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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.
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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 optical 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.
Emission from Atomic Hydrogen (H I) 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 radio wavelengths (21 cm).
Atomic Hydrogen in the Milky Way Radio waves are not blocked by dust, so we can see the emission from H I in the interstellar medium across the entire galaxy.
Molecular Hydrogen in the Milky Way The most common molecule is H2, but it does not produce any emission lines that are easy to detect. Fortunately, the molecule CO is relatively common, and has strong emission lines at radio wavelengths.
Ionized Hydrogen in the Milky Way When hydrogen is located near a hot (O or B) star, it can become ionized (HII), which produces optical emission lines.
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)
Measuring the Age of a Star Cluster The main sequence turn-off is fainter for older clusters.
Ages of Open Clusters For open clusters, the main sequence extends to bright magnitudes, so these clusters are young.
Ages of Globular Clusters In globular clusters, the main sequence turnoff is at spectral type G or K. These clusters are 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
Measuring Ages of Individual Stars 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 and helium when it was born. Stars have created heavier elements, or “metals”, over time through fusion and supernovae. Some of these metals return to 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 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 only within ~1000 light years from the Sun; parallaxes of more distant stars are too small to measure, even with modern telescopes. So we need another way to measure distances to stars across the Milky Way.
Measuring Distances in the Milky Way 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 / d2where bis the brightness seen from Earth, L is the luminosity, and dis the distance. Any object whose luminosity you know is astandard candle.
Pulsating Stars as Standard Candles Not all stars are stable. There is a narrow region in the HR diagram where stars cannot maintain a constant brightness. Instead, they pulsate, getting bigger and smaller (i.e., brighter and dimmer) over time. There are 2 types of pulsating stars: RR Lyrae and Cepheids.
RR Lyrae Pulsations RR Lyrae stars can double their brightness within a day. The period of this variation is constant.
Finding RR Lyrae Stars Globular clusters contain many RR Lyrae stars. They are easy to find because their luminosities change over time. Since we know the absolute luminosities of RR Lyrae stars, we can now measure the distance to this RR Lyrae star (and hence the cluster) with the inverse square law of light.
The Distribution of Globular Clusters 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. This is the center of the Milky Way, or the Galactic center. 25,000 light years 100,000 light years
The Halo and the Disk of the Milky Way Like globular clusters, many of the older stars in the Milky Way are distributed in a roughly spherical halo. Younger stars, open clusters, and most of the gas and dust are contained in a flattened disk. Within this disk, most of the stars and gas are found in spiral arms.
The Halo and the Disk 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. It takes the Sun and the other stars in the Milky Way about 200,000,000 yrs to complete one orbit around the Galactic center. The Milky Way contains 100 billion stars and is 100,000 light years in diameter.
