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Monday February 4 Astro.docx

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Bozidar Mitrovic

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Monday February 4, 2013 Astro Lecture: The Milky Way Galaxy: - a diffuse band of light across the sky - Greeks called it Galaxies Kuklos (milky circle) - Romans called it Via Lacte (Milky way) - Galileo used his telescope in 1609 to find that it consists of many faint (because they are very distant) stars th - In the 18 century Thomas Wright and Immanuel Kant proposed that we observe this band of light because we live in a disk of stars Size of the Milky Way and our Location in it: - after WWI Harlow Shaply measured our distance from globular clusters (clusters of 100,000 to 1,000,000 stars) - he noticed that they are located on one side of the sky near the constellation of Sagittarius - Shapley assumed that they are distributed in a spherical manner around the center of our galaxy - By measuring the distances to them he could deduce how far away we are from the center - Using similar method the astronomers have determined the size and the shape of the milky way - Region above and below the Milky Way is called the halo - He used the Cepheid variables to measure the distances to the globular clusters - The method was established by Henrietta Swan Levitt (1912) - Cepheid variables are variable yellow giants - L (luminosity) = const. R*2(radius)T*4(surface temperature) - Their luminosity will change with their radius in a periodic way - Leaviti found perpm Luminosity- period of luminosity variation correlation - Once we know the luminosity L, we measure the star’s brightness and deduce its distance D from: o B= L/4πd*2 - This method is used to find distances to clusters and galaxies when parallax method cannot be used th Monday February 11 2013 Contents of the Milky Way: Disk: - stars - star clusters - gas and dust of interstellar material - interstellar material is not distributed uniformly throughout the disk but is concentrated in spiral arms - how do we know that the milky way has spiral arms? o Look at the other spiral galaxies gives us a hint – their spiral arms are outlined by very hot (O- and B- spectral class), luminous young-stars which occur in associations (clusters of 10-100 stars) o Thus the astronomers look for such associations in our own galaxy o Observations have shown that indeed there are 4-s bands (called arms) of these associations in the vicinity of the Sun (which is in the Orion Arm) o Also the astronomers can map out the distribution of neutral (not ionized) hydrogen by looking at the sources of radio waves with the wavelength of 21 cm o They found that the neutral hydrogen gas is located in the same bands which contain the association of hot young stars Stars and star clusters in the disk: - associations - open clusters (contain from 100-1000 stars) Stars: 1) hot, luminous young stars (in associations) 2) medium mass stars like the sun and probably a very large number of 3) red dwarfs - the average age of the medium mass stars in the disk is about 7 billion years - How do we find the age of a star cluster? We use it HR diagram to determine its age: Very young cluster Halo: - no gas and dust (as if all of it was used in star formation) - stars occur mainly in the globular cluster (100,000 – 1,000,000 stars) and they tend to be lower main sequence stars (they are cool and not very bright) - thus, they are also very old (from 10 to 14 billion years) Bulge: - stars are similar to those in the halo and there are also some young luminous stars - the stars in the disk and the rest of the galaxy differ also in elements heavier than helium (so-called metals): o disk stars are population 1 stars (contain 2% or 3% of metals) o halo and bulge stars are population 2 stars (contain 0.1% of metals) Wednesday February 13 2012 th The movement of the - disk stars have nearly circular orbits and they move in the same directions - halo stars have highly elliptical orbits and they move in random directions - the Sun, being a disk star, has nearly circular orbit around the galactic center - its speed relative to the galactic center can be deduced from the Doppler shift of the light emitted by the globular cluster (the speed is about 220 km/s) - from the orbital speed and the radius of the orbit one can find the orbital period of the sun - it is about 230 million years - the sun completed o 4.6 billion years/230 million years per trip = 4.6X10*9 years/ 2.3X10*8 year per trip = 20 trips - using the newton’s laws of motion and his law of gravity one can determine the mass of a galaxy - Fg=G(gravitational constant) Mm/R2 (law of gravity) - Fg= m v*2/R (newton’s second law) - Then m v*2/R = G mM/R*2 - M= Rv*2/G - In the mid 70s it was found (Vera Rubin) that the galaxies (including the milky way) contain much more mass than what is contained in the stars of the galaxy - That missing mass is called dark matter (ordinary matter is on 4% of the total mass in the universe, dark matter accounts for 24% of the total mass in the universe, dark energy accounts for 72% of the mass in the universe) - Relation (1) can be rewritten as v*2= GM/R - Rubin measured the rotational curves of galaxies, i.e speed of star as a function of its distance from galactic center - From (2) the only way that v can be constant with increasing R only if M increases with the radius of the orbit
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