AST201 FINAL

You fall asleep on a train. How can you tell if the train is moving?

You couldn't tell

A plane is going 900 km/hr. Someone on it rolls a bowling ball towards the front of the plane at 10 km/hr. How fast does the person on the plane see the ball moving relative to the plane?

10 km/hr

How fast does the person on the ground see the ball moving relative to the ground?

910 km/hr

The same person now flashes a light towards the front of the plane. If a beam of light behaved the same way as the ball, what would a person on the ground measure its speed to be relative to the ground?

c + 900 km/hr

If a beam of light obeys the law of physics, what would a person on the ground measure its speed to be relative to the ground?

c (the speed of light)

Jackie is moving away from you at speed 0.9c. You shine a beam of light at her (at speed c, of course). How fast does Jackie measure the light to be moving?

c

Fritz is on Earth. Vera is flying past in a spaceship moving at 90% of the speed of light (0.9c). Which of the following best describes how they see each others' clocks as Vera passes Fritz?

They both see each other's clock running slower than their own

Someone on Earth would measure the star Vega to be 25 light years away. If you fly a spaceship to Vega at 0.999c, you will measure the distance to be:

much less than 25 light years

The person on Earth sees you travel 25 light years at 0.999c. They think your trip takes

a little more than 25 years

YOU measure the distance between Earth and Vega as only 1 light year. Thus, at a speed of 0.999c, for you the trip to Vega takes only:

about one year

Are stars on the our side of the galaxy affected by the gravitational pull of stars on the other side of the galaxy?

Yes

Imagine a universe which is empty except for you and a baseball. You throw the baseball. If we ignore gravity, what will ultimately happen to the baseball?

it will keep moving forever at a constant speed

The moment the baseball loses contact with your hand, it stops:

accelerating

After you throw the baseball, what will happen to you?

you will move in the direction opposite to the ball's motion, but slower than the ball

If the distance between two stars triples, the force of gravity between them:

decreases by a factor of 9

Imagine you're in a spaceship, far away from any object with mass. The spaceship is coasting upward at a constant speed. You let go of a baseball at shoulder height. What happens to the ball?

it hovers at shoulder height

Imagine you're in a spaceship, far away from any object with mass. The spaceship accelerates upward. You let go of a baseball at shoulder height. What happens to the ball?

it falls toward your feet

If we could squeeze the Earth, so it got smaller, what do you think would happen to the escape speed from the surface?

It would go up

If you are just inside the event horizon of a black hole, can you get away from the hole?

No

If we add mass to a black hole, the black hole will:

Get larger

If we replaced the Sun with a black hole having exactly the same mass as the Sun, what would happen to Earth?

Earth would continue to orbit as normal, but it would get very cold

Which of the following would allow us to locate a black hole today?

we could look for objects that appear to be orbiting an invisible object

Why do we believe there is a black hole in Cygnus X-1?

X-rays are observed originating from just outside the event horizon

A light year is the distance that light can travel in one year through empty space. How many light years do you think the Earth is from the Sun?

Much less than 1 light year

If different elements have different orbitals,
they must also differ in the:

colours of light they can absorb

Rebecca is in her ship floating motionless in empty space. She sees Ariel fly by at 70% of the speed of light (0.7c). What is Ariel's velocity in her own reference frame?

0c

Rebecca is in her ship floating motionless in empty space. She sees Ariel fly by at 70% of the speed of light (0.7c). She also sees Alysa flying by in the opposite direction at 0.8c. What is Alysa's velocity from Ariel's reference frame?

between 0.8c and 1c

General relativity says gravity...

can be explained by curved spacetime

To determine a star's surface temperature,
we would need to measure:

the wavelength at which it emits the most light

To determine a star's chemical composition,
we would need to measure:

the wavelengths at which its spectrum shows absorption lines

If the Sun is made of plasma (very hot gas), why doesn't its gravity crush it into a tiny ball?

the plasma particles are moving very quickly

If a reaction creates energy via E\=mc2, then the mass
of the products is\____ the mass of the inputs.

less than

Which of these phrases most accurately describes
how the Sun generates energy?

it fuses particles, converting mass into energy

A star that is 1 light year away has a luminosity of 2 L¤. How would its luminosity change when viewed from
2 light years away?

