ASTRONOMY MIDTERM

Question 1

VELOCITY

Answer 1

A measure of how fast something is moving in a specific direction

Question 2

ACCELERATION

Answer 2

how quickly the speed or direction of something is changing. It’s not just about speeding up; it can also mean slowing down (decelerating) or changing direction.

Question 3

WHAT DOES THE SPEED OF LIGHT BEING INVARIANT MEAN?

Answer 3

it means it is a number that does not change.

Question 4

TIME DILATION

Answer 4

“moving clocks run slow”

if you perfectly synchronize two clocks and the none one on an airplane and fly it around at a very high speed - when they are brought back ti rest relative to one another you will find that the moving clock has passed through less time.

Question 5

TWIN PARADOX

Answer 5

Pair of twins called fritz and vera - fritz is on earth as vera flies past him in a spaceship moving at 60% of the speed of light. Which of the following best describes how they see each other’s clocks as they pass by one another.?

They each see each other’s clocks running slower than their own.

Vera is going to move away from earth at 0.6c (60% the speed of light) therefore they see each other’s clocks running slower than their own.

The different in rate of flow of their two clocks is a factor of 0.8

Each of the twins will say that the other twin’s clock is running at 80% the speed of their own clock.
(for every one second fritz sees on his clock, 0.8 seconds will tick by on veras clock) vise versa

They both assume that they will be older because the other twin’s clock is moving slower (paradox) - but they cannot both be right..

Question 6

TRAIN EXPLANATION

Answer 6

We have no way of knowing if a well isolated train is moving - there is no sense in which a train is moving.

Question 7

THEORY OF RELATIVITY

Answer 7

we must always speak about motion with respect to a given frame of. a reference - e.g., (cannot say ball is moving 10km/hr, you have to say “the ball is moving 10km/hr relative to the plane”)

Question 8

SPEED OF LIGHT

Answer 8

C= the speed of light - the speed of light does not depend on the frame of reference it is ALWAYS the same - the speed of light is Invariant - a number that does not change.

Question 9

WHAT DOES IT MEAN FOR THE SPEED OF LIGHT TO BE INVARIANT

Answer 9

it means the number does not change. - the speed of light will always be the same. - that the speed of light in a vacuum is constant and does not change regardless of the relative motion of the source and the observer.

Question 10

Translate descriptions of motion between the reference frames of different observers, such as a person on a moving train vs. a person on the ground

Answer 10

Imagine a ball, a person on a train is holding the ball, It is thrown up - bounces off the roof of the train and is caught back in the hand of the person who threw it.
goes up a meter, comes down a meter. That takes one second. -> overall it’s travelled 2m/S
Perspective of person on the tracks. from their perspective the ball went diagonally

up and diagonally back down. - Up and down 2m/s = 4m/s
what aspect do the two observers agree about?
the time between throwing and catching.

Question 11

Use an example to show how the invariance of the speed of light gives rise to time dilation, using the example of the two observers on and off the train

Answer 11

Both people agree that the action (throwing and catching the ball) takes 1 second. But the paths they see are different: a short path for the person on the train, and a longer path for the person on the ground.
The catch is: for both paths to be completed in the same amount of time, time itself must be working differently for each person. This is time dilation.
In simpler terms, time seems to pass at different rates for the person on the train and the person on the ground. This difference in how time passes is due to the fact that the speed of light is constant and the same in all reference frames. It’s like having two clocks that tick at different rates.

time measured on the moving train appears to be passing slower from the perspective of the observer on the ground.

“moving clocks run slow”

Question 12

Explain how the invariance of the speed of light binds space and time together into “spacetime”

Answer 12

Space and time are woven together into spacetime because the speed of light is always constant

If you move really fast, time actually slows down for you compared to someone who’s not moving. And distances can seem shorter.

Question 13

Explain the concept of time dilation and give real-world examples of its effects

Answer 13

If you perfectly synchronize two clocks and then load one on an airplane, and fly it around at a very high speed, when they are brought back to rest relative to one another a you will find that the moving clock has passed through less time.

If your friend jackie gets into a spaceship that moves at a
Constant speed relative to you the time between events that she measures on her ship will be less than the time you would measure while watching her

Question 14

Explain the Twin Paradox and its resolution

Answer 14

Pair of twins called fritz and vera - fritz is on earth as vera flies past him in a spaceship moving at 60% of the speed of light.
They each see each other’s clocks running slower than their own.
Vera is going to move away from earth at 0.6c (60% the speed of light) therefore they see each other’s clocks running slower than their own.

