Spoiler Alert: The Universe Is Weird

How Do We Know What We Know?

Dr. Anna Rosen

January 20, 2026

Join iClicker

Credit: iClicker

join.iclicker.com/ASES

If you can’t scan: open the link and enter the course code ASES.

Take 30–60 seconds at the very start to get everyone connected. Then move on immediately — don’t troubleshoot at the podium; have students flag you after class if needed.

When you look up at the night sky,
what do you see?

What do you assume you’re seeing?

0–1 min. Let this sit for 5 seconds in silence.

Don’t answer out loud. Just let students look at the image.

This is about activating their prior knowledge and assumptions before we challenge them.

Advance to the video without explanation.

Credit: NSF-DOE Vera C. Rubin Observatory

The Cosmic Treasure Chest

1–3 min. Play video without introduction.

Made from over a thousand images captured by NSF-DOE Vera C. Rubin Observatory.

Begins with two galaxies, zooms out to reveal millions of galaxies — just a tiny fraction of what Rubin will capture over its survey.

Let the video do the work. Don’t narrate over it.

Most of Those Weren’t Stars

Credit: Rubin Observatory/NSF/AURA

When you look up, you see stars, planets, maybe a galaxy or two…

But most of those points of light? Galaxies.

Each one containing hundreds of billions of stars.

You just saw millions of galaxies — and that’s only a tiny fraction of what one telescope will find.

Interactive Image (Click)

THE REVEAL (3 min):

Say: “When you look up at the night sky, you see stars, planets, maybe the Andromeda galaxy if you know where to look. But in that video, most of those points of light weren’t stars at all — they were galaxies. Each one containing hundreds of billions of stars.”

Let that land. Pause.

Then: “And we’re going to learn how to read them.”

How Do We Know That?

We’ve never been to another galaxy.

We’ve never touched a star.

We can’t send a probe to most of what we study.

So how do astronomers know anything about objects trillions of kilometers away?

(Later today: why astronomers switch to light-years (and parsecs) for cosmic distances.)

4–5 min.

This is the central question of the course.

Don’t answer it yet — just plant the question.

“This course is about answering that question. Not just what we know, but how we know it.”

Today Is a Trailer

By the end of today, you’ll be able to…

• State the course thesis: pretty pictures → measurements → models → inferences
• Name the 4 things we can directly measure: brightness, position, wavelength, timing
• Explain why wavelength is the most powerful: it encodes temperature, composition, and motion

5–6 min.

Say: “Today is just a trailer — like a movie preview. You’re not expected to master anything today. You’re expected to recognize these ideas when they come back.”

Emphasize: Recognition, not retention.

The Course Thesis

Pretty pictures → measurements →
models → inferences

What this means:

• Astronomical images aren’t just beautiful — they’re data
• We extract measurements from that data (how bright, what color, where, when)
• We apply physical models to interpret those measurements
• The result is an inference — a conclusion about something we can’t directly access

6–7 min.

This is the single most important idea of the entire course.

Repeat it: “Pretty pictures are data. Data becomes measurements. Models interpret measurements. The result is an inference.”

An inference is a claim about reality based on evidence + reasoning.

We Cannot Touch the Stars

Astronomy is the art of inferring physical reality from constrained measurements.

Constraint: We can only study the light that reaches us.

• No thermometers in the Sun
• No tape measures to galaxies
• No scales to weigh

Everything we claim to know is inferred from light.

7–8 min.

This is a constraint, not a limitation — it’s what makes astronomy a science of clever inference.

“We cannot touch the stars. We can only decode the light they send us.”

8–9 min. Don’t read the infographic aloud. It’s a map, not a quiz.

Quick tour (point to each region):

1. Left: “The Four Observables” — brightness, position, wavelength, timing. These are the only things we can directly measure.

