Slide 1 M31: The Andromeda Galaxy 2 million light years from us

M31: The Andromeda Galaxy 2 million light years from us

http://antwrp.gsfc.nasa.gov/apod/

200 billion stars

Milky Way Galaxy

25,000 light years,Or ~ 8 kpc 1 pc = 3.26 ly

Galactic year = 225 million yrOur sun is 4.6 billion yr old

“Milky Way” – a milky patch of stars that rings the Earth

Galactos = milk in Greek

Galileo found that the Milky Way is made up of stars

The Structure of the Milky Way (1)

Disk

Nuclear Bulge

HaloSun

Globular Clusters

Local group: ~ 30 galaxies

Galaxy M31 in Andromeda – similar to the Milky Way Galaxy

1 Mpc from us

The Structure of the Milky Way (2)

Galactic Plane

Galactic Center

The structure is hard to determine because:1) We are inside2) Distance measurements are difficult3) Our view towards the center is obscured by gas and dust

William and Caroline Herschel, 1785

Herschel could not see very far because of the interstellar gas and dust. He concluded that we live near the center of a relatively small disk of stars.

Strategies to Explore the Structure of Our Milky Way

I. Select bright objects that you can see throughout the Milky Way and trace their directions and distances

II. Observe objects at wavelengths other than visible (to circumvent the problem of optical obscuration), and catalogue their directions and distances

III. Trace the orbital velocities of objects in different directions relative to our position

The key to determining the size and the shape of the Galaxy: measure the distances to the most distant stars and star clusters

Parallax works only for the closest stars within 500 pc

To probe the distance at larger scales, we must find standard candles – very bright objects of known luminosity (or absolute magnitude)

Then we can measure their intensities or apparent magnitudes and find the distance using the inverse square law:

R

d

)mJ/(s4

22 d

LI

5/)5(10pc)( Mmd

Henrietta Leavitt (1868-1921)

The key to the distance scale in the Universe: variable stars cepheids

Discovered and catalogued over 2000 variable stars in Small Magellanic Cloud

Delta Cephei Discovered in 1784 by John Goodricke (deaf-mute)

John Goodricke 1764-1786

Also explained a puzzle of Algol!

Leavitt noticed that the brightest variable stars had longest periods

Period-magnitude dependence for cepheids in SMC

Period-luminosity relation

Ave

rag

e m

agn

itu

de

M<

V>

Period (log P)

0.4 2.01.40.80.6 1.0 1.2 1.6 1.8

-2-7

-5-6

-4-3

Since all cepheids in SMC are at the same distance from us, the same relationship should be between their periods and absolute magnitudes!

Leavitt did not know the distance to SMC and could not calibrate this relation.It was done by Shapley

Harlow Shapley(1885-1972)

Proposed to use globular clusters as tracers of the mass distribution in our Galaxy

Exploring the Galaxy Using Clusters of Stars

Two types of star clusters:

1) Open clusters: young clusters of recently formed stars; within the disk of the Galaxy

Open clusters h and Persei

2) Globular clusters: old, centrally concentrated clusters of stars; mostly in a halo around the GalaxyGlobular Cluster M 19

Globular Clusters• Dense clusters of 50,000 – 1 million stars

• Old (~ 11 billion years), lower-main-sequence stars

• Approx. 200 globular clusters in our Milky Way

Globular Cluster M80

Locating the Center of the Milky Way

Distribution of globular clusters is not centered on the sun…

…but on a location which is heavily obscured from direct (visual) observation

Infrared View of the Milky Way

Interstellar dust (absorbing optical light) emits mostly infrared

Near infrared image

Infrared emission is not strongly absorbed and provides a clear view throughout the Milky Way

Nuclear bulge

Galactic Plane

Cepheid Distance Measurements

Comparing absolute and apparent magnitudes of Cepheids, we can measure their distances (using the 1/d2 law)!

The Cepheid distance measurements were the first distance determinations that worked out to distances beyond our Milky Way!

