Astronomy 2

8/3/2019 Astronomy 2

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The goals specific to Astronomy 162, Introduction to Stellar, Galactic and ExtragalacticAstronomy, are to explore the physical nature of the stars, galaxies and the universe as awhole, including their constituents, formation and evolution. Example questions we willanswer: Where do stars come from?, Are there really black holes and what are they?, Howdid galaxies get to be the way they are?, How did the universe form?. We will do many

demonstrations, hands-on activities, and computer based activities which will help youunderstand our universe and the physical laws that govern it. Hopefully, by the end of thecourse you will have a greater appreciation for our universe and our place in it.

Extragalactic Astronomy

a branch of astronomy studying celestial bodies and systems that lie beyond our stellar system, theMilky Way galaxy. The formation of this branch of astronomy was preceded by a long period for determining what types of celestial bodies make up our stellar system and what types are foundoutside it. At the end of the first quarter of the 20th century it was conclusively established that our stellar system has finite dimensions and at the same time does not exhaust the entire stellar universe.It was called the Galaxy (the Milky Way galaxy). Also established was the existence of other stellar systems which, because of their closed nature and independent position in space, were calledgalaxies. The totality of all galaxies, called the metagalaxy, is the most extensive system known toscience. The most distant of the brightest galaxies, whose distances it has been possible to establish,are located over 1 billion par-sees away from us. The exact value of this maximum distance cannot beindicated since, first, more and more remote objects become known almost annually and, second, theresult of computing distances based on quantities obtained directly from observations depends on theassumed proper-ties of space in the metagalaxy, which have not been sufficiently studied.Nevertheless, it may be asserted that the most distant of the known galaxies are not at the limits of the metagalaxy.

The results of investigations obtained by extragalactic astronomy are the main observational materialfor cosmology. In studying natural phenomena on a very large scale, extragalactic astronomyencounters new, previously unknown, phenomena and perhaps even new laws of nature. The resultsof extragalactic astronomy greatly assist the study of our galaxy. This is conditioned by the fact thatwe observe other galaxies from the outside and as a whole, but we must study our own galaxy fromwithin. This is more difficult for a number of reasons. The solar system is located within the dustyequatorial layer of our galaxy, which severely reduces our zone of visibility, especially in directionsclose to the plane of the galactic equator. Other galaxies are seen as a whole and from various pointsof view depending on their random orientation with respect to our line of sight. But be-cause of thegreat distances to the galaxies, the various-type stars that make up these galaxies can almost never be ob-served separately. However, data on the types of stars and their motions in our galaxycontribute to a better understanding of other stellar systems.

The distribution of galaxies in space is not uniform. Most of them are concentrated in compact or

scattered clusters of galaxies containing from dozens to tens of thousands of members. The rates of motion of galaxies in clusters, measured by spectrograms based on the Doppler effect, are random indirection and may be as high as 2,000 km/sec. In some cases these velocities are so great that theymay prove sufficient for galaxies to leave a cluster. The question as to what extent the distribution of galactic clusters in the metagalaxy may be considered uniform has not yet been answered. On theone hand, the majority of galaxies are concentrated in randomly scattered clusters and, on the other,no marked asymmetry in the distribution of clusters and no tight crowding are observed. The questionof whether the real universe is uniform or nonuniform is important to cosmology.

The metagalactic space between galaxies is not empty. There are many small stellar systems,individual stars, rarefied gas, and cosmic dust in it as well as cosmic rays. Moreover, the intensity of the fieldsincluding the gravitational and magnetic fieldsare nonzero. The study of these fields isalso part of the task of extragalactic astronomy.


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At the turn of the 19th century, the English astronomer W. Herschel was the first to compile extensivecatalogs of the bright nebulous spots visible in the sky. Investigations showed that some of them,when viewed through strong tele-scopes, proved to consist of stars. At the same time, how-ever, theexistence of nebulas consisting of a continuous dif-fuse medium was recognized. This was finallyproved in the second half of the 19th century by spectral analysis. The spectrum of some nebulasproved to consist of bright lines that belonged to rarefied gases. For other nebulas it resembled the

spectrum of star clusterscontinuous, with absorption lines; such nebulas constituted anoverwhelming majority. Later it was learned that a small percentage of nebulas with such a spectrumdo not constitute stellar systems but are clouds of cosmic dust shining with the reflected light of brightstars. In the 1920s, E. Hubble (of the United States) was able to prove that gaseous and dust nebulasare found even among objects comparatively close to us. Somewhat earlier H. Shapley succeeded indetermining the distances to globular star clusters, of which the most distant resolve into stars onlywith difficulty, even through the most powerful telescopes.

The nature of the remaining nebulous spots (and there is a tremendous number of them; the catalogscontain about 30,000 objects up to the 15th visual stellar magnitude) was clarified by the middle of the1920s. As early as the middle of the 19th century the English scientist W. Parsons (Earl of Rosse)observed a spiral structure in the largest of them, but the diversity and fineness of the structure of nebulas were brought to light only after the introduction into astronomical practice of photography andtelescopes of increased power. The Swedish astronomer K. Lundmark, observing in spiral nebulasthe scarcely noticeable nova outbursts, which actually were of tremendous luminosity, concluded thatspiral nebulas lie beyond our galaxy. Subsequently it was learned that stars whose explosions wereobserved in galaxies most often were not new stars but supernovas, hundreds of times brighter, as aresult of which the estimates of distances to spiral nebulas made by Lundmark had to be increased.Not a single supernova has been observed in our galaxy since the invention of the telescope.Therefore the study of these interesting celestial bodies rests mainly on the results of extragalacticastronomy.

