light pollution stellar evolution

Charles HakesFort Lewis College 1


Light Pollution

Stellar Evolution


Lab Notes

• Be sure you have started your “report” lab.• Constellation presentations next week.• Observatory field trips…


Night Lights

• http://apod.nasa.gov/apod/ap101104.html

http://apod.nasa.gov/apod/ap101104.html






Limiting Magnitude

and

Light Pollution


Limiting Magnitude

• Limiting Magnitude is a measure of the dimmest star visible from a given location.

• Center of a big city: ~2.0• Suburbs: ~4.5• Downtown Durango: ~5.5• La Plata County (FLC observatory): ~6.5

• Depends on observer experience• Depends on local glare• Depend on how dark adapted your eyes are. You

should wait 20-30 minutes to measure this.


Limiting Magnitude

• Use stars of known magnitude (e.g. Little Dipper)


Limiting Magnitude

• Count the stars in a well-defined region• Chose one of the predefined star regions that is

overhead• Count the number of stars visible within the region

boundary• Look up the number on the published tables to find

the corresponding limiting magnitude


Limiting Magnitude


Limiting Magnitude

Alpha-Epsilon-Beta Gemstars LM1 1.22 2.43 3.24 3.95 4.36 5.07 5.18 5.39 5.610 5.711 5.912 6.113 6.214 6.315 6.416 6.518 6.620 6.722 6.923 7.025 7.230 7.5


How many Stars Can You See?Magnitude Range Cumulative Stars % Increase Seen

-1 -1.50 to -0.51 2 -0 -0.50 to +0.49 8 400%1 +0.50 to +1.49 22 275%2 +1.50 to +2.49 93 423%3 +2.50 to +3.49 283 304%4 +3.50 to +4.49 893 316%5 +4.50 to +5.49 2,822 316%6 +5.50 to +6.49 8,768 311%7 +6.50 to +7.49 26,533 303%8 +7.50 to +8.49 77,627 293%9 +8.50 to +9.49 217,689 280%10 +9.50 to +10.49 626,883 288%11 +10.50 to +11.49 1,823,573 291%12 +11.50 to +12.49 5,304,685 291%13 +12.50 to +13.49 15,431,076 291%14 +13.50 to +14.49 44,888,260 291%15 +14.50 to +15.49 130,577,797 291%16 +15.50 to +16.49 379,844,556 291%17 +16.50 to +17.49 1,104,949,615 291%18 +17.50 to +18.49 3,214,245,496 291%19 +18.50 to +19.49 9,350,086,162 291%20 +19.50 to +20.49 27,198,952,706 291%


How many Stars Can You See?

• Dark-adapted naked eye (1x7 binoculars)• Can see to magnitude 6.5 -> ~104 stars

• Light gathering ability scales with area.• Magnitude Increase = log10(Area increase) / 0.4• 10x50 binoculars ~50x area -> +4.25 magnitudes• 16” SCT ~64x area -> +4.5 magnitudes• 10m Keck telescope ~625x area -> +7 magnitudes


How many Stars Can You See?

• So for naked eye observing• 10x50 binoculars

• 50x area• +4.25 magnitudes (to 10.75) > 106 stars

• 16” SCT• 64x area• +4.5 magnitudes (to 15.25) > 108 stars

• 10m Keck telescope• 625x area• +7 magnitudes (to 22.75) > 1011 stars


Figure 10.6Apparent Magnitude


Light Pollution

• Generally not an issue in La Plata county.• Durango has a dark sky ordinance, but

only for new construction.• Fort Lewis is making progress with outside

light fixtures.


Light Pollution

Earth at Night Credit: C. Mayhew & R. Simmon (NASA/GSFC), NOAA/ NGDC, DMSP Digital Archive


Light Pollution

From IDA Website: http://www.darksky.org/images/sat.html


Light Pollution

You Are Here

Observatory


Figure 10.15Hipparcos H–R Diagram

• Plot the luminosity vs. temperature.

• This is called a Hertzsprung-Russell (H-R) diagram


What fraction of the stars on an H-R diagram are on the main sequence

A. 0-50%

B. 50-70%

C. 70-80%

D. >80%


Distance Scale

• If you know brightness and distance, you can determine luminosity.

• Turn the problem around…


Distance Scale

• If you know brightness and distance, you can determine luminosity.

• Turn the problem around…• If a star is on the main sequence,

then we know its luminosity. So• If you know brightness and luminosity,

you can determine a star’s distance.


Distance Scale

• Spectroscopic Parallax - the process of using stellar spectra to determine distances.

• Can use this distance scale out to several thousand parsecs.


Figure 10.16Stellar Distances


Stellar Evolution


Figure 11.16Atomic Motions

• Low density clouds are too sparse for gravity.• A perturbation could cause one region to start

condensing.


Figure 11.17Cloud Fragmentation


Figure 11.20Interstellar Cloud Evolution


http://discovermagazine.com/2009/interactive/star-formation-game/

google “star formation game”




H-R diagram review

• The H-R diagram shows luminosity vs. temperature.

• It is also useful for describing how stars change during their lifetime even though “time” is not on either axis.

• How to do this may not be obvious.• Exercise - Get in groups of ~four and get

out a blank piece of paper.


Group Exercise

• As a group, create a diagram with “financial income” on the vertical axis, and “weight” on the horizontal axis.

• Use this graph to describe the past and future of a fictitious person (or a group member).

