Module 14: A Universe of Galaxies: Fundamentals of Cosmology

The balloon universe

A balloon is an easily visualized analogy for an expanding universe. Like the universe, the balloon can expand with no center and no edges. The balloon's surface represents all three dimensions of space. (The inside and outside of the balloon have no meaning in this model.) As the universe expands, the distances between the galaxies grow, but not the galaxies themselves. Observers on each "galaxy" would see the other "galaxies" receding from them. Our expanding universe can be imagined as a balloon that is being blown up. Galaxies that were once close together are moving apart because space itself is expanding.

Barred Spiral Galaxy

A barred spiral galaxy has a bar of stars across the bulge. Our Milky Way Galaxy is now thought to be a barred spiral. NGC 1300 is 61 million l-y (18.7 Mpc) distant in Eridanus (the River) and was discovered by John Herschel in 1835.

Galaxy Cluster

A galaxy cluster is a group of galaxies that are mutually gravitationally bound. The two types of galaxy clusters are regular and irregular. Regular clusters are huge groupings of galaxies with large numbers of galaxies concentrated in their centers. They tend to contain thousands of galaxies and to have many bright elliptical and S0 type galaxies. Irregular clusters (often called groups) are not as centrally condensed with a somewhat nonspherical shape. They contain a few to hundreds of galaxies. Our Local Group is an example of an irregular cluster.

Lenticular Galaxies

A lenticular galaxy has a disk like a spiral galaxy but much less dusty gas (intermediate between spiral and elliptical). (A) NGC 2787 is a barred lenticular galaxy that lies about 25 million l-y (7.7 Mpc) away toward the constellation of Ursa Major. (Marcella Carollo-ETHZ/Hubble Heritage Team/ NASA) (B) M84 (NGC 4374) is a lenticular galaxy in the constellation Virgo about 60 million l-y (18.4 Mpc) distant. M84 was discovered by Charles Messier in 1781 and contains a 1.5 billion-solar-mass supermassive black hole at its core.

Irregular Galaxies

A miscellaneous class that includes small galaxies such as the LMC and SMC and "peculiar" galaxies that appear in disarray. Usually quite white and dusty, like the disks of spirals. Distant galaxies are more likely to be irregulars than nearby galaxies, which means they were more common when the universe was young. The Small Magellanic Cloud (NGC 292)is found in the constellation Tucana (the Toucan), some 200,000 l-y (61.3 kpc) distant from the Milky Way Galaxy. The color in this image represents different wavelengths of infrared light. Blue and cyan represent light at wavelengths of 3.4 and 4.6 microns, mostly emitted from stars. Green and red represent light at 12 and 22 microns, which is mostly light from warm dust.

Plot of Cepheid Variables

A period-luminosity curve plotted for Type I Cepheid variables. Cepheid variable stars with longer periods have greater luminosities. A period-luminosity plot for a selected group of Type I Cepheid variable stars. The longer the period of a Cepheid variable, the more luminous it is.

Hubble and Lemaître

American astronomer Edwin Hubble (1889-1953) and (left) Belgian priest and cosmologist Georges Lemaître (1894-1966) are shown here. Based on new evidence, both scientists should share credit for independently uncovering evidence for the expanding universe in the late 1920s. Lemaître is also credited with proposing a theory for the origin of the universe that would later be called the "Big Bang." The telescope on the left is the 100-inch (2.5-m) Hooker Telescope on Mt. Wilson in California. The 98-inch (2.5-m) Hubble Space Telescope is on the right.

V. M. Slipher and Redshifts

American astronomer V. M. (Vesto Melvin) Slipher (1875-1969) worked at the Lowell Observatory in the early part of the 20th century. In 1912, using a 24-inch refractor, he was the first to record the shift of spectral lines of galaxies, thereby becoming the discoverer of galactic redshifts. Later, Edwin Hubble and Milton Humason combined their measurements of galaxy distances with Slipher's galaxy redshift measurements and showed that distances to galaxies are proportional to their redshifts. This redshift-distance correlation, now known as Hubble's law, was formulated by Hubble and Humason in 1929 and became the basis for the modern model of the expanding universe.

Refined Distance Measure

Astronomers in 2008 used ESO's New Technology Telescope to measure echoes of light from Cepheid variable RS Pup as they bounced off a distant nebula. RS Pup changes in brightness by a factor of 5 every 41.4 days. Because Cepheids pulse at a rate proportion to their size, astronomers can measure how far they are by how often they pulsate. But this only tells you how far they are relative to one another. So astronomers use parallax to measure distance as well. This new technique works by watching how light moves through the nebula of material shed by RS Pup in the past. Since light has a finite speed, it takes time to pass by various blobs of gas and dust in the nebula. The researchers calculated the light curve from an event on the star, and then watched as that same curve passed different parts of the nebula. Calculations show the star is 6,500 ± 90 light-years (1,991 ± 27.6 pc) distant. It is the most accurate distance to a Cepheid ever captured, with a 1% level of precision. Cephedi variable RS Pup is 10 times more massive than the Sun, 200 times larger and puts out 15,000 times more light. In order to avoid a heavy saturation of the detector, RS Pup was positioned in the gap between the detectors.

Nature of the Spiral Nebulae—The Great Debate of 1920

Before Hubble, some scientists argued that "spiral nebulae" were entire galaxies like our Milky Way, while others maintained they were smaller collections of stars within the Milky Way. The famous Shapley-Curtis debate in 1920 on the scale of the universe set the stage for Hubble's discoveries. In 1924, Hubble, using Cepheids, established the distance to the Andromeda Galaxy, proving that "spiral nebulae" were true galaxies outside of the Milky Way Galaxy. Using this technique, Hubble estimated the distances to numerous other galaxies. This work, in turn, led him to his greatest discovery: the universe is expanding.

