ASTRONOMY 202 Spring 2007: Solar System Exploration Instructor: Dr. David Alexander Web-site: www.ruf.rice.edu/~dalex/astr202_s07 Class 3: Our Place in the Universe [1/19/07] Announcements Scale in the Universe Our place in the Milky Way Spaceship Earth The Expanding Universe Discussion & Questions?
Announcements Email Textbook should be in bookstore on Thursday Homework 1 available online Observing Week of Jan 22
Our Place in the Universe: the speed of light Use fixed speed of light to define a handy unit of distance: the light year 1 light year DISTANCE traveled by a light beam in 1 year = 9.46 trillion km [9.46 x 10 12 km] 1 light minute = distance traveled by light in 1 minute = 18 million km 1 light second distance traveled by light in 1 second = 300,000 km We see all objects as they were in the past
Our Place in the Universe: the scale of the Milky Way The nearest star (Proxima Centauri) is 4.3 light years distant.
Our Place in the Universe: the scale of the Milky Way The Milky Way Galaxy contains 100 billion (10 11 ) stars is over 100,000 light years across. The Milky Way is just one of about 80 billion galaxies in the known Universe
Spaceship Earth: we are constantly in motion The rotating Earth Earth rotates from West to East at >1000 km/hr with its spin axis tilted at 23.5 o to the plane containing the Sun and the Earth ( ecliptic plane ).
Spaceship Earth: we are constantly in motion The revolving Earth Earth revolves ( orbits ) counter-clockwise around the Sun at an average distance of 150 million km. This yields an average orbital speed of 108,000 km/hr. Earth s orbit is an ellipse. The average distance of the Earth from the Sun is known as an Astronomical Unit or AU.
Spaceship Earth: we are constantly in motion And it doesn t stop there Stars nearby the Sun are all moving relative to each other at around 70,000 km/hr. Stellar collisions?
Spaceship Earth: we are constantly in motion And it doesn t stop there Stars nearby the Sun are all moving relative to each other at around 70,000 km/hr. The Solar System revolves around the Galactic Center completing a revolution in 230 million years. [825,000 km/hr]
Spaceship Earth: we are constantly in motion nor there The Milky Way itself is streaming towards the Andromeda Galaxy (M31) at a speed of 300,000 km/hr. For each million light years the observed speed increases by ~22 km/s H 0 =70 kms -1 /Mpc The WHOLE Universe is expanding with every galaxy outside the Local Group moving away from us at a velocity which increases with distance. SPACE itself is getting larger!
The Big Bang The idea that the Universe is expanding everywhere led to the notion of a Big Bang where the whole Universe started from a singular point. This has been borne out by the discovery and subsequent measurements of the Cosmic Microwave Background (recent Nobel prize for physics).
Expansion of the Universe Time = t 2 Time = t 1 t 2 > t 1
Expansion of the Universe Time = t 1 Time = t 2
Expansion of the Universe Time = t 1 Time = t 2
Dark Matter and Dark Energy Observations of the rotation of galaxies implied the presence of much more mass than could be accounted for by the visible stars and normal gas and dust. A significant chunk of the Universe is comprised of Dark Matter. Matter that does not interact very strongly with regular matter and so does not produce heat or light. Supernovae observations have suggested that the expansion of the Universe is accelerating. This has led to suggestions that space itself has energy which pushes the Universe apart. It is known as dark energy.
Main Concepts: Chapter 1 Space is BIG and everything in it is moving very fast. The speed of light is a large but finite number Earth is not the center of the Universe Heavier elements, such as carbon, are produced in stars The whole of human history occurred in an eye blink in the history of the Universe The universal presence of gravity is evident in the constant motion of the Earth, solar system, stars, galaxies and the Universe itself.