Later, Hubble determined more precisely the distances and dimensions of the spiral galaxies M31 (theGreat Nebula in Andromeda), M33 (in the Triangulum constellation), and NGC 6822 (in Sagittarius).He proved the great similarity of these stellar systems to our galaxy by establishing that they allcontain stars of identical types, identical star clusters, diffuse gaseous nebulas, and novas. Thesediscoveries, like many that followed in extragalactic astronomy, were accomplished with the aid of thelargest telescopes in the world, mounted in the United States.

In 1924-25 variable stars, including the cepheids, whose luminosity is known to be connected with theperiod of variation of their brightness, were detected in photographs of nearby spiral galaxies. Thus,by determining the luminosity on the basis of the observed variation in the brightness and bycomparing it with the visual stellar magnitude of these celestial bodies, it is possible to estimatedistances to the cepheids and hence to the galaxies containing them. (The dimensions of galaxies aresmall in comparison with the distances to them.) The cepheid method of determining distances toremote stellar systems is most accurate but is applicable only to the closest ones. For more remotesystems, including the most distant systems observed at present, the method of determiningdistances to galaxies from the line shift in the spectrum of the galaxies, the so-called red shift, is best.

In 1924, K. Lundmark and K. Wiertz (of Germany) discovered that the greater the distance to a galaxythe more strongly its spectrum is displaced toward the red end. Later, the magnitude of the red shiftcaused by movement away from us (the Doppler effect) was determined more precisely. Indetermining distances by this method it is assumed that for each million parsecs of distance the redshift increases by approximately 100 km/sec (Hubbles law). This systematic shift due to theexpansion of the metagalaxy has superposed on it the shift of spectral lines (toward the red or blueend) due to the individual velocities of the galaxies, which do not usually exceed 1,000 km/sec.Because of this, the method of determining distances by the red shift of spectral lines is unreliablewhen applied to nearby galaxies.

The tasks of extragalactic astronomy are to study photo-graphically the shapes and types of galaxies,to classify galaxies (the foundation for which was laid by Hubble), to measure the stellar magnitudesand colors of galaxies on the whole and of individual sections, and to investigate the principlesgoverning the structure and composition of galactic clusters. The number and distribution of variousobjects of various luminosities are studied in the closest galaxies. By means of spectral analysis the


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rates of motion and the laws governing the rotation of galaxies are studied. This provides material for determining their masses. The chemical composition of the stars making up the galaxies is alsostudied and compared. Electron image intensifies, which reduce the expo-sure time and make itpossible to photograph very faint objects, are used in the photography of galaxies.

New possibilities were offered to extragalactic astronomy by the methods of radioastronomy. Withtheir aid, fundamentally new objects and phenomena in the metagalaxy have been discovered.Among such objects are the so-called radio galaxies, characteristic of which is extraordinarily powerfulradiation in the radio band apparently originating from elementary particles of tremendous energymoving in the magnetic fields of some galaxies, and quasars, whose nature is still insufficientlystudied. On the basis of the very large red shifts in the spectra of most observed quasars, theconclusion is already being drawn, however, that many of them are at distances of several billionparsecs. So-called quasistellar galaxies, which are starlike objects that have no strong and perhapsnot even moderate radio emissions, are similar to quasars in luminosity and spectrum. They aredozens of times more numerous than quasars. At the same time there is much in common betweenthe turbulent processes in quasars and the nuclei of some galaxies.

In the USSR the most extensive theoretical and observational investigations in extragalacticastronomy are being conducted at the Biurakan Astrophysical Observatory of the Academy of Sciences of the Armenian SSR and at the P. K. Shternberg State Astronomical Institute of MoscowUniversity

Hubble Galaxy

may be viewed as the basic building blocks for the large-scale visible stucture of theUniverse. There are may galaxy types, having rather diverse features. Therefore, it isuseful to have a way to classify galaxies into different types.

The Tuning Fork Diagram

Hubble introduced the classification scheme illustrated in the following figure, whichseparates most galaxies into elliptical, normal spiral, and barred spiral categories, andthen sub-classifies these categories with respect to properties such as the amount of flattening for elliptical galaxies and the nature of the arms for spiral galaxies. Thegalaxies that do not fit into these categories are classified separately as irregular

galaxies .

This diagram is termed the Hubble classification scheme , or (because of its shape) the"tuning fork diagram".

Examples of Hubble Galaxy Types

Here are some examples of specific galaxies that illustrate some of the Hubbleclassification types.

M81 : Type Sb spiral NGC2997 : Type Sc spiral M95 : Type SBa barred spiral NGC1365 : Type SBb barred spiral

Leo I : Type E3 (dwarf) elliptical M110 : Type E6 elliptical
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Small Magellanic Cloud : Irregular type Many galaxies take the form of ellipsoids, with no spiral structure or flattened

disks. Elliptical galaxies constitute approximately 10% of observed galaxies. Examples of Elliptical Galaxies The adjacent image shows an example, the giant elliptical

galaxy M87 in the center of the Virgo cluster (click on theimage for a larger version). Some other examples of elliptical galaxies include M32 , which is an E2 dwarf elliptical near the Andromeda Galaxy, and the E6 ellipticalgalaxy M110 , another satellite of the Andromeda Galaxy.