The different in rate of flow of their two clocks is a factor of 0.8

Each of the twins will say that the other twin’s clock is running at 80% the speed of their own clock.
(for every one second fritz sees on his clock, 0.8 seconds will tick by on veras clock) vise versa
If Fritz and Vera are both 20 and Vera sets out on a trip at 0.6c, how old will each of them be when she returns to earth?

They both assume that they will be older because the other twin’s clock is moving slower (paradox) - but they cannot both be right..

This is called the twin paradox (that they expect the other twin to be the younger one.)

Question 15

Four fundamental forces of nature

Answer 15

Strong nuclear force (holds atomic nuclei together)

Electromagnetism (most of the forces you experience in your daily life are manifestations of this force) e.g., friction, tension etc…

Weak nuclear force (involved in radioactive decay) (encourages it to fall apart)

Gravity (makes masses attract one another)
It governs the motion of planets, stars, galaxies, and even light. Despite being the weakest force, it dominates at large scales due to its long-range and the fact that it only attracts

The strong nuclear force and the weak nuclear force we never encounter in our daily life. - they only function inside atoms.

(those are in order) -

Question 16

Describe how forces are related to accelerations

Answer 16

Force is that which produces acceleration - if you had an acceleration you must have had a force.

Newton’s second law.

Question 17

Distinguish between Newtonian gravity and Einstein’s general relativity

Answer 17

(Newton) The pull of gravity gets weaker the further apart things are. If you double the distance, the gravitational pull becomes four times weaker.

(Einstein)
Instead of an invisible rope, Einstein thought of space and time as a fabric (like a trampoline). Big objects with a lot of mass (like planets) make a dent in this fabric. Other objects fall into these dents, which we see as gravity. Instead of being pulled by a force, things move because they are following the curves and dents in the space-time fabric.

Newton’s theory says gravity is a pulling force between things with mass. Einstein’s theory says gravity is the result of massive objects bending space and time.

Question 18

Explain the Equivalence Principle and describe what it tells us about the nature of gravity

Answer 18

States that the effects of gravity are indistinguishable from the effects of acceleration. This means that being at rest in a gravitational field (like standing on Earth) is equivalent to being in a constantly accelerating spacecraft in space (where there’s no gravity).

Question 19

Describe how the presence of a mass deforms spacetime

Answer 19

Picture spacetime like a big, stretchy, two-dimensional sheet or trampoline. This sheet represents the fabric of spacetime.

Imagine placing a heavy ball (like a bowling ball) in the middle of this sheet. What happens? The ball causes the sheet to curve and sag around it. This sagging or curving is the “deformation” of spacetime.

When a mass like a star or planet exists, it causes the spacetime around it to curve in a similar way. The bigger the mass, the more spacetime curves.

Question 20

Describe gravity in terms of spacetime curvature

Answer 20

In space, when we have something really heavy like the Earth or the Sun, it makes a dip in this “space-time sheet.” Then, things like the Moon or satellites or even light move towards these dips.

So, when we see things falling to the ground or planets going around the sun, it’s like the marbles rolling towards the dips in the sheet. That’s gravity!

Question 21

Define the term “escape speed” and explain how it changes with the mass of a body and the distance from that body

Answer 21

The speed required for a projectile to break free from the surface of the earth (or free from an object’s gravity) is called the “escape speed” - from the surface of the earth the escape speed is about 11.2 km/s

The more massive the body (like a planet or star), the stronger its gravitational pull.
To escape from a more massive body, you need a higher escape speed. For example, escaping from Earth requires a higher speed than escaping from the Moon because Earth has more mass.

The closer you are to the planet (or any celestial body), the stronger its gravitational pull on you, and the faster you have to move (or throw the ball) to escape that gravity. The further away you are, the weaker the pull, and the slower your escape speed can be.

Question 22

Describe what happens to bodies which attempt to orbit with speeds above and below the escape speed

Answer 22

If a body is moving at a speed below the escape speed but still fast enough to avoid crashing into the celestial object, it will settle into a stable orbit. This is like a satellite orbiting the Earth.

Too slow, and it might fall towards the celestial body; too fast, and it will escape the gravitational pull and move away into space.

Question 23

Define the term “Schwarzschild radius” in terms of the escape speed and the speed of light.

Answer 23

If you shrink a star or planet down to fit inside this circle, it turns into a black hole. The circle is just big enough so that, if you’re on the edge and try to escape, you’d have to go as fast as light to get out. But since nothing can go that fast, nothing can escape once it gets inside this circle.

