]


"The other 3 red galaxies are devoid of strong emission lines, but they do show continuum breaks identifiable as the rest-frame mid-UV breaks at 2640 Å and 2900 Å (Figure 2)."[5]


Balmer lines can appear as absorption or emission lines in a spectrum, depending on the nature of the object observed. In the spectra of most spiral and irregular galaxies, AGNs, H II regions and planetary nebulae, the Balmer lines are emission lines.

"[T]he extended red emission (ERE) [is] observed in many dusty astronomical environments, in particular, the diffuse interstellar medium of the Galaxy. ... silicon nanoparticles provide the best match to the spectrum and the efficiency requirement of the ERE."[9]


This graph shows the power density spectrum of the extragalactic background. Credit:
This ROSAT image is an Aitoff-Hammer equal-area map in galactic coordinates with the Galactic center in the middle of the 0.25 keV diffuse X-ray background. Credit: The Max Planck Institute for Extraterrestrial Physics, Snowden et al. 1995, ApJ, 454, 643; Imagine the Universe! is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.
Map of the column density of Galactic neutral hydrogen in the same projection as the 0.25 keV SXRB. Note the general negative correlation between the 0.25 keV diffuse X-ray background and the neutral hydrogen column density shown here. Credit: Dickey and Lockman 1990, ARAA, 28, 215; The Imagine Team, NASA.
This 0.75 keV diffuse X-ray background map from the ROSAT all-sky survey in the same projection as the SXRB and neutral hydrogen. The image shows a radically different structure than the 0.25 keV X-ray background. At 0.75 keV, the sky is dominated by the relatively smooth extragalactic background and a limited number of bright extended Galactic objects. Credit: The Max Planck Institute for Extraterrestrial Physics, Snowden et al. 1995, ApJ, 454, 643; Imagine the Universe! is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

The diffuse infrared background has been surveyed by the Diffuse Infrared Background Experiment (DIRBE) aboard NASA's Cosmic Background Explorer (COBE).[10] The DIRBE instrument was an absolute radiometer with an off-axis folded-Gregorian reflecting telescope, with 19 cm diameter aperture.[10] Brightness maps of the universe at ten frequency bands ranging from the near to far infrared (1.25 to 240 micrometer) have been obtained.[10] Also, linear polarization was measured at 1.25, 2.2, and 3.5 micrometers.[10] During the mission, the instrument could sample half the celestial sphere each day.[10]

DIRBE also detected 10 new far-IR emitting galaxies in the region not surveyed by IRAS as well as nine other candidates in the weak far-IR that may be spiral galaxies. Galaxies that were detected at the 140 and 240 μm were also able to provide information on very cold dust (VCD). At these wavelengths, the mass and temperature of VCD can be derived.

When these data were joined with 60 and 100 μm data taken from IRAS, it was found that the far-infrared luminosity arises from cold (≈17–22 K) dust associated with diffuse HI cirrus clouds, 15-30% from cold (≈19 K) dust associated with molecular gas, and less than 10% from warm (≈29 K) dust in the extended low-density HII regions.[11]

The diffuse extragalactic background radiation (DEBRA) refers to the diffuse photon field from extragalactic origin that fill our Universe. It contains photons over ∼ 20 decades of energy from ~10−7 eV to ~100 GeV. The origin and the physical processes involved are different within every wavelength range. There are plenty of observational evidences that support the existence of the DEBRA.[12]

The figure at right shows a schematic picture, based on many different data sets, of the spectral intensity (also called spectral radiance) multiplied by wavelength of the DEBRA over all the electromagnetic spectrum. This representation is convenient because the area inside the curve is the energy. The nature and history of the universe is coded in this radiation field and any realistic cosmological model must be able to describe it. Understanding the DEBRA is a major challenge of modern cosmology with huge consequences in other fields of astrophysics, therefore extraordinary efforts are being put by theoreticians, observers, and instrumentalists to do so.

The diffuse extragalactic background light (EBL) is all the accumulated radiation in the Universe due to star formation processes, plus a contribution from active galactic nuclei (AGNs). This radiation covers the wavelength range between ~ 0.1-1000 microns (these are the ultraviolet, optical, and infrared regions of the electromagnetic spectrum). The EBL is part of the diffuse extragalactic background radiation (DEBRA), which by definition covers the overall electromagnetic spectrum. After the cosmic microwave background, the EBL produces the second-most energetic diffuse background, thus being essential for understanding the full energy balance of the universe.

The understanding of the EBL is also fundamental for extragalactic very-high-energy (VHE, 30 GeV-30 TeV) astronomy.[13] VHE photons coming from cosmological distances are attenuated by pair production with EBL photons. This interaction is dependent on the spectral energy distribution of the EBL. Therefore, it is necessary to know the SED of the EBL in order to study intrinsic properties of the emission in the VHE sources.

