Properties of Stars

• Star Color – the color of a star is a clue to its temperature

a. The coolest stars are red

b. The medium stars are yellow

c. The hottest stars are blue

Star Characteristics• The color of the star can be a clue to its temperature.

– The hotter the star the shorter the wavelength the star emits.

Measuring Distance to Stars

• Parallax – using the location of a star at two different times to calculate its distance from Earth– The nearest stars have the

largest parallax (seem to move the most) and the farthest stars have the smallest parallax (seem to move the least)

The closer the star the more it “moves” in the

sky!

Measuring Distance to Stars

• Light-year – the distance light travels in one year (9.5 trillion kilometers)

• We use this when determining astronomical distances since miles and kilometers would be way too small of a number

• The next closest star after our sun, Proxima Centauri, is 4.3 light years away from our sun

Stellar Brightness• Apparent magnitude – measure of how bright a

star appears from Earth• This varies due to the star’s temperature, size, and distance

away from Earth

• Absolute magnitude – measure of how bright a star actually is

• To get a stars absolute magnitude, astronomers have to compare all stars at an equivalent distance

These three stars can have the same absolute magnitude when their sizes, temperatures, and distances from Earth are taken into account

Comparison of Apparent Magnitude and Absolute Magnitude of Stars

Name Distance (light years)

Apparent Magnitude*

Absolute Magnitude*

Sun N/A -26.7 5.0

Alpha Centauri 4.27 0.0 4.4

Sirius 8.70 -1.4 1.5

Arcturus 36 -0.1 -0.3

Betelgeuse 520 0.8 -5.5

Deneb 1600 1.3 -6.9

The more negative, the brighter and the more positive, the dimmer

Astronomers estimate that there are 200-400 billion stars in our Milky Way Galaxy, but we can only see about 2,500 visible to the naked eye on Earth

Life Cycle of a Star1. Stars begin as large clouds of gas and dust called

nebula2. The nebula then contracts and shrinks and its

temperature increases– This process may take ~ 1 million years

3. Now the star is a protostar – still developing , large, red object – It can’t be classified as a star yet because it does not undergo

fusion

1. A star becomes a main-sequence star when fusion begins– Stars at this stage have a delicate balance between

gravity pushing inward and the gas pressure pushing outward

• Stable, main-sequence stars have varying lifespans:– Large, blue stars may only last a few million years– Small, red stars may last hundreds of billions of years

• As a star begins the end of its lifecycle it will become a red giant– Red giants are red in color because they are cooler– Red giants are HUGE in size compared to when they

were a main-sequence star

• When a small to medium sized star dies it becomes a white dwarf– White dwarf – a small, cool star near the end of its life

• When a large star dies it becomes a supernova– Supernova – an exploding star– When the largest stars explode, they can form black

holes

Hertzsprung-Russell Diagram

• H-R Diagram – shows the relationship between the absolute magnitude and temperature of stars