Welcome to Physics 1403 Stars and Galaxies Mr. Kris Byboth

Syllabus: http://www.blinn.edu/brazos/natscience/phys/kbyboth/ Homework: Visit my site and print out a copy of the free SFA star charts. Read chapters 1 & 2 Register for Mastering Astronomy and start the first assignment.

Pop Quiz One. Just for fun. What is the closest star to Earth? What is the brightest star in the sky? On what horizon do stars rise? Set? How many stars are there?

Answers What is the closest star to Earth? The Sun! Proxima Centauri is the second closest star.

Proxima Centauri

What is the brightest star in the sky? The Sun Sirius is the second brightest star.

On what horizon do stars rise? Set? East West

How many stars are there? Approximately 10 billion trillion stars “The number of stars in the universe is larger than the total number of grains of sand on all the beaches on Earth!”

What Do You Want to Know? On the bottom of your quiz please write one question you would like answered this semester. Then please pass your quiz to the front of the room

Definitions Star Planet Moon Asteroid Comet Solar System Nebula Galaxy Universe/Cosmos

Star A large, glowing ball of gas that generates heat and light through nuclear fusion

Planet Mars Neptune A moderately large object that orbits a star; it shines by reflected light. Planets may be rocky, icy, or gaseous in composition.

Why Pluto is no longer a planet? The Problem Eris, discovered in 2005, is slightly larger than Pluto. The Solution. A New Definition for Planet. Orbits a star Is massive enough for its gravity to make it nearly round It has cleared the neighborhood around its orbit. Is much larger than anything near it. (Charon is about ½ the size of Pluto) Pluto is now a dwarf planet

Moon (or satellite) An object that orbits a planet. Io

Asteroid A relatively small and rocky object that orbits a star. Ida Also worth pointing out: (1) Note the non-spherical shape; small objects are often non-spherical because their gravity is not strong enough to compress the material into a sphere. (2) Asteroids are sometimes called minor planets because they orbit much like planets but are smaller than anything we consider to be a true planet. Ida

Comet A relatively small and icy object that orbits a star. Also worth noting: (1) the basic difference between an asteroid and a comet is composition; (2) comets have tails ONLY when they come close to the Sun, not when they are much farther away.

Solar (Star) System A star and all the material that orbits it, including its planets and moons

Nebula An interstellar cloud of gas and/or dust Note: We do not include “nebula” in our list of basic definitions in ch. 1, because it is a less important term at this point in the course. However, you may find yourself talking about nebulae (e.g., Orion Nebula) if you are doing any early-term observing with your students, in which case you may find this slide useful. An interstellar cloud of gas and/or dust

Galaxy A great island of stars in space, all held together by gravity and orbiting a common center Remember that one of the most common student problems is confusion between the terms “solar system” and “galaxy.” You can use these slides of basic definitions to help combat this problem.

Universe The sum total of all matter and energy; that is, everything within and between all galaxies.

Chapter 1 Our Place in the Universe

What do we think we know and how do we know it? We must first assume all of the laws of physics are universal. Almost all that we know we have learned by observing light from distant sources. We will spend most of this semester looking at observations and trying to see how those observations lead to our understanding of the universe.

How did we come to be… briefly The universe is expanding in all directions If we run this expansion backwards the universe collapses to a single point (the big bang) in about 13.7 billion years Shortly after the big bang the universe was composed only of the simplest of elements. Hydrogen & Helium

All of the heavier elements are created in stellar death! As stars generate energy by nuclear fusion they create heavier elements. Up to Carbon All of the heavier elements are created in stellar death! We are all “star stuff”

How do we know about the past? Although nothing can travel as fast as light, there is a limit to how fast light can travel. 300,000km/s This means it takes time for light to travel from place to place. 8min from the Sun to Earth 2.5million years from the nearest galaxy to Earth

Light years a measure of distance We define the light year to be the distance light will travel in one year. 1 lyr ~ 10 trillion km ~ 6 trillion mi Thus when we see light from a distant galaxy that is 12 billion light years away we are looking 12 billion years in the past. Since the universe is only ~14 billion years old we can only observe parts of the universe within 14billion light years. This is called the observable universe

The scale of the universe. How big is the universe? Really BIG!

A scale model Let the Sun be a large grapefruit. This reduces the sun to one 10 billionth of its actual size. How big would the Earth be? an atom a ball point a marble a golf ball At this scale the Earth would be ~50ft (15m) away from the sun

Pluto would be 600 meters (1/3 of a mile) from the sun The nearest star would be about 2500mi or 4000km away This is approximately the distance from Washington D.C. to California. How hard would it be to find a planet at this distance?

How big is the Milky Way? In order to view our home galaxy our scale needs to be reduced by another factor of a billion At this scale the entire solar system would be microscopic The distance from the sun to Alpha Centauri would be 4.4 mm The Milky Way would be about the size of a football field.

How big is the universe?

How big is the universe? The Milky Way is one of only 100 billion galaxies in the observable universe. If we were to assume our galaxy is an average galaxy then there would be 10,000,000,000,000,000,000,000 stars in the universe.

Why I can’t sit still!! Contrary to our perception, we are not “sitting still.” We are moving with the Earth in several ways, and at surprisingly fast speeds… The Earth rotates around its axis once every day. Our first motion is ROTATION. Point out that most of us are moving in circles around the axis at speeds far faster than commercial jets travel, which is why jets cannot keep up with the Sun when going opposite Earth’s rotation…

Earth orbits the Sun (revolves) once every year: at an average distance of 1 AU ≈ 150 million km. with Earth’s axis tilted by 23.5º (pointing to Polaris for now) and rotating in the same direction it orbits, counter-clockwise as viewed from above the North Pole. Our second motion is ORBIT. Point out the surprisingly high speed of over 100,000 km/hr.

… And orbits the galaxy every 230 million years. Our Sun moves randomly relative to the other stars in the local Solar neighborhood… typical relative speeds of more than 70,000 km/hr but stars are so far away that we cannot easily notice their motion … And orbits the galaxy every 230 million years. Our third and fourth motions are MOTION WITH THE LOCAL SOLAR NEIGHBORHOOD and ROTATION OF THE MILKY WAY GALAXY.

More detailed study of the Milky Way’s rotation reveals one of the greatest mysteries in astronomy: Most of Milky Way’s light comes from disk and bulge … Although we won’t discuss dark matter until much later in the course, you might wish to mention it now to whet students’ appetites… …. but most of the mass is in its halo

How do galaxies move within the universe? Galaxies are carried along with the expansion of the Universe. But how did Hubble figure out that the universe is expanding? Describe the raisin cake analogy, and have students work through the numbers with you to make the table. (E.g., “How far away is Raisin 1 at the beginning of the hour? [1 cm] How far is it at the end of the hour? [3 cm] So how far would you have seen it move during the hour? [2 cm] So how fast is it moving away from you? [2 cm/hr]”

Hubble discovered that: All galaxies outside our Local Group are moving away from us. The more distant the galaxy, the faster it is racing away. Conclusion: We live in an expanding universe. Now relate the raisin cake analogy to the real universe…

Are we ever sitting still? Earth rotates on axis: > 1,000 km/hr Earth orbits Sun: > 100,000 km/hr Solar system moves among stars: ~ 70,000 km/hr Milky Way rotates: ~ 800,000 km/hr Milky Way moves in Local Group This slide summarizes our motion with spaceship Earth… Universe expands

Astronomy Picture of the Day A Star Cluster in the Rosette Nebula

Astronomy Picture of the Day Your Home The Milky Way Galaxy