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Characteristics of Stars
Notes
What is a spectrograph?
•Breaks up the light from an object
into a spectrum
•A spectrograph is a photograph of the
resulting spectrum
How do astronomers use spectrographs?
•To gather information about a
star
•Ex: chemical composition and
temperature
What is a galaxy?
• A cluster of stars
• Can contains hundreds of billions of stars
• We live in the Milky Way galaxy
• Distance across the Milky Way = 250 million billion
kilometers
• Even if you could travel at the speed of light it would still
take you 25,000 years to cross the galaxy
The Universe:
•Defined as space and everything
in it
•Contains billions of galaxies
Distances to the Stars:
•Distances are so large that kilometers
are not very practical
•Instead astronomers use a unit called
a light-year
What is a light-year?
• The distance that light travels in one year
• It travels about 9.5 million million km
• Travels at a speed of 300,000 km per
second
Parallax:
• The apparent change in position of an object
when you look at it from different places
• One of the ways astronomers measure the
distance between stars
How are stars classified?
1. By size
2. By color and temperature
3. By brightness
1. By Size
• Stars come in all different sizes
• Our sun is a medium sized star
• Betelgeuse is a supergiant star and would
fill our solar system all the way to Jupiter
2. By color and temperature
• A star’s color reveals its temperature
• The more white/blue a star is the hotter it is
• The more red a star is the cooler it is
• Ex: Rigel (blue-white star) ~ 15,000 degrees Celsius
• Ex: Betelgeuse (reddish star) ~ 5,500 degrees Celsius
3. Brightness of Stars
• Brightness: the amount of light a star gives off
• Depends on size and temperature
• How bright a star looks from Earth depends on how
far away it is
• Brightness is described in 2 different ways
Brightness: Apparent Magnitude
• Brightness of a star as seen from Earth
• This is not the most accurate method since the
closer a star is to Earth the brighter it will look
• Ex: the closer you are to a flashlight the
brighter the light looks
Brightness: Absolute Magnitude
• The brightness a star would have if it was a standard
distance from Earth
• This requires an astronomer to determine both the
apparent magnitude and distance from Earth
• Ex: calculating absolute magnitude by making all
stars 1 light-year away
Hertzsprung-Russell Diagram:
• Shows the relationship between surface
temperature and brightness
• There are many different versions of the diagram
but they all contain the same information
• Sometimes abbreviated as an “HR Diagram”
An Example of an HR Diagram:
Left-side Activity:
• Answer questions 1-4 on page 289
• Answer the following questions
using the HR diagram on page 289
1. Which star is hotter: Rigel or
Aldebaran?
2. Which star is brighter: Sirius B or
Betelgeuse?
3. Which star is a main sequence
star: Polaris or the sun?