Chapters 23-26; 28
Chapters 23-26; 28 Chapter 23 23-1— The Sun is located in the disk of our Galaxy, about 8000 parsecs from the galactic center. •
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A Galaxy is an immense collection of stars and interstellar matter, far larger than a star cluster.
Locating the Sun Within the Galaxy: Early attempts •
We have an edge-on view from inside the pancake-like disk of our own Milky Way Galaxy, which is why the Milky Way appears as a band around the sky.
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Until the twentieth century, the prevailing opinion was that the Sun and planets lie at the Galaxy’s center. One of the first to come to this conclusion was the eighteenth-century English astronomer William Herschel. Who discovered the planter Uranus and was a pioneering cataloger of binary star systems.
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Herschel found approximately the same density of stars all along the Milky Way. Therefore, he concluded that we are at the center of our Galaxy
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According the Kapteyn, the Milky Way is about 17 kpc (17 kiloparsecs = 17,000 parsecs = 55,000 light-years) in diameter, with the Sun near its center.
The Problem: Interstellar Extinctions •
The reason for their mistake was discerned in 1930 by Robert J. Trumpler of Lick Observatory.
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Like the stars themselves, dust is concentrated in the plane of the Galaxy. As a result, it obscures our view within the plane and makes distant objects appear dim, an effect called interstellar extinction.
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Thanks to interstellar extinction, Herschel and Kapteyn were actually seeing only the nearest stars in the Galaxy. Hence, they had no idea of either the enormous size of the Galaxy or the vast number of stars concentrated around the galactic center.
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To find our location in the Galaxy, we need to locate bright objects that are part of the Galaxy but lie outside its plane in un-obscured regions of the sky.
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The Breakthrough: Globular Clusters •
Globular Cluster, a class of star clusters associated with the Galaxy but which lie outside its plane.
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Cepheid variables are pulsating stars that vary periodically in brightness.
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A family of pulsating stars closely related to Cepheid variables called RR Lyrae variables.
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The longer a Cepheid’s period, the greater its average luminosity
The light curve of an RR Lyrae variable is similar to that of a Cepheid, but RR Lyrae variables have shorter pulsation periods and lower peak luminosities.
Modern-Day Measurements •
Galactic nucleus, the center of our Galaxy
23-2—Observations at non-visible wavelengths reveal the shape of the Galaxy •
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In other words, the longer the wavelength, the farther radiation can travel through interstellar dust without being scattered or absorbed.
Exploring the Milky Way in the Infrared •
The dust emits radiation predominantly at wavelengths from about 30 to 300 nanometers. These are called far-infrared wavelengths, because they lie in the part of the infrared spectrum most different in wavelengths from visible light.
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Near-infrared wavelengths, that is, short wavelengths closer to the visible spectrum.
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The disk of our Galaxy is about 50 kpc (160,000 ly) in diameter and about 0.6 kpc (2000 ly) thick.
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The center of the Galaxy is surrounded by distribution of stars, called the central buldge, which is about 2 kpc (6500 ly) in diameter.
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The spherical distribution of globular clusters traces the halo of the Galaxy.
The Milky Way’s Distinct Stellar Population •
It is estimated that our Galaxy contains 200 billion (2 x 10^11) stars.
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The globular clusters in the halo are composed of old, metal-poor, Population II stars.
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Most halo stars are single Population II stars in isolation, called high-velocity stars because of their high speed relative to the Sun. These ancient stars orbit the Galaxy along paths tilted at random angles to the disk of the Milky Way, as do the globular clusters.
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The stars in the disk are mostly young, metal-rich, Population I stars like the Sun. The disk of a galaxy like the Milky Way appears bluish because its light is dominated by radiation from hot O and B main-sequence stars.
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By contrast, O & B stars are present in the halo, which implies that star formation ceased there long ago.
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The central buldge contains both Population I stars and metal-poor Population II stars.
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The central bulge looks yellowish or reddish because it contains many red giants and red super giants, but does not contain luminous, short-lived, blue O or B stars. Hence, there cannot be ongoing star formation in the central buldge.
23-3—Observations of cold hydrogen clouds and star-forming regions reveal that our galaxy has spiral arms •
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Spiral arms, spiral-shaped concentrations of gas and dust that extend outward from the center in a shape reminiscent of a pinwheel.
Mapping Hydrogen in the Milky Way •
By looking for concentrations of hydrogen gas, we should be able to detect important clues about the distribution of matter in our Galaxy
Unfortunately, ordinary visible-light telescopes are of little use in this quest, because hydrogen atoms can only emit visible light if they are first excited to high energy levels.
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What makes it possible to map out the distribution of hydrogen in our Galaxy is the even cold hydrogen clouds emit radio waves.
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The hydrogen in these clouds is neutral—that is, not ionized—and is called H I
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In addition to having mass and charge, particles such as protons and electrons possess a tiny amount of angular momentum (that is, rotational motion) commonly called spin.
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If the spin of the electron changes its orientation from the higher-energy configuration to the lower-energy one—called spin-flip transition—a photon is emitted.
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The photon emitted in a spin-flip transition between these configurations has only a small energy, and thus its wavelength is a relatively long 21 cm—a radio wavelength.
Our Sun lies near the edge of an irregularly shaped region within which the interstellar medium is very thin but at very high temperatures (about 10^6 K) is called the Local Bubble.
Mapping the Spiral Arms and the Central Bulge •
Observations demonstrate that our Galaxy has at least four major spiral arms as well as several short arm segments.
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The central bulge of the Milky Way is not spherical, but elongated like a bar.
23-4—The rotation of our Galaxy reveals the presence of dark matter •
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Measuring How the Milky Way Rotates •
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Most of the mass of the Galaxy is in the form of dark matter, a mysterious sort of material that emits not light at all.
What keeps a star in its orbit around the center of the Galaxy is the combined gravitational force exerted on it by all of the mass (including stars, gas, and dust) that lies within the star’s orbit.
