Our Problem We are inside one of the spiral arms.
Our Parent Galaxy Name from the Greeks (galactos milk) via the Romans Our sun is in the galactic disk which contains most of the luminous stars and interstellar matter. 100-200 billion stars. The density of stars and interstellar matter makes it difficult to study the Galaxy from within. We see only about 10%. Yet except for Andromeda and the Large and Small Magallanic clouds every naked eye object belongs to the Milky Way.
Our Parent Galaxy cont Much of what we think we know of our galaxy comes from the study of other galaxies similar to our own. Ours is a spiral or more likely a barred spiral galaxy. All spiral galaxies have a Galactic bulge and disk embedded in a roughly spherical ball of faint old stars called the Galactic halo.
Andromeda Structure • Andromeda Galaxy , 2.5 million lyr. away, 30,000 pc across • b) more detail • c) Double core, 15 pc
Star Counts Early attempts William Herschel in late 18th century tried to estimate the size and shape of the galaxy (the universe at that time) by counting stars in different directions. ..found Sun at center of “grind stone” , flattened disk estimated later to be about 10 kpc diameter and 2 kpc thick. The error came from not knowing the role or the extent of interstellar gas and dust. Any objects more than a few kpc are hidden in the visible region. Herschel’s shape has more to do with visibility than stellar distribution.
Dusty gas clouds obscure our view because they absorb visible light This is the interstellar medium that makes new star systems
Early Galaxy markers Two classes of objects were the focus of study of the large scale structure of the universe: Globular clusters and Spiral nebulae Both are too far away for parallax measurement and the HR diagrams (1911) were useless since individual stars couldn’t be clearly identified. It was assumed globular clusters were within our galaxy , spiral nebulae were even less understood. Early photographs were thought to be stars forming.
We see our galaxy edge-on Primary features: disk, bulge, halo, globular clusters
From NASA NSN materials
If we could view the Milky Way from above the disk, we would see its spiral arms
NGC 4603 Spiral galaxy, 100 million light years away Blue (young stars) Red giants (older) Only brightest stars are resolved, others merge into haze.
Stars in the disk all orbit in the same direction with a little up-and-down motion
Orbits of stars in the bulge and halo have random orientations
Sun’s orbital motion (radius and velocity) tells us mass within Sun’s orbit: =1.0 x 1011MSun
Lower mass stars return gas to interstellar space through stellar winds and planetary nebulae. About ½ the original mass of the star is recycled into the interstellar medium.
X-rays from hot gas in supernova remnants reveal newly-made heavy elements
Supernova remnant cools and begins to emit visible light as it expands New elements made by supernova mix into interstellar medium. Interaction with the interstellar medium helps further slow and cool the remnant.
Radio emission in supernova remnants is from particles accelerated to near light speed Cosmic rays probably come from supernovae
Multiple supernovae create huge hot “super bubbles” that can blow out of disk in what is called a galactic fountain. Gas clouds cooling in the halo can rain back down on disk
Atomic hydrogen gas forms as hot gas cools, allowing electrons to join with protons Molecular clouds form next, after gas cools enough to allow to atoms to combine into molecules
Molecular clouds in Orion • Composition: • Mostly H2 ~70% • About 28% He • About 1% CO • Many other • molecules
Gravity forms stars out of the gas in molecular clouds, completing the star-gas-star cycle
Radiation from newly formed stars is eroding these star-forming clouds
Summary of Galactic Recycling • Stars make new elements by fusion • Dying stars expel gas and new elements, producing hot bubbles (~106 K) • Hot gas cools, allowing atomic hydrogen clouds to form (~100-10,000 K) • Further cooling permits molecules to form, making molecular clouds (~30 K) • Gravity forms new stars (and planets) in molecular clouds Gas Cools
Thought Question Where will the gas be in 1 trillion years? A. Blown out of galaxy B. Still recycling just like now C. Locked into white dwarfs and low-mass stars
We observe star-gas-star cycle operating in Milky Way’s disk using many different wavelengths of light
Infrared Visible Infrared light reveals stars whose visible light is blocked by gas clouds
X-rays X-rays are observed from hot gas above and below the Milky Way’s disk
Radio (21cm) 21-cm radio waves emitted by atomic hydrogen show where gas has cooled and settled into disk
Radio (CO) Radio waves from carbon monoxide (CO) show locations of molecular clouds
IR (dust) Long-wavelength infrared emission shows where young stars are heating dust grains
Gamma rays show where cosmic rays from supernovae collide with atomic nuclei in gas clouds
Ionization/emission nebulae are found around short-lived high-mass stars, signifying active star formation
Reflection nebulae scatter the light from stars Why do reflection nebulae look bluer than the nearby stars?
Reflection nebulae scatter the light from stars Why do reflection nebulae look bluer than the nearby stars? For the same reason that our sky is blue!
Halo: No ionization nebulae, no blue stars no star formation Disk: Ionization nebulae, blue stars star formation
Much of star formation in disk happens in spiral arms Whirlpool Galaxy
Much of star formation in disk happens in spiral arms Ionization Nebulae Blue Stars Gas Clouds Whirlpool Galaxy
Spiral arms are waves of star formation • Gas clouds get squeezed as they move into spiral arms • Squeezing of clouds triggers star formation • Young stars flow out of spiral arms