The Star

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The Stars:

Characteristics, Magnitude, Stellar

Classifications, H-R Diagram, Motions and

Significance

Laver Joy Lovido

Marjonlien A. Mahusay

Characteristics of stars

• Size

• Color

• Temperature

• Composition

• Brightness

Size of a Star

anywhere from 20km to 1 trillion km in diameter

Main Group by Star

1. Neutron Star

• **Can be the distance you travel to school

2. White Dwarf Star

• **Can be the size of Asia

3. Medium-sized Star

• **Our own Sun

4. Giant Star

• *The diameter 10 to 100X larger than our sun

5. Super-giant Star

• *The diameter 1000X larger than sun

Size of a Star

Color: star can be

Blue, White, Orange

or Yellow, red

Temperature:

Blue – hottest (35000⁰C)

Yellow – our Sun

(5500⁰C)

Red - coolest(3000⁰C)

Color and Temperature

Characteristics of stars

Color: stars can be red, blue, white, orange or yellow

Composition: contain different elements determined using

spectra

• Temperature: Blue is the hottest (35000

Yellow (our sun) (5500

Red is the coolest (3000

Composition

Astronomers use a spectroscope to determine what stars are made of by their spectral lines

• Hydrogen (70% of the total mass of a star )

• Helium (28% of the total mass of a star)

• Trace Elements- O, Ne, C, N and other elements (2% )

Magnitude of Star

1. Apparent magnitude

2. Absolute magnitude

Magnitude of Star

1. Apparent magnitude

is the brightness of a star when viewed from

The lower the number on the scale, the brighter

the star appears to us.

factors :

how big it is

how hot it is

how far away it is.

Analyzing the Apparent Magnitude of

Relating Magnitudes to Brightness Ratio

Magnitude Difference Ratio of Brightness

1 2.512:1

2 2.5122 = 6.31:1

3 2.5123 = 15.85:1

4 2.5124 = 39.8:1

5 2.5125 = 100:1

10 2.51210 = 104:1

EXAMPLE: Suppose stars 1 & 2 have magnitudes of m1=6 m2= 1: the ratio of their brightness is: SOLUTION:

=2.512m1-m2

=2.5126-1 =100.02

Analyzing the Apparent Magnitude

of Stars

• Most powerful telescopes can detect stars with apparent magnitude of +29

• Faintest star seen by eye has apparent magnitude of +6

• This is called a 6th magnitude star

2. Absolute Magnitude

the actual brightness of a star, assuming all stars

were set at a standard distance from Earth.

true brightness

If sun was 32.6 light-years away, it would be

5th magnitude

•So absolute magnitude of sun = +5

•Most stars are between -5 and +15

Magnitude of Star

Analyzing the Absolute Magnitude of

Relating Absolute Magnitude to Luminosity

Absolute Magnitude Approximate Luminosity

in Solar Units

-5 10,000

10 0.01

EXAMPLE:

If Sirius A has Absolute Magnitude of 1.5, how many times that sirius is brightre than the sun?

SOLUTION:

5-1.5=3.5

2.5123.5 = 25

Comparison of the Nearest Stars in terms of

Apparent and Absolute Magnitude

Name Distance (ly) Apparent

Visual

Magnitude

Absolute Visual

Magnitude

Sun -26.8 4.83

Proxima Centauri 4.23 11.09 15.5

Alpha Centauri A 4.35 0.01 4.4

B 4.35 1.34 5.7

Barnard’s Star 5.98 9.55 13.2

Wolf 359 7.80 14.45 16.6

Stellar Classification

1. Color

2. Temperature

2. Size

3. Brightness

Analyzing Starlight

The surface temperature

of a star is indicated by

its color.

Blue stars shine with the

hottest temperatures and

Red stars with the

coolest.

• The luminosity class is added in Roman numerals

after the temperature spectral class.

– Indicates the size of the star.

Stellar Luminosity Classes

Class Description

I Supergiants

II Bright Giants

III Giants

IV Subgiants

V Main-sequence Stars

VI Subdwarfs

VII White Dwarfs

Getting Familiar to the Spectral Sequence

Spectral type- a way of classifying a star by the

lines that appears in its spectrum; it is related

to surface temperature.

Basic spectral types are designated by letters :

OBAFGKM with O for the hottest and M

for the coolest.

subdivided with numbers from 0-9

Spectral classes are as follows:

Oh Be A Fine Girl/Guy, Kiss Me!

Brightest Star

Name Distance

Spectral

Luminosity

Apparent

Magnitude

Absolute

Magnitude

Sirius 8.6 A1 V -1.44 1.4

Canopus 310 F0 Ib/II -0.74 -5.65

Kentaurus

4.4 G2 V -0.01 4.3

Arcturus 36.7 K2 III -0.05 -0.31

Vega 25.3 A0 V 0.03 0.58

Capella 42 G8 III 0.08 -4.8

Hertzsprung-Russsell Diagram

developed by Einar Hertzsprung and Henry

Russell in the early 20th century.

graph that exhibits the intrinsic stellar

properties such as the sizes, colors and

temperatures

Plots the individual stars as points , with stellar

luminosity on the vertical axis and surface

temperature con the horizontal axis

Hertzsprung-Russsell Diagram

Stellar Motion

Two kinds of motion are associated with stars

• 1. APPARENT MOTION

• Earth’s rotation causes the illusion of stars moving around a

central star, Polaris, commonly known as the North Star.

• Earth’s revolution around the Sun causes stars to be visible

during different seasons.

• The apparent change of position of a star on the celestial sphere is

called the proper motion of the star.

• Angular Motion on the Celestial Sphere

• -Proper motion is usually denoted by the Greek symbol "mu", and

is a velocity that is usually quoted in units of seconds of arc per

Stellar Motion

2. ACTUAL MOTION

– First, they move slightly across the sky (only see the

closest ones).

– Second, they may revolve around another star

(binary system).

– Third, they may either move away from or toward

our solar system.

• The apparent shift in the wavelength of light emitted by

a light source moving away from or toward an observer

is called the Doppler Effect.

–Also used for sound waves on Earth.

Stellar Motion

Significance of Stars

Why is it important to learn about the stars?

1. studying stars is important because it helps

tell us how we got all the other elements that

make up things around us (and in us!).

2. To learn from other stars may help us

understand our own Sun, which is also a star.

The Sun only seems different to us because it

is so much closer to us than other stars.

3. we can also learn something about how they

are born and die. This helps us understand

how our own solar system was formed.

4. Stars contain a large fraction of all the visible

mass in galaxies. As a result, their combined

gravitational forces affect the 'dynamics' of

galaxies, i.e. the ways in which galaxies move

and evolve in shape.

The gravitational pull of one particular star, our

Sun, is especially important since it is the Sun's

gravitational attraction that keeps the Earth in

orbit.

-Scott Sandford

-Astrophysics Branch

-NASA/Ames Research Center

The Star

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