The Hubble Classification In the Hubble sequence E0, E1, E2, ... E7, the number is

related to how flattened the ellipse appears to be, with E0corresponding to no flattening and E7 to a very elongated ellipse. The Hubbleclassification scheme uses the apparent ellipticity, so it refers to the projection of the galaxy's shape on the celestial sphere, not its actual shape.

Properties of Elliptical Galaxies The masses of elliptical galaxies cover a large range: from about 10 7 up to 10 13

solar masses. The corresponding range of diameters is about 1/10 kpc up toabout 100 kpc, and the absolute blue magnitude varies over a correspondinglylarge range from -8 to -23. Thus, the smallest of the elliptical galaxies, which arecalled dwarf ellipticals , may be only a little larger than globular clusters, whilethe giant elliptical galaxies like M87 are among the largest galaxies in theUniverse. This is a much larger range in size than is seen for the spiral galaxies.

Elliptical galaxies exhibit far less evidence for young stars, gas, or dust than dospiral galaxies, and have larger random motion of stars than in spiral galaxies

where the motion is a more ordered rotation. Irregular Galaxies Approximately 3% of galaxies observed cannot be classified as either ellipsoidal

or spirals. These galaxies have little symmetry in their structure and are termedirregular galaxies . An example is Sextans A, shown in the image on the right.This irregular galaxy is a member of the Local Group, at a distance of about 10million light years (Ref) . The blue regions are clusters of young stars; thebrighter stars are members of our own Milky Way galaxy in the foreground.Other examples of irregular galaxies are the Large Magellanic Cloud (a satellitegalaxy of the Milky Way) and the Messier object M82.

Properties of Irregular Galaxies Irregular galaxies have masses in the range 10 8 to 10 10 solar masses, diameters

from 1 to 10 kpc, and blue magnitudes from -13 to -20. Other than that, theyhave few systematic features.

Peculiar Galaxies Hubble's original classification just lumped all galaxies that are not spirals or

elliptical into the irregular category, but it is more common today to makefurther distinction between more "normal" irregular galaxies and peculiar

galaxies , which are galaxies that look unusual in some respect ( M82 , forinstance). For example, some of these are objects that have been tidally distortedby interaction with another galaxy that we shall discuss in conjunction with

colliding galaxies . Others are active galaxies with some evidence of violentinternal processes taking place. Still others may be more normal galaxies, but
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given an unusual appearance because of obscuring dust. We shall discuss thesespecial kinds of irregular galaxies separately later.Spiral Galaxies

Galaxies come in a variety of shapes , with the shapes depending in a way not yetcompletely understood on the evolution of the galaxies . More than half of allobserved galaxies are spiral galaxies.

Examples of Spiral Galaxies The figure below right shows a nice spiral galaxy, M100 , which is in the Virgo

cluster Another beautiful example of a spiral galaxy is M83 . Presumably ourown galaxy would resemble these galaxies in appearance if we could view it fromthe outside. The below left image shows a class Sc spiral galaxy M101 (NGC5457; also called the Pinwheel Galaxy),which lies at a distance of about 7 Mpc or22 million light years (Ref) .

Properties of Spiral Galaxies

The range of masses for spiral galaxies is ~ 10 9 - 10 12 solar masses, with the typical massbeing ~ 10 11 solar masses. The typical range of luminosities corresponds to absolute bluemagnitude -16 to -23, and the typical diameter of the visible disk is 5-100 kpc.The Milky Way is a member of a group of galaxies termed the Local Group thatcontains approximately 20 bright galaxies and 30 galaxies total. The largest galaxies inthe local group are the spirals Andromeda (M31) and the Milky Way.

Some Galaxies in the Local Group

The two closest galaxies to the MilkyWay are called the Magellanic Clouds ,which may be viewed as satellitegalaxies to the Milky Way at a distanceof a little less than 200,000 light years.

They are only visible in the SouthernHemisphere, but can easily be seen bythe naked-eye and their brightest starscan be seen with binoculars. They areirregular galaxies and are much smaller

than the Milky Way.

The Large Magellanic Cloud
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Two galaxies are visible to the naked-eye in the Northern Hemisphere. TheAndromeda Galaxy (M31) is a greatspiral galaxy much like our own at adistance of about 3 million light years

(a little less than 3 Mpc). To the nakedeye it is a faint fuzzy patch thatappears, with binoculars, as a lensshaped object. It has two dwarf elliptical satellite galaxies visible through a small telescope.

The other galaxy of the local group that is visible to the naked eye is the spiral M33 inTriangulum at a distance comparable to that of Andromeda. It too is a spiral galaxy,but it is smaller than Andromeda and therefore is harder to see.

Some Other Nearby Groups of Galaxies

Some other nearby groups of galaxies are listed in the following table. All told, there areabout 20 small groups of galaxies lying nearer to us than the Virgo rich cluster .

Some Nearby Groups of Galaxies

Group Name

M81Numbe

r ofMembersDi

stance(Mpc)

8 3.1

Sculptor 6 1.8

Centaurus 17 3.5

M101 5 7.7

M66 + M96 ~10 9.4

NGC 1023 6 9.5

The Hubble constant H is one of the most important numbers in cosmology because itmay be used to estimate the size and age of the Universe . It indicates the rate at whichthe universe is expanding. Although the Hubble "constant" is not really constantbecause it changes with time (and therefore should probably more properly be called

The Andromeda Galaxy
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the "Hubble parameter"). The Hubble constant is often written with a subscript "0" todenote explicitly that it is the value at the present time, but we shall not do so.

