ASTR­1303 Chapter 24 Notes PDF

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Textbook notes (Astronomy Today) for George Marion's ASTR 1303 ACC Course
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(24.1) HUBBLE’S GALAXY CLASSIFICATION - We are able to take images of many different galaxies across the sky. We are able to tell these are galaxies due to their fuzzy edges and elongated shaped (stars are usually seen as sharp and pointed), - The first astronomer to categorize galaxies in an easy to understand way was Edwin Hubble, who, using a 2.5 m optical telescope, classified the galaxies he saw into 4 basic types: spirals, barred spirals, ellipticals, and irregulars (so this categorization was basically only based on physical appearance). These categories are called the Hubbleclassification scheme. Spirals - Examples of spiral galaxies: The Milky Way, Andromeda - all spirals have a galactic disk with spiral arms, a central galactic bulge with a dense nucleus, and an extended halo of faint, old stars. - The stellar density is greatest in the center of the galactic bulge - Spirals have different shapes - Spirals are classified by the letter S followed by type a, b, or c depending on the size of its central bulge (a is the largest bulge, c is the smallest, etc.) - The tightness of the spiral pattern is correlated with the size of the bulge - Type Sa spirals have tightly wrapped spiral arms - Types Sb and Sc have open or loosely defined arms - Ads the spirals become more open, the spiral looks more and more clumpy - The bulges and halos of spiral galaxies contain many red old stars and globular clusters - The white glow from these galaxies comes from a/g-type stars in the galactic disk - Galactic disks contain interstellar matter, and since the smaller the bulge means the larger the disk, the smaller the bulge also means more interstellar matter. - Star formation occurs in the spiral arms, therefore they appear blue - Galaxies do not have to have spiral arms to be classified as a spiral. They only need to have a galactic disk Barred Spirals - Characterized by a “bar” of stellar and interstellar matter passing through the center and into the disk. - The spiral arms project from the ends of the bar rather than the bulge - Categorized by the letters SB followed by a, b, and c once again depending on bulge size - Like the normal spirals, the tightness of the spiral pattern is correlated with the bulge size - Spirals and barred spirals are often difficult to tell apart Ellipticals - Elliptical galaxies have NO spiral arms and no obvious galactic disk - Have little internal structure - Stellar density is highest in the nucleus - Categorized with the letter E followed by a number 1-7 depending on how elliptical they look in the sky (circular are E0, most elliptical E7, etc.)

- Its appearance is difficult to measure - The largest elliptical galaxies (far larger than our galaxy), can hold trillions of stars, while dwarf ellipticals may contain fewer than a million stars - Contain little or no cool gas/dust therefore no obscuring dust lanes - No ongoing star formation, therefore no young stars--just red, old, low-mass stars - Some exceptions - Star orbits are irregular (no rotation, move in all directions) - Very VERY hot interstellar gas in the halo of elliptical galaxies which emit x-ray radiation - S0 galaxies: Objects that have a thin disk and flattened bulge but no gas/spiral arms Irregulars - Rich in interstellar matter and young/blue stars but lack a structure - Divided into two subclasses: Irr I and Irr II - Irr I galaxies look like misshapen spirals - Smaller than spirals but larger than the smallest ellipticals - Contain about 10^8 to 10^10 stars - Example of irregular galaxies: Magellanic Clouds, a part of Irr I galaxies that orbit the Milky Way. (24.2) THE DISTRIBUTION OF GALAXIES IN SPACE - Galaxies are not evenly distributed in space. We can use their uneven distribution to determine their appearance and evolutions Extending the Distance Scale - Some galaxies cannot be observed with the cepheid variable technique, therefore astronomers have tried to come up with another distance measurement--such as trying observations of standard candles, or intrinsically bright, easily recognizable astronomical objects whose luminosities are confidently known. - Comparing luminosity with apparent brightness yields distance - Standard candle must have a well-defined luminosity and must be bright enough to be seen at large distances in order to be useful. - Examples of standard candles: novae, emission nebulae, planetary nebulae, globular clusters, etc. - Best standard candles: planetary nebulae and type I supernovae - Astronomers have found a correlation between the rotational speeds and luminosities of spiral galaxies within close range of the milky way galaxy. - Tully-Fisher relation: correlation between a spiral galaxy’s luminosity and rotational speed (can derive luminosity from rotational speed) - The part of a nearby galaxy that is rotating away from us is redshifted, while the part of the galaxy rotating toward us is blueshifted (doppler effect). This motion results in a broadening of the radiation line. The faster the rotation, the more broadened the line is. Therefore by measuring the amount of broadening, we can determine the galaxy’s

