February 18, 2003 Lynn Cominsky - Cosmology A3501 Part I: Stars and Stellar Characteristics Part II:...

February 18, 2003

Lynn Cominsky - Cosmology A350

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Part I: Stars and Stellar Characteristics

Part II: Stars that Go Boom

July 22, 2004

February 18, 2003


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Stars and Stellar Characteristics

• Seeing Stars

• Physical characteristics (distance, luminosity, mass, temperature, etc.)

• Life Cycles

• Creation of Elements

• Supernovae and Gamma-Ray Bursts

Topics for the Day

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What do we know about stars?

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Case Study: Seeing Stars

In this interactive exercise, we will examine a “case study” of how astronomers study stars

We will use the scientific method to observe, ask questions, then re-observe, modifying our questions and knowledge as our investigation expands

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Case Study 1: The Stars, Like Sparks of Fire

The Mighty Hunter

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Case Study 1: The Stars, Like Sparks of Fire

• Stars are points of light• Different brightnesses• Different colors• Different distribution (many

along Milky Way, not many at 90 degrees from it)

• Need distances to get physics, more understanding of their nature

Some possible observations:

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How far away are the stars?

How can you figure this out, given the tools of the time?

Think of what you know from the first case study, and see if you can apply it.

Case Study 2: The Stars, My Destination

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0.12

0.51.6

2.1

4.7

6.0 10.1

Mag Lights

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• Parallax

• Brightness (relative) [yes! Make blink]

• Colors (redder are farther)

• Size (oh really?)

Need distances to get physics, more understanding of their nature

Some possible methods:

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Case Study 3: The Lights in the Sky Are Stars

Ogden claims to know the distance to Rigel!

What assumptions has he made? Which are good, which are not?

Is his distance estimate accurate? If not, do you think it’s too big or too small?

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Case Study 3: The Lights in the Sky Are Stars

• He remembers Rigel’s brightness accurately

• Being inside is same as outside

• Rigel is just like the Sun

Some assumptions:

Rigel is hotter, bluer, and bigger than the Sun. How does this affect his distance?

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Aside: Parallax Overview

• Parallax is an apparent shift• Given baseline, can determine distance• Gimme a thumbs-up!• My thumb is 2 cm across and 68 cm away from

my eye.• From small angle formula:

Apparent size (radians) = diameter/Distance2 / 68 * (180 / pi) = 1.5º

• So anything the size of my thumb (1.5º) is 34 times its own diameter away.

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Distances to Stars Parallax : determined by the change of position

of a nearby star with respect to the distant stars, as seen from the Earth at two different times separated by 6 months.

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Parallax, parsecs and light years

• 1 parsec is defined as the distance at which a star would have a parallax angle of 1 arc-second

• 1 arc-second = (1 degree/3600) = (1 degree/3600) (radians/ 180 degrees ) = 4.85 x 10-6 radians

• 1 parsec = (1.5 x 108 km)/(4.85 x 10-6 ) = 3.086 x 1013 km = 3.26 light years

• 1 light-year is the distance light will travel in one year• 1 light-year = (2.998 x 108 m/s)(86400 s/d)(365 d/y) = 9.46

x 1012 km = 9.46 x 1015 m • A LIGHTYEAR IS A DISTANCE, NOT A TIME!

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Parallax movie

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The Nearest Stars

Distance to Alpha Centauri system is~4 x 1011 km or ~4.2 light years or~1.3 parsecs

Parallax is a bit less than

1 arcsecond

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The Solar Neighborhood

Some stars within about 2 x 1014 km(~ 20 light years)

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BreakWhat is the closest naked-eye star?

Suppose you observe a star over the course of a year. Due to parallax, in what pattern does it appear to move?

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430

772

242

721

817

1342915

343

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Case Study 4: The Absolute Lightness of Being

Stars are far, har har har

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Absolute vs. Apparent magnitude• Apparent magnitude - How bright does the star appear (from the Earth)? Uses symbol “m”

• Absolute magnitude - the apparent magnitude of a star if it were located at 10 pc. Uses symbol “M”

• Absolute and apparent magnitude are related to the true distance “D” to the star by:

•m – M = 5 log (D/10 pc) = 5 log (D/pc) – 5 OR

•D = 10 pc * 10((m-M)/5)

• Magnitudes seem backwards – the bigger the number, the fainter the star.


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Stellar Spectral types• OBAFGKMLT (Oh, Be A Fine Guy/Girl,

Kiss Me, Like Totally)

• Describe spectral features of stars

• Linked to temperature (not necessarily mass!)