The Sun’s Orbital Motion and the Mass of the Galaxy •
Traveling at 790,000 km/h, it takes the Sun about 220 million years to complete one trip around the Galaxy.
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The mass of the matter inside the Sun’s orbit is about 9.0 x 10^10 (90 billion solar masses)
Rotation Curves and the Mystery of Dark Matter •
Rotation curve, a graph of the speed of galactic rotation measured outward from the galactic center.
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The Galaxy’s rotation curve is quite flat, indicating roughly uniform orbital speed well beyond the visible age of the galactic disk.
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The total mass of our Galaxy could exceed 10^12 solar masses or more, of which about 10% is in the form of stars.
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The unseen material, which is by far the predominant constituent of our Galaxy, is called dark matter (90%).
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Observations of star groupings outside the Milky Way suggest that our Galaxy’s dark matter forms a spherical halo centered on the galactic nucleus, like the halo stars and globular clusters.
Dark Matter Speculations •
One proposal is that the dark matter halo is composed, at least in part, of dim objects with masses less than 1 solar mass
These objects, which could include brown dwarfs, white dwarfs, or black holes, are called massive compact halo objects (MACHOs).
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The MACHOs gravity acts like a lens that focuses the light from the star. This effect called microlensing, makes the star appear to brighten substantially for a few days.
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One type of neutrino can transform into another, and these transformations can only take place if neutrinos have mass. While we don’t yet know the masses of these neutrinos have, we have come to understand they are unlikely to account for halo dark matter.
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Another possibility that has been proposed is a new class of subatomic particle called weakly interacting massive particles (WIMPs).
23-5—Spiral arms are caused by density waves that sweep around the Galaxy -
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The Winding Dilemma •
As we have seen, stars, gas, and dust all orbit the galactic center with approximately the same speed.
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Galaxy’s age is thought to be 13.5 billion (1.35 x 10^10) years
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Theory suggests that the Milky Way’s spiral arms ought to have disappeared by now. The fact that they have not is called the winding dilemma.
The Density-Wave Model
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In the 1940s, the Swedish astronomer Bertil Lindblad proposed that the spiral arms of a galaxy are actually a pattern that moves through the Galaxy like ripples on water.
This idea was elaborated on greatly in the 1960s by the American astronomers Chia Chiao Lin and Frank Shu.
They picture a pattern of density waves sweeping around the Galaxy. These waves make matter pile up in the spiral arm, which are the crest of the waves.
The Self-Propagating Star-Formation Model •
We do indeed see many so-called grand-design spiral galaxies, with this, graceful, and well-defined spiral arms.
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Flocculent spiral galaxies, the spiral arms are broad, fuzzy, chaotic, and poorly defined.
To explain such flocculent spirals, M. W. Mueller and W. David Arnett in 1976 proposed a theory of self-propagating star formation. Imagine that star formation begins in a dense interstellar cloud within the disk of a galaxy that does not yet have spiral arms. As soon as hot, massive stars form, their radiation and stellar winds compress nearby matter, triggering the formation of additional stars in that gas. When massive stars become supernovae, they produce shock waves that further compress the surrounding interstellar medium, thus encouraging still more star formation.
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Self-propagating star formation tends to produce flocculent spiral galaxies that have a chaotic appearance with poorly defined spiral arms.
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In the density wave model, star formation caused by the spiral arms; in the self-propagating star formation model, by contrast, the spiral arms are caused by star formation.
23-6—Infrared, radio, and X-ray observations are used to probe the galactic center •
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At the center of this empire of light, however, lies the darkest of all objects in the universe—a black hole millions of times more massive than the Sun.
Saggittarius A*: Heart of Darkness •
Sagittarius A* (say “A star”), lies at the very center of the Galaxy.
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High resolution infrared images using adaptive optics, shows hundred of stars crowded within 1 ly (0.3 pc) of Sagittarius A*.
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Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics & Andrea Ghez of UCLA have found a number of stars orbiting around Sagittarius A* at speeds in excess of 1500 km/s
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An object this massive and this compact can only be one thing: a supermassive black hole.
X-rays from Around a Super-massive Black Hole •
The X-ray flares are presumably emitted by blobs of material that were compressed and heated as they fell into the black hole.
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The X-rays from Sagittarius A* are relatively feeble, which suggests that the super-massive black hole is swallowing only relatively small amounts of material.
Chapter 24: Galaxies •
Only about 10% of a typical galaxy’s mass emits radiation of any kind; the remainder is made up of the mysterious dark matter.
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Just as most stars are found within galaxies, most galaxies are located in groups and clusters.
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Remarkably, remote clusters of galaxies are receding from us; the greater their distance, the more rapidly they are moving away.
This relationship between distance and recessional velocity, called the Hubble law, reveals that our immense universe is expanding
24-2—Hubble proved that the spiral nebulae are far beyond the Milky Way •
Straight forward calculations using modern data reveal that M31 (Andromeda galaxy) is some 750 kpc (2.5 million ly) from Earth.
24-3—Galaxies are classified according to their appearance •
Hubble classified galaxies into four broad categories based on their appearance. These categories form the basis for the Hubble classification, a scheme that is still used today.
The four classes of galaxies are:
1. Spirals; S 2. Barred spirals; SB 3. Elliptical; E 4. Irregulars; Irr. -
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Spiral Galaxies: Stellar Birthplaces •
Spiral galaxies are characterized by arched lanes of stars, just as is our own Milky Way Galaxy
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The lack of star formation also explains why the central bulges of spiral galaxies have a yellowish or reddish color; as a population of stars ages, the massive, luminous blue stars die off first, leaving only the longer-lived, lowmass red stars.
Barred Spiral Galaxies: Spirals with an Extra Twist •
In barred spiral galaxies, the spiral arms originate at the ends of a barshaped region running through the galaxy’s nucleus rather than from the nucleus itself.
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A bar will not develop if a galaxy is surrounded by a sufficiently massive halo of non-luminous dark matter.
Elliptical Galaxies: From Giants to Dwarfs •
Elliptical galaxies, so named because of their distinct elliptical shapes, have no spiral arms.
