Our Galaxy

The Milky Way Revealed Our goals for learning What does our galaxy look like? How do stars orbit in our galaxy? REVIEW: Recall from our Introduction that stars are the building blocks of huge conglomerations called galaxies. Our Sun is one star in a huge assembly of stars called the Milky Way.

All-sky picture taken with fish-eye lens. This hazy band of light is our galaxy seen from our vantage point inside it. Galileo Galilei was the first person to use a telescope to resolve the Milky Way into numerous individual stars. Living inside the Milky Way

All-Sky View This projection shows the entire sky A GALAXY is any gravitationally-bound conglomeration of billions of stars. The galaxy containing our Sun is called the Milky Way and we can only see it from the inside. It is a very large structure.

Edge-on, our galaxy looks like a flattened “disk” of stars with a central concentration or “bulge” of stars. Note the location of the Sun, very far from the Center. The disk is 1,000 light-years thick but 100x that is width – very thin! Primary features: disk, bulge, halo, globular clusters

If we could view the Milky Way from above the disk, we would see that it has Spiral Arms. Many other galaxies look the same, for example the nearby spiral galaxy (M31) in Andromeda.

Stars in the disk all orbit in the same direction with a little up-and-down motion (exaggerated here). How do stars orbit in our galaxy?

Orbits of stars in the Bulge and Halo have random orientations, even crossing through the disk plane. How do stars orbit in our galaxy? Disk plane Center Note: stars don’t collide – too small compared to distance between them.

Thought Question Why do orbits of Disk stars bob up and down? A. They’re stuck to interstellar medium B. Gravity of disk stars pulls toward disk C. Halo stars knock them back into disk

Thought Question Why do orbits of Disk stars bob up and down? A. They’re stuck to interstellar medium B. Gravity of disk stars pulls toward disk C. Halo stars knock them back into disk The net gravitational effect of a large flat disk of mass is to pull objects vertically toward the disk. Objects with a slight motion out of the plane of the disk are pulled back. Disk of stars

Orbital Velocity Law The orbital speed (v) and radius (r) of an object on a circular orbit around the center of the galaxy tells us the mass (M r ) within that orbit.

The Sun’s orbital motion (radius and velocity) tells us the mass within the Sun’s orbit: ~1.0 x M Sun THE MASS OF THE GALAXY

What have we learned? What does our galaxy look like? The Milky Way Galaxy consists of a thin disk of stars about 100,000 light- years in diameter with a central bulge and a spherical region called the halo that surrounds the entire disk. The disk also contains the gas and dust of the interstellar medium, while the halo contains very little gas.

What have we learned? How do stars orbit in our galaxy? Stars in the disk all orbit the galactic center in about the same plane and in the same direction. Halo and bulge stars also orbit the center of the galaxy, but their orbits are randomly inclined to the disk of the galaxy.

Galactic Recycling Our goals for learning How is gas recycled in our galaxy? Where do stars tend to form in our galaxy?

How is gas recycled in our galaxy? Star-Gas-Star cycle: gas from old stars is recycled into new stars Let’s follow the sequence from here.

High-mass stars have strong stellar winds that blow bubbles of hot gas …

Lower mass stars return gas to interstellar space through stellar winds and planetary nebulae …

X-rays from very hot gas in Supernova Remnants reveal newly- made heavy elements that get thrown back into the interstellar medium…

As the supernova remnant cools it begins to emit visible light as it expands. New elements made by the supernova mix into the interstellar medium and we detect them using spectroscopy.

Multiple supernovae create huge hot bubbles that can blow out of the galactic disk. Gas clouds cooling in the halo can “rain” back down on disk.

Molecular clouds in Orion Composition: Mostly H 2 About 28% He About 1% CO Many other molecules Atomic hydrogen gas (H) forms as hot gas cools, allowing electrons to join with protons to make normal atoms. Molecular clouds form next, after the gas cools enough to allow atoms to combine into molecules; mostly H 2.

Gravity forms stars out of the gas in the molecular clouds, thus 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 (~10 6 K) Hot gas cools, allowing atomic hydrogen clouds to form (~100-10,000 K) Further cooling permits molecules to form (mainly H 2 ), 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

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 1 trillion years = 1,000 billion years, much longer than the lifetime of the Sun

We observe star-gas-star cycle operating in Milky Way’s disk using many different wavelengths from gamma rays to radio waves. THE MILKY WAY AT DIFFERENT WAVELENGTHS

Radio (atomic hydrogen) Visible FOR EXAMPLE, 21-cm radio waves emitted by atomic hydrogen show where gas has cooled and settled into disk

Long-wavelength infrared emission shows where young stars are heating dust grains inside molecular clouds IR (dust) Visible

X-rays are observed from very hot gas above and below the Milky Way’s disk X-rays Visible

Ionization nebulae are found around short-lived high-mass stars, signifying active star formation Where do stars tend to form in our galaxy? Orion Nebula

Reflection nebulae scatter the light from stars. The radiation from less massive young stars does not ionize the cloud. Why do reflection nebulae look bluer than the nearby stars? For the same reason that our sky is blue! Blue light (shorter wavelength) is “scattered” more than red light.

