Marcus DuPont
Princeton University
Step outside and look up — not directly! — and you're looking at a star.
The Sun is an ordinary star. The only one close enough to study in detail.
Everything in this talk is happening right now, 93 million miles away, in the one star you already know.
Every atom in your body heavier than hydrogen was forged inside a star.
Stars are the factories that built the raw materials for everything.
A star is a giant ball of hot gas held together by gravity, powered by nuclear fusion in its core.
That glow? It's millions of degrees of hydrogen fusing into helium.
Think of it as a continuous hydrogen bomb held together by its own gravity.
A star survives by balancing two forces:
This is called hydrostatic equilibrium.
When this balance breaks, the star dies.
This same principle governs Earth's atmosphere — it's how engineers calculate the air pressure and density at any altitude, which is how we know how high airplanes can fly.
By mass, a typical star like our Sun is roughly:
~73% Hydrogen
~25% Helium
~2% Everything else
Nobody has ever been inside a star. So how do we know it's 73% hydrogen at millions of degrees?
Answer: we split its light into a rainbow.
Each element absorbs specific colors, leaving dark gaps — a barcode stamped into the starlight.
Read the barcode and you know what a star is made of, how hot it is, and how fast it's moving — all from light that left it years to millennia ago. This trick is called spectroscopy, and it's how we know almost everything in this talk.
Stars form inside vast clouds of gas and dust called nebulae.
These clouds are mostly hydrogen — the simplest element.
A typical star-forming cloud can span dozens of light-years across —
one light-year is about 6 trillion miles.
That's like driving coast-to-coast across the US... two billion times.
Something disturbs the cloud — maybe a nearby supernova — and gravity takes over.
The moment hydrogen begins fusing in the core, a star is officially born.
Fusion is when lightweight atoms are squeezed together so hard they merge into heavier ones.
Four hydrogen nuclei fuse into one helium nucleus.
A tiny bit of mass is converted into a huge amount of energy.
How huge? Fusing the hydrogen in a single glass of water would release as much energy as burning 20 tons of coal.
The energy released by fusion comes from mass itself:
Even a tiny amount of mass (m) produces enormous energy because the speed of light (c) is a very large number — and you're squaring it.
Converting just 1 kg of matter into energy would produce ~25 billion kWh —
enough to power New York City for about 3 years.
Every second, the Sun converts about 4 million tons of matter into pure energy.
That's equivalent to exploding ~90 billion megatons of TNT per second.
Put differently: one second of the Sun's energy output could power all of human civilization for ~500,000 years.
And the Sun has been doing this for 4.6 billion years. It hasn't even hit halftime yet.
That reaction — happening in the Sun's core right now — is the reason for basically everything:
Every calorie you've ever eaten traces back to hydrogen fusing into helium, 93 million miles away.
If you could somehow harness the Sun's energy output:
| 1 second | of sunlight could power the entire US for 9 million years |
| 1 pinhead | of the Sun's core material radiates enough heat to warm every home in New Jersey — from 150 km away |
| 1 cubic meter | of Sun-core has the energy output of a mid-sized power plant |
| 1 second | of a supernova emits more energy than the Sun will produce in its entire lifetime |
Stars are so far beyond human scale that even our best analogies barely scratch the surface.
A star's color tells you its surface temperature:
Red stars are the coolest. Blue stars are the hottest.
Yes, it's the opposite of your kitchen faucet.
A "cool" red star at 3,000 K is still hotter than any blast furnace on Earth (~2,300 K).
A blue star's surface could vaporize any known material instantly.
Your eye says "reddish" or "bluish." A telescope needs an actual number.
The trick: photograph the same star twice, through two different colored filters.
A filter is just a piece of glass that lets one band of color through and blocks the rest — like taping colored cellophane over a flashlight.
That brightness through each filter is measured in magnitudes. One catch trips up everyone:
The scale runs backwards — a smaller number means a brighter star. The brightest things have negative magnitudes.
