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Star Stuff. Hot solid, liquid, dense gas: no lines, continuous spectrum. Hot object through cooler gas: dark lines in spectrum. Cloud of thin gas: bright lines in spectrum. What is the physical explanation for these different spectra?. The energy levels of the Hydrogen atom:.

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Star Stuff

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Star Stuff


Hot solid, liquid, dense gas: no lines, continuous spectrum

Hot object through cooler gas: dark lines in spectrum

Cloud of thin gas: bright lines in spectrum

What is the physical explanation for these different spectra?


The energy levels of the Hydrogen atom:

  • Places where the electron is located

  • Fixed levels by quantum mechanics

  • Energy levels depend on atomic make-up


Spectral lines originate from electrons moving in atoms

In order to go UP a level, electron must absorb energy

In order to go DOWN a level, electron releases energy

  • ENERGY takes

  • the form of

  • EM radiation, or

  • “photons”

absorption of a photon

to go up energy levels

  • photons have

  • wavelength which

  • corresponds to the

  • energy change

emission of a photon

to jump down levels


Measuring the “spectrum” of light from a star

-divides the light up into its colors (wavelengths)

- use a smaller range of wavelengths than entire EM spectrum

because instrumentation is different

“spectrometer”

Filters the light into its different parts


The “spectrum”

of a star in

the visible part of

EM spectrum

A plot of

INTENSITY

vs.

WAVELENGTH


Features of stellar spectra:

Blackbody objects (wavelength of peak intensity)

Have additional features – “spectral lines”

no lines

bright lines

dark lines


A much closer look at the spectrum of the Sun

  • Wollaton (1802) discovered dark lines in the solar spectrum.

  • Fraunhofer (1817) rediscovered them, and noted some

  • were not present in stars - but other stars had more.

Image courtesy of the McMath-Pierce Solar Observatory


A much closer look at the spectrum of the Sun

Things to note in solar spectrum:

 brightest intensity at green/yellow wavelengths

 presence of many dark lines and features

Image courtesy of the McMath-Pierce Solar Observatory


Spectra for a variety of stars

1

  • Some stars have

  • fewer dark lines

  • in their spectra

  • than the Sun

  • and others have

  • more dark lines

  • than the Sun

  • The dark lines are

  • also at different

  • positions than

  • the Sun’s

2

3

4

5

6

7


Real Data

Simulated Data

Both of these plots show wavelength vs. intensity


Experiments on Spectra of the early 1900’s

Burned different elements

over a bunsen burner

 glow different colors!!


Scientists discovered that the bright lines also correspond to the dark lines


The three types of spectra:

no lines

bright lines

dark lines


Hot solid, liquid, dense gas: no lines, continuous spectrum

Hot object through cooler gas: dark lines in spectrum

Cloud of thin gas: bright lines in spectrum


Identifying the spectral lines in the Sun’s spectrum

There are many dark

absorption lines –

what does this mean??

The Sun’s cooler gaseous outer layers are absorbing

the photons arising from the hotter inside !

Mainly hydrogen absorption lines, but over 60 different

elements identified in small quantities


Identifying the spectral lines in the Sun’s spectrum

O2 at 759.4 to 726.1 nm (A)

O2 at 686.7 to 688.4 nm (B)

O2 at 627.6 to 628.7 nm (a)

H at 656.3 nm – Hydrogen alpha line Ha (C)  electron moves between n=3 and n=2

H at 486.1, 434.0 and 410.2 nm (F, f, h)

Ca at 422.7, 396.8, 393.4 nm (g, H, K)

Fe at 466.8, 438.4 nm (d, e)

Terrestrial Oxygen – in

Earth’s atmosphere!


Identifying chemical composition of the Sun’s spectrum


Procyon (F5)

Arcturus (K1)


Spectra for a variety of stars

1

  • Some stars have

  • fewer dark lines

  • in their spectra

  • than the Sun

  • and others have

  • more dark lines

  • than the Sun

  • The dark lines are

  • also at different

  • positions than

  • the Sun’s

2

3

4

5

6

7


Using spectra to identify chemical compositions


How many different kinds of spectral “signatures” are there?

What determines “signatures” of different kinds of stars?

Major research effort at Harvard in the 1920’s

Need to inspect many, many different stellar spectra

look for categories, patterns among them


The Harvard College Observatory: female “computers”

 under direction of Professor Henry N. Russell


Figuring out the various

types of stars

Cecelia

Payne-Gaposchkin

(1900-1979)

Annie Jump Cannon

(1863-1941)

1918-1924: she classified

225,000 stellar spectra!

PhD 1925 Harvard

(first Astronomy PhD)

Figured out that different

spectra were due to TEMP.


