The Milky Way
Our Home Galaxy We live in a large disk of stars, gas, and dust called a galaxy. From locations not affected by light pollution we can see a bright band of stars and interstellar material across the sky, known to us as the Milky Way Amateur photographer Eric Hines' incredible image captures the Milky Way in all its glory above the haloed Devil's Tower landmark in Wyoming
Our Location in the Galaxy Since we live IN the Milky Way it is a challenging problem to figure out exactly where we are located in it. Another problem is understanding the structure of the galaxy, because we cannot study it from outside. Early attempts by William Herschel and Jacobus Kapteyn counted stars and wrongly concluded that we are in the center of the galaxy. Analogy: can't see the forest for the trees
Early Attempts to Find Sun s Location (1700s) William Herschel counted stars all of the stars in 683 regions of the sky. Her reasoned that the center of the Milky Way would be in the direction where the most stars were found, but counted nearly the same number of stars in all directions. (1920s) Jacobus Kapteyn studied the brightness and motion of stars to come to the same conclusion as Herschel, but estimated the size of the galaxy to be 17,000 pc in diameter. Herschel using his 48 inch telescope (above) mapped the sky and performed a star counting exercise by looking along 683 lines of sight to find a model for the Milky Way. Kapteyn (1920's) used photographic star counts and estimated distances using parallaxes and examining the proper motion of nearby stars.
Why did the Early Attempts Fail? The interstellar medium contains dust that blocks star light at visible wavelengths! Only stars close to the Sun could be counted. Trumpler discovered in the 1930s that the space between stars was NOT a perfect vacuum, but rather contains dust. Interstellar Extinction Obscuring dust Out of galactic plane Galactic plane Out of galactic plane
Globular Clusters Type of star cluster that are part of the galaxy, but not found in the galactic disk. They are bright, and are commonly found above and below the galactic plane where we can see them. First task is to find the distances to these clusters! Globular Clusters M13 - The Great Globular Cluster in Hercules
Distances to Globular Clusters RR Lyrae variable stars obey a period-luminosity relationship and are commonly found in globular clusters (RR Lyrae stars are old and low mass) The above movie was created from 4 images in 3 filters (BVI) taken over the course of a single night.
Distances to Globular Clusters In the 1920s Harlow Shapley used these RR Lyrae stars to measure the distances of 93 globular clusters Shapley found that globular clusters are in a halo around the center of the Milky Way and that the Sun is NOT at the center! Right idea Wrong size by factor of 2 Ignored interstellar exinction (objects appeared fainter and thus farther away)
Observations of the Milky Way at At visible wavelengths dust in the galactic plane obscures our view of the galaxy. Longer wavelengths (infrared, millimeter, radio) can pass through the dust Infrared light is particularly useful for studying the shape of the galaxy and the location of dust. Far-infrared: 30-300 μm Good for finding warm dust heated by stars Near-infrared: 0.7-30 μm Good for finding stars Nonvisible Wavelengths
Visible Light Far-infrared from IRAS satellite shows where warm dust is located in the Milky Way Near-infrared from COBE satellite shows the locations of stars that are obscured by dust.
Structure of the Galaxy Disk of the galaxy is about 50 kpc in diameter and 0.6 kpc thick Milky Way has a bulge of stars near the center about 2kpc in size. The galactic halo contains stars and globular clusters
Stellar Populations in the Milky Way Globular clusters and halo stars (also called high-velocity stars because of their orbits) are metal-poor population II stars The galactic disk contains metal-rich population I stars The galactic disk also contains many young OB stars that must have formed relatively recently (why?) Older, population II stars Younger, population I stars
Galactic Structure Orbits of Stars in the Galaxy Stars in the disk of the galaxy all orbit nearly in the same plane Stars in the halo orbit in random directions at high inclinations from the plane. Distributions of stars in other galaxies Population I stars in disk Young OB stars indicate active star formation Population I and Population II are found in the bulge Lack of OB stars indicate no star formation M101 located in Ursa Major is 21 million light years away
Revealing Spiral Structure How can the structure within the disk of the Milky Way be determined? Other galaxies often have spiral structure does our galaxy? M33 M51 M101 NGC 4535
Searching for Hydrogen in Galaxies Neutral hydrogen emits radio waves at a very specific wavelength of 21 cm. To understand why we much look at properties of the proton and electron that make up a hydrogen atom. Both the proton and electron have a property called spin, which is like angular momentum. Because they are charged and spin they each act like tiny magnets and can interact with each other. When the hydrogen atom goes from high to low energy configuration the 21cm line is emitted, called the spin-flip transition.