Cepheid Variable Stars

Cepheid variable stars are very luminous and can therefore been seen from great distances. Cepheid variables lie within the instability strip, where several types of variable stars are found. Cepheid variables are stars whose luminosity varies, with a rapid rise in brightness followed by a slower decline. The period of a Cepheid is related to the luminosity of the Cepheid by the period-luminosity relationship: the more luminous the Cepheid, the longer the period. This property makes Cepheids useful for obtaining distances. One determines the pulsation period and uses the relationship to get the luminosity. The apparent brightness of the star then yields distance. Cepheid variable stars are divided into two subclasses: Type I (metal rich) or classical Cepheids, and Type II (metal poor) or W Virginis Cepheids. The former are young massive stars, whereas the latter are older fainter stars. Classical Cepheids are supergiants of spectral class F6-K2. The stars are 5-20 times more massive than the Sun and up to 30,000 times more luminous. Types I and II Cepheids follow different period-luminosity relationships. The luminosity of Type II Cepheids is, on average, less than classical Cepheids by about 1.5 magnitudes.

Hubble Used the Best Technology

Completed in 1917, the 100-inch (2.5-m) Hooker telescope on Mount Wilson was the largest and most advanced telescope of its time. Using this new technology, Edwin Hubble settled the debate about the "spiral nebulae" by measuring the distance to the Andromeda Galaxy using Cepheid variables as standard candles. He then observed that most galaxies are moving away from each other, proving that the universe is expanding. After being mothballed for several years, in 1992, the 100-inch (2.5-m) Hooker telescope was refitted with adaptive optics and was again in use. Because of worsening light pollution in the Los Angeles area, the telescope was converted for visual observing in 2014 and is now open for public viewing.

Shapley-Curtis Debate Curtis's View: Spiral Nebulae are Galaxies

Curtis believed the Sun was nearly in the center of a much smaller galaxy-only 30,000 l-y in diameter. (Wrong on both counts.) He argued that the spiral nebulae were island universes (initially proposed by Johannes C. Kapteyn)-external galaxies located beyond the MWG. (Correct.) Curtis agreed with Shapley that the globular clusters fell outside the disk of the MWG, but he did not agree with Shapley's estimates of the actual distances to the clusters. (Curtis thought the Cepheids Shapley used to determine distances to the globular clusters were different from nearby Cepheids.) (Correct that globular clusters fall outside the disk of the MWG; incorrect on usefulness of Cepheids as standard candles.) Curtis held that the globular clusters were much closer, implying that the MWG itself was much smaller. (Correct in principle on the size of the MWG as it relates to the "universe," but he woefully underestimated its size.) Curtis set forth the theory that spiral nebulae were objects similar in form to the Milky Way. He showed evidence that the optical spectrum of a spiral nebula was indistinguishable from the spectrum of the MWG. (True, but spectral evidence at the time was not overwhelming.) Curtis also argued that novae observed in spirals prove they contain mature individual stars and thus must be large collections of stars like our own Milky Way. (True, but again at the time the evidence was not overwhelming.) The yellow star represents the Sun and our position in the Milky Way. The light-blue area represents the Milky Way composed of all the individual stars we can see. The purple ovals symbolize other large collections of stars, each one having a size similar to our own Milky Way.

Formation of a Protogalaxy

Dark matter and ionized gas collapse under gravity to form a protogalaxy. Gravity separates the protogalaxy into a core and halo. The baryons (ordinary matter) that constitute the gas can interact to lose energy and fall to the core of the protogalaxy. The dark matter, which only weakly interacts with regular matter, remains in the halo. Formation of a protogalaxy was likely influenced by the presence of dark matter in the early universe.

Disk & Spherical Components

Disk: stars of all ages, many gas clouds Spherical Component: bulge & halo, old stars, few gas clouds

Lookback time

Distances between faraway galaxies change while light travels. Astronomers think in terms of lookback time rather than distance. The lookback distance and time are depicted pictorially (not to scale). Because light travels at a finite speed, a photon emitted in the past from a distant galaxy will take a long time to travel to an observer on Earth located in the Milky Way Galaxy. During the photon's journey, spacetime itself expands and redshifts the photon. By the time the observer detects the photon, the distant galaxy has moved farther away from the observer than is indicated by the lookback time.

Supercluster Redshift

Distances of farthest galaxies are measured from redshifts. Distant galaxies as well as galaxy clusters and superclusters show significant redshifts in their spectra. Absorption lines in the optical spectrum of a supercluster of distant galaxies known as BAS11 (bottom), as compared to those in the optical spectrum of the Sun (top). The supercluster is receding at a speed of 21,000 km/s. The BAS designation is from a catalog of superclusters compiled by Bahcall and Soneira in 1982. BAS 11 is situated just north of a vast void located in the direction of the constellation Boötes (Herdsman) at a distance of about 1 billion l-y (306.4 Mpc).

Distant Sources Show Redshifts

Distant sources, whether they be supernovae (in galaxies), individual galaxies, or galaxy clusters, show spectral features that are redshifted. That is, these galaxies are all moving away from us. The redshift of a galaxy tells us its distance through Hubble's Law: distance = velocity / H0. The spectrum of a distant supernova is compared to a laboratory reference spectrum. The supernova in a distant galaxy is receding at a high velocity while the laboratory spectrum is at rest (v = 0). All of the lines in the supernova spectrum are redshifted.

Another distant galaxy

EGSY8p7 is a distant galaxy, with a spectroscopic redshift of z = 8.68, a light travel distance of 13.2 billion l-y from Earth. At an age of 13.2 billion years, it is observed as it existed 570 million years after the Big Bang. The light of galaxy EGSY8p7 appears to have been magnified twofold by gravitational lensing in the light's travel to Earth, enabling its detection. This galaxy had been the previous record holder as most distant.

