UniverseUniverseEighth EditionEighth Edition

UniverseUniverseEighth EditionEighth Edition

Roger A. Freedman • William J. Kaufmann III

CHAPTER 17CHAPTER 17The Nature of StarsThe Nature of Stars

CHAPTER 17CHAPTER 17The Nature of StarsThe Nature of Stars

M 39 is an Open or Galactic Cluster

The The distancedistance (d) to a star can be determined from a (d) to a star can be determined from a measurement of the star’s measurement of the star’s parallaxparallax (p). (p).

Review of Previously Covered Concepts

Stellar ParallaxAs Earth moves from one side of the Sun to the other, a nearby star will seem to change its position relative to the distant background stars.

d = 1 / pd = distance to nearby star in parsecs

p = parallax angle of that star in arcseconds

Some Nearby StarsSome Nearby Stars

Proxima Centauri: p = 0.772 arcsec, d = 1/p = 1.3 pc

Barnard’s Star: p = 0.545 arcsec, d = 1/p = 1.83 pc

Sirius A/B : p = 0.379 arcsec, d = 1/p = 2.64 pc

1 pc = 206,265 AU = 3.26 LY

The The distancedistance (d) to a star can be determined from a (d) to a star can be determined from a measurement of the star’s measurement of the star’s parallaxparallax (p). (p).

The “intrinsic brightness” or The “intrinsic brightness” or luminosityluminosity (L) of a star can (L) of a star can be determined from a measurement of the star’s be determined from a measurement of the star’s apparent brightness apparent brightness (b) and a knowledge of the (b) and a knowledge of the star’s distance.star’s distance.

Review of Previously Covered Concepts

If a star’s distance is known, its luminosity If a star’s distance is known, its luminosity can be determined from its brightness.can be determined from its brightness.

As you get farther and As you get farther and farther away from a star, farther away from a star, it appears to get dimmer.it appears to get dimmer.

Luminosity, Luminosity, LL, doesn’t , doesn’t changechange

Apparent brightness, Apparent brightness, bb, , does change following does change following the inverse square law for the inverse square law for distance.distance.

bb = = LL / (4 / (4dd22))

A star’s luminosity can be determined from A star’s luminosity can be determined from its apparent brightness if its distance is its apparent brightness if its distance is known:known:

LL = 4 = 4 d d 22 bb

LL = 4 = 4 d d22 b b

LL/L/L = ( = (dd/d/d))22 ( (bb/b/b) )

Where LWhere L = the Sun’s luminosity = the Sun’s luminosity

If a star’s distance is known, its luminosity If a star’s distance is known, its luminosity can be determined from its brightness.can be determined from its brightness.

Example: The SunExample: The Sun

d = 1 AU = 1.51011 m

b = 1370 W/m2 (Solar Constant)

L = 4 d2 b = 1.256 101 2.251022 m2 1.37103 W/m2

L = 3.871026 W

Example: Example: Eridani Eridani

d = 3.22 pc = 3.22206,265 AU = 6.65105 AU

b = 6.7310-13 b

L/L = (6.65105)2 6.7310-13 = 0.3 Eri has a luminosity equal to30% of the solar luminosity.

The The distancedistance (d) to a star can be determined from a (d) to a star can be determined from a measurement of the star’s measurement of the star’s parallaxparallax (p). (p).

The “intrinsic brightness” or The “intrinsic brightness” or luminosityluminosity (L) of a star can (L) of a star can be determined from a measurement of the star’s be determined from a measurement of the star’s apparent brightness apparent brightness (b) and a knowledge of the (b) and a knowledge of the star’s distance.star’s distance.

The The surface temperature surface temperature (T) of a star can be (T) of a star can be determined from a measurement of the star’s determined from a measurement of the star’s colorcolor (or spectral type).(or spectral type).

Review of Previously Covered Concepts

17-7 How H-R diagrams summarize our knowledge of 17-7 How H-R diagrams summarize our knowledge of the starsthe stars

17-6 How stars come in a wide variety of sizes17-6 How stars come in a wide variety of sizes17-8 How we can deduce a star’s size from its spectrum17-8 How we can deduce a star’s size from its spectrum

Today we will learn

Let’s pause to examine the spread of “L” and “T” valuesamong the stars that arenearest to us (Appendix 4).