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Giant elliptical galaxies are about 20 times larger than an average galaxy
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Usually located near the middle of a large cluster of galaxies
For elliptical galaxies, studies of this kind show that star motions are quite random
This randomness in a very around elliptical is called isotropic, meaning “equal in all directions.”
In a flattened elliptical galaxy, the randomness of the stellar motion is anisotropic, which means that the range of star speeds is different in different directions.
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Hubble also identified galaxies that are midway in appearance between elliptical and the two kinds of spirals, denoted as S0 and SB0 galaxies (lenticular galaxies).
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Although they look elliptical, lenticular (“lens shaped”) galaxies have both a central bulge and a disk like spiral galaxies, but no discernible spiral arms.
Irregular Galaxies: Deformed and Dynamic •
Galaxies that do not fit into the scheme of spirals, barred spirals, and elliptical are usually referred to as irregular galaxies.
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Generally rich in interstellar gas and dust, and have both young and old stars.
Best known examples of Irr. I galaxies are the Large Magellanic Cloud and Small Magellanic Cloud.
Both these galaxies contain substantial amounts of interstellar gas.
Tidal forces exerted on these irregular galaxies by the Milky Way help to compress the gas, which is why both of them are sites of active star formation.
Irr. II galaxies, have asymmetrical, distorted shapes that seem to have been caused by collisions with other galaxies or by violent activity in their nuclei.
24-4—Astronomers use various techniques to determine the distance to remote galaxies -
Standard Candles: Variable Stars and Type Ia Supernovae •
To determine the distance to a remote galaxy, astronomers look instead for a standard candle—an object, such as a star, that lies within that galaxy and for which we know the luminosity.
Should have four properties: 1. They should be luminous, so we can see them out to great
distances.
2. We should be fairly certain about their luminosities, so we can be
equally certain of any distance calculated from a standard candle’ apparent brightness and luminosity. 3. They should be easily identifiable—for example, by the shape of
the light curve of a variable star. 4. They should be relatively common, so that astronomers can use
them to determine the distances to many different galaxies.
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Distance Determination without Standard Candles •
One was discovered in the 1970s by the astronomers Brent Tully and Richard Fisher.
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One class of standard candles that astronomers have used beyond 30 Mpc is Type Ia supernovae
They found that the width of the hydrogen 21-cm emission line of a spiral galaxy is related to the galaxy’s luminosity. This correlation is the Tully-Fisher relation—the broader the line, the more luminous the galaxy.
In geometry, three points define a plane, so the relationship among size, motion, and brightness is called the fundamental plane.
The Distance Ladder •
Because one measuring technique leads us to the next one like rungs on a ladder, the techniques are referred to collectively as the distance ladder.
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One distance-measuring technique that has broken free of the distance ladder uses observations of molecular clouds called masers.
Acronym for “microwave amplification by stimulated emission of radiation.”
Just as an electric current stimulates a laser to emit an intense beam of visible light, nearby luminous stars can stimulate water molecules in a maser to emit intensely at microwave lengths.
This radiation is so intense that masers can be detected millions of parsecs away.
24-5—The Hubble law relates the redshifts of remote galaxies to their distances from Earth -
Redshift, Distance, and the Hubble Law •
The more distant a galaxy, the greater its redshift and the more rapidly it is receding from us.
In other words, nearby galaxies are moving away from us slowly, and more distant galaxies are rushing away from us much more rapidly
This universal recessional movement is referred to as the Hubble flow.
The redshift, denoted by the symbol z, is found by taking the wavelength observed for a given spectral line, subtracting from it the ordinary, unshifted wavelength of that line to get the wavelength difference, and then dividing that difference.
This relationship tells us that we are living in an expanding universe. In 1929, Hubble published this discovery, which is now known as the Hubble law.
Hubble law= 73 km/s/Mpc
The greater the redshift of a distant galaxy, the greater its distance.
24-6—Galaxies are grouped into clusters and superclusters •
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Galaxies are not scattered randomly throughout the universe but are found in clusters.
Clusters of Galaxies: Rich and Poor, Regular and Irregular •
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A cluster is said to be either poor or rich, depending on how many galaxies it contains.
Poor clusters, which far outnumber rich ones, are often called groups.
For example, the Milky Way Galaxy, the Andromeda Galaxy (M31), and large and small magellanic cloud belong to a poor cluster familiarly known as the Local Group.
The Virgo cluster, for example, is called an irregular cluster, because its galaxies are scattered throughout a sprawling region of the sky.
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Regular clusters have distinctly spherical appearance, with a marked concentration of galaxies at its center.
Superclusters: Clusters of Clusters of Galaxies •
Clusters of galaxies are themselves grouped together in huge associations called superclusters.
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Observations indicate that unlike clusters, superclusters are not bound together by gravity. That is, most clusters in each supercluster are drifting away from most of the other clusters in that same supercluster. Furthermore, the superclusters are all moving away from each other due to the Hubble flow.
Cosmic Voids and Sheets: The Distribution of Superclusters •
Voids are where exceptionally few galaxies are found (First discovered in 1978 in a pioneering study by Stephen Gregory and Laird Thompson at the Kitt Peak National Observatory).
24-7—Colliding galaxies produce starbursts, spiral arms, and other spectacular phenomena -
High-Speed Galaxy Collisions: Shredding Gas and Dust •
The best evidence of such collisions take place is that many rich clusters of galaxies are strong sources of X rays.
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Gentle Galactic Collisions and Starbursts •
In a less violent collision or a near-miss between two galaxies, the compressed interstellar gas may have more time to cool, allowing many protostars to form.
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This emission reveals the presence of substantial amounts of hot intracluster gas at temps between 10^7 & 10^8 K
Such collisions may account for starburst galaxies, which blaze with the light of numerous new born stars.
Tidal Forces and Galaxy Mergers •
When two galaxies merge, the result is a bigger galaxy.
