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Week 9 Week 9

Week 9 - PowerPoint Presentation

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Week 9 - PPT Presentation

9 th grade science Unit 4 Measuring Stars Create a Unit cover page for unit 4 You will copy all of the unit 4 objectives on this page Create a Unit 4 WWK page Create a Unit 4 Must do page ID: 620838

star stars apparent magnitude stars star magnitude apparent sun temperature luminosity unit brightness sirius radius page dwarf earth classification

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Slide1

Week 9

9

th

grade scienceSlide2

Unit 4 – Measuring Stars

Create a Unit cover page for unit 4. (You will copy all of the unit 4 objectives on this page)

Create a Unit 4 WWK page

Create a Unit 4 Must do pageSlide3

Objectives (write on cover page)

Explain how stellar distances are determined.

Explain how physical laws are used to estimate stellar sizes.

Distinguish between luminosity and brightness and explain how stellar luminosity is determined.

Explain how stars are classified according to their colors, surface temperatures and spectral characteristics, and tell why such a classification is useful.

State how an H-R diagram is constructed, and summarize the properties of the different types of stars that such a diagram helps us to identify. Slide4

WWK

Stellar Parallax – the apparent motion of a relatively close object with respect to a more distant background as the location of the observer changes.

Parsec – distance at which a star must lie in order for its measured parallax to be exactly 1 arc second. 1 parsec equals 206,000 A.U.Slide5

Parallactic

angle is the parallax

Measured in

arcseconds

.

1° = 3600

arcsecondsSlide6
Slide7

Must Do

Please write a statement about what this diagram is trying to express.

Be ready to discuss.Slide8

WWK

Giants – star with a radius between 10 and 100 times that of the Sun

Supergiants

– A star with a radius between 100 and 1000 times that of the Sun

Red giant – A giant star whose surface temperature is relatively low, so that it glows red.

Dwarf – Any star with radius comparable to, or smaller than, that of the Sun (including the Sun itself)

White dwarf – A dwarf star with sufficiently high surface temperature that it glows whiteSlide9

Stellar Sizes

Measured directly - using geometry and knowing how far away the star is. Astronomers have measured a few dozen stars this way.

Most stars are too far away for this to work

Measured indirectly – luminosity and temperature gives us an indirect measurement as to the size of the star.

Luminosity is proportional to the radius squared times the temperature to the fourth power.

Called the radius-luminosity-temperature relationship.Slide10

SWK – Sirius (dog star)

Sirius is a two star system 8.6 light years from Earth.

It consists of the main sequence star Sirius A and its small white dwarf companion Sirius B.

White dwarfs are the core remains of stars that have exhausted their fuel and shed their outer layers.

Sirius B is the closest white dwarf star to Earth.

The force of gravity on Sirius B is 350,000 stronger than on Earth, meaning 3 grams of matter (roughly a sugar cube) would weigh 1,000 kilos (2,200 pounds)!

Sirius

is the brightest star in the night sky and the nearest that can be seen without the aid of a telescope

.

Radius and luminosity

A

r

=71%

larger than our Sun

A

l

= 25 x the sun

B

r

= smaller than earth (but more dense)

B

l

= 3% the sunSlide11

Luminosity and Apparent Brightness

Absolute brightness

– An intrinsic property, luminosity

Apparent brightness

– How much energy is striking a light detector per unit of time

Energy produced by the star as seen from earth

Uses the

magnitude scale - Slide12

Ancient Magnitude Scale

2

nd

century BC astronomer Hipparchus (6 groups)

Brighter stars were ranked first magnitude and fainter stars were classified 6

th

magnitude.

1-6 classification spans a factor of 100 in apparent brightness.

Each magnitude is a difference of 2.5 in apparent brightness. 1st magnitude stars are 2.5 times brighter than 2

nd

magnitude stars.Slide13

Modern M

agnitude

S

cale

A change in 5 magnitude represents a factor of 100 in apparent brightness.

Now called apparent magnitudes

No longer limited to whole numbers

4 - 4.5 - 5

1-6 barely even covers it

Hubble can see 30 magnitude stars

The sun is a -26.8 magnitude starSlide14

Mathematical R

elationship

Knowing a stars apparent magnitude and its distance allows us to compute its absolute magnitude

Alternatively, knowing a star’s apparent and absolute magnitude allows us to determine its distance.Slide15

Temperature and Color

Color index – ratio of its B (blue) to V (visible) intensitiesSlide16

Classification of Stars

Color and temperature can classify stars well enough but SPECTROSCOPY gives us spectral-line radiation which is a much more detailed classification theme.

The composition of these stars are the same the difference in absorption spectra is temperature.

Why do the hotter stars have less absorption lines? Slide17

Detailed Spectra

20,000 K and up show strong ionized Helium because

it takes a lot of energy to excite tightly bound atoms.

Hydrogen is very weak in these hot stars because

It is ionized so few hydrogen atoms have electrons

In cooler stars absorption lines are caused by molecules that are still able to maintain their bonds.Slide18

Spectral Classification

Between 1880 and 1920 stars were classified by their spectral analysis even before they knew how atoms worked.

They were categorized by the intensity of the hydrogen lines in a A, B, C, D, E

…P

Then it was realized that they could be organized by temperature so the new schema was born.

O

h

B

e

A

F

ine

G

uy,

K

iss

M

e.”Slide19

Our Sun is a G2 Star

(each letter is split into 10 subdivisions)