The categories of stars: O B A F G K M

Differences are due to the TEMPERATURE of star

  • TEMPERATURE can determine:

  • where the electrons are located (which energy levels)

  • which elements have absorption, emission lines

-- an O-star has a temperature of ~50,000 K

-- an A-star has a temp of ~10,000 K, enough for hydrogen to be ionized (spectral lines in the UV)

-- a G-star (like our Sun) has a temperature of ~6,000 K


  • Different stars have different spectral “signatures”

  • All stars fall into several categories: O-B-A-F-G-K-M

Hot 50,000 K

 Our Sun

Cool 4,000 K


Stellar Evolution is the study of

- how stars are born

- how stars live their “lives”

- how stars end their lives


The Hertzsprung-Russell Diagram (H-R Diagram)

Plots the relationship between TEMPERATURE (x) and

LUMINOSITY (y) of different stars

  • not a star

  • chart (positions)!

Sun 

  • shows that a

  • star’s T is related

  • to its Luminosity

  • in a certain way


The Hertzsprung-Russell Diagram (H-R Diagram)

Plots the relationship between TEMPERATURE (x) and

LUMINOSITY (y) of different stars

MAIN SEQUENCE

Most stars fall

along this line

The MORE LUMINOUS the

star, the HOTTER it is

The LESS LUMINOUS the

star, the COOLER it is


The Hertzsprung-Russell Diagram (H-R Diagram)

Plots the relationship between TEMPERATURE (x) and

LUMINOSITY (y) of different stars

color illustrates the

main sequence (MS)

BLUE MS stars are

LUMINOUS, HOT

RED MS stars are

DIM, COOL


The Hertzsprung-Russell Diagram (H-R Diagram)

Properties of stars on the Main-Sequence

  • fusing H  He in their cores

  • the length of time fusion can

  • last depends on how much

  • “fuel” is there for fusion

  • and the rate at which fusion occurs

  • amount of fuel = star’s MASS

  • rate of fusion = star’s LUMINOSITY


The Hertzsprung-Russell Diagram (H-R Diagram)

Properties of stars ON the Main-Sequence

The LUMINOUS stars

are more massive

5 to 50 times solar mass

The DIMMER stars

are less massive

0.1 – 1 times solar mass


Masses given

in Solar Masses


The Hertzsprung-Russell Diagram (H-R Diagram)

Properties of stars on the Main-Sequence

The more massive stars

have MORE FUEL

but also more LUMINOSITY

 They fuse H  He faster!

The LESS LUMINOUS stars

have less fuel but they fuse

H  He more slowly


A relationship

between MASS

and LUMINOSITY

For stars ON the

MAIN SEQUENCE

 direct relationship

LARGER MASS

Higher Luminosity

SMALLER MASS

Lower Luminosity


The Hertzsprung-Russell Diagram (H-R Diagram)

Two other categories

of where stars are:

SUPERGIANTS –

COOL but very very

LUMINOUS

WHITE DWARFS –

HOT but very very

DIM


Changes in a star’s

physical state

result in changes

on the H-R diagram

Red Giants are

LARGER

COOLER

than the Sun

UPPER RIGHT


Examples of Red Giants:

Arcturus, Betelgeuse


What happens next depends on initial mass

1. Stars ~ 1 solar mass

“Helium Flash” – explosive

consumption of He fuel

T ~ 300 million K

L ~1014 solar luminosity

2. Stars > 2 solar masses

Continue to fuse He and

carbon to make core rich

in oxygen and carbon


Red Giants: instabilities & brightness variations

  • very delicate balance between pressure, gravity

  • easily offset – causing star to expand, contract

  • these changes can be observed as a variation in

    • star’s brightness as it “pulses”


Instability of Red Giants – Variable Stars

  • brightness changes by many times

  • because of pulsations in the star

  •  factors of a few to 100 or more!

  • can see the “signature” of

  • changes over a few days to many

  • years

  • Long period variables: Miras

  • Shorter period variables: Cepheids

Optical images of a variable

star spaced over a few days


Instability of Red Giants – Variable Stars

  • Red Giants are constantly

  • changing their relationship

  • between Luminosity and

  • Temperature

“Instability Strip”

  • with every expansion,

  • some of the star’s outer

  • layers are lost into the

  • interstellar medium


Red Giant expanding into the interstellar medium

NGC 6826

3 minutes exposure

2.5 hours exposure


Planetary Nebula (PN)

  • remains of star: very hot core T~100,000 K

  • surrounded by thin, hot layers of expanding star

  • symmetric shape

  • shows how gas ejected

  • bad name: no planets

  • spectra of PN:

  • emission lines of

  • H, Oxygen, Nitrogen

  • common in our

  • Galaxy ~50,000 !

  • in the end, 80% of

  • star’s mass is lost


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