This is a specific measurement, a certain distance from the center of a black hole. (add more mass get more radius)

Question 24

Define the “event horizon” of a black hole

Answer 24

Black hole’s “point of no return.” It’s the invisible line around the black hole that, once you cross it, you can’t come back. It’s like a trapdoor that only opens one way. Once you’re in, you’re in for good and can’t get out or send any signals back.

This is the actual boundary around the black hole.

Question 25

Explain why black holes are not “cosmic vacuum cleaners”

Answer 25

Black holes have strong gravity, but their gravity doesn’t work differently than that of other objects. They pull things towards them the same way any celestial body (like a star or planet) does. If you replace the Sun with a black hole of the same mass, Earth would orbit it the same way it orbits the Sun now. The black hole wouldn’t start “sucking in” the solar system.

Their gravitational pull doesn’t reach out to actively drag objects in from afar. Objects fall into black holes when they get too close, just as they would fall towards any other massive object if they couldn’t maintain a stable orbit or weren’t moving fast enough to escape its gravitational pull.

Question 26

Describe the observational evidence for the existence of black holes, including the black hole at the center of our galaxy

Answer 26

It is very hard to obtain, because one of there defining features is they are invisible. - there are also super tiny.
Cygnus X 1
some of the earliest evidence is x-ray binaries - which is just light but with more energy.
A normal star orbits a compact object, which is often a black hole. Material from the star gets pulled towards the black hole, forming an accretion disk around it.

noticed that the star was orbiting something that could not be seen with a large mass but it was very compact .

we have a supermassive black hole in the center of the milky way (4 million times the mass of the sun) - telescope sees a bunch of stars in the milky way moving incredibly quickly around nothing.

and gravitational waves

Question 27

Draw a diagram to explain gravitational lensing and relate it to observational evidence for the existence of black holes

Answer 27

..

Question 28

Describe what would happen to Earth if the Sun was replaced by a 1 solar mass black hole

Answer 28

Replaced by a black hole of the same mass, Earth would continue to orbit at the same distance because the gravitational force exerted by the black hole would be the same as that of the Sun.

The Sun provides light and heat to the Earth, which are critical for life. A black hole, on the other hand, does not emit light or heat. Earth would become dark and cold very quickly without the Sun’s energy, leading to a rapid drop in global temperatures and a failure of the photosynthesis process, which is the basis of most food chains on Earth.

Question 29

Distinguish between stellar-mass and supermassive black holes

Answer 29

These black holes typically have masses that are up to 20 times greater than that of our Sun but can be as much as 100 times more massive. However, despite this mass, their size is relatively small, often just a few kilometers in diameter due to their immense density.
They are commonly found throughout galaxies and can be isolated or in binary systems, paired with a star.

Supermassive black holes are enormous, with masses ranging from hundreds of thousands to billions of times that of our Sun. Their event horizons can be as large as the orbits of entire solar systems Supermassive black holes are found at the centers of almost all large galaxies, including our own Milky Way, where Sagittarius A* resides.

stellar-mass black holes are “small” black holes that form from the deaths of massive stars, while supermassive black holes are gigantic, with mysterious origins, sitting at the centers of galaxies.

Question 30

Estimate the physical size of black holes of different masses (e.g. 1 solar mass, 25 solar masses, 1 million solar masses)

Answer 30

The Schwarzschild radius (event horizon) is directly proportional to the mass of the black hole. The formula for the Schwarzschild radius

A 1 solar mass black hole would have a Schwarzschild radius of about 2.95 kilometers.

A 25 solar mass black hole would have a Schwarzschild radius of about 73.75 kilometers (25 times 2.95 km).

A 1 million solar mass black hole (which would be a supermassive black hole) would have a Schwarzschild radius of about 2,950,000 kilometers (1 million times 2.95 km).

Question 31

Describe the general properties of the Sun in comparison to the Earth: size, mass, distance

Answer 31

The sun is much larger than the earth it is 100 times the radius of the earth. - this distance from the earth is 8 light minutes or 150 million km (1 Astronomical Unit AU)

the mass of the sun is 333,000 that of the earths

Question 32

Describe the main features of the solar spectrum: the underlying continuous or blackbody part and the absorption lines

Answer 32

spectrum of light emitted by the Sun

a black body emits a perfectly continuous spectrum - it is something that absorbs all light that hits it (and it re emits all the light that hits it)
anything dense is roughly a blackbody.
- as you heat a black body up it emits more light
Absorption lines are more of less continuous just as a black body but with gaps missing at wavelengths corresponding to the chemical elements present in the atmosphere of the star.

