slide1 l.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
How to classify a star and to place it on the H-R diagram correctly?? PowerPoint Presentation
Download Presentation
How to classify a star and to place it on the H-R diagram correctly??

Loading in 2 Seconds...

play fullscreen
1 / 73

How to classify a star and to place it on the H-R diagram correctly?? - PowerPoint PPT Presentation


  • 313 Views
  • Uploaded on

Need to know its luminosity, but it is difficult, because distance is unknown If you can estimate a star’s diameter and/or mass, you can figure out its luminosity Then you can also find the distance to this star. How to classify a star and to place it on the H-R diagram correctly??. 0.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'How to classify a star and to place it on the H-R diagram correctly??' - Sophia


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
how to classify a star and to place it on the h r diagram correctly
Need to know its luminosity, but it is difficult, because distance is unknown

If you can estimate a star’s diameter and/or mass, you can figure out its luminosity

Then you can also find the distance to this star

How to classify a star and to place it on the H-R diagram correctly??
the radii of stars in the hertzsprung russell diagram

0

The Radii of Stars in the Hertzsprung-Russell Diagram

Betelgeuse

Rigel

10,000 times the sun’s radius

Polaris

100 times the sun’s radius

Sun

As large as the sun

is there any spectral signature of giants

How to distinguish between

main-sequence stars and giants?

Is there any spectral signature of giants?

The width of spectral lines!

spectral lines of giants

0

Spectral Lines of Giants

Pressure and density in the atmospheres of giants are lower than in main sequence stars.

=> Absorption lines in spectra of giants and supergiants are narrower than in main sequence stars

=> From the line widths, we can estimate the size and therefore, the luminosity of a star.

Distance estimate (spectroscopic parallax)

luminosity classes
Luminosity Classes

Ia Bright Supergiants

Ia

Ib

Ib Supergiants

II

II Bright Giants

III

III Giants

IV Subgiants

IV

V

V Main-Sequence Stars

luminosity classes8
Ia bright supergiant

Ib Supergiant

II bright giant

III giant

IV subgiant

V main-sequence star

Luminosity classes
example luminosity classes
Example Luminosity Classes
  • Our Sun: G2 star on the Main Sequence:G2V
  • Polaris: G2 star with Supergiant luminosity: G2Ib
slide10

Measuring masses

Mass is the most important parameter.

Knowing masses of stars would allow us to calculate their luminosities, lifetime and all other properties.

But how to measure masses??

binary stars

Measuring masses

Binary Stars

More than 50 % of all stars in our Milky Way are not single stars, but belong to binaries:

Pairs or multiple systems of stars which orbit their common center of mass.

If we can measure and understand their orbital motion, we can estimate the stellarmasses.

the center of mass
The Center of Mass

center of mass = balance point of the system.

Both masses equal => center of mass is in the middle, rA = rB.

The more unequal the masses are, the more it shifts toward the more massive star.

center of mass
Center of Mass

(SLIDESHOW MODE ONLY)

slide14

m1

m2

estimating stellar masses
Estimating Stellar Masses

RecallKepler’s 3rd Law:

Py2 = aAU3

Valid for the Solar system: star with 1 solar mass in the center.

We find almost the same law for binary stars with masses MA and MB different from 1 solar mass:

aAU3

____

MA + MB =

Py2

(MA and MB in units of solar masses)

examples estimating mass
Examples: Estimating Mass

Binary system with period of P = 32 years and separation of a = 16 AU:

163

____

MA + MB = = 4 solar masses.

322

Arbitrary units:

How to measure period and separation?

visual binaries
Visual Binaries

The ideal case:

Both stars can be seen directly, and their separation and relative motion can be followed directly.

slide18

Visual binaries

The Castor system

The Sirius system

The two stars are separately visible in the telescope

slide19

Detecting the presence of a companion by its gravitational influence on the primary star.

Wobbling motion of Sirius A

spectroscopic binaries
Spectroscopic Binaries

Usually, binary separation a can not be measured directly because the stars are too close to each other.

Stars are seen as a single point

However:

1) their SPECTRA are different, like different fingerprints;

2) Their spectral lines shift periodically because of Doppler effect. This allows us to measure their orbital velocities

the doppler effect
The Doppler Effect

The light of a moving source is blue/red shifted by

Dl/l0 = vr/c

l0 = actual wavelength emitted by the source

Dl = Wavelength change due to Doppler effect

vr = radial velocity( along the line of sight)

Blue Shift (to higher frequencies)

Red Shift (to lower frequencies)

vr

slide22

(Observed wavelength - Rest wavelength)

Shift z =

(Rest wavelength)

The Doppler effect: apparent change in the wavelength

of radiation caused by the motion of the source

Doppler effect:

doppler effect
Doppler effect

The Doppler effect: apparent change in the wavelength

of radiation caused by the motion of the source

RADIAL velocity!!

the doppler effect24
The Doppler Effect

The Doppler effect allows us to measure the source’s radial velocity.

Dl/l0 = vr/c

vr

spectroscopic binaries25
Spectroscopic Binaries

The approaching star produces blue shifted lines; the receding star produces red shifted lines in the spectrum.

Doppler shift Measurement of radial velocities

Estimate of separation a

Estimate of masses

slide26

Spectroscopic binaries

Stars are seen as a single point

  • Spectra of both stars are distinguishable
  • Sometimes spectrum of only one star is seen
spectroscopic binaries 3
Spectroscopic Binaries (3)

Typical sequence of spectra from a spectroscopic binary system

Time

slide29
Measure the orbital period

Measure the radial component of the orbital velocities

Can estimate the orbit size

Can determine masses!

slide30

1. Below is a radial velocity curve for a spectroscopic binary. Estimate the mass of each star if the mass of the binary system is 6 solar masses.

