25.1 Properties of Stars

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25.1 Properties of Stars
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25.1 Properties of Stars
? Star Color and Temperature
Color is a clue to a stars temperature.

• A constellation is an apparent group of stars
  originally named for mythical characters. The sky
  contains 88 constellations.

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The Constellation Orion
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Constellations of the Winter Sky
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25.1 Properties of Stars
? Binary Stars and Stellar Mass
A binary star is one of two stars revolving
around a common center of mass under their mutual
gravitational attraction.
Binary stars are used to determine the star
property most difficult to calculateits mass.
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Common Center of Mass

1. For stars of equal mass, the center of mass lies
   in the middle.
2. A star twice as massive as its partner is twice
   as close to the center of mass. It therefore has
   a smaller orbit than its less massive partner.

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25.1 Properties of Stars
? Parallax
Parallax is the slight shifting of the
apparent position of a star due to the orbital
motion of Earth.
The nearest stars have the largest parallax
angles, while those of distant stars are too
small to measure.
To us, the clock reads as if it is 12 after the
hour, but to an observer in line with the light
that is responsible for the minute hands shadow,
the clock would appear as if it is 14 past the
hour.
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Parallax
The parallax angle shown here is exaggerated to
illustrate the principle. Because the distances
to even the nearest stars are huge, astronomers
work with very small angles.
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Light Year

• Light-Year

• Due to the tremendous distances between stars,
  measuring distances in kilometers or astronomical
  units just isnt practical.
• A light-year is the distance light travels in a
  year about 9.5 trillion kilometers.

Name Distance (light-years)
Sun -
Alpha Centauri A 4.3
Alpha Centauri B 4.3
Alpha Centauri C 4.2
Barnard's Star 6.0
Wolf 359 7.7
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25.1 Properties of Stars
? Apparent Magnitude
Apparent magnitude is the brightness of a star
when viewed from Earth.
Three factors control the apparent brightness
of a star as seen from Earth how big it is, how
hot it is, and how far away it is.
? Absolute Magnitude
Absolute magnitude is the apparent brightness
of a star if it were viewed from a distance of
32.6 light-years.
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Distance, Apparent Magnitude, and Absolute
Magnitude of Some Stars
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25.1 Properties of Stars
? A HertzsprungRussell diagram shows the
relationship between the absolute magnitude and
temperature of stars.
? A main-sequence star is a star that falls into
the main sequence category on the HR diagram.
This category contains the majority of stars and
runs diagonally from the upper left to the lower
right on the HR diagram.
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HertzsprungRussell Diagram
In this idealized chart, stars are plotted
according to their temperature and their absolute
magnitude.
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25.1 Properties of Stars
? A red giant is a large, cool star of high
luminosity it occupies the upper-right portion
of the HR diagram.
? A supergiant is a very large, very bright red
giant star.
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25.1 Properties of Stars
? Variable Stars

• A Cepheid variable is a star whose brightness
  varies periodically because it expands and
  contracts it is a type of pulsating star.
• A nova is a star that explosively increases in
  brightness.

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Images of a Nova Taken Two Months Apart
These two photographs show the decrease in
brightness that follows a nova flare-up
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25.1 Properties of Stars
? Interstellar Matter
A nebula is a cloud of gas and/or dust in
space.
There are two major types of nebulae
1. Bright nebula
- Emission nebula
- Reflection nebula
2. Dark nebula
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Interstellar Matter
The famous Horsehead Nebula found in the
constellation Orion.
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25.1 Properties of Stars

1. How is the wavelength of light emitted from a
   star related to its temperature?
2. What is parallax and how do scientists use it to
   calculate distance?
3. What unit of length do astronomers use to measure
   vast distances?
4. Compare absolute magnitude to apparent magnitude.
5. What does a Hurtzsprung-Russell diagram compare?
6. Name two types of variable stars.