It wouldn't change

If two stars have different parallax angles, the one with the smaller angle is:

farther

Consider a star that is 1 light year away. How would its apparent brightness change when viewed from
2 light years away?

1/4 as bright

Which pieces of information are the MINIMUM set required to measure a star's luminosity?

apparent brightness and distance

From Earth, Victor sees Ayushi fly past in a spaceship at 0.8c. From her spaceship, Ayushi sees the Earth (and Victor) fly past at 0.8c. Victor sees Ayushi's watch tick slower than his watch. What does Ayushi see?

Victor's watch ticks slower than hers

The equivalence principle states that

Acceleration and gravity have the same effect

TRAPPIST-1 is a nearby star only 40 light years away from the Earth. Thierry travels towards TRAPPIST-1 at 0.5c in search for life around one of its seven planets. Abhinav stays behind on Earth and sees that it takes 80 years for Thierry to reach his destination.
From Thierry's frame of reference, how long did the trip take and how far did he travel?

less than 80 years (time) and less than 40 light years (distance)

Margaret and Ariel are ready to test the Twin Paradox. Margaret travels away from Earth at 0.6c to a nearby star, turns around, then returns to Earth. Ariel stays on Earth the entire time. When Margaret and Ariel are finally reunited, who has aged less and why?

Margaret has aged less because she changed her reference frame

Peter is in a room without windows or doors, and doesn't have any way to communicate with anything outside the room. Peter is standing on the floor of the room and is not weightless. What can Peter do to determine if he is on a planet or in an accelerating spaceship?

He can't

Imagine two metal objects that are the same size. One has been heated so it is glowing red hot. They other is heated so it is glowing with a pale blue. What must be true?

The blue one is hotter

The "temperature" axis of the H-R diagram could also be labeled as:

colour

Stars can have weak absorption lines of a given element if that element is:

either too cold or mostly ionized

Which of the following objects emit blackbody radiation (The Sun, The Earth, An asteroid, You, All of the above)

All of the above

If the surface of the Sun were to cool down what would happen to its blackbody curve?

The peak wavelength of the blackbody curve would move towards redder wavelengths.

Imagine 2 stars that have the same colour and apparent brightness. What must they have in common?

They have the same temperature

Star A appears to move 10x more than Star B compared to background stars. We can conclude that:

Star B is 10 times further away than Star A

Star A and Star B have the same apparent brightness, but Star B is 10x further away. To have the apparent brightness, Star B must be

100x more Luminous

Where are the oldest stars on the main sequence found?

Bottom right

What property of a star determines its spectral class (O, B, A, F, G, K, M)?

Temperature

What are the two axes of an H-R diagram?

luminosity, temperature

What do two stars on the main sequence have in common?

They are both fusing hydrogen in their cores

What stops fusion in the Sun's core from happening faster and faster?

If it did, the core would expand, cool, and slow things back down.

When a star first forms, it is made of:

90% hydrogen, 10% helium

If nuclear fusion reactions in the Sun's core suddenly stopped, what would happen?

the core would cool and gravity would crush it

When a gas expands freely, it also cools down.
Thus, when the Sun's outer layers expand, they should:

appear redder

The luminosity of the Sun will increase a lot when it
becomes a red giant. Why?

Because it will be much larger

While the Sun is on its way up the sub-giant branch, its core still isn't undergoing nuclear fusion. What should be happening to it?

It should still be collapsing

Once core helium fusion begins, the whole core will heat up, back to the way it was when the Sun was 'alive'. What should then happen?

The core will expand, energy production will fall, and the whole Sun will shrink a bit.

What's the main difference between a white dwarf star and a main sequence star?

white dwarf stars are supported against gravity by electron degeneracy pressure, not by heat.

Which of the following statements is true?

fission and fusion can both either produce or consume energy

What would stop a low-mass star from becoming a black hole once it could no longer fuse any element?

electron degeneracy pressure

What will not occur during the Sun's
process of dying?

core-collapse supernova explosion

When a figure skater is spinning on the ice and they pull in their arms, what happens to them?

they spin faster

When a figure skater is spinning on the ice and they pull in their arms, they are reducing their:

radius

If the angular momentum of the figure skater, m x v x r, can't change, then when r goes down, v has to:

increase

When we take a gigantic star, 70 million km in diameter and shrink much of it down to 25 km in diameter, it MUST:

spin faster

Which of the following correctly describes the relationship between neutron stars and pulsars?

neutron star is sometimes seen from Earth as a pulsar

What two measurements can astronomers make to determine a star's luminosity?