The Hubble Expansion Law

In 1929, Edwin Hubble announced that almost all galaxies appeared to be moving awayfrom us. This phenomenon was observed as a redshift of a galaxy's spectrum. Thisredshift appeared to have a larger displacement for faint, presumably further, galaxies.Hence, the farther a galaxy, the faster it is receding from Earth. The Hubble constant isgiven by

H = v/d

where v is the galaxy's radial outward velocity, d is the galaxy's distance from earth,and H is the current value of the Hubble constant.

Determining the Hubble Constant

Obtaining a true value for H is complicated. Two measurements are required. First,spectroscopic observations reveal the galaxy's redshift, indicating its radial velocity.The second measurement, the most difficult value to determine, is the galaxy's precisedistance from Earth. The value of H itself must be derived from a sample of galaxiesthat are far enough away that motions due to local gravitational influences arenegligibly small (these are called peculiar motion , and they represent deviations fromthe Hubble Law).

Units for Hubble's Constant

The units of the Hubble constant are "kilometers per second per megaparsec." In otherwords, for each megaparsec of distance , the velocity of a distant object appears toincrease by some value. For example, if the Hubble constant was determined to be 50km/s/Mpc, a galaxy at 10 Mpc would have a redshift corresponding to a radial velocityof 500 km/s.

Current Value of the Hubble Constant

The value of the Hubble constant initially obtained by Hubble was around 500km/s/Mpc, and has since been radically revised because initial assumptions about starsyielded underestimated distances. For the past three decades, there have been twomajor lines of investigation into the Hubble constant. One team, associated with AllanSandage of the Carnegie Institutions, has derived a value for H around 50 km/s/Mpc.The other team, associated with Gerard DeVaucouleurs of the University of Texas, hasobtained values that indicate H to be around 100 km/s/Mpc.

Hubble Time

The inverse of the Hubble constant H has the units of time because the

Hubble law is

v = H d
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where v is the velocity of recession, H is the Hubble constant, and d is the distance.Thus, from this equation, we have that 1/H = d/v . but d/v is distance divided by velocity,which is time (e.g., if I travel 180 miles at 60 miles/hour, the time required is t = d/v =180/60 = 3 hours).

Thus, the Hubble time T is just the inverse of the Hubble Constant:

T = 1 / H

Taking a value of H = 20 km/s/Mly (where Mly means mega-light years),

where all the factors are necessary to convert the time units to years and I've roundedsome numbers to simplify the display.

The physical interpretation of the Hubble time is that it gives the time for the Universeto run backwards to the Big Bang if the expansion rate (the Hubble "constant") wereconstant. Thus, it is a measure of the age of the Universe. The Hubble "constant"actually isn't constant, so the Hubble time is really only a rough estimate of the age of the Universe.

Reasonable assumptions for the value of the Hubble constant and the geometry of the

Universe typically yield ages of 10-20 billion years for the age of the Universe. Forexample, H near 50 km/s/Mpc gives a larger value for the age of the Universe (around16 thousand million years), while a larger value of 80 km/s/Mpc gives a lower value forthe age (around 10 thousand million years). Therefore, we shall take this information,and additional information from other methods to estimate the age of the Universe thatwe have not discussed, to indicate an age of approximately 15 billion years for theUniverse.

A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite .Globular clusters are very tightly bound by gravity , which gives them their spherical shapesand relatively high stellar densities toward their centers. The name of this category of star cluster is derived from the Latin globulus a small sphere. A globular cluster is sometimesknown more simply as a globular .

Globular clusters, which are found in the halo of a galaxy, contain considerably more starsand are much older than the less dense galactic, or open clusters , which are found in the disk.Globular clusters are fairly common; there are about 150 [2] to 158 [3] currently known globular clusters in the Milky Way , with perhaps 10 to 20 more still undiscovered. [4] Large galaxiescan have more: Andromeda , for instance, may have as many as 500. [5] Some giant ellipticalgalaxies , particularly those at the centers of galaxy clusters, such as M87 ,[6] have as many as13,000 globular clusters. These globular clusters orbit the galaxy out to large radii, 40kiloparsecs (approximately 131,000 light-years ) or more. [7]
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Every galaxy of sufficient mass in the Local Group has an associated group of globular clusters, and almost every large galaxy surveyed has been found to possess a system of globular clusters. [8] The Sagittarius Dwarf and Canis Major Dwarf galaxies appear to be in the

process of donating their associated globular clusters (such as Palomar 12 ) to the Milky Way.[9] This demonstrates how many of this galaxy's globular clusters might have been acquired in

the past.

Although it appears that globular clusters contain some of the first stars to be produced in thegalaxy, their origins and their role in galactic evolution are still unclear. It does appear clear that globular clusters are significantly different from dwarf elliptical galaxies and wereformed as part of the star formation of the parent galaxy rather than as a separate galaxy .[10]

However, recent conjectures by astronomers suggest that globular clusters and dwarf spheroidals may not be clearly separate and distinct types

Types of galaxies

PRINCIPAL SCHEMES OF CLASSIFICATION

Almost all current systems of galaxy classification are outgrowths of the initialscheme proposed by Hubble in 1926. In Hubble's scheme, which is based on theoptical appearance of galaxy images on photographic plates, galaxies are dividedinto three general classes: ellipticals, spirals, and irregulars. His basic definitionsare as follows:

Elliptical galaxies .Galaxies of this class have smoothly varying brightnesses, with the degree of brightness steadily decreasing outward from the centre. They appear elliptical inshape, with lines of equal brightness made up of concentric and similar ellipses.These galaxies are nearly all of the same colour: they are somewhat redder thanthe Sun.