Plot “L Plot “L vsvs. T” for 27 Nearest Stars. T” for 27 Nearest Stars

Data drawn from Appendix 4 of the textbook.

L and T appear to be CorrelatedL and T appear to be Correlated

Nearest Stars

L and T appear to be CorrelatedL and T appear to be Correlated

A few of the brighteststars in the night sky

Hertzsprung-Russell (H-R) DiagramHertzsprung-Russell (H-R) Diagram

Hertzsprung-Russell (H-R) DiagramHertzsprung-Russell (H-R) Diagram

“main sequence”

More complete mapping of stars More complete mapping of stars onto the H-R Diagramonto the H-R Diagram

Stefan-Boltzmann lawStefan-Boltzmann law relates a star’s energy relates a star’s energy output, called Loutput, called LUMINOSITYUMINOSITY, to its temperature , to its temperature and size.and size.

LLUMINOSITYUMINOSITY = 4 = 4RR22TT44

LLUMINOSITYUMINOSITY is measured in joules per square meter of a is measured in joules per square meter of a surface per second and surface per second and = 5.67 X 10 = 5.67 X 10-8-8 W m W m-2-2 K K-4-4

Small stars will have low luminosities unless Small stars will have low luminosities unless they are very hot.they are very hot.

Stars with low surface temperatures must be Stars with low surface temperatures must be very large in order to have large luminosities.very large in order to have large luminosities.

Stars come in a wide variety of sizesStars come in a wide variety of sizes

Determining the Sizes of Stars from an H-R Determining the Sizes of Stars from an H-R DiagramDiagram

Main sequence stars are Main sequence stars are found in a band from the found in a band from the upper left to the lower upper left to the lower right.right.

Giant and supergiant Giant and supergiant stars are found in the stars are found in the upper right corner.upper right corner.

Tiny white dwarf stars are Tiny white dwarf stars are found in the lower left found in the lower left corner of the HR diagram.corner of the HR diagram.

Hertzsprung-Russell (H-R) diagrams Hertzsprung-Russell (H-R) diagrams reveal the different kinds of stars.reveal the different kinds of stars.

Main sequence starsMain sequence stars Stars in hydrostatic equilibrium Stars in hydrostatic equilibrium

found on a line from the upper found on a line from the upper left to the lower right.left to the lower right.

Hotter is brighterHotter is brighter Cooler is dimmerCooler is dimmer Red Dwarfs (on MS) & Brown Red Dwarfs (on MS) & Brown

Dwarfs (Dwarfs (notnot on MS): lower right on MS): lower right corner (small, dim, and cool)corner (small, dim, and cool)

Red giant starsRed giant stars Upper right hand corner (big, Upper right hand corner (big,

bright, and cool)bright, and cool) White dwarf starsWhite dwarf stars

Lower left hand corner (small, Lower left hand corner (small, dim, and hot)dim, and hot)

Details of a star’s spectrum reveal whether it is a giant, a white dwarf, or a main-sequence star.

Both of these stars are spectral class B8. However, star a is a luminous super giant and star b is a typical main-sequence star. Notice how the hydrogen absorption lines for the more luminous

stars are narrower.

LUMINOSITY CLASS

Based on the width of spectral lines, it is possible to tell whether the star is a supergiant, a giant, a main sequence star or a white dwarf.

These define the luminosity classes shown on the left occupying distinct regions on the HR diagram.

The complete spectral type of the Sun is G2 V. The “G2” part tells us Teff, the “V” part tells us to which sequence or luminosity class the star belongs.

Example: M5 III is a red giant with Teff ~ 3500K, M=0 (or L=100 Lsun).

HR Diagram

This template will be used in the upcoming test. Please become familiar with it. We will do a few examples in class of how to read off the temperature, luminosity and size of a star given a full spectral type.

HR Diagram

I expect you to know which of the gray sequences is which luminosity class. From top to bottom: Ia, luminous supergiants Ib, supergiantsIII, giantsV, main sequence

Examples:

G2V The Sun

M5III

B4Ib

M5Ia