I. General Properties of the Optically Observable Milky Way Galaxy II.The

Lecture 34 - 1 The Milky Way Galaxy: Our Home in the Universe 9 April, 2012 (Read Chapter 15 as background for lecture notes) Note: Our Galaxy – The Milky Way – is composed of billions of stars which were born over the past history of the galaxy. Our Galaxy also contains a fair amount of gas and dust out of which new stars are currently forming. I. General properties of the optically observable Milky Way galaxy Overall shape = a flattened disk, like the solar system, but less centrally concentrated + many more component parts. Disk diameter = ~40,000 parsecs (or 40 kpc) Stars vs. gas &dust = ~96% stars, ~4% gas &dust (observable matter - by mass) 11 Mass (40kpc) = ~500 billion Msun (5 x 10 Msun) # of stars (40kpc) = ~500 billion stars (5 x 1011 stars) 10 Luminosity (40 kpc) = ~50 billion Lsun (5 x 10 Lsun) 8 rotation period (Sun) = ~200 million years⊕ (10 years⊕) II.The constituent parts of the Milky Way • The Galactic Disk-- The disk contains a mixture of stars, gas and dust. Nearly all of the gas, dust and the youngest stars in the Galaxy reside in a thin disk (~ 0.5 kpc); these young stars are referred to as Population I stars. The older disk stars are more spread out in a thick disk (~2 kpc); these older stars are referred to as Population II stars. The gas and dust represents material in between the stars, hence are collectively referred to as the interstellar medium (or ISM). The ISM is normally divided into 3 parts, depending on the temperature, density and ionization state of the dominant atomic constituent, Hydrogen: ISM (molecular clouds) = the coldest and densest regions, where the hydrogen is in molecular form (H2); ISM (atomic clouds) = warmer regions where the hydrogen is in atomic form (H); and ISM (ionized regions, or HII regions) = very warm regions where the gas is ionized (H+ ,or alternatively HII). Approximately 50% of the gas and dust 5 6 is found in the coldest regions (H2) in Giant Molecular Clouds, whose masses range from 10 to 10 Msun ; these GMCs are the main birth sites of all new stars. The origin of the gas and dust is both material left over from the formation of the Galaxy, plus newer material expelled from massive stars in their red giant/death phases. Viewed face-on, the most active star-forming GMCs appear to be concentrated in a pinwheel pattern that is often referred to as “spiral arms”. • The Galactic Nucleus -- The nucleus is a central dense concentration of stars + gas and dust, ~ 0.1 kpc in diameter. The galactic nucleus as viewed from the Sun is toward the constellation Sagittarius, and its distance 8 7 from the Sun is ~8.5 kpc. The nucleus contains ~10 Msun of stars (both young and old), plus ~10 Msun of gas. 6 The innermost region of the nucleus appears to also contain a massive black hole of 10 Msun. • The Galactic Spheroid -- The galactic spheroid represents an approximately spherical distribution of older (i.e. Pop II) stars centered on the galactic nucleus, with a diameter of ~10 kpc. • The Galactic Halo -- The halo contains the oldest stars, some of which are distributed in a slightly flattened ellipsoid with major axis diameter ~80 kpc, and others of which are in a spheroidal distribution of ~1000 5 6 globular clusters. Each globular cluster contains anywhere from 10 to 10 Msun of stars. The distribution of globular clusters in the sky was what allowed astronomer Harlow Shapley in the 1920’s to estimate the true distance from the Sun to the galactic center, and hence to estimate the true size of our Milky Way galaxy. The halo also appears to contain a large fraction of “dark matter” which may be in the form of very low mass stars (i.e. brown dwarfs) or in some other currently unknown form of matter (i.e. exotic particles). • The Galactic Corona -- Until recently, this portion of our Galaxy was entirely unknown. Recent data have shown that our Galaxy may actually extend out to ~200 kpc diameter, or nearly 5 times the diameter of the optically observable disk. The Galactic corona may contain as much as 10 times the total mass of all of the other parts of the Galaxy put together. Until very recently, the nature of the “unseen matter” that makes up most of the galactic corona was unknown. The recent discovery that neutrinos have mass, gives added weight to the notion that at least some of this “dark matter” may be in neutrinos. .

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