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Stars. Star Field as seen through the Hubble Space Telescope. 2. Stars –. 1. Definition- a large gaseous body that generates energy through nuclear fusion in its core

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stars
Stars –

1. Definition- a large gaseous body that generates energy through nuclear fusion in its core

( Although the term is often also applied to objects that are in the process of becoming stars or to the remains of stars that have died.)

2. Spectra (light) of Stars-

- Allows astronomers to determine the star’s

a. Composition

b. Temperature

c. Luminosity

d. Velocity and Rotation rate in Space

e. Mass

There are three different types of spectra produced when light is passed through a prism depending on the source of the light:

stars cont
Stars (cont.)

2. Spectra (light) of Stars(cont.)

A. Continuous Spectra-

produced by a glowing solid, liquid, or very high density gas under certain conditions. (A normal light bulb produces a continuous spectra.)

B. Absorption Spectra (Dark Line)-

produced when a cooler gas lies between the observerand the object emitting a continuous spectra.

- The gas absorbs some of the wavelengths of light leaving behind dark lines. The wavelengths absorbed depends on the composition of the gas and the temperature of the light source.

-This is the spectra used to classify stars

stars cont5
Stars (cont.)

2. Spectra (light) of Stars(cont.)

C. Emissions Spectra (Bright Line) –

-produced when a glowing gas emits energy at specific wavelengths, characteristic of the element composing the gas.

- used to study nebulae (Clouds of gas)

stars cont8
Stars (cont.)

3. Classifications of Stars-

- Stars are essentially all made of the same material!!!

- So WHY don’t they all have the same color or absorption line spectra?

***The spectral difference is due to the difference in temperature of the star.

The different temperatures also leads to the difference in colors that we see:

- Hotter stars appear Blue

- Cooler Stars appear Red

A. Classification system

The classification scheme used today divides the star up into seven major spectral or temperature classes

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

O – Hottest Stars

M – Coolest Stars

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The Spectral Sequence

Spectral Class

Temperature

Color

Spectral Lines

Example

O

30,000 to 50,000 K

Blue-Violet

Ionized Helium

Minataka

B

11,000 to 30,000 K

Blue-White

Neutral HELIUM, Hydrogen

Rigel, Spica

A

7,500 to 11,000 K

White

Hydrogen (Strong)

Sirius, Vega

F

5,900 to 7,500 K

Yellow-White

Ionized Metals

Procyon

G

5,200 to 5,900 K

Yellow

Ionized CALCIUM,

Ionized and Neutral Metals

The Sun, Capella

K

3,900 to 5,200 K

Orange

Neutral Metals

Arcturus, Aldebaran

M

2,500 to 3,900 K

Red-Orange

Neutral Metals,

Molecular Bands

Betelgeuse, Antares

stars cont12
Stars (cont.)

3. Classifications of Stars (cont)-

A. Classification system (cont.)

Since 1995 Astronomers have found new stars with surface temps even lower than spectral class M. These bodies which are not truly stars are called Brown Dwarfs- Heat is generated by contraction of gases not Nuclear Fusion. (Give off a lot of light in the infrared range.)

B. H-R Diagram (Hertzsprung + Russel)

- In 1912 classification scheme for stars invented

- Stars are plotted according to:

1. Luminosity (Absolute Magnitude)

Brightest Stars at the Top

2. Temperature (Spectral Class)

Hotter Stars on the Left – Temperature Decreases as you move to the right

stars cont15
Stars (cont.)

3. Classifications of Stars (cont)-

B. H-R Diagram (cont.)

3. Super-giants:

- Very few rare stars that are bigger and brighter than typical giants

- 1000 times larger than the Sun

EX- Betelguese in Orion and Antares in Scorpius

4. White dwarfs-

- Remaining 9% of stars located in the lower left of the H-R Diagram

- Although Very Hot, they have low luminosities due to their small size.(About the size of Earth)

- (So dim can only be seen with a telescope)

**- NO nuclear Fusion in core, only shines due to stored heat remaining from contraction of core.

EX- Sirius B a companion star to Sirius A.

stars cont16
Stars (cont.)

3. Classifications of Stars (cont)-

B. H-R Diagram (cont.)

- Data points (Stars) on the diagram are NOT scattered randomly, but rather appear grouped in a few distinct regions:

1. Main Sequence Stars:

- About 90% of stars fall in this band stretching diagonally across the diagram.

-Extends from the hot, luminous blue stars to the cool, dim red stars

Ex- Sun is a Main sequence star

2. Giants:

- Upper right hand side of diagram

- Stars are both luminous and cool.

