Agenda

Review EM radiation How far away are stars? What can starlight tell us?

NASA

Sunlight carefully separated by wavelength

Absorption and emission

U. Tennessee

Blackbody spectrum

Emission spectrum(glowing material)

Absorption spectrum

Consider the following photo

Compare and contrast the stars in the photo

NASA

What if all stars had the same luminosity?

• Luminosity means total power output as light.

• Determining their distance from us would be easy if they all were the same luminosity!

Given only 25 Watt bulbs

The brightness depends only on their distance from us

Stars aren’t all 25 Watt bulbs!

A star’s distance can be determined: If we know how luminous it really is Compared to how bright it looks to us

Apparent Magnitude scale

Classifies objects by how bright they appear in the sky

Larger number means less bright 1 magnitude = difference in brightness

by a factor of 2.5

Apparent Magnitude scale

-25 -20 -15 -10 -5 0 5 10 15 20 25

SunNaked eye

Polaris

Faintest objectWith binoculars

Venus

Sirius

Full moon

Faintest object4 m telescope

Absolute magnitude

Luminosity = how much light a star is giving off (like wattage for light bulbs) Compared to the Sun (Lsun)

Absolute magnitude = how bright (what magnitude) a star would appear at 10 parsecs (32.6 light years)

Example: Antares versus Sirius

Apparent magnitudes: Antares -4, Sirius -1.5

Absolute magnitudes: Antares +2, Sirius +1.4

Which appears brighter in the sky?A. AntaresB. SiriusC. sameD. Cannot conclude

Example: Antares versus Sirius

Apparent magnitudes: Antares -4, Sirius -1.5

Absolute magnitudes: Antares +2, Sirius +1.4

Which is more luminous?A. AntaresB. SiriusC. sameD. Cannot conclude

So how do we determine

the distance to other stars?

Compare absolute and apparent magnitudes

• If we know the luminosity (or absolute magnitude) of a star, we can find its distance.

• A star of known luminosity is called a standard candle.

• More on this later...

So how do we determine

the distance to other stars?

There is another, more direct way...

Triangulation (parallax)Measure the two angles (a, b) to find how wide the Missouri river is!

Trigonometric parallax

KnownDistance(2 AU)

Sun

Earth

Some star

Unknown distance

Earth

p

What makes parallax measurement difficult?

A. It’s hard to read your protractor when it is dark outside.

B. The parallax angle is very small because the stars are so far away.

C. We can’t see any of the same stars six months apart.

D. It actually is not difficult!

Parsec (a unit of distance)

If p = 1 arcsecond

1 AU

This distance is 1 parsec (3.26 light years)

1 arcsecond = 1/60 arcminute = 1/3600 degree

Unfortunately, there are no stars this close to us.

Limits with this method

p

KnownDistance(1 AU)

Beyond 300 light years, p is too small for us to measure.(Our galaxy is about 100,000 light years across)

Sun

Earth

Some star

Unknown distance

What can we learn by looking at starlight?

Distance to star Ingredients Temperature

Exam one is Tuesday at 6 pm

Tuesday – optional review during class Individual portion

Multiple choice 2 essay questions

Group portion Multiple choice

Grab a review sheet! Challenge: write one exam question

(multiple choice), and email it to me!

Team activity # 4

Trigonometric parallax

Include last names and team number on all activities!

Part I: Do NOT place any measuring device beyond the line!! You must estimate the angles!

Part II: You may want to go into the hallway.

Grab your graded activities on the way out!

Nearby stars appear to move

(Not apparent to the naked eye)

Wikipedia