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STARS. Page Keeley Where do Stars Go?. Learning goal: Explain the physical properties of stars. Think About It. When you look at the night time sky, how many stars do you see? Hundreds? Thousands? Millions?

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Page Keeley

Where do Stars Go?

Learning goal:

Explain the physical properties of stars.

Think About It

When you look at the night time sky, how many stars do you see? Hundreds? Thousands? Millions?

  • Although there might be too many stars to count, all stars share similar characteristics? Can you come up with four different groups to characterize the stars?

How are stars classified?

  • Stars are classified according to their physical properties.

  • Color(Temperature)

  • Size

  • Distance from the Earth

  • Brightness – Apparent magnitude

    - Absolute magnitude

Color and Temperature

  • Color reveals a star’s temperature

  • Hot stars appear blue (short wavelength)

  • Cool stars appear red (long wavelength)

  • Other colors in between like orange, yellow and white

Size of Stars (Mass)

  • Many stars come in pairs called “Binary Stars”

  • Binary stars rotate around each other

  • Scientists use this property to calculate mass of stars

Brightness of a star

Apparent magnitude: How bright a star looks/appears from Earth depends on

  • Its size

  • Its temperature

  • Its distance from Earth

    Absolute magnitude: how bright a star actually is.

Distances of Stars from Earth

  • Measured in:

  • Light years

  • Use Parallax angles (apparent change in position of an object when you look at it from opposite sides of Earth’s orbit).

  • The nearest stars have a larger parallax angle

Apparent and Absolute Star’s Magnitude Lab

Star light, Star bright Lab

How is a star born?

  • A star is born when the contracting gas and dust from a nebula become so dense and hot that nuclear fusion starts

  • Nuclear fusion takes place when hydrogen atoms fuse forming helium.

  • What is a nebula?

What determines how long a star will live?

  • A star’s lifespan depends on its mass.

  • Small mass stars live longer because they use their fuel more slowly

  • Large mass stars live shorter

  • A young star has more hydrogen

  • An old star has more helium

  • When a star begins to run out of hydrogen it becomes a red giant or a supergiant.

  • When a star runs out of hydrogen it becomes a white dwarf; a neutron star or a blackhole

Life Cycle of a Star

Life Cycle of a Star

Prentice Hall page 710

H-R Diagram

  • A graph that shows relationship between absolute magnitude and temperature of stars.

  • About 90% of the stars are in the mainsequence stage

  • Hottest main sequence stars are the brightest and the coolest stars are the dimmest

  • Red Giants and Super Giants brightness is because of massive size, not temperature.

  • The Sun is in the main sequence stage

H-R diagram


  • A huge group of stars, dust and gas bound together by gravity.

    What are the main types of galaxies?

  • Spiral galaxies (Solar System is in a spiral galaxy called The Milky Way)

  • Elliptical galaxies

  • Irregular galaxies


  • A group of stars that form a picture

How can scientists determine whether a star is moving toward or away from the Earth?

  • They use Doppler shifts

  • The light from a star that is moving away from Earth appears red (its waves become longer)

  • The light from a star that is moving towards Earth appears blue (its waves become shorter)

The Expanding Universe

  • Most galaxies have Doppler Shifts towards the red end of the spectrum. This shows that the Earth and the source are moving away from each other.

  • The red shift of distant galaxies indicate that the universe is expanding.

The Big Bang Theory

  • States that :-

  • At one time the entire universe was confined to a dense, hot, super massive ball.

  • Then, about 13.7 billion years ago, a violent explosion occurred, hurling this material in all directions

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