What is a star?

A cloud of gas and plasma, mainly

hydrogen and helium

The core is so hot and dense that

nuclear fusion can occur.

The fusion converts light elements into

heavier ones

Relative Size of Planets and Stars

https://www.youtube.com/watch?v=HEheh1BH34Q

Every star is different Luminosity:

Tells us how much energy is being produced in the core

Can be calculated using apparent magnitude and distance

Color:

Tells us the surface temperature of the star

Determined by analyzing the spectrum of starlight

Mass:

Determines the life cycle of a star and how long it will last

Given relative to our sun’s mass (ex: 0.8 solar masses)

Units of luminosity

We measure the luminosity of every day

objects in Watts.

How bright is a light bulb?

By comparison, the Sun outputs:

380,000,000,000,000,000,000,000,000 Watts

This is easier to write as 3.8 x 1026 Watts

To make things easier we measure the

luminosity of stars relative to the Sun.

Units of temperature

Temperature is measured in Kelvin

The Kelvin temperature scale is the same as

the Celsius scale, but starts from -273o.

This temperature is known as “absolute zero”

-273 oC -173 oC 0 oC 100 oC

0 K 100 K 273 K 373 K

1000 oC

1273 K

Kelvin = Celsius + 273

Measuring the temperature

The temperature of a star is indicated by

its color

Blue stars are hot and red stars are

cooler

Colors of Stars

Stars appear different colors depending on the peak wavelength of light they emit.

The sun, whose data is depicted in this graph, appears yellow-orange to our eyes.

Spectral Class

(Oh Boy, A Failing Grade Kills Me)

Determined by analyzing a star’s spectra

O stars are the hottest and most massive

M stars are the coolest and least massive

Our Sun is a G star

The Hertzsprung Russell Diagram

We can also compare stars by showing

a graph of their temperature and

luminosity

Hertzprung-Russell Diagram

What information is plotted on the H-R

Diagram?

Temperature and luminosity

What are the main stages of stars?

Main sequence, giant, supergiant, dwarf,

Do stars always stay in the same stage?

No, they change throughout their “lifetimes”

“Birth” of Stars…

Stars (and their solar systems) are created in giant

molecular clouds of cosmic dust and gas

When gravity causes intense heat and pressure in

the core of the proto-star, it triggers fusion and a star

is “born” The planets and other solar system objects are formed from

the left-over materials in the proto-planetary disk surrounding

this new star

Mass and Stellar Evolution

The life cycle of a star is determined by its

mass

More massive stars have greater gravity,

and this speeds up the rate of fusion

O and B stars can consume all of their core

hydrogen in a few million years, while very

low mass stars can take hundreds of billions

of years.

Brown Dwarf– a “Failed Star”

If a proto-star does not have enough

mass, gravity will not be strong enough

to compress and heat its core to the

temperatures that trigger fusion

If the mass is less than 0.08 x solar

mass, it will form a Brown Dwarf

Brown Dwarfs are not true stars, but

they do give off small amounts of light

as they cool

The Main Sequence

Longest life stage of a star

Energy radiating away from star balances gravitational pull inward (hydrostatic equilibrium)

Main-sequence stars fuse hydrogen into helium at a constant rate

Star maintains a stable size as long as there is ample supply of hydrogen atoms

The Sun will spend a total of ~10 billion years on the main sequence

OUR SUN

A

Main

Sequence

Star

SDO image

When hydrogen in the core starts to

run low…

In stars with masses more than 0.4 x solar

mass, fusion slows down

Outer layers of the star begin to swell and

surface temperatures fall

The shell surrounding the core begins to

fuse hydrogen

Stars move out of the Main Sequence

Giants and Supergiants

“Old” stars

Helium produced through shell fusion becomes part of the core

Star’s core temperature increases as the more massive core contracts

The increased core temperature causes the helium left to fuse into carbon atoms (triple-alpha process)

A red supergiant nearing the end of it's

life

Betelgeuse

HR Diagram Activity

“Death” of Stars

Depends on MASS

“Low mass stars” are less than 8 solar

masses

“High mass stars” are greater than 8

solar masses

The End of Low-Mass stars

All stars spend most of their “lives” on

the main sequence

Near the end of their lives, low mass

stars (0.4 – 8.0 x solar mass) leave the

main sequence and become red giants

once they run out of hydrogen and begin

to fuse helium

Low-Mass Giants

In low mass stars (0.4 – 8.0 x solar mass)

strong solar winds and energy bursts from

helium fusion shed much of their mass

The ejected material expands and cools,

becoming a planetary nebula (which

actually has nothing to do with planets, but

we didn’t know that in the 18th century

when Herschel coined the term)

The core collapses to form a White Dwarf

White Dwarf Stars

The burned-out core of a star less than

8 x solar mass becomes a white dwarf

The carbon-oxygen core that remains is

about the size of earth, but much more

dense

Theoretically, after all of the stored

energy radiates out into space, these

dead stars will become giant crystals of

carbon and oxygen (Black Dwarfs)

Astronomers overexposed the image of Sirius A so that the dim Sirius B could be seen.

HST photo

White Dwarf

Stars

Massive stars continue fusion

Massive stars (> 8 x solar mass) have more gravity than low-mass stars

When helium fusion ends, gravity collapses the core and the temperature rises beyond 600 million K

Fusion of the atoms from heavier elements begins, and the star becomes a luminous supergiant

These stars produce neon, magnesium, oxygen, sulfur, silicon, phosphorous, and iron

Supernova explosions

The iron-rich core signals the impending

violent death of the massive star

The core collapses in seconds, and the

resulting temp. exceeds 5 billion K

Intense heat breaks apart the atomic

nuclei in the core, causing a shock wave

After a few hours, the shockwave

reaches the star’s surface, blasting

away the outer layers in a supernova

Crab Nebula (HST image)

Remnants of a Supernova recorded in 1064

11 ly across

Supernova remnants are strong sources of X-rays and radio waves

Supernova 1987A

This HST picture shows three rings of glowing gas encircling the site of supernova in February 1987.

The supernova is 169,000 ly away in the dwarf galaxy called the Large MagellanicCloud

HTUW: Supernovas

Death of a Star segment

Neutron Stars

The cores left over after Supernovae can become Neutron Stars-- very small, dense balls of NEUTRONS

1 teaspoon of this would be approximately 1 billion tons on Earth

Due to the great density it rotates very rapidly, and some become PULSARS

https://www.youtube.com/watch?v=ZW3aV7U-aik&feature=iv&src_vid=IXxZRZxafEQ&annotation_id=annotation_3729890613

Pulsars Rapidly-spinning neutron

stars with very strong magnetic fields.

Jets of charged particles are ejected from the magnetic poles of the star.

This material is accelerated, producing beams of light in all wavelengths from the magnetic poles.

We can see this “lighthouse effect” many times per second

Computer model

Pulsar

Chandra X-Ray Observatory image shows a pulsar at the center of the Crab Nebula

Black Holes

Supermassive stars (>25 x solar mass)

collapse into neutron stars too massive

to be stable

They collapse in on themselves, forming

a region of infinite density and zero

volume– a SINGULARITY at the center

of a Black Hole

Space “curves inward” and traps all

matter and electromagnetic radiation

Stellar life cycles video

https://www.youtube.com/watch?v=PM9

CQDlQI0A