le>GalaxiesGalaxies:Before the 1920"s, it was assumed that galaxies were in reality objectswithin our own Galaxy, maybe regions creating individual stars. Theywere provided the surname ``nebula"",which we currently use to represent regions that gas and dust in ~ galaxies.The exploration of `nebula", fuzzy objects in the skies that were no planets, comets orstars, is attributed come Charles Messier in the so late 1700"s. His arsenal of 103objects is the first galaxy catalog. Herschel (1792-1871) supplied a huge reflectingtelescope to produce the very first General magazine of galaxies.You space watching: includes mostly older stars through a red color
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At the turn of the century Cepheid variablestars, a special course of pulsatingstars that exhibit a certain period-luminosity relation, werediscovered. In other words, the was discovered that their intrinsicbrightness to be proportional to their duration of sport and, hence,could be offered for measure up the ranges to surrounding galaxies.In the late 1920"s, Hubble discovered similar Cepheid stars in neighboring galaxies together was foundin our very own Galaxy. Since they complied with the same period-luminosityrelation, and also they were very faint, climate this implied the theneighboring galaxies were very far away. This showed that spiral`nebula" were, in fact, exterior to our own Galaxy and also sudden theUniverse was vast in space and time.Although Hubble proved that spiral nebula were exterior to our Galaxy,his estimate of their ranges was turn off by a variable of 6. This to be dueto the truth that the calibration to Cepheids was bad at the time,combined with the primitive telescopes Hubble used. Now we understand that a galaxy is acollect the stars, gas and dust bound together by your commongravitational pull. Galaxies variety from 10,000 come 200,000 light-years in size and also between109 and 1014 solar luminosities in brightness.Galaxies have details features in common. Heaviness holds the billionsof stars together, and also the densest region is in the center, dubbed acore or bulge. Part galaxies have spiral or pinwheel arms. Allgalaxies have actually a pass out outer region or envelope and also a secret dark matterhalo.

You are watching: Contains mostly older stars with a red color


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nearly all galaxy types can be found in groups or clusters. Manyclusters of galaxies have a large, supergiant galaxy at itscenter i beg your pardon has prosper by cannibalizing its neighbors. Ours solarsystem is located in external regions of a spiral galaxy we contact theMilky Way. The nearest neighbor galaxy is Andromeda Galaxy (M31).
Above is a 3D plot of many of the Local group of galaxies, the population ofgalaxies in ~ 1000 kpc if the Milky Way. Clustering ofdwarf satellite galaxies approximately the an excellent Milky method and Andromeda spiralscan it is in seen. Hubble succession :Almost all current systems of galaxy group are outgrowths ofthe initial scheme proposed through American astronomer Edwin Hubble in1926. In Hubble"s scheme, i beg your pardon is based on the optical figure ofgalaxy pictures on photographic plates, galaxies are split into threegeneral classes: ellipticals, spirals, and also irregulars.
Elliptical galaxies :Galaxies of this course have smoothly varying brightnesses, steadilydecreasing outside from the center. They show up elliptical in shape,with present of equal brightness consisted of of concentric and also similarellipses. These galaxies space nearly every one of the same color: lock aresomewhat redder 보다 the Sun. Ellipticals are likewise devoid of gas ordust and contain just old stars.
NGC4881All ellipticals watch alike, NGC 4881 is a an excellent example (NGC standsfor brand-new General Catalog). Notice how smooth and also red NGC 4881 lookscompared come the blue spirals to the right.
M32A few ellipticals are close sufficient to us that we have the right to resolve the individual starswithin them, such together M32, a companion to the Andromedia Galaxy. Spiral galaxies :These galaxies space conspicuous for your spiral-shaped arms, which emanatefrom or close to the nucleus and gradually wind outward to the edge. Thereare usually 2 opposing eight arranged symmetrically approximately the center.The cell nucleus of a spiral galaxy is a sharp-peaked area of smooth texture,which deserve to be quite small or, in some cases, can comprise the mass of thegalaxy. The arms are embedded in a thin disk of stars. Both the arms andthe disc of a spiral system are blue in color, whereas its central areasare red like an elliptical galaxy.
M100Notice in the above photo of M100 native HST, that the facility of thespiral is red/yellow and also the arms room blue. Hotter, younger stars areblue, older, cooler stars space red. Thus, the center of a spiral ismade of old stars, through young stars in the arms formed recently out ofgas and also dust.
NGC 1365NGC 1365 is a barred spiral galaxy. Note the unique dark lanes ofobscuring dust in the bar pointing towards the bulge. A close-up ofthe spiral arms reflects blue nebula, website of existing starformation.
NGC 253 core and outer diskNGC 253 is a usual Sa form galaxy with an extremely tight spiral arms. Asspiral galaxies room seen edge-on the big amount the gas and also dust isvisible as dark lanes and filaments cross in front of the bulgeregions. Rarely often rare galaxies :Most to represent of this class consist that grainy, extremely irregularassemblages the luminous areas. They have actually no noticeable symmetry norobvious main nucleus, and also they are normally bluer in color than arethe arms and also disks of spiral galaxies.
NGC 2363NGC 2363 is an example of a nearby irregular galaxy. There is no welldefined form to the galaxy, nor space there spiral arms. A close-up ofthe bright an ar on the eastern side mirrors a swarm of brand-new starsembedded in the red light of ionized hydrogen gas.Galaxy Colors:The various colors in a galaxy (red bulge, blue disks) is due to the typesof stars discovered in those galaxy regions, dubbed its mainly population.Big, enormous stars burn their hydrogen fuel, by thermonuclear fusion,extremely fast. Thus, they space bright and hot = blue. Low mass stars,although much more numerous, room cool in surface ar temperature (= red) and also muchfainter. All this is shown in a Hertzsprung-RussellDiagram the the young star cluster.
The warm blue stars use their core fuel much quicker than the fainter,cooler red stars. Therefore, a young stellar population has a meancolor that is blue (the amount of the irradiate from all the stars in thestellar population) because most the the light is coming from the hotstars. An old stellar population is red, because all the hot stars havedied off (turned into red huge stars) leaving the faint cool stars.
The bottom line is the the red regions of a galaxy room old, through no hotstars. The blue sections of a galaxy space young, meaning the stellarpopulation the dominates this an ar is recently formed. Star formation :The one attribute that correlates through the shape, appearance and also colorof a galaxy is the amount of current star formation. Stars kind whengiant clouds of hydrogen gas and also dust please under your owngravity. As the cloud collapses it fragments into numerous smallerpieces, every section proceeds to collapse till thermonuclear fusionbegins.

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Initial conditions for a galaxy determines its price of star formation.For example, elliptical galaxies collapse beforehand and kind stars quickly.The gas is used up in its early years and today has the illustration of asmooth, red object v no current star formation.See more: petrol To Gallons every Minute come Kg/S ), Metric, Gallons us Per Minute to Kilograms per Second
Spirals, on the various other hand, type slower, with lower rates of starformation. The gas the `fuels" star formation is supplied slower and,thus, there is plenty approximately today to continue to type stars within thespiral arms.