• Topics• Ch. 7: How Stars Live• Ch. 8: How Stars Die

•Today: Special Stars– —Cepheids– —White Dwarfs

•The way a star lives and dies is controlled by one factor: its mass

A Star’s Fate Depends on its Mass

! Small Mass: Mass < 0.4 MSUN ! Medium Mass: 0.4 MSUN < Mass < 8 MSUN ! High Mass: 8 MSUN < Mass

! Small Stars live “forever”! Medium Stars (eg. Sun) grow to become Red Giants, then lose their outer layers (Planetary Nebula), leaving behind a White Dwarf

Before they die, some giant stars vary in size, growing larger then smaller (like a throbbing heart!)

Astronomers aren’t sure why, but we love it! These stars help us measure distance.

They are called: “Cepheids”

(The throbbing effect is a bit like a pot of boiling water: Pressure builds up, then a puff of steam is released.)

Cepheid Variable Stars

Leavitt

Delta Cephei

Delta Cephei

…are named for the constellation Cepheus (King of Ethiopia)

There are many types of variable star, but Cepheids are the most important.

Their significance was discovered by an astronomer at Harvard named Henrietta Leavitt

Delta CepheiHenrietta Leavitt (1868-1921)

Leavitt graduated from college & took a job as a “computer” (she was paid $10.50/week to do math)

Like Annie J. Cannon, (also at Harvard observatory) she was also deaf.She then studied variable stars in the “Magellanic Clouds”

In 1908 she made a huge discovery about Cepheids …that would impact astronomy for over a century and help us understand the universe.

She went on to become “Head of Stellar Astronomy” at Harvard.

She died @53 just before being nominated for a Nobel Prize.

Throbbing Cepheid StarsCepheids get brighter and dimmer

The time it takes to do this is the Period, P

mag

nitu

de

time (days)

P=5.36626 d

Leavitt measured P for several Cepheids all at the same distance.

She noticed something very interesting:

Large, bright Cepheids throb slowly (long “period”)

Small, dim Cepheids throb quickly (short period)

So, if we observe the period of, we can figure out exactly how bright it is...its luminosity.

(The North Star is a Cepheid with P = 4 days.)

If we know the star’s true luminosity then we can figure out its distance!

(even if parallax doesn’t work)

A New Way to Measure Distance

" Leavitt’s “Period-Luminosity Relation” for Cepheids

Cepheids were the first so-called “Standard Candle”

(a star of known brightness used to measure distance.)

“Period-Luminosity Relation”

This measuring “tool” has been used ever since!

" Stars “live” by fusing Hydrogen into Helium " When they run out of Hydrogen, the begin to “die” " Their temperature and luminosity change, so they

“move” off the HR diagram’s Main Sequence

" When a medium-mass star (like the Sun) begins to die, it turns into a Red Giant, a Planetary Nebula, then a White Dwarf.

" Cepheids are variable giant stars with a known Period-Luminosity relation

" This allows us to determine their distance.

•Summary: How Stars Live

Summary of the Sun’s Evolution

After leaving the Main Sequence, the Sun will first become a Red Giant, then a Planetary Nebula, and a White Dwarf

It will have a “quiet retirement”

How Stars Die(Chapter 8)

White Dwarfs" When a medium-mass star dies, its core is left behind. " It is very hot: around 10,000K " They are called: White Dwarfs

" The Sun will end its life as a White Dwarf, slowly cooling down.

" How do we know this? We have seen them.

The star Sirius appeared to be wobbling as it moved through the sky.

Eventually, astronomers found a small star orbiting it.

Sirius B (White Dwarf)

Sirius A (Main Sequence Star)

This new star (Sirius B) was bizarre!

It was dim, and tiny—about the size of the Earth!

But it had enough gravity to tug big bright Sirius A around!

Observing White Dwarfs

White dwarfs are about as big as the Earth, but have the mass of the Sun!

So, they must be extremely dense.

If you held something with the density* of a white dwarf, then one teaspoon would weigh 10 tons!!!

=*Density = mass/volume

White DwarfsNormal stars (on the Main Sequence) resist gravity with thermal pressure from fusion.

But White Dwarfs do not have fusion.

Why don’t they collapse?

Their extreme density is the key to survival.

The matter in a white dwarf is “Degenerate”: so dense that the electrons almost “touch”

Ele

ctro

n en

ergy

leve

lsThis “Electron Degeneracy Pressure” resists gravity and allows White Dwarfs to exist

Normal Matter Degenerate Matter

(all energy levels are filled)

White Dwarfs can ExplodeAstronomer S. Chandrasekhar used quantum physics to show that White Dwarfs with a mass over 1.4 Msun. cannot exist!

If a White Dwarf’s mass exceeds this “Chandrasekhar Limit” it will explode!!

This runaway thermonuclear reaction is called a Supernova (Type 1) and it destroy the star!

But how could a White Dwarf’s mass increase to become more than 1.4 MSUN ?

Chandra

White Dwarf

Type Ia Supernova

If another star orbits the White Dwarf, then matter will accrete (fall onto) it, adding to its mass.

Companion

If the W.D. goes over 1.4 Msun....

KABOOM

The Death of Stars

Massive stars start on the Main Sequence & quickly fuse their H. They eventually expand, and turn into SuperGiants

After a supergiant runs out of fuel it will cause a huge explosion called a supernova.

It could become neutron star or a black hole

Next: Death of Massive Stars (M>8 MSun)