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Ch. 18 Star Lecture The Interstellar Medium Standards • Understand the scale and contents of the universe, including nebulae Interstellar Space • Contains matter with extremely low density (a trillion trillion times less dense than matter in stars or planets) • Far more tenuous than the best vacuum attainable in labs. • This matter only amounts to anything because IS space is so vast. Interstellar = Between stars Chris Hetlage. 2005. Astronomy Picture of the Day. http://antwrp.gsfc.nasa.gov/apod/ap050804.html Why study it at all? • There is as much mass in the “voids” among the stars as there are in the stars themselves. • IS space is the region out of which new stars are born. • It is the region into which old stars explode at death. Interstellar Matter • Look at space. What do you see? • You see bright areas, regions with collections of stars • You also see dark areas, regions where IS matter obscures light from the stars beyond. • IS matter is made up of gas & dust, and is intermixed through all space. Photo: Marc Sylvestre. 2002. APOD. http://antwrp.gsfc.nasa.gov/apod/ap020923.html Milky Way over Mount Blanc Interstellar Gas • Made up of individual atoms • Density averages 1 atom per cm3 • Composition: ~90% atomic or molecular hydrogen (H or H2). ~9% helium 1% heavier elements Hydrogen gas in the Rosette Nebula Nick Wright. 2005. APOD. http://antwrp.gsfc.nasa.gov/apod/ap051223.html Interstellar Dust • Clumps of atoms & molecules • Similar to smoke or fog (light from distant stars can’t pierce IS dust any more than a car’s headlights can illuminate objects in a thick fog. • Density averages 1 dust particle for every trillion gas atoms. • Even though density is low, it’s enough to block starlight because there is so much IS space. • Composition: not well known. Evidence for silicates, carbon & iron Snake Nebula: Star Forming Region Jean-Charles Cuillandre. 2002. APOD. http://antwrp.gsfc.nasa.gov/apod/ap020409.html Temperature • Interstellar space is very cold • Temperature averages 100 K (water freezes at 273 K. • 100 K = -279 F Nebula • Historically used to refer to any “fuzzy” patch (bright or dark) on the sky – any region of space that can be seen through a telescope, but that is not sharply defined like a star or planet. • Bright if something in cloud (like a group of hot young stars) causes it to glow. • Dark if they obscure stars lying behind them. • Many nebulae are stellar nurseries. Emission Nebulae • Glowing clouds of hot IS matter. • The gas glows because one or more nearby young stars ionize the gas • Ionized means the atoms in the gas become charged (remember the glowing gas in the spectroscopy lab). • The gas becomes ionized from UV radiation emitted by the nearby stars. Examples of Emission Nebulae N44 Emission Nebula WFO. 2006. APOD. http://antwrp.gsfc.nasa.gov/apod/ap060213.html IC 410: The Tadpoles Ken Crawford. 2006. APOD. http://antwrp.gsfc.nasa.gov/apod/ap060111.html IC 1805: Heart & Soul Nebulae Richard Crisp. 2005. APOD. http://antwrp.gsfc.nasa.gov/apod/ap050810.html Dark Dust Clouds • 99% of IS space is devoid of nebular regions & contains no stars. It is simply dark. • Within these dark areas are dark dust clouds that are cooler than their surroundings (T’s as low as 10’s of K) and thousands to millions of times denser (still as tenuous as best lab vacuums). • Most are bigger than our solar system. • Primarily made of gas, but a large amount of dust absorbs starlight. Examples of Dark Dust Clouds BHR 71 Alves et al. VLT. 2003. APOD. http://antwrp.gsfc.nasa.gov/apod/ap030127.html IC 2944: Thackeray’s Globules HST. 2003. APOD. http://antwrp.gsfc.nasa.gov/apod/ap030816.html Horsehead Nebula J.-C. Cuillandre . 2003. APOD. http://antwrp.gsfc.nasa.gov/ apod/ap031007.html Various Nebulae Lagoon Nebula Michael Sherik. 2005. APOD. http://antwrp.gsfc.nasa.gov/apod/ap050803.html Flaming Star Nebula Jean-Charles Cuillandre (CHFT). 2006. APOD. http://antwrp.gsfc.nasa.gov/apod/ap060228.html Orion in Sulfur, Hydrogen & Oxygen Russell Croman. 2004. APOD. http://antwrp.gsfc.nasa.gov/apod/ap040325.html Wolf-Rayet Binary Nebula ESO. 2004. APOD. http://antwrp.gsfc.nasa.gov/apod/ap040421.html Eagle Nebula HST. 2005. APOD. http://antwrp.gsfc.nasa.gov/apod/ap050424.html Eagle Nebula: EGG’s (Evaporating Gaseous Globules) = star forming regions HST. 2001. APOD. http://antwrp.gsfc.nasa.gov/apod/ap010812.html