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Home , Galaxy X (galaxy), Milky Way, Milky Way (mythology)

Chapter 1 Our Place in the 1.1 Our Modern View of the Universe Topics we will explore: • What is our place in the universe? • How did we come to be? • How can we know what the universe was like in the past? • Can we see the entire universe? • First, let’s take a look to some basic astronomical objects (in general ) A , in our case the and all the material that orbits it, including its , , , , small bodies and gas and dust. Star (Sun) A large, plasma sphere that generates heat and light through nuclear fusion in its core. In the case of the sun, it fuses into

Plasma: Is a state of matter in which the matter is in ionized form. Composed of particles electrically charged. Normally composed of electrons (negative) and protons and ions (positive)

Mars

A moderately large object that orbits a star; it shines by reflecting light from the star. The constitution of planets may be rocky, icy, or gaseous in composition. We will discuss the formal definition given by the IAU (International Astronomical Union) later in the semester

Moon (or )

An object that orbits a planet

Ganymede (orbits )

A relatively small and rocky object that orbits a star

A relatively small and icy object that orbits a star What is outside the solar system? An of gas and/or dust. Parts of a nebula may reflect light from a star (s), some part may emit light from exited gas by UV radiation from a star (s). Part may absorb light . (In the picture, the nebula) The belongs to the (our ) Galaxy What is outside the Milky Way? A huge collection of in space, all held together by and orbiting a common center. The Milky Way and the are examples of

M31, the great galaxy in Andromeda Clusters of galaxies and • Groups of galaxies with more than a few dozen members are called galaxy clusters • Regions where galaxies and galaxy clusters are most tightly packed are called superclusters • Galaxy clusters and superclusters form giant chains and sheets with large voids between them Universe

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

• The portion of the universe than can be seen from . The observable Universe is probably only a portion of the entire Universe What is our place in the universe? The and the expanding of the Universe

• Observations of galaxies show that the entire universe is expanding, the average distance between galaxies is increasing with time. • This means that galaxies ( or at least matter) must have been close together in the past. • If we go back far enough, all the matter was concentrated in a small from which the expansion began. • That is called the Big Bang. From the rate of expansion it is estimated that it occurred about 14 billions year ago. • The universe has continued to expand and evolve since then. • On a small scale, the force of gravity has drawn matter together forming galaxies, stars formed inside the galaxies and planets formed around some of those stars. • The only two chemical elements created during the Big Bang were hydrogen and helium • The rest of the elements heavier than H and He are generated inside the stars • As Carl Sagan said: We are “star stuff”, all the chemical elements on our body came from stars How did we come to be? Let’s address the question: How can we know what the universe was like in the past?

• Light travels at a finite speed (300,000 km/s).

Destination Light travel time 1.2 second

Sun 8 minutes

Sirius 8 years Andromeda Galaxy 2.5 million years

How can we know what the universe was like in the past? • Light travels at a finite speed (300,000 km/s).

Destination Light travel time Moon 1.2 second

Sun 8 minutes

Sirius 8 years Andromeda Galaxy 2.5 million years

Thus, we see objects as they were in the past: The farther away we look in distance, the further back we look in time. The meaning of a Light-Year

• The distance light can travel in 1 year • About 10 trillion kilometers (10x10^12 km) or (6 trillion miles)

How far is a light-year?

Distance = Speed x Time 1light-year = () (1 year) km 365 days 24 hr 60 min 60 s = 300,000 s 1 yr 1 day 1 hr 1 min How far is a light-year?

1light-year = (speed of light) (1 year) km 365 days 24 hr 60 min 60 s = 300,000 s 1 yr 1 day 1 hr 1 min =9,460,000,000,000 km Example: The distance to the Orion Nebula is about 1500 light years.

We see the Orion Nebula as it looked 1500 years ago. Example: This photo shows the Andromeda Galaxy as it looked about 2.5 million years ago. Question: When will we be able to see what it looks like now? • At great distances, we see objects as they were when the universe was much younger.

We see a galaxy 7 billions light-years away as it was 7 billion years ago.