Cepheids are up to ~ 40,000 times more luminous than our sun

=> can be identified in other galaxies.

Cepheids help to determine distances to other galaxies

Cepheid Variables: The Period-Luminosity Relation

The variability period of a Cepheid variable is correlated with its luminosity.

=> Measuring a Cepheid’s period, we can determine its absolute magnitude!

The more luminous it is, the more slowly it pulsates.

What are cepheids??

Cepheids: what happens when the balance between the thermal pressure and gravity becomes unstable

Pulsating Variables: The Instability Strip

For specific combinations of radius and temperature, stars can maintain periodic oscillations.

Those combinations correspond to locations in the Instability Strip

Cepheids pulsate with radius changes of ~ 5 – 10 %.

2

0

2

4

6

5.0 4.5 4.0 3.5

log Tefflog Teff

log

(L

/L)

log

(L

/L)

Main Sequence

Solar-type stars

Mira LPVs

Cepheids

Irregular LPVs

DBVs

DDVs

PNNVs

Instability Strip

Classical Cepheids

RR Lyrae Scutis

VW Virginis

ZZ Ceti (DAVs)

There are several different types of variable stars

Mechanism of pulsations: the battle between opacity, the temperature and gravity

Explained by Sergei Zhevakin in 1950s

A simple pulsation cycle

•At one point in the pulsation cycle, a layer of stellar material loses support against the star’s gravity and falls inwards.

•This inward motion tends to compress the layer, which heats up and becomes more opaque to radiation.

•Since radiation diffuses more slowly through the layer (as a consequence of its increased opacity), heat builds up beneath it.

N.B. These diagrams are definitely not to scale!!

N.B. These diagrams are definitely not to scale!!

•The pressure rises below the layer, pushing it outwards.

•As it moves outwards, the layer expands, cools, and becomes more transparent to radiation.

•Energy can now escape from below the layer, and pressure beneath the layer drops.

•The layer falls inwards and the cycle repeats.

•This animation illustrates two stellar pulsation cycles.

General idea is OK, but it does not work – pulsations will be damped

•In most stars there are two main partial ionisation zones.

Partial ionisation zones

•The hydrogen partial ionisation zone is a broad region with a characteristic temperature of 1 to 1.5 104 K, in which the following cyclical ionisations occur:

•The helium II partial ionisation zone is a region deeper in the stellar interior with a characteristic temperature of 4 104 K, where further ionisation of helium takes place:

Pulsating Variables: The Valve Mechanism

Partial He ionization zone is opaque and absorbs more energy than necessary to balance the weight from higher layers.

=> Expansion

Upon expansion, partial He ionization zone becomes more transparent, absorbs less energy => weight from higher layers pushes it back inward. => Contraction.

Upon compression, partial He ionization zone becomes more opaque again, absorbs more energy than needed for equilibrium => Expansion

•The pulsation properties of a star depend primarily on where its partial ionisation zones are found within the stellar interior.

•For stars hotter than Teff ~

7500K, the partial ionisation zones are located too close to the star’s surface, where there is not enough mass to drive the oscillations effectively.

•The location of the partial ionisation zones is determined by the star’s temperature.

-5

-7

-9

-10

-3

-6

-8

-4lo

g (

1-M

r/Mst

ar)

log

(1-

Mr/M

star)

He

H Su

rfac

eC

entr

e

Teff ~ 7500K

•However at low temperatures, energy transport via convection becomes quite efficient in the stellar interior, preventing the build-up of heat and pressure beneath the driving pulsation layer.

-5

-7

-9

-10

-3

-6

-8

-4

log

(1-

Mr/M

star)

log

(1-

Mr/M

star)

He

H

Su

rfac

eC

entr

e

Teff ~ 5500K

•For stars cooler than Teff ~ 5500K, on

the other hand, the partial ionisation zones are deep in the stellar interior.

