Astro Mid Term Sem. 2 (Book 1/7)

Question 1

What is Hydrostatic Equilibrium?

Answer 1

The balance of gravity pulling in and pressure from heat pushing out.

Con’t: A star becomes stable when the outward forces of expansion from the energy released in Nuclear fusion reactions balance the inward forces of gravity

Question 2

What is Luminosity?

Answer 2

The total energy radiated by a star.

Question 3

What is Nuclear Fusion?

Answer 3

The joining of two nuclei together to form a different one.

Question 4

Very massive stars are common. TRUE or FALSE?

Answer 4

FALSE.

Very massive stars are rare. Reference the HR Diagram.

Question 5

Low-mass stars are common. TRUE or FALSE?

Answer 5

TRUE.

Low-mass stars are very common. Reference the HR Diagram.

Question 6

Why are we “star-stuff”?

Answer 6

Because the elements necessary for life were made in stars.

Question 7

Where does our sun, Sol, sit relative to our galaxy?

Answer 7

2/3 of the way from the center to the edge of the disk (galaxy).
~25,000 Light years (Ly)

Question 8

How often does our sun revolve around the center of our galaxy?

Answer 8

Once every 250 Million years.

Question 9

Where are stars born?

Answer 9

Stars are born in Nebulae.

Question 10

What are Nebulae?

Answer 10

Nebulae are swirling clouds of hydrogen gas

Question 11

What is the gas, and dust, called that are between stars?

Answer 11

It is called The Interstellar Medium

Question 12

What happens to these clouds, Nebulae, when hot and bright new stars are present?

Answer 12

The clouds/Nebulae become illuminated.

Question 13

What is The Interstellar Medium comprised of specifically? What is the scientific term for these clouds?

Answer 13

Gas (75% Hydrogen, 25% Helium) and Dust

It is called a Nebula (Plural - Nebulae)

Question 14

What is an Emission Nebula?

Answer 14

A Nebula with the characteristic emission line spectrum of a hot, thin gas.

Question 15

What is a Dark Nebula?

Answer 15

A Nebula so opaque that is blocks visible light that are emitted from stars behind the Nebula.

Question 16

What is a Reflection Nebula?

Answer 16

It does not produce its own light like Emission Nebulae, but scatters star light.

This scattering is due to the dust grains.

Lower concentration of dust grains than Dark Nebulae

Scattering gives rise to blue color

Question 17

What is Interstellar Extinction?

Answer 17

The intensity of star light is reduced as light passes through the Interstellar Medium.

Question 18

What is Interstellar Reddening?

Answer 18

When light from a star pass through Interstellar Medium, dust particles absorb or scatter blue light allowing red light to pass through (like a sunset on Earth).

Question 19

What is Interstellar Reddening?

Answer 19

When light from a star pass through Interstellar Medium, dust particles absorb or scatter blue light allowing red light to pass through (like a sunset on Earth).
The star appears red.

Cont:
Long-Wavelength infrared light passes through a cloud more easily than visible light.

Observations of Infrared light reveal stars on the other side of the cloud.

Question 20

What are Giant Molecular Clouds?

Answer 20

In certain cold regions of interstellar space atoms combine to form molecules.

Question 21

What happens in Molecular Clouds?

Answer 21

Stars are born in Molecular Clouds consisting mostly oh hydrogen Molecules.

Question 22

Stars form in places where gravity can overcome thermal pressure in a cloud. TRUE or FALSE?

Answer 22

TRUE

Question 23

Star forming clouds DO NOT emit infrared light. TRUE or FALSE?

Answer 23

FALSE.

They do; this is because of the heat generated as stars form.

Question 24

Infrared observations can directly detect dust in the Interstellar Medium. TRUE or FALSE?

Answer 24

TRUE

Con’t:
Some molecules in the cold gas emit in the infrared - infrared observations can detect very cold clouds of gas.

Question 25

The birth and life of any star can be described as a battle between WHAT 2 forces?

