Stars: Luminosity, Temperature, Radii Hertzsprung-Russell Diagram Phys 1830: Lecture 30 Shapely 1, Malin Registrars Office: Exam Friday April 24 1:30pm Frank Kennedy Gold Gym Seats 1-135 Previous Classes: Stars: Luminosity, Temperature, Radii Hertzsprung-Russell Diagram This class: Radii, Mass, Lifetime on Main Sequence Stellar evolution: star birth and death of a star with 1 solar mass. Next Classes: Stellar Evolution: 1 solar mass and more massive stars. black holes ALL NOTES COPYRIGHT JAYANNE ENGLISH

Can we have a low surface temperature star with a high luminosity? Review: Can we have a low surface temperature star with a high luminosity? Yes, if the radius is large. No, if the star’s surface is cool it must also be dim. No, since the temperature at the surface doesn’t tell us about the luminosity produced in the core.

If a high T and a low L , then the radius must decrease. Stellar Radii If the T is low, we can see that increase in radius leads to increase the star’s L If a high T and a low L , then the radius must decrease.

Stars: Red Giants E.g. Antares by David Malin. Dust grains floating away  very tenuous surface. Low T, high L  R very large (e.g. 100s x radius of sun). Masses are up to 100 solar masses. Nuclear fusion creates carbon, silicon, oxygen which are expelled into space, polluting the interstellar medium.

E.g. Sirius B (Bond et al.). Dot in left corner. Stars: White Dwarf HST Binary system of Sirius A and Sirius B Diffraction spikes and concentric rings are due to the optics. The large star is Sirius A, the brightest star in the night time sky. Sirius A is a Spectral class A and the size of a main sequence star. E.g. Sirius B (Bond et al.). Dot in left corner. High T, low L  R very small (e.g. size of Earth). Masses ~ 1 solar mass, but a million times denser.

review One can find the radius of a point source star using which of the following relationships: a) b) c) d) e) One can’t measure the radius of a point source even if it is a star.

Stars: Luminosity, Temperature and Radius on H-R diagram The position of a star on the H-R diagram will depend on its mass, composition, and stage of evolution. The lifetime of a star on the main sequence depends on its mass.

The majority of stars are in pairs – binary systems. Stars: Mass The majority of stars are in pairs – binary systems. (Do this derivation as homework.) Recall we did this to find the mass of planets using their moons.  Mass of a star is determined by the velocity of the companion (squared) times the distance between the stars (r = radius of of the orbit).

Most stars are in binaries – a pair of stars in one system. protostars forming as a pair emit flashes – probably as disk material falls on them when they are close together.

Visual binary  r and velocity Velocity from distance/time Stars: Mass This can be done for any set of stars – they do not have to be on the main sequence. Visual binary  r and velocity Velocity from distance/time v = circumference of ellipse/Period of orbit. Also can use Doppler shifts and light curves to constrain masses.

Masses of stars can only be determined for stars on the main sequence. Review: Masses of stars can only be determined for stars on the main sequence. True False The only thing you need is a companion star.

Stars: Masses on Main Sequence Determined from measurements: Mass is related to radius and luminosity (low mass, low luminosity). Few percent are giants and supergiants.

Stars: MS Lifetimes Depend on Mass Main sequence star: H burning in the core Energy and radiation Hydrostatic equilibrium When the fuel in the core is consumed then the star is no longer in hydrostatic equilibrium. It evolves off the MS, through various stages. Most of its life is on the MS – what is its MS lifetime? Hydrostatic equilbrium is the balance between the outward pressure due to radiation and the inward pressure due to gravity.

Stars: MS Lifetimes Depend on Mass Lifetime = amount of fuel --------------------- rate of energy liberation Both the fuel and rate are determined by mass. In a more massive star the core has higher density  higher temperature This gives more nuclear reactions so the fuel is burned faster.

Stars: MS Lifetimes Depend on Mass Lifetime = amount of fuel --------------------- rate of energy liberation On MS only, the rate of energy liberated is the luminosity M is the mass of the star in this equation. Therefore

Stars: MS Lifetimes Depend on Mass Star P is ten times as massive as star Q. Compared to star Q: Star P has a longer life time. Star P has a shorter life time. They both have the same life time. Massive stars live fast and die young!

