observational astronomy 5

7/28/2019 Observational Astronomy 5

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19-Apr-13

IESO

Observational Astronomy

Part 5


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19-Apr-13

A stars color, temperature, size,

brightness and distance are all

related!


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The Beginnings

Late 1800s, early 1900s how light is produced by

atoms is being intenselystudied by Gustav Kirchoff & Robert Bunsen

Max PlanckJosef Stefan...

Ludwig BoltzmannAlbert Einstein


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Black Bodies

In 1862, Kirchoff coins the phrase black body to

describe an imaginary object that would perfectly

absorb any light (ofany wavelength) that hit it.

No light transmitted through, no light reflected off,just totally absorbed.


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a perfect absorber of light would also be a perfectemitter

amount of light energy given off each second (itsbrightness or luminosity) and the color of its light are

related to the objects temperature.


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Molten lava and hot iron are two good examples ofblack bodies, but

a star is an excellent black body emitter.


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Max Planck, a German physicist, was able to maketheoretical predictions of how much light of each color

or wavelength would be given off by a perfect black

body at any given temperature.

These predictions or models are today called Planck

Curves.


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Ohio University - Lancaster Campus

Spring 2009 PSC 100 slide 7 of 47


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What 2 characteristics of the curves change as thetemperature increases?

(1) The size of the curve increases.

(2) The peak of the curves shift to the

left, to shorter wavelengths & higher

energies.


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Can we draw some conclusions?

Hotter stars should be brighter than cooler stars.

Hotter stars should emit more of their light at shorter

wavelengths (bluer light)

Cooler stars should emit more of their light at longer

wavelengths (redder light).

All stars emit some energy at allwavelengths!


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In 1879, Josef Stefan discovered that the luminosity of astar was proportional to the temperature raised to the

4th power.

In 1884, Stefans observations were confirmed whenLudwig Boltzmann derived Stefans equation from

simpler thermodynamic equations.


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Stefan-Boltzmann Law Today, we honor both scientists by naming the

equation after themthe Stefan-Boltzmann Law:

At the surface of the star, the energy thats given offper square meter (Watts / m2) called the luminous

flux is...

W / m2 = 5.67 x 10-8 T4

Ohio University - Lancaster Campus slide 11 of 47

Spring 2009 PSC 100


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At 100 K (cold enough to freeze you solid in justseconds), a black body would emit only 5.67 W/m2.

At 10x hotter, 1000 K, the same black body would emit104 times as much light energy, or 56,700 W/m2.


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If the temperature of a star were to suddenly double,how much brighter would the star become?

If the temperature of a star somehow fell to 1/3 of what

it was, how much fainter would the star become?

24 = 16 times brighter

(1/3)4 = 1/81, or 81 times dimmer


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In 1893, Wilhelm Wien (pronounce vine) discoveredby experiment the relationship between the main

color of light given off by a hot object and its

temperature.

This main color is the peak wavelength, called max ,

at the top of the Planck Curve.


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For each curve, the

top of the curve is the

peak wavelength.


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Wiens Law Wiens Law says that the peak wavelength is

proportional to the inverse of the temperature:

max = 2.9 x 106 T = 2.9 x 106

T max

T must be in Kelvin, and max in nanometers.


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What is the peak wavelength of our sun, with a T =5750 K?

What is the peak wavelength of a star with a surface

temperature of 3500 K?

2.9 x 106 = 504 nm (yellowish-green)

5750 K

2.9 x 106 = 829 nm (this star emits the

3500 K majority of its light as

infrared, IR).


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A reddish star has a peak wavelength of 650 nm. Whatis the stars temperature?

A star has a peak wavelength in the ultra-violet of 300 nm.

What is the stars temperature?

2.9 x 10

6

= 4462 K (cooler than the sun)650 nm

2.9 x 106 = 9667 K

300 nm


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We now have a color thermometer that we can use

to determine the temperature of any astronomical

object, just by examining the light the object gives

off.

We know that different classes of objects are at

different temperatures and give off different peak

wavelengths.


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What kinds of objects?