O B A F G K M L T

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The Classy Hunter

B8I

M2I B2III

B0I

K5III

A5V

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After several thousand of years, Ogden finally has several stellar characteristics for a list of stars.

What can he do with this information?

How can he find correlations?


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Classifying Stars

Hertzsprung-Russell diagram

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Main Sequence

• Stars spend most of their lives on the Main Sequence

• How do they get there?

• What happens when they leave?

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Life Cycles of Stars

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Molecular clouds and protostars

• Giant molecular clouds are very cold, thin and wispy– they stretch out over tens of light years at temperatures from 10-100K, with a warmer core

• They are 1000s of time more dense than the local interstellar medium, and collapse further under their own gravity to form protostars at their cores

BHR 71, a star-forming cloud(image is ~1 light year across)

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Protostars• Orion nebula/Trapezium stars (in the sword)• About 1500 light years away

HST/ 2.5 light years Chandra/10 light years

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Disks around stars

• There is much evidence of disks with gaps (presumably caused by planets) around bright, nearby stars, such as HD141569 (shown here)

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Stellar nurseries• Pillars of

dense gas

• Newly born stars may emerge at the ends of the pillars

• About 7000 light years away

HST/EagleNebula in M16

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Pleiades Star Cluster

• A star cluster has a group of stars which are all located at approximately the same distance

• The stars in the Pleiades were all formed at about the same time, from a single cloud of dust and gas

• Roughly 108 years old D = 116 pc

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Logan’s Run

You’re such a card…

you should be dealt with.

• Game teaches about star formation

• Build a star before your opponent does!

• Offense and defense based on real star-formation phenomena

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Lunch! Renewal!

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Composition of the Universe

Actually, this is just the solar system.

Composition varies from place to place in universe, andbetween different objects.

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“What’s Out There?”

A classroom activity that demonstrates the different elemental compositions of different objects in the universe.

Demonstrates how we estimate the abundances.

(Developed by Stacie Kreitman, Falls Church, VA)

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Top 10 Elements in the Human Body

Element by # atoms

10. Magnesium (Mg) 0.03%

9. Chlorine (Cl) 0.04%

8. Sodium (Na) 0.06%

7. Sulfur (S) 0.06%

6. Phosphorous (P) 0.20%

5. Calcium (Ca) 0.24%

4. Nitrogen (N) 1.48%

3. Carbon (C) 9.99%

2. Oxygen (O) 26.33%

1. Hydrogen (H) 61.56%

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Spectral Analysis

We can’t always get a sample of a piece of the Universe.

So we depend on light !

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Spectral Analysis

• Each element has a unique spectral signature:

• Determined by arrangement of electrons.

• Lines of emission or absorption arise from re-arrangement of electrons into different energy levels.

Hydrogen

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Nickel-odeon Classroom Activity

Spread a rainbow of color across a piano keyboard

(Developed by Shirley Burris, Nova Scotia)

Then, “play” an element

Hydrogen

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More Musical Elements

Now play another elementHelium

Carbon

And Another

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Getting a Handel on Water

Oxygen

All together now ...

Hydrogen

Water

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Elemental, My Dear Watson

So stars have heavy elements.

Where did they come from?

… and how did they get here?

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HR Diagram again as a reminder

Hertzsprung-Russell diagram

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Main Sequence Stars• Stars spend most of their lives on the “main sequence”

where they burn hydrogen in nuclear reactions in their cores

• Burning rate is higher for more massive stars - hence their lifetimes on the main sequence are much shorter and they are rather rare

• Red dwarf stars are the most common as they burn hydrogen slowly and live the longest

• Often called dwarfs (but not the same as White Dwarfs) because they are smaller than giants or supergiants

• Our sun is considered a G2V star. It has been on the main sequence for about 4.5 billion years, with another ~5 billion to go

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Solar Power• The Sun is powered by nuclear fusion

reactions in its core

• The gravity from the Sun’s mass squeezes the nuclei together so that they can overcome electrostatic repulsion and fuse

… but high pressure and temperature encourage

impact

Electrostatic repulsion stops impact

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Solar Power• Hydrogen nuclei fuse to Deuterium and then Helium,

releasing about 7 MeV each• The released radiation keeps the Sun from collapsing due

to its own gravity• Energy comes from conversion of mass: E=mc2

Start with 4 protons under enormous

pressure and temperature

End up with a “normal” helium nucleus,

two gamma rays, two positrons and

two neutrinos

Several Reactions

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Pro Fusion or Con Fusion?