Stars: MS Lifetimes Depend on Mass Star Q has a life time of 10 billion years. What is the life time of star P that has 10 times the mass of star Q. Substitute in mass of P and life time of Q Star Q’s MS lifetime is like our own sun’s. In (10 Mq) to the third power, each component has the power of 3 applied to it. That is, (10 to the power of 3) times (Mq to the power of 3). Therefore the (Mq to the power of 3) in the numerator and denominator cancel out. Try this for a star ½ the mass of the sun. Re-arrange the equation. The MS life time of star P is only 10 million years!

Stars: Stellar Populations When a massive star explodes as a supernova at the end of its life, it also pollutes the interstellar medium with elements, many fused during the explosion.

Stars: Stellar populations Finished here New stars form out of the polluted interstellar medium. These stars have more elements.

Stars: Stellar Populations - classification Population I: Age of our sun or younger. Most enriched in chemical elements . Population II: Older, previous generation. Less enriched. Population III: Have the chemical abundances of the early universe (only H, He, Li and traces of other elements). Died long ago so not observed.

Review: Stars that have only the elements that existed in the very early universe are called Population I stars. True False

Sizes range from several million stars up to 100 billion stars. Galaxies: Definition: A large group of stars held together by the stars’ mutual gravitational attraction. Sizes range from several million stars up to 100 billion stars.

Mainly older Pop II stars. Galaxies: Mainly older Pop II stars.

Spiral galaxies also contain significant amounts of gas and dust.

Pop II and older Pop I stars: throughout the nucleus, bulge and disk. Galaxies: Pop II and older Pop I stars: throughout the nucleus, bulge and disk. Younger Pop I stars: in the Spiral arms.

The Story of Supernovae It starts with star birth.

Star Formation in Galaxies: These HII regions are sites of star formation in galaxies. The gas is being converted into stars. Hot, young stars heat surrounding gas, ionizing it  HII regions.

Star Formation: Examples in our Milky Way Galaxy Interstellar medium (ISM). HI gas and dust between stars. Where the gas is dense molecular clouds form.

Do this exercise outside of class. At the end of this section you will describe to your neighbour the 5 main stages of evolution for a star like our sun, starting with star birth. Star Birth Main Sequence Red Giant Planetary Nebula White Dwarf

Blastwave of supernova. Spiral density wave. Star Formation: Pressure applied to dense, cold ISM clouds causes them to gravitationally collapse and form stars. Blastwave of supernova. Spiral density wave.

Orion by : Reinhold Wittich Narrow band and broad band filters.

WISE: Near-IR (NIR) & Far-IR (FIR) Orion Nebula in IR Color in this image represents specific infrared wavelengths. Blue represents light emitted at 3.4-micron wavelengths and cyan (blue-green) represents 4.6 microns, both of which come mainly from hot stars. Relatively cooler objects, such as the dust of the nebulae, appear green and red. Green represents 12-micron light and red represents 22-micron light. WISE: Near-IR (NIR) & Far-IR (FIR)

Star Formation: Classic example of an HII region is the Orion Nebula. To show the nebula, Malin masked out the bright light from the central hot young stars. Several hundred stars are forming, along with protoplanetary disks around 1/3-1/2 of the stars.

Star Formation: Orion Nebula in IR young stars FIR detects more young stars. left: Herschel Far IR right: Spitzer Near IR

Star Formation – cold, dense dusty clouds: The bits of HII regions that are going to turn into proplyds are the dense, cold clouds. Horsehead Nebula Bok Globules

Evaporating Gaseous Globules (EGGs) Star Formation: Leads us back to the HR diagram Eagle Nebula. Evaporating Gaseous Globules (EGGs)

Herbig Haro Objects – the jets emanating from protostars. Star Formation: Protostar == first stage of becoming a star. Herbig Haro Objects – the jets emanating from protostars.

Note the jet coming out of the molecular cloud. Star Formation: Protostar buried in the molecular cloud. Note the jet coming out of the molecular cloud.

Star Formation: Herbig Haro Jets Radius can be more than 0.5 to 15 ly long!

Hot young stars  massive ones are blue. Fusion occurring in the core. Star Formation: The Pleiades . Hot young stars  massive ones are blue. Fusion occurring in the core. Still surrounded by dust which reflects the blue light. Form in a group, stars with different masses.