Stars and Universe Study Guide

good luck man

albedo

The percentage of light reflected from an object

Frequency

the number of wave cycles that pass by per second

wavelength

the distance between two corresponding points on a wave

energy

amount of energy carried by a light wave

Light with a higher frequency has a…

higher energy and a shorter wavelength

Light with a shorter wavelength has a…

higher frequency and a higher energy

Are radio waves from the sun or stars absorbed by clouds?

No. radio waves go through clouds relatively well, making it possible for radio telescopes to observe astronomical objects even on cloudy days

When an object is heated, it glows in this order

red, orange, yellow, white, blue

Speed of light

300,000 km/s

Absorption Spectrum

most useful for studying stars

the spectrum where specific colors are missing after light passes through a substance

Emission Spectrum

the spectrum that shows specific colors emitted by excited atoms

Light from an object moving toward Earth

shifts to the blue end of the spectrum

Light from an object moving away from Earth

shifts to the red end of the spectrum

Which has a longer wavelength: blue or radio?

Radio

Which has a longer wavelength: orange or x-ray?

Orange

Which has a greater frequency: red or purple?

Purple

Which has a greater frequency: gamma or microwave?

Gamma

Which has a greater frequency: yellow or ultraviolet?

Ultraviolet

Which has greater energy: x-ray or green?

X-ray

Which has greater energy: blue or red?

Blue

Which has greater energy: ultraviolet or radio?

Ultraviolet

Why do astronomers want telescopes in space?

To avoid the Earth's atmosphere, which blocks or blurs certain light (like UV, X-rays, and gamma), and to get clearer, sharper images of space

ESSAY ANSWER

The life cycle of an average sun-like star begins in a nebula, a large cloud of hydrogen gas where gravity pulls the gas together to form a protostar. As the protostar condenses, its temperature rises, and it may begin forming a planetary disc around it. When the core temperature reaches about 10 million Kelvin (1×10⁷ K), the star enters the main sequence phase. During this long, stable period, the inward pull of gravity is balanced by the outward pressure from nuclear fusion as hydrogen atoms fuse into helium. This phase can last around 10 billion years. As the hydrogen fuel in the core is used up, fusion slows down, and gravity begins to dominate again, causing the star to expand and cool into a red giant. In this phase, hydrogen fusion continues in a shell around the core, making the outer layers swell while the surface temperature drops. Eventually, the core becomes hot enough (about 100 million Kelvin, or 1×10⁸ K) to start helium fusion, marking the horizontal branch phase. Once helium in the core is also depleted, the outer layers of the star are pushed away, forming a planetary nebula, leaving behind just the hot core. This remnant becomes a white dwarf—a very dense, hot, but non-fusing object. Although it no longer produces energy, a white dwarf can continue glowing for up to 20 billion years as it slowly cools.

6 basic physical changes

melting (solid to liquid)

freezing (liquid to solid)

condensation (gas to liquid)

Evaporation (liquid to gas)

sublimation (solid to gas)

deposition (gas to solid)

3 basic subatomic particles

Proton – Positive charge (+)

Neutron – No charge (neutral)

Electron – Negative charge (−)

What type of change involves sharing or transferring electrons?

Chemical change (or reaction)

This is when atoms bond by sharing or transferring electrons, forming new substances

Convex lens

causes light rays to converge (come together)

Concave lens

causes light rays to diverge (spread apart)

Concave mirror

causes light rays to converge

Convex mirror

causes light rays to diverge

If a light ray hits a specular mirror at a 36° angle (angle of incidence), what is the angle of reflection?

36°

Reflection Rule: The angle of incidence = the angle of reflection
(Both measured from the normal line — a line perpendicular to the surface)

difference between an isotope and an ion

Isotope = different neutrons

Ion = different electrons

chemical reaction examples

Burning, rusting, baking, vinegar + baking soda, digestion

How many naturally occurring elements exist?

92

What is nuclear fission?

What is nuclear fusion?

Splitting of heavy atomic nuclei

(e.g. Uranium-235)

Combining of atomic nuclei

(e.g. hydrogen into helium)

Nuclear reactions

involves loss of mass and creation of energy

What famous equation describes mass-to-energy conversion? (nuclear reaction)

E = mc²

(E = energy, m = mass, c = speed of light)

What is sub-critical mass?

