Astronomy Picture of the Day

Recall:

Luminosity - Intrinsic property of a star.

Apparent Brightness – the brightness we perceive a star to be from Earth.

Important Relationship, Another Inverse Square Law:

Apparent Brightness α Luminosity/ Distance2

Also

Luminosity α Radius2 * Temperature4

And

We can measure the spectral type of a star (on the main sequence – we'll get to this in a minute) we can determine its Luminosity

Color of a Star

Why are stars different colors?

Center of our Galaxy

Color of a Star

Why are stars different colors?

They are different Temperatures! Recall – maximum intensity of radiated E-M waves depends on Temperature. So Astronomers can measure the apparent brightness of a star at several intensities (“colors”) to determine the temperature.

Stars are Born in Nebulae

(video clip)

Hertzsprung-Russell Diagram

- Stars begin their lives as red giants and move onto the main sequence where they remain until H supply is exhausted.- Red dwarfs are most common type of star.- 90% of stars are on the main sequence.-Most stars are members of multiple star systems (i.e. Binary)

Nearby stars Brightest visible stars

What happens if a star isn't massive enough to begin fusing H into He?

Brown Dwarf - a “failed” star

Question

When a star runs out of hydrogen, what happens next?

Evolution of a Low-Mass Star(< 8 M

sun , focus on 1 M

sun case)

- Helium ash collects in core.

- Too cool for He burning. Why? Larger electric repulsion.

- Core contracts. Heats up. H burning shell

- Higher temp. => Brighter! Star expands!

- "Red Giant". Diameter ~ 1 AU!

- Does fusion rate at this stage increase or decrease? Why?

Red Giant

Evolution of a Low-Mass Star(< 8 M

sun , focus on 1 M

sun case)

Does fusion rate at this stage increase or decrease? Why?

Rate increases. Phase lasts ~ 1 billion years

Red Giant

Creation of Heavier Elements

- Core shrinks and heats up to 108 K, => Helium fuses into Carbon.

- All He -> C.

- Core shrinks and heats up.

- Onion-like structure

Each phase shorter than the last.

Red Giant

Death of a Low Mass Star

What factor(s) eventually determine when this process stops?

"Planetary Nebulae"

-- Low mass star (< 8 Msun) cannot achieve 600 Million K temp. needed for Carbon fusion

-- Contraction stopped by the Pauli exclusion principle: two objects cannot occupy the same space. “Supported by the resistance of its electrons to further contraction, the core contractin stops.”

- Star becomes unstable. Ejects outer layers (high velocity: 10's km/sec). "Planetary Nebula" (Historical name, nothing to do with planets.) Shine due to ionizing radiation from the hot core of the star embedded in a cool gas cloud.

-- Carbon core called a “White Dwarf” - shines only by stored heat, no more nuclear reactions. About the size of Earth. Cools to become black dwarf, remaining about the size of Earth.

Ring

Cat's eye

m2-9

Stellar Explosions

Novae

White dwarf in binary system

WD steals mass from companion. Eventually, a burst of fusion, H → He. Brightens by 10'000's! Cycle may repeat every few decades => recurrent novae.

Nova Cygni with Hubble

May 1993 Jan 1994

1000 AU

Is all of the accreted matter expelled into space during a nova?

A Carbon-Detonation or “Type I” Supernova

Despite novae, mass continues to build up on WD.

At 1.4 MSun

(the "Chandrasekhar limit"), gravity overwhelms the Pauli exclusion pressure supporting the WD => contraction and heating.

Carbon fusion everywhere at once.

Tremendous energy makes star explode. No core remnant.

Stellar Lifetimes

Is the lifetime of a high mass star shorter or longer than that of a lower mass star? Why?

Evolution of Stars > 8 MSun

Higher mass stars burn out faster and fuse heavier elements.

Example: 20 MSun

star lives "only" ~10 million years.

Heaviest element made in core of any star is iron – iron nuclei will not fuse to release energy.

Products of outer layers become fuel for inner layers.

Eventual state of > 8 MSun

star

Red Supergiant

H burns for 10 million years, Iron for less than 1 day!

Death of a Very High-Mass Star

M > 8 MSun

Iron core at T ~ 1010 K radiation photodisintegrates iron nuclei into protons and neutrons.

Core collapses in < 1 sec.

Neutrons “rebound”. Shock ejects outer layers => Core-collapse or Type II Supernova

Ejection speeds 1000's to 10,000's of km/sec!

Remnant is a “neutron star” or “black hole”.

Supernova 1987A in the Large Magellanic Cloud

In 1000 years, the exploded debris might look something like this:

Crab Nebula: debris from a stellar explosion observed in 1054 AD.

Vela Nebula: debris from a stellar explosion in about 9000 BC.

Or in 10,000 years:

2 pc

50 pc

Remember, carbon-detonation (Type I) and core-collapse (Type II) supernovae have very different origins

How long will a Star Live?

Stellar lifetime α mass/luminosity

Making the Heaviest Elements

Since iron is the heaviest element that can be made by stellar fusion, where do the heavier elements come from?

Making the Elements

H and some He were made in Big Bang. Many made in stars, and distributed by supernovae.

Heaviest elements made in supernovae.

Solar System formed from such "enriched" gas 4.6 billion years ago.

Testing our Theories

How can we test our theories of stellar evolution when the lifetimes of stars are so long?

Star Clusters

Two kinds:

1) Open Clusters

-Example: The Pleiades

-10's to 100's of stars

-Young (10's to 100's of millions of years)

2) Globular Clusters

- few x 10 5 or 10 6 stars

- Billions of years old

Why are star clusters useful for stellar evolution studies?

Clusters are useful for stellar evolution studies because all of the stars:

1) formed at about same time

2) are at about the same distance

3) have same chemical composition

The ONLY variable property among stars in a cluster is mass!