21-cm Radio Emission Emission from neutral hydrogen is concentrated in the disk of the galaxy, and also in spiral arms in external galaxies Three color composite image of the nearby spiral galaxy M101. The green color represents emission from neutral hydrogen (HI), emitted at 21 cm. The HI observations are part of VLA The HI Nearby Galaxy Survey (THINGS) which is based at MPIA (image credit: Fabian Walter, MPIA). Blue shows UV emission due to recent (<10 8 yr) star formation as seen by the Galaxy Evolution Explorer (GALEX). Red indicates warm dust emission as traced by infrared emission at 24 microns as seen by SPITZER (image credit: Karl Gordon, Steward Observatory).
Mapping Our Galaxy Using the 21-cm line of neutral hydrogen we can map the galaxy Since this is an emission line we can detect Doppler shifts of gas clouds moving relative to Earth Using the Doppler shifts we can separate different clouds from one another and map their location within the galaxy. Gas clouds 1 and 3 have a moderate blueshift (moving towards us) Cloud 4 has no measured shift We are here Cloud 2 has a high blueshift
Mapping Our Galaxy Map of 21-cm emission in the Milky Way shows that neutral hydrogen is concentrated in arcs which suggests spiral structure as observed in other galaxies. We can also map the structure of the galaxy by looking at OB associations, and CO emission from giant molecular clouds. Sun No Doppler shift detected
Milky Way Spiral Structure The Milky Way may look like this from above Solar System
Measuring the Rotation of the Galaxy Measurements show that the gas, stars, and dust in the galaxy rotates around the galactic center. 21-cm emission indicates that everything in the disk rotates in the same direction Orbital speeds are nearly the same for all locations in the disk This tells us additional important properties of the galaxy. If the galaxy rotates like a solid disk. Stars in larger orbits move faster than stars In smaller orbits
Measuring the Rotation of the Galaxy Measurements show that the gas, stars, and dust in the galaxy rotates around the galactic center. 21-cm emission indicates that everything in the disk rotates in the same direction Orbital speeds are nearly the same for all locations in the disk This tells us additional important properties of the galaxy. If the galaxy rotates like the solar system and Kepler s third law. Larger orbits mean slower orbital speeds
Measuring the Rotation of the Galaxy Measurements show that the gas, stars, and dust in the galaxy rotates around the galactic center. 21-cm emission indicates that everything in the disk rotates in the same direction Orbital speeds are nearly the same for all locations in the disk This tells us additional important properties of the galaxy. The way the galaxy actually rotates Stars orbit with nearly the same orbital speed independent of distance
Measuring the Rotation of the Galaxy A star s orbital velocity is determined by the total mass of the galaxy that is inside its orbit. For the solar system this means that in each orbit the total mass the planet orbits is the sum of the Sun and all interior planets, which is basically just the mass of the Sun. The Sun s orbital velocity is 220 km/s and orbits once every 220 million years!
Rotation Curves If we measure the rotation speed at different distances from the center of the galaxy we can plot the result. A plot of orbital speed vs. distance from the center of the galaxy is called a rotation curve. Rotation curves of the Milky way show that there exists a lot of material outside of the Sun s orbital radius If there were no material beyond the visible edge of the galaxy the rotation curve should be keplerian. Actual rotation curve of Milky Way
Dark Matter The rotation curve is NOT observed to drop off at the edge of the visible disk. This implies there is additional material beyond the disk that we cannot see Dark Matter Dark matter is thought to exist in a spherical halo that surrounds the Milky Way
What is Dark Matter? Nobody knows. Yet. Numerous ideas floating around: MACHOS (Massive compact halo objects) Small halo objects that are very dim. Microlensing surveys look at background stars and watch for momentary brightening when a MACHO passes in front of the star. (gravitational lensing effect) Can only account for 10-20% of the dark matter halo MACHO moving through space may pass in front of background star as seen from Earth Background star Light rays are lensed (bent) by the gravity of the MACHO Telescope on Earth
What is Dark Matter? WIMPS (weakly interacting massive particles) A new type of subatomic particle yet to be discovered WIMPS do not emit or absorb electromagnetic radiation May have masses of 10 to 10,000 times the mass of a proton The HESS array of four Cherenkov telescopes (one is shown here) has pinpointed high-energy gamma emission to the galactic centre, and cast doubt on its origin in dark-matter particles. Ways to search for evidence of dark matter
Origin of Spiral Structure Spiral structure is observed in the Milky Way and in numerous other galaxies. Spiral structure seems to be a persistent feature that isn t something very shortlived. If spiral structure formed early in galaxy evolution and then remained fixed the galaxy would have to rotate like a solid disk. But, the Milky way does not rotate this way, and in fact no galaxy has been observed to rotate like a solid body.
