ASTR-1014/4/2018

Stellar Evolution:Part II

Lecture 19

WHEN’S THE NEXT TEST ?!?!?!?

• If anyone is following the syllabus, you know that it says there is a test today.

• The test will be on April 11th (a week from today)

• Topics will include:• The Sun

• Measuring the Stars

• Interstellar medium

• Star Evolution

• Star Death

• Obstacle course for extra credit-any injures will result in F

Possible Extra Credit Option

• Kinematics of Parsec-scale Jets of Gamma-Ray Blazars and Connection between Jet events and Gamma-Ray activity 2:00 pm, Thursday, Room 190, Physics & Astronomy

• Please let me know if you are going so that I can attend as well.• Summarize:

• What is the goal of the presented research• Describe the experimental setup• What issues they are encountered• What are the results.

• At least a half a page• Points will be awarded based on completeness and at my discretion.• Up to 5% of a single test grade

More Massive Stars

Quantum Degeneracy Pressure

• Quantum mechanics states that particles (electron, protons, and neutrons) exist in discrete states.

• If all possible states for a given system are filled, no more particles can be added.

• The result is an emergent pressure against compression of matter into smaller volumes of space.

• This is the pressure that holds together a White Dwarf:• White Dwarfs are in hydrostatic equilibrium

• White Dwarfs have no fusion

• With quantum degeneracy pressure, gravity would crush the star

• If the Main-Sequence binary companion is close enough, the white dwarf is able to ‘steal’ the upper layers from its companion.

• As the hydrogen envelope is accreted onto the white dwarf, fusing begins and the accretion disk is quickly ignited.

• This process can happen many times, depending on the mass of the companion star.

Novae

Type1a Supernovae

• As the hydrogen envelope is accreted onto the white dwarf, fusing begins and the accretion disk is quickly ignited.

• This process can happen many times, but eventually comes to a stop once the mass becomes large enough.

• If the mass gets above the Chandrasekhar limit, 1.4 solar masses, the star cannot support the force of gravity.

• The star suddenly fusses all the carbon at once.

Type II supernovae-Core Collapse Super Novae

• Hydrostatic equilibrium is lost because Iron does not burn.

• Before this point core was held up by electron degeneracy pressure and nuclear fusion.

• Core is compressed, due to gravity, by overlying layers.

• Electron degeneracy pressure is not strong enough and the core collapses, compressing objects even further.

• Photodisintegration occurs and many neutrons are created as heavy nuclei are ripped apart.

• Neutrons get squeezed inside of the core.

• Collapsing star is halted by neutron degeneracy pressure, resulting in a violent, fast, rebound of the collapsing material.

Death of Massive Starts

• Stars with a mass greater than 8 solar masses will end their lives in a massive explosion called a core collapse supernova.

• This happens when the core starts to fuse Silicon in Iron.

• Unlike lighter elements, fusing Iron into heavier elements does not produce energy. It takes energy away from the system.

Core-burning nuclear fusion stages for a 25-solar mass star

ProcessMain fuel

Main products

25 M☉ star

TemperatureDuration

(K)

hydrogen burning hydrogen helium 7×107 107 years

triple-alpha process

heliumcarbon,oxygen

2×108 106 years

carbon burning process

carbonNe, Na, Mg, Al

8×108 103 years

neon burning process

neon O, Mg 1.6×109 3 years

oxygen burning process

oxygen Si, S, Ar, Ca 1.8×109 3.5 months

silicon burning process

silicon iron 2.5×109 5 days

Star Clusters

• Stars that form in the same molecular cloud have very similar initial conditions.

• Groups of stars gravitationally bound are called star clusters.

• Often they are moving around a common central point.

• Stars will differ in mass, but be compositionally the same.

• Massive stars evolve fast.

• Low mass stars evolve slow.

16,000 light-years away, Omega Centauri, most massive globular cluster

Globular Clusters • nearly symmetrical round systems of, typically, hundreds of thousands of stars.

• Globular clusters are the oldest part of our Milky Way.

• If Earth orbited one of the inner stars in a globular cluster, the nearest stars would be light-months, not light-years, away.

• Found mostly in the Halo of our galaxy.

Open Cluster• Open clusters contain up to a

few hundred stars and are not much older than our Sun.

• The youngest open clusters are still associated with the interstellar matter from which they formed. 30 light year diameter

• The average speed of the member stars may be higher than the cluster’s escape velocity,

• The stars will gradually “evaporate” from the cluster

Stellar Association• Stellar Associated stars are a

group of less than 50 hot bright young stars scattered over 100-500 light year diameter.

• the constellation Orion is an example.

• Also contains many low mass proto-stars.

• Always shrouded in dense ISM.

Main Sequence Turnoff

• Once a star stops burning hydrogen, it will turn off of the main sequence.

• The more massive the star the quicker it will make this turn.

• What population is still left on the Main Sequence dictates the age of the cluster.

• Open clusters turn out to be as young as 1 to a few 100 million years old.

• Globular clusters have main sequence stars that turn off at a luminosity less than that of the Sun (10 billions years)

What type of cluster is this?

Heavy Metals in Clusters

• Like our Sun, open clusters contain 1-4% metals (things heavier than hydrogen and helium).

• Globular clusters contain much less heavy metals (< 1%).

• Heavy elements are created from star death, which enriches the ISM.

• The first generation of stars were only hydrogen and helium because not enough time had pasted for stars to die.

• This means that the first generation of stars that formed in our Galaxy would not have been accompanied by a planet like Earth

• Gaia, launched by ESA, has been measuring the position and distances to almost one billion stars with an accuracy of a few tenmillionths of an arcsecond.

• Giving us a three-dimensional map of a large fraction of our own Milky Way Galaxy.

• These precision measurements help improve the distances measurement techniques using the H-R diagram.

Using Parallax to map out the Milky Way

Roughly ~30,000 parsecs

Variable Stars• Recall that the apparent brightness of an object decreases with the

square of the distance to that object.

• A light curve represents how bright a star is as a function of time

• What would the light curve of our Sun look like?

Pulsating Variable Stars

• Cepheid and RR Lyrae variables are pulsating variable stars, meaning they change diameter as a function of time.

• The expansion and contraction can be measured using Doppler shift.

• This physical phenomenon happens during the brief Red Giant phase of some more massive stars.

• As the star expands, it cools.

• Once it becomes cool enough, it contracts (gravity dominates pressures).

The Period-Luminosity Relation

• Cepheid have a very consistent period of luminosity fluctuation occurring between 3-50 days.

• Because they are in the red giant phase, they are 1000-10,000 times brighter than the Sun (easy to spot).

• The amazing part: The brighter the star, the longer the period.

• Measuring the period of the fluctuations gives the intrinsic brightness of the star.

• By comparing absolute brightness and relative brightness measured on Earth and using the inverse square law, an distance can be measured!

Henrietta Leavitt

• Leavitt discovered hundreds of variable stars in the Large MagellanicCloud and Small Magellanic Cloud.

• She concluded that all variable stars in these galaxies were at about the same distance.

• Any difference in their apparent brightnesses must be caused by differences in their intrinsic luminosities.

• Leavitt found that brighter stars have longer periods and concluded that their period must be related to the absolute brightness of the star.

Type1a Supernovae

• Type1a supernovae always explode at 1.4 solar masses -> always have the same brightness.

• Crucially they also have an easily identified light curve.

• Using Cephied variable stars to measure the distance to supernovae, astronomers can use typ1a supernovae light curves to measure distances.