If this new galaxy is located in a rich cluster, it may capture and devour additional galaxies, growing to enormous dimensions by galactic cannibalism.
24-8—Most of the matter in the universe is mysterious dark matter -
The Dark-Matter Problem and Rotation Curves •
The problem is to determine what form the invisible mass takes.
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A partial solution to this dark-matter problem, is it consist of hot, Xray-emitting gas within clusters of galaxies
“Seeing” Dark Matter with Gravitational Lensing •
A powerful source of gravity that distorts background images is called a gravitational lense.
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For this to work, the alignment between Earth, the massive galaxy, and a remote background light source must be almost perfect.
Because dark matter is so dominant, “ordinary” visible matter—including this book, the air that you breathe, all the planets and stars, and your own body— is in fact relatively rare. Thus the vast majority of mass in the universe is of completely unknown composition.
Chapter 25: Quasars and Active Galaxies •
Quasar—one of many thousands of distant objects whose luminosity is far too great to come from starlight alone.
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Active galaxies—galaxies whose tremendous luminosity can fluctuate substantially over a periods of months, weeks, or even days.
25-1—Quasars look like stars but have huge redshifts -
Quasars: High Redshifts, Extreme Distances •
Because of their strong radio emission and starlike appearance, quasars were originally dubbed quasi-stellar radio sources (shortened to quasars)
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“Radio-quiet” quasars were originally called quasi-stellar objects, or QSOs, to distinguish them from radio emitters.
Today, however, the term “quasar” is often used to include both types.
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Only about 10% of quasars are “radio-loud”
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More than 100,000 quasars are known.
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The universe has expanded at different rates a different times in the past
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Because there are no quasars with small redshifts, it follows that there are no nearby quasars.
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The absence of nearby quasars means that there have been no quasars for 800 million years.
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Quasars were a common feature of the universe in the distant past, but there are none in the present-day universe
25-2—Quasars are the ultraluminous centers of distant galaxies •
While quasars are not galaxies, we will see compelling evidence that the two are intimately related:
Quasars turn out to be ultraluminous objects located at the centers of remote galaxies.
Quasars and Their Luminosities
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Protection effect—whereby a distant quasar just happens to be in the same part of the sky as a nearby galaxy.
Chapter 26—Cosmology: The Origin & Evolution of the Universe Key Ideas
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The expansion of the Universe: the Hubble law describes the continuing expansion of space. The redshifts that we see from distant galaxies are caused by this expansion, not by the motions of galaxies through space.
The redshift of a distant galaxy is a measure of the scale of the universe at the time the galaxy emitted its light.
It is meaningless to speak of an edge or center to the universe or of what lies beyond the universe
The Cosmological Principle: Cosmological theories are based on the idea that on large scales, the universe looks the same at all locations and in every direction.
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The Big Bang: The universe began as in infinitely dense cosmic singularity that began its expansion in the event called the Big Bang, which can be described as the beginning of time. The observable universe extends about 14 billion ly in every direction from
Earth. We cannot see objects beyond this distance because light from these objects has not had enough time to reach us. During the first 10^-43 seconds after the Big Bang, the universe was too
dense to be described by the known laws of physics. •
Cosmic Background Radiation and the Evolution of the Universe: The cosmic microwave background radiation, corresponding to radiation from a blackbody radiation, corresponding to radiation from a blackbody at a temperature of nearly 3 K, is the greatly redshifted remnant of the hot universe as it existed about 380,000 years after the Big Bang. The background radiation was hotter and more intese in the past. During
the past 380,000 years of the universe, radiation and matter formed an opaque plasma called the primordial fireball. When the temperature of the radiation fell below 3000 K, protons and electrons could combine to form hydrogen atoms and the universe became transparent. The abundance of helium in the universe is explained by the high
temperatures in its early history •
The Geometry of the Universe: the curvature of the universe as a whole depends on how the combined average mass density compares to a critical density If CAMD is greater than CD, the density parameter omega-0 has a value
less than 1, the universe is open, and space is sphericle (w/ positive curvature) If CAMD is greater than CD the density parameter omega-0 has a value
less than 1, the universe open, and space is hyperbolic (w/ negative curvature) •
Cosmological Parameters and Dark Energy: Observation of temperature variations in the cosmic microwave background indicate that the universe is flat or nearly so, with a combined average mass density equal to the critical density. Observations of galaxy clusters suggest that the average density of matter in the universe is about .24 of the critical density. The remaining contribution to the average density.
Chapter 28: The Search for Extraterrestrial Life Organic Molecules in the Universe: All life on Earth, and presumably on other worlds, depends on organic (carbon-based) molecules. These molecules occur naturally throughout interstellar space.
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Life in the Solar System: Besides Earth, only two worlds in our solar system—the planet Mars and Jupiter’s satellite Europa—may have had the right conditions for the origin of life.
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The organic molecules needed for life to originate were probably brought to the young Earth by comets, meteorites, and interplanetary dust particles. Another likely source for organic molecules is chemical reactions in Earth’s primitive atmosphere. Similar processes may occur on other worlds.
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Mars once had liquid water on its surface, though it has none today. Life may have originated on Mars during the liquid water era.
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The Viking Lander spacecraft searched for microorganisms on the Martian surface, but found no conclusive sign of their presence. The Mars Science Laboratory is designed to assess the past and present suitability of Mars for microbial life.
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An ancient Martian rock that came to Earth as a meteorite shows circumstantial evidence that microorganisms once existed on Mars. Additional rock samples are needed for corroboration.
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Europa appears to have extensive liquid water beneath its icy surface. Future missions may search for the presence of life there. Radio Searches for Extraterrestrial Intelligence: Astronomers have carried out a number of searches for radio signals from other stars. No signs of intelligent life have yet been detected, but searches are continuing and using increasingly sophisticated techniques. •
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The Drake equation is a tool for estimating the number of intelligent, communicative civilizations in our Galaxy
Telescopes Searches for Earthlike Planets: A new generation of orbiting telescopes may be able to detect numerous terrestrial planets around nearby stars. If such planets are found, their infrared spectra may reveal the presence or absence of life.