Light from nearly 14 billion light years away shows the universe as it looked shortly after the Big Bang, before galaxies existed Can we see the entire universe?

Question Why can’t we see a galaxy 15 billion light-years away? (Assume the universe is 14 billion years old.)

A. Because no galaxies exist at such a great distance. B. Galaxies may exist at that distance, but their light would be too faint for our telescopes to see. C. Because looking 15 billion light-years away means looking to a time before the universe existed. Question

Why can’t we see a galaxy 15 billion light-years away? (Assume the universe is 14 billion years old.)

A. Because no galaxies exist at such a great distance. B. Galaxies may exist at that distance, but their light would be too faint for our telescopes to see. C. Because looking 15 billion light-years away means looking to a time before the universe existed. What have we learned?

• What is our physical place in the universe? – The Earth is a small planet in the solar system. – The solar system is one of many planetary systems in the Milky Way Galaxy – The Milky Way galaxy is one of about 40 galaxies in the of galaxies. – The Local Group is one a many groups in the Local of galaxies • How did we come to be? – Part of the matter in our bodies came from the Big Bang, which produced hydrogen and helium. – All other elements were constructed from H and He in the core of the stars and then recycled into the interstellar space from which new star systems were formed, including our solar system. What have we learned?

• How can we know what the universe was like in the past? – When we look to great distances, we are seeing events that happened long ago because light travels at a finite speed. • Can we see the entire universe? – No. The observable portion of the universe is about 14 billion light-years in radius because the universe is about 14 billion years old.

How big is Earth compared to our solar system?

Let’s reduce the size of the solar system by a factor of 10 billion; the Sun is now the size of a large grapefruit (14 cm diameter).

How big is Earth on this scale?

A. an atom B. a ball point C. a marble D. a golf ball

Let’s reduce the size of the solar system by a factor of 10 billion; the Sun is now the size of a large grapefruit (14 cm diameter).

How big is Earth on this scale?

A. an atom B. a ball point C. a marble D. a golf ball Our Solar System - Sizes 1 mile = 1.6 km

Earth diameter =12,756 km = 1.28 x 104 km

Jupiter diameter = 142,984 km = 1.43 x 105 km

Sun diameter = 1,292,000 km = 1.292 x 106 km

Diameter of Sun ~ 109 times diameter of Earth Diameter of Jupiter ~ 12 times diameter of Earth The scale of the solar system • On a 1-to-10- billion scale: – The Sun is the size of a large grapefruit (14 cm). – Earth is the size of a ball point, 15 meters away. – The Earth is 8 light-minutes away from the Sun How far away are the stars? On our 1-to-10-billion scale, it’s just a few minutes’ walk to . How far would you have to walk to reach (one of the closes start to the solar system)? (Alpha Centauri is 4.3 light-years away) A. 1 mile B. 10 miles C. 100 miles D. the distance across the United States (2500 miles) Answer: D, the distance across the United States How big is the Milky Way Galaxy?

The Milky Way has about 100 billion stars. Its diameter is about 100,000 light years On the same 1-to-10- billion scale, how big is the Milky Way?

Question Suppose you tried to count the more than 100 billion stars in our galaxy, at a rate of one per second.

How long would it take you? A. a few weeks B. a few months C. a few years D. a few thousand years

Suppose you tried to count the more than 100 billion stars in our galaxy, at a rate of one per second.

How long would it take you?

A. a few weeks B. a few months C. a few years D. a few thousand years How big is the universe? • The Milky Way is one of about 100 billion galaxies. • 1011 stars/ 1011 galaxies = 1022 stars

There are as many stars as grains of (dry) sand on all Earth’s beaches. How do our lifetimes compare to the ? • The : a scale on which we compress the history of the universe into 1 year.

What have we learned?

• How big is Earth compared to our solar system? – The distances between planets are huge compared to their sizes—on a scale of 1-to-10-billion, Earth is the size of a ball point and the Sun is 15 meters away. • How far away are the stars? – On the same scale, the stars are thousands of kilometers away. • How big is the Milky Way Galaxy? – It would take more than 3000 years to count the 100 billion stars in the Milky Way Galaxy at a rate of one per second, and they are spread across 100,000 light- years. What have we learned?