Spiral galaxies .These galaxies are conspicuous for their spiral-shaped arms, which emanatefrom or near the nucleus and gradually wind outward to the edge. There are

usually two opposing arms arranged symmetrically around the centre. Thenucleus of a spiral galaxy is a sharp-peaked area of smooth texture, which canbe quite small or, in some cases, can make up the bulk of the galaxy. The armsare embedded in a thin disk of stars. Both the arms and the disk of a spiralsystem are blue in colour, whereas its central areas are red like an ellipticalgalaxy.

Irregular galaxies.Most representatives of this class consist of grainy, highly irregular assemblagesof luminous areas. They have no noticeable symmetry nor obvious centralnucleus, and they are generally bluer in colour than are the arms and disks of
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spiral galaxies. An extremely small number of them, however, are red and havea smooth, though nonsymmetrical, shape.

Hubble subdivided these three classes into finer groups according tosubtle differences in shape, as described in detail below. Otherclassification schemes similar to Hubble's follow this pattern but subdividethe galaxies differently. A notable example of one such system is that of Gerard de Vaucouleurs. This scheme, which has evolved considerablysince its inception in 1959, includes a large number of codes for indicatingdifferent kinds of morphological characteristics visible in the images of galaxies. The major Hubble galaxy classes form the framework of deVaucouleurs's scheme, and its subdivision includes different families,varieties, and stages, as shown in Table 1.

Examples of the de Vaucouleurs classification scheme are for galaxy M33,the Triangulum Nebula, which is classified as SA(s)cd, and the nearbysmall galaxy NGC 6822, classified as IB(s)m.

An entirely different kind of classification scheme is the luminosityclassification developed in 1960 by Sidney van den Bergh. Based onmorphological considerations, luminosity classes are assigned toindividual galaxies within the Hubble classes. Those that are the mostluminous are given a luminosity class of I, and the intrinsically faintestmembers of a class are assigned a V or VI, recalling the general approachof the luminosity class scheme used for stellar spectra . Thus a very

luminous galaxy with open, resolved arms would be an Sc I galaxy, whilea somewhat intrinsically fainter object with the same basic structurewould be an Sc II or Sc III galaxy. To assign a luminosity class, a galaxy'simage has to be compared with a set of standard images of galaxies forwhich distances are known and for which luminosity classes have beenestablished by van den Bergh.

Classification schemes based on criteria other than optical appearancehave been proposed. There is, for example, the Morgan scheme(proposed by W.W. Morgan), which combines information on the

spectrum of a galaxy with its general shape. Here, a class is coded with aletter that indicates the spectral type of the galaxy in the blue (either asmeasured or as determined from the galaxy's bulge morphology, whichcorrelates with the spectral type): e.g., a, af, f, fg, g, gk, k, for increasingdominance by cooler stars. The code then includes a capital letter toindicate general morphology-- e.g., E, S, or I--in accordance with Hubble'sgeneral classes. This is followed by a number that indicates the overalloptical shape of the image, with 0 representing a circular image and a 10(never actually realized) standing for a linear, infinitely thin image. Anexample is the galaxy M31, the Andromeda Nebula, which is classified askS5 in the Morgan system.


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Systems that separate galaxies according to the character of their radiostructure and the strength of their radio emissions also have beendevised. For example, radio galaxies can be classified according to thefollowing scheme:

g: galaxies with normal radio fluxes.

R: galaxies with strong radio emission. Many have distorted morphology,with evidence of explosive events or interactions with companions.

cD: galaxies with abnormally large, distended shapes, always found in thecentral areas of galaxy clusters and hypothesized to consist of mergedgalaxies.

S: Seyfert galaxies, originally recognized by the American astronomer

Carl K. Seyfert from optical spectra. These objects have very bright nucleiwith strong emission lines of hydrogen and other common elements,showing velocities of hundreds or thousands of kilometres per second.Most are radio sources.

N: galaxies with small, very bright nuclei and strong radio emission,probably similar to Seyfert galaxies but more distant.

Q: quasars, small, extremely luminous objects, many of which are strongradio sources. Quasars apparently are related to Seyfert and N galaxies

but have such bright nuclei that the underlying galaxy can be detectedonly with great difficulty.

Although such schemes are sometimes used for special purposes,including, for example, certain kinds of statistical studies, the generalscheme of Hubble in its updated form is the one most commonly used andso will be described in detail in the following section.

CLASSES OF GALAXIES

In The Hubble Atlas of Galaxies (1961), Allan R. Sandage drew on Hubble'snotes and his own research on galaxy morphology to revise the Hubbleclassification scheme. Some of the features of this revised scheme are subject toargument because of the findings of very recent research, but its generalfeatures, especially the coding of types, remain viable. A description of the

classes as defined by Sandage is given here, along withobservations concerning needed refinements of some of thedetails.

Elliptical galaxies.

These systems exhibit certain characteristic properties. They havecomplete rotational symmetry; i.e., they are figures of revolution

M104,

SombreroGalaxy inVirgo, anelliptical galaxysurrounded bya disc of dustand gas.
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with two equal principal axes. They have a third smaller axis that is thepresumed axis of rotation. The surface brightness of ellipticals at opticalwavelengths decreases monotonically outward from a maximum value at thecentre, following a common mathematical law of the form:

where I is the intensity of the light, I o is the central intensity, r is theradius, and a is a scale factor. The isophotal contours exhibited by anelliptical system are similar ellipses with a common orientation, eachcentred on its nucleus. No galaxy of this type is flatter than b / a = 0.3,with b and a the minor and major axes of the elliptical image,respectively. Ellipticals contain neither interstellar dust nor bright stars of spectral types 0 and B. Many, however, contain evidence of the presenceof low-density gas in their nuclear regions. Ellipticals are red in colour,and their spectra indicate that their light comes mostly from old stars,especially evolved red giants.