2. Center: the telescope — collects photons and turns them into numbers.

3. Right: “The Decoder Ring” — the repeated move: signal → measurement → model → inference.

4. Bottom: two key ideas — wavelength is physics (different wavelengths = different processes); distance is a time dial (looking far = looking back in time).

Close with: “Today is just the trailer. If you recognize these ingredients later, you’re doing it right.”

NASA’s Three Big Questions

Credit: (A. Rosen/NotebookLM)

Think: Which question are you most curious about?

9–10 min.

Quick hand poll or pair-share: “Which of these three questions draws you in?”

Then: “Here’s the amazing part — we answer all three using the same method: Measure → Infer → test with models.”

Don’t linger. This is motivation, not content.

What Can We Actually Measure?

From billions of kilometers away, we can directly measure only four things.

Credit: (A. Rosen/NotebookLM)

10–12 min.

1. How bright? — brightness (energy emitted per second per area)
2. Where is it? — position (angles on the sky)
3. What color? — wavelength (spectrum)
4. When does it change? — timing (variability)

Everything else — distance, mass, temperature, age — is calculated.

Emphasize: “These are the only inputs the universe gives us. Everything else is a calculation.”

Technical note: Astronomers call brightness “flux” — the amount of energy passing through a unit area per second (e.g., W/m²). The “per area” matters because a bigger telescope collects more light.

Don’t over-explain yet. Just name them and let the spoiler reel show examples.

The Four Observables

Credit: (A. Rosen/NotebookLM)

Direct observables (what we measure):

• Brightness — how much energy arrives per second
• Position — where on the sky, and how it moves
• Wavelength — what colors/spectrum are present
• Timing — how brightness or position changes over time

12–13 min.

Point to each icon in the figure as you say the name.

“These four are our entire toolkit. Every claim about the universe is built from these four measurements plus physics.”

Quick Check: Direct Measurement

Which of these can astronomers directly measure for a distant star?

• Temperature
• Apparent brightness
• Mass
• Age

13–14 min. Give ~30 seconds.

Correct: apparent brightness is a direct measurement.

Temperature, mass, and age are all inferences — they require models to calculate from what we actually measure.

The Spoiler Reel

For each image, don’t memorize details — identify the pattern.

What do we measure?

(Which of the 4 observables?)

What do we infer?

(What physical claim do we make?)

Same pattern every time:
Observable → Model → Physical Reality

14–15 min.

Orientation: “This is the repeatable structure for the whole course. I’m going to show you 7 examples. For each one, ask yourself: what did we measure, and what did we conclude?”

Spoiler 1: Nebulae — Colors Are Chemistry

Credit: (A. Rosen/NotebookLM)

We measure: Colors at specific wavelengths

We infer: Chemical composition and dust structure

The physics: Each element emits/absorbs at unique wavelengths — a “spectral fingerprint”

Red = hydrogen (656 nm) Blue-green = oxygen (500 nm) Dark lanes = dust blocking light

15–17 min.

Key insight: “Colors aren’t decoration — they’re encoded physics. The red glow is hydrogen, the blue-green is oxygen, and the dark lanes are dust silhouettes.”

This is spectroscopy in action: different elements = different colors = different wavelengths.

Credit: NSF-DOE Vera C. Rubin Observatory

17–18 min. Trifid and Lagoon Nebulae from Rubin Observatory.

Made from 678 exposures in just over seven hours.

Point out:

• Reds and pinks = hydrogen emission (656 nm) — active star formation
• Blue and turquoise = reflected starlight and oxygen emission
• Dark lanes = dense dust absorbing visible light

“This is exactly what we just discussed: color = physics.”

Quick Check: Why Specific Colors?

Why do nebulae glow at specific colors instead of a continuous rainbow?

• Telescopes filter out other colors
• Each chemical element emits at specific wavelengths
• Hot gas always glows red
• The colors are added artificially by scientists

18–19 min. Give ~30 seconds.

Correct: Each element has a unique set of wavelengths it can emit or absorb — like a fingerprint.