There are empirical approaches that predict the overall SED of the EBL in the local Universe as well as its evolution over time. These types of modeling can be divided in four different categories according to.[14]

(i) Forward evolution, which begins with cosmological initial conditions and follows a forward evolution with time by means of semi-analytical models of galaxy formation.[15][16][17]

(ii) Backward evolution, which begins with existing galaxy populations and extrapolates them backwards in time.[18][19][20]

(iii) Evolution of the galaxy populations that is inferred over a range of redshifts. The galaxy evolution is inferred here using some quantity derived from observations such as the star formation rate density of the universe.[21][22][23]

(iv) Evolution of the galaxy populations that is directly observed over the range of redshifts that contribute significantly to the EBL.[24]

In addition to discrete sources which stand out against the sky, there is good evidence for a diffuse X-ray background.[25] During more than a decade of observations of X-ray emission from the Sun, evidence of the existence of an isotropic X-ray background flux was obtained in 1956.[26] This background flux is rather consistently observed over a wide range of energies.[25] The early high-energy end of the spectrum for this diffuse X-ray background was obtained by instruments on board Ranger 3 and Ranger 5.[25] The X-ray flux corresponds to a total energy density of about 5 x 10−4 eV/cm3.[25] The ROSAT soft X-ray diffuse background (SXRB) image shows the general increase in intensity from the Galactic plane to the poles. At the lowest energies, 0.1 - 0.3 keV, nearly all of the observed soft X-ray background (SXRB) is thermal emission from ~106 K plasma.

By comparing the soft X-ray background with the distribution of neutral hydrogen, it is generally agreed that within the Milky Way disk, super soft X-rays are absorbed by this neutral hydrogen.

"[T]he instrumental response, sky background, and seeing conspire to provide the best images. All the available bands are used to define colors with which we separate cluster members from the background in order to minimize dilution of the weak-lensing signal by unlensed objects."[27]

"We select red galaxies with colors redder than the color-magnitude sequence of cluster E/SO galaxies. The sequence forms a well-defined line due to the richness and relatively low redshifts of our clusters. These red galaxies are expected to lie in the background by virtue of k-corrections which are greater than for the red cluster sequence galaxies".[27]

"Typically the proportion of blue galaxies used is around 50% of the red background."[27]

"[T]he gravitational shear field [is derived] by locally averaging the corrected distortions of color-selected background galaxies of each cluster. ... Maps of the surface number-density distribution of color-selected cluster member galaxies [have been produced], with the gravitational shear of background galaxies overlaid ... Profiles of background red galaxy counts, whose intrinsic slope is relatively shallow [have been observed] ... the utility of the background red galaxies for measuring magnification [has been established]."[27]

"The distortion profiles measured are among the most accurate constructed to date and the great depth of the Subaru imaging permits magnification profiles to be established with unprecedented detail using the background red galaxy counts."[27]


The secondary antiprotons (p) then propagate through the galaxy, confined by the galactic magnetic fields. Their energy spectrum is modified by collisions with other atoms in the interstellar medium. The antiproton cosmic ray energy spectrum is now measured reliably and is consistent with this standard picture of antiproton production by cosmic ray collisions.[28]


As of December 5, 2011, "Voyager 1 is about ... 18 billion kilometers ... from the [S]un [but] the direction of the magnetic field lines has not changed, indicating Voyager is still within the heliosphere ... the outward speed of the solar wind had diminished to zero in April 2010 ... inward pressure from interstellar space is compacting [the magnetic field] ... Voyager has detected a 100-fold increase in the intensity of high-energy electrons from elsewhere in the galaxy diffusing into our solar system from outside ... [while] the [solar] wind even blows back at us."[29]


This image captures the core of Messier 31 (M31) in X-rays using the Chandra X-ray Observatory. Credit: S. Murray, M. Garcia, et al., Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA) NASA.


This beautiful galaxy is tilted at an oblique angle on to our line of sight, giving a "birds-eye view" of the spiral structure. Credit: Hubble data: NASA, ESA, and A. Zezas (Harvard-Smithsonian Center for Astrophysics); GALEX data: NASA, JPL-Caltech, GALEX Team, J. Huchra et al. (Harvard-Smithsonian Center for Astrophysics); Spitzer data: NASA/JPL/Caltech/S. Willner (Harvard-Smithsonian Center for Astrophysics.


Still much further away from the Earth than the Sun or Neptune are the many stars and nebulae that make up the Milky Way. Beyond the confines of our galaxy is the Andromeda Galaxy shown at the top of the page.

Of the Local Group, “[i]ts two dominant galaxies, the Milky Way and Andromeda (M31), are separated by a distance of ~700 kpc and are moving toward each other with a radial velocity of about -117 km s-1 (Binney & Tremaine 1987, p. 605).”[30] making Andromeda one of the few blueshifted galaxies. The Andromeda Galaxy and the Milky Way are thus expected to collide in about 4.5 billion years, although the details are uncertain since Andromeda's tangential velocity with respect to the Milky Way is only known to within about a factor of two.[31] A likely outcome of the collision is that the galaxies will merge to form a giant elliptical galaxy.[32] Such events are frequent among the galaxies in galaxy groups. The fate of the Earth and the Solar System in the event of a collision are currently unknown. If the galaxies do not merge, there is a small chance that the Solar System could be ejected from the Milky Way or join Andromeda.[33]


This image shows a cluster of yellow galaxies near the middle of the photograph. Credit: W.N. Colley and E. Turner (Princeton University), J.A. Tyson (Bell Labs, Lucent Technologies) and STScl/NASA.