Question 33

Describe atoms in terms of their constituent particles: protons, neutrons, and electrons

Answer 33

atoms consist of nuclei made of protons and neutrons which are orbited by electrons in discrete shells

protons and neutrons in the nucleus

electrons orbit in shells (they can only ever be in those shells)
each shell has corresponding energy low closer to the nucleus and higher as you get further.

electrons jump to other cells but in order to move closer to the nucleus they must lose energy - and it does that by emitting light

and then to go to higher energy shell it must absorb light.

Question 34

Explain how light generated by the Sun is absorbed to produce absorption lines

Answer 34

Some of the light being produced by the body of the star is being absorbed by atoms in its outer atmosphere (and they eat those gaps out of the spectrum)

Question 35

Explain how we can use absorption lines to determine the chemical composition of the Sun, or any celestial object

Answer 35

when you look at the spectrum of the sun and you find the gaps you pair them with the particular colors of light that different elements can absorb - certain chemicals emit specific colors so that allows us the determine the composition of the sun.

Question 36

Wavelengths and color

Answer 36

a long wavelength corresponds to a red color, (cooler)

a short wavelength corresponds to a blue color (hotter)

Infrared light (increasing wavelength) “redder”
Ultraviolet light (increasing energy and frequency) “bluer”

Question 37

continues spectrum

Answer 37

some of every color in the wavelength

Question 38

Explain how we can use the shape of a celestial object’s blackbody curve to determine its temperature

Answer 38

shorter wavelengths are also known as blue lights which emit a hotter temperature - it would be the same black body diagram except it would peak in the blue section of the spectrum - this would also be the same for red or long wavelengths which are considered cooler and these peak in the red spectrum.

Question 39

Explain why we don’t see green stars

Answer 39

A white star is in a spectrum kind of in between - which passes more through light rather than any other color - but it emits equal amount of blue, green, and red - our brains perceive it as white ish

Question 40

Describe how the Sun produces energy

Answer 40

mass and energy are equivalent - mass converts to energy and energy converts to mass.

protons are positively charged and try to go together but they try to repel one another - this is where strong nuclear force comes in, it is much stronger than electromagnetic forces and can bind protons together, - but except when they are combined the amu would be smaller

when you bind protons together it causes their masses to fall, and the excess comes out as energy

Question 41

Define nuclear fusion and explain how nuclear reactions are different from chemical ones

Answer 41

you start with 4 protons - you end with a helium nucleus, anti electron and neutrino, and light energy out.

where hydrogen nuclei fuse to form helium under extreme temperatures and pressures.

Question 42

List the main inputs and outputs of the p-p chain

Answer 42

you start with 4 hydrogen atoms - you get two helium atoms - two bits of anti matter (positrons) - 2 neutrinos - and 2 gamma rays (high energy form of light)

the main source of energy in the sun

hydrogen - helium

Question 43

Explain why neutrinos can escape the Sun so easily, focusing on their interactions via the four fundamental forces

Answer 43

Neutrinos only interact with gravity and the weak nuclear force - they have no knowledge or electromagnetic or strong nuclear forces

because of this they pass through most ordinary matter without difficulty.

they are made in the core during nuclear fusion and then flow right out because they barely know that the sun is there .

Question 44

Distinguish between a star’s apparent brightness and its luminosity

Answer 44

luminosity just means the brightness of the sun - but it is a very specific definition of brightness (how much energy is it outputting per second)

the luminosity of the star is 1 solar luminosity.

luminosity tells us the intrinsic brightness of a light source - it has nothing to do where you are relative to the star

apparent brightness means - how much light you perceive from these light bulbs

(even if they look the same it does not mean they are the same)

Question 45

Draw a diagram illustrating how to use the parallax method to measure the distance to a nearby star

Answer 45

(check the picture that was sent to lexie with the triangle)

larger the angle the closer the distance.

Question 46

Define the term light year and use it correctly, distinguishing it clearly from measures of time

Answer 46

Unit of distance used in astronomy to describe the distance that light travels in a vacuum in a one-year period. It’s important to distinguish this from measures of time, as it represents distance, not the duration of time.

