Transcript Chapter 15
Galaxies The Universe • Early in the history of the universe, hydrogen and helium (and other forms of matter) clumped together by gravitational attraction to form countless trillions of stars. Billions of galaxies, each a cluster of billions of stars, now form most of the visible mass in the universe. When stars are viewed through a telescope, they appear as points of light. However some objects in the sky look like "fuzzy" clouds. Some are nebulae (star-forming regions) Others are actually islands of stars that are much farther from us than the individual stars we see in the night sky. These islands are galaxies. Hubble’s Galaxy Classification This pair of images shows the Coma cluster of galaxies. Almost every object visible is a galaxy. © 2011 Pearson Education, Inc. Galaxies • Galaxies are systems of stars, gas and dust • They exist in a wide variety of shapes and sizes Galactic Inquiry Purpose: • Recognize that galaxies are a collection of stars • Understand that galaxies take a variety of forms • Learn to develop your own classification scheme to understand how scientists classify • Comprehend the four major categories of galaxies Galaxy Classification Intro Activity • Follow the instructions carefully, in the first part you will be developing your own classification of galaxies, then you will look at what Hubble thought. Galaxies • Galaxies are systems of stars, gas and dust • They exist in a wide variety of shapes and sizes • 4 basic types Let’s Review Four basic types elliptical, spiral, barred spiral, and irregular Hubble Tuning Fork Scheme The Milky Way is a spiral galaxy. Most galaxies that we can see are spiral galaxies. Looking at other spiral galaxies, we can begin to learn more about how the spirals work. Spirals Hubble’s Galaxy Classification Spiral galaxies are classified according to the size of their central bulge Hubble’s Galaxy Classification Type Sa has the largest central bulge, Type Sb is smaller, and Type Sc is the smallest. Type Sa tends to have the most tightly bound spiral arms with Types Sb and Sc progressively less tight, although the correlation is not perfect. © 2011 Pearson Education, Inc. Hubble’s Galaxy Classification The Sombrero galaxy, with its large central bulge, is a type Sa. We cannot see the spiral arms, as they are edge-on. © 2011 Pearson Education, Inc. Turn to your elbow partner One of you hold up a text book or notebook for your partner with the edge facing your partner. Now hold the book with the title facing your partner. How does the book look different. Which is more recognizable as a book? Try this with a pen. Look down the top of the pen. Now look at it from the side. How is the image different? Variety of Spiral Arms Flocculent spirals (fleecy) Grand-design spirals (highly organized) What do you notice about the colors of the stars in the spiral arms of this galaxy? The arms of spiral galaxies contain most of the bright, blue O and B type stars. Stellar Lifetimes • Which live longer, Red or Blue stars? Stellar Lifetimes • So the stars in the spiral arms are mostly bright, hot, and young stars! Since O stars have such a short lifetime, it must be that: the arms of spiral galaxies are where star formation happens! If stars form in the spiral arms, how do they end up getting spread out over the whole disk? Spiral arms are not permanent structures, rather they are locations where stars are forming. If spiral arms are not permanent structures, then what is making these locations become active star forming regions? Spiral arms maybe caused by spiral density waves – a spiral shaped disturbances traveling through the disk of the galaxy that compressed gas and . triggers star formation. Elliptical galaxies Barred Spirals Elliptical Galaxies • About 60% of the galaxies are round to oval in shape Hubble’s Galaxy Classification Elliptical galaxies have no spiral arms and no disk. They come in many sizes, from giant ellipticals of trillions of stars, down to dwarf ellipticals of less than a million stars. Ellipticals also contain very little, if any, cool gas and dust, and they show no evidence of ongoing star formation. Elipticals have large clouds of hot gas, extending far beyond the visible boundaries of the galaxy. © 2011 Pearson Education, Inc. Hubble’s Galaxy Classification Ellipticals are classified according to their shape from E0 (almost spherical) to E7 (the most elongated) © 2011 Pearson Education, Inc. What would you classify this galaxy? Tell your elbow partner why. © 2011 Pearson Education, Inc. Galaxies that don’t seem to look like spirals or ellipticals are called…. Irregular Irregular Galaxy These galaxies have little symmetry in their structure About 10% of the galaxies Hubble’s Galaxy Classification The irregular galaxies have a wide variety of shapes. The small and large Magellanic Clouds are close neighbors to our own Milky Way. © 2011 Pearson Education, Inc. Hubble’s Galaxy Classification Here are several other irregular galaxies: AM 0644-741 and its neighbors on the left, and NGC 1569 on the right. © 