This is one of the most powerful facts in astronomy: we can identify what things are made of from light-years away.

Spoiler 2: Spectroscopy — The Master Key

Credit: (A. Rosen/NotebookLM)

We measure: Brightness at each wavelength (a spectrum)

We infer: Temperature, composition, and motion

The physics: Atoms have quantized energy levels — electrons can only jump between specific rungs

Why it’s the master key:

A single brightness measurement = 1 data point. A spectrum = thousands of data points.

19–21 min.

CRITICAL SLIDE:

“This is one of the most important ideas in the entire course. Spectroscopy is how astronomy stops being pictures and starts being physics.”

A prism reveals that white light contains many wavelengths. By measuring brightness at each wavelength, we get temperature, composition, and velocity — all from light.

The Quantum Barcode

Credit: (A. Rosen/NotebookLM)

21–22 min.

Walk through the three types of spectra:

1. Continuous — hot dense source produces a rainbow
2. Absorption — cool gas in front absorbs specific wavelengths (dark lines)
3. Emission — hot thin gas emits specific wavelengths (bright lines)

The pattern of lines is unique to each element — like a barcode or fingerprint.

“If you see these lines in starlight, you know that element is present. We don’t have to go there — the light carries the information.”

Spoiler 3: The Cosmic Distance Ladder

Credit: (A. Rosen/NotebookLM)

We measure: How bright an object appears (flux)

We infer: How far away it is

The physics: Light spreads out as it travels
— if you know the true brightness, you can calculate distance

A dim nearby source can look identical to a bright distant one
— physics breaks the tie.

22–24 min.

The key insight: “Distance is not something we measure directly. We measure brightness and use physics to calculate distance.”

The “ladder” means each method calibrates the next — nearby stars calibrate variable stars, which calibrate supernovae, which reach across the universe.

Standard Candles: Know the Brightness, Calculate the Distance

Credit: (A. Rosen/NotebookLM)

• Pulsating stars (period → luminosity)
• Exploding white dwarfs
  (same mass → same brightness)

The key idea: If you know how bright something actually is (luminosity), and you measure how bright it appears (flux), you can calculate distance.

Standard candles: Objects whose true brightness we can predict from physics.

24–25 min.

Don’t introduce equations. Just the concept: “If I know a lightbulb is 100 watts and I measure how bright it appears, I can calculate how far away it is.”

Cepheids pulse with a period related to their luminosity. Type Ia supernovae have consistent peak brightness because physics sets a limit on how massive a white dwarf can be.

Credit: NSF-DOE Vera C. Rubin Observatory

25–26 min. 46 pulsating RR Lyrae stars from Rubin Observatory.

Watch the stars pulse. RR Lyrae are standard candles — their pulsation period tells us their true luminosity.

Over 10 years, Rubin will detect up to 100,000 of these stars extending more than a million light-years away.

“This is the cosmic distance ladder in action. Measure how bright they appear, know how bright they are, calculate how far.”

Spoiler 4: Same Galaxy, Different Physics

Credit: NASA/ESA/STScI/AURA (Optical); NRAO/AUI (Radio)

We measure: The same galaxy at different wavelengths

We infer: Different physical components are visible at different wavelengths

The physics: Different emission processes dominate at different wavelengths

Left (optical): Stars and dust lanes

Right (radio): Cold hydrogen gas

26–28 min.

Key insight: “Same object, different wavelength, different structures visible.”

Optical light comes from stellar surfaces. Radio waves (21-cm) come from cold hydrogen gas. The gas extends beyond the visible stellar disk.

This is why astronomers need many kinds of telescopes — not for “better” pictures, but for different physics.

Spoiler 5: Infrared Reveals Hidden Stars

Credit: Illustration: NASA, ESA, CSA

We measure: Optical vs infrared light from the same region

We infer: Newborn stars can hide inside dusty clouds

The physics: Dust blocks/scatters short wavelengths more than long wavelengths

Left (Hubble): Dark, opaque pillars

Right (JWST): Thousands of hidden newborn stars revealed

28–30 min.

