THE MILKY WAY

Intro Info

The Universe has aboutThe Universe has about 100 billion to 1 trillion100 billion to 1 trillion

galaxies, and each galaxy has on galaxies, and each galaxy has on the average the average

100 billion and 1 trillion numbers 100 billion and 1 trillion numbers of starsof stars ..

So how many stars are there roughly???

• Minimum Average: 1011 x 1011

• Maximum Average: 1012 x 1012

• BETWEEN 1022 AND 1024 STARS!!!• 10 sextillion and 1 septillion STARS!

Dwarf Galaxies

• About 10 million stars

Irregular Galaxies

• Can have up to 10 trillion stars

The Milky Way

• Barred spiral galaxy• Over 200 billion stars• One of tens of billions of galaxies in the

observable universe

The Milky Way is about 100 000 The Milky Way is about 100 000 light-years across, about 10 000 light-years across, about 10 000 light-years thick. light-years thick.

Our Solar System is located near Our Solar System is located near the edge of one of the arms. It is the edge of one of the arms. It is located about 30 000 located about 30 000 light-years light-years from the middle.from the middle.

Size and Shape

• Clouds of interstellar medium (dust and gas) fill the galactic disk and prevent our ability to see through it.– This long fooled astronomers to think we lived near

the centre of our galaxy

• In the 1920s, Harlow Shapley found that the globular clusters orbit a point tens of thousands of light years from our Sun. – This point is the centre of our galaxy, not our Sun!!– Also not the centre of our Universe!!!

Size and Shape

• Radio and infrared technologies allow us to see through the interstellar dust.

• Our Sun lies about 28 000 light-years from the centre of the Milky Way

Size and Shape• Flat disk of stars with a

bright central bulge (bar)• Spiral arms• Dimmer round halo

surrounding everything• A few hundred globular

clusters of stars orbit the galaxy’s centre

• In 2005, the Spitzer Space Telescope collected evidence that the Milky Way is a Barred Spiral

Terms

Gas Halo: also called galactic haloA gas halo is a ring of gas and stars surrounding

a galaxy.

A globular cluster is a spherical collection of stars that orbits a galaxy core as a satellite. They are older stars.

The Milky Way

• Has satellite galaxies (it is very large)

The Local Group

• The Milky Way is part of the Local Group.• The Milky Way and Andromeda are the

two most massive galaxies in this group (both spiral galaxies).

• Composed of 30 galaxies with the gravitational centre between the Milky Way and Andromeda.

• 10 million light years in diameter.

Virgo Cluster• The Milky Way is

part of the Local Group which is part of the Virgo Cluster.

• 1300 galaxies in this group.

• Many members of this galaxy are visible with a telescope.

• It is located in the constellation Virgo.

Virgo Supercluster (Local Supercluster)

• An irregular supercluster of galaxies that contains the Milky Way, Local Group and Virgo Cluster.

• Contains 100 galaxy groups and clusters• Diameter of 110 million light years

http://www.atlasoftheuniverse.com/virgo.html

Comparison of Solar System to Virgo Supercluster

http://upload.wikimedia.org/wikipedia/commons/a/a7/Universe_Reference_Map_%28Location%29_001.jpeg

It appears that we are in the It appears that we are in the middle of the Universe. That isn’t middle of the Universe. That isn’t true. We are only in the middle of true. We are only in the middle of

what we canwhat we can seesee..

We can see the same distance in We can see the same distance in all directionsall directions.

The next closest galaxy to us is Andromeda and is about

2 million light years away.

In 1842, Christian Doppler explained In 1842, Christian Doppler explained why an object making a noise sounds why an object making a noise sounds more shrill when it is coming towards more shrill when it is coming towards

you, and sounds deeper when it is you, and sounds deeper when it is going away from you. When an going away from you. When an

object is coming towards you, the object is coming towards you, the sound vibrations in the air are sound vibrations in the air are

travelling in the same direction as the travelling in the same direction as the object.object.

When the object passes you and When the object passes you and moves away, the sound vibrations moves away, the sound vibrations

are travelling in the opposite are travelling in the opposite direction from the moving object.direction from the moving object.

Every star sends out light waves. The Every star sends out light waves. The light appears bluer if the star is light appears bluer if the star is

coming towards us, and redder if it coming towards us, and redder if it is moving away. is moving away.

In the 1930’s, astronomers found thatIn the 1930’s, astronomers found that most galaxies show a most galaxies show a red shiftred shift. .

This means that they are moving This means that they are moving awayaway from our galaxy. The from our galaxy. The farthest galaxies are moving farthest galaxies are moving

away at thousands of kilometres away at thousands of kilometres a minute!a minute!

In the 1950s, astronomers discovered In the 1950s, astronomers discovered that some galaxies sent out radio that some galaxies sent out radio

waves. Their light was very strange waves. Their light was very strange because the light waves were because the light waves were

stretched out. These galaxies were stretched out. These galaxies were called called

quasarsquasarsand they have the largest red shifts and they have the largest red shifts known. They are the farthest from known. They are the farthest from

us. Looking at quasars is like looking us. Looking at quasars is like looking into the past.into the past.

The farthest known quasar is aboutThe farthest known quasar is about12 12 billion light-years away.billion light-years away.

When we look out into space, out beyond When we look out into space, out beyond the quasars, we are really looking back the quasars, we are really looking back

intointotimetime..

Why are all galaxies moving away from us? Why are all galaxies moving away from us? Why should the galaxies farthest away Why should the galaxies farthest away

from us move away the fastest? What’s so from us move away the fastest? What’s so special about us?special about us?

Quasars

• A very energetic and distant ACTIVE galaxy. They are the most luminous objects in the universe.

• QUASAR: quasi-stellar radio source• Quasars were first identified as being high

redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than extended sources similar to galaxies.

Rotation of Galaxies – The Missing Mass Problem

Distance from galaxy center

Orb

ital

sp

e ed

Observed

Expected

•Doppler Effect used to measure the speed of material orbiting around the center of a galaxy.•Photographs of galaxies show that luminous material appears to be concentrated towards the center and drops off with increasing distance.

Rotation of Galaxies – The Missing Mass Problem

Distance from galaxy center

Orb

ital

sp

e ed

Observed

Expected

If matter were really concentrated in this fashion, we would see “rotation curves” following the “expected” path in the diagram at right. What is observed instead is that rotation curves tend to remain high as far out as they can be measured. This means the existence of massive halos of dark matter in galaxies. The nature of the material comprising this dark matter is completely unknown at present, making this one of the greatest problems of contemporary astronomy.

Gravitational Lensing in Abell 2218 Cluster

Hubble Space Telescope Image

As predicted by Einstein’s General Theory of Relativity, a compact intervening object is bending and distorting light from individual members of this cluster so that we see a halo effect.

A Lensed Quasar

National Optical Astronomy Observatories Image

An intervening galaxy between us and this distant quasar is causing light from the quasar to be bent along curved paths that give rise to an Einstein cross, a phenomenon predicted by Einstein’s General Theory of Relativity.