rotation speed. This will tell us the luminosity with the tully-fisher relation. - 21-cm radiation is normally used for the line radiation. - Line measuring only works up to a distance of 200 Mpc. Clusters of Galaxies - All galaxies within the proximity of the milky way are called the local group, a new level of structure in the universe about the scale of our galaxy (includes spirals, dwarf irregulars and dwarf ellipticals). A group of galaxies held together by its own gravity - Galaxy cluster: a collection of galaxies held together by their mutual gravitational attraction (24.3) HUBBLE’S LAW - Galaxies and galaxy clusters move in a very ordered way Universal Recession - American astronomer Vesto Slipher found that nearly every spiral galaxy was receding from our galaxy because they were redshifted. We now know that with the exception of the closest galaxies to us, every galaxy moves away from us (recedes) in all directions - The greater the redshift, the more distance the galaxy has moved away from us - A few galaxies within our local group are blueshifted, meaning they are moving towards us, but this is due to their apparent motion within the cluster. - Hubble diagrams, which plot the distance against the recessional velocity, show that the rate at which a galaxy recedes is directly proportional to its distance from us (HUBBLE’S LAW) - Hubble’s law demonstrates that the universe itself is expanding over time. However, the galaxy clusters are not getting bigger and the galaxies/objects within these clusters are not drifting apart. The space between these clusters is expanding. - The universal recession described by the hubble diagram is called the hubble flow. - To distinguish recessional redshift from redshifts caused by the motion within an object (galactic orbits, etc.), the redshift resulting from the Hubble flow is called the cosmological redshift. Hubble’s Constant - Hubble’s constant: the constant of proportionality between recessional velocity and distance in Hubble’s law, represented by the symbol H0. - Once again, Hubble’s law states that the distance to an object is directly proportional to its recessional velocity - Hubble’s Law: Recessional velocity = H0 x distance - The value of Hubble’s constant is the slope of the Hubble diagram (recessional velocity divided by distance), and is equal to about 70 km/s/Mpc - The Hubble’s constant basically tells us the rate of expansion of the universe Top of the Distance Ladder - We can derive the distance to an object by dividing its recessional velocity by the hubble constant. This makes it the next rung in the distance ladder

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Many redshifted objects have very high recessional velocities, some even a fraction of the speed of light, meaning that as they travel through space, the length of their wavelength increases by a LOT, some even shifted all the way to the infrared part of the spectrum.

(24.4) ACTIVE GALACTIC NUCLEI - The galaxies that fall under the hubble classification scheme are called normal galaxies. Their luminosities range from about a million times that of the sun to more than a trillion solar luminosities. Galactic Radiation - Many bright galaxies do not fit into the normal category because their spectra are very different from normal galaxies. They luminosities are EXTREMELY large, and they are known as active galaxies. - Look like normal galaxies with optical viewing, but are seen as extremely luminous when viewed in other wavelengths. - Unlike normal galaxies, active galaxies’ spectra do NOT peak in visible light, therefore their radiation is known as nonstellar. - Most galaxies with nonstellar radiation are starburst galaxies, which were previously normal systems characterized by widespread episodes of star formation (likely due to interactions with neighbours.) - Active galaxy: a system whose abnormal activity is related to violent events occurring in or near the galactic nucleus. - 3 basic types of active galaxies: seyfert, radio, and quasars - Astronomers believe that the black hole system in our galaxy is what other galaxies, particularly active galaxies, have as well. Seyfert Galaxies - Discovered by Carl Seyfert - Seyfert galaxies are a class of objects whose properties lie between normal galaxies and very active galaxies. - They look like normal spiral galaxies (have a galactic disk and spiral arms), and produce about the same level of visible radiation as spiral galaxies. However, most of their energy is emitted from their galactic nucleus (10,000x that of our nucleus). - Emitraidation in the form of multiple wavelengths, from infrared to x-rays. However, most of them (75%) radiate in the infrared part of the spectrum. - Very broad spectral lines demonstrate very rapid motion in the nuclei. However, some seyferts have no broad lines at all. - The energy emission of seyferts varies rapidly over time, leading us to believe that the source of energy emissions in seyferts are very compact. Radio Galaxies - Active galaxies that emit most of their radiation in the radio part of the spectrum. - As opposed to seyferts, whose energy is emitted by the active nucleus, radio galaxies emit energy through two huge extended regions called radio lobes, roundish clouds of