"This Hubble Space Telescope image [at right] shows several blue, loop-shaped objects that actually are multiple images of the same galaxy. They have been duplicated by the gravitational lens of the cluster of yellow, elliptical and spiral galaxies - called 0024+1654 - near the photograph's center. The gravitational lens is produced by the cluster's tremendous gravitational field that bends light to magnify, brighten and distort the image of a more distant object. How distorted the image becomes and how many copies are made depends on the alignment between the foreground cluster and the more distant galaxy, which is behind the cluster."[34]

"In this photograph, light from the distant galaxy bends as it passes through the cluster, dividing the galaxy into five separate images. One image is near the center of the photograph; the others are at 6, 7, 8, and 2 o'clock. The light also has distorted the galaxy's image from a normal spiral shape into a more arc-shaped object. Astronomers are certain the blue-shaped objects are copies of the same galaxy because the shapes are similar. The cluster is 5 billion light-years away in the constellation Pisces, and the blue-shaped galaxy is about 2 times farther away."[34]

"Though the gravitational light-bending process is not new, Hubble's high resolution image reveals structures within the blue-shaped galaxy that astronomers have never seen before. Some of the structures are as small as 300 light-years across. The bits of white imbedded in the blue galaxy represent young stars; the dark core inside the ring is dust, the material used to make stars. This information, together with the blue color and unusual "lumpy" appearance, suggests a young, star-making galaxy."[34]

"The picture was taken October 14, 1994 with the Wide Field Planetary Camera-2. Separate exposures in blue and red wavelengths were taken to construct this color picture."[34]


Huge, cold clouds of gas and dust in our own galaxy, as well as in nearby galaxies, glow in far-infrared light. This is due to thermal radiation of interstellar dust contained in molecular clouds.

"One of the most interesting discoveries made by IRAS was of galaxies with far-infrared luminosities of 1011 - 1012 L, and LIR/LB ~ 10 - 100 (Soifer et al. 1984). [...] Only a few examples of this type of object, namely Arp 220, NGC 3690, Mrk 231, and NGC 6240, have ever been studied from the ground in detail."[35]


Terahertz radiation is emitted as part of the black body radiation from anything with temperatures greater than about 10 kelvin. While this thermal emission is very weak, observations at these frequencies are important for characterizing the cold 10-20K dust in the interstellar medium in the Milky Way galaxy, and in distant starburst galaxies. Telescopes operating in this band include the James Clerk Maxwell Telescope, the Caltech Submillimeter Observatory and the Submillimeter Array at the Mauna Kea Observatory in Hawaii, the BLAST balloon borne telescope, the Herschel Space Observatory, and the Heinrich Hertz Submillimeter Telescope at the Mount Graham International Observatory in Arizona. The Atacama Large Millimeter Array, under construction, will operate in the submillimeter range. The opacity of the Earth's atmosphere to submillimeter radiation restricts these observatories to very high altitude sites, or to space.


"The [microwave] detection of interstellar formaldehyde provides important information about the chemical physics of our galaxy. We now know that polyatomic molecules containing at least two atoms other than hydrogen can form in the interstellar medium."[36]


The pseudo-colour image is of the large-scale radio structure of the FRII radio galaxy 3C98. Lobes, jet and hotspot are labelled. Credit: Mhardcastle.
Another pseudo-colour image is of the large-scale radio structure of the FRI radio galaxy 3C31. Jets and plumes are labelled. Credit: Mhardcastle.

"Over the past 30 years, radioastronomy has revealed a rich variety of molecular species in the interstellar medium of our galaxy and even others."[37]

These regions are non-luminous, save for emission of the 21-cm (1,420 MHz) region spectral line. ... Mapping H I emissions with a radio telescope is a technique used for determining the structure of spiral galaxies.

In 1974, radio sources were divided into two classes Fanaroff and Riley Class I (FRI), and Class II (FRII).[38]

The distinction was originally made based on the morphology of the large-scale radio emission (the type was determined by the distance between the brightest points in the radio emission): FRI sources were brightest towards the centre, while FRII sources were brightest at the edges.

There is a reasonably sharp divide in luminosity between the two classes: FRIs were low-luminosity, FRIIs were high luminosity.[38]

The morphology turns out to reflect the method of energy transport in the radio source. FRI objects typically have bright jets in the centre, while FRIIs have faint jets but bright hotspots at the ends of the lobes. FRIIs appear to be able to transport energy efficiently to the ends of the lobes, while FRI beams are inefficient in the sense that they radiate a significant amount of their energy away as they travel.

The FRI/FRII division depends on host-galaxy environment in the sense that the FRI/FRII transition appears at higher luminosities in more massive galaxies.[39] FRI jets are known to be decelerating in the regions in which their radio emission is brightest,[40]

The hotspots that are usually seen in FRII sources are interpreted as being the visible manifestations of shocks formed when the fast, and therefore supersonic, jet (the speed of sound cannot exceed c/√3) abruptly terminates at the end of the source, and their spectral energy distributions are consistent with this picture.[41]


Centaurus A in X-rays shows the relativistic jet. Credit: NASA.