Each 5 steps = exactly 100× the brightness. Blame the ancient Greeks: Hipparchus ranked the brightest stars "1st magnitude," the faintest his eye could see "6th" — we never let it go.
Two stars can share the same magnitude in your data for opposite reasons:
The Sun is the brightest thing in your sky (apparent −27) —
but tow it 30 light-years out and it's a faint speck you'd struggle to spot.
Same star, wildly different "brightness." That's why you need distance (hello, parallax) before comparing stars fairly. Your g and r are apparent magnitudes.
The dataset you'll work with measures each star's brightness in two filters:
r catches the redder light. g catches the bluer/greener light.
Those are the two "brightness" columns in your table. Gaia's own versions are named BP and RP — same trick, different labels.
Subtract the two brightnesses and you get a single number: the color index.
| Hot, blue star | brighter in g ➔ small g − r |
| Cool, red star | brighter in r ➔ large g − r |
One subtraction = a thermometer you can read from light-years away.
And g − r is exactly the horizontal axis of the real H-R diagram you can build from this data.
If the Sun were the size of a basketball, a red supergiant would fill a football stadium.
And Earth? A grain of sand sitting on the 50-yard line.
| Star Type | Mass (vs Sun) | Lifetime |
|---|---|---|
| Red Dwarf | 0.1 – 0.5 | Trillions of years |
| Sun-like | 0.8 – 1.2 | ~10 billion years |
| Blue Giant | 10 – 50 | ~10 million years |
Massive stars burn through their fuel much faster.
They live fast and die spectacularly.
If the Sun's lifetime were a single 24-hour day, a blue giant's entire life would be over in ~1.5 minutes.
Energy generated in the core can take 100,000+ years to reach the surface.
The sunlight warming your face today began its journey when woolly mammoths still roamed the Earth.
A photon (a particle of light) created in the core doesn't fly straight out.
It bounces off particle after particle in a random walk — like trying to cross Times Square on New Year's Eve, blindfolded, bumping into someone every fraction of a millimeter.
Once it reaches the surface, it takes only 8 minutes to cross the emptiness of space to Earth.
So when you step outside and feel the sun on your face:
that light is 8 minutes old — but the energy in it took
100,000 years to claw its way out of the core.
Plot every star by its temperature and brightness, and patterns emerge:
Find the highlighted dot — that's our Sun, sitting in the middle of the main sequence. Perfectly average.
Where a star sits on this diagram tells astronomers its age, mass, and fate.
Gaia is a European Space Agency (ESA) satellite that has been surveying the sky since 2013.
If you mapped every star Gaia has measured to a grain of sand, you'd fill roughly 15 dump trucks.
The same trick you use without thinking about it: parallax.
Hold your thumb out at arm's length. Close one eye, then the other.
Your thumb appears to shift against the background. The closer your thumb, the bigger the shift.
Gaia does the same thing, but instead of two eyes it uses Earth's orbit — measuring a star from opposite sides of the Sun, six months apart.
The baseline? ~300 million km. Like measuring the width of a human hair from 1,000 km away.
From Gaia's raw measurements, we can derive real physical properties:
| Gaia Measures | ➔ | We Derive |
|---|---|---|
| Parallax angle | ➔ | Distance (in parsecs or light-years) |
| Apparent brightness (G magnitude) | ➔ | True luminosity (how bright it really is) |
| Blue & red photometry (BP – RP) | ➔ | Surface temperature and color |
| Position over time | ➔ | Velocity (how fast and where it's going) |
With distance + apparent brightness + color, you have everything you need to place a star on the H-R diagram.
The theoretical diagram we saw earlier? Gaia lets us build it from actual data.
from astroquery.gaia import Gaia
# grab the 10,000 nearest stars with good parallax
query = """
SELECT TOP 10000
source_id, parallax, phot_g_mean_mag,
bp_rp, teff_gspphot
FROM gaiadr3.gaia_source
WHERE parallax > 10
AND parallax_over_error > 10
ORDER BY parallax DESC
"""
job = Gaia.launch_job(query)
stars = job.get_results()
That's it. ~10 lines to pull real measurements of 10,000 nearby stars. No telescope required.
