stars, our friends in the universe. the nearest stars distance to alpha or proxima centauri is ~4 x...

Stars, our Friends in the Universe

The Nearest Stars

Distance to Alpha or Proxima Centauri is~4 x 1013 km or ~4.2 light years

Distance between Alpha and Proxima Centauri is ~23 AU

The Solar Neighborhood

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

What are Stars?

What are they made of?

What are their life cycles?

How do we know what we know about them?

What is a Star?

Stars are huge balls of hot gas, heated

from inside by nuclear energy. Many are similar to

our Sun, but there are giants as big as our solar

system and dwarfs the size of Earth.

Life Cycles of Massive (> 8 Suns) Stars

QuickTime™ and aCinepak decompressor

are needed to see this picture.

Life Cycles of Stars

Classifying Stars

Hertzsprung-Russell diagram

Classes of Stars

Bigger stars are brighter than smaller stars because they have more surface area

Hotter stars make more light per square meter. So, for a given size, hotter stars are brighter than cooler stars.

• White dwarfs - small and can be very hot

• Main sequence stars - range from hotter and larger to smaller and cooler

• Giants - rather large and cool

• Supergiants - cool and very large

How Stars are Born

Pillars of dense gas

Newly born stars seen to emerge at the ends of the pillars

About 7000 light years away

HST/EagleNebul

a in M16

How Stars are Born

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

HST/ 2.5 light years Chandra/10 light years

Main Sequence Stars

Stars spend most of their lives on the “main sequence” where they burn hydrogen in nuclear reactions in their coresBurning rate is higher for more massive stars - hence their lives are much shorter and they are rather rareRed dwarf stars are the most common as they burn hydrogen slowly and live the longestOften called dwarfs (but not the same as White Dwarfs) because they are smaller than giants or supergiants

Properties of Stars Temperature (degrees K) - color of star light.

All stars with the same blackbody temperature are the same color. Specific spectral lines appear for each temperature range classification. Astronomers name temperature ranges in decreasing order as:

Surface gravity - measured from the shapes of the stellar absorption lines. Distinguishes classes of stars: supergiants, giants, main sequence stars and white dwarfs.

O B A F G K M

Properties of Stars

Luminosity (Watts) - absolute brightness; independent of distance. Derived from spectral type and surface gravity classification.

Mass (kg or Solar mass units) - can be derived from spectrum or measured directly in binaries

Radius (m) - usually derived, but can be measured directly for close, very large stars.

Brightness

Luminosity = absolute brightness (How much energy does the star emit each second?)

Flux - How much energy from the star hits a square meter located at a distance d?

Apparent brightness (or magnitude) - How bright does the star appear (from the Earth)?

Absolute magnitude - the apparent magnitude of a star if it were located at 10 pc. A logarithmic measure of its absolute brightness.

Sun Facts

Mass of Sun 1.989 x 1030 kg

Diameter of Sun 1,390,000 km

Distance to Sun 1 A. U. or 93 x 106 miles or ~1.5 x 1011 m

Rotation Rate of Sun 25.4 d (equator) 36 d (poles)Surface Temperature of Sun 5800 K

(yellow visible light)

Star Power

A star is powered by nuclear fusion reactions in its core

The gravity from the star’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

Star Power

Hydrogen nuclei fuse to Deuterium and then Helium, releasing about 7 MeV each

The released radiation keeps the star from collapsing due to its own gravity

Start with 4 protons under enormous

pressure and temperature

End up with a ìnormalî helium nucleus,

two gamma rays, two positrons and

two neutrinos

Several Reactions

Features of a Main Sequence Star

Regions of a Main Sequence Star

Core - dense region consisting of plasma of electrons and protons which undergo nuclear fusion reactions to power the star. Temperature is greater than 15,000,000 K.Radiation zone - region containing both plasma and atoms. The atoms slowly (170,000 y) absorb and reradiate the energy created in the core, transporting it to the outer layers. Temperature is around 5,000,000 K.Convection zone - turbulent region where the solar material “boils” to quickly (1 week) move heat to the outer layers. T ~ 2,000,000 K


Photosphere - “surface” of the star that radiates visible light. Convection cells can be seen as granules - T ~ 5800 K Sunspots - highly variable, dark, cool regions in the photosphere. T ~ 3500 KChromosphere - thin (2000 km) layer outside photosphere in which Hydrogen absorbs radiation and reemits it as red light (H-alpha). Jagged outer edge has dancing “flames” or spicules.