"The structure of relativistic jets in [active galactic nuclei] AGN on scales of light days reveals how energy propagates through jets, a process that is fundamental to galaxy evolution."[42]

Their lengths can reach several thousand[43] or even hundreds of thousands of light years.[44] The hypothesis is that the twisting of magnetic fields in the accretion disk collimates the outflow along the rotation axis of the central object, so that when conditions are suitable, a jet will emerge from each face of the accretion disk. If the jet is oriented along the line of sight to Earth, relativistic beaming will change its apparent brightness. The mechanics behind both the creation of the jets[45][46] and the composition of the jets[47] are still a matter of much debate in the scientific community; it is hypothesized that the jets are composed of an electrically neutral mixture of electrons, positrons, and protons in some proportion.

A relativistic jet emitted from the AGN of M87 is traveling at speeds between four and six times the speed of light.[43]

"The term 'superluminal motion' is something of a misnomer. While it accurately describes the speeds measured, scientists still believe the actual speed falls just below the speed of light."[43]

"It's an illusion created by the finite speed of light and rapid motion".[43]

"Our present understanding is that this 'superluminal motion' occurs when these clouds move towards Earth at speeds very close to that of light, in this case, more than 98 percent of the speed of light. At these speeds the clouds nearly keep pace with the light they emit as they move towards Earth, so when the light finally reaches us, the motion appears much more rapid than the speed of light. Since the moving clouds travel slightly slower than the speed of light, they do not actually violate Einstein's theory of relativity which sets light as the speed limit."[43]

Gaseous objects[edit]

GMCs are so large that "local" ones can cover a significant fraction of a constellation; thus they are often referred to by the name of that constellation, e.g. the Orion Molecular Cloud (OMC) or the Taurus Molecular Cloud (TMC). These local GMCs are arrayed in a ring in the neighborhood of the Sun coinciding with the Gould Belt.[48] The most massive collection of molecular clouds in the galaxy forms an asymmetrical ring around the galactic center at a radius of 120 parsecs; the largest component of this ring is the Sagittarius B2 complex. The Sagittarius region is chemically rich and is often used as an exemplar by astronomers searching for new molecules in interstellar space.[49]

Interstellar medium[edit]

The interstellar medium is the matter that exists in the space between the star systems in a galaxy.

Milky Way[edit]

Milky Way is viewed by H-Alpha Sky Survey. Credit: David Brown and Douglas Finkbeiner.
This is a composite image of the central region of our Milky Way galaxy. Credit: NASA/JPL-Caltech/ESA/CXC/STScI.
This is a Chandra X-ray Observatory image of the Galactic Central region. Credit: NASA/CXC.
This is a 400 by 900 light-year mosaic of several Chandra X-ray Observatory images of the Galactic center region. Credit: NASA/UMass/D. Wang et al.
This is a visible image of the Galactic Central region from the Hubble Space Telescope. Credit: NASA/ESA/STScI.
This is an infrared image of the Galactic Central region using the Spitzer Space Telescope. Credit: NASA/JPL-Caltech.
This is a radio image of the central region of the Milky Way galaxy. Credit: NRL/SBC Galactic Center Radio Group.

"The Milky Way fills the background of the image [at upper right] with countless yellowish older stars. Some of them appear fainter and redder because of the dust in NGC 6559."[50]

At the lower right is a composite image. "In celebration of the International Year of Astronomy 2009, NASA's Great Observatories -- the Hubble Space Telescope, the Spitzer Space Telescope, and the Chandra X-ray Observatory -- have produced a matched trio of images of the central region of our Milky Way galaxy. Each image shows the telescope's different wavelength view of the galactic center region, illustrating the unique science each observatory conducts."[51]

"In this spectacular image, observations using infrared light and X-ray light see through the obscuring dust and reveal the intense activity near the galactic core. Note that the center of the galaxy is located within the bright white region to the right of and just below the middle of the image. The entire image width covers about one-half a degree, about the same angular width as the full moon."[51]

"Although best known for its visible-light images, Hubble also observes over a limited range of infrared light [Figure 2 (middle frame of poster) at third lower right]. The galactic center is marked by the bright patch in the lower right. Along the left side are large arcs of warm gas that have been heated by clusters of bright massive stars. In addition, Hubble uncovered many more massive stars across the region. Winds and radiation from these stars create the complex structures seen in the gas throughout the image.This sweeping panorama is one of the sharpest infrared pictures ever made of the galactic center region."[51]

"Spitzer's infrared-light observations provide a detailed and spectacular view of the galactic center region [Figure 1 (top frame of poster) lowest left]. The swirling core of our galaxy harbors hundreds of thousands of stars that cannot be seen in visible light. These stars heat the nearby gas and dust. These dusty clouds glow in infrared light and reveal their often dramatic shapes. Some of these clouds harbor stellar nurseries that are forming new generations of stars. Like the downtown of a large city, the center of our galaxy is a crowded, active, and vibrant place."[51]

At lowest right is a radio image of the central region of the Milky Way galaxy. The arrow indicates a supernova remnant which is the location of a newly-discovered transient, the bursting low-frequency radio source GCRT J1745-3009.