• The core of the Sun is 15 million degrees Celsius

• Fusion occurs 1038 times a second• Sun has 1056 H atoms to fuse• 1018 seconds = 32 billion years• 2 billion kilograms converted every second• Sun’s output = 50 billion megaton bombs per

second

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1018 seconds is a long time…

but it’s not forever.

What happens then?

Don’t Let the Sun Go Down on Me

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A Red Giant You Know

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The Beginning of the End: Red Giants

After Hydrogen is exhausted in core...Energy released from nuclear fusion

counter-acts inward force of gravity.

Core collapses, and kinetic energy of collapse

converted into heat.

This heat expands the outer layers.

Meanwhile, as core collapses, Increasing Temperature and Pressure ...

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More Fusion !

At 100 million degrees Celsius, Helium fuses:

3 (4He) --> 12C + energy

(Be produced at an intermediate step)

(Only 7.3 MeV produced)

Energy sustains the expanded outer layers of the Red Giant

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Stellar evolution made simple

Stars like the Sun go gentle into that good night

More massive stars rage, rage against the dying of the light

Puff!

Bang!

BANG!

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How stars die• Stars that are below about 8 Mo form red giants

at the end of their lives on the main sequence• Red giants evolve into white dwarfs, often

accompanied by planetary nebulae• More massive stars form red supergiants• Red supergiants undergo supernova

explosions, often leaving behind a stellar core which is a neutron star, or perhaps a black hole (more in later lectures about these topics)

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Red Giants and Supergiants

Hydrogen burns in outer shell around the core

Heavier elements burn in inner shells

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Aside: Planetary nebulae• Planetary nebulae

are not the origin of planets

• Outer ejected shells of red giant illuminated by a white dwarf formed from the giant’s burnt-out core

• Not always formed HST/WFPC2Eskimo nebula5000 light years

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Fate of high mass stars• After Helium exhausted, core collapses

again until it becomes hot enough to fuse Carbon into Magnesium or Oxygen.

12C + 12C --> 24Mg

OR 12C + 4H --> 16O

• Through a combination of processes, successively heavier elements are formed and burned.

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Heavy Elements from Large Stars

• Large stars also fuse Hydrogen into Helium, and Helium into Carbon.

• But their larger masses lead to higher temperatures, which allow fusion of Carbon into Magnesium, etc.

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Supernova !

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Supernova Remnants: SN1987A

a b

c d

a) Optical - Feb 2000Illuminating material

ejected from the star thousands of years before the SN

b) Radio - Sep 1999c) X-ray - Oct 1999d) X-ray - Jan 2000The shock wave from

the SN heating the gas

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Supernova Remnants: Cas AOptical

X-ray

Radio

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Elements from Supernovae

All X-ray Energies Silicon

Calcium Iron

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Crab nebula• Observed by Chinese

astronomers in 1054 AD

• Age determined by tracing back exploding filaments

• Crab pulsar emits 30 pulses per second at all wavelengths from radio to TeV

movie

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Crab nebula

Radio/VLA Infrared/Keck

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Crab nebula

Optical/HST WFPC2Optical/Palomar

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Crab nebula and pulsar

X-ray/Chandra

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Not all explosions are created equal

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The Terrestrial Zoo

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The Cosmic Zoo

• Need to categorize the objects seen• By sorting into different categories,

distinguishing characteristics are found• Potentially misleading differences can be

determined• Further investigation can reveal

the nature of the phenomenon

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Classifying Bursts• In this activity, you will be given twenty cards

showing different types of bursts• Pay attention to the lightcurves, optical

counterparts and other properties of the bursts given on the reverse of the cards

• How many different types of bursts are there? Sort the bursts into different classes

• Fill out the accompanying worksheet to explain the reasoning behind your classification scheme

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Pick a Card…

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Aitoff Projection & Galactic Coordinates (1)

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Let’s get busy!

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Sorting out the Cosmic Zoo

• How many groups did everyone get?

• What was your reasoning?

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Aitoff answer key

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What makes Gamma-ray Bursts?

• X-ray Bursts• Properties• Thermonuclear Flash Model

• Soft Gamma Repeaters • Properties• Magnetar model

• Gamma-ray Bursts• Properties• Models• Afterglows• Future Mission Studies

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X-ray Bursts

• Thermonuclear flashes on Neutron Star surface – hydrogen or helium fusion

• Accreting material burns in shells, unstable burning leads to thermonuclear runaway

• Bursts repeat every few hours to days• Bursts are never seen from black hole binaries (no surface for unstable nuclear burning) or from (almost all) pulsars (magnetic field quenches thermonuclear runaway)

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X-ray Burst Sources

• Locations in Galactic Coordinatesbursters non-bursters Globular Clusters

• Most bursters arelocated in globularclusters or near theGalactic center• They are therefore relatively older systems

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Soft Gamma Repeaters

• There are four of these objects known to date• One is in the LMC, the other 3 are in the Milky

Way

LMC

SGR 1627-41

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Soft Gamma Repeater Properties

– Superstrong magnetic field: B~1014-15 G

– NS spin period seen in bursts ~5-10 sec,

– No orbital periods – not in binaries!