•Computer modelling of stellar pulsation suggests that it is primarily the helium II ionisation zone which is responsible for the observed oscillations of stars on the instability strip.

Modelling pulsations

•The hydrogen ionisation zone, however, still plays an important role, producing an observable phase lag between the star’s maximum brightness and its minimum radius.

Time (days)Time (days)

rR

/Rm

inT

eff

V

Observed properties of a classical Cepheid

Note the phase lag between the star’s maximum brightness and

its minimum radius.

Note the phase lag between the star’s maximum brightness and

its minimum radius.

The structure of the Galaxy

Two components:• Disk • Spherical (halo and bulge)

Spiral arms

Methods to map out the spiral arms: • galactic (or open) clusters

(found in spiral arms)• O & B stars: bright, short lived• HI 21 cm line

(HI is the astronomer's name for hydrogen atoms) when the electron "flips its spin", it emits a 21 cm radio wave (radio frequency passes through dust)

• HII regions HII ionized Hydrogen (found preferentially in spiral arms)

• Molecular clouds ( CO)

The puzzle of spiral arms

M 51

Density wave theory of the spiral arms

Star Formation in Spiral Arms

Shock waves from supernovae, ionization fronts initiated by O and B stars, and the shock fronts forming spiral arms trigger star formation

Spiral arms are stationary shock waves, initiating star formation

Star Formation in Spiral Arms (2)

Spiral arms are basically stationary shock waves

Stars and gas clouds orbit around the Galactic center and cross spiral arms

Shocks initiate star formation

Star formation self-sustaining through O and B ionization fronts and supernova shock waves

Winds from other stars drive new star formation cycles

Measuring the mass of the Galaxy

Key idea: to measure rotation velocities

Orbital Motion in the Milky Way

Differential Rotation• Sun orbits around Galactic center with 220 km/s

• 1 orbit takes approx. 240 million years

• Stars closer to the galactic center orbit faster

• Stars farther out orbit more slowly

r

GMv

r

rGMv

)(

3

3

4~)( rrM

)(~ rrv

Matter extends beyond the visible disk!

There is much more matter than we see!

Dark matter halo

Fritz Zwicky 1898-1974

"spherical bastards”

Walter Baade 1893-1960

What is dark matter???

• White dwarfs

• Brown dwarfs

• Black holes

• Exotic particles

• ???

• Revision of the Standard Model of elementary particles may be necessary

The age and origin of our Galaxy

Age of the star clusters from turnoff points

The age of our Galaxy

12-13 billion years for oldest globular clusters in the halo

9-10 billion years for oldest open clusters in the disk

Disk seems to be younger than halo (?!)

Two populations of stars

Walter Baade1893-1960

Ages of the stars

Their main difference is in chemical composition

Population I – metal-richPopulation II – metal-poor

Metals: all elements heavier than helium

The Abundance of Elements in the Universe

Logarithmic Scale

All elements heavier than He

are very rare.

Linear Scale

Metals in StarsAbsorption lines almost exclusively from hydrogen: Population II

Many absorption lines also from heavier elements (metals): Population I

At the time of formation, the gases forming the Milky Way consisted exclusively of hydrogen and helium. Heavier elements (“metals”) were later only produced in stars.

=> Young stars contain more metals than older stars

Continuing cycle of stellar evolution

Metal abundance in stars is related to their age!

More metals Younger star

Stellar Populations

Population I: Young stars: metal rich; located in spiral

arms and disk

Population II: Old stars: metal poor; located in the halo (globular clusters) and

nuclear bulge

There were no metals in the early universe, before the first stars were born

All heavy elements were synthesized in stars

That is why metals are so rare in the universe

The lack of metals in Population II stars also suggests that the disk is younger than the halo

History of the Milky Way

The traditional theory:

Quasi-spherical gas cloud fragments into smaller pieces, forming the first, metal-poor stars (pop. II);

Rotating cloud collapses into a disk-like structure

Later populations of stars (pop. I) are restricted to the disk of the Galaxy

Problems with the traditional model

• Some of the oldest stars are in the bulge, and some younger clusters are in the halo

• Many Population II stars in the bulge are metal-rich• Even the oldest stars have some metals• Maybe the bulge formed first and the halo later?• Our Galaxy may have absorbed many small galaxies

in the past (galactic cannibalism)• Many globular clusters have been destroyed and gave

their stars to the halo

What is in the Galactic Center??