"As the sun moves in its path through the galaxy, it will not always be immersed in the tenuous intercloud region of the interstellar medium."[52]

At top left is an image of the Galactic central region using the Chandra X-ray Observatory.

"X-rays detected by Chandra expose a wealth of exotic objects and high-energy features [Figure 3 (bottom frame of poster)]. In this image, pink represents lower energy X-rays and blue indicates higher energy. Hundreds of small dots show emission from material around black holes and other dense stellar objects. A supermassive black hole -- some four million times more massive than the Sun -- resides within the bright region in the lower right. The diffuse X-ray light comes from gas heated to millions of degrees by outflows from the supermassive black hole, winds from giant stars, and stellar explosions. This central region is the most energetic place in our galaxy."[51]

The second image at left is a "400 by 900 light-year mosaic of several Chandra images of the central region of our Milky Way galaxy ... [It] reveals hundreds of white dwarf stars, neutron stars, and black holes bathed in an incandescent fog of multimillion-degree gas. The supermassive black hole at the center of the galaxy is located inside the bright white patch in the center of the image. The colors indicate X-ray energy bands - red (low), green (medium), and blue (high)."[53]

Andromeda galaxy[edit]

This is an X-ray image of the Andromeda galaxy. Credit: ESA/XMM-Newton/EPIC/W. Pietsch.
Here the center of M31 is imaged by the Chandra X-ray Observatory. Credit: S. Murray, M. Garcia, et al., Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA) NASA Technical Rep.: Jay Norris.

The Andromeda Galaxy is also known as M31, NGC 224, UGC 454, PGC 2557, 2C 56 (Core),[1] LEDA 2557

Local Voids[edit]

This is a Hubble Space Telescope image of NGC 6503, which sits at the edge of a giant, hollowed-out region of space called the Local Void. Credit: ESA/Hubble and NASA.

"NGC 6503 sits at the edge of a giant, hollowed-out region of space called the Local Void. The Hercules and Coma galaxy clusters, as well as our own Local Group of galaxies, circumscribe this vast, sparsely populated region. Estimates for the void’s diameter vary from 30 million to more than 150 million light-years — so NGC 6503 does not have a lot of galactic company in its immediate vicinity."[54]

The Local Void is a vast, empty region of space, lying adjacent to our own Local Group.[55][56]

The Local Void is now known to be composed of three separate sectors, separated by bridges of "wispy filaments".[56] The precise extent of the void is unknown, but it is at least 150 million light years across[57] and may have a long dimension of up to 70 Mpc (230 million light years).[56] The Local Void also appears to have significantly fewer galaxies than expected from standard cosmology.[58]

The Milky Way sits in a large, flat array of galaxies called the Local Sheet, which bounds the Local Void.[55] The Local Void extends approximately 60 megaparsecs, beginning at the edge of the Local Group.[59] It is believed that the distance from Earth to the centre of the Local Void must be at least 23 megaparsecs (75 Mly).[56]

The size of the Void was calculated due to an isolated dwarf galaxy located inside it. The Void may be growing and the Local Sheet, which makes up one wall of the void, is rushing away from the void's centre at 260 kilometres per second.[60]

"Fresh starbirth infuses the galaxy NGC 6503 [at right] with a vital pink glow in this image from the NASA/ESA Hubble Space Telescope. This galaxy, a smaller version of the Milky Way, is perched near a great void in space where few other galaxies reside."[54]

The Local Void is surrounded uniformly by matter in all directions, except for one sector in which there is nothing.

The Milky Way's velocity away from the Local Void is 270 kilometres per second (600,000 mph).[55][57]

"This new image [at right] from Hubble’s Advanced Camera for Surveys displays, with particular clarity, the pink-coloured puffs marking where stars have recently formed in NGC 6503's swirling spiral arms. Although structurally similar to the Milky Way, the disc of NGC 6503 spans just 30 000 light-years, or just about a third of the size of the Milky Way, leading astronomers to classify NGC 6503 as a dwarf spiral galaxy."[54]

"NGC 6503 lies approximately 17 million light-years away in the constellation of Draco (the Dragon)."[54]

"This Hubble image was created from exposures taken with the Wide Field Channel of the Advanced Camera for Surveys. The filters were unusual, which explains the peculiar colour balance of this picture. The red colouration derives from a 28-minute exposure through a filter that just allows the emission from hydrogen gas ([H-alpha,] F658N [, 658 nm]) to pass and which reveals the glowing clouds of gas associated with star-forming regions. This was combined with a 12-minute exposure through a near-infrared filter (F814W) [814 nm], which was coloured blue for contrast. The field of view is 3.3 by 1.8 arcminutes."[54] A combination of H-alpha and infrared is also used and is green in color.[54]

Intergalactic medium[edit]

The intergalactic medium (IGM) is "a rarefied plasma[61] that is organized in a cosmic filamentary structure.[62]

Def. occurring between galaxies is called intergalactic.