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A Galaxy is an immense collection of stars and interstellar matter, far larger than a star cluster.
Locating the Sun Within the Galaxy: Early attempts •
We have an edge-on view from inside the pancake-like disk of our own Milky Way Galaxy, which is why the Milky Way appears as a band around the sky.
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Until the twentieth century, the prevailing opinion was that the Sun and planets lie at the Galaxy’s center. One of the first to come to this conclusion was the eighteenth-century English astronomer William Herschel. Who discovered the planter Uranus and was a pioneering cataloger of binary star systems.
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Herschel found approximately the same density of stars all along the Milky Way. Therefore, he concluded that we are at the center of our Galaxy
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According the Kapteyn, the Milky Way is about 17 kpc (17 kiloparsecs = 17,000 parsecs = 55,000 light-years) in diameter, with the Sun near its center.
The Problem: Interstellar Extinctions •
The reason for their mistake was discerned in 1930 by Robert J. Trumpler of Lick Observatory.
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Like the stars themselves, dust is concentrated in the plane of the Galaxy. As a result, it obscures our view within the plane and makes distant objects appear dim, an effect called interstellar extinction.
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Thanks to interstellar extinction, Herschel and Kapteyn were actually seeing only the nearest stars in the Galaxy. Hence, they had no idea of either the enormous size of the Galaxy or the vast number of stars concentrated around the galactic center.
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To find our location in the Galaxy, we need to locate bright objects that are part of the Galaxy but lie outside its plane in un-obscured regions of the sky.
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The Breakthrough: Globular Clusters •
Globular Cluster, a class of star clusters associated with the Galaxy but which lie outside its plane.
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Cepheid variables are pulsating stars that vary periodically in brightness.
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A family of pulsating stars closely related to Cepheid variables called RR Lyrae variables.
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The longer a Cepheid’s period, the greater its average luminosity
The light curve of an RR Lyrae variable is similar to that of a Cepheid, but RR Lyrae variables have shorter pulsation periods and lower peak luminosities.
Modern-Day Measurements •
Galactic nucleus, the center of our Galaxy
23-2—Observations at non-visible wavelengths reveal the shape of the Galaxy •
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In other words, the longer the wavelength, the farther radiation can travel through interstellar dust without being scattered or absorbed.
Exploring the Milky Way in the Infrared •
The dust emits radiation predominantly at wavelengths from about 30 to 300 nanometers. These are called far-infrared wavelengths, because they lie in the part of the infrared spectrum most different in wavelengths from visible light.
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Near-infrared wavelengths, that is, short wavelengths closer to the visible spectrum.
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The disk of our Galaxy is about 50 kpc (160,000 ly) in diameter and about 0.6 kpc (2000 ly) thick.
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The center of the Galaxy is surrounded by distribution of stars, called the central buldge, which is about 2 kpc (6500 ly) in diameter.
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The spherical distribution of globular clusters traces the halo of the Galaxy.
The Milky Way’s Distinct Stellar Population •
It is estimated that our Galaxy contains 200 billion (2 x 10^11) stars.
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The globular clusters in the halo are composed of old, metal-poor, Population II stars.
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Most halo stars are single Population II stars in isolation, called high-velocity stars because of their high speed relative to the Sun. These ancient stars orbit the Galaxy along paths tilted at random angles to the disk of the Milky Way, as do the globular clusters.
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The stars in the disk are mostly young, metal-rich, Population I stars like the Sun. The disk of a galaxy like the Milky Way appears bluish because its light is dominated by radiation from hot O and B main-sequence stars.
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By contrast, O & B stars are present in the halo, which implies that star formation ceased there long ago.
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The central buldge contains both Population I stars and metal-poor Population II stars.
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The central bulge looks yellowish or reddish because it contains many red giants and red super giants, but does not contain luminous, short-lived, blue O or B stars. Hence, there cannot be ongoing star formation in the central buldge.
23-3—Observations of cold hydrogen clouds and star-forming regions reveal that our galaxy has spiral arms •
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Spiral arms, spiral-shaped concentrations of gas and dust that extend outward from the center in a shape reminiscent of a pinwheel.
Mapping Hydrogen in the Milky Way •
By looking for concentrations of hydrogen gas, we should be able to detect important clues about the distribution of matter in our Galaxy
Unfortunately, ordinary visible-light telescopes are of little use in this quest, because hydrogen atoms can only emit visible light if they are first excited to high energy levels.
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What makes it possible to map out the distribution of hydrogen in our Galaxy is the even cold hydrogen clouds emit radio waves.
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The hydrogen in these clouds is neutral—that is, not ionized—and is called H I
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In addition to having mass and charge, particles such as protons and electrons possess a tiny amount of angular momentum (that is, rotational motion) commonly called spin.
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If the spin of the electron changes its orientation from the higher-energy configuration to the lower-energy one—called spin-flip transition—a photon is emitted.
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The photon emitted in a spin-flip transition between these configurations has only a small energy, and thus its wavelength is a relatively long 21 cm—a radio wavelength.
Our Sun lies near the edge of an irregularly shaped region within which the interstellar medium is very thin but at very high temperatures (about 10^6 K) is called the Local Bubble.
Mapping the Spiral Arms and the Central Bulge •
Observations demonstrate that our Galaxy has at least four major spiral arms as well as several short arm segments.
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The central bulge of the Milky Way is not spherical, but elongated like a bar.
23-4—The rotation of our Galaxy reveals the presence of dark matter •
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Measuring How the Milky Way Rotates •
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Most of the mass of the Galaxy is in the form of dark matter, a mysterious sort of material that emits not light at all.
What keeps a star in its orbit around the center of the Galaxy is the combined gravitational force exerted on it by all of the mass (including stars, gas, and dust) that lies within the star’s orbit.
The Sun’s Orbital Motion and the Mass of the Galaxy •
Traveling at 790,000 km/h, it takes the Sun about 220 million years to complete one trip around the Galaxy.
