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Understanding the Stars How can we learn about the lives of stars, which last millions to billions of years? How can we learn about the lives of stars,

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Presentation on theme: "Understanding the Stars How can we learn about the lives of stars, which last millions to billions of years? How can we learn about the lives of stars,"— Presentation transcript:

1 Understanding the Stars How can we learn about the lives of stars, which last millions to billions of years? How can we learn about the lives of stars, which last millions to billions of years? Consider the story of the Ephemera Consider the story of the Ephemera Image from: http://www.startunzflutes.com/graphics Image from: http://www.startunzflutes.com/graphics

2 How can we Study the Life Cycles of Stars? A star can live for millions to billions of years. A star can live for millions to billions of years. we will never observe a particular star evolve from birth to death we will never observe a particular star evolve from birth to death The key is that all stars were not born at the same time. The key is that all stars were not born at the same time. the stars which we see today are at different stages in their lives the stars which we see today are at different stages in their lives we observe only a brief moment in any one stars life we observe only a brief moment in any one stars life by studying large numbers of stars, we get a snapshot of one moment in the history of the stellar community by studying large numbers of stars, we get a snapshot of one moment in the history of the stellar community we can draw conclusions just like we would with human census data…we do stellar demographics! we can draw conclusions just like we would with human census data…we do stellar demographics!

3 Classification of Stars Stars were originally classified based on: Stars were originally classified based on: their brightness their brightness their location in the sky their location in the sky This classification is still reflected in the names of the brightest stars…those we can see with our eyes: This classification is still reflected in the names of the brightest stars…those we can see with our eyes: Orionis Geminorum Order of brightness within a constellation Latin Genitive of the constellation

4 Classification of Stars The old classification scheme told us little about a stars true (physical) nature. The old classification scheme told us little about a stars true (physical) nature. a star could be very bright because is was very close to us; not because it was truly bright a star could be very bright because is was very close to us; not because it was truly bright two stars in the same constellation might not be close to each other; one could be much farther away two stars in the same constellation might not be close to each other; one could be much farther away In the 20 th Century, astronomers developed a more appropriate classification system based on: In the 20 th Century, astronomers developed a more appropriate classification system based on: a stars luminosity a stars luminosity a stars surface temperature a stars surface temperature A stars stage of life A stars stage of life

5 Summer Triangle Which of these three Stars in the Summer Triangle is the brightest (to the naked eye) Which is the most Luminous? Which of these three are the closest?

6 Dark Sky image of Summer Triangle

7 Dont always trust your eyes! Note that Deneb is a bit larger than the other two Its also a bit farther away2250 light years! Vega is 22 light years awayits in the neighborhood Note that Altair (about 30 lyrs) is probably flattened..its rotating very fast! Apparent brightness depends on size, temp, and distance!

8 Luminosity and Intensity What is luminosity and how do we determine it? What is luminosity and how do we determine it? How do we measure the distance to nearby stars? How do we measure the distance to nearby stars? How does the magnitude of a star depend on its apparent brightness? How does the magnitude of a star depend on its apparent brightness? Our goals for learning:

9 Luminosity of Stars Apparent brightness refers to the amount of a stars light which reaches us per unit area. Apparent brightness refers to the amount of a stars light which reaches us per unit area. the farther away a star is, the fainter it appears to us the farther away a star is, the fainter it appears to us how much fainter it gets obeys an inverse square law how much fainter it gets obeys an inverse square law its apparent brightness decreases as the (distance) 2 its apparent brightness decreases as the (distance) 2 Apprent brightness is also known as the Intensity Apprent brightness is also known as the Intensity App Bright = L / 4 d 2 Luminosity – the total amount of power radiated by a star into space. The apparent brightness of a star depends on two things: How much light is it emitting: luminosity (L) [watts] How far away is it: distance (d) [meters]

10 Apparent Brightness The Inverse Square Law for LightWhat Determines Apparent Brightness?

11 Measuring Distances to Stars parallax – apparent wobble of a star due to the Earths orbiting of the Sun

12 Measuring Distances to Stars p = parallax angle d = 1 AU/ p Gives distance in Parsecs. convert p into arcsec d = 206,265 AU/ p

13 Measuring Distances to Stars d = 1 / p If p is in arcsec and d is in parsecs A star with a parallax of 1 arcsec is 1 parsec distant lets define 1 parsec 206,265 A.U. = 3.26 light years

14 The Brightness of Stars Astronomers still use an ancient method for measuring stellar brightness which was proposed by the Greek astronomer Hipparchus (c. 190 – 120 B.C.) This scale runs backwards: The bigger the number, the fainter the star Brightest stars are #1, next brightest are #2, etc.

