04/20/2304/20/23

Stars CharacteristicsStars Characteristics

Classifying Stars• Color

• Temperature

• Size

• Composition/Mass

• Brightness

04/20/23

Star Mass and Composition

• Mass and composition determine most of the properties of a star.

• The more massive a star is, the greater the gravity, the hotter and denser a star must be.

04/20/23

Star Size

• The diameters of stars range from as little as 1/10 the Sun’s diameter to hundreds of times larger.

• The mass of stars can be from less than 1/100 to 20 or more times that of our Sun.

• Stars over 50 times the Sun’s mass are extremely rare.

• Our Sun is a medium sized star.

04/20/23

Size ComparisonSize

ClassificationApproximate

Size (km) Examples

Neutron Star 16 km

Less than the distance to

school

pulsar

White Dwarf Less than the distance across

Asia

Sirius B

Van Maanen’s star

Medium star 109 X size of Earth or

1,392,000 km

Sun

Giant star 100 X size of Sun

Aldebaran

Supergiant 1000 X size of Sun

Rigel, Betelgeuse, Arcturus

04/20/23

Star Energy

• The enormous pressure and heat in a star’s core converts matter into energy.

• Stars consist of controlled atomic reactions called nuclear fusion in which hydrogen (nuclei) atoms fuse to form helium (nuclei) atoms.

• During each step of the process, mass is lost and energy is released.

04/20/23

Star Temperature

• Stars in the sky show tinges of different colors which reveal the stars’ temperatures.

• Blue stars shine with the hottest temperatures and red stars shine with the coolest temperatures.

• Our Sun is a yellow star having a surface temperature of about 5,500ºC

Stop and Think

• The hottest flame color in a campfire is?

Color and TemperatureStar Color Surface Temp (oC)

Blue or Blue-White 35,000 oC

White 10,000 oC

Yellow 6,000 oC

Red-Orange 5,000 oC

Red 3,000 oC

Chemical Composition

• Most stars are made of –73% Hydrogen–25% Helium–2% Other elements

Chemical Composition• Astronomers view stars with a spectroscope

that splits the light (much like a prisme) to determine elements found in stars

Spectrograph (line spectrum)

• Spectrograh (line spectrum) is created. It is an an image of coloured lines separated by bands of darkness that is like a bar code of finger print unique to every element on earth.

Other Elements

Brightness of Stars• The brightness of a star depends upon both its size

and temperature• How bright a star looks from Earth depends on

both its distance from Earth and how bright the star truly is

04/20/23

Stop and Think

If person A has a pen flashlight and If person A has a pen flashlight and person B has a mega flashlight and both person B has a mega flashlight and both were standing at the front of the were standing at the front of the classroom, which light would appear classroom, which light would appear brighter?brighter?

If person A (pen light) stood in the classroom and person B If person A (pen light) stood in the classroom and person B (mega light) stood at the baseball fields across the street, (mega light) stood at the baseball fields across the street, which light would appear brighter?which light would appear brighter?

You cannot tell by looking in the sky how bright a star You cannot tell by looking in the sky how bright a star truly is. The farther away the star is, the less bright ittruly is. The farther away the star is, the less bright it

will appear.will appear.

04/20/23

Apparent Magnitude

The brightness of a star as it appears from Earth with the naked eye.

As you can see, the magnitude numbers are bigger for faint stars, and magnitudes are negative for very bright stars.

04/20/23

Absolute Magnitude

The true brightness of a star if all stars were at a uniform distance from Earth

The absolute magnitude of stars is reported in comparison to our Sun.

Ex. AM of Sun = 1

AM < 1 : brighter than Sun

AM > 1 : less bright than Sun

Brightness of Stars

• Apparent magnitude- The brightness of a star as it appears on Earth

• Absolute magnitude- The amount of light a star actually gives off (uses formulas)

04/20/23

Star MassPosition on H-R depends on mass

• The larger the amount of mass in a star, the more luminous it is.

• Giants: more mass, more luminous

• Dwarfs: less mass, less luminous

Why do stars twinkle?

• The scientific name for the twinkling of stars is stellar scintillation

• Stars twinkle when we see them from the Earth's surface because we are viewing them through thick layers of turbulent (moving) air in the Earth's atmosphere.

Measuring Distances to Stars

• Astronomers use a unit called the light year to measure distances between the stars

• Light travels at a speed of 300,000 km/s

• Light year- distance that light travels in one year =9.5 trillion km

• Light year=unit of distance

Parallax• Astronomers use parallax to measure

distances to nearby stars

• Parallax= the apparent change in position of an object when viewed from two locations

Parallax is larger for closer objects