A105 Stars and Galaxies

Homework due TODAY 2nd Exam on Thursday, Nov. 2

Today’s APOD

Announcements…• Kirkwood Obs. open Weds

night 8-10 PM

• Rooftop Session, TONIGHT, 8 PM (weather permitting)

EXAM ON THURSDAY• UNITS 49-69

• IN CLASS, MULTIPLE CHOICE

• REVIEW GUIDE: Oncourse

• REVIEW SESSIONS IN CLASS ON TUESDAY – Bring questions

Compact Objects

White Dwarfs•Formed from the cores of stars less than 8 times the mass of the Sun•up to 1.4 solar masses•made of compressed He, C, or Fe•about the size of the Earth•One cc would weigh about 3 tons

Neutron Stars•Formed in supernova explosions•from 1.4 to about 3 solar masses•Made of pure neutrons – a giant atomic nucleus•About 20 km in diameter•One cc would weigh about a million tons

Black Holes•Formed in Supernova explosions•Usually a few times the mass of the Sun•A solar mass black hole is about 3 km in diameter•Density is infinite

Key Ideas – Black Holes

• What is a black hole?• A singularity? • An event horizon? • A Schwarzschild radius?

• Properties of black holes

• Relativity

What is a Black Hole?

•Black holes are objects with such strong gravity that not even light can escape•Since nothing can travel faster than light, nothing can escape from inside a black hole•Objects of any mass can (in principle) become black holes if sufficiently compressed•How compressed? – depends on mass

radius = 3 km x mass (in solar masses)

•The Sun would become a black hole if compressed to a radius of 3 kilometers

What is a Black Hole?•Since nothing can travel faster than light, nothing can escape from inside a black hole

•Even in Isaac Newton’s time, scientists speculated that such objects could exist

•Space and time near a black hole become so warped that time practically stops (from Theory of Relativity)

Defining Terms

• Singularity – The place at the center of a black hole where, in principle, gravity crushes all matter to an infinitely tiny and dense point.

• Event Horizon – The boundary that marks the “point of no return” between a black hole and the outside universe. Events that occur within the event horizon can have no influence on our observable universe.

• Schwarzschild Radius – A measure of the size of the event horizon of a black hole.

Forming Black Holes

• Matter must be compressed to a high density

• This happens in supernova explosions of massive stars when the iron core implodes

Black holes can have mass, angular

momentum, and electric charge.

MASS: At a distance a black hole exerts the same gravitational force on something as any other object of the same mass would.

Black holes form spinning rapidly.Their fast rotation distorts space and time near the boundary of the black hole.

Exploring the Schwarzschild Radius

•The size of a black hole depends on mass•A black hole can have any mass, from billions of times the mass of the Sun to very small (even your mass!)

0.000010.0001

0.0010.01

0.11

10100

100010000

1000001000000

10000000

1.0E-06 1.0E-04 1.0E-02 1.0E+00 1.0E+02 1.0E+04 1.0E+06 1.0E+08

Solar Masses

Schw

arzs

child

Rad

ius

(km

)

Any object with mass can become a black hole if it can be crushed to a small enough radius!

The graph shows the radius at which a given mass (in solar masses) will become a black hole if all of the mass is compressed into a sphere that size or smaller.

What evidence do astronomers use to infer the presence of a black hole?

GRAVITY!!! Matter flowing into a black hole emits X-rays before it crosses the event horizon Astronomers look for black holes in X-ray binariesCompact objects in binary systems with masses greater than about 3 solar masses can’t be neutron starsThey must be black holes

An Example: Cygnus X-1

8000 light years from EarthX-ray source orbits a hot, blue supergiant every

5.9 daysSupergiant’s mass is about 30 x solar massX-ray source mass exceeds 6 x solar mass

What would you see if you went right up to a Black Hole?

•The black hole has such strong gravity that light is noticeably bent towards it - causing some very unusual visual distortions.• In the distorted frame, every star in the normal frame has at least two bright images - one on each side of the black hole.•Near the blackhole, you can see the whole sky - light from every direction is bent around and comes back to you.

Approaching a Black Hole

Two computer generated images:

left: normal star field (find Orion’s belt)

right: a black hole has been added at the center of the field

Falling into a Black Hole

spagettification!

Space ship orbiting 10 solar mass black hole

•black hole diameter = 30 km•you are 15,000 km above the surface

TIDES: you can feel the gravity is stronger on your feet than on your head. It feels like you are being stretched.

•The space ship’s orbit spirals in to 3000 km.•Now the tide is VERY uncomfortable.•Head back out into space!

Try a much more massive black hole!•one million solar masses•about the size of the Solar System•tides are weaker

As you cross the event horizon, a distant observer will see your clocks slow down (so slow that it will appear as if you never cross into the black hole), your color will appear reddish, and you will look stretched out.

Black holes change the paths of light rays

Researchers at Humboldt State University have figured out how images of landscapes and planets would be distorted by having a black hole sitting in the foreground. (Ignoring, of course, the effect on Earth!)

And what do you see if you fall into a black hole’s event horizon?

…the black void is suddenly replaced by an unimaginable array of views.

We don’t know what you may see inside the black hole and, unfortunately, you can never tell us about your discoveries. Any signal you send to us is sucked into the black hole with you.

You are lost to our universe forever.

And within a few seconds, you are swept into the massive singularity at the center of the black hole!

Black Hole Concepts to Think About:1.Why would an isolated black hole in

space be difficult to detect?

2.Why is Cygnus X-1 thought to be a black hole?

3.How big is a black hole with twice the mass of the Sun?

4.How would the period of the Earth’s orbit around the Sun change if the Sun suddenly collapsed into a black hole? (Note that this can never happen!)

5. In what ways does matter in white dwarfs, neutron stars, and black holes differ from the matter in the Sun?

• How does a white dwarf form?

• Why are pulsars observed to emit regular bursts of radio light?

• Professor X just announced the discovery of a neutron star with a mass of 7.6 times the mass of the Sun.  Do you think Professor X is right?  Why?

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!)

Mike Stanfill, Private Hand - Flash Animation - The Elements, by Tom Lehrer

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

Unit 69 News Quiz on Tuesday Homework Due EACH

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