The Pillars of Creation in the Eagle Nebula.

Key insight: “What you can’t see at one wavelength might be brilliant at another. Infrared penetrates dust that blocks visible light.”

This is why JWST was built to see infrared — to peer into star-forming regions where visible light can’t reach.

Spoiler 6: Hidden Mass — The Dark Matter Problem

Credit: Rubin Observatory/NSF/AURA

We measure: how galaxies and stars move (from spectra over time)

We infer: extra unseen mass (“dark matter”)

The physics: gravity links motion to mass; if objects move too fast, there must be more mass than we can see

30–31 min.

“Dark matter is an inference from motion: we measure how things move (using Doppler shifts), and gravity tells us how much mass must be there.” “When the required mass is bigger than the stars + gas we can see, we’ve discovered missing mass: dark matter.”

Spoiler 7: The Cosmic Web — Mapping the Universe in 3D

Credit: DESI Collaboration/NOIRLab/NSF/AURA/Kitt Peak

We measure: spectra (wavelength shifts) → redshifts → distances → 3D positions

We infer: the cosmic web (filaments + voids) and expansion-history constraints

The physics: gravity + cosmic expansion shape large-scale structure; mapping it tests cosmological models

31–33 min.

Point to: filaments (where galaxies cluster), voids (empty spaces).

“We measure spectra and get redshifts — wavelengths stretched by cosmic expansion. Redshift gives us distance, so we can map galaxies in 3D.” “The pattern isn’t random — gravity and expansion sculpt this web.”

Spoiler 8: The Universe Is “Dark”

Dark Matter + Dark Energy

We measure: how the expansion rate changes with time (distance + redshift)

We infer: the expansion is accelerating → “dark energy”

The physics: gravity + cosmic expansion link energy content to expansion.

Acceleration implies a component that acts like “repulsive gravity”

Credit: NASA

Big question: How does dark energy evolve?

33–34 min.

“We don’t start with a ‘dark energy meter’. We measure distances + redshifts, build an expansion history, and then infer what kind of energy content could produce it.”

Quantitative anchor (as shown in the NASA graphic): ~5% normal matter, ~27% dark matter, ~68% dark energy.

Emphasize: “We can describe the universe with numbers — but we still don’t know what ~95% of it is.”

History of the Universe

Credit: NASA

Say: “This is the punchline: the universe has a history — and by the end of this course, you’ll be able to explain the story on this timeline, and how we know it.”

Spoiler Synthesis: The Pattern Repeats

Every spoiler followed the same structure:

Observable → Model → Physical Reality

Which observable appeared most often?

(Hint: it starts with “W”)

33–34 min. SYNTHESIS MOMENT

Ask: “Which of the four observables kept showing up?” → Wavelength (spectrum).

“Spectroscopy — measuring brightness at each wavelength — is the workhorse of astronomy. It shows up in almost everything we do.”

Lookback Time: Distance Is a Time Machine

Credit: (A. Rosen/NotebookLM)

Object	You see it as it was…
The Moon	1.3 seconds ago
The Sun	8 minutes ago
Andromeda Galaxy	2.5 million years ago
Distant galaxies	Billions of years ago

Looking far away means looking into the past.

34–36 min.

Light takes time to travel. When we look at the Moon, we see it as it was 1.3 seconds ago. The Sun: 8 minutes ago.

For distant galaxies, we’re literally seeing the past — billions of years ago.

“This isn’t a limitation — it’s a feature. Distance is a time dial that lets us see cosmic history.”

What’s a Light-Year?

A light-year is a unit of distance, not time.

Definition: The distance light travels in one year.

When we say a galaxy is “100 million light-years away,” we mean its light traveled for 100 million years to reach us.

35–36 min.

Common misconception: Students often think light-year is a time unit because it has “year” in the name.