Def. originating

  1. outside the Milky Way galaxy or
  2. outside of a galaxy

is called extragalactic.

Globular clusters[edit]

The NASA/ESA Hubble Space Telescope has produced this beautiful image of the globular cluster Messier 56 (also known as M 56 or NGC 6779). Credit: NASA & ESA. Acknowledgement: Gilles Chapdelaine.
This NASA/ESA Hubble Space Telescope image shows a compact and distant globular star cluster that lies in one of the smallest constellations in the night sky, Delphinus (The Dolphin). Credit: ESA/Hubble & NASA.

"The NASA/ESA Hubble Space Telescope has produced this beautiful image [at right] of the globular cluster Messier 56 (also known as M 56 or NGC 6779), which is located about 33 000 light years away from the Earth in the constellation of Lyra (The Lyre). The cluster is composed of a large number of stars, tightly bound to each other by gravity."[63]

"Astronomers typically infer important properties of globular clusters by looking at the light of their constituent stars. But they have to be very careful when they observe objects like Messier 56, which is located close to the Galactic plane. This region is crowded by “field-stars”, in other words, stars in the Milky Way that happen to lie in the same direction but do not belong to the cluster. These objects can contaminate the light, and hence undermine the conclusions reached by astronomers."[63] Bold added.

"A tool often used by scientists for studying stellar clusters is the colour-magnitude (or Hertzsprung-Russell) diagram. This chart compares the brightness and colour of stars – which in turn, tells scientists what the surface temperature of a star is."[63]

"By comparing high quality observations taken with the Hubble Space Telescope with results from the standard theory of stellar evolution, astronomers can characterise the properties of a cluster. In the case of Messier 56, this includes its age, which at 13 billion years is approximately three times the age of the Sun. Furthermore, they have also been able to study the chemical composition of Messier 56. The cluster has relatively few elements heavier than hydrogen and helium, typically a sign of stars that were born early in the Universe’s history, before many of the elements in existence today were formed in significant quantities."[63]

"Astronomers have found that the majority of clusters with this type of chemical makeup lie along a plane in the Milky Way’s halo. This suggests that such clusters were captured from a satellite galaxy, rather than being the oldest members of the Milky Way's globular cluster system as had been previously thought."[63]

"This image consists of visible [blue, centered at 606 nm] and near-infrared [red, centered at 814 nm] exposures from Hubble’s Advanced Camera for Surveys. The field of view is approximately 3.3 by 3.3 arcminutes."[63]

At lower right the "NASA/ESA Hubble Space Telescope image shows a compact and distant globular star cluster that lies in one of the smallest constellations in the night sky, Delphinus (The Dolphin). Due to its modest size, great distance and relatively low brightness, NGC 7006 is often ignored by amateur astronomers. But even remote globular clusters such as this one appear bright and clear when imaged by Hubble’s Advanced Camera for Surveys."[64] The visual portion is centered at 606 nm (blue), a visual + infrared is green, and the infrared is centered at 814 nm (red).[64]

"NGC 7006 resides in the outskirts of the Milky Way. It is about 135 000 light-years away, five times the distance between the Sun and the centre of the galaxy, and it is part of the galactic halo. This roughly spherical region of the Milky Way is made up of dark matter, gas and sparsely distributed stellar clusters."[64] Bold added.

"Like other remote globular clusters, NGC 7006 provides important clues that help astronomers to understand how stars formed and assembled in the halo. The cluster now pictured by Hubble has a very eccentric orbit indicating that it may have formed independently, in a small galaxy outside our own that was then captured by the Milky Way."[64]

"Although NGC 7006 is very distant for a Milky Way globular cluster, it is much closer than the many faint galaxies that can be seen in the background of this image. Each of these faint smudges is probably accompanied by many globular clusters similar to NGC 7006 that are too faint to be seen even by Hubble."[64]

"This image was taken using the Wide Field Channel of the Advanced Camera for Surveys, in a combination of visible and near-infrared light. The field of view is a little over 3 by 3 arcminutes."[64]

Starburst galaxy[edit]

This mosaic image taken by the Hubble Space Telescope of Messier 82 combines exposures taken with four colored filters that capture starlight from visible and infrared wavelengths as well as the light from the glowing hydrogen filaments. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA).

