A105 Stars and Galaxies

ROOFTOP TONIGHT AT 9 PM HAND IN HOMEWORK Exam coming on Nov. 2

Today’s APOD

Upcoming Events• Orionid meteor shower peaks

Saturday night, view from 11:45 onward – if weather is clear, watch for at least 20 minutes from a dark site

• Transit of Mercury–Nov. 8, 2:15 PM – Sunset–From Sample Gate

Let’s talk about…• Mid-Term Grades

• The Next Exam

Temperature, Diameter,

and Brightness

Star formation brings stars to the main sequence

…What happens next?

Explaining the HR Diagram

•Energy

•Gravity

•Energy Transport

Review:

• Why was the Sun’s energy source a major mystery?– Chemical and gravitational energy sources

could not explain how the Sun could sustain its luminosity for more than about 25 million years

• Why does the Sun shine?– The Sun shines because gravitational

equilibrium keeps its core hot and dense enough to release energy through nuclear fusion.

How does nuclear fusion occur in the Sun?

• The core’s extreme temperature and density are just right for nuclear fusion of hydrogen to helium through the proton-proton chain

• Gravitational equilibrium acts as a thermostat to regulate the core temperature because fusion rate is very sensitive to temperature

Stellar Mass and Fusion

• The mass of a main sequence star determines its core pressure and temperature

• Stars of higher mass have higher core temperature and more rapid fusion, making those stars both more luminous and shorter-lived

• Stars of lower mass have cooler cores and slower fusion rates, giving them smaller luminosities and longer lifetimes

Low Mass Stars

Massive Stars

Sun-like Stars

Star Clusters and Stellar Lives

• Our knowledge of the life stories of stars comes from comparing mathematical models of stars with observations

• Star clusters are particularly useful because they contain stars of different mass that were born about the same time

Evolution of a Very Low Mass Star

(~0.3 solar masses)

• The entire star is convective.

• As hydrogen is consumed, the core shrinks and heats, the luminosity rises along the main sequence.

• Since convection occurs through the whole star, all the star’s hydrogen is burned.

• Leaves a helium remnant

๏

Lifetime: 300 Billion Years

What are the life stages of a Sun-like star?

A star remains on the main sequence as long as it can fuse hydrogen into helium in its core

What happens next?

Life Track after Main Sequence

• Observations of star clusters show that a star becomes larger, redder, and more luminous after its time on the main sequence is over

Sun-like stars become red giants

When the helium core contracts, the surrounding hydrogen puffs up and the star becomes a red giant.

Broken Thermostat• As the core contracts, H

begins fusing to He in a shell around the core

• Luminosity increases because the core thermostat is broken—the increasing fusion rate in the shell does not stop the core from contracting

Helium fusion does not begin right away because it requires higher temperatures than hydrogen fusion—larger charge leads to greater repulsion

Fusion of two helium nuclei doesn’t work, so helium fusion must combine three He nuclei to make carbon

Once helium burning begins the “thermostat” starts to work again. Helium burning stars neither shrink nor grow because core thermostat is temporarily fixed.

End of Fusion

• Fusion progresses no further in a Sun-like star because the core temperature never grows hot enough for fusion of heavier elements

• Electron pressure from quantum mechanics supports the core against further gravitational contraction

The End of Solar-type Stars

When the carbon core reaches a density that is high enough, the star blows the rest of its hydrogen into space.

Main Sequence Red

Giant

PlanetaryNebula

White Dwarf

The hot, dense, bare core is exposed!

Surface temperatures as hot as 100,000 degrees

The hot core heats the expelled gas and makes it glow

Planetary Nebulae

• Fusion ends with a pulse that ejects the H and He into space as a planetary nebula

• The core left behind becomes a “white dwarf”

Planetary

Nebulae!

Life Track of a Sun-Like Star

Earth’s Fate

• Sun’s luminosity will rise to 1,000 times its current level—too hot for life on Earth

Earth’s Fate

• Sun’s radius will grow to near current radius of Earth’s orbit

Summary

• The life stages of a Sun-like star– H fusion in core (main sequence)– H fusion in shell around contracting core

(red giant)– He fusion in core

• How does a Sun-like star end?– Ejection of H and He in a planetary

nebula leaves behind an inert white dwarf

Life Stages of High-Mass Stars

• Late life stages of high-mass stars are similar to those of low-mass stars:– Hydrogen core

fusion (main sequence)

– Hydrogen shell burning (supergiant)

– Helium core fusion (supergiant)

What about Massive Stars?

• Massive stars continue to generate energy by nuclear reactions until they have converted all the hydrogen and helium in their cores into iron.

• Once the core is iron, no more energy can be generated

• The core collapses and the star explodes

Iron builds up in core until degeneracy pressure can no longer resist gravity

Core then suddenly collapses, creating supernova explosion

A “Recent” Supernova in Our Galaxy

• A new star in Taurus observed by the Chinese in 1054 A.D.

