11Galaxies 2Guiding Questions• How did astronomers first discover other galaxies?• How did astronomers first determine the distances to galaxies?• Do all galaxies have spiral arms, like the Milky Way?• How do modern astronomers tell how far away galaxies are?• How do the spectra of galaxies tell astronomers that the universe is expanding?• Are galaxies isolated in space, or are they found near other galaxies?• What happens when galaxies collide with each other?• Is dark matter found in galaxies beyond the Milky Way?• How do astronomers think galaxies formed?3When galaxies were first discovered, it was notclear that they lie far beyond the Milky Way45M51This is NOT a true color image6The “Discovery” of GalaxiesAt the beginning of the 20thcentury, what we now call spiral galaxies were referred to as “spiral nebulae” and most astronomers believed them to be clouds of gas and stars associated with our own Milky Way. The breakthrough came in 1924 when Edwin Hubble was able to measure the distance to the “Great Nebula in Andromeda” (M 31) and found its distance to be much larger than the diameter of the Milky Way. This meant that M 31, and by extension other spiral nebulae, were galaxies in their own right, comparable to or even larger than the Milky Way. Edwin P. Hubble (1889-1953)27M31 - The Great Spiral Galaxy in AndromedaThis nearby galaxy in the Local Group of galaxies, of which the Milky Way is a member, is 2.9 million light years away. (NOAO/AURA Photos)8The Nuclear Bulge of M31(NOAO/AURA Photos)Note that you find young stars along the spiral arms. M32 and NGC 205, both dwarf elliptical galaxies, are in the bottom center and upper right.9The Outer Disk of M31(NOAO/AURA Photos)10Hubble demonstrated that the spiral nebulae are far beyond the Milky Way• Edwin Hubble used Cepheid variables to show that spiral nebulae were actually immense star systems far beyond our Milky Way Galaxy11Types of Galaxies SpiralsSpiral galaxies are so-named because of the graceful shapes of arms emanating from a bright central nucleus. Spirals are classified according to how tightly or loosely wound the arms are, and it turns out that the brightness of the central nucleus is correlated to the tightness of the arm. The galaxies M 104 (below) and M 51 (right) respectively show tightly and loosely wounds. Notice the effects of dust in both galaxies. (NOAO/AURA Photos)12Barred Spiral GalaxiesThe spiral galaxies M 91 (left) and M 109 (right) have bars across their nuclei from which spiral arms unwind. In virtually all spirals (barred or not) the galaxies rotate such that the spiral arms trail behind in the rotation. The Milky Way is known to be a barred spiral galaxy. (NOAO/AURA Photos)313Types of GalaxiesEllipticalsElliptical galaxies lack spiral arms and dust and contain stars that are generally identified as being old. The elliptical galaxies M 32 (below) and M 110 (right) show varying degrees of ellipticity. (NOAO/AURA Photos)14Types of GalaxiesIrregularsIrregular galaxies lack any specific form and contain stars, gas and dust generally associated with youth. The irregular galaxy at top right is the Large Magellanic Cloud, a satellite of the Milky Way located about 170,000 light years from the Sun. It is about 60,000 light years across. The bright reddish feature in the upper right is a region of star formation. The galaxy at the bottom is an irregular galaxy in Sagittarius.(NOAO/AURA Photo)15Galaxies were classified according to theirappearance by Hubble and placed in a diagram.16Galaxies can be grouped into four major categories: spirals, barred spirals, ellipticals, and irregularsLenticular (SB0) galaxies are intermediate between spiral and elliptical galaxiesFrom this table, consider which galaxies are the most and least massive, most and least luminous, and largest and smallest in size.17The disks of spiral and barred spiral galaxies are sites of active star formation18Elliptical galaxies are nearly devoid of interstellar gas and dust, and so star formation is severely inhibited419 2021Irregular galaxies have ill-defined, asymmetrical shapesThey are often found associated with other galaxies22Astronomers use various techniques to determinethe distances to remote galaxiesStandard candles, such as Cepheid variables and the most luminous supergiants, globular clusters, H II regions, and supernovae in a galaxy, are used in estimating intergalactic distances23• The Tully-Fisher relation, which correlates the width of the 21-cm line of hydrogen in a spiral galaxy with its luminosity, can also be used for determining distance• A method that can be used for elliptical galaxies is the fundamental plane, which relates the galaxy’s size to its surface brightness distribution and to the motions of its starsThe Distance Ladder24Masers• One distance-measuring technique that has broken free of the distance ladder uses observations of molecular clouds called masers• “Maser” is an acronym for “microwave amplification by stimulated emission of radiation”525The Hubble law relates the redshifts of remotegalaxies to their distances from the EarthThere is a simple linear relationship between the distance from the Earth to a remote galaxy and the redshiftof that galaxy (which is a measure of the speed with which it is receding from us)26The Hubble law is v = H0dThe value of the Hubble constant, H0, is not known with certainty but is close to 71 km/s/Mpc27Galaxies are grouped into clusters and superclustersGalaxies are grouped into clusters rather than being scattered randomly throughout the universe2829 30Clusters of GalaxiesRather than occurring individually in space, galaxies are grouped in clusters ranging in size from a few dozens to thousands of galaxies. The Coma Cluster, shown at right, is 300 million light years from the Milky Way and contains more than 1,000 (and possibly as many as 10,000) galaxies.The Milky Way is a member of a small cluster called the Local Group which contains about 40 galaxies. The two largest members of the Local Group are M 31 and the Milky Way.(NOAO/AURA Photo)631• A rich cluster contains hundreds or even thousands of galaxies• A poor cluster, often called a group, may contain only a few dozen• A regular cluster has a nearly spherical shape with a central concentration of galaxies• In an irregular cluster, galaxies are distributed asymmetrically32Our Galaxy is a member of a poor, irregular cluster called the Local