1 Ch. 25 In-Class Notes: Beyond Our Solar System ES2a. The solar system is located in an outer edge of the disc-shaped Milky Way galaxy, which spans 100,000 light years. ES2b. Galaxies are made of billions of stars and comprise most of the visible mass of the universe.
2 Elliptical galaxies have a uniform distribution of stars and are shaped somewhat like a football.
3 Irregular galaxies are clusters of stars that have no distinct shape, such as Barnard s Galaxy and the Magellanic Clouds. Barnard's Galaxy or NGC 6822 is a small irregular galaxy only 14 degrees east of the galactic plane in the constellation of Sagittarius. A member of the Local Group of Galaxies, it is quite similar to the Small Magellanic Cloud in composition and structure. Though it has a fairly low surface brightness, NGC 6822 is one of the easiest galaxies to resolve
4 Spiral galaxies consist of a nuclear bulge in the center of a disk of stars orbiting it in a spiral shape. Globular clusters of stars surround the bulge and disk in a spherical shape, called the halo. A view of our Milky Way galaxy (from our position inside the disk, of course), obtained by the DIRBE (Diffuse Infrared Background Experiment) instrument on NASA's COBE (Cosmic Background Explorer) satellite. This infrared image allows us to see through obscuring dust clouds, revealing a more accurate view than is possible using visible light.
5 Theory 1: spiral arms form as materials build up when they catch up to and get caught behind one another. Theory 2: spiral arms are continually changing due to disturbances such as supernovae explosions or gravitational pull of nearby galaxies.
6 There is a supermassive black hole in the center of the Milky Way Galaxy, whose gravity is keeping billions of stars orbiting it on spiral arms. Our Sun & solar system is located on a minor spiral arm of the Milky Way Galaxy, which is about 100,000 ly across.
7 The older stars of the Milky Way are near the center in the nuclear bulge and the halo. Newer stars are found in the outer parts of the spiral arms. We are located closer to the outer edge of one of the spiral arms.
8 To estimate how far away stars are from Earth, astronomers looked at the parallax shift of stars at opposite positions in its orbit around the Sun. Parallax is the apparent change, or shift, in the position of a star compared to other stars around it when viewed from different locations.
9 The Hertzsprung-Russell (H-R) diagram organizes stars based on their absolute magnitude, temperature, and spectral type. The mass and composition of a star determine its temperature, luminosity, and density.
10 Absolute magnitude gives the brightness of a star compared to others as if they were all the same distance away from the Earth (10 parsecs). 1 parsec (pc) = 3.26 ly or x km
11 Most H-R diagrams measure temperature in Kelvins (K). Temperature can usually be found along the x-axis. 1 K = C C = 5/9( F 32) F = 9/5 C + 32
12 Stars are classified according to the pattern of their absorption lines (spectra), which corresponds to their temperature and composition. In order, from hottest to coldest, spectral types are: O, B, A, F, G, K, and M The Sun is a type G star.
14 Stars are not actually alive, but they do have a beginning, middle and end to their existence.
15 Stars begin as a cloud of mostly Hydrogen gas and dust called a nebula.
16 As the nebula contracts, it begins to flatten into a disk. The center of the disk becomes very hot and dense and is called a protostar.
17 As the temperature and pressure continue to increase, nuclear fusion of Hydrogen into Helium begins, giving off extra energy as heat and light. Once there are enough nuclear reactions pushing out to balance the gravity pulling in, it is officially considered a stable, main sequence star.
18 As nuclear reactions build up Helium in the core and Hydrogen gets used up, the main sequence star becomes a Red Giant with heavier elements in the core.
19 As nuclear reactions build up larger elements in the core of a Red Giant, it stops at Carbon. The outer layers of the Red Giant expand and may be released to leave a small, hot White Dwarf star about the size of Earth.
20 Some of the large stars may give in to their gravity and collapse into a Neutron Star instead of becoming a White Dwarf. Neutron Stars are extremely dense.
21 Massive stars may collapse so quickly that the outer layers rebound off of the core and explode off as a supernova. Supernovae make and release elements heavier than iron into the universe.
22 Some massive stars collapse in on themselves so violently that they continue to collapse forever, which we call a black hole. The gravitational pull of a black hole is so great that not even light can escape it.