– 4 well studied systems + several other candidate systems

– Several SGRs are located in or near SNRs

– Soft gamma ray bursts are from magnetic reconnection/flaring like giant solar flares

– Lx = 1042 - 1043 erg/s at peak of bursts

Young Neutron Stars near SNRs

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SGR 1900+14• Strong burst

showing ~5 sec pulses

• Change in 5 s spin rate leads to measure of magnetic field

• Source is a magnetar!

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SGR burst affects EarthOn the night of August 27, 1998 Earth's upper

atmosphere was bathed briefly by an invisible burst of gamma- and X-ray radiation. This pulse - the most powerful to strike Earth from beyond the solar system ever detected - had a significant effect on Earth's upper atmosphere, report Stanford researchers. It is the first time that a significant change in Earth's environment has been traced to energy from a distant star. (from the NASA press release)

http://www.spaceweather.com/

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Gamma Ray Burst Properties

– Unknown magnetic field

– No repeatable periods seen in bursts

– No orbital periods seen – not in binaries

– Thousands of bursts seen to date – no repetitions from same location

A cataclysmic event of unknown origin

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The GRB GalleryThe GRB Gallery

When you’ve seen one gamma-ray burst, you’ve seen….one gamma-ray burst!!

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GRB the Third: The Plots Thicken

Sometimes, the distribution of objects can tell you a lot about them

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Two Views of the Universe

Isotropic In the Milky Way

Which do your plots look like?

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CGRO/BATSE Gamma-ray Burst Sky

This is the realdistribution of 2704 as seen bythe BATSE instrument on board CGRO

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Near or Far?

Isotropic distribution implications:

Silly or not, the only way to be sure was to find the afterglow.

Very close: within a few parsecs of the Sun

Very far: huge, cosmological distances

Sort of close: out in the halo of the Milky Way

Why no faint bursts?

What could produce such a vast amount of energy?

A comet hitting a neutron star fits the bill

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Breakthrough!

In 1997, BeppoSAX detects X-rays from a GRBafterglow for the first time, 8 hours after burst

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The View From Hubble/STIS The View From Hubble/STIS

7 months later

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On a clear night, you really On a clear night, you really cancan see see forever!forever!

990123 reached 9th magnitude for a few moments!

First optical GRB afterglow detected simultaneously

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GRB the Fourth: Beam me Up

• So we got the distance, so we’re all done, right? Right?

• Oops. The energy needed is a tad high

• So can we still save the day?

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Insert Aurore’s images of sphere of lightAnd light house

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Gimme an “M”! Gimme an “E”!Gimme a “G”!

Megaphone!

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Spin the Flashlight!

Note that the beam can only be seen by a few people at once.

What can we learn from this?

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The Supernova ConnectionThe Supernova Connection

GRB011121

Afterglow faded like supernova

Data showed presence of gas like a stellar wind

Indicates some sort of supernova and not a NS/NS merger

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Hypernova

• A billion trillion times the power from the Sun• The end of the life of a star that had 100 times

the mass of our Sun

movie

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Iron lines in GRB 991216

Chandra observations show link to hypernova model when hot iron-filled gas is detected from GRB 991216

Iron is a signature of a supernova, as it is made in the cores of stars, and released in supernova explosions

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Catastrophic Mergers

• Death spiral of 2 neutron stars or black holes

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Which model is right?Which model is right?

The data seem to indicate two kinds of GRBs

• Those with burst durations less than 2 seconds• Those with burst durations more than 2 seconds

Short bursts have no detectable afterglows so far as predicted by the NS/NS merger model

Long bursts are sometimes associated with supernovae, and all the afterglows seen so faras predicted by the hypernova merger model

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Gamma-ray Bursts

Either way you look at it – hypernova or merger model

GRBs signal the birth of a black hole!

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Gamma-ray Bursts

Or maybe the death of life on Earth?

No, gamma-ray bursts did not kill the dinosaurs!

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Reflection and Debrief

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• Now what do we know?

• What are the big ideas here?

• What do our students need to know?

• Is there anything else we need to know?