Emphasize: “Light-year sounds like time, but it’s actually distance. It’s how far light can travel in a year.”

The scientific notation here previews Thursday’s math boot camp: - 3 × 10⁵ = 300,000 (move decimal 5 places right) - 9.5 × 10¹² = 9,500,000,000,000 (9.5 trillion)

This is why we need scientific notation — these numbers are unwieldy otherwise.

Note: astronomers often use parsecs in practice; light-years are common in outreach because they’re intuitive.

How big is a light-year?

• Light speed: 300,000 km/s (3 × 10⁵ km/s)
• One year: ~31.5 million seconds (3.15 × 10⁷ s)
• 1 light-year ≈ 9.5 trillion km (9.5 × 10¹² km)

Do this. This is the first “math moment” and sets up Thursday’s math boot camp. Time-box to ~60–90 seconds.

Quick Check: Lookback Time

You observe a galaxy 100 million light-years away.

When did the light you’re seeing leave that galaxy?

• 100 million years ago
• Right now
• 13.8 billion years ago
• It depends on the telescope

36–37 min. Give ~30 seconds.

Correct: 100 million years ago.

A light-year is the distance light travels in one year. If something is 100 million light-years away, its light took 100 million years to reach us.

Wavelength Is Energy

Credit: (A. Rosen/NotebookLM)

The key relationship:

For thermal light: shorter wavelength = higher photon energy = hotter source

• Gamma/X-ray: Million-degree plasma, black holes
• Visible: Stellar surfaces (3,000–50,000 K)
• Infrared/Radio: Dust and cold gas (10–100 K)

This is why different telescopes see different physics — not just “better pictures.”

37–39 min.

“The electromagnetic spectrum isn’t arbitrary — it’s organized by energy. Hot, violent processes produce short wavelengths (X-rays, gamma rays). Cold, quiet processes produce long wavelengths (radio).”

This is why we need X-ray telescopes for black holes and radio telescopes for cold gas.

Note: some emission is non-thermal (not a simple thermometer). We’ll learn those cases later.

What You Can Recognize Now

After today, you should recognize these ideas when they return:

• The course thesis: Pretty pictures → measurements → models → inferences
• The four observables: Brightness, position, wavelength, timing
• The spoiler pattern: Observable → Model → Physical Reality
• Lookback time: Looking far away = looking into the past
• Wavelength: Encodes temperature, composition, and motion

39–40 min. Read the list once, then say: “Recognition, not mastery.”

Recognition, Not Retention

You are not expected to remember details yet.

Today was a trailer — a preview of coming attractions.

When these ideas return, you’ll recognize them. That’s the goal.

40–41 min.

EXACT SCRIPT:

“If this felt overwhelming at moments, that’s a sign this worked. You are not expected to remember details yet. You are expected to recognize ideas when we return to them — and by the end of the semester, you’ll be able to say these things with confidence.”

Questions We’ll Answer This Semester

• How do we know the Sun’s temperature if we can’t touch it?
• How do we weigh a star?
• How do we know the universe is 13.8 billion years old?
• What IS dark matter?
• How do we search for life on other planets?

Write down YOUR question. We’ll return to it.

41–42 min.

Give students a moment to write their own question.

“Keep your question somewhere. We’ll come back to it during the semester.”

Next Time: Math Boot Camp

Thursday: The Math We Need

• Scientific notation (writing very big and very small numbers)
• Powers of 10 and orders of magnitude
• Ratio reasoning (“if X doubles, what happens to Y?”)
• Dimensional analysis (do the units make sense?)

The math is not the obstacle — it’s the microscope.

42–43 min.

“Don’t worry if math isn’t your favorite subject. The math we use is a tool for seeing clearly, not a barrier to understanding.”

Emphasize: “If you can do ratios and powers of 10, you can do the math in this course.”

Questions?

Leave remaining time for questions about the big picture.

Good closing: “Welcome to ASTR 101. Let’s decode the cosmos together.”