"The presence of ERE has been established spectroscopically in ... the starburst galaxy M82 (Perrin, Darbon, & Sivan 1995)."[9]

"This mosaic image [at right] is the sharpest wide-angle view ever obtained of M82. The galaxy is remarkable for its bright blue disk, webs of shredded clouds, and fiery-looking plumes of glowing hydrogen blasting out of its central regions."[65]

"Throughout the galaxy's center, young stars are being born 10 times faster than they are inside our entire Milky Way Galaxy. The resulting huge concentration of young stars carved into the gas and dust at the galaxy's center. The fierce galactic superwind generated from these stars compresses enough gas to make millions of more stars."[65]

"In M82, young stars are crammed into tiny but massive star clusters. These, in turn, congregate by the dozens to make the bright patches, or "starburst clumps," in the central parts of M82. The clusters in the clumps can only be distinguished in the sharp Hubble images. Most of the pale, white objects sprinkled around the body of M82 that look like fuzzy stars are actually individual star clusters about 20 light-years across and contain up to a million stars."[65]

"The rapid rate of star formation in this galaxy eventually will be self-limiting. When star formation becomes too vigorous, it will consume or destroy the material needed to make more stars. The starburst then will subside, probably in a few tens of millions of years."[65]

"Located 12 million light-years away, M82 appears high in the northern spring sky in the direction of the constellation Ursa Major, the Great Bear. It is also called the "Cigar Galaxy" because of the elliptical shape produced by the oblique tilt of its starry disk relative to our line of sight."[65]

"The observation was made in March 2006, with the Advanced Camera for Surveys' Wide Field Channel. Astronomers assembled this six-image composite mosaic by combining exposures taken with four colored filters that capture starlight from visible and infrared wavelengths as well as the light from the glowing hydrogen filaments."[65]

Messier 100[edit]

This image of Messier 100 is from the NASA/ESA Hubble Space Telescope. Credit: ESA/Hubble & NASA.
This is a colour-composite image of the central 5,500 light-years wide region of the spiral galaxy NGC 1097, obtained with the NACO adaptive optics on the VLT. Credit: European Southern Observatory.

"This [visual] image [at right] from the NASA/ESA Hubble Space Telescope, the most detailed made to date, shows the bright core of the galaxy and the innermost parts of its spiral arms. Messier 100 has an active galactic nucleus — a bright region at the galaxy’s core caused by a supermassive black hole that is actively swallowing material, which radiates brightly as it falls inwards."[66] Bold added.

"Messier 100 is a perfect example of a grand design spiral galaxy, a type of galaxy with prominent and very well-defined spiral arms. These dusty structures swirl around the galaxy’s nucleus, and are marked by a flurry of star formation activity that dots Messier 100 with bright blue, high-mass stars."[66]

"The galaxy’s spiral arms also host smaller black holes, including the youngest ever observed in our cosmic neighbourhood, the result of a supernova observed in 1979."[66]

"Messier 100 is located in the direction of the constellation of Coma Berenices, about 50 million light-years distant."[66]

"This image, taken with the high resolution channel of Hubble’s Advanced Camera for Surveys demonstrates the continued evolution of Hubble’s capabilities over two decades in orbit. This image, like all high resolution channel images, has a relatively small field of view: only around 25 by 25 arcseconds."[66]

The visual data is centered at 555 nm (blue), the visual + infrared is in green, and additional infrared centered at 814 nm is red.[66]

At lower right is a "[c]olour-composite image of the central 5,500 light-years wide region of the spiral galaxy NGC 1097 [45 million light years away], obtained with the NACO adaptive optics on the VLT. More than 300 star forming regions - white spots in the image - are distributed along a ring of dust and gas in the image. At the centre of the ring there is a bright central source where the active galactic nucleus and its super-massive black hole are located. The image was constructed by stacking J- (blue), H- (green), and Ks-band (red) [infrared] images. North is up and East is to the left. The field of view is 24 x 29 arcsec2, i.e. less than 0.03% the size of the full moon!"[67]

NGC 5728[edit]

This is a visual astronomy image of NGC 5728 from the Hubble Space Telescope. Credit: Fabian RRRR.
This is a Hubble Space Telescope image of the green continuum in the nuclear region of NGC 5728. The observation was made on September 4, 1992, using the Planetary Camera with appropriate filters. Credit: A. S. Wilson, J. A. Braatz, T. M. Heckman, J. H. Krolik, & G. K. Miley/NASA-Hubble.
This is a Hubble Space Telescope image of the green [O III] emission line in the active galactic nuclear region of NGC 5728. The observation was made on September 4, 1992, using the Planetary Camera with appropriate filters. Credit: A. S. Wilson, J. A. Braatz, T. M. Heckman, J. H. Krolik, & G. K. Miley/NASA-Hubble.

"[H]igh-resolution (0.1") observations of the Seyfert 2 galaxy NGC 5728 with the Hubble Space Telescope",[68] in the images at right, show the full color of the galactic nucleus and a green continuum image of the active galactic nucleus.

At left is a Hubble Space Telescope image in the light of the green [O III] emission line in the active galactic nuclear region of NGC 5728. "The emission-line images reveal a spectacular biconical structure with overall extent [of] 1.8 kpc. The two cones share a common axis and apex. The cones' axis coincides to within ≃ 3° with that of the one-sided nuclear radio continuum emission but is not aligned with the rotation axis of the galaxy disk."[68]

These "ionization cones" are "conical regions of high-excitation emission-line gas extending from an active nucleus. ... The generally accepted interpretation is that partially collimated ionizing radiation shines out from the nucleus and ionizes gas in its vicinity. ... two exposures were taken through the F492M filter (4906 Å/364 Å) to cover the [O III] λλ4959, 5007 emission. ... The high-excitation gas can be traced to ≃ 8.5" (1.6 kpc) from the apex in the SE cone, but only to 1.5" (270 pc) in the NW one."[68]

Arp 220[edit]

This is a Hubble Space Telescope image of Arp 220. Credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University).