• Misconceptions

(take notes)

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• What are some effective ways to teach students about galaxies?

• Standards???

(take notes)

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END OF SLIDES!!!!!!!!!

All slides after this are spares, removed from original show

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How to study Gamma rays?

• Absorbed by the Earth’s atmosphere

• Use rockets, balloons or satellites

• Can’t image or focus gamma rays

• Special detectors: crystals, silicon-strips GLAST

balloon test

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HETE-2

• Launched on 10/9/2000

• Operational and finding about 2 bursts per month

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Swift Mission

• Burst Alert Telescope (BAT)

• Ultraviolet/Optical Telescope (UVOT)

• X-ray Telescope (XRT)

To be launched in 2003

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Swift Mission

• Will study GRBs with “swift” response• Survey of “hard” X-ray sky• To be launched in 2003• Nominal 3-year lifetime• Will see ~150 GRBs per year

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Gamma-ray Large Area Space Telescope

• GLAST Burst Monitor (GBM)

• Large Area Telescope (LAT)

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GRBs and Cosmology

• GRBs can be used as standard candles, similar to Type 1a supernovae

• However, the supernovae are only seen out to z=0.7 (and one at z=1.7), whereas GRBs are seen to z=4.5, and may someday be seen to z=10

• Schaefer (2002) has constructed a Hubble diagram for GRBs, using the cosmological parameters from supernova data. When more burst redshifts become available (e.g., from Swift), the parameters can be determined independently

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White dwarf stars

• Red giants (but not supergiants) turn into white dwarf stars as they run out of fuel

• White dwarf mass must be less than 1.4 Mo

• White dwarfs do not collapse because of quantum mechanical pressure from degenerate electrons

• White dwarf radius is about the same as the Earth• A teaspoon of a white dwarf would weigh 10 tons• Some white dwarfs have magnetic fields as high as 109

Gauss• White dwarfs eventually radiate away all their heat and end

up as black dwarfs in billions of years

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Periodic Table

16O + 16O 32S + energy4He + 16O 20Ne + energy

Light Elements Heavy Elements

4 (1H) 4He + energy 3(4He) 12C + energy 12C + 12C 24Mg + energy4He + 12C 16O + energy28Si + 7(4He) 56Ni + energy 56FeC-N-O Cycle

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Making a Neutron StarMaking a Neutron Star

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Neutron Stars

• Neutron stars are formed from collapsed iron cores• All neutron stars that have been measured have

around 1.4 Mo (Chandrasekhar mass)

• Neutron stars are supported by pressure from degenerate neutrons, formed from collapsed electrons and protons

• A teaspoonful of neutron star would weigh 1 billion tons

• Neutron stars with very strong magnetic fields - around 1012-13 Gauss - are usually pulsars due to offset magnetic poles

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Neutron Stars: Dense cinders

Mass: ~1.4 solar massesRadius: ~10 kilometersDensity: 1014-15 g/cm3

Magnetic field: 108-14 gauss Spin rate: from 1000Hz to 0.08 Hz

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The End of the Line for Massive Stars

• Massive stars burn a succession of elements.

• Iron is the most stable element and cannot be fused further.– Instead of

releasing energy, it uses energy.

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Supernovae• Supergiant stars

become (Type II) supernovae at the end of nuclear shell burning

• Iron core often remains as outer layers are expelled

• Neutrinos and heavy elements released

• Core continues to collapse

Chandra X-ray image of Eta

Carinae, a potential supernova

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Cas A• ~320 years old• 10 light years across• 50 million degree shell

Radio/VLAX-ray/Chandra

neutron star

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X-ray Burst Source Properties

– Weaker magnetic dipole: B~108 G– NS spin period seen in bursts ~0.003

sec. – Orbital periods : 0.19 - 398 h from X-ray

dips & eclipses and/or optical modulation

– > 15 well known bursting systems– Low mass companions– Lx = 1036 - 1038 erg/s

Neutron Stars in binary systems

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X-ray EmissionX-ray Emission

• X-ray emission from accretion can be modulated by magnetic fields, unstable burning and spin

• Modulation due to spin of neutron star can sometimes be seen within the burst

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The first Gamma-ray Burst

Discovered in 1967 while looking for nuclear test explosions - a 30+ year old mystery!

Vela satellite

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Compton Gamma Ray Compton Gamma Ray ObservatoryObservatory

• Eight instruments on corners of spacecraft• NaI scintillators

BATSE

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February 18, 2003 Lynn Cominsky - Cosmology A3501 Part I: Stars and Stellar Characteristics Part II:...

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