In order to be as luminous as they are they must be large or giants

- Approximately 10 to 100 times larger than our Sun

Ex- Aldebaran in Taurus

stars cont20
Stars (cont.)

4. Stellar Evolution-

- Stars DO NOT Live forever

- Eventually the fuel which powers the nuclear reactions will run out and the star will cease to shine.

- Changes that a star undergoes is referred to as its LIFE CYCLE

A. Pre-Main Sequence Stage Star

- Stars form in a dense cold, cloud of dust and gas (Mostly Hydrogen and Helium) called a Cocoon Nebula that begins to condense and form a Proto-star

Possible Reasons for Condensation-

a. Nearby Supernova Outburst

b. Stellar Winds from hot nearby stars

1. Proto-Star- Forms as the cloud condenses by the gravitational accretion of gas and dust. As it grows the contraction of the particles causes it to heat and begin to glow.

stars cont21
Stars (cont.)

4. Stellar Evolution (cont.)

A. Pre-Main Sequence Stage (cont.)

2. Protostar(cont.)

- As protostar begins to heat and glow, it spins faster. Which starts Bipolar Outflow

- NO FUSION YET – Heat only generated by contraction

- Evidence of star formation:

a. T-Tauri Stars

b. Herbig-Haro Objects- Bipolar outflow collides with surrounding interstellar medium

c. EGG’s (Evaporating Gaseous Globules) smalll dense clouds in the act of contracting

d. Protoplanetary disks (PROPLYDS)

- If you see any of these there would most likely be a star forming there, but no planets and no fusion yet!!!!

stars cont33
Stars (cont.)

4. Stellar Evolution (cont.)

A. Pre-Main Sequence Stage (cont.)

2. Protostar(cont.)

-Eventually contraction of gasses produces a high enough temperature at the core so that Nuclear Fusion Starts.

***-Once Hydrogen fusion begins  A MAIN SEQUENCE STAR IS BORN

-Time frame for formation:

A. The more mass there is, the more heat generated by contraction, the faster the Star forms

(15- solar masses takes about 60,000 years to form)

B. The less mass there is, the less heat generated by contraction, the slower the star forms

( .5 solar masses takes 150 million years to form)

C. Our sun probably took about 50 million years to form

slide34

15MSun

9MSun

3MSun

1MSun

0.5MSun

34

stars cont36
Stars (cont.)

4. Stellar Evolution (cont.)

B. Main Sequence Stars-

- Once Hydrogen fusion begins the temperature and pressure in the core become strong enough to resist further contraction

***- Hydrostatic Equilibrium is reached and the star becomes a stable Main sequence Star

slide38

Hydrostatic Equilibrium – The outward pressure of Nuclear Fusion is EQUAL to the inward Pull of Gravity

Our Sun- A Main Sequence Star

stars cont40
Stars (cont.)

4. Stellar Evolution (cont.)

B. Main Sequence Stars (cont.)-

- Time frame for Main sequence Star:

1. More Massive Stars have to burn hotter and faster to resist gravitational contraction and therefore use up their fuel quicker.

( A 15 solar mass star will burn for about 10 million years)

** Higher internal temps makes these stars more luminous

2. Less massive stars burn cooler and therefore can last longer

( A .5 solar mass star will live for 100 billion years)

** Low temps mean they are NOT as luminous

3. Our Sun will fuse hydrogen (burn) for about 10 billion years

stars cont41
Stars (cont.)

4. Stellar Evolution(cont.)

B. Main Sequence Stars (cont.)-

- The short life span of massive stars implies that observed ones MUST be YOUNG!!! -> Would you expect to find Life around planets that orbit these massive stars???

C. Post Main Sequence Stage-

- Core’s Hydrogen supply runs out Fusion stops and core begins to contract under gravity.

- The release of heat from contraction causes outer layers of hydrogen to fuse at an incredible rate and outer layer expands to become a RED GIANT STAR

1. Red Giant or Super-giant:

Very luminous due to its size but heat is spread out over a larger area so cooler than main sequence star….That’s why it turns red!!!

Ex- Betelguese in Orion is a Star that has left the Main sequence stage and become a Red Supergiant.

stars cont46
Stars (cont.)

4. Stellar Evolution (cont.)

C. Post main Sequence Stage (cont.)

what happens to a star after Fusion stops depends on the original mass of the star.

a. Low mass stars such as our sun will become Red giants

b. Higher Mass stars will expand much further to become Red Super-giants. (ex- Betelguese)

stars cont47
Stars (cont.)

