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 WayAt 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 WayAt 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 WayAt 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

Open Clusters

Open Clusters

Open Clusters

Globular Clusters

Globular Clusters

Globular Clusters

Globular Clusters

Motions of Stars in 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 ClustersIn 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 WayParallaxes 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.

Suppose you knew the luminosity of a star. If so, you could determine the distance to the star simply from the inverse square law of light.

b = L / d2 whereb is the brightness seen from Earth, L is the luminosity, andd is the distance.

Any object whose luminosity you know is a standard candle.

Measuring Distances in the Milky Way

Pulsating Stars as Standard CandlesThere 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 StarsPulsating 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 WayGas 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 WayThe Sun is in the disk between 2 spiral arms about halfway from the Galactic center to the edge of the galaxy. It takes the Sun and the other stars in the Milky Way about 200,000,000 yrs to complete one orbit around Galactic center. The Milky Way contains 200 billion stars and is 100,000 light years in diameter.