• Label significant events, for example• birth• college• retirement• death


Stellar Evolution

1 - interstellar cloud - vast (10s of parsecs)2(and 3) - a cloud fragment may contain 1-2

solar masses and has contracted to about the size of the solar system

4 - a protostar• center ~1,000,000 K• Too cool for fusion, but hot enough to

see. (photosphere ~3000 K)• radius ~100x Solar


How would the luminosity of a one-solar-mass protostar compare to the sun?

A) Less than .1x as bright

B) A little lower.

C) About the same.

D) A little brighter

E) More than 10x brighter


Figure 11.19Protostar on the H–R Diagram


Figure 11.21Newborn Star on the H–R Diagram

5 - Gravity still dominates the radiation pressure, so the star continues to shrink.


Figure 11.18Orion Nebula, Up Close


Figure 11.23Protostars


Stars A and B formed at the same time. Star B has 3 times the mass of star A. Star A has an expected lifetime of 3 billion years. What is the expected lifetime of star B?

A) more than 9 billion years

B) about 9 billion years

C) 3 billion years

D) about 1 billion years

E) less than 1 billion years





C) 3 billion years




Stellar Lifetimes

• Proportional to mass• Inversely proportional to luminosity• Big stars are MUCH more luminous,

so they use their fuel MUCH faster.• The distribution of star types is

representative of how long stars spend during that portion of their life.

• Example - snapshots of people.


Figure 10.21Stellar Masses


Figure 11.24Prestellar Evolutionary Tracks


Figure 11.25Brown Dwarfs


Figure 11.22Protostellar Outflow


Stellar Evolution




Can have violent “winds” streaming outwards; often bipolar flow from poles;

T-Tauri phase




Can have violent “winds” streaming outwards; often bipolar flow from poles;

T-Tauri phase

6 - a newborn star

Core temperature high enough to ignite nuclear fusion.





C) 3 billion years




Stellar Lifetimes

• Proportional to mass• Inversely proportional to luminosity• Big stars are MUCH more luminous,

so they use their fuel MUCH faster.• The distribution of star types is

representative of how long stars spend during that portion of their life.

• Example - snapshots of people.


Figure 12.1Hydrostatic Equilibrium



The final location on the main sequence depends entirely on the size (mass) of the condensing cloud.


Figure 11.25Brown Dwarfs

Not big enough to start fusion.

Mass <~ 0.08 solar masses

~=80x mass of Jupiter.

These are likely very numerous


Stellar Evolution

7 - the star stays on the main sequence for most (~90%) of its lifetime.


Figure 10.15Hipparcos H–R Diagram





C) 3 billion years




What forces a star like our Sun to evolve off the main sequence?

A) It loses all its neutrinos, so fusion must cease.

B) It completely runs out of hydrogen.

C) It builds up a core of inert helium.

D) It explodes as a violent nova.

E) It expels a planetary nebula to cool off and release radiation.


Figure 12.2Solar Composition Change

7 - fusion of H to He occurs in the core until the H is used up.


After hydrogen fusion stops in the core of a star, the core…

A) expands and cools

B) expands and heats

C) contracts and cools

D) contracts and heats


After hydrogen fusion stops in the core of a star, the star as a whole…

A) expands

B) contracts


Figure 12.3Hydrogen Shell Burning

7 - fusion of H to He occurs in the core until the H is used up.

8 - the He core begins to shrink (and heat!), while the H-burning region moves out into a shell


Figure 12.4Red Giant on the H–R Diagram

9 - the He core continues to shrink (just a few times bigger than Earth) and heat (to 108 K)

Heat pressure

demo


Figure 12.1Hydrostatic Equilibrium


Figure 12.5Horizontal Branch

9 - the Helium flash is when the He in the core begins to fuse into carbon. This happens when the core is ~108K.

Core expands and cools.

New equilibrium on the “horizontal branch.”

10 - New equilibrium on the “horizontal branch.”


Figure 11.27bGlobular Cluster

Note the “horizontal branch”


Question

What happens when the He in the core is used up?


Figure 12.6Helium Shell Burning

In the He shell burning stage, the star expands just like in the H shell burning stage


Figure 12.7Reascending the Giant Branch

11 - Helium shell burning begins.

Core shrinks and heats.

Exterior expands and cools.


Figure 12.10White Dwarf on H–R Diagram

12 - For 1 solar mass stars, that is all that will fuse.

(need 600 million K for the next reactions to occur.)

The outer shell gets “blown off” by the hot, dense, core.

Result is a planetary nebula around a white dwarf (13).


Figure 12.9Planetary Nebulae


White Dwarf stage

Just the core of the star remains Very small - about the size of Earth Very dense - about half as massive as the

sun. Will eventually fade and become a black

dwarf (stage 14).


Figure 12.8G-Type Star Evolution


Three Minute Paper

• Write 1-3 sentences.• What was the most important thing

you learned today?• What questions do you still have

about today’s topics?


Earth Hour

• Saturday, March 27, 2010.• 8:30 P.M. • Turn off lights - save energy.• http://www.myearthhour.org/

http://www.myearthhour.org/


How many Stars Can You See?• But who uses naked eye observing any more?• High quantum efficiency CCD cameras greatly

extend the depth of a telescope.• Image stacking lets you go even deeper.

• Depth scales linearly until star brightness is about the same as the background sky brightness.

• A dark sky on Earth is about magnitude 21/arcsec2.• For film, this would be about as deep as you could go.

• 16” SCT should reach magnitude 21.5 with a 5 minute exposure using SBIG full frame CCDs

• For deeper images, the signal only scales as the ~sqrt(time)

light pollution stellar evolution

Documents

magnitude increase

magnituderangecumulative

number of stars visible

light pollutionearth

light pollutiongenerally

magnitudelimiting magnitude

stars16 sct64x area

outside light fixtures