Astronomy 1020-H Stellar Astronomy Spring_2015 Day-32

Course Announcements Smartworks Chapter 14: Friday 2 Dark night observing sessions left: Mon. Apr. 13 & Thurs. Apr. 16 Reports are due Wed. Apr. 22 Solar Rotation Project due Mon. Apr. 27

Astronomy in the Fall, 2015 ASTR-1010/ Planetary Astronomy + Lab (H,R) ASTR-1020/ Stellar Astronomy + Lab (R) ASTR Problems in Planet Astronomy ASTR Intro. to Observational Astronomy ASTR-4000/4001 – Astrophotography & Lab ASTR-4170 – Special Topics in Astronomy Photometry and Filter Systems TR; 3:30-5:00; B310

Where do Stars Form? Spirals

Even in the Galactic Center

Where don’t Stars Form? Ellipticals

 The interstellar medium: gas and dust between the stars.  Near the Sun, chemical composition is similar.  Most is gas; 1 percent is interstellar dust.

 The gas is very tenuous: about 1 atom per cubic centimeter.  It emits various kinds of light, depending on its temperature.

 Dust is in the form of solid grains.  “Interstellar soot” (iron, silicon, carbon, and more).  Dust blocks visible light from stars, galaxies, etc.: interstellar extinction.

 Size of dust particles: large molecules up to 300 nanometers.  Therefore, dust blocks short wavelengths more efficiently.  More red light is let through: interstellar reddening.  Long wavelengths (infrared and radio) penetrate dust.

 A star viewed through dust is fainter and redder due to interstellar extinction and reddening.  The position of absorption lines are not affected.

 Dust also emits light when it blocks a star.  Energy from absorbed light is transferred to the dust, heating it.  Dust grains are typically cool (10–300 K).  Therefore, they emit infrared radiation.

 Most gas and dust is concentrated in relatively dense interstellar clouds.  The material found between clouds is called intercloud gas.

 Some regions can be very hot (10 6 K).  X-rays emitted, but extremely tenuous.  The Sun resides in a local bubble of million- degree gas.  Most intercloud gas is 8000 K.

 H II regions (about 10 4 K):  Hydrogen heated and ionized by ultraviolet light from hot, luminous stars (O and the hottest B).  Ionized: stripped of one or more electrons.

 At lower temperatures, hydrogen is in single, neutral atoms.  This gas emits radio waves with = 21 cm.  Light of this wavelength penetrates the dust.  Good for mapping the Milky Way.

 Many clouds are cold enough for hydrogen to be in the H 2 molecule.  These are called molecular clouds.  Dense and cold, and appear dark.

 Temperatures are around 10 K, with densities as high as molecules/cm 3.  Emit radio waves.  Many other molecules are in the mix.

 Some clouds can have masses as large as 10 million times that of the Sun: giant molecular clouds.  On average, 120 light-years in size.  Stars form in these molecular clouds.

 Molecular clouds are cold and dense.  Some places in the cloud are denser than average.  Self-gravity will make these regions collapse.

 Rate of collapse is slowed by magnetic fields, turbulence, and angular momentum (spin).  Collapse and fragmentation lead to dense star-forming molecular-cloud cores.  The Sun began in one of these cores.

 Molecular cores collapse under their own gravity.  Center shrinks fastest; outer layers later.  This produces a dense protostar.

 Spin of core produces a disk of material around the protostar.  Material falls onto the growing protostar from the disk.  Our Solar System began this way.

 The interstellar material that becomes stars and planets must have the key elements for life if those planets are going to have it.  Water and oxygen have been detected in some star-forming regions.

Concept Quiz—H II Regions In H II regions, the hydrogen gas is in what form? A.doubly ionized hydrogen B.once-ionized hydrogen C.neutral hydrogen atoms D.hydrogen molecules