The Winding Problem The rotation curves of most galaxies are flat meaning that the orbital speed is constant with increasing radius. If spiral arms were made of a fixed group of stars the inner stars have a shorter orbital period than the stars that are further out in the disk Spiral arms would wind tighter and tighter!
The Density Wave Model Swedish astronomer Bertil Lindblad proposed the idea that spiral arms are a pattern that moves through the galaxy like ripples in water. These patterns are called density waves. Spiral arms are NOT made of some fixed group of stars, just like ripples in a pond are NOT made of the same water molecules
The Density Wave Model Traffic jam analogy: Imagine a slow truck moving on the highway. A traffic jam is created behind the truck, but cars can overtake and pass the slow moving truck. The traffic jam moves along the highway slowly, but the cars that are in the traffic jam are constantly changing.
The Density Wave Model In a galaxy stars play the role of cars in the traffic jam analogy. The spiral arms move around the galaxy but at a slower rate than the stars and gas do. Since the spiral arms are compressed and have a higher density than the surrounding regions of the disk, stars are more likely to form here. New stars appear on the downstream side of a spiral arm.
Spiral Galaxy M51 Example Infrared Visible Star A Star A Densest part of spiral arm has not passed star A Newly formed OB stars have already moved through spiral arm and past star A Star B Star B Densest part of spiral arm has not passed star B Newly formed OB stars have already moved through spiral arm and past star B
Where are the Stars? Visible Near-infrared Hot OB stars found in spiral arms where they recently formed Lower mass red stars are more evenly spread out through the disk of the galaxy.
Different Types of Spiral Galaxies Grand Design Spirals M101 Flocculent Spirals NGC 5055 Prominent, well defined spiral arms Short, poorly defined spiral arms Density Wave theory cannot adequately describe these galaxies.
Self-Propagating Star Formation Theory proposed to explain flocculent spiral structure observed in galaxies. First generation of stars form in a dense region of ISM Massive stars evolve and blow up as supernovae triggering new star formation Since the disk rotates differentially, these density enhancements and new OB stars are pulled into short spiral arms Top and bottom of this group of stars have different orbital periods and will be stretched out into short spiral arms
The Galactic Center Center of the Milky Way is crowded with stars and gas Very center of galaxy is a supermassive black hole called Sagittarius A* This point corresponds t the gravitational center of the galaxy and also is a very strong source of radio emission Sag A* itself is not visible in infrared images. Cannot see much in visible light A view of the center of the Milky Way from a groundbased telescope, with arrows pinpointing the location of Sgr A*. [European Southern Observatory]
The Galactic Center Using infrared telescopes astronomers have detected the motions of stars that surround Sag A* The star SO-16 came to within 45 AU of Sag A* and was traveling at 12,000 km/s! We can use Kepler s laws to calculate the mass of the object that these stars orbit. Mass of central object (that we cannot see) is 3.7 million solar masses. SO-2 orbits Sag A* with an orbital period of 15.56 ± 0.35 years
The Galactic Center In the X-rays, Sag A* is an active region of the sky. Flares are observed to brighten in 10 minutes, indicating a compact source. Gas is falling into black hole in relatively small amounts. A dramatic example of this will come later when we talk about quasars. Chandra X-ray Observatory image of Sagittarius A*. Credit: NASA/CXC/Caltech/M. Muno et al. Taken by the VLA, this is a 1 meter radio wave image of the galactic center. Sgr A is much more luminous in this image than in the previous image. Visible also in this image are cloud of gas and the supernova remnant.