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ASTRO 101 Principles of Astronomy Instructor: Jerome A. Orosz (rhymes with “boris”) Contact: • Telephone: 594-7118 • E-mail: [email protected] • WWW: http://mintaka.sdsu.edu/faculty/orosz/web/ • Office: Physics 241, hours T TH 3:30-5:00 Homework/Announcements • For Chapter 11, skip sections 11.9, 11.11, 11.14, 11.16, 11.17, 11.18, 11.19 • For Chapter 12, sections 12.1-12.7 • Tuesday, May 7: wrap-up and review • Tuesday May 14, Final Homework/Announcements • Chapter 12 homework Due May 7: Question 5 (What observations led Harlow Shapley to conclude we are not at the center of the Galaxy?) NEXT: Our Galaxy and Other Galaxies A Sense of Scale • So far, we have discussed things that are relatively nearby: • The Sun, Moon, and solar system planets. The size is several “light hours”. • Stars: many stars that you can see without a telescope are within a few hundred light years. A Sense of Scale • The Sun (and its planets) and these nearby stars are part of a vast collection of stars bound by gravity: Such a collection of stars is called a “galaxy”. This structure contains roughly 1011 stars and is 100,000 light years across. • There are about 50 billion galaxies similar to our own in the observable universe! And now for something completely different: The Meaning of Life Defining the Milky Way • The Milky Way is only one of many galaxies… Island Universes • We know today that galaxies are the fundamental building blocks of the Universe. • Billions of them are known, and we often talk of them as if they are single objects (rather than a collection of objects). Island Universes • You see galaxies almost everywhere you look, provided you expose for a long enough time. Island Universes: The History • Astronomers in the 1700s were very interested in comets. These objects appear as fuzzy blobs when seen through a telescope. Island Universes: The History • Comets look obviously non-stellar, especially in modern photographs. • It is much harder to see find detail visually (i.e. without a photograph). Island Universes: The History • Astronomers in the 1700s were very interested in comets. These objects appear as fuzzy blobs when seen through a telescope. • Many workers undertook surveys for comets. Many fuzzy objects were found that were not comets. Charles Messier made a catalog of them, for which he is famous. His comet discoveries have been forgotten. Island Universes: The History • Many workers undertook surveys for comets. Many fuzzy objects were found that were not comets. Charles Messier made a catalog of them, for which he is famous. His comet discoveries have been forgotten. • Some of these “fuzzy blobs” turned out to be clouds of gas relatively nearby the Sun. Other blobs showed “pinwheel” structure. Island Universes: The History • This object from Messier’s catalog (the Orion Nebula) turned out to be a cloud of gas and dust. Island Universes: The History • This object from Messier’s catalog (the Andromeda Galaxy), turned out to be something different… Island Universes: The History • Many workers undertook surveys for comets. Many fuzzy objects were found that were not comets. Charles Messier made a catalog of them, for which he is famous. His comet discoveries have been forgotten. • Some of these “fuzzy blobs” turned out to be clouds of gas relatively nearby the Sun. Other blobs showed “pinwheel” structure. • The nature of the “spiral nebulae” was not solved until the 1920s. Island Universes: The History • This figure illustrates how objects of very different sizes can appear to have the same angular sizes. Island Universes: The History • The nature of the “spiral nebulae” was not solved until the 1920s. • Why was it so hard? Early astronomers had no way to judge the distances to these objects. As a result, it was not obvious if these things were very large and very far away, or relatively small objects seen close by. • Edwin Hubble solved the problem in the 1920s using a certain class of variable stars. Island Universes: The History • Edwin Hubble used the 100 inch telescope at Mount Wilson (just outside L.A.), which was the largest telescope available in the 1920s. He is shown here at the 60 inch telescope at Palomar Observatory, just north of here. • He could see individual stars in some of the spiral nebulae! Inverse Square Law Luminosity F = L/(4d2) Brightness or flux Distance L=4d2F One usually measures F, d (parallax) to find L Inverse Square Law Luminosity F = L/(4d2) Brightness or flux Distance d2=L/(4F) Sometimes one can measure F and L to find d… Distance Indicators • Some stars vary in brightness on a regular basis. In many cases, this is because they are pulsating. The radius and temperature changes over the cycle, giving rise to brightness variations. Distance Indicators • Some stars vary in brightness on a regular basis. In many cases, this is because they are pulsating. The radius and temperature changes over the cycle, giving rise to brightness variations. Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com) Distance Indicators • Some stars vary in brightness on a regular basis. In many cases, this is because they are pulsating. The radius and temperature changes over the cycle, giving rise to brightness variations. Distance Indicators • For a certain class of pulsators, the luminosity is proportional to the period of pulsation. • If you observe the star over a long enough time, you can measure the period and then compute the luminosity. Distance Indicators • For a certain class of pulsators, the luminosity is proportional to the period of pulsation. • If you observe the star over a long enough time, you can measure the period and then compute the luminosity. Distance Indicators • For a certain class of pulsators, the luminosity is proportional to the period of pulsation. • If you observe the star over a long enough time, you can measure the period and then compute the luminosity. So what? Distance Indicators • If you observe the star over a long enough time, you can measure the period and then compute the luminosity. So what? • If you know how bright a star appears, and also its luminosity, then you can compute the distance! Distance Indicators • If you observe the star over a long enough time, you can measure the period and then compute the luminosity. So what? • If you know how bright a star appears, and also its luminosity, then you can compute the distance! Island Universes • Hubble found that the great nebula in Andromeda was well over 1,000,000 light years away (Modern measurements give 2,400,000 light years). • This is much larger than the size of the Milky Way. The spiral nebulae are outside our galaxy! Island Universes • Hubble found that the great nebula in Andromeda was well over 1,000,000 light years away (Modern measurements give 2,400,000 light years). • Also, given the distance and the angular size on the sky, one can compute the physical size. Island Universes • Hubble found that the great nebula in Andromeda was well over 1,000,000 light years away (Modern measurements give 2,400,000 light years). • Also, given the distance and the angular size on the sky, one can compute the physical size. >100,000 light years The Structure of Our Galaxy • How do we measure the structure of the Milky, given that we are inside it? • To appreciate why this is so hard, let us briefly revisit stellar evolution… Stellar Evolution • The basic steps are: Gas cloud Main sequence Red giant Rapid mass loss (planetary nebula or supernova explosion) Remnant • The stars form from clouds of gas and dust, and much of this material is put back into space before the star “dies.” So what? Interstellar Dust • The space between the stars often contains lots of gas (e.g. hydrogen, oxygen, etc) and dust particles. This interstellar dust can inhibit our ability to observe at certain wavelengths. This point was not fully understood or appreciated until the mid 1930s. Interstellar Dust • The dust is composed of tiny slivers of graphite and silicates, possibly coated with water ice. • Note that the scale on this diagram is 10-7 meters! Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com) Interstellar Dust • Light passing through an interstellar dust cloud will be dimmed. • However, the amount of dimming depends on the wavelength of the light: blue light is scattered more easily than red light. The object appears redder. Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com) Why is the Sky Blue? • Blue light travels a relatively short distance before it is scattered by molecules in the air. Red light goes much further before being scattered. Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com) Interstellar Dust • Interstellar dust makes a star appear dimmer and redder if that star is behind a cloud of dust. Intersteller Dust • Interstellar dust makes a star appear dimmer and redder if that star is behind a cloud of dust. • This will complicate matters when we try to judge the structure of our galaxy… Finding the Structure • We are inside a large collection of stars. How do we make sense of the structure of this system from the inside? It is not easy… Finding the Structure • A band of diffuse light going around the sky can be seen from a dark location. This band was known to the ancient Greeks and Romans and was called the “Milky Way”. Finding the Structure • Using a telescope Galileo discovered that the Milky Way is actually made up of millions of stars, each individually too faint to see without a telescope. • Away from this band, fewer stars are seen. Finding the Structure • Astronomers in the 1800s did large “star gauging” surveys all over the sky. They essentially counted stars using visual observations through a telescope. Finding the Structure • Relatively large numbers of stars are seen in a band cutting across the sky. This band was known to the ancient Greeks and Romans and was called the “Milky Way”. • Away from this band, fewer stars are seen. Finding the Structure • From the star counts, it was concluded that the “Universe” is a flattened structure (aspect ratio of 5:1) with a diameter of about 8000 light years. The Sun was essentially at the center. Finding the Structure • From the star counts, it was concluded that the “Universe” is a flattened structure (aspect ratio of 5:1) with a diameter of about 8000 light years. The Sun was essentially at the center. Finding the Structure • From the star counts, it was concluded that the “Universe” is a flattened structure (aspect ratio of 5:1) with a diameter of about 8000 light years. The Sun was essentially at the center. • Either we are at a very special place, or something is wrong. Finding the Structure • From the star counts, it was concluded that the “Universe” is a flattened structure (aspect ratio of 5:1) with a diameter of about 8000 light years. The Sun was essentially at the center. • Either we are at a very special place, or something is wrong. Interstellar dust was not accounted for. Finding the Structure • From the star counting observations and other observations, we know the galaxy is a flattened structure. Finding the Structure • From the star counting observations and other observations, we know the galaxy is a flattened structure. • However, interstellar dust obscures the distant parts of it, leading to severe observational biases. Finding the Structure • From the star counting observations and other observations, we know the galaxy is a flattened structure. • However, interstellar dust obscures the distant parts of it, leading to severe observational biases. • Is there another way to probe the structure? Finding the Structure • From the star counting observations and other observations, we know the galaxy is a flattened structure. • However, interstellar dust obscures the distant parts of it, leading to severe observational biases. • Is there another way to probe the structure? There are several: the earliest alternative was a study of globular clusters… Finding the Structure • In 1918 Harlow Shapley estimated the distances to 93 globular clusters. Shapley found a strong concentration of globular clusters in the direction of Sagittarius. Finding the Structure • In 1918 Harlow Shapley estimated the distances to 93 globular clusters. Shapley found a strong concentration of globular clusters in the direction of Sagittarius. • If the GC’s are uniformly distributed through the galaxy, then the Sun is well off the center! Finding the Structure • In 1918 Harlow Shapley estimated the distances to 93 globular clusters. Shapley found a strong concentration of globular clusters in the direction of Sagittarius. • If the GC’s are uniformly distributed through the galaxy, then the Sun is well off the center! Finding the Structure Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com) • In 1918 Harlow Shapley estimated the distances to 93 globular clusters. Shapley found a strong concentration of globular clusters in the direction of Sagittarius. • If the GC’s are uniformly distributed through the galaxy, then the Sun is well off the center! Finding the Structure • Counting individual stars in the optical leads to biases if interstellar dust is not accounted for. One has the illusion that we are at the center. • The globular clusters give a much more unbiased view. The center of the galaxy is roughly 25,000 light years away in the direction of Sagittarius. Finding the Structure • In 1918 Harlow Shapley estimated the distances to 93 globular clusters. Shapley found a strong concentration of globular clusters in the direction of Sagittarius. • If the GC’s are uniformly distributed through the galaxy, then the Sun is well off the center! Finding the Structure • There are three main parts: – The bulge/nucleus at the center (roughly spherical with a radius of about 3000 light years). Finding the Structure • There are three main parts: – The disk which is about 100,000 light years across and about 2000 light years thick. It contains the young stars, the gas, and the dust. The Sun is in the disk about 2/3 the way out. Finding the Structure • There are three main parts: – The halo, which is a roughly spherical and relatively diffuse region a few hundred thousand light years across. The halo contains very old stars and the globular clusters. Finding the Structure Finding the Structure Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com) The Structure of the Milky Way • The bulge contains relatively old stars. • The disk contains gas and dust, young stars, and some older stars. The gas and dust are confined to a relatively thin region. • The halo contains very old stars, and essentially no gas and dust. Next • Structure in the Disk • The Galactic Center Finding the Structure of the Disk • In the 1920s and 1930s, Jan Oort studied the motions of thousands of stars. He concluded that most stars are on roughly circular orbits about the center of the galaxy (as one would expect from Newton’s laws). Finding the Structure of the Disk • In the 1920s and 1930s, Jan Oort studied the motions of thousands of stars. He concluded that most stars are on roughly circular orbits about the center of the galaxy (as one would expect from Newton’s laws). • In addition, he showed that stars closer to the center have higher angular speeds. Finding the Structure of the Disk • In addition, he showed that stars closer to the center have higher angular speeds. • If one can estimate the orbital speeds and the distance to the center, then one can find the mass of the galaxy (or at least its central parts). Finding the Structure of the Disk • In addition, he showed that stars closer to the center have higher angular speeds. • If one can estimate the orbital