Subclasses of elliptical galaxies are defined by their apparent shape,which is of course not necessarily their three-dimensional shape. Thedesignation is E n , where n is an integer defined by

A perfectly circular image will be an E0 galaxy, while a flatter object mightbe an E7 galaxy. (As explained above, elliptical galaxies are never flatterthan this, so there are no E8, E9, or E10 galaxies.)

Although the above-cited criteria are generally accepted, current high-quality measurements have shown that some significant deviations exist.Most elliptical galaxies do not, for instance, exactly fit the intensity lawformulated by Hubble; deviations are evident in their innermost parts andin their faint outer parts. Furthermore, many elliptical galaxies haveslowly varying ellipticity, with the images being more circular in thecentral regions than in the outer parts. The major axes sometimes do notline up either, their position angles varying outward. Finally, astronomershave found that a few ellipticals do in fact have small numbers of luminous 0 and B stars as well as dust lanes.

Spiral galaxies.Spirals are characterized by circular symmetry, a bright nucleus

surrounded by a thin outer disk, and a superimposed spiralstructure. They are divided into two parallel classes: normalFigure 1:Hubble'ssystem of classificationfor galaxies(see text).
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spirals and barred spirals. The normal spirals have arms that emanate from thenucleus, while barred spirals have a bright linear feature called a bar straddlingthe nucleus, with the arms unwinding from the ends of the bar. The normalspirals are designated "S" and the barred varieties "SB." Each of these classes issubclassified into three types according to the size of the nucleus and the degreeto which the spiral arms are coiled. The three types are denoted with thelowercase letters "a," "b," and "c." There also exist galaxies that areintermediate between ellipticals and spirals. Such systems have the disk shapecharacteristic of the latter but no spiral arms. These intermediate forms bear thedesignation "S0" (Figure 1).

S0 galaxies.These systems exhibit some of the properties of both the ellipticals and thespirals and seem to be a bridge between these two more common galaxy types.

Hubble introduced the S0 class long after his original classification scheme hadbeen universally adopted largely because he noticed the dearth of highlyflattened objects that otherwise had the properties of elliptical galaxies.Sandage's elaboration of the S0 class yielded the characteristics described here.

S0 galaxies have a bright nucleus that is surrounded by a smooth,featureless bulge and a faint outer envelope. They are thin; statisticalstudies of the ratio of the apparent axes (seen projected onto the sky)indicate that they have intrinsic ratios of minor to major axes in the range0.1 to 0.3. Their structure does not generally follow the luminosity law of

elliptical galaxies, but it has a form more like that for spiral galaxies.Some S0 systems have a hint of structure in the envelope, either faintlydiscernible armlike discontinuities or narrow absorption lanes produced byinterstellar dust. Several S0 galaxies are otherwise peculiar, and it isdifficult to classify them with certainty. They can be thought of as peculiarIrr galaxies ( i.e., Irr II galaxies [see below]) or simply as some of the 1 or2 percent of galaxies that do not fit easily into the Hubble scheme. Amongthese are such galaxies as NGC 4753 that has irregular dust lanes acrossits image and NGC 128 that has a double, almost rectangular, bulgearound a central nucleus. Another type of peculiar S0 is found in NGC

2685. This nebula in the constellation Ursa Major has an apparently edge-on disk galaxy at its centre, with surrounding hoops of gas, dust, andstars arranged in a plane that is at right angles to the apparent plane of the central object.

Sa galaxies.These normal spirals have narrow, tightly wound arms, which usually are visibledue to the presence of interstellar dust and in many cases bright stars as well.Most of them have a large, amorphous bulge in the centre, but there are somethat violate this criterion, having a small nucleus around which is arranged anamorphous disk with superimposed faint arms. NGC 1302 is an example of the


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normal type of Sa galaxy, while NGC 4866 is representative of one with a smallnucleus and arms consisting of thin dust lanes on a smooth disk.

Sb galaxies.

This intermediate type of spiral typically has a medium-sized nucleus. Its armsare more widely spread than those of the Sa variety and appear less smooth.They contain stars, star clouds, and interstellar gas and dust. Sb galaxies showwide dispersions in details in terms of their shape. Hubble and Sandageobserved, for example, that in certain Sb galaxies the arms emerge at thenucleus, which is often quite small. Other members of this subclass have armsthat begin tangent to a bright, nearly circular ring, while still others reveal asmall, bright spiral pattern inset into the nuclear bulge. In any of these cases,the spiral arms may be set at different pitch angles. (A pitch angle is defined asthe angle between an arm and a circle centred on the nucleus and intersecting

the arm.)

Hubble and Sandage noted further deviations from the standard shapeestablished for Sb galaxies. A few systems exhibit a chaotic dust patternsuperimposed upon the tightly wound spiral arms. Some have smooth,thick arms of low surface brightness, frequently bounded on their inneredges with dust lanes. Finally, there are those with a large, smoothnuclear bulge from which the arms emanate, flowing outward tangent tothe bulge and forming short arm segments. This is the most familiar typeof Sb galaxy and is best exemplified by the giant Andromeda Galaxy.

