Astronomy 1020Spring_2015

Day-25Stellar Astronomy

Course Announcements• Smartworks Chapter 13: Next week

sometime• Apr. 2 – Last day to drop a class.• Midterm grades never made it … IF you have

questions, come and talk to me.• Thursday: 1st Quarter Moon Observing – 7:30pm

• Last one for the semester (still 2 dark nights to go).

• Reports are due Wed. Apr. 22

• Thursday: Hot Topic Movie & Discussion• 6-8pm – E106-B• Topic this time: The Big Bang

Astronomy in the Fall, 2015

ASTR-1010/1011 - Planetary Astronomy + Lab (H,R)ASTR-1020/1021 - Stellar Astronomy + Lab (R)ASTR-2010 - Problems in Planet AstronomyASTR-2011 - Intro. to Observational AstronomyASTR-4000/4001 – Astrophotography & LabASTR-3030/3031 – Methods & Techniques in Astronomy

The parallax (p) of a star is inversely proportional to the distance (d) to a star.

Let p be the parallactic angle in arcseconds.• 1 arcsecond = 1/3,600 of a degree.

Let d be the distance in parsecs.• 1 parsec = 206,205 AU = 3.26 light-years.

Then:

Parsec: distance at which p = 1 arcsecond. Even the closest star to the Sun has a

parallax of only about ¾ arcsecond.

MATH TOOLS 13.1MATH TOOLS 13.1

pd

1

Parallax and Distance Lecture Tutorial pg. 41

Work with a partner!Read the instructions and questions carefully.Discuss the concepts and your answers with

one another. Take time to understand it now!!!!Come to a consensus answer you both agree on

and write complete thoughts into your LT.If you get stuck or are not sure of your answer,

ask another group.

How Bright?We use the quantity “magnitude” to rank an

object’s brightness.Apparent magnitude: How bright the object

appears to the observer.Absolute magnitude: How bright the object

would be at a set distance (10 pc).There is a factor of 2.512 X in brightness

between any 2 magnitudes.Definition = Fifth root of 100 X in brightness.

The magnitude system was developed by Hipparchus in ancient Greece.

Divides stars into categories of brightness (originally 1st through 6th).

The greater the magnitude, the dimmer the star.

Apparent magnitude: the brightness of a star as it appears in the sky from Earth.

Absolute magnitude: the brightness of a star if it were 10 pc from Earth.

CONNECTIONS 13.1CONNECTIONS 13.1

Not many stars are near the Sun. Obtaining distances is essential. Luminosity: total energy radiated by a star

each second. Brightness: rate at which we receive that

energy (depends on observer’s perspective).

Brightness depends on both luminosity and distance.

A dim star could have a low luminosity or be far away.

A bright star could be close or have a high luminosity.

From distance and brightness, we know a star’s luminosity.

Idea: How much light must the star emit to be as bright as it is at its distance?

Luminosity = 4d2 brightness.

Usually the luminosity is expressed as the solar luminosity = 1 L.

The most luminous stars are 106 L. The least luminous are 104 L. More low-luminosity stars than high.

Brightness and DistanceFrom distance and brightness, we know a

star’s luminosity.Idea: how much light must the star emit to

be as bright as it is at its distance?Luminosity = 4d 2 Brightness.Usually the luminosity is expressed as the

solar luminosity = 1 L.

The most luminous stars are 106 L.

The least luminous are 104 L.

Inverse-Square Law of Brightness

Brightness is the amount of light arriving at a particular place.Decreases as the distance from a light source increases.Light obeys an inverse square law

Measuring the color of a star tells us the surface temperature.

We can measure stellar surface temperatures from Wien’s law.

peak is the wavelength at which a star is brightest.

“Hotter means bluer” (the spectrum shifts to shorter wavelengths at higher temperatures).

peak

Knm000,900,2

T

The Stefan-Boltzmann law allows you to estimate the sizes of stars.

The luminosity (L) of a star is related to its temperature (T) and radius (R):

Rearranging, you get:

Called the luminosity-temperature-radius relationship for stars.

MATH TOOLS 13.2MATH TOOLS 13.2

Concept Quiz—Getting Brighter

Suppose a star gets more luminous but does not change its

temperature. What is happening?

A. The star is expanding.

B. The star is contracting.

C. The star is getting more massive.

D. The star is changing its spectral type.

Spectrum: the amount of light emitted as a function of wavelength.

Some light leaving the staris absorbed by atoms or molecules in the star’s atmosphere.

Absorption lines in the spectrum result.

Sometimes emission lines are also seen.

Both are superimposed on a Planck (or continuous) spectrum.