speeds and the distance to the center, then one can find the mass of the galaxy (or at least its central parts). Modern measurements yield a mass of about 1011 solar masses for the Milky Way galaxy. The Structure of the Disk • Is the disk of the Milky Way galaxy axisymmetric, or are there patterns within it? Finding the Structure of the Disk • It is very hard to study the structure of the disk of the galaxy because we are inside it. We cannot travel above it as in the above example. Is there another way? Finding the Structure of the Disk • It is very hard to study the structure of the disk of the galaxy because we are inside it. We cannot travel above it as in the above example. Is there another way? Yes. Observe at radio wavelengths. The 21 cm line of Hydrogen • For the study of Galactic structure, radio observations have some advantages: The 21 cm line of Hydrogen • For the study of Galactic structure, radio observations have some advantages: Radio photons can easily penetrate the interstellar dust. You can essentially see every part of the disk. The 21 cm line of Hydrogen • For the study of Galactic structure, radio observations have some advantages: Radio photons can easily penetrate the interstellar dust. You can essentially see every part of the disk. Neutral hydrogen, which is the most abundant element in the universe, has an emission line at 21.1 cm, so you can measure radial velocities from Doppler shifts. The 21 cm line of Hydrogen • In quantum mechanics, electrons and protons have a property called “spin”, although it is really not the same as a spinning ice skater. The 21 cm line of Hydrogen • In quantum mechanics, electrons and protons have a property called “spin”, although it is really not the same as a spinning ice skater. • This “spin” has only two values: “up” or “down”. The 21 cm line of Hydrogen • In quantum mechanics, electrons and protons have a property called “spin”, although it is really not the same as a spinning ice skater. • This “spin” has only two values: “up” or “down”. Why is this important? The 21 cm line of Hydrogen • A hydrogen atom where the proton and the electron have the same “spin” has a slightly higher energy than an atom where the “spins” are opposite. The 21 cm line of Hydrogen • A hydrogen atom where the proton and the electron have the same “spin” has a slightly higher energy than an atom where the “spins” are opposite. • An isolated hydrogen atom can spontaneously go from one state to the other, giving off a photon with a wavelength of 21.1 cm in the process. The 21 cm line of Hydrogen • Hydrogen is very abundant, so 21 cm line emission comes from all over the Milky Way galaxy’s disk. • Just build a large antenna with a good radio receiver, and make a spectrum near a wavelength of 21 cm in many lines-of-sight throughout the disk… The 21 cm line of Hydrogen • The spectrum gives you the intensity as a function of wavelength (or radial velocity). • Using some assumptions about how the clouds orbit, one can compute the distances to the clouds in that line-of-sight. Image from Nick Strobel’s Astronomy Notes The Structure of the Disk • Detailed radio observations show that the hydrogen gas is not uniformly distributed in the disk. • There are distinct “spiral arms”, but with branches and spurs, similar to what is seen in other galaxies. The Structure of the Disk • This map is made from infrared observations of dust. • There are distinct “spiral arms”, and possibly a “bar” in the center. Spiral Arms • Spiral structure is seen in many galaxies outside the Milky Way galaxy. • What causes this structure? Differential Rotation • Stars near the center take less time to orbit the center than stars further out. Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com) Differential Rotation • Stars near the center take less time to orbit the center than stars further out. • A linear structure can quickly turn into a spiral pattern. Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com) Differential Rotation • Stars near the center take less time to orbit the center than stars further out. • A linear structure can quickly turn into a spiral pattern. However, it is a bit too quick… Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com) Differential Rotation • Stars near the center take less time to orbit the center than stars further out. • A linear structure can quickly turn into a spiral pattern. However, it is a bit too quick… Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com) Differential Rotation • Stars near the center take less time to orbit the center than stars further out. • A linear structure can quickly turn into a spiral pattern. However, it is a bit too quick… ??? Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com) Density Waves • Spiral arms are not “material” waves, but rather “density” waves. In other words, the same stars do not stay in the arm. • The “spiral density wave” is a compression wave. Density Waves • Spiral arms are not “material” waves, but rather “density” waves. In other words, the same stars do not stay in the arm. • The “spiral density wave” is a compression wave. Density Waves Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com) Next: • The Galactic Center • Dark Matter