Hubble's Law

Earlier work had shown that most of the "spiral nebulae" (galaxies) showed spectra that tended to be redshifted Hubble showed that redshifted spectra in galaxies meant that the galaxies were moving away from us. The greater the redshift, the greater is the distance By plotting velocity vs. distance for numerous galaxies Hubble showed that the more distant a galaxy, the greater the redshift and hence the faster it moves away from us. Hubble's law: v=Ho x d Ho is called Hubble's constant Hubble's law relates a galaxy's distance to its velocity of recession. The velocity of recession of a galaxy is equal to its distance times a constant known as Hubble's constant. For over 80 years, astronomers have been working to accurately determine H0.

Clusters of Galaxies

Elliptical galaxies are much more common in huge clusters of galaxies with hundreds to thousands of members. Massive galaxy cluster Abell 1689 is 2.459 billion l-y (753.7 Mpc) distant in Virgo. It has a redshift of z = 0.1832. Abell 1689 is one of the biggest and most massive galaxy clusters known and acts as a gravitational lens, distorting the images of galaxies that lie behind it. It has the largest system of gravitational arcs ever found. Abell 1689 shows over 160,000 globular clusters, the largest population ever found. There is evidence of merging gases in excess of 100 million K. The very large mass of this cluster makes it useful for the study of dark matter and gravitational lensing.

The Hubble Constant H0

Even in the 21st century, determining H0 is a difficult task. Though the range of values for H0 has narrowed greatly from Hubble's time, the HST Key Project data show that different methods return different values.

Transitional Galaxy Populations

Evidence from NASA's Galaxy Evolution Explorer supports the long-held notion that many galaxies begin life as smaller spirals before transforming into larger, elliptical-shaped galaxies. Examples of young, teenage and adult galaxies are shown here from left to right. The data making up these photos come from both GALEX and visible-light telescopes. Long-wavelength ultraviolet light is blue; short-wavelength ultraviolet light is green; and visible red light is red. NGC 300 (7 million l-y distant in Sculptor). Younger galaxies like this one tend to form more stars, and since new stars give off more ultraviolet and blue light, the galaxies appear blue. NGC 1316 (62 million l-y distant in Fornax) is an older elliptical. Older stars emit more red light, so this galaxy appears red.

M Number

French astronomer Charles Messier became famous for his discovery of 20 comets. In our time, he is better known for his catalog of Nebulae and Star Clusters. He developed his catalog of what we call deep space objects (DSO) to help comet hunters avoid mistaking bright clusters and galaxies for comets. In the 18th and 19th centuries, telescopes were of modest aperture, so comets (which look like fuzzy blobs before forming tails as they approach the Sun) were easy to confused with DSOs.

Islands of Stars

From the Hubble Ultra Deep Field, astronomers estimated that the observable universe contains 100 billion galaxies. Cosmology is the study of the overall structure and evolution of the universe as a whole. Galaxies are categorized as one of three major types: spiral galaxies elliptical galaxies irregular galaxies

Parts of the Spiral Galaxy

Grand design spiral galaxy NGC 4414 is located 62.3 million l-y (19.1 Mpc) distant in the constellation Coma Berenices. The image shows that the central regions of this galaxy, as is typical of most spirals, contain primarily older yellow and red stars. The outer spiral arms are considerably bluer due to ongoing formation of young blue stars. The arms are also very rich in clouds of interstellar dust, seen as dark patches and streaks silhouetted against the starlight.

Cepheid Variable V-1

HST image of Cepheid variable star named Hubble variable number one, or V1, which is located in the outer regions of the Andromeda Galaxy. Edwin Hubble used V1 to prove that Andromeda was external to the Milky Way Galaxy. Hubble's work showed that the universe is a much, much bigger place than anyone had ever imagined, His research led the way to our modern understanding of the universe.

Galatic Redshifts

High z-number (z > 0.1) redshifts seen in distant galaxies are the result of the cosmological or Hubblered shift (rather than the Doppler Effect). Cosmological redshift is based on general relativity and is mainly a result of the expansion of space. This means that the farther away a galaxy is from us, the more the space has expanded in the time since the light left that galaxy, so the more the light has been stretched, the more redshifted the light is, and so the faster it appears to be moving away from us. As the universe expands, photons of radiation are stretched in wavelength, giving rise to the cosmological redshift.

Which kind of stars are best for measuring large distances?

High-luminosity stars

Why does ongoing star formation lead to a blue-white appearance?

Hot, short-lived blue stars outshine others.

Hubble's Classification System

Hubble invented a system for classifying galaxies that remains widely used. His system is often referred to as the "Hubble Tuning Fork." E for elliptical + a number (larger the number, the flatter the galaxy). S for spiral + B if barred + a, b, c to designate size of bulge and dustiness of disk (increasing a to c). S0 for lenticular galaxies to designate their intermediate position between ellipticals and spirals. Irr for irregular galaxies. The Hubble classification does not represent the evolutionary sequence of galaxies (as he once thought). NGC 1566 is 40 million l-y (12.3 Mpc) distant in the constellation of Dorado (Dolphinfish or Swordfish). NGC 1566 is an intermediate spiral galaxy: it does not have a well-defined bar-shaped region of stars at its center—like barred spirals—nor is it quite an unbarred spiral either.

Age of the Universe

Hubble's constant tells us the age of the universe because it relates velocities and distances of all galaxies. Based on the value of Hubble's constant, the age of the universe is somewhere between 12 and 15 billion years. This value has been refined using additional observational techniques and is now (March 2013) set at 13.798 billion years with a margin of error of 0.27% (± 0.037 billion years, which is 37 million). Lookback time is the difference between the current age of the universe and the age of the universe when the light left the object. An object's lookback time is directly related to its redshift.

Henrietta Swan Leavitt

In 1912, Henrietta Swan Leavitt discovered the period-luminosity relation in a class of intrinsically bright variable stars—the Cepheids. The brighter the star, the longer is its period. Soon after her discovery, Danish astronomer Ejnar Hertzsprung realized that, once calibrated, the relation could be used to estimate the intrinsic brightness of a star solely on the basis of its period. In 1926, Edwin Hubble discovered Cepheids in the Andromeda Galaxy (M31) and proved that the spiral nebulae were actually galaxies—"island universes"—that reside outside of our galaxy and are millions of light years away. In 2012, working in the near-infrared (which greatly reduces the effect of dust present in the Milky Way), Riess et al. used 68 Cephedis to determined a refined distance to M31 of 2,450,000 light-years (751 kpc).

