Characteristics of Stars

Section 27.1

Definition of a star

• It’s a body of gases that gives off lots of energy in the form of light and heat

• They vary in size and mass, some are smaller that 20 km in diameter or up to a 1000 times the size of our sun!

Variations in composition and temperature

• Using a spectrometer, scientists can determine what type of gases compose a star

• Every chemical element has a characteristic spectrum

Temperature

• The temperature of a star is indicated by its color

• The red ones are the coolest and the blues are the hottest

• Most Temps range between 2800°C to 24,000°C.

Star Temperatures

Motion

• Actual motion can only be measured by high powered telescopes since stars are so far away

• Apparent motion of stars is what we see and is only due to the motion of the earth

Actual Motion of Stars

• They rotate on an axis, they revolve around other stars, and they move towards or away from earth

• Don’t forget the red shift

Distance to the Stars

• It’s measured in light years- the distance that light travels in one year ( about 9.5 trillion km)

• Light from the sun takes about 8 minutes to reach earth

Parallax

• Scientists can use this to measures stars within 1000 light years of accuracy by measuring the angle between a stars’ apparent position over a period of 6 months

• The closer the star, the larger the angle

Stellar Magnitudes

• The visibility of a star depends on its brightness and distance from earth

• Over 3 billion stars can be seen through a telescope, but only 6000 can be seen with the naked eye

• The 2 scales that scientists use to measure brightness are apparent magnitude and absolute magnitude

Apparent Magnitude

• The brightness of a star as is appears from earth

• The most powerful telescopes can detect stars with an apparent magnitude of about 29

• Those stars are 1.5 billion times fainter than those seem with a naked eye

Apparent magnitude

Absolute Magnitude

• How bright a star would be if it were 32.6 light years away

• Scientists use this distance because it’s equal to one parsec

Classification of stars

• Main-sequence stars- stars that are visible in the night sky, this includes the sun

• Super giants – are bright because they have so much surface area to project light

• White dwarfs- these stars are hot and dim because of their size