Transition region - very thin (100 km) layer in which temperature rises from 20,000 to 106 KCorona - very sparse outer ionized gas region with loops and streamers of magnetic field. Temperature ~ 106 K

Solar Movie shows:

1) Photosphere

2) Chromosphere

3) Corona

QuickTime™ and a decompressor


Sunspot and Convection Cells

Optical sunspot image from the Vacuum Tower telescope at the Sacramento Peak National Solar Observatory with100 km resolutionShows granules from convection - each is about 1000 km across and lasts for about 10 minutes

Solar Chromosphere

Maps of the solar chromosphere are made by observing light in the H-alpha lineLight is emitted in the H-alpha line when an electron jumps down from the n=3 shell to the n=2 shell in Hydrogen

Solar Corona

Only easily visible during solar eclipseEclipses can be created artificially in coronographs

SOHO/LASCO movie



Eruptions on the Sun

Sunspots - concentrations of magnetic flux on the solar disk, which appear dark because they are coolerProminences - loops and streamers of magnetic field which channel electrons in the coronaCoronal Mass Ejections -(CMEs) violent flares which eject particles from the sun at millions of miles per hour

Solar magnetic field loops

Solar Flares

Solar prominence seen by Skylab in 1973

SOHO/MDI 11th magnitude

earthquake on Sun following solar flare



Coronal Mass Ejections

CMEs are the cause of major geomagnetic storms on Earth

CMEs are NOT caused by solar flares, although they may both be signatures of rapid changes in the magnetic field

1015 - 1016 g of material is ejected into space at speeds from 50 to >1200 km/s

Can only be observed with coronagraphs

Coronal Mass Ejections

Coronal mass ejection in UV from SOHO

Solar Maximum Mission CME in 1989

Let’s Take a Break

QuickTime™ and aSorenson Video decompressorare needed to see this picture.

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

Red Giants and Supergiants

Hydrogen burns in outer shell around the core

Heavier elements burn in inner shells

Planetary Nebulae

Planetary nebulae are not the origin of planetsOuter ejected shells of red giant illuminated by a white dwarf formed from the giant’s burnt-out coreNot always formed

HST/WFPC2Eskimo nebula5000 light years

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 electronsWhite dwarf radius is about the same as the EarthA teaspoon of a white dwarf would weigh 10 tonsSome white dwarfs have magnetic fields as high as 109 GaussWhite dwarfs eventually radiate away all their heat and end up as black dwarfs in billions of years

Supernovae

Supergiant stars become (Type II) supernovae at the end of nuclear shell burningIron core often remains as outer layers are expelledNeutrinos and heavy elements released Core continues to collapse

Chandra X-ray image of Eta

Carinae, a potential supernova

Three Views of a Supernova



Neutron StarsNeutron stars are formed from collapsed iron coresAll 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 tonsNeutron stars with very strong magnetic fields - around 1012-13 Gauss - are usually pulsars due to offset magnetic poles

Cas A

~320 years old10 light years across50 million degree shell

Radio/VLA X-ray/Chandra

neutron star

Binary Star Systems

Often stars are formed in binary systemsSince they have unequal masses, the more massive star will evolve faster - and reach the end of its main sequence lifetimeIn some cases, the supernova of the primary star will not disrupt the binary system and a COMPACT BINARY is formedMass transfer can then occur from the main sequence star onto the collapsed, compact companion star - which can be a WHITE DWARF, NEUTRON STAR or BLACK HOLE

X-ray Binary

QuickTime™ and aCinepak decompressor


Pulsars

Radio pulses are powered by the energy released as the pulsar’s spin slows

We see the brightness change in a periodic way….we see this in their light curves!

Crab Nebula

Observed by Chinese astronomers in 1054 ADAge determined by tracing back exploding filamentsCrab pulsar emits 30 pulses per second at all wavelengths from radio to TeV

QuickTime™ and aIntel Indeo® Video R3.2 decompressor


Crab Nebula

Radio/VLA Infrared/Keck

Crab Nebula

Optical/HST WFPC2Optical/Palomar

Crab Nebula and Pulsar

X-ray/Chandra

Black Holes

Final state of stellar collapse

After supernova, if cores are larger than 3 Mo , a black hole will be formed

Escape velocity from a black hole is greater than the speed of light, once inside the event horizon

Some Other Stellar Types and Groups

…and a little reminder

Variable Stars

Most stars vary in brightnessPeriodic variability can be due to: Eclipses by the companion star Repeated flaring Pulsations as the star changes size or temperature

Novae are stars which repeatedly blow off their outer layers in huge flares – they are NOT supernovae!Flare stars have regions which explodePulsating stars have an unstable equilibrium between the competing forces of gas pressure and gravity

Cepheid Variables

Henrietta Leavitt studied variable stars that were all at the same distance (in the LMC or SMC) and found that their pulsation periods were related to their brightnesses

L =K P1.3

Polaris (the North Star)

is not constant, it

is a Cepheid variable!

Open Star Clusters

Open ClusterNGC 3293

d = 8000 c-yr 20 -1000 stars

young stars

mostly located in spiral arms of our Galaxy and other galaxies

solar metal abundance

Globular Star Clusters

Globular Cluster 47 Tuc

d=20,000 c-yr 104 - 106 stars

centrally condensed

old stars

galaxy halo

low in metals

Pleiades Star Cluster

A star cluster has a group of stars which are all located at approximately the same distanceThe stars in the Pleiades were all formed at about the same time, from a single cloud of dust and gas

Light-years

1 light-year is the distance light will travel in one year1 light-year = (2.998 x 108 m/s)(86400 s/d)(365 d/y) = 9.46 x 1012 km

A LIGHTYEAR IS A DISTANCE, NOT A TIME!

stars, our friends in the universe. the nearest stars distance to alpha or proxima centauri is ~4 x...

Documents