Many of these variations in shape remain unexplained. Theoretical modelsof spiral galaxies based on a number of different premises can reproducethe basic Sb galaxy shape (see below The Milky Way Galaxy ), but manyof the deviations noted above are somewhat mysterious in origin andmust await more detailed and realistic modeling of galactic dynamics.

Sc galaxies.These galaxies characteristically have a very small nucleus and multiple spiralarms that are open, with relatively large pitch angles. The arms, moreover, are

lumpy, containing as they do numerous irregularly distributed star clouds, stellarassociations, star clusters, and gas clouds known as emission nebulas.

As in the case of Sb galaxies, there are several recognizable subtypesamong the Sc systems. Sandage has cited six subdivisions: (1) galaxies,such as the Whirlpool Nebula (M51), that have thin, branched arms thatwind outward from a tiny nucleus, usually extending out about 180before branching into multiple segments; (2) systems with multiple armsthat start tangent to a bright ring centred on the nucleus; (3) those witharms that are poorly defined and that span the entire image of the

galaxy; (4) those with a spiral pattern that cannot easily be traced andthat are multiple and punctuated with chaotic dust lanes; (5) those with


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thick, loose arms that are not well defined-- e.g., the nearby galaxy M33,the Triangulum Nebula; and (6) transition types, which are almost solacking in order that they could be considered irregular galaxies.

Some classification schemes, such as that of de Vaucouleurs, give the lastof the above-cited subtypes a class of its own, type Sd. It also has beenfound that some of the variations noted here for Sc galaxies are related tototal luminosity. Galaxies of the fifth subtype, in particular, tend to beintrinsically faint, while those of the first subtype are among the mostluminous spirals known. This correlation is part of the justification for theluminosity classification discussed earlier.

SB galaxies.The luminosities, dimensions, spectra, and distributions of the barred spiralstend to be indistinguishable from those of normal spirals. The subclasses of SBsystems exist in parallel sequence to those of the latter.

There are SB0 galaxies that feature a large nuclear bulge surrounded by adisklike envelope across which runs a luminous, featureless bar. SomeSB0 systems have short bars, while others have bars that extend acrossthe entire visible image. Occasionally there is a ringlike feature externalto the bar. SBa galaxies have bright, fairly large nuclear bulges andtightly wound, smooth spiral arms that emerge from the ends of the baror from a circular ring external to the bar. SBb systems have a smoothbar as well as relatively smooth and continuous arms. In some galaxies of this type, the arms start at or near the ends of the bar, with conspicuousdust lanes along the inside of the bar that can be traced right up to thenucleus. Others have arms that start tangent to a ring external to the bar.In SBc galaxies, both the arms and the bar are highly resolved into starclouds and stellar associations. The arms are open in form and can starteither at the ends of the bar or tangent to a ring.

Irregular galaxies.Hubble recognized two types of irregular galaxies, Irr I and Irr II. The Irr I type

is the most common of the irregular systems, and it seems to fall naturally on anextension of the spiral classes, beyond Sc, into galaxies with no discernible spiralstructure. They are blue, highly resolved, and have little or no nucleus. The Irr IIsystems are rare objects. They include various kinds of chaotic galaxies forwhich there apparently are many different explanations. Table 2 comparesvarious subgroups of this rather confusing assembly of objects.

Some irregular galaxies, like spirals, are barred. They have a nearlycentral bar structure dominating an otherwise chaotic arrangement of material. The Large Magellanic Cloud is a well-known example. The

Hubble system does not normally recognize this as a subtype, though the


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de Vaucouleurs classification scheme includes it and its related types asIm and IB (see Table 1).

The Hubble Distance - Redshift Relationship

When Hubble plotted the redshift vs. the distance of the galaxies, he found a surprisingrelation: more distant galaxies are moving faster away from us. Hubble concluded that thefainter and smaller the galaxy, the more distant it is, and the faster it is moving away from us,or that the recessional velocity of a galaxy is proportional to its distance from us:

v = H o d ,

where v is the galaxy's velocity (in km/sec), d is the distance to the galaxy (in megaparsecs; 1Mpc = 1 million parsecs), and H o proportionality constant, called "The Hubble constant".

Hubble's Law states that the galaxy's recession speed = H o * distance, where H o is known asthe Hubble constant and is a measure of the slope of the line through the distance versusrecession velocity data. The line goes through the origin (0,0) because that represents our home position (zero distance) and we are not moving away from ourselves (zero speed).

To determine a galaxy's distance, we must rely on indirect methods. For instance, oneassumption used by Hubble, and other early 20th century astronomers, is to assume allgalaxies of the same type are the same physical size , no matter where they are. This isknown as "the standard ruler" assumption. To determine the distance to a galaxy one wouldonly need to measure its apparent (angular) size, and use the small angle equation: a = s / d ,where a is the measured angular size (in radians!), s is the galaxy's true size (diameter), and d is the distance to the galaxy.

In order to precisely determine the value of H o, we must determine the velocities and

distances to many galaxies. Hubble's law has been confirmed by subsequent research and provides the cornerstone of modern relativistic cosmological theories of our expanding
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universe. In 1963 astronomers discovered cosmic objects known as quasars that exhibit larger redshifts than any of the remotest galaxies previously observed. The extremely large redshiftsof various quasars suggest that they are moving away from the Earth at tremendous velocities(i.e., approximately 90 percent the speed of light) and thereby constitute some of the mostdistant objects in the universe.