Disk: Ionization nebulae, blue stars  active star formation Halo: No ionization nebulae, no blue stars  no star formation

Much of the star formation in the disk happens in the Spiral Arms. Whirlpool Galaxy Ionization Nebulae Blue Stars Gas Clouds Spiral Arms are best explained as “density waves” – a bunching up of moving material as it orbits the Galactic Center

1.Gas clouds get squeezed as they move into spiral arms 2.Squeezing of clouds triggers star formation 3.Young stars flow out of spiral arms Spiral arms are waves of star formation Density Wave: think of traffic! Cars on the freeway bunch up as they approach a slow moving construction crew working along the hard shoulder. Bunching up is the density wave. The wave keeps moving forward, and each car gets through … eventually.

What have we learned? How does our galaxy recycle gas into stars? Stars are born from the gravitational collapse of gas clumps in molecular clouds. Near the ends of their lives, stars more massive than our Sun create elements heavier than hydrogen and helium and expel them into space via supernovae and stellar winds. The supernovae and winds create hot bubbles in the interstellar medium, but the gas within these bubbles gradually slows and cools as they expand. Eventually, this gas cools enough to collect into clouds of atomic hydrogen. Further cooling allows atoms of hydrogen and other elements to collect into molecules, producing molecular clouds.These molecular clouds then form stars, completing the star–gas–star cycle. Stacie Kent: Insert thumbnail of 14.8 Stacie Kent: Insert thumbnail of 14.8

What have we learned? Where do stars tend to form in our galaxy? Active star-forming regions, marked by the presence of hot, massive stars and ionization nebulae, are found preferentially in spiral arms. The spiral arms represent regions where a spiral density wave has caused gas clouds to crash into each other, thereby compressing them and making star formation more likely.

The History of the Milky Way Our goals for learning What clues to our galaxy’s history do halo stars hold? How did our galaxy form?

Halo Stars: Contain only % heavy elements (O, Fe, …)  only old stars, not much reprocessing by gas-star-gas cycle Disk Stars: 2% heavy elements,  stars of all ages, lots of reprocessing of material Conclusion: Halo stars formed first then stopped; disk stars formed later and kept forming.

Our galaxy probably formed from a really giant gas cloud How did our galaxy form?

Halo stars formed first as gravity caused cloud to contract

Remaining gas settled into a spinning disk

Stars continuously form in the disk as galaxy grows older Warning: This model is oversimplified

Detailed studies: Halo stars formed in clumps that later merged to form a single, larger protogalactic cloud. In other words, the early Milky Way suffered collisions and mergers.

What have we learned? What clues to our galaxy’s history do halo stars hold? –Halo stars are all old, with a smaller proportion of heavy elements than disk stars, indicating that the halo formed first How did our galaxy form? –Our galaxy formed from a huge cloud of gas, with the halo stars forming first and the disk stars forming later, after the gas settled into a spinning disk

The Mysterious Galactic Center Our Goals for Learning What lies in the center of our galaxy? A BLACK HOLE!

Infrared light from centerRadio emission from center The Galactic Center lies 28,000 light-years from the Sun, hidden by layers of dusty spiral arms. Visible light cannot penetrate. But … Zooming in …

Radio emission from centerSwirling gas near center Strange radio sources in galactic center (Sgr A*).

Swirling gas near center Orbiting stars near center Infrared image obtained by Adaptive Optics methods. Stars at galactic center: we can now map out the motions over many years, and use Kepler’s 3rd Law to get orbits. Location of Sgr A*

Stars appear to be orbiting something very massive but it is invisible … a black hole? The orbits of stars indicate a mass of about 4 million M Sun We will see later that this is not unusual; many galaxies harbor very massive Black Holes.

Further Evidence: X-ray flares from the Galactic Center suggest that the tidal forces of the suspected black hole occasionally tear apart chunks of matter that are about to fall in. X-ray image from the Chandra X-ray telescope in space GALACTIC CENTER

What have we learned? What lies in the center of our galaxy? –Orbits of stars near the center of our galaxy indicate that it contains a black hole with about 4 million times the mass of the Sun –This is evidence for black holes much larger than stellar-mass objects