Galaxies Guiding 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 1 2 3 4 When galaxies were first discovered it was not clear that they lie far beyond the Milky Way The Discovery of Galaxies M51 At the beginning of the 20th century 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 This is NOT a true color image 5 Edwin P Hubble 1889 1953 6 1 M31 The Great Spiral Galaxy in Andromeda The Nuclear Bulge of M31 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 This 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 7 NOAO AURA Photos 8 Hubble demonstrated that the spiral nebulae are far beyond the Milky Way The Outer Disk of M31 Edwin Hubble used Cepheid variables to show that spiral nebulae were actually immense star systems far beyond our Milky Way Galaxy NOAO AURA Photos 9 Spiral 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 10 Barred Spiral Galaxies Types of Galaxies Spirals The 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 NOAO AURA Photos 11 12 2 Elliptical 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 Irregular 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 Types of Galaxies Ellipticals NOAO AURA Photos Types of Galaxies Irregulars NOAO AURA Photo 13 Galaxies were classified according to their appearance by Hubble and placed in a diagram 14 Galaxies can be grouped into four major categories spirals barred spirals ellipticals and irregulars 15 The disks of spiral and barred spiral galaxies are sites of active star formation 17 Lenticular SB0 galaxies are intermediate between spiral and elliptical galaxies From this table consider which galaxies are the most and least massive most and least luminous and largest and smallest in size 16 Elliptical galaxies are nearly devoid of interstellar gas and dust and so star formation is severely inhibited 18 3 19 Irregular galaxies have ill defined asymmetrical shapes 20 Astronomers use various techniques to determine the distances to remote galaxies Standard 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 distances 21 22 They are often found associated with other galaxies The Distance Ladder The Tully Fisher relation which correlates the width of the 21cm 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 23 brightness distribution and to the motions of its stars Masers 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 24 stimulated emission of radiation 4 The Hubble law relates the redshifts of remote galaxies to their distances from the Earth The Hubble law is v H0d There is a simple linear relationship between the distance from the Earth to a remote galaxy and the redshift of that galaxy which is a measure of the speed with which it is receding from us 25 The value of the Hubble constant H0 is not known with 26 certainty but is close to 71 km s Mpc Galaxies are grouped into clusters and superclusters Galaxies are grouped into clusters rather than being scattered randomly throughout the universe 27 28 Rather 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 29 Clusters of Galaxies NOAO AURA Photo 30 5 Our Galaxy is a member of a poor irregular cluster called the Local Group 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 asymmetrically 31 32 33 34 35 36 Rich regular clusters contain mostly elliptical and lenticular galaxies Irregular clusters contain spiral barred