Historical Note: It is not common for any other astronomers to be mentioned along with Edwin Hubble as being responsible for figuring out how the distance to a galaxy is related toits recession velocity. However, Hubble did not work alone and many other astronomersdeserve credit for establishing the distance--redshift relationship.

The trick to determining the distance to a galaxy is to find in that galaxya standard candle, an object that has a known luminosity. If such a class of objects can be found, and if it can be calibrated, preferably by measuring the parallax of one such object within our own galaxy, we can calculate thedistance to the galaxy by measuring the brightness of the object andapplying the inverse square law.

The primary standard candle in astronomy is the Cepheid variable , astar with a luminosity that is set by its pulsation period. A second importantstandard candle is the type 1a supernova , which has a peak luminosity thatcan be used as a standard candle. Because type 1a supernovae are rare inany given galaxy, their use is limited to testing theories of cosmology andcalibrating a third important distance measurethe cosmological redshift .Very distant galaxies are moving away from us with a velocity that isproportional to distance. The redshift of the light from these galaxies is

therefore a measure of their distance. This distance measure, however, canonly be calibrated against standard candle distance indicators.

Cepheid Variables

The best standard candle for determining the distance to the nearbygalaxies is the Cepheid variable star. These are bright and reasonablycommon, with strong identifying signatures, so their observation in othergalaxies is not too difficult. Many of those observed in our own Galaxy havemeasured parallaxes, so this standard candle is calibrated to physical units;273 Cepheid type variables having been observed by the Hipparcos satellite .

The Cepheid variable has a luminosity that is a function of period alone.If you observe one and determine its period of variability, then you canassign it the luminosity of nearby Cepheids with similar periods. Bymeasuring the brightness of the Cepheid in the distant galaxy, one canderive the distance using the inverse square law; in terms of absolutemagnitude M and apparent magnitude m ,1 the distance is given by R = 10 1 +0.2( m - M ) parsecs .

Type 1a Supernovae
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The standard candle of choice in cosmological studies is the type 1asupernova. It is as bright as any event in the universe, so it can be seen inthe most distant galaxies. A supernova is brighter than its host galaxy, andon many occasions, the host galaxy of an observed type 1a supernova is toodim to observe.

The characteristic of type 1a supernovae that make them standardcandles is that low redshift supernovae with similar durations and spectrahave similar peak luminosities. Those observed at low redshift can becalibrated with Cepheid variables.

The basic theory behind this type of supernova is that we are seeing theaftermath of the explosion of a carbon-oxygen thermonuclear bomb. Theprogenitor, a degenerate dwarf (white dwarf), is pushed over theChandrasekhar mass limit ; as the star starts to collapse, the oxygen andcarbon in the star undergo nuclear fusion, releasing the energy in the

supernova. Theory, however, cannot provide the observed behavior from first

principles, so it is unable to show whether the standard candle behavior,which is seen in nearby supernovae, persists at large redshift, where theuniverse is younger, and therefore somewhat different from the currentuniverse in its galactic structure and chemical composition. If changingconditions within the universe make the luminosity of a type 1a supernovachange with redshift, then the distance that is derived will be systematicallytoo large or too small. This has a direct bearing on the application of thisstandard candle to cosmology.

Beyond the question of whether conditions in the early universe affectthe luminosity of a type 1a supernova, a second problem besets thisstandard candle that severely limits its use: supernovae are rare in anygiven galaxy. They therefore cannot be used to determine the distance toany galaxy that we may be interested in. Their only uses are in calibratingother distance measures, such as the cosmological redshift, in testingcosmological theories, and in studying the surrounding of these supernovae.Current studies with these standard candles are examining the expansion of the universe at redshifts between z = 0.01 and 1.

Cosmological Redshift

The light from distant galaxies is shifted to lower frequencies. Thisobserved behavior is well established, and is the motivation behind thetheory that the universe is expanding: the redshift is interpreted as aconsequence of the galaxies moving away . Independent of theory, themagnitude of the shift to lower energies, which is called a redshift, is ameasure of the distance to a galaxy that can be calibrated throughcomparison to standard candle measures of distance.

The standard way of expressing the redshift of a galaxy in astronomy is

through the variable z , which is defined by the equation ob = emit /( 1 +
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s z ) ,

where obs is the observed frequency of an emission line, and emit is the emittedfrequency of the emission line.

For z much less than unity, the distance is found to be proportional to z ; This relationship is given by the Hubble constant H 0 , which is the ratio of theimplied velocity to the distance. The distance is then related to the redshiftby d = c z/H 0 , where c is the speed of light. The value of the Hubble constantthat is determined using type 1a supernovae as standard candles is H 0 = 65km s -1 Mpc -1 (Mpc stands for megaparsec), a value that is believed correct to10%. From this we see that objects with a redshift of 0.1 are about 4.6gigaparsecs way.

For redshifts approaching unity, the dependence of distance on redshiftis set by the precise nature of our cosmology. The distance versus theredshift at large redshift is an active field of research that impacts ourtheories for the evolution of the universe.1 The magnitude scale is a logarithmic scale of brightness. As the magnitude of a starincreases, its brightness decreases. The apparent magnitude m is the brightness of a star measured at Earth, and it depends both on the luminosity and the distance of the star. The absolute magnitude M is the brightness of a star that is placed 10parsecs from Earth. Normally one must state the frequency range over which themeasurement is made. The Sun has an absolute visual magnitude of 4.83, and thebrightest star in the sky, Sirius, has an absolute visual magnitude of 1.4 and anapparent magnitude of -4.6.

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