11: A Trip Through Our Universe

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Astronomy
2-1 Stars
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Introduction

• Most stars look like faint dots of light in the
  night sky.
• But stars are actually huge, hot, bright spheres
  of gas that are trillions of kilometers away from
  Earth.

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Color of Stars

• Although red and yellow may be thought of as
  warm colors and blue may be thought of as a
  cool color, scientists consider the opposite.
• BLUE HOT
• RED (and Yellow) COOL
• Betelgeuse, which is red, and Rigel, which is
  blue, are the stars that form two corners of the
  constellation Orion.
• Because the stars are two different colors, we
  can conclude that they have different
  temperatures.

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Cool
Hot
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Composition of Stars

• When you look at white light through a glass
  prism, you see a rainbow of colors called a
  spectrum including red, orange, yellow, green,
  blue, indigo, and violet.

Continuous Spectrum
Roy G. Biv
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Composition of Stars

• Astronomers use an instrument called a
  spectrograph to break a stars light into a
  spectrum (discontinuous).
• Because some light has been absorbed by the star,
  the spectrum provides information about the
  stars composition AND temperature.

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Emission Lines
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Most stars are made of
Hydrogen
and
Helium
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Temperature of Stars

• In the 1800s, astronomers started to collect and
  classify the spectra of many stars.
• At first, letters were assigned to each type of
  spectra.
• Stars were classified according to the elements
  of which they were made.

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Brightness of Stars

• With only their eyes to aid them, astronomers
  created a system to classify stars based upon
  their brightness.
• They called the brightest stars in the sky
  first-magnitude stars and the dimmest stars
  sixth-magnitude stars.
• When they began to use telescopes, astronomers
  were able to see many stars that had been too dim
  to see before.
• Rather than replace the old system, of
  magnitudes, they added to it.

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Brightness of Stars

• Positive numbers represent dimmer stars and
  negative numbers represent brighter stars.
• The brightest star in the night sky, Sirius (the
  dog star), has a magnitude of -1.4.

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Brightness of Stars
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Brightness of Stars

• The brightness of a light or star is called
  apparent magnitude.
• Absolute magnitude is the actual brightness of a
  star.

What is the brightest star on Earth?
What factors determine a star's apparent
magnitude?
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Distance to Stars

• A light-year is the distance that light travels
  in one year.
• The closest star to Earth is the SUN
• 93 million miles or 150 million km away
• The next closest star is the Alpha Centauri
  triple star system.
• 4.3 light-years away (Proximi Centauri 4.2?)
• The North Star (Polaris), which is part of Ursa
  Minor is 431 light-years away.

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Distance to Stars

• Astronomers use parallax and simple trigonometry
  to find the actual distance to stars that are
  close to Earth.
• A stars apparent shift in position is called
  parallax.
• This can only be seen through telescopes.

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Motions of Stars

• Daytime and nighttime are caused by the Earths
  rotation.
• The Earths tilt and revolution around the sun
  cause the seasons.
• During each season, the Earth faces a different
  part of the sky at night.
• This is why you see a different set of
  constellations at different times of the year.
• Because stars are so distant, their actual motion
  is hard to see.

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Motions of Stars

• Because of Earths rotation, the sun appears to
  move across the sky.
• Likewise, if you look at the night sky long
  enough, the stars also appear to move.
• All of the stars appear to rotate around Polaris,
  the North Star, which is almost directly above
  the Earths North Pole.
• Because of Earths rotation, all of the stars
  appear to make one complete circle around Polaris
  every 24 hours.

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Apparent Motion
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Astronomy
2-2 Life Cycle of Stars
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