gas spanning about half a Mpc and are not detectable in visible light. - Example of a radio galaxy: Centaurus A - Radio lobes are roughly symmetrical, jutting out from the center of the galaxy (may contain material from the nucleus) - The angle at which we view a radio galaxy determines both the type of galaxy we see and the type of radiation we see. Looking at the jets of a radio galaxy head on will cause it to appear to us as a core-dominated system. Also, if the viewer is looking at this jet’s radiation head on, it will appear very intense and doppler shifted toward shorter wavelengths (this is called a blazar) and the radiation is seen as x-rays or gamma rays. Quasars - In earlier times, radio sources were detected without any corresponding visible object. We did not know what was emitting this radiation. Eventually, as astronomers were attempting to find a visible source, they found a faint blue star at the location of a source and obtained its spectrum. Its spectrum had unusually broad emission lines, and was unable to be interpreted. Eventually these astronomers found that the spectrum had been redshifted, over 16 percent! This means its recession velocity is over 48,000 km/s! - Despite it being difficult to see them in optical light, these objects are the most luminous in the universe, shining over 20 trillion times brighter than the sun! - These objects are NOT stars, and became known as quasi-stellar radio sources or quasars. - Most do not emit in the radio part of the spectrum, but rather the optical and infrared (24.5) THE CENTRAL ENGINE OF AN ACTIVE GALAXY - Seyferts, radio galaxies, and quasars (and also normal galactic nuclei) share a common energy-generation mechanism - The have high luminosities - Their energy emission is most nonstellar - Their energy output can be very variable - They may exhibit jets and other signs of explosive activity - Their optical spectra may show broad emission lines (rapid internal motion) - Their activity may be associated with interactions between galaxies Energy Production - The main system used in astronomy for energy production in galaxies is a central supermassive black hole with an accretion disk of matter swirling inward, heating up, and releasing HUGE amounts of energy. - In an active galaxy, the origin of the accreted gas is stars and clouds of interstellar gas (probably diverted into the galactic center by an interaction with a neighbouring galaxy) that come too close to the black hole and are destroyed by its gravity - Figure 24.31!!! - Accretion efficiently converted the infalling interstellar gas into electromagnetic radiation

(energy). About 10-20% of the total gas radiates away before falling into the black hole and being lost forever. - Average size galaxies (billion solar masses) consumes about 1 solar mass of gas per decade. - These central galactic black holes are very compact and their accretion disks are rather small. - Broadening of spectral lines seen in active nuclei are often caused by rapid orbital motion of the gas in the black hole's gravity. - Commonly seen as parts of the accretion disk are jets, which are material (electrons and protons) blasted into space from the inner regions of the disk. May be formed by strong magnetic fields produced within the accretion disk, which accelerates particles to nearly the speed of light and ejects them into space Energy Emission - Theory says that the radiation emitted by the accretion disk should be in a variety of different wavelengths, however, it seems this radiation is absorbed and reemitted at longer wavelengths by nearby matter. Astronomers think this matter is a fat, donut-shaped ring of gas/dust surrounding the inner accretion disk (where this energy is produced) - If our line of sight is not between this donut of gas and the black hole then we see the unchanged radiation. If the donut is obstructing our view, we see infrared radiation. - Another theory of how the energy is reprocessed is that when a charged particle encounters a magnetic field, it spirals around the magnetic field lines, and as the particle whirls around, it emits radiation (synchrotron radiation) which is nonthermal. - Eventually, this jet is slowed/stopped by the interstellar medium, the flow becomes turbulent, and the magnetic field gets tangled, resulting in a radio lobe radiating synchrotron radiation....