Answer 25

Gravity VS. Internal Pressure

Con’t:
Gravity wants to collapse the star and Internal Pressure holds up the star

Remember Newton’s Law of Gravity:
The amount of gravitational force depends on the mass.
Gravitational potential energy is turned into heat as a star collapses.

Question 26

In the densest clouds, Hydrogen CANNOT exist as molecules (Hª2) rather than as atoms. TRUE or FALSE?

Answer 26

FALSE

Hydrogen CAN exist as molecules rather than atoms. These clouds are called Molecular Clouds

Question 27

AN INTERSTELLAR MOLECULAR CLOUD - Flip Card

Answer 27

• Star formation begins when part of the interstellar molecular cloud contracts under its own gravitational attraction - denser regions in the clouds are favourable for star formation.
• The gravitational collapse overwhelms the pressure - colder regions are more favourable since they are low pressure regions.
• These cold dense regions of clouds collapse under its own weight to form clumps, future stars.

Question 28

What are Contracting Clouds?

Answer 28

Star formation is triggered when a sufficiently massive pocket of gas is squeezed by some external event.

• Material flowing out of protostars cause shock waves that trigger regions nearby to collapse.
• A Supernova explosion of a dying star can compress the surrounding gas triggering a collapse.

Question 29

A star-forming cloud colliding with a shock wave can be compressed and break into fragments. TRUE or FALSE?

Answer 29

TRUE

Some of these fragments can become dense enough to collapse under gravity and form stars.

Question 30

GO TO MIDDLE OF PAGE 8 OF BOOK 1

Answer 30

Large Clouds can go on to make a whole cluster of stars

Question 31

What is a Protostar?

Answer 31

It is the dense and opaque region at the center of a cloud

- an embryonic object at the dawn of star birth

Question 32

What happens when observing Infrared light in a cloud?

Answer 32

It can reveal the newborn star embedded inside it.

Question 33

Why is Solar-System formation a good example of star birth?

Answer 33

• The rotation speed speed of the cloud from which a star forms increases as the cloud contracts.
• Rotation of a contracting cloud speeds up for the same reason a skater speeds up as she pulls in her arms.

Question 34

What is Flattening?

Answer 34

It is collisions between particles in the cloud cause it to flatten into a disk.

Question 35

Formation of Jets (slide)

Answer 35

Jets are observed coming from the centers of disks around protostars.

Question 36

From Protostar to Main Sequence: Protostars look star-light after what? Where does its Thermal Energy come from?

Answer 36

After the surrounding gas is blown away. But its thermal energy comes from gravitational contraction, not fusion

Question 37

From Protostar to Main Sequence: Contraction must continue until the core becomes hot enough for what? What is the temp.?

Answer 37

Until the core becomes hot enough for nuclear fusion at a temperature of 10,000,000 K.

Question 38

From Protostar to Main Sequence: When does the contraction stop?

Answer 38

It stops when the energy released by core fusion balances energy radiated from the surface - the star is now a Main-Sequence star.

Question 39

At what point do we say a new star is born?

Answer 39

When thermonuclear reactions start at the center of a protostar.

Question 40

SUMMARY OF STAR BIRTH

Answer 40

1) Gravity causes gas cloud to shrink and fragment
2) Core of shrinking cloud heats up
3) When core gets hot enough, fusion begins and stops the shrinking
4) New star achieves long-lasting state of balance

Question 41

H-R Diagram: What does the life track illustrate?

Answer 41

It illustrates a star’s surface temperature and luminosity at different moments in time.

Question 42

H-R Diagram: What have we observed?

Answer 42

We have observed;

• bipolar low from you stars
• star forming regions (e.g. Orion Nebula)
• Young Star clusters

Question 43

What have we learned? LAST ONE

Answer 43

How do stars form?

• Stars are born in cold, relatively dense molecular clouds.
• As a cloud fragment collapses under gravity, it becomes a protostar surrounding by a spinning disk of gas
• The protostar may also fire jets of matter outward along its poles. Protostars rotate rapidly, and some may spin so fast that they split to form close binary star systems