ASTR 1120 The Milky Way General Astronomy: Stars & Galaxies � omework #6 on Mas � ring As � onom � due on Tuesday, � ov. 03, by 5p � If your CU clicker grade is 0 and you have been in class, please send your clicker # to TA Thomas Rogers Milky Way Anatomy – Spiral Galaxy Size of the Milky Way • 100-400 billion stars • 100,000 light years in diameter • Sun (and us) are located ~28,000 light years from the center, in the ‘Orion Arm’ Artist’s sketch
Disk, Bulge & Halo Disk is very thin! • Disk: includes spiral arms -- young, new star formation • Bulge & Halo: older stars, globular clusters Artist’s sketch What Milky Way might look like! Spiral galaxy NGC 4414 Galaxy NGC 4565 � nearly edge-on
Clicker Question Clicker Question What kind of object lie in the halo of our What kind of object lie in the halo of our Galaxy? Galaxy? A. O and B stars. A. O and B stars. B. Gas and dust. B. Gas and dust. C. Globular clusters C. Globular clusters D. Open clusters. D. Open clusters. E. All of the above. E. All of the above. Clicker Question Clicker Question Where does most star formation occur Where does most star formation occur in the Milky Way today? in the Milky Way today? A. In the halo A. In the halo B. In the bulge B. In the bulge C. In the spiral arms C. In the spiral arms D. In the Galactic center D. In the Galactic center E. Uniformly throughout the Milky E. Uniformly throughout the Milky Way Way
Mapping the Milky Way An important Question • Galileo "For the Galaxy is nothing else than a congeries of innumerable stars distributed in clusters." William & Caroline Herschel (1785): star counts • – Counted stars along 683 lines of sight using their 48-inch telescope. Conclusion: Sun is in the center How do we know all we know and MW width is about 5 times its thickness about the Milky Way? Shapley’s globular clusters 6,500 ly 30,000 ly • Harlow Shapely measured distances to globular clusters – These appeared to be centered on a location tens of thousands of light-years from the Sun. Conclusion: Sun not in center, about 2/3 out
Stars in the disk all orbit in the same How Do Stars Orbit in Our Galaxy? direction with a little up-and-down motion • If they get too far above or below the disk, the gravity of everything in the disk pulls them back in Orbits of stars in the bulge and halo have random orientations • Evidence points to bulge and halo formed before the disk existed – Their orbits not affected (much) by the gravity of the disk
Clicker Question Why spiral arms? Why do orbits of disk stars bob up and “Density waves” – stars move in and down? out of denser regions Like ripples in a pond A. They’re stuck to the interstellar medium which moves like that because of its density. B. The gravity of disk stars pulls toward disk In dense regions, star formation is more C. The halo stars push them back into disk intense, so “arms” D. Their orbits carry them out but then they are brighter bounce off the edge of the Galaxy and head back in. E. The density waves in the spiral arms kick M51 - Whirlpool them out of the disk. Clicker Question Galaxies: Ultimate Recyling Plants Why do orbits of disk stars bob up and down? A. They’re stuck to the interstellar medium which moves like that because of its density. B. The gravity of disk stars pulls toward disk C. The halo stars push them back into disk D. Their orbits carry them out but then they bounce off the edge of the Galaxy and head back in. E. The density waves in the spiral arms kick them out of the disk.
Clicker Question Clicker Question Which generation of stars do you Which generation of stars do you expect to be more metal-rich? expect to be more metal-rich? A. Older population of stars (i.e. stars A. Older population of stars (i.e. stars formed a very long time ago) formed a very long time ago) B. Younger population of stars (i.e. B. Younger population of stars (i.e. formed more recently) formed more recently) C. No difference C. No difference Molecular clouds = Contents: Cold stuff star forming regions • Molecular CLOUDS – Mostly atomic hydrogen, some helium and other CO molecules Doppler • Dark, dusty, cold Images – 10-30K • Emit molecular emission lines in far IR, radio • Orion image here in carbon monoxide (CO) – colors are Doppler shifts
Semi-Warm stuff Mapping Cold Hydrogen • Dust: • Even the coldest – absorbs visible hydrogen emits and UV light faint emission lines – Transparent to in the RADIO long wavelengths (red, IR, radio) • Change in energy levels of nuclear • Emits IR light configuration All sky 21 cm radio mapping • Wavelength: 21 cm Horsehead Nebula Hot stuff Dust+dark Ionization nebulae molecular “O & B star clouds associations” Horsehead • Hot stars excite in close-up atomic transitions in hydrogen and other light elements VLT in the gas (Very Large Telescope) • T~ 10,000 K near hot young stars Lagoon Nebula
Clicker Question If we took a spectrum of the gas in an ionization nebula, what should we expect to see? A. A continuous spectrum B. Emission lines of hydrogen plus a few other slightly heavier elements A. A continuous spectrum C. Emission lines of elements all the way up to B. Emission lines of hydrogen plus a few other slightly heavier iron and a lot of heavier elements elements D. Absorption lines of a number of elements C. Emission lines of elements all the way up to iron and a lot of heavier elements E. Absorption lines from hydrogen only D. Absorption lines of a number of elements E. Absorption lines from hydrogen only Stellar nursery (Sharpless 140) Trifid nebula (M20) visible Spitzer infrared Spitzer IR image shows deeply embedded O-type stars within dark dust cloud encasing them
Really Hot Stuff More hot stuff Planetary Supernova & Hot star Nebula Remnants winds from aging stars (10 ly across) Superbubbles & Fountains Some stuff is REALLY HOT • When multiple bubbles join (from a cluster) they • Bubbles of hot gas can create superbubbles. blown out by SUPERNOVAE • Superbubbles can blast hot gas even out of the • T = tens of millions Galaxy! of degrees K • “Enriches” gas between • Mixing with rest of galaxies galactic gas � enrichment with • Some will rain back down heavy elements and mix into the Galaxy Artists’ conceptions!
Superbubbles in spiral galaxy NGC 3079 Edge-on view of spiral galaxy NGC 4013 • Dust/gas in disk obscures light • Plumes and fuzz sticking out are “fountains” & “superbubbles” from supernovae HST HST Inside our Galaxy: Material is Constantly Being Recycled
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