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The mass of the matter inside the Sun’s orbit is about 9.0 x 10^10 (90 billion solar masses)
Rotation Curves and the Mystery of Dark Matter •
Rotation curve, a graph of the speed of galactic rotation measured outward from the galactic center.
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The Galaxy’s rotation curve is quite flat, indicating roughly uniform orbital speed well beyond the visible age of the galactic disk.
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The total mass of our Galaxy could exceed 10^12 solar masses or more, of which about 10% is in the form of stars.
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The unseen material, which is by far the predominant constituent of our Galaxy, is called dark matter (90%).
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Observations of star groupings outside the Milky Way suggest that our Galaxy’s dark matter forms a spherical halo centered on the galactic nucleus, like the halo stars and globular clusters.
Dark Matter Speculations •
One proposal is that the dark matter halo is composed, at least in part, of dim objects with masses less than 1 solar mass
These objects, which could include brown dwarfs, white dwarfs, or black holes, are called massive compact halo objects (MACHOs).
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The MACHOs gravity acts like a lens that focuses the light from the star. This effect called microlensing, makes the star appear to brighten substantially for a few days.
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One type of neutrino can transform into another, and these transformations can only take place if neutrinos have mass. While we don’t yet know the masses of these neutrinos have, we have come to understand they are unlikely to account for halo dark matter.
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Another possibility that has been proposed is a new class of subatomic particle called weakly interacting massive particles (WIMPs).
23-5—Spiral arms are caused by density waves that sweep around the Galaxy -
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The Winding Dilemma •
As we have seen, stars, gas, and dust all orbit the galactic center with approximately the same speed.
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Galaxy’s age is thought to be 13.5 billion (1.35 x 10^10) years
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Theory suggests that the Milky Way’s spiral arms ought to have disappeared by now. The fact that they have not is called the winding dilemma.
The Density-Wave Model
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In the 1940s, the Swedish astronomer Bertil Lindblad proposed that the spiral arms of a galaxy are actually a pattern that moves through the Galaxy like ripples on water.
This idea was elaborated on greatly in the 1960s by the American astronomers Chia Chiao Lin and Frank Shu.
They picture a pattern of density waves sweeping around the Galaxy. These waves make matter pile up in the spiral arm, which are the crest of the waves.
The Self-Propagating Star-Formation Model •
We do indeed see many so-called grand-design spiral galaxies, with this, graceful, and well-defined spiral arms.
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Flocculent spiral galaxies, the spiral arms are broad, fuzzy, chaotic, and poorly defined.
To explain such flocculent spirals, M. W. Mueller and W. David Arnett in 1976 proposed a theory of self-propagating star formation. Imagine that star formation begins in a dense interstellar cloud within the disk of a galaxy that does not yet have spiral arms. As soon as hot, massive stars form, their radiation and stellar winds compress nearby matter, triggering the formation of additional stars in that gas. When massive stars become supernovae, they produce shock waves that further compress the surrounding interstellar medium, thus encouraging still more star formation.
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Self-propagating star formation tends to produce flocculent spiral galaxies that have a chaotic appearance with poorly defined spiral arms.
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In the density wave model, star formation caused by the spiral arms; in the self-propagating star formation model, by contrast, the spiral arms are caused by star formation.
23-6—Infrared, radio, and X-ray observations are used to probe the galactic center •
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At the center of this empire of light, however, lies the darkest of all objects in the universe—a black hole millions of times more massive than the Sun.
Saggittarius A*: Heart of Darkness •
Sagittarius A* (say “A star”), lies at the very center of the Galaxy.
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High resolution infrared images using adaptive optics, shows hundred of stars crowded within 1 ly (0.3 pc) of Sagittarius A*.
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Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics & Andrea Ghez of UCLA have found a number of stars orbiting around Sagittarius A* at speeds in excess of 1500 km/s
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An object this massive and this compact can only be one thing: a supermassive black hole.
X-rays from Around a Super-massive Black Hole •
The X-ray flares are presumably emitted by blobs of material that were compressed and heated as they fell into the black hole.
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The X-rays from Sagittarius A* are relatively feeble, which suggests that the super-massive black hole is swallowing only relatively small amounts of material.
Chapter 24: Galaxies •
Only about 10% of a typical galaxy’s mass emits radiation of any kind; the remainder is made up of the mysterious dark matter.
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Just as most stars are found within galaxies, most galaxies are located in groups and clusters.
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Remarkably, remote clusters of galaxies are receding from us; the greater their distance, the more rapidly they are moving away.
This relationship between distance and recessional velocity, called the Hubble law, reveals that our immense universe is expanding
24-2—Hubble proved that the spiral nebulae are far beyond the Milky Way •
Straight forward calculations using modern data reveal that M31 (Andromeda galaxy) is some 750 kpc (2.5 million ly) from Earth.
24-3—Galaxies are classified according to their appearance •
Hubble classified galaxies into four broad categories based on their appearance. These categories form the basis for the Hubble classification, a scheme that is still used today.
The four classes of galaxies are:
1. Spirals; S 2. Barred spirals; SB 3. Elliptical; E 4. Irregulars; Irr. -
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Spiral Galaxies: Stellar Birthplaces •
Spiral galaxies are characterized by arched lanes of stars, just as is our own Milky Way Galaxy
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The lack of star formation also explains why the central bulges of spiral galaxies have a yellowish or reddish color; as a population of stars ages, the massive, luminous blue stars die off first, leaving only the longer-lived, lowmass red stars.
Barred Spiral Galaxies: Spirals with an Extra Twist •
In barred spiral galaxies, the spiral arms originate at the ends of a barshaped region running through the galaxy’s nucleus rather than from the nucleus itself.
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A bar will not develop if a galaxy is surrounded by a sufficiently massive halo of non-luminous dark matter.
Elliptical Galaxies: From Giants to Dwarfs •
Elliptical galaxies, so named because of their distinct elliptical shapes, have no spiral arms.
