The Milky Way Galaxy

Our home in the Universe

Overview• Galaxies = groupings of matter within empty

Universe– contain stars, dust, gas– formed in early Universe– many types of galaxies – groups of galaxies form clusters– groups of clusters form superclusters– some galaxies collide– some galaxies have very active (energetic) cores.

Black hole at center?

Finding the center of the galaxy

Finding the center of the galaxy

Finding the center of the galaxy• The interstellar dust is concentrated in the

plane of the galaxy, and therefore obscures our view of stars within the galactic plane. – Interstellar extinction

• Like a motorist determines his position on a foggy day by looking for tall buildings that extend above the fog and are visible, astronomers determine the location of the galactic center from our view point by looking at stars (Globular Clusters) that are lie outside the galactic plane.

Finding the center of the galaxy

• Astronomers use globular clusters (a class of star clusters associated with the galaxy but lie outside) in order to locate the galactic center.

• However, one needs to know the distance to these clusters in order to find the center.

• Astronomers use pulsating variable stars (Cepheid variables and RR Lyrae stars) that are found in globular clusters to find the distance to these clusters.

Finding the center of the galaxy• In early 90s Henrietta Levitt discovered that

the pulsation period of a Cepheid is directly related to it Luminosity.

• Shortly after Harlow Shapley discovered an identical relation in RR Lyrae stars.

• So, by measuring the pulsation period, we can know the luminosity, and from observations we can determine the apparent brightness of the star. Once luminosity and apparent brightness and luminosity are known, the distance can be determined.

Finding the center of the galaxy• Using the distance to globular clusters,

astronomers have determined the galactic center to be approx. 8pc away from us.

• Before, these calculations (the first such attempts were made by Shapely in 1920) it was believed that the Sun (and thus the Earth) was at the center off the Galaxy.

• Just as Copernicus & Galileo dislodged Earth from is long held special place at the center of the solar system, Shapley managed to show that Earth is nowhere near the galactic center.

Physical characteristics of the Milky Way• Disk shaped with central bulge

• Spherical halo of stars and globular clusters

• Spiral arm pattern

• About 1011 M

• REALLY BIG! – If Milky Way were size of Earth, solar system

would be size of a cookie

• Sun’s location: 2/3 from edge to center– 240 million year revolution period at Sun’s location

Physical characteristics of the Milky Way

Milky Way in infrared(IR) light: a) far IR from IRAS spacecraft. b) near IR from COBE observations. The light in b) come from stars in the plane of the galaxy

Things in the Milky Way• Star clusters

– globular– open (Pleiades)– associations

• Gas (form dark or emission nebulas)• Dust (form dark or reflection nebulas)• Stars

– Population I - young stars– Population II - old stars

Star classification: Young vs. old

• Population I– younger– bluer– located in spiral arms in disk

• Population II– older– redder– located in globular clusters (halo) and galactic bulge

Star clusters• Globular

– 105 to 106 stars, large, located in halo and nucleus (bulge) of galaxy, older stars (Pop II), no gas or dust

• Open– few hundred stars, small, located in spiral arms,

young (Pop I) stars, no gas or dust

• Associations– very large, lie along spiral arms, very young blue

stars, still contain gas and dust

Interstellar Matter:Gas and Dust

• Dust – extremely sparse– very efficient at blocking visible light– blocks most of Milky Way from view in disk– “zone of avoidance” is region of sky along

galactic disk where our view is blocked by dust– only infrared and radio can penetrate dust (see

COBE photo of galaxy in infrared)

NGC 4565: How we would see the milky way if we could see it edge on. Gas & dust can be seen in the plane of the galaxy

Interstellar Matter:Gas

• Gas– 90% hydrogen, 10% helium (by number)– can have molecules– cold gas: detected by absorption spectrum– emission nebula: emission spectrum, gas heated

by nearby hot stars. Typically look red in pictures (hydrogen emission lines in the red part of spectrum). Called HII region.

Interstellar Matter:Dust

• Dust– very small (nano- to micro- meter sizes)– consists of silicate core with icy surface– blocks light (makes distance objects dimmer and

seem more distant), causes reddening of light from objects behind it by scattering blue light more than red light

– reflection nebula: reflect starlight. Typically look blue in pictures (reflecting light from hot blue star)

Spiral Arms• How do we see them?