With that data, the color-magnitude diagram is a few lines away:
import numpy as np
import matplotlib.pyplot as plt
# convert apparent magnitude to absolute magnitude
# using distance from parallax
distance_pc = 1000.0 / stars['parallax']
abs_mag = (stars['phot_g_mean_mag']
- 5 * np.log10(distance_pc) + 5)
color = stars['bp_rp']
plt.figure(figsize=(8, 10))
plt.scatter(color, abs_mag, s=0.5, c=color,
cmap='RdYlBu_r', alpha=0.5)
plt.gca().invert_yaxis()
plt.xlabel('Color (BP - RP)')
plt.ylabel('Absolute Magnitude (brighter ↑)')
plt.title('H-R Diagram from Gaia DR3')
plt.show()
The main sequence, red giants, white dwarfs — they all emerge naturally from the data. No hand-drawing required.
| Column | What It Is | Units |
|---|---|---|
parallax |
Angular shift over 6 months | milliarcseconds |
phot_g_mean_mag |
Apparent brightness (broad filter) | magnitudes |
bp_rp |
Color index (blue minus red) | magnitudes |
teff_gspphot |
Estimated surface temperature | Kelvin |
parallax_over_error |
Signal-to-noise of parallax | dimensionless |
Rule of thumb: filter on parallax_over_error > 10 to keep only stars with reliable distances.
You'll get ~40,000 real stars in a table. For temperature, age, mass, and radius:
Spoiler: four graphs, one answer — the Sun is gloriously, reassuringly average.
That's the whole arc of this talk, proven by your own hands with real data.
Everything we learned today — verified with real starlight.
Every star begins on the main sequence. Its birth mass decides what comes next.
a quiet, slow fade over billions of years
a violent, spectacular explosion
When a massive star runs out of fuel, its core collapses in milliseconds.
Death and birth, linked in a cosmic recycling loop.
White Dwarf
Earth-sized, incredibly dense.
A teaspoon weighs ~5 tons —
about as much as a hippo.
Neutron Star
Manhattan-sized, unimaginably dense.
A teaspoon weighs ~6 billion tons —
roughly every car on Earth combined.
Black Hole
Gravity so strong, not even light escapes.
If you compressed Earth into a black hole,
it would be the size of a marble.
| Age | ~4.6 billion years (middle-aged) |
| Type | G-type main sequence (yellow dwarf) |
| Surface Temp | ~9,900 °F (hot enough to melt any metal) |
| Core Temp | ~27 million °F (hotter than any nuclear weapon detonation) |
| Diameter | ~1.4 million km (you could line up 109 Earths across it) |
| Fuel left | ~5 billion years |
The Sun is a perfectly ordinary star. That's actually good news for us.
It weighs about 330,000 Earths — and contains 99.86% of all mass in the solar system.
As a red giant, the Sun will expand past the orbit of Earth —
imagine standing in New York and the Sun's edge reaching past Los Angeles.
Except it's on fire. Because it's a star.
Does this doom us? Honestly, long before that. The Sun slowly brightens as it ages — in roughly 1 billion years it will be hot enough to boil off Earth's oceans. The dramatic red-giant ending is real, but life here runs out of road well before the finale.
Not because the star flickers — it's rock-steady.
Starlight passes through Earth's turbulent atmosphere, which bends it slightly this way and that.
The twinkle is happening a few miles above your head — not light-years away.
Fun test: planets (like Venus or Jupiter) barely twinkle. They're close enough to look like tiny disks instead of points, so the atmosphere can't jiggle them as much.
Nowhere. They're still there.
The Sun — our one local star — is just so overwhelmingly bright that it floods the sky and drowns the others out.
