Transcript 19ism1s
The Interstellar Medium Astronomy 315 Professor Lee Carkner Lecture 19 Exercise 18 – Dark Matter Use Kepler’s Third Law to find the total mass of the galaxy M = a3/P2 a = (20000 pc)(206000 AU/pc) = 4.12X109 AU P = 460X106 years M = (4.12X109)3/(460X106)2 = 3.3X1011 solar masses Luminosity of galaxy F = L/4pd2, L = F4pd2 L = (3.88X10-12)(4)(p)(5.58X1023)2 = 1.52X1037 W 1.52X1037/3.8X1026 = 4X1010 Lsun Mass-to-light ratio M/L = 3.3X1011/4X1010 = 8.25 The Stuff Between the Stars Called the interstellar medium Concentrated in the disks of spiral galaxies Exists with different temperatures, densities, size scales, and radiation environments Composition We can get an idea of the composition of the interstellar medium through spectroscopy There are two components: gas and dust Gas Dust Account for most of the opacity Gas Interstellar gas is abundant in hydrogen and helium Heavier elements have condensed to form the dust What form is the gas in? Radiation Environment Different parts of the ISM experience different external radiation fields Location of bright stars Density of cloud The inner part of a dense cloud is shielded from most external radiation Effects of Radiation Clouds that experience little radiation are known as HI regions Clouds in higher radiation environment are known as HII regions Some clouds have so little radiation that they can form some molecules Called giant molecular clouds Dust The clouds are much more opaque than you would expect just from the gas Dust particles are small (~1 micron or 1 millionth of a meter) Some different types of dust: All these different types identified from complex absorption properties Extinction Curve What Does Dust Do? Absorption This causes interstellar extinction Scattering Dust scatters blue light better than red Stars behind the cloud look redder Reddening in the ISM Reddening Nebulae What is a nebula ? There are three basic types of nebula, each with a distinct appearance based on the way it interacts with light Dark Nebulae Dense clouds can completely block out the light of stars behind them Can see stars in front of the cloud projected on it Smallest are called Bok globules Star Cluster and Dark Cloud Emission Nebulae If a cloud is near bright high mass stars it may shine as an emission nebula The UV light ionizes the gas Like a florescent light Emission nebulae are HII regions Reflection Nebulae Dust preferentially scatters blue light Same reason sky is blue Need bright star fairly near-by to produce effect Observing the ISM With optical telescopes we see the effects of the ISM in different ways We also need to observe at other wavelengths Multiwavelength ISM Radio Penetrates dust and allows us to map much of the galaxy Millimeter Some are very complex and must be protected from UV radiation More Multiwavelength ISM Infrared Dust is still very cold (<100K or -150 C) X-ray Formed from supernova Types of Clouds HI clouds Molecular Clouds Coronal gas Intercloud medium Hot, low density, about 50% of total mass A lot of mass between the well-defined clouds Structure of ISM If we look in any direction we see many different parts of the ISM The sun is actually in a large region of hot gas called the local bubble The denser parts of the ISM are the sites of star formation Next Time Read Chapter 24.1-24.2