2011 Pearson Education, Inc. Hubble’s Galaxy Classification Hubble’s “tuning fork” is a convenient way to remember the galaxy classifications. © 2011 Pearson Education, Inc. Tutorial: Galaxy Classification • Work with a partner! • Read the instructions and questions carefully. • Discuss the concepts and your answers with one another. Take time to understand it now!!!! • Come to a consensus answer you both agree on. • If you get stuck or are not sure of your answer, ask another group. Galaxy Classification and Properties: Lecture Tutorial (handout) 1 2 5 6 3 7 4 8 A galaxy that appears to be populated by mostly red stars, likely: 1. never had blue stars in the galaxy 2. had blue stars that are not present anymore but were at one time long ago 3. has been around long enough for the blue stars to have already evolved into red stars 4. never contained enough gas to have blue stars develop 5. has blue stars that are being blocked by dust 0/0 Cross-Tab Label Why are the arms of spiral galaxies typically blue in color? 1. They are usually moving toward us and are Doppler shifted to blue wavelengths. 2. The gas and dust in the arms filter out all but the blue light from stars in the arms 3. Stars are forming in the spiral arms so there are high mass, hot blue stars in the arms. 4. Almost all the stars are in the arms of the disk of the galaxy and their light makes the arms appear blue. See if you can locate a few galaxies on Stellarium M31 in Andromeda a spiral galaxy M81 in Ursa Major a spiral galaxy M95 in Leo a barred spiral M87 in Virgo (near Mars) an elliptical © 2011 Pearson Education, Inc. The Local Group The Andromeda Galaxy • The Andromeda Galaxy is the nearest large galaxy to the Milky Way. • It is 2.5 million light years away. • It takes the light from the stars and gas in Andromeda 2.5 million years to get to us. • That means, we see Andromeda as it was 2.5 million years ago. Tutorial: Looking At Distant Objects • Work with a partner! • Read the instructions and questions carefully. • Discuss the concepts and your answers with one another. Take time to understand it now!!!! • Come to a consensus answer you both agree on. • If you get stuck or are not sure of your answer, ask another group. Imagine that you simultaneously receive the satellite transmission of two pictures of two people that live on planets orbiting two different stars. Each image shows the people at their 21st birthday parties. Consider the following possible interpretations that could be made from your observations. Which do you think is the most plausible interpretation? 1. Both people are the same age but at different distances from you. 2. The people are actually different ages but at the same distance from you. 3. The person that is closer to you is actually the older of the two people. 4. The person that is farther from you is actually the older of the two people Fifteen years ago, a quasar was observed that was found to be located 8 billion light years away. If our universe is approximately 15 billion years old, when did the quasar emit the light that we observe? 1. 2. 3. 4. 5. 0/0 15 years ago 7 billion years ago 8 billion years ago 15 billion years ago 23 billion years ago Cross-Tab Label Colliding galaxies • Take notes on the following • https://www.youtub e.com/watch?v=U QNo1hkcrYU 24.4 Active Galactic Nuclei About 20–25% of galaxies don’t fit well into the Hubble scheme—they are far too luminous. Such galaxies are called active galaxies. They differ from normal galaxies in both the luminosity and type of radiation they emit. Active galactic nuclei have some or all of the following properties: high luminosity • nonstellar energy emission • variable energy output, indicating small nucleus • jets and other signs of explosive activity • broad emission lines, indicating rapid rotation 24.4 Active Galactic Nuclei This active galaxy has star-formation rings surrounding a very luminous core Many active galaxies have jets, and most show signs of interactions with other galaxies. Quasars—quasistellar objects—are starlike in appearance, but have very unusual spectral lines Quasars (QSO’s) • A quasi-stellar radio source is a very energetic and distant active galactic nucleus. • Quasars are extremely luminous. • A quasar is a compact region in the center of a massive galaxy surrounding a supermassive black hole. • 10–10,000 times the Schwarzschild radius of the black hole. Quasars The Central Engine of an Active Galaxy This is the leading theory for the energy source in an active galactic nucleus: a black hole, surrounded by an accretion disk. The strong magnetic field lines around the black hole channel particles into jets perpendicular to the magnetic axis. This pair of images shows evidence for a black hole at the center of NGC 4261. Make your own Active Galaxy Read through the introduction and background information to Active Galaxies. Highlight anything that might make a good test question or is an important point. With our partners follow the instructions on the student worksheet to make your own active galaxy.