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Giant elliptical galaxies are about 20 times larger than an average galaxy
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Usually located near the middle of a large cluster of galaxies
For elliptical galaxies, studies of this kind show that star motions are quite random
This randomness in a very around elliptical is called isotropic, meaning “equal in all directions.”
In a flattened elliptical galaxy, the randomness of the stellar motion is anisotropic, which means that the range of star speeds is different in different directions.
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Hubble also identified galaxies that are midway in appearance between elliptical and the two kinds of spirals, denoted as S0 and SB0 galaxies (lenticular galaxies).
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Although they look elliptical, lenticular (“lens shaped”) galaxies have both a central bulge and a disk like spiral galaxies, but no discernible spiral arms.
Irregular Galaxies: Deformed and Dynamic •
Galaxies that do not fit into the scheme of spirals, barred spirals, and elliptical are usually referred to as irregular galaxies.
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Generally rich in interstellar gas and dust, and have both young and old stars.
Best known examples of Irr. I galaxies are the Large Magellanic Cloud and Small Magellanic Cloud.
Both these galaxies contain substantial amounts of interstellar gas.
Tidal forces exerted on these irregular galaxies by the Milky Way help to compress the gas, which is why both of them are sites of active star formation.
Irr. II galaxies, have asymmetrical, distorted shapes that seem to have been caused by collisions with other galaxies or by violent activity in their nuclei.
24-4—Astronomers use various techniques to determine the distance to remote galaxies -
Standard Candles: Variable Stars and Type Ia Supernovae •
To determine the distance to a remote galaxy, astronomers look instead for a standard candle—an object, such as a star, that lies within that galaxy and for which we know the luminosity.
Should have four properties: 1. They should be luminous, so we can see them out to great
distances.
2. We should be fairly certain about their luminosities, so we can be
equally certain of any distance calculated from a standard candle’ apparent brightness and luminosity. 3. They should be easily identifiable—for example, by the shape of
the light curve of a variable star. 4. They should be relatively common, so that astronomers can use
them to determine the distances to many different galaxies.
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Distance Determination without Standard Candles •
One was discovered in the 1970s by the astronomers Brent Tully and Richard Fisher.
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One class of standard candles that astronomers have used beyond 30 Mpc is Type Ia supernovae
They found that the width of the hydrogen 21-cm emission line of a spiral galaxy is related to the galaxy’s luminosity. This correlation is the Tully-Fisher relation—the broader the line, the more luminous the galaxy.
In geometry, three points define a plane, so the relationship among size, motion, and brightness is called the fundamental plane.
The Distance Ladder •
Because one measuring technique leads us to the next one like rungs on a ladder, the techniques are referred to collectively as the distance ladder.
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One distance-measuring technique that has broken free of the distance ladder uses observations of molecular clouds called masers.
Acronym for “microwave amplification by stimulated emission of radiation.”
Just as an electric current stimulates a laser to emit an intense beam of visible light, nearby luminous stars can stimulate water molecules in a maser to emit intensely at microwave lengths.
This radiation is so intense that masers can be detected millions of parsecs away.
24-5—The Hubble law relates the redshifts of remote galaxies to their distances from Earth -
Redshift, Distance, and the Hubble Law •
The more distant a galaxy, the greater its redshift and the more rapidly it is receding from us.
In other words, nearby galaxies are moving away from us slowly, and more distant galaxies are rushing away from us much more rapidly
This universal recessional movement is referred to as the Hubble flow.
The redshift, denoted by the symbol z, is found by taking the wavelength observed for a given spectral line, subtracting from it the ordinary, unshifted wavelength of that line to get the wavelength difference, and then dividing that difference.
This relationship tells us that we are living in an expanding universe. In 1929, Hubble published this discovery, which is now known as the Hubble law.
Hubble law= 73 km/s/Mpc
The greater the redshift of a distant galaxy, the greater its distance.
24-6—Galaxies are grouped into clusters and superclusters •
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Galaxies are not scattered randomly throughout the universe but are found in clusters.
Clusters of Galaxies: Rich and Poor, Regular and Irregular •
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A cluster is said to be either poor or rich, depending on how many galaxies it contains.
Poor clusters, which far outnumber rich ones, are often called groups.
For example, the Milky Way Galaxy, the Andromeda Galaxy (M31), and large and small magellanic cloud belong to a poor cluster familiarly known as the Local Group.
The Virgo cluster, for example, is called an irregular cluster, because its galaxies are scattered throughout a sprawling region of the sky.
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Regular clusters have distinctly spherical appearance, with a marked concentration of galaxies at its center.
Superclusters: Clusters of Clusters of Galaxies •
Clusters of galaxies are themselves grouped together in huge associations called superclusters.
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Observations indicate that unlike clusters, superclusters are not bound together by gravity. That is, most clusters in each supercluster are drifting away from most of the other clusters in that same supercluster. Furthermore, the superclusters are all moving away from each other due to the Hubble flow.
Cosmic Voids and Sheets: The Distribution of Superclusters •
Voids are where exceptionally few galaxies are found (First discovered in 1978 in a pioneering study by Stephen Gregory and Laird Thompson at the Kitt Peak National Observatory).
24-7—Colliding galaxies produce starbursts, spiral arms, and other spectacular phenomena -
High-Speed Galaxy Collisions: Shredding Gas and Dust •
The best evidence of such collisions take place is that many rich clusters of galaxies are strong sources of X rays.
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Gentle Galactic Collisions and Starbursts •
In a less violent collision or a near-miss between two galaxies, the compressed interstellar gas may have more time to cool, allowing many protostars to form.
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This emission reveals the presence of substantial amounts of hot intracluster gas at temps between 10^7 & 10^8 K
Such collisions may account for starburst galaxies, which blaze with the light of numerous new born stars.
Tidal Forces and Galaxy Mergers •
When two galaxies merge, the result is a bigger galaxy.
If this new galaxy is located in a rich cluster, it may capture and devour additional galaxies, growing to enormous dimensions by galactic cannibalism.
