ASTRONOMY FINAL Chapters 11 15 11 Galaxies The milky way looks like a band of light crossing the sky We are a spiral galaxy because of the spiral arms that would be visible in a face on view From the side the arms blend into a thin disk that contains more than 100 billion stars In the center of the disk we find a bright central bulge surrounded by a vast roughly spherical halo that is nearly invisible except it contains about 200 globular clusters of stars Disk is about 100 000 light years in diameter but only 1000 light years thick Our sun is located about 28 000 light years from the center Interstellar medium interstellar gas and dust that the galactic disk is filled with This obscures our view when we try to peer through in hiding most of our galaxy which long fooled astronomers into believing we lived near the galaxy s center Disk Stars same plane Orbit in roughly circular paths that all go in the same direction on the This arises from its gravitational attraction toward the galactic center A star far above the disk is pulled back by gravity The up and down motions of disk stars give the disks its 1000 light years thickness Each star takes more than 200 million years to orbit The orbital velocities of stars near the edge and those near the center are about the same Halo and Bulge Stars These orbits are much less organized than those of disk stars Travel on more or less elliptical paths but orientations of the paths are random Neighbors can circle the galactic center in opposite directions Plunge through the disk at such high velocities that the disk s gravity hardly alters their trajectories These swooping orbits explain why the bulge and halo are much rounder and puffier Soar to heights about the disk far greater than those by disk stars Don t find many young stars Spectra of halo stars show they contain fewer heavy elements than disk Star gas star cycle Stars are continually forming and dying Once their fuel for fusion is exhausted they die and return much of material back ot the interstellar medium through either a planetary nebula or supernova depending on mass of star Remaining stages take place in the interstellar medium Gas enters interstellar medium in the form of a hot bubble Atomic hydrogen gas warm gas can be mapped by using radio observations at spectral line at a wavelength of 21 and is distributed through the galactic disk Gravity slowly draws blobs of atomic gas into tighter clumps which radiate energy Once the temperature drops below 100 K hydrogen atoms pair up with hydrogen molecules and become molecular clouds which go on to form stars This process gradually changes the chemical composition of the interstellar medium New stars are not spread evenly across the galaxy Environments rich in molecular clouds tend to spawn new stars easily compared to gas poor environments Hot massive stars signify an area of active star formation Ionization nebulae Colorful wispy globs of glowing gas that glow because elections in their atoms are raised to high energy levels or ionized when they absorb ultraviolet photons from the hot stars Spiral arms are the hallmarks of star formation and are full of newly forming stars They have enhanced amounts of molecular and atomic gas Hot blue stars and ionization nebulae trace out the arms and the stars between are redder and older Spiral arms do not move with the stars because stars near the center of the galaxy complete an orbit in much less time than the stars in the center It is a moving pattern rather than a permanent structure Stars are too spread to collide with each other but large gas clouds due which enhances the force of gravity within them and triggers the formation of new star clusters Disk population contains young and old stars all have heavy element proportions Spheroidal roughly spherical in shape always old and low in mass Only 02 of heavier elements The halo does not contain the cold dense molecular clouds needed for star formation The small amount of gas is generally hot Lack of heavy elements indicates halo stars much have formed early in the galaxy s history Only ones that still survive are old low mass stars Our galaxy began as a giant protogalactic cloud Gravity caused the cloud to contract and fragment Stars of the spheroidal population formed first The gravity formed a cloud that was blobby in shape and had no measurable rotation This caused the randomly oriented orbits of stars in the halo Later the remaining gas contracted settled into a flattened spinning disk because of conservation of angular momentum Collisions tended to average out the random motions causing them to orbit in the same direction plane Stars that formed in this spinning disk became members of the disk population of stars Another theory is the earliest stars formed in small protogalactic clouds with a few globular clusters and the clouds later collided combined to create the full cloud that became the Milky Way Spiral Galaxies Flat white disks with yellowish bulges at the center Cool gas and dust interspersed with hotter ionized gas and spiral Proportions in the different populations can vary Barred Spiral spiral arms curling away from the ends of the bar Lenticular intermediate class between spiral and elliptical Like spiral galaxies but lacking the arms Most large galaxies in the universe are spiral or lenticular Tend to be found in loose collections called groups Redder rounder and often longer in one direction than in the other Contain very hot ionized gas very little cool gas and dust Lack a significant disk population Look much like the bulge and halo of a spiral galaxy that is missing its Elliptical Galaxies arms disk Can contain more than a trillion stars and are most common in clusters of galaxies which can contain thousands of galaxies Dwarf elliptical galaxies are often found near large spiral galaxies Some large elliptical galaxies can contain amounts of very hot gas that can emit x rays Generally have little or no ongoing star formation Irregular galaxies are neither disk like nor round Spiral and Irregular look white because they contain stars of all different colors and ages but old reddish stars produce most of the elliptical light Distant galaxies are more likely to look irregular than nearly galaxies These were more common when the universe was younger Edwin Hubble s Galaxy Classes Shaped like a tuning fork Elliptical on the handle The larger the number the flatter the elliptical galaxy E0 galaxy is a sphere and the