ASTR 1 Fall 2026

Stars and Black Holes

Celestial Sphere/Geocentric Model

Earth is put at the center of the universe and everything orbits around it; Greek interpretation of space

ecliptic

path the sun follows as it circles the celestial sphere

zenith

center, right above the head

meridian

separates from east to west from observer’s view; goes through the zenith and the North and South poles; when the sun hits the meridian, it is noon

circumpolar

remains above the horizon, circling the north celestial pole

latitude

measurement north to south; can be determined by location of Polaris in the sky

longitude

measurement east to west

angular size

angle object appears in the span of your field of view

precession

gradual wobble of a rotating object around a verticle line; 26,000 year cycle for Earth, constrained by the moon’s gravity

North Star location

straight up (zenith) at north pole, so going more south it would be closer to the horizon

Sidereal Day

23 hours and 56 minutes

solar day

24 hours

sidereal day and solar day

combined / the mean of them is the length of a day, which means the stars rise and set 4 minutes less each night

Matter in the Universe

Dark matter is 21 percent of all matter in the universe, dark energy is 75 percent

Mass of galaxy

Most of it is located on the outside of the disk; dark matter

Constellation

Region of sky with defined borders

Synchronous rotation

Moon rotates exactly once with each orbit

waxing

new moon to full moon

Waning

full moon to new moon (no moon)

tidal forces

moon and earth pull on each other, this force is the difference between the two. depending on where the moon is it pulls the water of Earth to it, causing lower or higher tides in the oceans;

epicycles

when a planet or other object moves in a small circle while also travelling the path of a larger circle; circling itself while circling the other object

Ptolemy

came up with epicycles in the Greek Geometric Model, which also helped with calculations and predict motions of planets

scientific/natural law

pattern observed to repeat and believed to hold true; how things work

scientific theory

why and why things work; attempts to explain origin or physical laws through evidence-based explanations including observations, empirical laws, and experimental results

Kepler’s laws

1. orbit of planet is an ellipse with the sun at 1 focus

2. as a planet moves in its orbit, it moves fastest when closest to the sun

3. p²=a³ is the numerical relationship between period of orbit and its size

velocity

length/time and involves direction

Speed

Length/time and has no direction

acceleration

change in velocity with time

scalar

only has a magnitude; speed

vector

has both magnitude and direction; velocity

Newton’s 3 Laws

1. objects tend to be at a constant velocity unless acted on by a force (law of inertia)

2. acceleration of an object depends on its mass and the force applied (a=F/m)

3. For every action there is an equal and opposite reaction

momentum

mass times velocity

Mass

density times volume; amount of matter in an object

weight

force that acts on an object

force

interactions

force acting through contact

• striking force

• tension - pulling force

• air resistance

• spring force - pushing/pulling by a compressed or stretched spring

• friction - sliding or rubbing of objects against each other

• upthrust/buoyancy - upward force on object placed inside a fluid

force acting at a distance

• gravitational force - pull of objects with mass

• electrostatic force - attractive or repulsive force between charged particles

• magnetic force -attractive or repulsive interaction between magnetic poles or materials

• electromagnetic force

• forces of nature

universal law of gravitation

1. every mass attracts every other mass

2. attraction directly proportional to product of their masses

3. inversely proportional to the square of the distance between objects’ centers

How does gravitational force between 2 objects change if distance between them triples?

the force decreases by a factor of 9

2 planets, 1 is 3x as massive as the other. how much bigger is the force on that object as compared to the lower mass one?

Force remains the same

a compact car and a large truck have a head-on collision. which statement is false?

change in velocity of car is the same as change in velocity of the truck

angular momentum

rotational equivalent to inertia; rotation is faster as radius shrinks

angular resolution

spectrum that a telescope can distinguish; when something is further away the angular separation of light is closer and looks like one and the opposite is true for closer light; detail in images

Why do we use telescopes?

Two reasons:

1. more light is collected so we can see fainter objects

2. clearer and sharper images through angular resolution

radial velocity

velocity along the line of sight; determines is a star has planets

red shifted absorption lines

astronomical object is getting further from us and has a lower frequency and longer wavelengths

blue shifted absorption lines

an astronomical object is getting closer to us and light has a higher frequency but shorter wavelengths

focal plane

light from different directions comes into focus

light wave

vibration of electric and magnetic fields

how is energy stored in atoms?

electrons absorbs light to enter a higher energy state and then releases it to return back to ground state; jumps between different states which also determines the atomic fingerprint

atomic fingerprint

the unique patterns of light emitted by an element, shown in the emission line spectrum

Suppose we look at two distant galaxies. Galaxy 1 is twice as far away as Galaxy 2. How are we seeing the galaxies as we look at the sky?

We see Galaxy 1 as it looked at an earlier time in the history of the universe than Galaxy 2

When we see an object that is 1000 light years away, what does that mean?

We see the object how it was 1000 years ago

Could we see a galaxy that is 20 billion light years away?

No we cannot because that would be beyond the bounds of the observable universe.

If you could count stars at a rate of about one per second, how long would it take to count all the stars in the Milky Way?

Several thousand years

The total number of stars in the observable universe is about the same as what material on Earth?

Same as the number of grains on sand on all of Earth’s beaches

How are we “star stuff”?

Helium and Hydrogen were the first elements, so all other elements came from them in stars

If we imagined the history of the universe compressed into a year, with the present as midnight at the end, when did dinosaurs become extinct?

Yesterday morning

How old is our solar system?

41/2 billion years old

How old is our solar system relative to the age of the universe?

About 1/3 the age

How did we learn of the expanding universe?

The further other galaxies are from us, the faster they move

How did the Ptolemaic model explain retrograde motion of planets?

The planets moved in small circles that also moved on larger circles around Earth

Copernicus created what model of the universe and how do it not lead to better predictions of planetary positions?

He created the Sun- centered model that used perfect circles for planetary orbits

How does a planet’s mass effect the orbit around the sun?

It has no effect