Apparent brightness and distance

Elaria has measured two stars, star A and star B, which both have the same apparent brightness. She measures that star B's parallax angle is larger than star A's parallax angle. What can she determine about the intrinsic properties of star A and star B?

Star A has a higher luminosity than star B

Order the following from largest to smallest: galaxy, star, solar system, universe

universe, galaxy, solar system, star

If you stood on a planet in the center of a symmetric sphere of stars, you would see the stars:

filling your sky roughly equally in all directions

If you stood somewhere to one side of a disk of stars, you would see the stars:

as a thin band of light circling your sky

Where does the carbon in the milky way's dust come from?

Stars and Supernovae

What can we conclude from the fact that almost all O stars in the galaxy are within or very close to the disk

Most star formation occurs in the disk.

If fusion stops happening in a region of a star's interior, what will happen to that region?

It will contract because pressure no longer balances against gravity

By mass, the Earth is approximately 30% iron and most of this iron is in the Earth's core. Where did iron in the Earth originally form?

It formed in high mass stars

Why will the Sun move up and to the right on an HR-diagram as it evolves from the main sequence stage to the red giant stage?

Shell burning pushes the envelope out, and the envelope cools as it grows

What are the different stages of the Sun's lifecycle?

Main sequence -\> H shell burning -\> He core burning -\> He shell burning -\> Planetary nebula

Why is iron the heaviest element that can be produced in the interiors of stars during their lifetime?

Iron fusion consumes energy

If we know the Earth's orbital period and its orbital semi-major axis, we can calculate:

The mass of the Earth + Sun

The Ring is bluer than the bulge. From this we can conclude:

There has been more recent star formation in the ring

When the Milky Way and Andromeda collide, what is the most likely effect on the Solar System?

Not much at all -though the night sky will look different

What property of a star is most responsible for determining whether that star becomes a black hole, neutron star or white dwarf at the end of its main sequence lifetime?

mass

Which of the following currently prevents the Sun from collapsing under its own gravity?

Nuclear fusion

What prevents white dwarfs from collapsing under their own gravity?

Electron degeneracy pressure caused by packing electrons close together

Mei is orbiting a black hole at a distance of 1AU, and determines that the event horizon of the black hole is 0.1 AU. She wants to approach the black hole, but also wants to be able to escape. How will Mei's escape speed change as she moves towards the event horizon?

her escape speed is much less than c at 1 AU and c at the event horizon, so she shouldn't get too close

Consider two stars, A and B. They have the same luminosity, but star B has 1⁄4 the apparent brightness of star. Assuming there is nothing in the way, what can we conclude?

Star B is twice as far away as Star A

According to Hubble's Law, if galaxy A is twice as far from us as galaxy B, then galaxy A's redshift will be:

twice galaxy B's

If we see all distant galaxies "expanding" away from us with distances proportional to their redshifts, then:

people in other galaxies should also see all distant galaxies moving away from them too

If you could go far enough away, you could leave the universe, and look back at it expanding towards you. True or false?

False, because the universe does not have an edge.

How can we describe dark matter?

Dark matter is something with mass that doesn't glow

Why is our view of the Milky Way not obstructed by dust at long-wavelength infrared light?

Dust primarily absorbs visible starlight and glows brightest at long-wavelength infrared light

A very distant galaxy has a peculiar velocity towards us. What will the redshift be in relation to Hubble's Law?

It will be a smaller redshift than predicted by Hubble's law

Can we see Galaxies further away than the plasma that produces the Cosmic Microwave Background (CMB)?

No, because the CMB comes from a time before there were any galaxies.

Imagine you are floating in the Universe after the Universe has cooled enough for the plasma to become gas, but before the first stars form. You look around. What do you see.

An yellow/orange/reddish glow coming from all directions.

When does the light from the cosmic microwave background come from?

At the end of the era of Nuclei, before the era of atoms

According to Hubble's Law, distant galaxies are:

getting farther away at speeds proportional to their distances