4. Stellar Evolution (cont.)

D. Death of a Star – 4 Solar Masses or less

- Core of Red Giant will heat up due to contraction and start fusing helium to carbon at a very high rate.

- When Helium runs out Fusion stops and Carbon Core begins to contract which again causes outer layers to heat up and expand.

- Outer layers of gas will be ejected into space to form a Planetary Nebula:

a huge shell of brightly glowing gas and dust lighted by the very hot exposed core of a star. (Will become White Dwarf Star)

stars cont58
Stars (cont.)

4. Stellar Evolution (cont.)

D. Death of a Star - 4 Solar Masses or less (cont.)

- Due to lack of mass carbon will not be able to condense enough to fuse into oxygen.

- After Planetary Nebula Gases Spread out all that remains is a

White Dwarf “Star”:

- Stellar Core Remnant that has about 1.4 Solar Masses or less

(About the mass of the SUN in what will shrink down to the size of the Earth – 1 teaspoon of matter would weigh 5 tons on earth)

- Generates light and heat from contracting of matter under gravity (NOT FUSION)

- Very hot but not luminous because of small size

- Eventually will stop shrinking (electrons prevent further collapse) and will slowly cool off over 10’s of billions year and become a black dwarf.

slide60

White Dwarf Star and Companion Star which wandered to close to white Dwarf will probably lead to a Type I Supernova

60

stars cont61
Stars (cont.)

4. Stellar Evolution (cont.)

E. Death of a Star - 4 Solar Masses or more

- Eventually due to extremely high mass of the Star, the core will eventually become hot enough to have fusion all the way to Iron

- As it tries to fuse into heavier elements it actually loses energy that is supporting the core against gravity.

- The core shrinks very rapidly and rebounds with a tremendous shock wave that blows apart the entire shell of the star in an explosion called a Supernova (Type II)

stars cont62
Stars (cont.)

4. Stellar Evolution (cont.)

E. Death of a Star - 4 Solar Masses or more (cont.) Supernova (Type II)-

- An explosion that causes a star to suddenly increases dramatically in brightness

- Energy released is more than 100 times what the sun will radiate over ts entire 10 billion year lifetime

- Very rare only about 1 every hundred years per galaxy (But there are billions of galaxies in the universe)

- Star will outshine ALL the stars in its own galaxy COMBINED!!

- May even be visible on earth during daylight hours

-Nucleosynthesis- creation of heavier elements from lighter elements. (All elements heavier than Iron could only be created in Supernova Explosions)

slide69

Crab Nebula- A Remnant of a Supernova Explosion observed in 1054 AD which was bright enough to be seen during the day for over three weeks and during the night for 6 months

69

stars cont70
Stars (cont.)

E. Death of a Star - 4 Solar Masses or more (cont).

  • -After Supernova explosion, stellar remnant is dependant upon how much of core is left.
  • 1. Neutron “Star”-
    • - Core remnant is between 1.4 and 3.0 solar masses
    • - Compression will be so great that protons and electrons of matter in core will combine to form neutrons – Atoms will be able to become very close together (Neutrons prevent further collapse)
    • - Only Massive stars 5-10 solar masses can become Neutron stars
    • - More Massive than a white dwarf star BUT only the size of a large city!!!!! (A paper clip made from a Neutron star would outweigh Mt. Everest )
    • - Emit strong radio waves
    • - Pulsars (Pulsating Radio waves) are evidence for the existence of Neutron Stars
    • **- Pulsars detected in at Center of Both Crab and Veil Nebula
    • (Remnants of a Supernova)
stars cont74
Stars (cont.)

E. Death of a Star - 4 Solar Masses or more (cont).

2. Black Hole

- Core remnant is greater than three solar masses

- Compression of core is so great that even neutrons cannot hold the core up against its own gravity.

- Gravity squeezes three solar masses into an infinitesimally small point (Smaller than the size of a pinhead) called a singularity

-The area that separates the black hole from the surrounding space is called the Event Horizon. -> Within the event horizon gravity is so strong that even light does not travel fast enough to escape the gravity.

(At the singularity the infinite gravity causes space and time to be jumbled and the laws of physics as we know them do not apply.)

stars cont75
Stars (cont.)

E. Death of a Star - 4 Solar Masses or more (cont).

2. Black Hole (cont.)

- Black holes are usually detected in binary star systems where one of those stars has become a black hole

- Only massive main sequence star (10 solar masses or more) will become black holes

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