Clouds of coldhydrogen gas

(nebulae) emit

radio waves

http://www.narrowbandimaging.com/images/vdb142_small.jpg

l d f l l h f


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Warmer clouds of molecules where stars form

emit microwaves and IR.


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Protostars emit IR.

http://www.antonine-education.co.uk/Physics_GCSE/Unit_3/Topic_10/protostar.jpg


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Sun-like stars emit mostly visible light, while

hotter stars peak in the UV.

http://www.nasa.gov/images/content/138952main_whywe16full.jpg

N d bl k h l k i h X


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Neutron stars and black holes peak in the X-

ray.


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Star cores emit gamma rays.

http://aspire.cosmic-ray.org/labs/star_life/images/star_pic.jpg


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Where would the peak wavelength be for

your body

a lightning bolt

the coals from a campfire


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A stars spectrum is also influenced byits temperature.

In 1872, Henry Draper obtained the firstspectrum of a star, Vega, in the

constellation Lyra.

photojournal.jpl.nasa.gov/jpeg/PIA04204.jpgCredit: Lick Observatory Archives


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In 1885, Edward Pickering began a project

at Harvard University to determine the

spectra of many stars. Drapers widow

funded the work.

The first 10,000 spectra obtained were

classified by Williamnia Fleming, using theletters A through Q.


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From 1901 to 1919, Pickering & his assistant

Annie Jump Cannon classified and publishedthe spectra of 225,000 stars (at the rate of

about 5000 per month!)

When Pickering died in 1919, Cannon

continued the work, eventually classifying

and publishing the spectra of 275,000 stars.

Credit: amazing-space.stsci.edu


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Hotter stars have

simpler spectra.

Cooler stars have

more complexspectra, since most

atoms are not ionized.


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Class O >30,000 K bluish

He lines in spectrum.(These stars are so hot that H is mostly ionized &

doesnt shows lines.) Pleiades

Class B 11,000-30,000 K bluish

He lines, weaker H lines

Rigel, Regulus, Spica

Class A 8,000-11,000 K blue-

white H lines (Balmer Series)

Sirius, Vega


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Class F 6,000-8,000 K white

H, Ca lines, weaker H lines Procyon

Class G 5,000-6,000 K yellow

Ca, Na lines, + other metalsSun, Capella, -Centauri

Class K 3,500-5,000 K orangeCa & other metals

Arcturus, Aldebaran


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Class M


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The stellar classes (OBAFGKM) are further

subdivided with a number 0 to 9 following the

letter.

Our sun, a G2 star, is slightly cooler than the

F range. A G9 star would be just a bit warmer

than the K range.


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1910-1913, Henry Russell, a professor atPrinceton, and Ejnar Hertzsprung, an

astronomer at Leiden Observatory in the

Netherlands, used the data from the Draper

catalog to plot the temperature of the stars

vs. their brightness or luminosity.

What kind of result would you expect, arandom scatter, or a pattern?


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universe-review.ca/I08-01-HRdiagram.jpg


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Betelgeuse and Antares show on the diagram

as being red stars, and red stars should be

faint.

Both stars are also hundreds of light

years distant, so why do they appear so

bright in our sky?


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Red Dwarfs

Red

Red


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The H-R Diagram makes a lot moresense when you realize that the

different regions dont show different

kinds of stars

but stars at different stages

of their lives.


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Determining distance using the HR Diagram

From a stars color-temperature, determine

its absolute magnitude (M).

Observe the stars apparent magnitude (m)

through a telescope.

Use the distance modulus equation to

calculate the distance.


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How far away is an F1 star that has a surface

temperature of 8000 K, if its apparent

magnitude is +9.6?


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distance in parsecs =

10^[(9.6 - 3.0 +5) 5] =

10^[11.6 5] =

10^2.32 =

209 parsecs (or 681 light years)


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Where might this method run into trouble?

Red & Orange star come in 2 varieties:

giants & dwarfs.

The spectrum of the star must be used todetermine if the star is large or small.

The presence of what element(s) in higherthan normal percentages might indicate

that the star is a giant, not a dwarf?

observational astronomy 5

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