Possible Extra Credit Option Attend an advanced seminar on Astrophysics or Astronomy held by the Physics and Astronomy department. There are seminars held every 2:00 pm, Thursday, Room 190, Physics & Astronomy Please let me know if you are going so that I can attend as well. Summarize: What is the goal of the presented research Describe the experimental setup What issues they are encountered What are the results. At least a half a page Points will be awarded based on completeness and at my discretion. Up to 5% of a single test grade
Lecture 17. The Interstellar Medium 3/28/2018
The Interstellar Medium (ISM) of the Milky Way Galaxy Why study it? Stars form out of it. Stars end their lives by returning gas to it. The ISM has: A wide range of structures A wide range of densities (10-3 10 7 atoms/cm 3 ) A wide range of temperatures (10 K 10 7 K) Comparative density of ISM Sun and Planets: 1-5 g / cm 3 ISM average: 1 atom / cm 3
Interstellar Matter Two components: Gas (98 %) and Dust (2%) Not surprisingly gas is mostly hydrogen and helium, but there are also trace elements of carbon monoxide (CO), water (H 2 O), and formaldehyde (H 2 CO). Dust is hard to study but most likely made of silicates, graphite and iron. Dust grains are about 10-7 m in size, which is comparable in size to the wavelength of visible light.
Interstellar Gas Gas near hot stars is easily visible because of the ionized hydrogen gas. 10,000 K stars produce a lot of UV rays. UV radiation ionizes ISM (strips the electron from the atom) When the atom find another electron, visible light is produced as the electron decays to the ground state. A cloud of ionized hydrogen is called an H II region
Emission nebulae are glowing (hot) clouds of ISM surrounding groups of young, bright stars. They most likely will end up on the upper left portion of the H-R diagram. Emission nebulae absorb the high energy UV light emitted by young stars and reemit it as red light.
Reflection nebula happen to be in the line of sight of the emission nebula. The blue color comes from the dust grains scattering blue light, but not red light. The IR image shows that the reflection nebula are much cooler and indeed only reflecting light and not emitting. Dust lanes are a part of the emission nebula and are very dense regions of the ISM that have not been heated by the forming stars
The Hourglass nebula is an extreme example of dust lanes. As the young star heats up the surrounding gas, the gas begins to dissipate. The most dense regions of dust remain the longest, yielding these amazing structures.
Interstellar Gas Most interstellar gas is neutral hydrogen that is very far away from any hot stars. Neutral hydrogen (1 proton, 1 electron, no net electric charge) does not emit or absorb visible light at typical ISM temperatures. Strongest interstellar absorption lines are produced by calcium and sodium. The ISM was first detected by viewing these absorption lines
1904 Detection of ISM When observing a bright star, astronomers noticed several faint absorption lines. Unlike the Doppler shifted lines associated with the star, what ever create these lines was not moving. Astronomers proposed that there was some dilute gas, mostly hydrogen, between all stars in our Milky Way
Direct Detection of Neutral Hydrogen in the ISM Quantum Mechanics predicted that neutral hydrogen will produce a photon with a wavelength of 21 cm. This photon is produced through a spin interaction rather than an electron orbital transition. Typically, a neutral hydrogen atom will produce 1 photon at 21 cm every 10 million years! Using the 21 cm line, astronomers have been able to map out neutral hydrogen in our milky way.
21-cm photon: The proton and electron each have spin, a result from quantum mechanics. Similar to the spin of the Earth around the Sun. If both spinning the same way, atom s energy is slightly higher. Eventually will make transition to state of opposite spins. Energy difference is small => radio photon emitted, wavelength 21 cm.
Map of the 21 cm atomic hydrogen line for the Milky Way. It is amazing how much structure exists in the ISM!
UV Radiation and the Local Bubble ISM can be a wide range of temperatures, depending on how long ago it was created and how it has interacted with other objects in space. By looking at UV emissions of the ISM, we have discovered a hot bubble of gas around our local neighborhood. This gas is 500,000 K and 100 pc across, which implies it was created from a star exploding about 300,000 years ago. Ancient humans would of seen this as an object in the sky as bright as the moon.
Ultra hot UV bubbles created from super nova
Gas Structures in the ISM Molecular Gas In the form of cold (~10 K), dense (~10 3-10 7 molecules/cm 3 ) clouds. Molecular cloud masses: 10 3-10 6 M with size of 10-100 pc Dense cloud regions may be dense gas that is being temporarily compressed by some large galactic flow of the ISM. Kind of like a slinky.
Molecular Clouds Because of the high density of these clouds, molecules are much more likely to form. Astronomers have found: acetic acid (the prime ingredient of vinegar a simple sugar (glycolaldehyde) molecular starting point for amino acids We can observe emission from molecules in the radio and IR. Molecules photons with rotational energy level transition.
False-color of CO emission from Orion molecular cloud complex. Best studied case. 500 pc away. 400,000 M of gas. Note complicated structure! Molecular Clouds important because stars form out of them!
Cosmic Dust The completely opaque regions of the milky way are due to the presence of cosmic dust Dust particles typically between 10-100 nm. This size makes these particles excellent at absorbing blue wavelengths and shorter.
Redding due to Dust The opacity of the ISM increases with decreasing wavelength. More likely to absorb blue than red If a spectrum of a star can be seen through the ISM, the blue component will be significantly decreased. The degree of redding of the spectrum can be used to measure how much ISM is between us and the star.
Interstellar Grains Dust particles can be characterized as sootlike (rich in carbon) or sandlike (containing silicon and oxygen) The grains has a rocky cores (soot like or sand like)soot (rich in carbon) In the dark clouds where molecules can form, these cores are covered by icy mantles ( (H 2 O), methane (CH 4 ), and ammonia (NH 3 The ice mantles are sites for some of the chemical reactions that produce complex organic molecules.
Interstellar Grains Most dust grains start by condensing on the cool surface of red giant stars. This is confirmed by the IR emissions from dust grains on these stars surfaces. Conditions in interstellar space are generally not suitable for the formation of silicate cores. Some dust must come from destroyed exoplanets, but that is a hard thing to confirm.
ISM Life Cycle Where does all this stuff come from?! Hydrogen and Helium were produced in the Big Bang. The Milky Way is accreting more from intergalactic space Everything else was created from nuclear fusion. Star death (supernova) dispersed it into the ISM. Dust forms when grains can condense in regions where gas is dense and cool. Molecular clouds eventually condense to form stars and planets. The process repeats.
Intergalactic Medium The intergalactic medium is the hot (1,000,000 K), X-ray emitting gas that permeates the space between galaxies. Contains less than one atom per cubic meter. Estimated to contain 40-50% of all normal matter in our universe. Astronomers now believe that the intergalactic medium is enriched through the action of galactic winds. Galactic wind is the flow of energetic particles from super massive black holes.
Once space travel is available, we will have humans who are interstellar Psychic mediums!