– Mapped by looking for gas and dust concentrations using optical and radio information

• 21 cm spin flip radio emission line

• What are they?– Higher density regions - more stars per unit volume

than other regions of Milky Way

• Why are they?– Good question! Density waves?

21 centimeter (radio) line of hydrogen

• Special radiation emitted by cold hydrogen allows us to map out cold hydrogen clouds (otherwise they are invisible)– detected at radio wavelengths of 21 cm– “spin-flip transition”

• When the spin of the proton and the electron in a hydrogen atom are in the same direction they are in a higher energy state.

• Then an electron can flip its spin and get to a lower energy state, and in the process emit radiation(photon) with a wavelength of 21cm.

21-cm radio emission from interstellar hydrogen gas.

The entire sky at 21 cm: We see that most of the 21cm emission, and therefore, abundance of hydrogen gas in the galactic plane.

•Our galaxy has 4 major spiral arms.

• Our Sun lies in one of the shorter spiral arms - the Orion arm

Why are there spiral arms?

• Density waves - these denser regions are created when fast moving interstellar gas & dust moves into slow moving density waves or spiral arms. Then, this fast moving gas and dust gets compressed into a nebula. – This compression begins the process by which new

stars are formed.

A density wave on a highway

This process is similar to how a slow moving truck causes a traffic jam on a highway, creating a region of higher density of cars.

Density Wave model

Importance of spiral arms

• Star formation definitely occurs in spiral arms– dust clouds seen going in on one side, stars

going out on the other– spiral arms are home to youngest stars

(including short-lived hot blue stars)

A spiral galaxy: M83

We see evidence of star formations in the spiral arms in other spiral galaxies.

Visible light

21-cm radio wavelength.

The Galactic Center

• In the very middle of our galaxy, something wild is happening– very powerful radio source (other radiations

absorbed by bulge and disk) at very center only about 10AU in size

• towards constellation of Sagittarius

– Million solar mass black hole?• Streams of antimatter also detected coming out in jets

from center of galaxy - only black hole can do this

Formation of the Milky Way

• Large cloud of swirling gas collapses to form disk and bulge (similar to Solar System)– localized collapses form early stars which enrich interstellar

medium with heavy elements

– Pop II stars formed early

– Collapse complete, disk formed, spiral arms form, Pop I stars form

– munching of smaller galaxies occurs

• Eventually all gas and dust used up and star formation will cease

The Question of Dark Matter

• The spiral arms of our Galaxy suggest that the disk rotates– The stars, dust and the gas is orbiting the

galactic center.

• The rotation rate of our galaxy has been measured.

• These measurements can be used to find out the mass and the mass distribution of our galaxy.

The Question of Dark Matter• Recent calculations indicate that the mass

of our Galaxy to exceed 1012 M.

• However, observations from all forms of EM radiation (from radio waves to gamma rays) tells us that all the stars, dust and gas in the galaxy only account for 10% of the mass of the galaxy.

• Then, what makes up the balance 90% of the mass of our galaxy?– Missing matter or Dark Matter problem

The Question of Dark Matter• This material, which seems not to emit any

type of EM radiation (not even 21cm radio waves), but which is by far the predominant constituent of our Galaxy is called Dark Matter.

• The only way we “know” of its presence is due to the gravitational effects this matter exerts, and the corresponding effect on the rotation of our Galaxy.

The Question of Dark Matter• The nature of this mysterious dark matter

has been and still is one of the leading research problems in Astrophysics. Current proposals are:– MACHOs: massive compact halo objects. Less

than 1M, dim objects like brown dwarfs, white dwarfs, or black holes.

– Non zero Neutrino masses: Latest research in elementary particle physics sees signs that the elementary particle, the neutrino may have a small but a non zero mass as contrary to the predictions of the Standard model.

The Question of Dark Matter

– Non zero Neutrino masses: Latest research in elementary particle physics sees signs that the elementary particle - the neutrino may have a small but a non zero mass. This is contrary to the predictions of the Standard model of particle physics.

– WIMPs: Weakly interacting massive particles. This is a new class of subatomic particles predicted by a new theory (not yet confirmed by experiment). These particles have a mass but does not emit or absorb any kind of EM radiation.