The Hubble Tuning Fork

In 1936, Edwin Hubble created a morphological classification scheme for galaxies that is often called the "Hubble Tuning Fork" because of its shape. Hubble considered and then abandoned the idea that this method of classification also represents how galaxies evolve.

Hubble Finds Farthest Galaxy

In 2016, HST astronomers, studying the Ursa Major region from the Great Observatories Origins Deep Survey (GOODS), have measured the distance to the farthest galaxy ever seen. The survey field contains tens of thousands of galaxies stretching far back into time. Galaxy GN-z11, shown in the inset, is seen as it was 13.39 billion years in the past, just 400 million years after the Big Bang, when the universe was only 3% of its current age. GN-z11 has a spectroscopic redshift of z = 11.09, which corresponds to a proper distance of approximately 32 billion l-y (9.8 billion pc) from Earth. GN-z11's light travel distance measurement is 13.39 billion l-y (4.1 billion pc). The galaxy is ablaze with bright, young, blue stars, but looks red in this image because its light has been stretched to longer spectral wavelengths by the expansion of the universe.

Discovering the Redshift

In the early 20thcentury, Hubble and some of his contemporaries noticed that certain lines in the spectra of galaxies appeared shifted toward the red end of the spectrum. Further study indicated that the farther away the galaxy, the more the spectral line was redshifted. Understanding the relationship between distance and redshift required more research during the 1920s. In the 1920s, Hubble and others began to understand the relationship between redshift and distance—that the amount a spectral line has shifted is directly proportional to its recessional velocity. Hubble is generally given credit for this discovery (though others had found the same relationship) because he verified the finding using several methods and published the results in widely read journals. In 1931, the published reports of Hubble and Humason, which extended Hubble's earlier work, firmly established what is now referred to as Hubble's law.

The Universe Has No Center and No Edge

In this example of a very smalluniverse containing only 48 stars, a spaceship flying among these stars cannot find the edge of this universe. If the ship exits on one side of the universe, it reemerges on the other side. The people in the spaceship see an infinite number of stars all around them. This universe has no boundary and no center. This micro universe illustrates what is true with our very large real universe—there is no way for us to determine either a physical boundary or edge or where the center is.

"First" Galaxies Imaged by Spitzer

Initially discovered by the Hubble Space Telescope, two distant, early galaxies ("964" and "1417") have been imaged in the infrared by the Spitzer Space Telescope. (See Fig. 14-14.) The two galaxies are seen just 700 million years after the Big Bang when the universe was 5 percent of its current age. The two galaxies were between 50 and 300 million years old and had masses about 100 times less than our full-grown Milky Way. The ages and masses suggest they were already in place much earlier, around 500-600 million years after the Big Bang. They could be among the first galactic systems formed in the universe, shortly after the first stars formed. The first 300-400 million years after the Big Bang is known to astronomers as "The Dark Ages," because the universe initially had no stars and then as stars formed, clouds of neutral hydrogen gas (the intergalactic medium) obscured early stars and galaxies from our view.

M81/M82 Galaxies

Just off the bowl of the Big Dipper, lie two bright galaxies, M81/M82, that are accessible to binoculars and small telescopes. They form the nucleus of a small group of galaxies, which at a distance of around 8 million light years (2.45 Mpc), may be the nearest group of galaxies beyond the "Local Group." (A) M82 ("Bode's Galaxy"), 11.6 million l-y (3.6 Mpc) distant, is a classic spiral galaxy. (B) M81 ("Cigar Galaxy"), 12 million l-y (3.7 Mpc) distant, is an irregular starburst galaxy. (Both images from NASA/ESA/The Hubble Heritage Team) (C) Both galaxies can be seen in the same field of view in a small telescope. (D) A finder chart for M81/M82 shows their position just off of the Big Dipper's bowl.

Ellipical Galaxy

Lack a significant disk component. Only have a spheroidal component. Also known as spheroidal galaxies. Contain very little cool gas and dust.More common in clusters. IM is mostly low-density, hot, X-ray-emitting gas (like the gas in bubbles and superbubbles). More common among small galaxies—dwarf elliptical galaxies—and are often found near large spiral galaxies. 10 dwarf elliptical galaxies belong to the Local Group. Elliptical galaxies present only a spheroidal component, with virtually no disk component. Red-yellow color indicates older star population. Elliptical galaxy M87 is located at the center of the nearby Virgo cluster of galaxies, 53.5 million l-y (16.4 Mpc) distant. This galaxy spans a diameter of 120,000 l-y but is spherical rather than flat like spiral galaxies. The mass of M87 is estimated at 2.4 billion solar masses. M87 contains one of the largest supermassive black holes discovered, with a mass 3.5-7.2 billion solar masses. An elliptical galaxy is all spheroidal component, with virtually no disk component. M86 is found in the constellation Virgo and is a member of the Virgo cluster of galaxies. M86 is classed as either an elliptical (type E3) or a lenticular galaxy (type S0). M86 is 60 million l-y (18.4 Mpc) distant. M87, a giant peculiar elliptical galaxy 53.5 million l-y (16.4 Mpc) distant in Virgo. (Overlay) Stellar motions are more random in elliptical galaxies.

Inverse-Square Law

Luminosity passing through each sphere is the same.Area of sphere: 4π (radius)<2Divide luminosity by area to get brightness. As light moves away from a source, it spreads out over an area that is the square of the distance traveled.