Night isn't when the stars turn on. It's when our star turns off (for you).
After the Sun, the nearest star is Proxima Centauri — about 4 light-years away.
Remember the basketball Sun? Here's the catch about space:
If the Sun were a basketball in New York City,
the next nearest star would be another basketball — in Chicago.
Stars are enormous, but the space between them is almost unimaginably empty. That's why your night sky looks calm and still.
Look at Orion this winter. The bright red-orange shoulder is Betelgeuse.
Everything in this talk is sitting in your sky right now, waiting for you to look up.
None of these words are magic. They're just old Greek and Latin, glued together:
Decode the roots and the universe stops sounding like jargon.
For almost all of human history, we knew of exactly one planetary system: ours.
As of today, we've confirmed more than 5,000 — and counting.
Any planet orbiting a star that isn't our Sun.
The first one circling a Sun-like star was found in 1995 (a planet called 51 Pegasi b). The discovery won the 2019 Nobel Prize in Physics. This is brand-new science — younger than most of the people in this room.
A star is roughly a billion times brighter than the planet next to it.
Spotting the planet directly is like spotting a firefly hovering next to a stadium floodlight — from across the country.
So we almost never see them. Instead we do exactly what we did with stars themselves: we never went inside a star — we read its light. To find planets, we watch what they do to their star's light.
Point a telescope at one star and just measure its brightness over time — that's photometry, the same trick behind g and r.
If a planet's orbit is edge-on to us, it passes in front of the star and blocks a sliver of light: a tiny dip that repeats once per orbit. That repeating dip is a world.
The star dims by the fraction of its disk the planet covers — a ratio of radii, which you already know how to think about:
| Jupiter across the Sun | ~1% dip |
| Earth across the Sun | ~0.008% dip (84 parts per million) |
Measuring an 84-ppm dip means counting starlight to a few parts per million. Earth's twinkling atmosphere ruins that — which is exactly why these telescopes (Kepler, TESS) sit in space.
| How deep the dip is | ➔ | the planet's size |
| Time between dips | ➔ | its orbit — how far from the star |
| + the star's temperature | ➔ | how warm the planet is |
Notice the catch: every planet answer leans on the star's properties. You can't know the planet until you know the star — which is the whole reason we spent two days on stars.
A planet's gravity tugs back on its star. The star traces a tiny circle — it wobbles.
We catch the wobble in the star's barcode: as it moves toward us then away, its spectral lines slide blue then red (the Doppler shift).
Same spectroscopy from earlier, plus the same idea as the star velocities Gaia measures. This was how 51 Pegasi b was found — and it gives the one thing a transit can't: the planet's mass.
Transit (size) + wobble (mass) ➔ density ➔ is it rock, water, or gas?
The ring around a star where it's not too hot, not too cold — where liquid water can exist.
Where the zone sits depends on the star's color — the thing color taught us, temperature. A hot blue star's zone is far out and wide; a cool red dwarf's hugs in tight and narrow.
During a transit, a thin sliver of starlight filters through the planet's atmosphere on its way to us.
The gases in that air stamp their own barcode onto the light — the exact spectroscopy trick we used to find what stars are made of, now aimed at a planet's atmosphere.
The James Webb Space Telescope is doing this right now: detecting water, carbon dioxide, and methane in the air of worlds light-years away.
The dream: find a barcode that only life could produce — a biosignature.
What we've already found out there:
On average, there is more than one planet per star. That's hundreds of billions of worlds in the Milky Way alone.
Remember the punchline of this whole talk: the Sun is an ordinary star.
Earth looks like an ordinary planet around it.
Ordinary × ordinary × hundreds of billions of chances…
We don't have the answer yet. But humans are the first generation with the tools to ask it seriously — and the data, like Gaia's, is public. The next people to find something could be sitting right here.
Questions?
Marcus DuPont
Princeton University
marcus.dupont@princeton.edu