Open cluster M45

the Pleiades

(also Subaru)

Globular cluster M13

What can we see? Stellar observables

• Stars have different colors• Stars have different brightness• Stars can be distributed randomly or in

groups.

What are some things we need to know? Stellar parameters

What are some things we need to know? Stellar parameters

• Mass and radius

• Surface temperature

• Luminosity (energy emitted per second; unit watts)

• Chemical composition (what stars are made of)

• Age

• Distance

• Velocity

• We can not directly observe the stellar parameters. We have to determine them using our observables. We analyze the observables and use our understanding of physics (the study of matter and energy and how they interact) to calculate stellar parameters.

• Check out the stellar parameter toolbox on the class website

How do we find out masses in astronomy?

• We use Newton’s laws of motion and Newton’s law of gravity.

• Stars orbiting the • Central black hole in • Our galaxy• Mass is over 1 million• Solar masses

The International Space Station (ISS)

Why are the astronauts in the ISS weightless?

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1. There is no gravity in outer space

2. Earth is so far away that gravity is very weak

3. There is no gravity in a vacuum

4. They are falling

5. They are too small for gravity to effect them

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5

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• Before answering the question:

Why are the astronauts in the ISS weightless?

Let’s look at a little physics.

What the demonstration shows

• Galileo – Objects fall at the same rate in a gravitational field regardless of their mass.

What the demonstration shows

• Galileo – Objects fall at the same rate in a gravitational field regardless of their mass.

• Newton’s First Law of Motion – An object at rest (or in motion) stays at rest (or in motion) unless acted on by a NET external force.

What the demonstration shows

• Galileo – Objects fall at the same rate in a gravitational field regardless of their mass.

• Newton’s First Law of Motion – An object at rest (or in motion) stays at rest (or in motion) unless acted on by a NET external force.

• Newton’s Second Law of Motion – The force applied to an object is equal to the mass of the object times the object’s acceleration. F = ma

What the demonstration shows

• Galileo – Objects fall at the same rate in a gravitational field regardless of their mass.

• Newton’s First Law of Motion – An object at rest (or in motion) stays at rest (or in motion) unless acted on by a NET external force.

• Newton’s Second Law of Motion – The force applied to an object is equal to the mass of the object times the object’s acceleration. F = ma

• Newton’s Third Law of Motion – For every force there is an equal and opposite reactionary force. F1 = -F2

Let’s examine our answers

• There is no gravity in outer space

Let’s examine our answers

• There is no gravity in outer space• Question: Why is the ISS going around

(orbiting) the Earth?

Let’s examine our answers

• There is no gravity in outer space• Question: Why is the ISS going around

(orbiting) the Earth?• Earth is so far away that gravity is very

weak• There is no gravity in a vacuum

Let’s examine our answers

• There is no gravity in outer space• Question: Why is the ISS going around

(orbiting) the Earth?• Earth is so far away that gravity is very

weak• There is no gravity in a vacuum• Question: Why is the ISS orbiting the

Earth?

Now a little more thinking

• They (the astronauts) are too small for gravity to effect them.

Now a little more thinking

• They (the astronauts) are too small for gravity to effect them.

• Question: Gravity is holding the ISS in orbit. What does Newton’s First Law tell us should happen to the astronauts if gravity isn’t effecting them? Would they be weightless?

The correct answer is…

• The astronauts are falling. The ISS is falling around the Earth and (from Galileo) we know that the astronauts are falling at the same rate as the ISS.

Why does the Earth orbit the Sun instead of the Sun orbiting the Earth?

1. The Sun is more massive so the Earth has a larger acceleration

2. The Sun is more massive so it exerts a greater force on the Earth than the Earth exerts on the Sun

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Quiz #1• Suppose you are on an imaginary planet called E2 that is

identical to Earth. It orbits a star that is identical to our Sun and it is at an average distance from its sun that is identical to Earth.

• Observables:

• E2’s rotation axis is perpendicular to its orbital plane (No tilt).

• E2 has seasons similar to Earth but the entire planet has summer at the same time and winter at the same time.

• PROBLEM: What must be causing the seasons? Explain your answer using the two observables.

• How does E2’s orbit compare to Earth’s? Explain your answer.

How light spreads out

Newton’s big realization