"Arp 220 [at right] appears to be a single, odd-looking galaxy, but is in fact a nearby example of the aftermath of a collision between two spiral galaxies. It is the brightest of the three galactic mergers closest to Earth, about 250 million light-years away in the constellation of Serpens, the Serpent. The collision, which began about 700 million years ago, has sparked a cracking burst of star formation, resulting in about 200 huge star clusters in a packed, dusty region about 5,000 light-years across (about 5 percent of the Milky Way's diameter). The amount of gas in this tiny region equals the amount of gas in the entire Milky Way Galaxy. The star clusters are the bluish-white bright knots visible in the Hubble image. Arp 220 glows brightest in infrared light and is an ultra-luminous infrared galaxy. Previous Hubble observations, taken in the infrared at a wavelength that looks through the dust, have uncovered the cores of the parent galaxies 1,200 light-years apart. Observations with NASA s Chandra X-ray Observatory have also revealed X-rays coming from both cores, indicating the presence of two supermassive black holes. Arp 220 is the 220th galaxy in Arp's Atlas of Peculiar Galaxies."[69] Bold added.

Compact galaxy groups[edit]

Compact groups of galaxies are tight associations of galaxies.[70] Their compactness suggests extremely short crossing times and a very rapid evolution.[70] Computer simulations suggest that a compact "group coalesces into a giant dominant galaxy in a small number of crossing times."[70] "Alternately, compact groups may be transient unbound cores of loose groups".[70] A third alternative is that they are mostly chance alignments within larger loose groups of galaxies.[70]

"In a physically dense group one would expect that the majority of the galaxies would exhibit visible signs of interaction."[70]

"At the same time, the dominant group members are as likely to be spirals as ellipticals, hence suggesting that systematic merging has not (yet) occurred".[70]

Galaxy clusters[edit]

IDCS J1426.5+3508 is a large galaxy cluster about 1010 light years from Earth. Credit: NASA/ESA/University of Florida, Gainsville/University of Missouri-Kansas City/UC Davis.
Comparison of the Chandra X-ray Observatory image of the X-ray emission from the intracluster medium in the core of the Abell 2199 galaxy cluster against the optical emission of the galaxies (from the Digitized Sky Survey (DSS). Credit: .

A "new galaxy cluster, IDCS J1426.5+3508 [has] hundreds to thousands of galaxies bonded by gravity [...] about 10 billion light-years from Earth".[71]

"The galaxy behind the cluster is a typical run-of-the-mill galaxy with a lot of young stars, but the galaxy cluster in front of it is a whopper for that range. However, it's really the way that the two systems are lined up that makes the occurrence truly remarkable."[72]

The intracluster medium (ICM) is the superheated plasmas present at the center of a galaxy cluster. This is gas heated to temperatures of between roughly 10 and 100 megakelvins and consisting mainly of ionized hydrogen and helium, containing most of the baryonic material in the cluster. The ICM strongly emits X-ray radiation.

Studying the composition of the ICM at varying redshift (which results in looking at different points back in time) can therefore give a record of element production in the universe if they are typical.[73]

Galaxy filaments[edit]


The universe within 1 billion light-years (307 Mpc) of Earth is shown to contain the local superclusters, galaxy filaments and voids. Credit: Richard Powell.

In astronomy, voids are the empty spaces between filaments (the largest-scale structures in the Universe), which contain very few, or no, galaxies. Voids located in high-density environments are smaller than voids situated in low-density spaces of the universe.[74]


This animation depicts the collision between our Milky Way galaxy and the Andromeda galaxy. Credit: Visualization Credit: NASA; ESA; and F. Summers, STScI; Simulation Credit: NASA; ESA; G. Besla, Columbia University; and R. van der Marel, STScI.

Astronomical radiation mathematics is the laboratory mathematics such as simulations that are generated to try to understand the observations of radiation astronomy.


  • Third Reference Catalogue of Bright Galaxies[75]


  1. The study and classification of galaxies has progressed little in two centuries.

"Keel (1980) used a sample of 91 field spirals as the control group in a study of the effect of inclination on the recognition of Seyfert nuclei. Of these 91 galaxies, approximately 50% had a/b [axial ratio, major over minor axis, at 23 R band mag.arcsec-2] ≥ 2.0."[35]

Six of thirty "edge-on spirals [...] with a [boxy/peanut shape] B/PS bulge [...] with bulges larger than 0.6' in diameter and disks smaller than about 7.0' (at the 25 B mag arcsec-2 level). [...] All objects are accessible from the south (δ ≤ 15°). [sample galaxies ...] have a more spheroidal bulge morphology [...] have emission lines extending far enough in the disk to apply the diagnostics developed by BA99 and AB99 with the ionised gas; all galaxies in the control group fulfill [these conditions]."[76]

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External links[edit]

{{Principles of radiation astronomy}}