MA dA = MB dB

V ~ 2d/P

slide33

THE PLANET CANNOT BE SEEN

...BUT

MOTIONS

OF THE STAR

BETRAY

ITS PRESENCE !

slide34

450 km

9 cm/s

150 000 000 km

30 km/s

X

750 000 km

13 m/s

JUPITER

X

780 000 000 km

13 km/s

EARTH

slide35

2020

1995

2010

1990

2015

2005

2000

0.002”

MOTIONS

OF THE SUN VIEWED FROM A STAR 30 LIGHT YEARS AWAY

0.002’’ IS THE ANGULAR SIZE OF A MAN ON THE MOON OR A STANDARD NEWSPAPER FONT 300 KM AWAY

Unobservable!

slide36

STELLAR

WOBBLE

RECEDING:

REDDER

APPROACHING:

BLUER

slide39

EXPECTED:

NEARLY CIRCULAR ORBITS

BIG PLANETS FAR AWAY FROM THE STAR

NO PLANETS BIGGER THAN JUPITER

DISCOVERED:

STRONGLY ELONGATED ORBITS

BIG PLANETS VERY CLOSE TOTHE STAR

MANY PLANETS BIGGER THAN JUPITER

slide40

Planetarysystem of u And

0.85 AU

242 days

2 MJ

2.5 AU

3.5 years

4 MJ

0.06 AU

4.5 days

0.75 MJ

0.73 AU

228 days

1 AU

1 year

0.39 AU

89 days

1.54 AU

1.9 years

Solar system

Source: Harvard-Smithsonian CfA

slide45

John Goodricke 1764-1786

Explained Algol puzzle in 1783

eclipsing binaries
Eclipsing Binaries

Usually, inclination angle of binary systems is unknown uncertainty in mass estimates.

Special case:

Eclipsing Binaries

Here, we know that we are looking at the system edge-on!

slide51

Measuring diameters

D = Vorb(t2 – t1)

slide52

Specific segments of the main sequence are occupied

by stars of a specific mass

L~ M3.5 dependence, but

Cutoff at masses > 100 M and < 0.08 M

puzzles of h r diagram
Puzzles of H-R diagram
  • Why > 90% of stars are on the main sequence?
  • Reason for mass-luminosity dependence and mass cutoff
  • Same stars at different stages of life or just different stars?
slide54

How can we learn about the life of stars??

  • Our life span is ~ 80 years
  • Human civilization exists ~ 5000 years
  • Our Sun exists at least 4.6 billion years!
star clusters school classes for stars
Star Clusters – “School Classes” for Stars

They consist of stars of the same age !

Globular clusters

100,000 of stars

Open clusters

100’s of stars

slide56

Pleiades

p. 188

slide58

Age of the cluster from turnoff point

Turnoff point: stars of that mass are going to die and move away from the main sequence

slide62

Stars spent most of their lives on the Main Sequence. That is why it is so populated!

At the end of its life the star moves away from the

Main Sequence

More massive and more luminous stars die faster

Hypothesis: Stars on the Main Sequence live due to nuclear fusion of hydrogen!

  • Stars stay on the main sequence until all hydrogen in
  • the core is consumed
  • Then something should happen
h r diagram
H-R diagram
  • 90% of stars are on the main sequence and obey the mass-luminosity dependence L ~ M3.5
  • Stars on the main sequence generate energy due to nuclear fusion of hydrogen
  • In the end of their lives stars move to the upper right corner of the H-R diagram
check this hypothesis
Mass should be most important parameter

It determines the pressure in the star center and the central temperature

It determines the surface temperature

Check this hypothesis

How to get this dependence?

slide65

Gravity Holds a Star Together

Stars are held together by gravity. Gravity tries to compress everything to the center. What holds an ordinary star up and prevents total collapse is thermal and radiation pressure. The thermal and radiation pressure tries to expand the star layers outward to infinity.

  • Newton’s gravitation law
  • Hydrostatic equilibrium
  • Equation of state
  • Energy transport

Mass determines all star’s properties

slide66

Amount of hydrogen fuel

Lifetime =

Rate of energy loss

Lifetime T ~ M/L ~ 1/M3.5-1 = 1/M2.5 ; p ~ 3.5

T ~ 3x108 years

M = 4M;

maximum masses of main sequence stars
Maximum Masses of Main-Sequence Stars

Mmax ~ 50 - 100 solar masses

a) More massive clouds fragment into smaller pieces during star formation.

b) Very massive stars lose mass in strong stellar winds

h Carinae

Example: h Carinae: Binary system of a 60 Msun and 70 Msun star. Dramatic mass loss; major eruption in 1843 created double lobes.

slide70

High-mass cutoff at M ~ 100 Msun

Too massive and luminous stars throw off their outer

layers due to radiation pressure

Eta Carinae

minimum mass of main sequence stars
Minimum Mass of Main-Sequence Stars

Mmin = 0.08 Msun

At masses below 0.08 Msun, stellar progenitors do not get hot enough to ignite thermonuclear fusion.

Gliese 229B

Brown Dwarfs

slide72

Low-mass cutoff of the main sequence: M ~ 0.08 Msun

Brown dwarfs: temperature is too low to ignite nuclear fusion

Gliese 229B: only 0.02 M

slide73

Conclusion

  • Based on this evidence, we conclude:
  • Stars spend most of their lives as main sequence stars.
  • During its lifetime, the surface temperature and luminosity stays almost constant.
    • Something else could happen in the star birth process.
    • Something else could happen in the star death process.
  • The star's mass determines what the temperature and luminosity is during the star's main sequence lifetime.
    • More mass -> hotter.
    • More mass -> more luminous.
    • Also, more mass -> bigger.