•Visible in the daytime

•Gradually faded; gone after about two years

•The Crab Nebula is a supernova remnant

The Crab Nebula Continues to Expand

• The Crab Nebula is about 7000 LY away

• The Nebula is about 10 LY across

• Expanding at a speed of about 1,400 kilometers per second

• The Crab Nebula - Then and Now

• Images taken in 1973 and recently

The Large Magellanic

Cloud

•Distance: about 150,000 LY

•Part of the Local Group

•“Irregular” galaxy

•Lots of star formation

Super-nova 1987a

•Feb. 1987

•Star previously known – 18 solar masses

•Study formation of supernova remnant

Rings around Supernova 1987A

• The supernova’s flash of light caused rings of gas around the supernova to glow

Summary• The life stages of a high-mass star are

similar to the life stages of a low-mass star

• Higher masses produce higher core temperatures that enable fusion of heavier elements

• A high-mass star ends when the iron core collapses, leading to a supernova

Is life on Earth safe from harm caused by supernovae?

Earth is safe at the present timebecause there are no massive stars

within 50 light years of the Sun.

Sun-like Star Summary

1. Main Sequence: H fuses to He in core

2. Red Giant: H fuses to He in shell around He core

3. Helium Core Burning: He fuses to C in core while H fuses to He in shell

4. Planetary Nebula leaves white dwarf behind

Not to scale!

Life Stages of High-Mass Star

1. Main Sequence: H fuses to He in core

2. Red Supergiant: H fuses to He in shell around He core

3. Helium Core Burning: He fuses to C in core while H fuses to He in shell

4. Multiple Shell Burning: Many elements fuse in shells

5. Supernova leaves neutron star behind

Not to scale!

Role of Mass• A star’s mass determines its entire life story

because it determines its core temperature• High-mass stars with >8MSun have short lives,

eventually becoming hot enough to make iron, and end in supernova explosions

• Sun-like stars with <2MSun have long lives, never become hot enough to fuse carbon nuclei, and end as white dwarfs

• Intermediate mass stars can make elements heavier than carbon but end as white dwarfs

The Evolution of Stars

The Composition of Stars

everythingelse

90% hydrogen atoms

10% helium atoms

Less than 1% everything else(and everythingelse is made in stars!)

Abundance of Elements in the Galaxy

Goals:

• Know how chemical elements are created

• in the Early Universe

• in Stars

• in Supernovae

• Know how the Galaxy is enriched in chemical elements

The Origin of Elements

• The process by which elements (nuclei) are created (synthesized) is called nucleosynthesis

• Nucleosynthesis has occurred since the creation of the universe and will essentially go on forever

• The elements created come together to form everything material we know, including us

Hydrogen and helium were created during the Big Bang while the Universe was cooling from its initial hot, dense state.

About 10% of the lithium in the Universe today was also created in the Big Bang. We’re still not surewhere the rest comes from.

The first stars formed from this material.

Primordial Nucleosynthesis

Hydrogen Burning

Stars burn hydrogen in their interiors to produce helium.

Hydrogen burning also rearranges carbon, nitrogen, and oxygen.

HeliumBurning

Three helium atoms combine to form carbon

LightElements

The Iron Peak Metals

In the cores of massive stars just beforesupernova explosions, atomic nuclei exchange protons and neutrons to form the iron peak metals.

Making Elements Up

to Iron

• Hydrogen – from big bang nucleosynthesis.

• Helium – from big bang and from hydrogen burning via the p-p chain and CNO cycle.

• Nitrogen – from CNO cycle.

• Carbon, Oxygen – from helium burning.

• Light elements (Neon, Magnesium, Calcium – from carbon and oxygen burning.

• Iron metals – from the final burning

Heavy MetalsAll heavier elements are formed

when iron peak elements captureneutrons

Elements Heavier than Iron …• Once iron is formed, it is no longer possible to create

energy via fusion.

Elements heavier than iron require a different process (Iron is atomic number 26.)

• The heaviest naturally occurring nucleus is uranium (atomic number 92). How do we get to uranium then?

•Elements heavier than iron are created byneutron capture

•The neutron is converted into a proton and added to the nucleus, increasing the atomic number to make the next element in the periodic table.

Making Heavy Metals in Stars

• In low mass stars like the Sun, heavy metals are created when the star is a giant

• Massive stars make heavy metals when they become supernovae

Stellar Nucleosynthesis

• We know now that all chemical elements heavier than atomic number 5 (Boron) were produced in stars.

• The light elements are essentially ashes of nuclear burning during the normal stellar evolution process.

• The heavier elements are produced in the envelopes of giants and during explosive nucleosynthesis that occurs during supernovae.

Chemical Enrichment of the Universe

• We know now that massive stars act as factories for creating heavy elements– Massive stars end their lives in supernova

explosions– The explosion scatters the new elements into

interstellar space

• Elements synthesized inside stars are also brought to the surface and expelled via stellar winds

• A new generation of stars recycle this material, enriching it further

The Galaxy (and the universe) is gradually enriched in heavy elements

Despite all the nucleosynthesis that has occurred since the creation of the universe, only 2% of the ordinary matter in the universe is now in the form of heavy elements. Most is still hydrogen and helium

Star Death – Units 67, 68, 69 News Quiz on Tuesday Homework Due EACH

THURS.EXAM NOV. 2nd