Lecture 24 Basic Properties of the Milky Way Chapter 19 • Basic Properties and Environments • Recycling Activity • Structure of the Milky Way
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Basic Properties of the Milky Way We live in a galaxy(galactos – Greek for milk) Milky Way contains over 100 billion stars. Disk size is ~100,000 light years. The Sun is ~28,000 light years from the center. It consists of aflat disk with acentral bulge. The disk has a spiral shape with a few arms. Interstellar gas and dust collectively form the interstellar medium.
Never mind... Recycling Activity
The Gas-Star-Gas Cycle Chemical composition of the Milky Way is 70% hydrogen, 28% helium, ~2% heavy elements. Generations of stars recycle the same matter. The process of adding new elements is called chemical enrichment. Synthesis of elements is just one part of the galactic recycling.
Role of Supernovae Supernovae eject matter with speeds, exceeding the galactic escape speed. The ejected gas collides with the interstellar medium, slows down, and eventually stops. Multiple supernovae stir and heat the interstellar medium. It cools down and forms clouds of atomic and then molecular hydrogen, where stars form.
Star - Gas Stars return most of their original mass through stellar winds and death events. Death events: planetary nebulae and supernovae. Low-mass stars return about half of their mass. High-mass stars return almost all their mass. The high-speed gas from the winds and explosions sweeps up interstellar matter and creates a bubble around mass-losing stars.
Star - Gas The bubbles are expanding shells of hot, ionized gas. These bubbles fill ~2050 % of the galactic disk. They emit radiation in different spectral regions. Supernovae generate shock waves waves of pressure, where gas moves faster than the speed of sound. Multiple supernovae from a star cluster can create a superbubble, a cavity of ~1000 light years in size which may shoot into the halo.
Atomic Hydrogen Gas Atomic gas is more common than ionized gas in bubbles and superbubbles. It emits radio waves with a wavelength of 21 cm. From measurements of this emission, we can say that ~5 billion solar masses of gas is distributed throughout the galactic disk. Gravity pulls gaseous blobs together. They radiate energy more efficiently and cool. The process is slow a lot of atomic gas in the disk. Once cooled, gas becomes molecular.
Completing the Cycle When a few stars form in a cluster, their radiation begins to heat and ionize the surrounding gas. This prevents the entire cloud to turn into stars. Only the densest knots can resist evaporation. Star formation can continue in these knots. The newly formed massive stars will explode in a few million years, filling the region with hot gas enriched with heavy elements.
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Disk and Halo Most of the galactic gas is stored in the disk. The halo contains old, red, and dim stars, and almost no gas. The halo stars have much less heavy elements that the stars in the disk. These properties indicate that the bulk of the galactic gas settled down into the disk long ago.
The Solar Neighborhood We live in the galactic disk (near its mid-plane). There are >300 stars within ~33 light years. Most of them are Mdwarfs. However, a few stars are younger than the Sun (Sirius, Vega, Altair, Fomalhaut). There are no O or B type star near the Sun. This means that no star clusters have formed recently here. We live inside a hot bubble.
Motion in the Milky Way Stars in the galactic disk rotate around the galactic center in the same direction. Stars in the halo have more elliptical orbits and random directions. Halo stars cross the galactic disk with speeds, which are not affected by the disk gravity. Arcturus, one of the brightest stars in the sky, is a halo star crossing the disk near the Sun. The Sun orbits the center at a speed of ~220 km/s (230 million years one revolution).
Distribution of Mass A method, similar to using Newton’s and Kepler’s laws, can be used to weigh the galaxy. It involves the orbital velocity law (the dependence of the star’s orbital speed on distance from the galactic center). The solar parameters suggest that the total mass within the solar circle is 2 1041 kg or 100 billion solar masses.
The Galactic Mass The galactic rotation curve is flat. It shows that most of the mass of the Milky Way is not concentrated in its center. This result implies that there is additional, invisible, matter in the galaxy. It cannot be in the form of stars, since we do not see the light. We call it dark matter. It may account for up to 90% of the galactic mass.
Spiral Arms The disk of spiral galaxies show sweeping spiral arms. The explanation for the spiral arms is not clear. Young hot stars are usually seen in the arms. Enhanced amount of gas also resides there. Theoretical models suggest that spiral arms are formed by spiral density waves. They are generated by disturbances from close encounters with other galaxies. They do not carry out stars or gas with them.
The Galactic Center The galactic center is located in the constellationSagittarius and is not seen in the visible sky. We can detect it in the infrared and radio region. There is a bright radio source in the center (Sagittarius A*), a lot of gas clouds, and a large star cluster. Motions in Sgr A* indicate that it contains a few million solar masses in ~3 light years across.
Summary The Milky Way is a dynamic system of stars and gas, recycling material and creating new stars. Orbital speeds can be used to determine the distribution of mass in the Milky Way. It was discovered that most of the galactic mass is in its halo (dark matter). Galactic mixing and recycling made us live.
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