Measuring Galactic Distance-1

Measuring distances to galaxies is extremely challenging. We can measure distances to the farthest galaxies to an accuracy of within 15%. 1st distance link: radar measurements of the solar system. 2nd distance link: parallax measurements to nearby stars. A standard candle is a known standard luminosity. Standard candles are used beyond 1,600 light-years (0.49 kpc) (limit of the Hipparchos satellite) for which we can measure distances by parallax. To measure distances beyond 1,000 l-y (0.306 kpc), we need standard candles brighter than stars like the Sun. Knowing the true distance to a star (or cluster of stars) via parallax, we can then use the luminosity-distance formula to calibrate the true luminosities. Main-sequence fitting determines distances by comparing main sequences in different star clusters. Main-sequence fitting works well for measuring distances to star clusters throughout the MWG but not for measuring the distances to other galaxies. The most useful bright stars that can function as long distance standard candles are Cepheid variables. Cepheids are pulsating variable stars that follow a simple period-luminosity relation: The longer the time period between peaks in brightness, the greater the luminosity of the Cepheid variable star. There are actually 2 types of Cepheids, with each type obeying a different period-luminosity relation.HST has permitted use of Cepheids out to about 55 million l-y (16.85 Mpc), which is the distance to the Virgo Cluster. In 2012, Cepheid variables in the Milky Way and LMC were measured in the infrared by Sptizer and resulted in a value for H0 of 74.3 ± 2.1 km/s/Mpc. The Tully-Fisher relation holds that the faster a spiral galaxy rotates, the more luminous it is. Both luminosity and rotation speed depend on the galaxy's mass; a galaxy's luminosity depends on the number of stars it contains, which is related to the total amount of matter within it, and the total amount of matter determines a galaxy's rotation speed. Cepheids, in turn, help astronomers calibrate even brighter standard candles—supernovae. White dwarf supernovae(Type Ia) make excellent long-distance standard candles. Using Cepheid variables to calibrate white dwarf supernovae indicate Hubble's constant lies between 65 and 75 km/s/Mpc. A 2016 determination gives the Hubble constant as 73.24 ± 1.74 km/s/Mpc. The concept of the standard candle. If we know how luminous the candle is and how bright it appears, we can calculate how far away it is using the inverse-square law for light.

A Balloon is a Good Analog to Our Expanding Universe

One example of something that expands but has no center or edge is the surface of a balloon. As the balloon expands, the dots, which represent galaxies in our expanding universe, move apart.

Spiral galaxies and elliptical galaxies both have spheroidal components (bulges and halos). What component do they not share?

Only spirals have a disk component.

Three galaxy types

Spiral galaxies (S) M101 flat white disks with yellowish central bulges spiral arms with a mix of cool gas and dust and regions of hot, ionized gas Elliptical galaxies (E) MGC1132 redder, more rounded, and often longer in one direction than the other contain very little cool gas and dust, though often contain hot, ionized gas Irregular galaxies (Irr) Irr appear neither disk-like or rounded often blue and white with many hot, new stars

Spiral Galaxies-2

Others have the disk shape but lack spiral arms: these are called lenticular galaxies. Among large galaxies in the universe most (75%-85%) are spiral or lenticular. Spirals are often found in loose collections called groups. Our Local Group has two large spirals—MWG and Andromeda Galaxy (M31). Lenticular galaxies are common in clusters of galaxies, which can contain hundreds or even thousands of galaxies. Clusters can extend over more than 10 million l-y (3.06 Mpc). Lenticular galaxy NGC 4866 in the constellation Virgo is 80 million l-y (24.5 Mpc) distant. Lenticular galaxies are some-where between spirals and ellipticals in terms of shape and properties. Note the bright central bulge of NGC 4886, which contains primarily old stars, but no spiral arms are visible. The galaxy is seen from Earth as almost edge-on, meaning that the disk structure—a feature not present in elliptical galaxies—is clearly visible.

Galaxies and Cosmologies

Our deepest images of the universe show a great variety of galaxies, with some of them billions of light-years away. A galaxy's age, its distance, and the age of the universe are all closely related. The study of galaxies is thus intimately connected with cosmology—the study of the structure and evolution of the universe.

Cosmic Distance Ladder

Radar ranging: distances within solar system (10<4 l-y) Parallax: distances to nearby stars (10<3 l-y) Main-sequence fitting: distances to star clusters in the MWG (10<5 l-y) Cepheid variables: distances to nearby galaxies (10<8 l-y) Distant standards (white dwarf supernovae and Tully-Fisher relation): distances to galaxy clusters (10<10 l-y) Hubble's law: distances to the "edge" of the universe (10<10 l-y) Statue of an astronomer and the concept of the cosmic distance ladder by the parallax method, made from the azimuth ring and other parts of the Yale-Columbia Refractor (telescope) (c. 1925) wrecked by the 2003 Canberra bushfires which burned out the Mount Stromlo Observatory; at Questacon, Canberra, Australian Capital Territory. The sculpture was made by Tim Wetherell in 2003. The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an astronomical object is possible only for those objects that are "close enough" (within about 1,000 parsecs) to Earth. The techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at close distances with methods that work at larger distances. Several methods rely on a standard candle, which is an astronomical object that has a known luminosity.

Hybrid Hierarchical Model of Galaxy Formation and Evolution

Recent work on the role of cold dark matter in the early universe has led to the hierarchical or bottom-up model of galaxy formation. In this model, gravity plays the dominant role in forming galaxies. First clumps of matter form after the Big Bang-->Mergers and collapse--> Massive stars form protogalaxies with small disks-->Mergers--> Protogalaxies merge: their disks are destroyed leaving spherical structures-->Mergers and interactions to Ellipical Galaxies or Evolution in Isolation-->Spiral Galaxies

Red Shift number Z

Redshift and blueshift may be characterized by the relative difference between the observed and emitted wavelengths (or frequency) of an object. In astronomy, it is customary to refer to this change using a dimensionless quantity called z. After Z is measured, the distinction between redshift and blueshift is simply a matter of whether z is positive or negative

Sagittarius Dwarf Elliptical

Sagittarius Dwarf Elliptical is a satellite galaxy of the Milky Way and the second closest external galaxy. Obscured by large amounts of dust in the galactic plane, it was discovered as recently as 1994. SagDEG orbits our galaxy in less than one billion years It is, however, apparently now in the process of being disrupted by tidal forces of the Milky Way.