Not enough to sustain a chain reaction

What is critical mass?

Just enough to sustain a chain reaction

What is super-critical mass?

Causes an uncontrolled chain reaction

Where was the first fission experiment done?

Was it the first in Earth's history?

University of Chicago, 1942

No – natural one happened in Africa 2 billion years ago

What element is most used in fusion?

Hydrogen (Deuterium + Tritium)

How is a fission reaction controlled in a reactor?

With control rods that absorb neutrons

What are the products of fusion?

Helium, energy, and a neutron

What temp is needed for fusion?

About 100 million °C

What energy sources are used in modern fusion attempts?

Lasers, magnetic fields, plasma heating, particle beams

What first generated fusion temps?

Atomic bomb (to ignite hydrogen bomb)

What is the energy source for the Sun?

Nuclear fusion — hydrogen atoms fuse into helium, releasing energy

Why doesn’t the Sun blow up?

Because there’s a balance between gravity pulling in and pressure from fusion pushing out — this balance is called hydrostatic equilibrium

What is the approximate age of the Sun?

About 4.6 billion years old

Where in the sun do nuclear reactions occur?

the core of the Sun, where temperatures and pressure are high enough for nuclear fusion to take place.

Where on the sun could you find a sunspot?

the photosphere, the visible surface of the Sun

Why are sunspots darker than the surrounding area?

They are cooler than the surrounding surface (still very hot, just not as hot), so they give off less light, making them appear darker.

What is the length of the sunspot cycle, and what point are we at now?

The sunspot cycle lasts about 11 years.

As of 2025, the Sun is in the rising phase of Solar Cycle 25, expected to peak around 2025–2026, which means we're near the solar maximum (lots of sunspots and solar activity)

apparent magnitude

How bright a star appears from Earth

absolute magnitude

How bright a star would be if it were 10 parsecs (32.6 light-years) away

How much brighter is a magnitude 2 star than a magnitude 7 star?

100× brighter

A red giant is 1/100th as bright as Aldebaran. How much farther is it?

10× farther away

What has a bigger impact on brightness: size or temperature?

Temperature

(because brightness is proportional to temperature⁴ × surface area)

apparent stellar motion

The way stars seem to move due to Earth's rotation and orbit

ex:

Stars rising/setting (Earth’s rotation)

Seasonal constellations (Earth’s orbit)

actual stellar motion

A star’s real movement through space

EX: Proper motion, Radial motion (toward/away), Orbiting the galaxy

Which star shows almost no apparent motion? Why?

Polaris (North Star)

It’s aligned with Earth’s axis, so it stays in the same spot in the sky

3 Ways to Detect Binary Stars

Visual binary — You can see two stars orbiting each other with a telescope.

Spectroscopic binary — Detected by Doppler shifts in the star’s spectrum (lines move back and forth as stars orbit each other).

Eclipsing binary — One star passes in front of the other, causing the system’s brightness to dip regularly.

Star Size Ranges

Diameter: From about 20 kilometers (neutron stars) to over 1,000 times the Sun’s diameter (supergiant stars)

Mass: From about 0.1 solar masses (small red dwarfs) up to about 100 solar masses (massive blue stars)

What type of spectrum helps identify a star’s chemical makeup?

Absorption spectrum

(You see dark lines where specific elements absorb light at certain wavelengths.)

Star Classification System

Astronomer: Annie Jump Cannon

Based on: Temperature (and indirectly color)

She developed the modern OBAFGKM system:

O = hottest (blue)

M = coolest (red)

H-R Diagram Explained

The Hertzsprung-Russell (H-R) Diagram plots:

Luminosity (brightness) on the vertical axis

Surface temperature (or spectral class) on the horizontal axis (hot stars on the left, cool stars on the right)

Most important use: It shows the relationship between a star’s brightness, temperature, color, and life stage

Most blue stars are hot and bright

Most red stars are cool and dim

Name of the S-shaped curve

Main Sequence

(Where most stars, including the Sun, are found.)

Two Ways to Determine the Age of Stars

Look at a star cluster’s H-R diagram — the location of the "turn-off point" tells the cluster’s age.

Measure the composition — older stars have fewer heavy elements (they formed earlier in the galaxy’s history)

Nebula

the large gas cloud where stars form

What gas must be present for star formation?