• How big is the universe? – The observable universe is 14 billion light- years in radius and contains over 100 billion galaxies with a total number of stars comparable to the number of grains of sand on all of Earth’s beaches. • How do our lifetimes compare to the age of the universe? – On a cosmic calendar that compresses the history of the universe into 1 year, human civilization is just a few seconds old, and a human lifetime is a fraction of a second. 1.3 Spaceship Earth

Let’s explore these topics: • How is Earth moving in our solar system? • How is our solar system moving in the Milky Way Galaxy? • How do galaxies move within the universe? • Are we ever sitting still? How is Earth moving in our solar system? • Contrary to our perception, we are not “sitting still.” • We are moving with Earth in several ways, and at surprisingly fast speeds.

The Earth rotates around its axis once every day. How can we calculate the speed of a person located at the due to earth rotation?

• Rotation speed at equator = equatorial circumference/length of day (Remember that Speed = Distance/Time)

The Earth’s equatorial radius is 6378 km The circumference is 2 x Pi x radius = 2 x 3.1416 x 6378 = 40,074 km One day is equal to 24 hours

Rotation speed at equator = 40,074/24 = 1670 km/hr

Earth orbits the Sun (revolves) once every year

• The time it takes the Earth to complete one orbit around the Sun is called the ( ~365 days) •The average distance between the Earth and the Sun is 1 AU ≈ 150 million kilometers. (AU:, the mean distance between the Earth and the Sun) • The Earth orbit the Sun at an average speed of 107,000 km/hr • With Earth’s axis tilted by 23.5º (pointing to close to Polaris) It rotates in the same direction it orbits, counterclockwise as viewed from above the North Pole. The Earth orbital path defines a flat plane called the . The ecliptic is also the apparent path of the Sun in the How is our Sun moving in the Milky Way Galaxy?

Our Sun moves relative to nearby stars in the local solar neighborhood…

•At typical relative speeds of more than 70,000 km/hr • but stars are so far away that we cannot easily notice their

The Sun is located at about 28,000 light-years from the center of the Milky Way … and the Sun orbits the galaxy every 230 million years. The speed at which our solar system rotates around the center of the galaxy is about 800,000 km/hr More detailed study of the Milky Way’s rotation reveals one of the greatest mysteries in : Most of the galaxy’s light comes from stars and gas in the galactic disk and central bulge But suggest that most of the lies unseen in the spherical halo that surrounds the entire disk

This reveals the presence of a mass that we cannot see. It is called 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? The analogy of the raisin cake: The analogy of the raising cake

• A raisin that was at 1 cm, after baking for one hour is at 3 cm It moves at a speed of 2 cm/hr • Another raising that was at 2 cm, after baking for one hour is at 6 cm It moves at a speed of 4 cm/hr

The more distant the raisins are, they move at larger speed.

Edwin Hubble discovered in the 1920’s by measuring the velocity and distance of galaxies that:

• All galaxies outside our Local Group are moving away from us. • The more distant the galaxy, the faster it is racing away. Hubble plotted the velocity and the distance of the galaxies. It gives a linear relationship between the two parameters. It is called the Hubble law

Conclusion: We live in an expanding universe. Are we ever sitting still? What have we learned?

• How is Earth moving in our solar system? – It rotates on its axis once a day (24 hours) and orbits the Sun in ~365 days at a distance of 1 AU = 150 million kilometers. • How is our solar system moving in the Milky Way Galaxy? – Stars in the Local Neighborhood move randomly relative to one another and orbit the center of the Milky Way in about 230 million years. What have we learned?

• How do galaxies move within the universe? – All galaxies beyond the Local Group are moving away from us with the expansion of the universe: the more distant they are, the faster they’re moving. • Are we ever sitting still? – The answer is No! Earth is constantly in motion, even though we don’t notice it.