Day 1 Claude Astro SG

Chemical enrichment: who makes what?

Planetary nebulae → carbon, nitrogen · type I supernovae → iron · type II supernovae → oxygen

Tycho (1572) and Kepler (1604) supernovae

Type I supernovae in our Galaxy, seen with the naked eye

The stars-gas cycle

Gas clouds → stars → gas returned (planetary nebulae, supernovae, winds) with heavier elements → new stars

Compact vs large-scale stellar remnants

Compact: white dwarf, neutron star, black hole. Large-scale: planetary nebula, supernova remnant (gas).

Life of a Sun-like star

Main sequence → red giant → horizontal branch → asymptotic giant branch → planetary nebula + carbon-oxygen white dwarf

White dwarf

Remnant of a star under 8 M☉: about the Sun's mass, the Earth's size, 1–100 tons/cm³, no fusion

Chandrasekhar limit

1.44 M☉: the maximum mass of a white dwarf

Tolman-Oppenheimer-Volkoff (TOV) limit

2–3 M☉: the maximum mass of a neutron star

Type I supernova

A white dwarf passes 1.44 M☉ (accretion or a white-dwarf collision). Leaves NO remnant. Tycho 1572, Kepler 1604.

Type II supernova

Iron-core collapse of a star above 8 M☉. Leaves a neutron star or black hole. SN 1054 (Crab), SN 1987A.

Supernova remnant vs compact stellar remnant

Supernova remnant = expanding gas nebula (few–100 ly). Compact remnant = white dwarf, neutron star or black hole.

Neutron star

~20 km across, 1–3 M☉, ~1 billion tons/cm³. Not a star: no fusion.

Pulsar

A fast-spinning neutron star. Its beams along the tilted magnetic axis sweep past us like a lighthouse.

Crab Nebula

Supernova remnant of the 1054 supernova; 6,500 ly away; its pulsar spins 30 times per second

Brown dwarfs

13–80 Jupiter masses. Burn deuterium, never hydrogen. “Failed stars”, not stars or remnants, and not brown.

Planetary nebula

Expanding gas shell (~1 ly) thrown off by a dying low-mass star; the star becomes a white dwarf. Nothing to do with planets.

Sirius B

The nearest white dwarf: the Sun's mass, the Earth's size, 25,000 K

Where did hydrogen, helium and lithium come from?

The Big Bang: all hydrogen, 91% of helium, 23% of lithium

Where did gold, platinum and uranium come from?

Mergers of neutron stars

Where did most oxygen through rubidium come from?

Type II supernovae

Threshold temperatures for hydrogen and helium burning

~10 million K for hydrogen; ~100 million K for helium

Which stars never ignite helium?

Very-low-mass stars, under ~0.5 M☉. They end as helium white dwarfs.

Which stars burn helium into carbon and oxygen?

Stars above ~0.5 M☉

Which stars do NOT make planetary nebulae?

Very-low-mass stars (under 0.5 M☉) and high-mass stars (over 8 M☉, which explode instead)

Minimum starting mass to explode as a type II supernova

~8 solar masses

The Sun's density: center and average

Center ~150 g/cm³ (150× water); average ~1.4 g/cm³. Water = 1; densest Earth materials (osmium) ~22 g/cm³

Range of stellar radii

Main sequence: 8% of the Sun's to ~60 R☉. Giants and supergiants: hundreds to ~2,000 R☉.

What does fast rotation do to a star?

Centrifugal force flattens it at the poles (ellipsoid). Achernar's equator is 35% wider than its poles.

How is energy carried out of a star?

By radiation or by convection. Sun: radiative zone inside, convective zone outside.

What are stars made of?

Ionized gas (plasma): ions + free electrons, not atoms or molecules

When will the Sun become a giant, and how will it change?

In ~5 billion years: ~200× bigger (≈ 1 AU), thousands of times more luminous, cooler and redder surface

Rigel, Deneb, Betelgeuse

Blue supergiant · white supergiant · red supergiant

Aldebaran

Orange giant in Taurus, 65 ly away, 45× the Sun's diameter

What do giant stars burn?

First hydrogen in a shell around a dead helium core; later helium → carbon + oxygen in the core

Range of stellar masses

0.08 to ~150 solar masses

Low-, intermediate- and high-mass stars

0.08–2 · 2–8 · 8–150 solar masses

Range of stellar surface temperatures

~2,000 K to ~50,000 K (Wolf-Rayet stars and central stars of planetary nebulae exceed 200,000 K)

Spectral classes, hot → cool

O B A F G K M (“Oh Be A Fine Girl/Guy, Kiss Me”)

Luminosity classes I–V

I supergiant · II bright giant · III giant · IV subgiant · V main sequence (dwarf)

What is a G2V star?

A main-sequence star like the Sun (a yellow dwarf; the Sun's true color is white)

What is a B8I star?

A hot blue supergiant

What is a K1III star?

An orange giant

What is Sirius A's spectral class?

A1V: a white main-sequence star, 10,000 K, 2 M☉, 25 L☉, 9 ly away

Axes of the Hertzsprung-Russell diagram

Luminosity (up) vs surface temperature, which increases to the LEFT

What fraction of stars are on the main sequence?

~95%

Red dwarfs

M-type main-sequence stars under 0.6 M☉; ~80% of all stars; live up to trillions of years

Blue dwarfs

A hypothetical late stage of very-low-mass stars. None exist yet.

Black dwarfs

Cooled-down white dwarfs. None exist yet: cooling takes longer than the age of the Universe.

Largest stars? Most luminous stars?

Largest: red supergiants. Most luminous: blue supergiants.

Lifetime of the Sun

~12 billion years

Lifetime of a 150 M☉ star vs a 0.08 M☉ star

~3 million years vs ~11 trillion years

Initial Mass Function (IMF)

Distribution of masses at birth: low-mass stars dominate, median ~0.1 M☉, under 1% are high-mass

Composition of a newborn star

~¾ hydrogen, ~¼ helium, a few % heavier elements

Hydrostatic equilibrium

Gravity pulling in is balanced by gas and radiation pressure pushing out

Proton-proton cycle

4 protons → 1 helium-4 nucleus; 0.7% of the mass becomes radiation. The Sun's main energy source.

CNO cycle

Hydrogen burning with carbon as a catalyst. Dominates above ~17 million K (massive stars); only 0.8% of the Sun's energy.

Triple-alpha process

3 helium-4 → carbon-12, at ~100 million K, only in stars above ~0.5 M☉

Why does fusion stop at iron?

Iron has the highest binding energy per nucleon, so fusing it costs energy instead of releasing it

Onion-skin structure

Shells of H, He, C, Ne, O, Si around an iron core in a high-mass star near the end of its life

How much matter does the Sun turn into radiation?

Nearly 5 million tons every second

The Sun's core

Inner 25% of the radius, ~1% of the volume, 99% of the energy

Radiative and convective zones in a Sun-like star

Radiative zone inside, convective zone near the surface (reversed above 1.5 M☉; fully convective below 0.5 M☉)

Giant molecular clouds

Where stars are born: 10–100 ly across, 10–20 K, dark in visible light