Hydrogen

Protostar

a hot, star-forming region before a star is born

What force acts on the gas cloud?

Gravity

Brown dwarfs

star-like objects too small for fusion

At what temperature does hydrogen fusion begin?

About 10 million Kelvin (1×10⁷ K)

If a nebula makes 5 Sun-sized stars, how many small M-class stars will form?

About 500 small red M-class stars

How does a star maintain a stable size?

Balance between gravity pulling in and pressure from fusion pushing out (hydrostatic equilibrium)

How long will a main sequence star last?

about 10 billion years.

Two changes when a star becomes a Red Giant:

Hydrogen fusion slows down in the core as fuel runs out.

Outer layers expand and surface cools as fusion continues in a shell around the core.

Planetary Nebula

the atmosphere blown away from a star after fusion stops

Why does the expanding cloud of gas (planetary nebula) shine?

The gas shines because it is heated by ultraviolet light coming from the hot core (the white dwarf) left behind.
The UV light energizes the gas, making it glow.

size, composition, and temperature of a white dwarf

Size: About the size of Earth (even though it has roughly the Sun’s mass!)

Composition: Mostly carbon and oxygen (the leftover core)

Temperature: Extremely hot — around 100,000 Kelvin at the surface when newly formed, but it cools slowly over billions of years

Will the Sun become a Nova?

No.

A nova happens when a white dwarf (the leftover core of a dead star) pulls in matter from a companion star (like in a binary system).

The Sun is single — it doesn’t have a nearby companion star to steal matter from.

Instead, after becoming a red giant and then a white dwarf, the Sun will gradually cool down without any nova explosion.

Summary: The Sun can't be a nova because it’s not in a binary system.

Why do large stars have shorter lives than average stars?

Large stars are hotter and burn through their nuclear fuel much faster than smaller stars.

Even though they start with more fuel, they use it up at a much faster rate (higher pressure = faster fusion).

EX: A star 10× the mass of the Sun might only live 10 million years (compared to the Sun's 10 billion years).

Summary: Bigger stars = faster fusion = shorter life.

After He → C fusion, what elements may fuse when the temperature hits 6×10⁸ to 2.7×10⁹ K?

Oxygen (O), Neon (Ne), Magnesium (Mg), and Silicon (Si)

What force stops balancing the star when Iron fusion starts?

Pressure from nuclear fusion stops — only gravity remains

Supernova

the violent outward explosion after core collapse

Specifically, Type II supernova for massive stars

Difference between Type II and Type Ia Supernova

Type II	Type Ia
Core collapse of a massive star	White dwarf explosion in binary system
Hydrogen is present in spectrum	No hydrogen in spectrum

How does a neutron star form?

After the core collapses, protons and electrons combine into neutrons, making an incredibly dense ball of neutrons

What is the size of a neutron star?

About 10–20 kilometers wide (about the size of a city)

Pulsar

Rotating neutron star with sweeping radio beams

After a supernova, if the core mass > 2 solar masses, the leftover is a:

black hole

Three basic properties of a black hole

Mass

Spin (rotation)

Electric charge

What’s the Schwarzschild radius of a 10 solar mass (10 M☉) black hole?

About 30 kilometers
(Formula: Schwarzschild Radius ≈ 3 km × mass in solar masses → 3 km × 10 = 30 km)

Open Cluster

Sparse grouping of up to 1,000 stars bound by gravity

Globular Cluster

Star clusters in the halo with 100,000–1,000,000 stars

How would the night sky look if the Sun were closer to the Milky Way's center?

The sky would be much brighter with far more stars visible, especially concentrated toward the center of the galaxy.

What objects did Harlow Shapley map to find the Milky Way's center?

Globular Clusters

(He realized they were clustered around the true galactic center, not around Earth.)

Spiral Nebula Debate (Shapley vs Curtis):

Shapley: "Spiral nebulae" are inside the Milky Way (WRONG)

Curtis: "Spiral nebulae" are separate galaxies (CORRECT)

Edwin Hubble proved it by measuring Cepheid variables

Basic size and structure of the Milky Way

Diameter: about 100,000 light-years across

Thickness: about 1,000 light-years thick

Shape: Spiral Galaxy with a central bulge, spiral arms, and a halo of globular clusters