15 The Modern Magnitude System apparent magnitude brightness of a star as it appears from Earth = -2.5 log (app bright) each step in magnitude is 2.5 times in brightness absolute magnitude the apparent magnitude a star would have if it were 10 pc away

16 What good is this? If you know apparent brightness, you can find magnitude. If you know apparent brightness, you can find magnitude. If you know magnitude, you can use another relationship to find distance If you know magnitude, you can use another relationship to find distance M – m = 5 – 5log(d) M – m = 5 – 5log(d) M= Absolute magitude M= Absolute magitude M = m when distance is ten parsecs. M = m when distance is ten parsecs.

17 An example of how this works! Deneb has an apparent visual magnitude of Deneb has an apparent visual magnitude of 1.26 (see chart of brightest stars at end of your text) Deneb has an Absolute visual magnitude of -8.73 (this is about the same brightness as the quarter moon---but at 32.6 light years away!) Using the weird equation, the distance to deneb can be calculated: 2500 light years (M – m = 5 – 5log(d)) One last obvious question: How did we ever know the Absolute visual magnitude to Deneb without knowing its distance in the first place?

18 16.3 Classifying Stars Hypothysis: Hypothysis: the Luminoisty and Abolsute Magnitude of Stars can be known if we know their Classification. the Luminoisty and Abolsute Magnitude of Stars can be known if we know their Classification. There Classification is completely revealed in their Spectra. There Classification is completely revealed in their Spectra. Comparing the spectra of nearby stars allows us to test this hypothis. Comparing the spectra of nearby stars allows us to test this hypothis. The Classification is known as Spectral Type The Classification is known as Spectral Type

19 Spectral type is revealed in the Colors of Stars Stars come in many different colors. The color tells us the stars temperature according to Wiens Law. Bluer means hotter!

20 Spectral Type Classification System O B A F G K M Oh Be A Fine Girl/Guy, Kiss Me! 50,000 K 3,000 K Temperature (L)

21 Spectral Types of Stars

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23 Spectral types are defined by the: Spectral types are defined by the: existence of absorption lines belonging to various elements, ions, & molecules in a stars spectrum existence of absorption lines belonging to various elements, ions, & molecules in a stars spectrum the relative strengths of these line the relative strengths of these line However, spectral type is not determined by a stars composition. However, spectral type is not determined by a stars composition. all stars are made primarily of Hydrogen & Helium all stars are made primarily of Hydrogen & Helium Spectral type is determined by a stars surface temperature. Spectral type is determined by a stars surface temperature. temperature dictates the energy states of electrons in atoms temperature dictates the energy states of electrons in atoms temperature dictates the types of ions or molecules which exist temperature dictates the types of ions or molecules which exist this, in turn, determines the number and relative strengths of absorption lines in the stars spectrum this, in turn, determines the number and relative strengths of absorption lines in the stars spectrum this fact was discovered by Cecilia Payne-Gaposchkin in 1925 this fact was discovered by Cecilia Payne-Gaposchkin in 1925

24 Spectroscopic parllax Summary: Summary: If we know distance (d) from parallax measurements and.. If we know distance (d) from parallax measurements and.. If we know apparent visual magnitude (m) from photometry or image size (apparent brightness is also measured in this way). If we know apparent visual magnitude (m) from photometry or image size (apparent brightness is also measured in this way). Then we can calculate Absolute visual magniutde (M). Luminosity is also measured in this way. Then we can calculate Absolute visual magniutde (M). Luminosity is also measured in this way. We can obtain spectra and spectral type for all these nearby stars (about 10,000!) We can obtain spectra and spectral type for all these nearby stars (about 10,000!) We can make a table that provides Absolute visual magnitude for stars given their spectral type. We can make a table that provides Absolute visual magnitude for stars given their spectral type. With this table, we can find the distance to distant stars simply by obtaining their spectra and apparent visual magnitude. With this table, we can find the distance to distant stars simply by obtaining their spectra and apparent visual magnitude. In a strange way, we have extended the parallax measurements out way beyond the one hundred parsec limit! In a strange way, we have extended the parallax measurements out way beyond the one hundred parsec limit!


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