The Galactic Center (1)

Wide-angle optical view of the GC region

Galactic center

Our view (in visible light) towards the galactic center (GC) is heavily obscured by gas and dust

Extinction by 30 magnitudes

Only 1 out of 1012 optical photons makes its way from the GC towards Earth!

Need to observe the GC in the radio, infrared, or X-ray range

Infrared imagesCentral 2 pc

Synchrotron radiation of relativistic particles

The central radio emission consists of three parts: Sagittarius A East (blue): a supernova remnant, which was produced by a violent explosion only several tens of thousands of years ago. The origin is unknown. Explanations range from a star disrupted by a black hole to a chain reaction of ordinary supernovae or even a gamma-ray burst. Sagittarius A West or Minispiral (red): Gas and dust streamers ionized by stars and spiraling around the very center, possibly feeding the nucleus. Sagittarius A *: A bright and very compact radio point at the intersection of the arms of the Minispiral (difficult to see in this image)

Fast rotation of spiral filaments around Sgr A*

If one looks at this region with big telescopes and near-infrared cameras one can see lots of stars. If one takes pictures every year it seems that some stars are moving very fast (up to 1500 kilometers per second). The fastest stars are in the very center - the position marked by the radio nucleus Sagittarius A* (cross).

Distance between stars is less that 0.01 pc

Explanation: the dark mass ~ 2.6 million solar masses

Rotation curve for the Galactic Center

Is this a black hole?!

A Black Hole at the Center of Our Galaxy

By following the orbits of individual stars near the center of the Milky Way, the mass of the central black hole could be determined to ~ 2.6 million solar masses

Radio observations with Very Long Baseline Interferometry (VLBI) which are thousands of times more precise than optical observations (good enough to easily pin-point a source the size of a pea in New York when sitting in Paris)

Resolution limit:

D

~rad)(

Determined by diffraction

For visible light ~ 0.5 m and Dmax ~ 10 m

D is the telescope mirror

For radio waves ~ 1 cm = 104 m but Dmax ~ 107 m !

The size of Sagittarius A* as measured with VLBI at different wavelengths of the observed radiation

Race to measure the size of Sgr A*0.1 mas = 100x106 km2 Rs = 8x106 km

Size ~ 1 AU (12 Schwarzschild Radii)

Density ~ 7x1021 Msun/pc3

Recent VLBI observations (Nature 2005)

No other explanation but a black hole!

Indicate heavier black hole ~ 4 million solar masses?

“Shadow” of a black hole

Black hole vicinity is probably very messy …

Cores of other galaxies show an accretion disk with a possible black hole

What about X-ray emission due to accretion?

Rather weak X-ray source

Chandra X-ray image of the Sgr A West region

Another Chandra X-ray image

Rapid X-ray variability and minute-long flares!

X-ray View of the Galactic Center

Chandra X-ray image of Sgr A*

Supermassive black hole in the galactic center is unusually faint in X-rays, compared to those in other galaxies

Galactic center region contains many black-hole and neutron-star X-ray binaries

Evidence for a black hole of ~ 3-4 million solar masses:

• Rotation curve indicating an ultra-compact object• No motion of the central object• Rapid variability• Dense stellar population• Radio jets

Radio jets but rather weak X-ray emission

Other galaxies contain much heavier black holes and stronger activity

Slide 1 M31: The Andromeda Galaxy 2 million light years from us

Documents

Transcript of Slide 1 M31: The Andromeda Galaxy 2 million light years from us