More on galaxies

Sizes range from dwarf galaxies (10<8 stars) to giant galaxies (10<12 stars). Spiral and irregular galaxies look white because they contain stars of all different colors and ages. Elliptical galaxies look redder because old, reddish stars produce most of their light.

Grouping of Galaxies

Spiral galaxies are often found in loose collections of up to a few dozen galaxies; our Local Group is an example. Elliptical galaxies are common in clusters of galaxies, which can contain hundreds or thousands of galaxies. Elliptical galaxies make up about 50% of the large galaxies in the central regions of clusters; outside of clusters they represent about 15% of large galaxies. Spiral galaxies are often found in groups of galaxies, with up to a few dozen members. Galaxy group Hickson 44 is located about 60 million l-y (18.4 Mpc) away toward the constellation of Leo. Many galaxies in Hickson 44 and other compact groups are either slowly merging or gravitationally pulling each other apart.

What are the three major types of galaxies?

Spirals, ellipticals, irregulars.

Use Radar to Determine Distance

Step 1 Determine the size of the solar system using radar. Three 34-m (110-ft) diameter Beam Waveguide antennas. These antennas are the latest addition to the NASA's Deep Space Network. The Goldstone Deep Space Communications Complex, located in the Mojave Desert in California, is one of three complexes which comprise NASA's Deep Space Network (DSN). The DSN provides radio communications for all of NASA's interplanetary spacecraft and is also utilized for radio astronomy and radar observations of the solar system and the universe.

Use Parallax to Determine Distance

Step 2 Determine distances of stars out to perhaps 1,000 light-years using parallax. As Earth orbits the Sun, the position of a nearby star appears to shift against the background of more distant stars. This principle of stellar parallax was known to the ancient Greeks, but they lacked the technology to detect it.

Main Sequence Fitting

Step 3 Apparent brightness of a star cluster's main sequence stars (compared to a calibrated main sequence) tells us the distance to the cluster. This technique is known as main sequence fitting. Another term for this method is spectroscopic parallax. The physics of stars tells us how luminous a main sequence star of a certain temperature (or spectral type) should be. The black dots show a portion of the main sequence where the absolute magnitude of the stars is known. (Here absolute magnitude is plotted instead of luminosity). The blue triangles plot the apparent magnitudes for main sequence stars in the Pleiades cluster (M45), whose distance is to be determined. The difference in the apparent and absolute magnitudes of a star gives its distance. (Mathematically, the absolute magnitude M = m − 5log(D/10 pc), where m is the apparent magnitude and D is the distance in parsecs.) Using the two plots, one just needs to measure the vertical size of the gap between the two plots to find the distance to the cluster.

Using Cepheid Variables as Standard Candles

Step 4 Because the period of a Cepheid variable star tells us its luminosity, we can use these stars as standard candles.

Galaxies as Standard Candles: The Tully-Fisher Relation

Step 5a Tully-Fisher RelationEntire galaxies can also be used as standard candles because galaxy luminosity is related to rotation speed. The Tully-Fisher Relation, L∝Wα, is a correlation that holds for galaxies with disks stabilized by rotation, between the intrinsic luminosity L of the galaxy in optical or near-infrared bands and the rate of rotation W. It is effective out to about 200 Mpc (650 l-y). (A) A rotating spiral galaxy's speed of rotation is related to its luminosity. (B) This integrated H I spectrum of galaxy UGC 11707 shows the typical two-horned profile of a spiral galaxy. (NRAO/AUI) (C) A plot of magnitude (luminosity) vs. log linewidth gives a straight line. Thus, the faster a galaxy rotates, the more luminous it is.

Use White Dwarf Supernovae as Standard Candles

Step 5b The apparent brightness of white-dwarf supernova tells us the distance to its galaxy. This method works up to 10 billion light-years.

Galaxies-The Numbers

Superclusters within 1 billion light years = 100 Galaxy groups within 1 billion light years = 240,000 Large galaxies within 1 billion light years = 3 million Dwarf galaxies within 1 billion light years = 60 million Stars within 1 billion light years = 250,000 trillion The Black Eye Galaxy (also called Sleeping Beauty Galaxy; designated Messier 64, M64, or NGC 4826) was discovered by Edward Pigott in March 1779, and independently by Johann Elert Bode in April of the same year, as well as by Charles Messier in 1780. It has a spectacular dark band of absorbing dust in front of the galaxy's bright nucleus, giving rise to its nicknames of the "Black Eye" or "Evil Eye" galaxy. M64 is a spiral galaxy in the Coma Berenices (Berenice's Hair) constellation about 24 million l-y (7.4 Mpc) distant.

Hubble and the Two Types of Cepheid Variables

That there are two types of Cepheid variables was unknown to Edwin Hubble when he made his historic distance measurement to the Andromeda Galaxy. Though his initial measurement was in error (less than half the actual distance), Hubble showed that the "spiral nebulae" are distant star systems like the Milky Way. Period-luminosity relationship for Cepheids and RR Lyrae stars. Type II Cepheids are less luminous than Type I Cepheids. The two types can be differentiated by their respective spectra.

Andromeda Galaxy and Constellations

The Andromeda Galaxy (M31) and the Triangulum Galaxy (M33, also nicknamed Pinwheel) are located just under 15° apart on either side of the constellation Andromeda. M33 is the third spiral galaxy in the Local Group and is about 2.74 million light-years (0.84 Mpc) distant.

Even More Galaxies: The Hubble UDF Revised

The Hubble Ultra Deep Field displays nearly 10,000 galaxies across the observable universe in both visible and near-infrared light. The smallest, reddest galaxies are among the youngest known, existing when the universe was just 800 million years old. In 2014, with the addition of ultraviolet light information, this iconic image is even more striking. The new observations, also taken with the Hubble Space Telescope, were combined with the previous image and now gives one of the most comprehensive pictures of galaxy evolution ever obtained.

