Characteristics of Stars

Chapter 3 Section 4ISN p. 33

What are stars?• The objects that heat and light the planets in a system• A star is a ball of plasma and gas held together by its own gravity;

they are much larger and hotter than planets due to nuclear fusion– Nuclear reactions occur in stars (H He)– Energy from the nuclear reactions is released as electromagnetic

radiation• The basic properties of stars include color, size, temperature, and

brightness

Classifying Stars: Temperature and Color

– The color of a star indicates the temperature of the star• Decreasing temperature =

bright to dim• Class Types: • O, B, A, F, G, K, M [Oh Be A

Fine Girl, Kiss Me ]

Class Temperature Color

O 20,000- 60,000 K

Blue

B 10,000 – 30,000 K

Blue-white

A 7,500 – 10,000 K

White

F 6,000 – 7,500 K

Yellow-white

G 5,000 – 6,000 K

Yellow

K 3,500 – 5,000 K

Orange

M 2,000 – 3,500 K

Red

Spectra• A spectrum is visible light arranged according to wavelengths (Roy

G. Biv) • If we look at visible light with a spectrograph we would see emission

lines.• Just as people have a unique fingerprint, each element has unique

spectral lines• Astronomers use spectrographs to determine the elements found in

stars. • Spectrograph- a device that breaks light into colors and produces

and image of the resulting spectrum (Most large telescopes have spectrographs)

Chemical CompositionThe lines on the spectrums below are from four different elements. Which elements have the strongest lines in Stars A, B, and C?

How do we know what stars are made of?Spectra Practice

Classifying Stars: Brightness• Luminosity is the

energy output from the surface of a star per second.

– The brightness we observe for a star depends on both its luminosity and its distance.

– Luminosity is measured in units of energy emitted per second, or watts.

– The Sun’s luminosity is about 3.85 × 1026 W.

Classifying Stars: Magnitude• 1 of the most basic properties of a star is how bright it appears. • Apparent brightness is based on the ancient Greek system of

classification which rates how bright a star appears to be.

• Apparent magnitude does not actually indicate how bright a star is, because it does not take distance into account.

• Absolute brightness is the brightness an object would have if it were placed at a distance of 10 pc.

H-R Diagram

– An H-R diagram shows the relationship between mass, luminosity, temp., and diameter of stars.

– Main sequence stars are the big group of stars that form a band from the upper left to the lower right.

– 90% of stars are in the main sequence.

Draw this on ISN p. 32

Groups of Stars• Binary Stars- 2 stars

gravitationally bound that orbit the same center of mass

• Star Clusters- larger groupings of stars.

• Constellations- the 88 groups of stars named after animals, mythological characters, or everyday objects. – Circumpolar constellations

can be seen all year long as they appear to move around the north or south pole.

– Summer, fall, winter, and spring constellations can be seen only at certain times of the year because of Earth’s changing position in its orbit around the Sun.