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Star StuffStar 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
emission of a photonto jump down levels
absorption of a photonto go up energy levels
In order to go UP a level, electron must absorb energy
In order to go DOWN a level, electron releases energy
Spectral lines originate from electrons moving in atoms
• ENERGY takesthe form of EM radiation, or “photons”
• photons havewavelength whichcorresponds to theenergy change
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 ofEM spectrum
A plot of INTENSITYvs. 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
Image courtesy of the McMath-Pierce Solar Observatory
• 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.
A much closer look at the spectrum of the Sun
Image courtesy of the McMath-Pierce Solar Observatory
Things to note in solar spectrum: brightest intensity at green/yellow wavelengths presence of many dark lines and features
Spectra for a variety of stars
• Some stars havefewer dark linesin their spectrathan the Sunand others havemore dark linesthan the Sun
• The dark lines arealso at different positions thanthe Sun’s
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Both of these plots show wavelength vs. intensity
Simulated Data Real Data
Experiments on Spectra of the early 1900’s
Burned different elementsover 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 absorbingthe 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 HC) 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 – inEarth’s atmosphere!
ElementElement Number %Number % Mass % Mass %
HydrogenHydrogen 92.092.0 73.4 73.4
HeliumHelium 7.87.8 25.0 25.0
CarbonCarbon 0.020.02 0.20 0.20
NitrogenNitrogen 0.0080.008 0.09 0.09
OxygenOxygen 0.060.06 0.8 0.8
NeonNeon 0.010.01 0.16 0.16
MagnesiumMagnesium 0.0030.003 0.06 0.06
SiliconSilicon 0.0040.004 0.09 0.09
SulfurSulfur 0.0020.002 0.05 0.05
IronIron 0.0030.003 0.140.14
Identifying chemical composition of the Sun’s spectrum
Arcturus (K1)
Procyon (F5)
Spectra for a variety of stars
• Some stars havefewer dark linesin their spectrathan the Sunand others havemore dark linesthan the Sun
• The dark lines arealso at different positions thanthe Sun’s
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Using spectra to identify chemical compositions
What determines “signatures” of different kinds of stars?
Need to inspect many, many different stellar spectralook for categories, patterns among them
How many different kinds of spectral “signatures” are there?
Major research effort at Harvard in the 1920’s
The Harvard College Observatory: female “computers” under direction of Professor Henry N. Russell
Annie Jump Cannon (1863-1941)
1918-1924: she classified 225,000 stellar spectra!
Figuring out the various types of stars
CeceliaPayne-Gaposchkin (1900-1979)
PhD 1925 Harvard (first Astronomy PhD)
Figured out that differentspectra 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
Cool 4,000 K
Our Sun
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 starchart (positions)!
• shows that astar’s T is relatedto its Luminosityin a certain way
Sun
The Hertzsprung-Russell Diagram (H-R Diagram)Plots the relationship between TEMPERATURE (x) and
LUMINOSITY (y) of different stars
MAIN SEQUENCE
Most stars fallalong 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 canlast 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 starshave MORE FUEL
The LESS LUMINOUS stars have less fuel but they fuse H He more slowly
but also more LUMINOSITY They fuse H He faster!
A relationship between MASSand LUMINOSITY
For stars ON the MAIN SEQUENCE direct relationship
LARGER MASS Higher Luminosity
SMALLER MASSLower Luminosity
The Hertzsprung-Russell Diagram (H-R Diagram)
SUPERGIANTS – COOL but very very LUMINOUS
WHITE DWARFS –HOT but very very
DIM
Two other categoriesof where stars are:
Changes in a star’sphysical state
result in changeson the H-R diagram
Red Giants areLARGERCOOLER
than the Sun
UPPER RIGHT
Examples of Red Giants: Arcturus, Betelgeuse
What happens next depends on initial mass
1. Stars ~ 1 solar mass
2. Stars > 2 solar masses
“Helium Flash” – explosiveconsumption of He fuel
T ~ 300 million KL ~1014 solar luminosity
Continue to fuse He and carbon to make core richin 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” ofchanges over a few days to manyyears Long period variables: Miras Shorter period variables: Cepheids
Optical images of a variablestar spaced over a few days
Instability of Red Giants – Variable Stars
“Instability Strip”
• Red Giants are constantlychanging their relationshipbetween Luminosity and Temperature
• with every expansion,some of the star’s outerlayers are lost into theinterstellar medium
Red Giant expanding into the interstellar medium
3 minutes exposure 2.5 hours exposure
NGC 6826
Planetary Nebula (PN)• remains of star: very hot core T~100,000 Ksurrounded by thin, hot layers of expanding star
• symmetric shapeshows how gas ejected
• in the end, 80% ofstar’s mass is lost
• bad name: no planets
• spectra of PN: emission lines ofH, Oxygen, Nitrogen
• common in ourGalaxy ~50,000 !