24-8—Most of the matter in the universe is mysterious dark matter -
The Dark-Matter Problem and Rotation Curves •
The problem is to determine what form the invisible mass takes.
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A partial solution to this dark-matter problem, is it consist of hot, Xray-emitting gas within clusters of galaxies
“Seeing” Dark Matter with Gravitational Lensing •
A powerful source of gravity that distorts background images is called a gravitational lense.
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For this to work, the alignment between Earth, the massive galaxy, and a remote background light source must be almost perfect.
Because dark matter is so dominant, “ordinary” visible matter—including this book, the air that you breathe, all the planets and stars, and your own body— is in fact relatively rare. Thus the vast majority of mass in the universe is of completely unknown composition.
Chapter 25: Quasars and Active Galaxies •
Quasar—one of many thousands of distant objects whose luminosity is far too great to come from starlight alone.
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Active galaxies—galaxies whose tremendous luminosity can fluctuate substantially over a periods of months, weeks, or even days.
25-1—Quasars look like stars but have huge redshifts -
Quasars: High Redshifts, Extreme Distances •
Because of their strong radio emission and starlike appearance, quasars were originally dubbed quasi-stellar radio sources (shortened to quasars)
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“Radio-quiet” quasars were originally called quasi-stellar objects, or QSOs, to distinguish them from radio emitters.
Today, however, the term “quasar” is often used to include both types.
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Only about 10% of quasars are “radio-loud”
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More than 100,000 quasars are known.
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The universe has expanded at different rates a different times in the past
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Because there are no quasars with small redshifts, it follows that there are no nearby quasars.
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The absence of nearby quasars means that there have been no quasars for 800 million years.
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Quasars were a common feature of the universe in the distant past, but there are none in the present-day universe
25-2—Quasars are the ultraluminous centers of distant galaxies •
While quasars are not galaxies, we will see compelling evidence that the two are intimately related:
Quasars turn out to be ultraluminous objects located at the centers of remote galaxies.
Quasars and Their Luminosities
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Protection effect—whereby a distant quasar just happens to be in the same part of the sky as a nearby galaxy.
Chapter 26—Cosmology: The Origin & Evolution of the Universe Key Ideas
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The expansion of the Universe: the Hubble law describes the continuing expansion of space. The redshifts that we see from distant galaxies are caused by this expansion, not by the motions of galaxies through space.
The redshift of a distant galaxy is a measure of the scale of the universe at the time the galaxy emitted its light.
It is meaningless to speak of an edge or center to the universe or of what lies beyond the universe
The Cosmological Principle: Cosmological theories are based on the idea that on large scales, the universe looks the same at all locations and in every direction.
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The Big Bang: The universe began as in infinitely dense cosmic singularity that began its expansion in the event called the Big Bang, which can be described as the beginning of time. The observable universe extends about 14 billion ly in every direction from
Earth. We cannot see objects beyond this distance because light from these objects has not had enough time to reach us. During the first 10^-43 seconds after the Big Bang, the universe was too
dense to be described by the known laws of physics. •
Cosmic Background Radiation and the Evolution of the Universe: The cosmic microwave background radiation, corresponding to radiation from a blackbody radiation, corresponding to radiation from a blackbody at a temperature of nearly 3 K, is the greatly redshifted remnant of the hot universe as it existed about 380,000 years after the Big Bang. The background radiation was hotter and more intese in the past. During
the past 380,000 years of the universe, radiation and matter formed an opaque plasma called the primordial fireball. When the temperature of the radiation fell below 3000 K, protons and electrons could combine to form hydrogen atoms and the universe became transparent. The abundance of helium in the universe is explained by the high
temperatures in its early history •
The Geometry of the Universe: the curvature of the universe as a whole depends on how the combined average mass density compares to a critical density If CAMD is greater than CD, the density parameter omega-0 has a value
less than 1, the universe is open, and space is sphericle (w/ positive curvature) If CAMD is greater than CD the density parameter omega-0 has a value
less than 1, the universe open, and space is hyperbolic (w/ negative curvature) •
Cosmological Parameters and Dark Energy: Observation of temperature variations in the cosmic microwave background indicate that the universe is flat or nearly so, with a combined average mass density equal to the critical density. Observations of galaxy clusters suggest that the average density of matter in the universe is about .24 of the critical density. The remaining contribution to the average density.
Chapter 28: The Search for Extraterrestrial Life Organic Molecules in the Universe: All life on Earth, and presumably on other worlds, depends on organic (carbon-based) molecules. These molecules occur naturally throughout interstellar space.
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Life in the Solar System: Besides Earth, only two worlds in our solar system—the planet Mars and Jupiter’s satellite Europa—may have had the right conditions for the origin of life.
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The organic molecules needed for life to originate were probably brought to the young Earth by comets, meteorites, and interplanetary dust particles. Another likely source for organic molecules is chemical reactions in Earth’s primitive atmosphere. Similar processes may occur on other worlds.
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Mars once had liquid water on its surface, though it has none today. Life may have originated on Mars during the liquid water era.
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The Viking Lander spacecraft searched for microorganisms on the Martian surface, but found no conclusive sign of their presence. The Mars Science Laboratory is designed to assess the past and present suitability of Mars for microbial life.
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An ancient Martian rock that came to Earth as a meteorite shows circumstantial evidence that microorganisms once existed on Mars. Additional rock samples are needed for corroboration.
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Europa appears to have extensive liquid water beneath its icy surface. Future missions may search for the presence of life there. Radio Searches for Extraterrestrial Intelligence: Astronomers have carried out a number of searches for radio signals from other stars. No signs of intelligent life have yet been detected, but searches are continuing and using increasingly sophisticated techniques. •
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The Drake equation is a tool for estimating the number of intelligent, communicative civilizations in our Galaxy
Telescopes Searches for Earthlike Planets: A new generation of orbiting telescopes may be able to detect numerous terrestrial planets around nearby stars. If such planets are found, their infrared spectra may reveal the presence or absence of life.
