The Milky Way

The Milky Way: Our Home Galaxy What are the different components of the Milky Way? How do we see those components? What does a map of each component look like from our point of view?

Stars in the Milky Way At visible wavelengths, we see mostly light from stars. Some of that starlight is blocked by huge clouds of gas and dust.

Stars in the Milky Way At the shortest infrared wavelengths (slightly redder than the visible spectrum), dust becomes transparent, so light from distant stars reaches us more easily.

Dust in the Milky Way At longer infrared wavelengths, we see thermal emission from interstellar dust rather than light from stars.

Radio emission from Hydrogen The lowest orbital of the hydrogen atom is not one level – it is actually 2 levels separated by a miniscule amount of energy. A transition from the upper level to lower level produces a photon at very low energies (radio wavelengths).

Hydrogen in the Milky Way Radio waves are not blocked by dust, so we can see the emission from hydrogen in space across the entire galaxy. Note: the colors in this image are not real. Different colors represent different radio brightnesses

The Milky Way across the Electromagnetic Spectrum

Star Clusters in the Milky Way Open Clusters  Hundreds to thousands of stars  Gravitational attraction between the stars is not strong enough to hold them in the same area of space, so the stars escape and the cluster disappears  Relatively young (<2 billion years) Globular Clusters  10,000 to millions of stars  Because there are so many stars, gravity is strong enough to keep stars from wandering away  Relatively old (up to 13 billion years)

Open Clusters

Globular Clusters

Motions of Stars in Clusters

Measuring the Age of a Star Cluster The main sequence turn-off is fainter for older clusters.

Open clusters contain stars high on the main sequence. Since those stars don’t live long, these clusters must be fairly young. Ages of Open Clusters

Ages of Globular Clusters In globular clusters, only the low-mass stars are still on the main sequence, so these clusters must be very old (>10 billion years).

Structure of the Milky Way How large is the Milky Way? What is its shape? Where are we in the Milky Way? To answer these questions, we need to construct a 3-D map of the Milky Way, and for this we need to measure distances to lots of stars It also will help if we can distinguish old stars from young stars, so we need to measure ages of stars

Metals = elements heavier than H and He

For individual stars that aren’t in clusters (like the Sun), we can’t use the cluster turnoff method to measure an age. For instance, a lone G star might be young, or it might be 10 billion years old. How do we measure its age? The universe contained only hydrogen, helium, and one other element (lithium) when it was born. Stars have created heavier elements, or “metals”, over time through fusion and supernovae. Some of these metals are sent into space when stars die. The cloud of gas and dust enriched by those metals can then form a new generation of stars. As a result, a star born more recently has a higher fraction of metals, or a higher metallicity, than a star born long ago. So we can estimate the ages of stars by measuring their metallicities. Measuring Ages of Individual Stars

Measuring a Star’s Metallicity If the absorption lines from metals in the spectrum of a star are strong, then the star has a high metallicity, and it must be young. If the metal lines are weak, then the metallicity is low, and the star must be old.

Measuring Distances in the Milky Way Parallaxes can be used to measure distances for stars within ~1000 light years from the Sun; parallaxes of more distant stars are too small to measure, even with modern telescopes. Most stars in the Milky are farther than 1000 light years, so we need another way to determine the diameter of the Milky Way and our location within it.

Imagine that you find a star with an unknown distance, but you notice that it has a distinctive characteristic that tells you that it is a specific type of star. And let’s imagine that all stars of that type have the same luminosity, and you happen to know the value of that luminosity. You can then estimate the distance for the star you found from the inverse square law of light: b = L / d 2 where b is the brightness seen from Earth L is the luminosity that all stars of this type are known to have d is the distance. Objects that are distinctive and have known luminosities are standard candles. Measuring Distances in the Milky Way

Pulsating Stars as Standard Candles There is a narrow region in the HR diagram where stars pulsate, getting bigger and smaller (i.e., brighter and dimmer) over time: There are 2 types of pulsating stars: RR Lyrae and Cepheids.

Finding RR Lyrae Stars Pulsating stars are good standard candles because they have a distinctive signature (pulsating light) that makes it easy to identify them. Globular clusters contain many RR Lyrae stars. Since we know that all RR Lyrae stars have a specific luminosity, we can measure the distance to a RR Lyrae star (and hence the cluster containing it) with the inverse square law of light.

When we measure distances to globular clusters with RR Lyrae stars and map their distribution, we find that they are not centered around the Sun. Instead, the globular clusters are scattered about a point 25,000 light years from us, which we assume is the center of the Milky Way. The Center of the Milky Way 100,000 light years 25,000 light years

Shape of the Milky Way Gas and dust clouds, open clusters, and most stars are concentrated in a narrow band wrapping around the sky. So these parts of the galaxy must form a flattened disk. However, globular clusters are found all across the sky, not just in that narrow band, so they must have a spherical distribution surrounding the disk, called a halo. Stars/gas/ dust/open clusters

Size of the Milky Way The Sun is in the disk between 2 spiral arms about halfway from the Galactic center to the edge of the galaxy. The Milky Way contains 200 billion stars and is 100,000 light years in diameter.

Rotation of the Milky Way Objects that are gravitationally bound to each other orbit around a point called the center of mass, which is the average position of those 2 objects. If they have equal masses, the center of mass is the point halfway between the objects. x center of mass

Rotation of the Milky Way If the objects have different masses, the center of mass is closer to the more massive object. x center of mass

Rotation of the Milky Way In the Milky Way, stars orbit around its center, which is the center of mass of all of its stars, gas, and dust. The Sun completes one orbit in roughly 250 million years. x center of mass

Rotation of the Milky Way An alternative way of visualizing the rotation of the Milky Way is in terms of its gravity well. Just as a planet orbits in the gravity well of a star, the Sun and other stars orbit in the gravity well produced by the sum of all matter in the galaxy.

Slides beyond this point contain extra material that you might find interesting but is not covered on homeworks and exams

The Milky Way’s Spiral Structure The Milky Way has a distinctive spiral pattern. Each spiral arm is a concentration of stars (especially young ones) and clouds of gas and dust. Why does the Milky Way have this spiral structure?

The Milky Way’s Spiral Structure If the Milky Way’s spiral arms were rigid concentrations of stars, the arms would rotate as the stars orbited the galaxy’s center. Also, the outer stars would move faster than the inner stars, but that’s not what we observe (and it would violate with Kepler’s 2 nd Law). So the arms are not rigid concentrations of stars.

The Milky Way’s Spiral Structure If the spiral arms were concentrations of stars that always stay together, and if outer stars orbit slower than inner stars like we expect, then the arms would become more tightly wound, and would dissolve after a few rotations. But this seems unlikely since it would mean we got lucky and lived at just the right time to see spiral arms.

The Milky Way’s Spiral Structure In reality, the spiral arms are produced by a density wave that compresses gas/dust clouds and stars as it sweeps around the disk, analogous to a concentration of cars in congested traffic. When the gas/dust clouds are compressed, many stars are born, which is why young stars are often found in the arms. Those stars eventually drift behind the density wave. Meanwhile, the wave sweeps up new stars and clouds to compress into spiral arms.