Hubble Ultra Deep Field

The Hubble Ultra-Deep Field (HUDF) is an image of a small region of space in the constellation Fornax, composited from Hubble Space Telescope data accumulated over a period from September 24, 2003, through January 16, 2004. Looking back approximately 13 billion years (between 400 and 800 million years after the Big Bang), the image has been used to search for galaxies that existed at that time. The HUDF image was taken in a section of the sky with a low density of bright stars in the near-field, allowing much better viewing of dimmer, more distant objects. The image contains an estimated 10,000 galaxies.

Modern Hubble Plot

The Hubble constant, Ho, describes how fast objects appear to be moving away from our galaxy as a function of distance. If you plot apparent recessional velocity against distance, the Hubble constant is usually expressed in units of "kilometers per second, per megaparsec" (1 Mpc=10<6 pc=3,262,000 l-y=3.09 x 10<22 m). Data from the past 40 years indicate Ho is between 50 and 100, with current research pointed to a value around 67

Hubble eXtreme Deep Field (XDF)

The Hubble eXtreme Deep Field (XDF) of 2012 combines Hubble observations taken over the past decade of a small patch of sky in the center of the original Hubble Ultra Deep Field from 2004. The XDF, located in the constellation Fornax, is a small fraction of the angular diameter of the full Moon. With a total of over two million seconds (555.6 hours or 23.1 days) of exposure time, it is the deepest image of the universe ever made—13.2 billion light-years (4,044 Mpc). The youngest galaxy found in the XDF existed just 450 million years after the big bang.

Large Magellanic Cloud

The Large Magellanic Cloud, (LMC) is an irregular galaxy 163,000 l-y (49.97 kpc) distant from the Milky Way Galaxy. It has a mass of approximately 10 billion M, making it roughly 1/100 as massive as the Milky Way. Its diameter is about 14,000 l-y (~ 4.3 kpc). The LMC is the fourth largest galaxy in the Local Group, after the Andromeda Galaxy (M31), our own Milky Way Galaxy, and the Triangulum Galaxy (M33). While the LMC is often considered an irregular type galaxy, it contains a very prominent bar in its center, suggesting that it may have previously been a barred spiral galaxy. The LMC's irregular appearance is possibly the result of tidal interactions with both the Milky Way and the Small Magellanic Cloud (SMC). It straddles the border between the constellations of Dorado and Mensa.

Observing M104 at Different Wavelengths

The Sombrero (M104) is one of the largest galaxies in the nearby Virgo cluster, 29.3 million l-y (8.98 Mpc) from Earth. The main figure shows the combined image from the three telescopes, while the three inset images show the separate observatory views.

Co Evolution of SMBH and Galaxy

The almost linear correlation between the SMBH mass (M) and velocity dispersion (σ, sigma) of the galactic bulge (the "M-sigma relation") suggests that there is some kind of co-evolution going on between a SMBH and its host galaxy. The only way an SMBH can get bigger is if its host galaxy gets bigger—and vice versa. The left chart shows data points derived from different objects in a galaxy; the right chart shows data points derived from different types of galaxies.

Expansion of the Universe

The expansion of the universe stretches out all the photons within it, shifting them to longer, redder wavelengths. We call this effect a cosmological redshift. Conceptually, it is better to think of space itself as expanding, carrying the galaxies along for the ride, rather than to think of the galaxies as flying through a static universe. The redshift of a galaxy is a measure of how much space has expanded during the lookback time to that galaxy. The cosmological horizon is a boundary in time, not in space. Cosmological Redshift in the Balloon Universe: While light from a distant galaxy travels across space, space itself, like this balloon, is expanding. The wavelength of the light increases, and its spectrum shifts toward red.

Cosmological Redshift

The expansion of the universe stretches out all the photons within it, shifting them to longer, redder wavelengths. We call this effect a cosmological redshift. In the "balloon universe," photons move and redshift galaxies spread apart, but the galaxies stay the same size. As the universe (spacetime) expands, light waves get stretched out. Stretching makes the wavelength longer, hence redder. The result is a "Cosmological Redshift."

Our Expanding Universe

The expansion rate appears to be the same everywhere in space. The universe has no center and no edge (as far as we can tell). There is one aspect of cosmic expansion that the animation cannot show: In this expansion, the points of view of all the galaxies are equally valid. Had we chosen a different galaxy to form the immobile center point of our animation, the animation would look just the same: all galaxies move away from the observer, and their average speeds and distances follow the same Hubble relation. The expansion has no center, and every observer on a galaxy sees the same expanding cosmos.

Timeline of Cosmic History

The first galaxies began to form before 500 million years after the Big Bang. These "first" galaxies were small but grew larger through collision and merger. (slide 58)

Size of Galaxies

The largest Local Group galaxy is M31, the Andromeda Galaxy, which is about 140,000 l-y (42.9 kpc) across, a diameter ~40% greater than that of our own Milky Way Galaxy. Far more massive than any member of the Local Group is M87, a giant elliptical galaxy. Andromeda (M31) Milky Way Galaxy Whirlpool Galaxy (M51) Triangulum Galaxy (M33) Nubecula Minor Small Magellanic Cloud Virgo A (M87)

Apparent Brightness and Luminosity

The relationship between apparent brightness and luminosity depends on distance: Brightness=Luminosity/4(3.14)(distance)<2 We can determine a star's distance if we know its luminosity and can measure its apparent brightness: Distance=Luminosity/(squareroot)4piBrightness A standard candle is an object whose luminosity we can determine without measuring its distance.

Two sources of redshift

The two primary sources of redshift are caused by two different effects. The Doppler Effect can result in either a redshift or a blueshift. The expansion of spacetime produces only a cosmological redshift. Two different sources of redshift: (A)Doppler effect: The star moving to the left emits light that is blue-shifted in the direction of the receiving antenna that the star approaches, and red-shifted in the direction of the receiving antenna that the star is leaving. (B) Cosmological redshift: The distance between the emitting star and both antennas increases while the light is propagating, increasing the wavelength of the light seen by both antennas. This effect is due to the expansion of spacetime.

