Basic Facts about the Milky Way

Transcription

1 THE MILKY WAY

2 Basic Facts about the Milky Way The Sun is one of about 200 billion stars in the Milky Way Galaxy The Milky Way is a spiral galaxy, with a flat disk, central bulge and bar, and spherical halo The disk contains stars, gas, and dust. The halo contains stars and globular clusters

3 First, a lijle history: The first efforts to map the structure of the Milky Way through star counts concluded that we were in the center of the Milky Way, because exoncoon of dust was neglected William and Caroline Herschel's Picture of the Milky Way (18th century):

4 What he didn t know: Interstellar Space in the disk of the Milky Way is filled with Gas and Dust Dust in the gas clouds make stars look RED and FAINT.

5 Harlow Shapley (Harvard Observatory) used RR Lyrae Stars in Globular Clusters to map the halo of the Milky Way in about 1915 X = Center of The Milky Way He realized that the Sun is not in the center of the Milky Way

6 The modern view of the structure of the Milky Way uses IR and radio maps to sort out the structure of the disk, and map out the spiral arms through the dust.

7 Milky Way RotaOng disk of gas, dust and stars Bulge of stars Central bar of gas, dust and stars Spherical Halo of stars and globular clusters

8 Stars rotate in the disk, with a lijle up and down mooon

9 In the halo, the orbits of the stars are in random planes

10 Here's the actual structure of the spiral arms of the Milky Way: The Milky Way s disk is ROTATING The Sun travels once around the Milky Way disk in 220 Million years

11 If you could fly out of the Milky Way and look back on it, it might look something like this: M83

12 Amazing Fact About the MILKY WAY #1: Stars are forming today out of the gas clouds in the disk. Molecular Clouds: Mostly H 2 and CO Supernovae Red Giant Mass Loss Planetary Nebulae

13 Star formaoon is triggered by spiral density waves, which gives a spiral galaxy its characterisoc spiral arms".

14 Why are there spiral arms? Are the spiral arms in galaxies just the result of stars closer to the center orbiong faster than those further out? NO The spiral arms would have wound up and disappeared long ago

15 SPIRAL DENSITY WAVES Instead, the spiral arms are standing waves, and the gas and stars streams through (traffic accident analogy, see above) The spiral arms are rotaong, but very slowly As gas streams through the spiral arm, it gets shocked, and starts to make stars So the young, bright blue stars are bunched up in the spiral arms

16 Tucsonans are familiar with standing waves in flooding rivers

17 Standing (staoonary) Wave in river

18 Back to spiral arms and Spiral Density Waves The spiral arms are a pajern, that rotates more slowly than the gas and dust in the disk When the gas moves through the spiral arms, it is compressed and forms stars The young, bright blue stars are therefore found in the arms. By the Ome the disk moves through the spiral arm, the young stars have lei the main sequence and are red.

19 Green gas cloud passes through red arm Blue stars form, then explode as supernovae AnimaOon from G. Rieke

20 Star formaoon regions Brown Dust

21 What starts the spiral density waves? Maybe the waves are excited by a satellite galaxy Computer model Real galaxy and its satellite

22 A few words about the gas and dust in the disk of the Milky Way

23 Most of the interstellar gas is hydrogen. Astronomers map the hydrogen using radio telescopes, using the 21- cm line energy released by the spin flip of electron

24 The Milky Way mapped in 21cm: atomic hydrogen In this picture, the sky is projected such that the plane of the Milky Way disk is running horizontally across the center Some streams of hydrogen above the disk may be bubbles blown out of the disk by supernova explosions

25 21- cm Map of Spiral arms in the Milky Way Yellow arrow: The Sun Blue Dot: Center of The Milky Way

26 In the densest gas clouds, most of the gas is H 2 = molecular hydrogen Ultraviolet photons from hot stars can destroy the molecular hydrogen, dividing the molecules into individual hydrogen atoms So only the densest clouds have molecular clouds

27 Carbon Monoxide Molecule Spectrum has lines at wavelength of 2- mm Interstellar Molecules like CO (Carbon Monoxide) emit radiaoon by rotajon and vibrajon rather than electrons changing energy levels

28 RotaOon and vibraoon of molecules at specific rates: Changes in rotaoon and vibraoon changes in energy of the molecule spectral lines, observed by mm telescopes

29 Giant Molecular Clouds RotaOng in disk of the Milky Way 100,000 a million solar masses in molecules light years across Cold: degrees Kelvin DensiOes: million molecules per cubic cm If they pass through a spiral density wave, they start to collapse and form stars

30 Dust Mostly carbon and silicon Dust parocles parocles are a few micro- meters in size Dust which is deep in molecular clouds has frozen water and CO 2 Atomic collisions and ultraviolet light heats the dust, so the dust grains radiate like black bodies, in the infrared Mass in dust is only 1% of the mass in gas Interstellar dust was formed in the cool outer regions of red giants Distributed into interstellar space by planetary nebulae, stellar winds

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