Welcome to Physics 1403Stars 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.

1. What is the closest star to Earth?

2. What is the brightest star in the sky?

3. On what horizon do stars rise? Set?

4. 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

StarA large, glowing ball of gas that generates heat

and light through nuclear fusion

Planet

A moderately large object that orbits a star; it shines by reflected light. Planets may be rocky,

icy, or gaseous in composition.

Mars Neptune

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.

Asteroid

A relatively small and rocky object that orbits a star.

Ida

Comet

A relatively small and icy object that orbits a star.

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

GalaxyA great island of stars in space, all held

together by gravity and orbiting a common center

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

• 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?A. an atomB. a ball pointC. a marbleD. 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.

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 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.

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 …

…. 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?

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.

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

Universeexpands

Astronomy Picture of the DayA Star Cluster in the Rosette Nebula

Astronomy Picture of the DayYour HomeThe Milky Way Galaxy