Therefore, in one year, light travels about 9.46 trillion kilometers

Question 47

Explain how the inverse square law of light relates a star’s luminosity to its apparent brightness

Answer 47

the amount of light you receive from a star - falls off with the square of the distance

Question 48

Explain how we can measure a star’s luminosity

Answer 48

star’s luminosity can be measured directly by determining its apparent brightness and distance

I = luminosity/ d squared

Question 49

Order stellar spectra according to the strengths of their spectral lines

Answer 49

O B A F G K M

in this sequence individual absorption lines become stronger or fainter in a regular way as we move from one class to another

goes from hotter to cooler

Question 50

Identify the main physical property which distinguishes stars of different spectral types and describe how it gives rise to the patterns of spectral line strengths

Answer 50

fundamentally what it is is a temperature sequence - it tells us the surface temp of the star

the stars differ from hotter to cooler

Question 51

Come up with your own mnemonic for remembering the sequence of stellar spectral types

Answer 51

Our Best Animal Friends Give Kind Moments.

Question 52

Discuss stellar masses in terms of the solar mass unit

Answer 52

Stellar masses are often discussed in terms of the solar mass unit, which is a standard unit of measurement in astronomy. This unit is based on the mass of our Sun and is denoted by M (o with dot)

Using the solar mass as a unit helps astronomers to easily compare the masses of other stars to that of the Sun. Here’s how stellar masses are generally categorized
​

Question 53

Draw and label an H-R diagram, indicating the physically meaningful ranges for stellar surface temperatures and luminosities

Answer 53

Sent to lexie

Question 54

Identify and draw the main sequence on the H-R diagram and explain its physical origin

Answer 54

Stars on the main sequence are still considered to be alive - the hotter the star the furthers left

Sent to lexie

Question 55

Describe the different types of stars found on the main sequence, carefully distinguishing what they have in common and what is different about them

Answer 55

there are lines of constant stellar radii on the diagram
every star where the circles are - 0.1 solar radii (10% of the sun)

the squares are 10 sqaured solar radii which is 100 times the sun

starts span a lot of sizes on the main sequence

any star on the main sequence is considered a dwarf star (some red, some yellow)

Question 56

Describe and draw how the ages and sizes of stars can be represented in an H-R diagram

Answer 56

as you go up the sequence and masses of stars get larger - their lifetimes also shrink

M stars- have a long life (trillions)

the sun has 10 billion years

higher class stars only have millions of years

• the lifetime means how long it will burn hydrogen.

(sent to lexie)

Question 57

Identify and name patterns or groupings of stars in the H-R diagram

Answer 57

Main sequence, Giants/ super giants, dwarf stars (white dwarfs)….

subgiants, supergiants, and giants are in the process of dying.

white dwarfs are dead

Question 58

Explain what a star’s luminosity class tells us about a star

Answer 58

it sounds like it tells you the luminosity of a star but it actually doesn’t - there are five classes
1- super giants
2-bright giants
3 - giants
4- subgiants
5 - main sequence stars

they break up stars into classes

Question 59

Distinguish between a star’s luminosity and luminosity class

Answer 59

Luminosity is a measure of the total amount of energy emitted by a star per unit of time. It’s a measure of the star’s intrinsic brightness, irrespective of its distance from us.

Luminosity class is a classification that indicates a star’s size (radius) and stage in its evolutionary cycle

Question 60

Given a star’s luminosity class and spectral class, state or estimate its surface temperature, main-sequence lifetime, luminosity, age, mass, colour, likely end-state (not all factors can be determined from all combinations of luminosity class and spectral class)

Answer 60

do it make an example -

Question 61

Explain how and why a star’s main-sequence lifetime is related to its mass

Answer 61

A star’s main-sequence lifetime, which is the period during which it fuses hydrogen into helium in its core, is closely related to its mass. The key relationship is that the more massive a star is, the shorter its main-sequence lifetime.

Question 62

Explain why some stars live longer than others and identify those types which live the longest

Answer 62

While a more massive star has more hydrogen fuel, it burns through this fuel much faster due to the higher fusion rate. This leads to a shorter overall lifespan.

Less massive stars, like red dwarfs, have lower core temperatures and pressures. This results in a slower rate of hydrogen fusion. Because of their efficient fuel usage, these stars can burn for much longer periods. Even though they have less fuel than larger stars, they use it so sparingly that they have the longest lifespans of all stars.

Question 63

Estimate the age of a star cluster by examining the H-R diagram of its members

Answer 63

if you have a star cluster with a full main sequence you know that it is young (if it’s missing stars then its older)