Cosmological Principle

The universe looks about the same no matter where you are within it. Matter is evenly distributed on very large scales in the universe. The universe has no discernable center and no edges (in terms of spatial dimensions). This principle is not proved but is consistent with all observations to date. The cosmological principle states that no matter where we look in the universe (D1 or D2) we still see the same distribution of objects.

Spiral Galaxies-1

Thin disk extends outward from a central bulge that merges smoothly into a halo that can extend to a radius of 100,000 l-y. Bulge and halo make up its spheroidal component. Disk component slices through bulge and halo and can extend 50,000 l-y (15.3 kpc) or more from the center. Disk contains interstellar medium of gas and dust, but relative compositions will differ spiral galaxy to spiral galaxy. Some spirals have a central bar and are called barred spirals. New data reveal that the Milky Way is a barred spiral. Barred spiral galaxy NGC 1672 in the constellation Dorado (Dolphinfish or Swordfish) is 60 million l-y (18.4 Mpc) distant.

"First Galaxies"

This galaxy is seen just 700 million years after the Big Bang (~13 billion years ago). The galaxy disappears at optical wavelengths (left), but is seen clearly in the infrared (right), as shown in the image boxes at the bottom.

Earliest Population of Galaxies

This image of the Hubble Ultra Deep Field (HUDF) 2012 campaign reveals a previously unseen population of seven faraway galaxies, which are observed as they appeared in a period 350 million to 600 million years after the Big Bang. The numbers refer to redshifts (z).

Hubble Frontier Galaxy Field

This long-exposure Hubble Space Telescope image of massive galaxy cluster Abell 2744 (foreground) is the deepest ever made of any cluster of galaxies. It shows some of the faintest and youngest galaxies ever detected in space. Abell 2744's gravity is being used as a lens to warp space and brighten and magnify images of more distant background galaxies. The more distant galaxies appear as they did longer than 12 billion years ago. The Hubble exposure reveals almost 3,000 of these background galaxies interleaved with images of hundreds of foreground galaxies in the cluster. Their images not only appear brighter, but also smeared, stretched, and duplicated across the field. Because of the gravitational lensing phenomenon, the background galaxies are magnified to appear as much as 10 to 20 times larger than they would normally appear. Furthermore, the faintest of these highly magnified objects is 10 to 20 times fainter than any galaxy observed previously. Without the boost from gravitational lensing, the many background galaxies would be invisible. The Hubble exposure will be combined with images from Spitzer and NASA's Chandra X-ray Observatory to provide new insight into the origin and evolution of galaxies and their accompanying black holes.

Galaxy Formation

This movie shows the formation of an individual galaxy that has been extracted from a much larger simulation. The region shown here is a 200 kpc (652.8 thousand l-y) cube centered on the forming galaxy. The simulated galaxy exhibits a rotationally supported disk structure, a feature that is characteristic of observed spiral galaxies. During the sequence, a satellite galaxy orbits about the main galaxy similar to the way that the large and small Magellanic clouds orbit our own Milky Way galaxy.

Constraints with Hubble's Law

Two practical difficulties with using Hubble's constant and redshifts to determine distances: Galaxies do not obey Hubble's law perfectly because they have velocities relative to one another caused by gravitational tugs. Even when galaxies do obey Hubble's law well, distances found with it are only as accurate as our best measurement of Hubble's constant. The search for an accurate determination of Hubble's constant has been a focus of astronomers for 85 years. The accuracy of the Hubble constant (H0) varies with the method used to determine it. The WMAP data have returned a value with the least amount of uncertainty.

Timeline to the first galaxies

Universe began with the Big Bang 13.8 billion years ago. Atomic hydrogen (H) formed 380,000 years later (Recombination), but the universe remained dark (Dark Ages) as no stars had yet formed in the neutral intergalactic medium (IGM). Molecular hydrogen (H2) clouds collapsed 300-400 million years later and formed the first stars and galaxies—the Cosmic Dawn. The light (radiation) from the first stars broke hydrogen into protons and electrons and started the Cosmic Reionization. Galaxies then grew in mass and size, chemical elements were synthesized in stars, and life developed giving us our present universe.

Shapley-Curtis Debate Shapley's View: MWG Is the Universe

Using globular clusters, Shapley estimated the size of the MWG as 300,000 l-y across and the Sun about 50,000 l-y from the center. (Generally correct, though we now know the MWG is 100,000-120,000 l-y across, and the Sun is about 27,000 l-y from the center.) Adriaan van Maanen had measured a rotational speed of 0.02 seconds of arc per year for M101; coupled with an angular size of half a degree, this meant that M101 would have had to rotate faster than the speed of light if it lay beyond the Milky Way. Furthermore, the high recessional velocities of spiral nebulae implied that they must be close enough to the Milky Way to be influenced by it. (Van Maanen's measurements were later shown to be simply incorrect.) Shapley did not believe that spiral nebulae lay beyond the Milky Way. He thought that they were located within the boundaries of the MWG. One popular theory was that they were late-forming solar systems, in the process of formation. (Completely wrong.) The yellow star represents the Sun. The light blue area represents all of the individual stars in the Milky Way (aka "universe"), some which we can see from our position (the darker blue shaded area around the Sun). The rest are obscured from our view. The blue dots represent the globular clusters that swarm around the Milky Way. The purple ovals represent other solar systems in formation—the spiral nebulae.

A face-on spiral galaxy

Young stars are found in the spiral arms (disk component) where gas is plentiful. The spheroidal component has little gas, and most of its stars are old (primarily long-lived K and M stars). Blue-white color indicates ongoing star formation Red-yellow color indicates older star population NGC 1232 is a spiral galaxy about 65 million l-y (19.9 Mpc) distant in the constellation Eridanus (the River).